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1 Foundry technology CONTENTS INTRODUCTION Manufacturing of model complexes General information Equipment and tools Molding materials and mixtures General information Properties of molding sands Preparation of molding and core sands Production of molds General information Tools for manual molding Flasks Molding in flasks Machine molding Gating systems Production of cores Assembly of molds

2 INTRODUCTION General information about the mold Casting is obtained by filling the mold cavity with liquid metal. Once poured, the liquid metal cools in the mold and solidifies to form a casting. We will consider the sequence of the technological process for manufacturing a casting using the example of casting a cast-iron bushing 1 According to the drawing of the bushing, a wooden model 2 is made. Models are made of wood, metal, plaster, plastic and other materials. The hub model consists of two halves, which are mutually centered using spikes and sockets. The hole of the sleeve 1 is made by the rod 3. The rod is part of the mold. It is made from a core mixture compacted in box 4. After being removed from the box, the core is dried in an oven. When assembling the mold, the dry core is installed with core marks in the corresponding nests of the mold, obtained using the marks 5 of model 2. The length of the core is greater than the length of the casting cavity by the value of the marks. The mold for the sleeve is assembled from two half-moulds: the upper 6 and the lower 7. The half-moulds are made from molding sand, which is compacted in cast-iron or steel frames 8, which are called flasks. Manufacture of the casting mold of the sleeve Half of the model 2 is installed on the mold plate 9, on which it is necessary to obtain an imprint in the lower half-mold, as well as the flask 8. The surface of the model 2 and the shield 9 is sprinkled or sprayed with a separating liquid, after which the molding sand is poured into the flask and compacted. Excess molding sand is removed from the surface of the compacted mold half, the flask is turned 180 degrees and placed on the bottom plate 9. Then, the upper half of the model is placed on the lower half of the model, and the upper half on the lower flask. The surface of the model is again sprinkled with separating sand, the models of the gating system are placed, the molding sand is poured into the upper flask and compacted. The upper half of the mold is removed, the halves of the models are removed, the core is installed and the mold is assembled. To assemble the molds, the flasks have special bushings 10, which include centering pins. Liquid metal

3, when filling the mold, presses on the walls of the mold, as a result of which the upper flask can rise, and then a gap is formed along the parting plane, through which the metal can flow out of the mold. To prevent this, the upper half mold is attached to the lower bracket 12, and sometimes a load is placed on the upper flask. When pouring, liquid metal enters the mold cavity 13 through gating channels. The system of channels that bring metal into the mold is called gating. The gating system consists of riser 14 (vertical channel), slag trap 15 and feeder 16, through which the metal enters the mold cavity. The sprue system also includes an upstream 17. The upstream serves to exit the mold of air and gases, as well as to control the filling of the mold with metal. After solidification and cooling of the metal, the mold is destroyed and the casting is released from the molding sand, sprues are cut off and the surface of the casting is cleaned from the sand. The above-described casting mold is called disposable, since it is used once. Usually, disposable casting molds are made from molding sands, the main component of which is quartz sand. Clay is used as a binding additive that gives strength to the mixture. The strength of such mixtures is relatively low, and the pressure of the liquid metal on the walls of the mold is quite high, so the molds from sandy-clay mixtures have to be made thick-walled. However, if special materials are used as a binder, which impart high strength to the molding sand, then the mold can be made shell (thin-walled). This makes it possible to drastically reduce the consumption of the molding sand, and also, due to its special properties, to increase the accuracy and cleanliness of the surface of the castings. In one-time thick-walled molds from sand-clay mixtures, it is possible to obtain castings of a very complex configuration weighing from several grams to tens of tons from various alloys, both in single-unit and serial and mass production. This is explained by the relative simplicity of the technological process, the low cost of the materials used, the sufficient accuracy of castings, good surface finish, and the possibilities of mechanization and automation of the manufacturing process. AT foundry molds made from special highly refractory masses, for example, based on graphite, are also used. In such molds, up to several dozen castings can be obtained without significant wear of the mold. These forms are called semi-permanent. They are used in small-scale production of castings from cast iron and non-ferrous alloys (aluminum, copper). For mass and large-scale production

4, the stability of these forms is insufficient, and for a single production, the cost of their manufacture is high. Metal molds, also called permanent molds, are widely used. In these forms, it is possible to obtain from several tens to several thousand castings from steel, cast iron and non-ferrous alloys. Castings can have a complex configuration and weigh several tons. Most often, castings of small and medium (up to several tens of kilograms) mass from light non-ferrous alloys are made in metal molds. Castings obtained in metal molds have a clean surface and increased dimensional accuracy. The use of permanent molds makes it possible to eliminate molding sand, improve working conditions, mechanize and automate production. However, the cost of metal molds is quite high, so they are used in large-scale and mass production of castings. The technological process of manufacturing castings in disposable molds is widely used in the foundry industry. It consists of various processes that are carried out in special workshops or departments of the foundry. The technological process of manufacturing a casting begins with the preparation of a model set: models or model plates, model shields, core boxes, drying plates, templates for checking the dimensions of the mold and cores, conductors to templates for checking the correct installation of cores in the mold, flasks, pins, etc. . A model set is made in a model shop or a model department of a foundry. An equally important link in the technological chain is the preparation of materials for the manufacture of a mold. Molding materials are materials used to make one-time and semi-permanent molds. These are sands, binders and special additives. Initial molding materials are stored in the warehouse of molding materials in special containers and bunkers. Upon receipt at the warehouse, they must check the compliance of their quality with a certificate. Quality control of molding materials is carried out in special laboratories. The process of making molds is called molding. In foundry production, manual and machine molding is used: in single-piece and small-scale production, manual molding (molds are usually made from wooden models), in mass-flow and batch production, machine molding (moulds are made on machines from metal models).

5 Rods are obtained using boxes or templates. Finished rods are dried in special ovens (dryers) to increase their strength, gas permeability, and also reduce gas generating capacity. The rods before installation in the mold are painted with paints consisting of refractory materials: graphite, powdered quartz, iron-free zircon, etc., which is necessary to improve the purity of the casting surface. Before assembly, the raw mold halves are dusted (with graphite, talc, charcoal, etc.) and painted to obtain a clean casting surface. If the casting has a cavity, then a core is installed in the mold before assembly. Then the mold is assembled, the flasks are fastened with bolts or staples and fed to the pouring with liquid metal. As starting materials for the production of liquid iron and steel, ingot foundry and pig irons, iron and steel scrap are used. Briquetted chips, ferroalloys, fuels and fluxes. These raw materials are called charge materials. They are stored in the warehouse of the charge, where they also prepare the raw materials for melting: sorting, crushing to the required size, blending, weighing individual portions various materials in accordance with the calculation to obtain a given chemical composition of the metal. Prepared mixture with special vehicles served in the melting compartment for the preparation of liquid metal (metal melting). Melting furnaces are called units designed to melt and overheat ferrous and non-ferrous metals and alloys. For melting cast iron, special cupola furnaces, electric furnaces and flame furnaces are used; for melting steel, open-hearth furnaces, converters, electric furnaces, for melting non-ferrous alloys, electric furnaces and flame furnaces. The molten metal must be superheated in a furnace to a certain temperature so that it fills the mold well. After melting and overheating, the metal is poured from the furnace into various ladles and transported to the mold casting area. The metal poured into the mold, giving off heat to the mold, cools and hardens. After the casting has cooled, the molds are destroyed (knocked out) and the castings are removed from the molds. The molds are knocked out only after the casting has cooled to a certain temperature, since at high temperatures the alloys are not strong enough and the casting can be destroyed. The knockout of molds is carried out on special installations located in the department or at the knockout area. Castings have sprues, bulges, sometimes burrs and metal fills, their surface can be contaminated with molding sand burnt to it.

6 Cutting off or cutting sprues, sprues, burrs, cleaning the surface of castings is carried out in the cleaning department and cutting off castings with a special tool, on shot-blasting and shot-blasting machines, in hydraulic, sand-hydraulic and cleaning drums. After that, the castings go to the department technical control(OTK). Castings are monitored here: their dimensions and tightness, the presence of internal and external defects (shrinkage cavities, gas cavities, cracks, etc.), mechanical properties and metal structure are checked. Castings with minor defects are corrected in various ways: gas and electric welding, impregnation with various resins, putty, etc. Very often, in order to obtain the required structure and mechanical properties, to relieve internal stresses, castings are subjected to heat treatment by heating and cooling according to strictly specified modes (according to time and temperature) in thermal ovens. This operation is carried out in the thermal department of the foundry. Then the castings are again subjected to cleaning and control. Accepted by Quality Control Department or foundry foreman. castings are sent to the warehouse of finished products, and from there to machining. Some castings are painted before being sent to the machine shop to prevent corrosion. During machining, the castings are given the final geometric shape, the required accuracy and surface finish, provided for by the drawings and specifications for the finished part. This is the most time-consuming process in mechanical engineering, since the cost of machining is 40-60% of the cost of manufacturing a machine. Therefore, it is necessary to strive to obtain castings with minimal machining allowances or so accurate and clean that machining is not required. Manufacture of model kits General information For the manufacture of castings, a large number of different devices are used, which are called foundry equipment. A part of the casting equipment, which includes all the technological devices necessary to obtain a casting model in the form of an imprint, is called a model set. The model set consists of casting models and elements of the gating system; core boxes; pattern plates for installation or

7 fastening of casting models and gating system; drying plates and devices for fine-tuning and control of forms and cores. When molding, in addition to the model kit, flasks and various devices are used - filling frames, shields, pins, brackets, etc. Therefore, with the concept of a molding kit, i.e. a complete set of equipment necessary to obtain a one-time form. Model kits are made by model workers, as a rule, of high qualification. The model set must meet the following basic requirements: 1) Provide a casting of a certain geometric shape and size; 2) Possess high strength and durability, i.e. ensure the manufacture of the required number of molds and cores; 3) Be technologically advanced in manufacturing; 4) Have a minimum weight and be easy to use; 5) Have a minimum cost, taking into account the cost of repairs; 6) Maintain dimensional accuracy and strength for a certain period of operation. The required accuracy, strength and durability of the model kit depend on the conditions for the production of a single, serial, mass. In single and small-scale production, wooden pattern sets are most often used; in mass and large-scale production - metal model kits, which, although more expensive, are much more durable than wooden ones. In serial production, in many cases, models made of plastics, such as epoxy resins, as well as plaster and cement, are successfully used. Metal and plastic models retain dimensional accuracy for a long service life, contribute to obtaining a clear casting configuration, are strong and durable. However, the cost of manufacturing metal and plastic models is 3-5 times higher than the cost of manufacturing wooden ones, so their use must be justified. economic calculation. The correct, economically justified choice of material for a model set can significantly reduce the cost of castings.

8 Equipment and tools Equipment. For wood processing, circular and band saws are used, machines: jointing, thicknessing, milling, grinding, tenoning. A circular saw is used for longitudinal and transverse sawing of boards and bars. The tape machine is used for rectilinear and curvilinear sawing of boards. Lumber is fed manually under the cutting edge of a moving vertically closed band. For safe operation, the belt web, together with the pulleys, is surrounded by a metal mesh casing. The jointing machine is used for processing the planes of bars and boards. On the table plate of the machine there is a shaft with lamellar knives, which rotates from the electric motor. By moving the table plate with the help of screws, a certain thickness d of the cut chips is set. Boards are fed to the jointer manually, with the board pressed against the plate. Thickness planer is used for planing the surface of the board and for leveling its thickness. Usually, boards are planed on thicknessing machines, one of the surfaces of which is processed on a planer. Boards are planed, one of the surfaces of which is processed on a planer. Thickness planer has a table that moves vertically to set a given thickness of the planed board, a shaft with knives that rotates from an electric motor. The board is fed to the knife shaft with special rollers and rollers. On milling machines, the curved surfaces of wooden blanks are processed, especially for core boxes, which have a large number of curved surfaces. There are several types of milling machines: vertical, horizontal and copying. The grinding machine is used for grinding with a tape or sandpaper wooden blanks of models and core boxes. Grinding machines come in various designs: belt, disc and combined. The lathe is used to process workpieces of models and core boxes that have the shape of bodies of revolution. The workpiece is strengthened in the centers of the machine on a faceplate or in a special chuck. Billets with a diameter of mm with the arrangement of wood fibers perpendicular to the axis of rotation are fixed on the faceplate with screws. Blanks for models of pulleys, flywheels and

9 other models with a diameter of 3000 mm or more are processed on lathes. For planing, milling, drilling, grinding, screwdriving, etc. use an electrified tool that greatly facilitates the work of the modeler. The following tools are most common: a circular electric saw of the I-78 model with a gearbox for trimming workpieces, sawing grooves and other work, a band saw, an electric planer, an electric cutter, an electric reamer, and a tool for electric grinding of model surfaces. Measuring tool. In the manufacture of models and core boxes, a measuring tool is used: a shrinkage meter, square, bevel, thickness gauge, compass, caliper, inside gauge and caliper. A shrink gauge measures the dimensions of the blanks of models and core boxes. Shrinkage meters (rulers) are made longer than an ordinary simple meter by the amount of shrinkage of the casting alloy. Right angles are checked with a square and perpendicular lines are marked on the bars and boards; it consists of a block and a thin ruler inserted into it at a right angle. When using a square, the block is applied to the plane of the workpiece, taken as the base. Malka, metal or wooden, serves to check various angles and for marking, consists of a block and a ruler (pen) connected to the block with a hinge screw. The thickness gauge is necessary for drawing parallel lines on bars and boards. Two wooden or metal bars are inserted into the thickness gauge block, having metal studs at the ends. During operation, the block is pressed against the base plane of the board, and each block is fixed at a certain distance from the plane of the block to the metal pin. When moving the block, the metal pin puts a risk on the surface of the board. The outer dimensions of the bodies of revolution, as well as the thickness of the products, are measured with a caliper, the diameters of the holes, recesses and distances between the individual parts of the model are measured with a caliper. Large circles are marked with a caliper.

10 Cutting and planing tools. In the manufacture of models and core boxes, planing and cutting tools are used: chisels, sherhebels, planes, jointers, tsinubels, drills and devices for folding. Flat chisels process flat and convex surfaces. Semicircular chisels cut out internal curved surfaces. Clukarzes process surfaces that cannot be processed with ordinary chisels. With the help of a chisel, recesses are obtained in models and core boxes. Sherhebel is used for rough wood processing. A plate with a semicircular blade, fixed with a wedge, is inserted into the slot of the sherhebel block at an angle of 45 0. To obtain a cleaner surface, single or double planers are used. Planes with double cutters process the end and share surfaces of the workpieces. Planes longer than 300 mm, when you need to get a flat surface of the product, are planed with a jointer. The jointer device is similar to the planer device. Molding materials and mixtures General information Molding materials are materials used to make molds and cores. Molding materials are divided into initial molding materials, molding and core mixtures, auxiliary molding compositions. The initial molding materials are divided into two groups: 1) the main refractory base of the mixture (quartz sand, etc.), binders (clay, various resins, other binders); 2) auxiliary, for example, various additives (coal, wood flour, peat, etc.), which impart certain properties to the molding or core sand. Molding and core sands are prepared from the original molding materials and from waste mixtures (mixtures that were in

11 usage). The composition of the mixtures depends on the purpose, the method of molding, the type of metal poured into the mold. Auxiliary molding compositions are materials (paints, adhesives, putties) necessary for finishing and correcting molds and cores. Properties of molding sands In order to obtain high-quality molds, cores and suitable castings, molding and core sands must have technological properties that meet certain requirements. For good compaction of the molding sand in the flask, the plasticity of the mixture is of great importance - the ability to deform under the action of external forces or its own weight, which ensures the imprint of the model or the filling of the cavity of the core box. The plasticity of the molding and core sand depends on the properties of the mixture components and the binders used. For example, a mixture with an oil binder has great plasticity; sand-clay mixtures have little plasticity. The casting mold must have sufficient strength so that it does not collapse during assembly, transportation and pouring with metal. Therefore, the molding sand must also have a certain strength - the ability to resist destruction under the action of a load. The strength of the sand is dependent on sand grain size, moisture content, density, and clay or binder content in the sand. With an increase in density, a decrease in the size of sand grains, and an increase in clay content, the strength of the mixture increases. The flowability of the mixture affects its hanging in the bunkers, the filling and uniformity of the distribution of the mixture when filling into the flask, the quality and duration of mixing the mixture in mixers. Flowability is associated with clumping - the ability of a mixture to form lumps. Flowability and compressibility depend on the strength of the bonds of sand grains at the points of contact. The initial (bulk) density of the mixture increases the uniformity of the mold compaction. Therefore, the mixture should have good flowability - minimal lumpiness. Of great importance is surface strength - the resistance of the surface layer of the form or rod to abrasion. Surface strength is characterized by shedding. In the process of pouring and cooling the casting, the walls of the mold are heated by the metal to high temperatures, practically equal to the temperature of the metal,

12 Therefore, molding materials must have high refractoriness. This is one of the main requirements for molding materials. Refractoriness - the ability of a mixture to resist softening or melting under the action of a high temperature of a liquid metal - depends on the refractoriness of the components of the mixture and their quantitative ratio. The more impurities in sand and clay, the lower the refractoriness of molding and core sands. The coarser the sand and the less impurities, dust and more silica in it, the more refractory the mixture. In the process of pouring a mold with metal, organic materials that are part of the molding sand (binders, sawdust) burn and emit gases, moisture evaporates and forms a large amount of vapor. The ability of a mixture to release gases during pouring is called gas production. It is determined by the amount of gases released from 1 kg of the mixture. The resulting gases, vapors and air tend to exit the mold through the pores of the sand. Therefore, it must have sufficient gas permeability. Gas permeability - the property of a mixture to pass gases through itself depends on the quality and quantity of clay components and quartz sand. The more sand in the sand and the coarser it is, the higher the gas permeability of the sand, and vice versa. The gas permeability also depends on the shape of the sand grains, humidity, the presence of dust, coal, the degree of compaction, etc. The more dust in the sand, the less gas permeability. With rapid gas formation and insufficient gas permeability of the mixture, the gas pressure exceeds the pressure of the poured metal, and the gas tends to exit the mold not through the mixture, but through the metal. In this case, gas shells may also appear in the castings. In the process of solidification and cooling, the dimensions of the casting decrease due to shrinkage of the metal. However, the shape prevents shrinkage, as a result, stresses and cracks can appear in the casting. Therefore, the molding sand must have pliability - the ability to contract in volume and move under the action of casting shrinkage. The high strength and gas permeability of the molding sand are ensured by the homogeneity of the uniform distribution of the constituent components in the molding sand as a result of thorough mixing. Molding and core sands should have minimal adhesion to the model or core box, depending on the moisture content, binder additive and its properties. The stickiness of the mixture increases with the increase in the amount of liquid in the mixture. Sulfite-alcohol bard increases the stickiness of the mixture, oil binders reduce it.

13 Hygroscopicity The ability of the molding and core sand to absorb moisture from the air depends on the properties of the binder additive. Rods made from mixtures on sulphite stillage have high hygroscopicity. Therefore, the assembled molds with such rods cannot be kept before pouring the metal, otherwise the marriage of gas shells increases. Durability - the ability of the mixture to retain properties when refilled. The more durable the mixture, the less fresh molding materials are added to the used mixture during its processing. The release of the used mixture from dust, the introduction of fresh sand and clay make it possible to restore the properties of the mixture. Knockout - the ability of the core mixture to be easily removed when it is knocked out of a cooled casting - depends on the amount of sand, clay and the type of binder in the core mixtures. Preparation of molding and core sands Molding and core sands are prepared from fresh sand-clay molding materials, additives and spent sand. Depending on the weight of the castings, the consumption of molding sands ranges from 500 to 1300 kg, and fresh materials from 500 to 1000 kg per 100 kg of suitable castings. The technological process of preparing molding sands consists of the following main operations: 1) pre-treatment of fresh molding materials and additives; pre-treatment of the spent sand; 3) preparation of a mixture from pre-prepared fresh and used molding sands, additives and binders. Pre-treatment of fresh molding materials includes sand drying, coal fine grinding, sand and coal screening. The spent mixture is cooled, loosened, subjected to magnetic separation and screened before reuse. Drying of sand and clay is carried out in various ovens (tubular, vertical and horizontal) and on slabs. The most common are vertical and horizontal drying kilns. Vertical kilns are used for drying quartz and low clay sands. For greasy sands and clays, they are not used due to sticking of materials to disks and plows. Fluidized bed sand dryers are widely used. In mechanized workshops, sand and clay are dried in drums with water

14 cooling the sand after drying. Fresh sand is dried at 250 C. The productivity of such dryers is from 5 to 20 t/h and more. Recently, installations with hot air drying of sand have been used. Sand from the bunker is loaded into a pipe into which air heated to C is supplied from below. Raw sand is carried upwards at a speed of m / s and dries quickly. The capacity of the plant can reach up to 15 t/h of dry sand. Dry clay is ground and sifted to a powder. Clay is ground in runners or in ball mills. Fine grinding of clay and coal is achieved in ball mills. The ball mill is a metal drum lined with steel tiles with gaps between them. Clay or coal is loaded into the drum through a funnel. When the drum rotates, the steel balls inside it grind clay or coal. The ground material falls through the gaps between the tiles and is sifted through a sieve. The finished material is poured out of the drum. Ball mill capacity kg/h. Instead of dry clay, a clay and clay-coal emulsion is often used (a solution of clay or clay and coal powder in water). When using an emulsion, clay and bentonite can not be dried or ground, and therefore a number of operations for the preparation and transportation of these materials are eliminated. Clay emulsion should have a density of 1.09 1.15 g/cm 3, it is prepared as follows: clay is loaded into a mixing tank with water and stirred for a certain time until the emulsion reaches a given density. The finished emulsion is released through the valve of the mixing tank. Clay emulsion is prepared in a concentrator tank, into which a certain amount of clay and clay-coal emulsion. After filling the concentrator tank, the emulsion is mixed to the desired density (1.1-1.5 g/cm 3 ) and then automatically fed into the runners or mixers with special dosing pumps. Processing of used sand Waste sand knocked out of flasks must be pre-processed before being reused. In non-mechanized foundries, it is screened on a conventional sieve or on a mobile mixing plant, where metal particles and other impurities are separated. In mechanized shops, the spent mixture is fed from under the knockout grate by a belt conveyor to the mixture preparation department. Large lumps of the mixture formed after the molds are knocked out are usually kneaded with smooth or corrugated rollers. Metal particles are separated by magnetic

15 separators installed in the areas of transfer of the used mixture from one conveyor to another. Regeneration (recovery) consists in the extraction of sand from waste mixtures and bringing its properties in line with the established technical requirements for foundry sands. Depending on the working conditions of the workshop, the regeneration of the used mixture is carried out in various ways: wet, electrocorona and special for mixtures prepared on liquid glass. The wet regeneration method is used mainly in workshops with hydraulic or sand-hydraulic installations for cleaning castings. In the wet method, sand grains are washed with water from clay and fine dust, which are carried away by a stream of water into settling tanks and then into waste. The washed and dust-free sand settles to the bottom of the collector, from where it is fed into the drying oven by a grapple, and then it is sifted and used to prepare molding sands. During electrocorona regeneration, the spent mixture is separated into particles different sizes using high voltage. Sand grains placed in the field of electrocorona discharge are charged with negative charges. If the electric forces acting on a grain of sand and attracting it to the collecting electrode are greater than the force of gravity, then the grains of sand settle on the surface of the electrode. By changing the voltage on the electrodes, it is possible to separate the sand passing between them into fractions. Regeneration of molding mixtures with liquid glass is carried out in a special way, since with repeated use of the mixture, more than 1 1.3% of alkali accumulates in it, which increases burn, especially on cast iron castings. The mixture and pebbles are simultaneously fed into the rotating drum of the regeneration plant, which, pouring from the blades onto the walls of the drum, mechanically destroy the film liquid glass on grains of sand. Through adjustable shutters, air enters the drum, which is sucked out together with dust into a wet dust collector. Then the sand, together with pebbles, is fed into a drum sieve to screen out pebbles and large grains with films. Suitable sand from the sieve is transported to the warehouse. Preparation of molding and core sands Humidification and mixing of the sand are very important operations. Thorough mixing of the mixture is necessary for uniform distribution of its components. When mixed, clay and binder envelop sand grains, clods of individual components are destroyed and moisture is evenly distributed. A well-mixed mixture has maximum strength and gas permeability. To mix the mixture, paddle mixers or runners are used.

16 The paddle mixer is a continuous machine and can be integrated into an automated mixing system. The mixer is often used to prepare mixtures with a low content of clay (filler mixtures, free-flowing, etc.) or mixtures with liquid binders. Mixtures with a high content of clay in a paddle mixer are poorly mixed and therefore have low technological properties. Such mixtures are usually prepared in skating runner mixers. The order of loading the components of the mixture. First, dry materials are loaded: sand, clay and spent sand. The dry mixture is stirred for about 1-3 minutes and then moistened. In the case of using a clay emulsion (a solution of clay in water or a clay-coal emulsion), the humidity is regulated by adding a solution of the emulsion and water. After moistening, the mixture is stirred again for several minutes. Binders are usually loaded last. The duration of mixing is for the mixture: filling 2-3 minutes, 3-5 minutes and facing 5 10 minutes. For quick-drying facing mixtures, the order of loading and the duration of mixing the mixtures are of particular importance. Usually quick-drying mixtures are prepared in mixing runners. When preparing these mixtures, first, dry materials (used mixture, sand, additives, etc.) are loaded into the runners and mixed for 5 minutes, then a binder and water are introduced, everything is mixed for another 7-10 minutes. The finished mixture should be aged before use for several hours to evenly distribute moisture in it. When preparing quick-drying mixtures with liquid glass, sand, clay are first loaded and mixed for 2-3 minutes, then caustic soda is added and the mixture is stirred again for 3-4 minutes, then liquid glass is introduced and again mixed for minutes. After that, fuel oil is added and mixed again for 4-5 minutes. Making Molds General Information The process of making molds is called molding. It is carried out in the molding departments of the foundry. The cores are manufactured in the core department and fed to the mold assembly during the mold assembly. The manufacture of molds, cores and mold assembly are the most critical stages in the production of castings. More than 80% of castings are produced in one-time casting molds, since the cost of their manufacture is quite low, however, they can be used to obtain almost any

17 configurations, complexity and mass casting from the most common ferrous and non-ferrous alloys. The following molding methods are used: 1) in soil and caissons; 2) in flasks; 3) flaskless; 4) according to the template; 5) by skeletal models and control sections; 6) in rods; 7) with the use of fast-hardening mixtures. Depending on the degree of mechanization of the process of making molds, there are three types of molding: manual, machine and automatic. At machine-building plants, manual molding is used to obtain one casting or several, for example, in pilot production, in the manufacture of unique castings, and also for repairs. Machine molding is used in the conditions of serial and mass production of castings or to automate the process of manufacturing molds of any one casting (specialized machines). Tools for manual molding In the manufacture and finishing of casting molds, a variety of tools are used. Depending on the purpose, it can be divided into two groups. The first group is the tools used to fill the flask with a mixture, compact the mixture and ventilate the mold (shovels, sieves, rammers, manual and pneumatic rammers, ventilation needles, etc.), as well as to check the position of the model in a horizontal plane (level or spirit level) The second group is tools designed to extract the model from the mold and finish the mold (hemp brushes and brushes, threaded and screw lifts, hooks, hammers, heavy and light trowels, hooks with blades of different sizes, lancets, spoons, slides of different profiles ). Flasks Molds in the foundry are produced mainly in flasks. Flasks are called rigid frames (rectangular, square, round, shaped) made of cast iron, steel, aluminum alloys, which protect the sand mold from destruction during its assembly, transportation and pouring. The flasks are made from cast iron grades SCH 15-32, SCH and steel grades 20L, 25L and 30L. Cast and welded steel flasks are considered the most perfect, since they are stronger than cast iron flasks. Usually the form is made in two flasks - upper and lower. The surfaces of the flasks that face each other during assembly (planes

18 of the connector), planed, and sometimes polished in order to ensure the tightness of the half-moulds. Handles are provided for transportation and overturning of flasks during molding (on small flasks), pins on large (crane) flasks. Ventilation holes are made in the walls of the flask for the release of gases formed during mold casting. The molding sand in large flasks is held by ribs-crosses (veneers). Molding in flasks In the foundry, molding in flasks is widespread, mainly in split models, and molding is most often carried out in two and less often in three or more flasks. When molded in flasks, the castings are more accurate than when molded in the soil, since the flasks are centered using pins. Molding in flasks is more productive than molding in the soil. Several molding methods are used: 1) in two flasks; 2) with pruning; 3) with a false flask; 4) with a flip block; 5) in several flasks; 6) according to the model with detachable parts. Molding in two flasks according to a detachable model. The mold manufacturing process begins with the installation of a model or its half on a model plate. Then an empty lower flask is placed on the plate and the surface of the model is moistened with a mixture of kerosene and fuel oil or dusted with fine sand. After that, the facing mixture is sifted through a manual sieve. The thickness of the layer of the facing mixture for small castings is mm, and for large ones it is mm. When forming large castings with high sheer walls, the facing mixture is sieved through a sieve only to cover the horizontal plane of the model. Sheer walls are lined with the same facing mixture. The filling mixture is poured into the flask and it is compacted. To achieve a uniform density of the form, the filling mixture is poured into the flask in layers (57-75 mm) and compacted with a manual or pneumatic rammer. When compacting, do not hit the model with a rammer, as the molding sand at the points of impact will be strongly compacted and gas pockets may form in the castings. Particular care should be taken to compact the mixture in the corners and at the walls of the flask. After compaction, the excess molding sand is raked with a ruler flush with the edges of the flask and the ventilation ducts are pierced so that the air vent does not reach the model by mm. Then the flask together with the model plate is turned on and the second half of the model is installed.

19 To eliminate the sticking of the molding sand of the upper half of the mold to the lower, the parting plane of the lower half of the mold is sprinkled with dry separating sand. This sand is blown off the surface of the model with compressed air. The upper flask is placed on the lower flask and a layer of facing mixture is poured on the model through a sieve, the riser model is installed and the filler mixture is poured. The mixture is then compacted. Excess mixture is raked and pricked with a stuffing box. The mold is opened and its surface near the model is wetted with water. To prevent the liquid metal from leaving the mold when pouring wet on the plane of the parting of the mold, risks (cutting) are made around the model at a distance of mm from it. When pouring metal into dry molds, and especially when the condition of the flasks is unsatisfactory, in most cases a thin layer of clay is placed on the parting plane of the mold, which, when pairing the half-molds, completely eliminates the breakout of the metal of their mold. Models should not be placed close to the flask edge; the distance from the model to the wall of the flask must be at least mm, depending on the mass of the casting and the overall dimensions of the flask. The lift is screwed into the model or clogged. Then it is slightly pushed aside by blows of a hammer on the rise and their forms are removed. Models of elements of the gating system, riser, upstream, feeder are also extracted. Small models are removed from the mold by hand, and large ones - with a crane. Removing the model from the mold is a critical operation, and it must be done very carefully so as not to destroy the mold. Strongly pushing the model is not recommended, since in this case the castings are obtained with increased dimensions and weight. After removing the model, the surface of the mold is finished. Damaged areas of the form are corrected with trowels, spoons, lancets, etc. Some parts of the form are reinforced with studs. The finished form, made damp, is sprinkled with powdered graphite or charcoal powder before assembly. When molding in a dry way, the surface of the mold is not dusted, but painted. Forms are usually painted after drying, when the form has not yet cooled. Sometimes forms are painted 2 times: before and after drying. Then the rod is installed and the mold is assembled.

20 Molding in two flasks on a one-piece model A small cover is obtained on a wooden one-piece model. First, the lower flask is formed. The model and the lower flask are placed on the wooden slab, and then the molding sand is poured and compacted. The flask with the plate is turned over by 180 0, the upper flask and models of the gating system are installed, and the molding sand is also poured into the upper flask and compacted. After that, the upper half-mold is lifted, turned over and the models of their shape are removed. Then the form is trimmed, assembled and filled with metal. Machine molding Machine molding is mainly used in serial and mass production and much less frequently in small-scale and single-piece production. Machine molding is usually carried out in two flasks, with the exception of stack molding and flaskless molding. The form usually consists of two half-forms - upper and lower. When making molds on machines, it is necessary to have models, pattern plates, paired steel flasks, pins. In mass and large-scale production, metal models are used, in mass production - wooden models fixed on coordinate plates. In all cases, molding on machines is carried out according to models mounted on metal plates, which increases the accuracy of castings, and mechanization of the main operations (mould compaction and model extraction) completely frees molders from laborious manual operations. Machine molding has a number of advantages compared to manual molding: high productivity, casting accuracy and, as a result, smaller machining allowances, uniform compaction of the mold, and the ability to perform work by molders with lower qualifications. The dimensional accuracy of castings during machine molding is ensured by the use of more accurate (with smaller slopes) models, the replacement of the operation of pushing the models apart by vibration when they are removed from the mold, and good centering of the flasks. For machine molding, three types of model plates are used: one-sided - the lower part of the model is mounted on one plate, and the upper part of the model is mounted on the other; 2) double-sided - on one side of the plate, a model of the top is mounted, and on the other - of the bottom (molding on one machine); reversible - the lower and upper flasks are molded on the same plate, and during assembly, the upper flask is turned by

21 Permanent fastening of models to plates is used in mass and large-scale production. Prefabricated model plates, consisting of inserts with models, are used in small-scale production; coordinate model plates - in single and small-scale production. The coordinate plates have holes for mounting the model and determining the correct position of the model. The hole on the plate is indicated by a cipher consisting of their letters and numbers. With the help of this cipher, models are installed on the stove. The technological process of manufacturing molds on machines consists of a number of operations. The main operations - compaction of the molding sand in the flask and extraction of the model from the mold - determine the quality of the future casting: the presence of blockages, gas pockets, cracks in it; the correctness of the geometry; surface cleanliness. Auxiliary and transport operations - installation of an empty flask on the machine, spraying and blowing of the pattern plate, filling the molding sand into the flask, transportation of finished molds - are carried out by special auxiliary and transport mechanisms of the machine. Depending on the degree of mechanization of auxiliary and transport operations, there are: 1) mechanized molding, when the worker manually controls the operation of the mechanisms that perform the main, auxiliary and transport operations, and 2) automatic molding, when the machine controls the operation of the mechanisms. The most time-consuming and responsible operations are mold compaction and model extraction. There are several ways to compact molding sands on machines: pressing, pressing with vibration, shaking, shaking with pre-pressing, sand thrower, press-sand blower.

22 Gating systems One of the most important conditions for obtaining a quality casting is the correct design of the gating system. Gating system is used for smooth supply of liquid alloy into the cavity of the casting mold and supply of castings in the process of crystallization. The location of the alloy supply to the casting largely determines its density, appearance and the formation of various casting defects. Choosing a gating system that produces good quality castings is the most difficult part of foundry technology. Therefore, the molder, foreman and technologist, when choosing a gating system, must take into account the features of the foundry technology. A correctly constructed gating system must meet the following requirements: 1) ensure good filling of the mold with metal and feed the casting during its solidification; 2) contribute to the production of a casting with accurate dimensions, without surface defects (blockages, uzhimin, slag inclusions, etc.); 3) promote directional solidification of the casting; 4) metal consumption for the gating system should be minimal. The gating funnel for small castings and the gating bowl for large castings are designed to receive a jet of metal flowing out of the ladle and retain slag that enters the bowl along with the metal. When the bowl is full to the brim, pure metal enters the riser, and light slag is at the top. In addition, a continuous supply of metal into the mold at the same pressure is ensured. To retain slag, the openings of the risers are sometimes closed with cast-iron plugs, thin tin plates. Corks are opened after the entire bowl is filled with metal, while the plates are melted with hot metal. The mold must be filled with metal as quickly as possible, while the metal must have a sufficient temperature. During the pouring of metal, the gating cup must be full. If the depth of the metal is not deep enough, a funnel is formed in the bowl, through which air and slag floating on the surface of the metal can enter the riser and then into the casting. For small castings, especially in conditions of mass production, the slag in the bowl is retained by filter grids, which are made from the core mixture. Riser - a vertical channel that transfers the metal of their funnel to other elements of the gating system. It is performed somewhat tapering downwards for ease of molding and providing hydraulic pressure in the gating system. The taper of the riser is 2-4%. In the manufacture of large castings, the riser and other elements of the gating system are often made from standard fireclay tubes-bricks.

23 The slag trap serves to retain slag and transfer metal, free from slag, from the riser to the feeders; is located in the horizontal plane. Usually, the slag trap is made in the upper half of the mold, and the feeders - in the lower one. The cross section of the slag traps is made trapezoidal. In the process of filling the mold with metal, for better retention of slag, the slag trap must be filled with metal. This is ensured by the appropriate ratio of the sections of the riser, slag trap and feeder. If the flow of metal through the riser is greater than the flow through the feeders, then the slag trap is filled with metal and the slag, floating up, lingers in it. If the flow through the riser less consumption through the feeders, the slag trap will be empty and the slag enters the casting. Thus, in order to retain slag, the cross section of the riser must be greater than the cross section of the slag trap, and the cross section of the slag trap must be greater than the total cross section of the feeders. Such a gating system is called locked. Feeders (sprues) are channels for supplying liquid metal directly into the mold cavity. The cross section of the feeders should be of such a configuration that the metal enters the mold cavity smoothly, cools a little on the way from the slag trap to the casting, and after hardening, its feeders easily break off from the casting. It has been established by practice that the best cross-sectional configuration of feeders is a trapezoid with a transition to a wide rectangle at the point of conjugation with the casting. For better separation of feeders from castings, if the thickness of its body is less than one and a half height of the feeder at the place of its supply to the casting, a pinch is made on the feeders at a distance of 2-2.5 mm from the casting. The vents are used to remove gases from the mold cavity and to feed the casting. They also reduce the dynamic pressure of the metal on the mold and signal the end of pouring. Depending on the size of the form, one or more bulges are placed. The section of the bulge at the base is usually 1/2-1/4 of the section of the wall of the casting. Above the base, the section of the bulge increases. Among the elements of the gating system that supply the casting with liquid metal in the process of its solidification, are feeding upstreams and risers. Profits and feed risers are used for castings from white low-carbon, high-strength cast iron, as well as for thick-walled castings from gray cast iron. They serve to feed the thickened places of the casting, which are the last to solidify. Profits are arranged so that the metal in them freezes last. The thickness of the profit must be greater than the thickness of the place of the casting over which it is placed. Profits of large sizes are economically unprofitable, since the consumption of metal for profits and the cost of castings increase.

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Foundry

Casting is made:

Liquid metal

FOUNDRY FOUNDATIONS

Schematic diagram of casting (on the example of a disposable detachable casting mold) is shown in fig. 2.1, a.

Rice. 2.1 Scheme of the casting process in disposable detachable molds: a - schematic diagram; b - casting after knockout; 1,2 - lower and upper flasks; 3 - shape; 4 - mold cavity; 5 - extrusion; 6 - casting core; 7 - ventilation channel; 8 - vertical channel (riser); 9 - sprue bowl (funnel); 10 - molten metal; 11 - bucket; 12 - slag trap; 13 - horizontal channels (feeders).

Liquid metal 10 from a bucket 11 is poured into the mold and enters the cavity through a system of channels 4 forms 3 , fills it and hardens. After solidification, extraction from the mold and processing, a casting is obtained (Fig. 2.1, b). To get holes, cavities and other complications of the configuration of castings, rods are used 6 , which are set when the form is assembled.

To obtain a casting, three technological elements are required: foundry material, technological foundry equipment and technological foundry equipment.

foundry materials.

Casting materials(metal alloys, plastics, rubber, ceramics) must have high casting (technological), mechanical and operational properties.

Casting properties of alloys

When designing any part, the mechanical properties of the alloy are primarily taken into account, but it is also necessary to take into account the casting properties of the alloy, which determine the possibility of obtaining a high-quality casting, i.e. castings that meet the specified requirements. The most important casting properties of alloys are:

a) fluidity;

b) shrinkage;

c) tendency to segregation and absorption of gases.

Fluidity - this is the ability of liquid metals and alloys to fill its cavities and clearly reproduce the relief of the casting. The fluidity of an alloy depends on its physical and chemical properties (first of all, the liquidus temperature) and technological factors, first of all, the pouring temperature. For alloys hardening at a constant temperature (pure metals and eutectic alloys) or in a narrow temperature range (up to 30 ° C), sequential solidification is characteristic with the formation of a continuous solid crust on the surface of the mold channel. A liquid phase remains inside this crust, capable of flowing into the channel. Alloys with a wide range of crystallization temperatures solidify with the formation of branched dendrites over the entire flow cross section. These melts lose their ability to flow in the mold channel in the presence of a solid phase of 20 ... 30% by volume. With an increase in the overheating temperature of the alloy, the fluidity increases.

Shrinkage- this is the property of casting alloys to decrease in volume and linear dimensions during solidification and cooling. Shrinkage processes occur from the moment the molten metal is poured into the mold until the casting is completely cooled. The amount of shrinkage is influenced, first of all, by the chemical composition of the alloy, the pouring temperature and the properties of the casting mold. With an increase in the temperature of the poured alloy, the shrinkage of the casting increases. Distinguish linear and volumetric shrinkage.

Linear shrinkage - this is a decrease in the linear dimensions of the casting when it is cooled from the pouring temperature to the ambient temperature b The value of linear shrinkage can vary from 1% for gray cast iron to 1.5 ... 2% for steels and non-ferrous alloys

Volumetric shrinkage is the decrease in the volume of the alloy when it is cooled in the mold. Volumetric shrinkage leads to the formation of shrinkage cavities, shrinkage porosity, cracks and warping in castings.

Many difficulties in the production of castings are associated with the shrinkage of the alloy. When shrinkage is retarded (by protrusions of the mold, rods), stresses arise in the casting, which can cause it to warp or crack.

Used to relieve shrinkage stresses annealing. To do this, the castings are heated to high temperatures (but below the solidus temperature) and slowly cooled together with the furnace. With slow cooling (20 degrees per hour), the temperature difference in different points there will be no casting, and, therefore, all its parts will shrink evenly. Often castings after preliminary (rough) machining are simply aged for a long time before final processing.

Segregation - this is the heterogeneity of the chemical composition over the cross section of the casting. Segregation occurs during the solidification of the casting due to the different solubility of the individual components of the alloy in its liquid and solid phases.. The greater this difference, the more heterogeneously distributed the components.

There are two main types of segregation: intracrystalline (or dendritic ), which is characterized by heterogeneity of the metal grain, and zonal when different zones of the casting have different chemical composition. The simplest case of segregation is associated with a large difference in the densities of the alloy components. So, when casting lead bronze in the lower parts of the casting, the lead content will be higher than in the upper parts. Conversely, the concentration of copper (the lighter component) will increase in the upper parts of the casting.

During the solidification of the casting, gases are released from the metal, which are dissolved in the melt. As a result, gas porosity or gas shells are formed in the casting, which significantly reduce the set of mechanical properties of the casting. The source of gas shells can also be a form with insufficient gas permeability and when it releases a large amount of gases. Gas sinks are most often formed near the surface of the horizontally located walls of the casting, as well as in places where gas removal is difficult.

Foundry equipment.

Foundry equipment is a set of machines, automatic machines, installations, melting furnaces, pouring devices, manipulators designed to perform processes and operations for the manufacture of castings. Each foundry process (forming, melting, pouring, etc.) uses its own types of equipment (forming, melting, etc.) in accordance with GOST 18111-93*.

foundry tooling.

Technological equipment of foundry production - means of technological equipment that complement the foundry technological equipment to perform a certain part of the casting process. Casting equipment according to GOST 17819-84* includes molds, cores, models and other means of technological equipment.

Casting shape – this is a system of elements that form a working cavity, when poured with liquid metal, a casting is formed. The main requirements for casting molds are as follows:

Strength- so that under the action of the pressure of the melt, the mold not only does not collapse, but also does not change its dimensions.

Gas permeability- so that gases both in the mold cavity and in the melt can be removed (to avoid gas porosity and gas pockets in castings).

Resistance to chemical interaction with the melt- to ensure ease of extraction and cleaning of castings.

The casting mold is made using the following casting equipment:

1. Casting model. For the manufacture of one-time molds, casting models are used that provide the formation of an imprint in the form corresponding to the configuration and dimensions of the casting.

Models are made of wood, plastics or metal alloys and can be detachable and one-piece, one-time and multiple.

2. Casting core – a mold element designed to form a hole, cavity or other complex contour in a casting. Largely due to the use of cores during casting, it is possible to obtain blanks of the most complex configuration. Rods, as well as forms, can be single and multiple, solid and prefabricated.

General casting technology

Casting technology is a combination of a large number of individual foundry processes that can be combined into four stages:

1. Technological preparation of the casting manufacturing process. Based on the design (drawing of the part, technical requirements, working conditions of the part in the assembly ...) and technological documentation(lot size, technological processing route ...) the following is carried out: the choice of a casting method, the development of a casting drawing, the design of technological equipment (model, rods, mold ...) the development of technology, including the determination of the sequence and technological parameters of individual processes, operations and transitions.

2. Form making. Based on the developed drawings, a model, rods, and the necessary technological equipment are made. When casting into non-metallic molds, the most responsible and time-consuming process is molding - the manufacture of molds and cores from molding and core mixtures. Most often, the casting mold is split (consists of two parts), which allows you to get most of the castings. One-time molds, like cores, are made by compacting the molding (core) mixture in flasks (core boxes). Finished mold parts (half-molds) and cores are fed to the mold assembly operation, which includes installation, connection and fixing of casting cores in the mold and mold parts to each other. In parallel with molding and assembly, melting is carried out in the melting department of the foundry - obtaining a melt of the desired chemical composition and temperature.

3. Pouring of molds and cooling of metal castings. The assembled molds are poured with a melt using a ladle or a casting machine. The melt temperature is: t fills = t liquidus + (100…150)ºС. Filling the mold with the melt requires a certain time, which limits the productivity of the casting as a whole. To improve the quality of castings, the influence of centrifugal forces, electromagnetic fields, ultrasound, etc. is used.

4. Extraction from molds and processing of castings. Cooled to a given temperature, the casting is removed from the mold, and the rods are removed from it.

knockout – removal of the casting from the mold. When casting into disposable molds, this operation is carried out on vibrating gratings.

stump - separation from castings of elements of the gating system, bays on the form connector and surface irregularities. It is carried out with the help of pneumatic hammers (iron castings) and pneumatic chisels, cutting with gas or plasma (steel castings), abrasive and other tools.

Casting Surface Cleaning from burn, residues of molding and core sand. For cleaning, methods such as tumbling, shot blasting, electrochemical, etc. are used. In rotating tumbling drums, burns are removed by rubbing the surfaces of the castings against each other and against additionally loaded sprockets made of white cast iron. In the shot blasting method, the casting surface is cleaned under the influence of a stream of iron or steel shot with a diameter of 1-3 mm. The high-speed shot flow is created using compressed air (in shot blasting machines) or rotating blades (in shot blasting machines).

Cleanup - casting surface machining in order to bring it into line with the requirements for surface quality. At the same time, the remains of feeders, bays along the plane of the parting of the mold and at the iconic parts of the rods are removed. Cleaning is most often carried out using grinding wheels and trimming presses.

Heat treatment of castings produced when it is necessary to increase strength (hardening), ductility, machinability, removal of internal stresses (annealing). In the latter case, they are often limited to long-term storage of castings in a warehouse.

Casting quality control provides for checking the conformity of products to specifications, including the absence of structural defects. An external examination allows you to identify external defects (shells on the surface, through cracks, skew, etc.). Dimensional accuracy and surface roughness are determined using a measuring tool (calipers, templates, gauges) and special equipment (profilometers, coordinate measuring machines). Metallography is used to control the structure of castings. With the help of non-destructive methods of control, such as ultrasonic, eddy current, X-ray, the internal structure of the metal is controlled.

CASTING METHODS

In industry, many casting methods are used, which can be classified according to various criteria. Most often, all types of casting are divided into casting in single forms(casting in sand-clay molds, shell molds, investment casting...) and casting in multiple forms(die casting, pressure casting, centrifugal casting...).

The choice of casting method is determined by the type of casting alloy (low-melting, refractory), the volume of production, the specified parameters of dimensional accuracy and surface roughness of the resulting castings, and other factors.

Hand molding technology.

Depending on the casting configuration and production conditions, various manual molding methods are used:

according to models in the soil(caisson),

in flasks, with cutting, with detachable parts, with a flip or lifting block, with a false flask, according to a template, in rods, according to skeletal models. Let's take a look at some shaping methods.

Split pattern molding- the most common molding method. In manual molding, the lower half mold is first made (Fig. 2.5, a). Then the upper half-mould is made (Fig. 2.5, b). To do this, the lower half of the mold is turned over 180 °, the upper half is installed on the lower half of the model with spikes, the models of the slag trap, riser and upstream are mounted. The parting plane (mode of form) is sprinkled with a separating mixture - graphite, finely grated quartz sand. The upper flask is installed on the lower flask along the guide pins. The molding sand is poured into it and compacted, and the upper half-mould is obtained. After compaction of the mixture, the riser and upstream models are removed. The upper half-mould is removed, turned over 180 °, removed from both half-moulds of the model (Fig. 2.5, in), and start assembling the form. The assembled and ready-to-fill mold is shown in Fig. 2.5, G.

Rice. 2.5. Molding according to the detachable model:

a-d – molding sequence; MF - connector model and shape

The casting shown in Fig. 2.6, during molding, it is impossible to remove part of the mold without destroying it (volume "K") (Fig. 2.6, a). In such cases, undercut molding is used. The lower half mold is compacted in the usual way and turned over. Cut off the volume "K" (Fig. 2.6, b). The resulting surface is carefully smoothed. When forming the upper half-mould (Fig. 2.6, in), in place of this volume, a blockhead is formed. The shape connector is not flat, but shaped. Next, set the model of the upper sign 2 , the cutting surface is sprinkled with a release mixture. Further molding operations do not differ from conventional molding operations.

Rice. 2.6. Molding with trimming: a - compacted lower half-mould; b - the lower half-form after removing excess mixture; in - the form in the collection; g - molding on a shaped pattern plate; 1 - model; 2 - model of the upper mark; 3 - cargo; 4 - model plate; 5 - nest of signs; 6 - trimming model; K - the volume of the mixture that prevents the extraction of the model; Ф - the plane of the half-mould parting; n - the direction "top" and "bottom" in relation to the plane of the parting of the half-forms.

Form molding with detachable parts it is used in the manufacture of complex configuration castings with protruding elements, when the use of a model with one connector does not allow removing its halves from the half-moulds after compaction of the mixture. Detachable parts of the view 1 (Fig. 2.7, a, b) is used if there is a cavity in the model sufficient to remove the pin 8 . When forming a model with detachable parts of the form 2 (Fig. 2.7, a, in), the mixture is compacted to the upper plane of the detachable part, then the pin is removed 9 and complete compaction. Removable part fastening 3 (Fig. 2.7, a, g) is called "dovetail".

Rice. 2.7. Molding according to the model with detachable parts:

a – model; b, c, d, - the sequence of operations 4, 5, 6, 7 molding; 1, 2, 3 - types of detachable parts; 8 - pin, 9 - hairpin; the arrows show the movements of detachable parts, pins and studs.

If it is impossible to place the model on a flat slab, molding with a false flask is used. Metal is not poured into a false flask. It serves only for molding as a figured pattern plate.

Lecture 2. FOUNDRY

Foundry- a branch of mechanical engineering that manufactures blanks or parts (castings) by pouring molten metal (melt) of a given chemical composition into a mold cavity having a casting configuration. Upon cooling, the melt solidifies and retains the shape of the mold cavity. Casting can produce products of a very complex configuration, which are difficult or impossible to obtain by other types of processing - forging, stamping, welding.

Casting is made: blanks for general-purpose parts that do not have special requirements for mechanical and operational properties; blanks of critical parts, such as parts of internal combustion engines (cylinder blocks, pistons), impellers and blades of gas turbines, etc. The mass of castings can be from a few grams (instrument parts) to tens of tons (machine bed, turbine generator rotors).

Liquid metal poured into single molds (after the casting is made, they are destroyed) and multiple (in one mold, you can get from ten to several tens of thousands of castings). One-time molds are made from non-metallic materials (moulding sands). Multiple molds are made from metal-based alloys.

INSTITUTE OF MODERN KNOWLEDGE

VITEBSK BRANCH

Department: "Informatics and Management"

Discipline: "Production technologies"

Test

On the topic: "Technology of foundry
production"

2nd year student

Group ZE 00/4

Vitebsk

Topic: Foundry technology

1. The essence of the foundry and its development .............................................. 2

2. Casting in disposable sand-clay molds .............................................. .3

3. Special casting methods .................................................. ...................... ten

4. Literature ............................................... ................................................. . fifteen

1. The essence of the foundry and its development

Foundry is the process of obtaining shaped products (castings) by pouring molten metal into a hollow mold that reproduces the shape and dimensions of the future part. After solidification of the metal in the mold, a casting is obtained - a workpiece or part. Castings are widely used in mechanical engineering, metallurgy and construction.

It is possible to obtain castings of various weights (from a few grams to hundreds of tons), simple and complex shapes from cast iron, steel, copper and aluminum alloys, zinc and magnesium, etc. Especially effective is the use of castings for the production of shaped products of complex configuration, which are impossible or economically unreasonable to be manufactured by other methods of metal processing (pressure, welding, cutting), as well as for the production of products from low-plastic metals and alloys.

With all the variety of casting techniques that have developed over a long period of development of its technology, the basic scheme of the casting process has not changed much over more than 70 centuries of its development and includes four main stages: melting metal, making a mold, pouring liquid metal into a mold, extracting a hardened casting from the form.

Until the middle of our century, the casting method was considered one of the most important methods for obtaining shaped blanks. The mass of cast parts was about 60% of the mass of tractors and agricultural machines, up to 70% of rolling mills, up to 85% of machine tools and printing machines. However, along with such advantages of foundry production as the relative ease of obtaining and low cost of castings (especially from cast iron), the possibility of manufacturing complex parts from brittle metals and alloys, it also has a number of significant disadvantages: first of all, rather low labor productivity, composition heterogeneity and reduced the density of the material of the workpieces, and, consequently, their lower strength characteristics than the workpieces obtained by pressure treatment.

During the years of the 11th Five-Year Plan, the output of foundry equipment increased significantly in the USSR. The production of automatic lines for forming, pouring and knocking out castings has been mastered, sets of modern sand preparation equipment have been created, the production of a whole range of machines for special casting methods has been mastered, the level of mechanization and automation has significantly increased technological processes.

The main directions of the economic development of the USSR for the period up to the year 2000 provide for a significant acceleration in the development of mechanical engineering. A significant contribution to the solution of the tasks set can be made by the reconstruction and modernization of the foundry, the replacement of outdated equipment with high-performance automatic and semi-automatic casting machines, robotic complexes. A large reserve for saving metal, reducing the material consumption of engineering products consists in increasing the share of casting from alloyed steels and high-strength cast iron, as well as precision casting obtained by special methods.

The main technical and economic indicators of the work of foundries are: annual production of castings in tons; production of castings per one worker (production); eat casting from 1m 2 of the production area of ​​the workshop; yield of suitable metal (as a percentage of the mass of metal filling and liquid metal); share of casting rejects (in percent), level of mechanization; the proportion of castings obtained by special methods; cost of 1 ton of casting.

In the structure of the cost of casting, the main share is the cost of metal (up to 80%). When performing a technical and economic analysis of foundry production, special attention must be paid to those stages and elements of the technological process that are directly related to possible metal losses due to waste, spatter, rejects, etc.

The cost of casting depends on the volume of production, the level of mechanization and automation of technological processes.

2. Casting in disposable sand-clay molds

Casting in disposable sand-clay molds is the most common and relatively simple way to obtain castings. One-time sand-clay molds can be prepared either directly in the soil (in the floor of the foundry) according to templates, or in special flask boxes according to models. Castings of large parts (frames, columns, etc.) are obtained in the soil, smaller castings are usually obtained in flask molds.

The external outline of the castings corresponds to the recesses of the mold, the holes are obtained by means of rods inserted into the mold cavity.

The technological process for the production of castings in flask molds (Fig. 1) consists of three stages: preparatory, main and final.

Model equipment, made in model shops, is a device with which molds and cores are made. Tooling includes models of parts, under-model shields, core boxes, models of gating system elements and flasks.

Models (Fig. 2, a) serve to obtain a cavity in an earthen form, which, in size and external outlines, corresponds to the future casting. Since the metal shrinks (reduces in volume) after solidification, the dimensions of the model are made somewhat larger.

Models are made of wood, plastic or metal. The choice of material depends on the conditions of production and the requirements that apply to the casting in terms of dimensional accuracy and surface finish. In order for the models to be easily removed from the mold, they are made with molding slopes and are often detachable, from two or more parts, easily fastened with spikes.

To obtain castings with holes or recesses on the models, protrusions are provided in the appropriate places - rod marks that leave prints in the mold for installing the rods. The place occupied by the core in the mold is not filled with metal, and after the core is removed, a hole or depression is formed in the casting. The rods are made from a special core mixture, stuffing it manually or by machine into core boxes (Fig. 2, b). In this case, the change in the dimensions of the casting during the solidification of the metal is taken into account. The dimensions of the rods must be smaller than the holes by the amount of shrinkage of the metal. Depending on the complexity of manufacturing, core boxes are made solid and split. With small batches of rods, boxes are made of wood, in mass production, especially with increased requirements for casting accuracy, metal boxes (cast iron or aluminum alloys) are used.

Models of the gating system are designed to form channels and cavities in the form that serve to supply metal, retain slag and exit air from the mold cavity (Fig. 2, c). The device of the gating system provides a calm, shock-free flow of metal into the mold, protecting it from damage.

Under-model shields-plates are used to place models on them and install the flask when making a mold manually.

In mass production during machine molding, it is effective to use carefully processed wooden or metal pattern plates with models of parts and elements of the gating system firmly fixed on them or made in one piece (Fig. 2, d).

Molds are wooden or metal frames, frames, the main purpose of which is to hold the sand-clay mixture, to ensure sufficient strength and rigidity of the form during its manufacture, transportation and pouring of metal.

Molding and core mixtures mainly consist of quartz sand of a certain grain size and heat resistance.

Fig.1 Technological process for the production of castings in flask molds

Molding and core mixtures must have plasticity and gas permeability, and the molds and cores made from them must have sufficient strength. These properties are achieved by adding clay, linseed oil, dextrin, liquid glass, as well as sawdust or peat chips to the base material. Moistened clay is added as a binder.

Sawdust, peat chips, burning out after pouring the metal into molds, form additional pores that increase the gas permeability of the mixture.

Fig.2 Model equipment:

a - part model; b - core box; c – gating system model;
g - under-model plate; d - flask.

Oil binders (linseed oil, linseed oil) are usually added to core mixtures, which should have a higher strength compared to molding ones. The preparation of molding and core sands is carried out in the ground preparation departments of the foundry and includes operations pre-training(drying, grinding), dosing of raw materials and thorough mixing until a homogeneous composition is obtained. In modern foundries, these operations are mechanized. The prepared mixtures are subjected to aging in the bunker for a more uniform distribution of moisture, and then, after loosening and control of the obtained properties, they are transported to the workplaces of the molders.

According to their purpose, molding sands are divided into facing, filling and uniform. Facing mixtures in direct contact with liquid metal are prepared from higher quality fresh materials. The used (burnt) mixture serves as a filler. In mass production, molds are made from a single mixture, the material for which is a used mixture with fresh additions of sand, clay, binders, etc.

The production of cores can be done by stuffing the core mixture into a box and ramming manually or by machine. Machine preparation of cores is carried out on pressing, shaking, sand-throwing and other core machines. In mass production, cores are produced on production lines consisting of core machines, drying ovens, and various conveying devices. Molded raw rods are dried at a temperature of 160 ... 300 ° C in drying ovens or chambers to give them high strength.

Recently, most factories have been using the method of making rods from quick-drying mixtures on liquid glass. Drying or chemical hardening of the rods in this case is achieved by blowing them with carbon dioxide for two to three minutes. Some plants have introduced high-speed drying of the rods using high-frequency currents.

The use of these drying methods helps to reduce production cycle production of castings by 2...5 times, increase in the removal of castings from Hm 2 of the production area of ​​the shop, and reduce the cost of transport and energy.

Fig.3 Technological process of sleeve molding

Molding is the most complex and time-consuming operation for the production of castings in disposable sand-clay molds. The complexity of manufacturing casting molds is 40 ... 60% of the total complexity of obtaining castings.

In the conditions of mass and large-scale production of small and medium-sized castings, machine molding is used. Hand molding is used in individual and small-scale production, as well as in the production of large castings. The concept of "manual molding" is somewhat outdated, since many of the work (feeding the molding sand, tamping, extracting models, turning and moving flasks) is currently mechanized.

Consider the sequence of manual molding for castings of a bushing type part.

Half of the model 2 is placed on the under-model shield 3 (Fig. 3, a) and the lower flask is installed, then through the sieve 4, non-stick dust is applied to the surface of the model - charcoal dust, graphite powder (Fig. 3, b). With a shovel 5, a facing molding sand is applied to the model, and then the entire flask is covered with a filling molding sand (Fig. 3, c). The mixture is compacted with a manual or pneumatic rammer 6 (Fig. 3, d), its residues are raked up and holes are pricked with a stuffing box (awl) 7 for better exit gases (Fig. 3, e). Then the lower flask with the molded model is turned over by 180°, the second half of the model 8 and the upper flask 9 are installed (Fig. 3, f). After installing the models of the gating system 10, the upper flask is molded in the same sequence (Fig. 3, g). At the end of the molding, the flasks are disassembled, the models are carefully removed, the collapsed places of the mold are corrected, dusted from the inside and, having laid the rod 12 in the lower half-mould in place of the signs 11 (Fig. 3, h), re-install the upper half-mould on the lower one and fasten them with bolts, clamps or simply press down with a load to prevent the metal from breaking through the mold parting plane. In this form, the mold is ready for pouring metal.

To obtain large castings, half-molds are dried at a temperature of 350 °C for 6–20 hours before assembly, depending on the dimensions of the mold.

Machine molding is economically feasible in conditions of serial and mass production, when molding machines are loaded for at least 40 ... 60% of the working time. However, the experience of leading factories in our country shows that machine molding is also economically justified in individual production if quick-change pattern plates are used. In this case, the change of models is carried out in 1.5 ... 2 minutes, i.e. in a short time it is possible to rebuild to obtain new castings.

The essence of machine molding lies in the mechanization of the main operations: installation of pattern plates and flasks, filling flasks with molding sand, compaction of the mixture and removal of models from molds. Separate designs of molding machines also make it possible to mechanize some auxiliary operations: turning the flasks, removing finished half-molds from the machine table, transferring them to assembly, etc.

According to the method of compacting the mixture, molding machines are divided into pressing, shaking, sand-throwing (Fig. 4) and combined (shaking with pre-pressing or pressing with a vibrator).

Pressing machines are the simplest and most productive, but they give uneven compaction of the mixture along the height of the flask, shaking machines are less productive, but in combination with pre-pressing, they allow more uniform compaction of the earth even in tall and large flasks. Sand throwers are used for stuffing medium and large flasks. They are distinguished by high productivity (up to 50...70 m 3 /h) and provide the most uniform compaction of the earth along the height of the flask.

Molding machines combined with transport devices with other machines and mechanisms make it possible to create in-line (mechanized, semi-automatic and automatic) molding sections.

Fig.4 Molding machines:

a - press; b - shaking; c - sand throwers.

Machine molding not only facilitates the work of mold workers, but also makes it possible to increase labor productivity, obtain more accurate castings with smaller machining allowances, and reduce rejects.

In the total labor intensity of manufacturing castings, the processes of melting and pouring metal into molds account for about 7 ... 10%. However, these processes are especially responsible, as they have a decisive influence on the quality and cost of castings.

The most important casting alloys are cast iron (gray, high-strength), steel (carbon, alloy), copper alloys (bronze, brass), aluminum, magnesium, zinc alloys, etc.

Gray cast iron, bronze, aluminum alloys with silicon (silumins) have the best set of casting properties. Cast iron is smelted mainly in cupolas and shaft furnaces. The cupola furnace is a vertical shaft-furnace of continuous operation, operating on foundry coal coke and air blast. The performance of the cupola, depending on its size, is 1...30 t/h, the maximum attainable temperature is 3400...1420 °C. The intensification of the melting process in the cupola is carried out by using hot (400...500 °C) blast with oxygen-enriched air.

Recently, coke oven and gas cupolas have become widespread, which make it possible to further improve the technical and economic indicators of processes by improving the quality of the metal, better use of charge materials and reducing the duration of melting.

Induction furnaces for iron smelting, operating at industrial frequency currents, are the most promising melting units. Their use makes it possible to melt cast iron of a homogeneous composition with high mechanical properties and thereby significantly reduce the mass of castings. The high heating temperature in induction furnaces makes it possible to use inexpensive steel waste and, by carburizing it, to obtain cast iron of the required chemical composition.

In the shops of large and medium-sized castings from carbon and low-alloy steel (partially high-alloyed), acid and basic open-hearth furnaces with a capacity of up to 80 tons are used. Electric arc furnaces are used to produce small and medium-sized castings from carbon and low-alloy steel, for non-critical castings - small Bessemer converters, cast iron into which it comes from cupolas. High-frequency induction furnaces and electroslag remelting units are used in the shops of especially critical steel casting.

Alloys of non-ferrous metals, depending on their properties (melting point, chemical activity, etc.) and the scale of production, are melted in crucible furnaces, flame and electric reverberatory furnaces, induction, vacuum arc, vacuum electron beam furnaces.

All melting units used in the foundry must meet certain general requirements: provide the temperature necessary for melting and overheating of the metal, have sufficient productivity, be economical (minimal fuel and energy consumption per 1 ton of liquid metal and minimal metal waste), more or less reliably protect the molten metal from contamination by gases and non-metallic inclusions.

The molten metal is fed to the casting area in pouring ladles of various capacities.

The quality of castings largely depends on compliance with the rules of casting. The metal is poured into the mold smoothly, in a continuous stream until it appears in bulges and profits. The casting temperature is always higher than the melting temperature of the alloy, however, its overheating should be minimal to ensure good filling of the mold. If the pouring temperature is too high, abundant gas evolution occurs, the molding sand burns to the surface of the casting, and its shrinkage increases. The temperature of the poured metal is controlled by optical pyrometers or thermocouples.

After solidification and cooling to a certain temperature, at which the castings acquire sufficient mechanical strength, they are knocked out of the molds; the rods are knocked out later, after additional cooling of the castings.

The knockout of castings is one of the most difficult operations in the foundry, accompanied by large emissions of heat and dust. According to the complexity of the operation of knockout, trimming and cleaning, they account for 30 ... .40% of the total labor intensity of manufacturing castings.

The essence of the knockout process is the destruction of the mold, the release of castings from the surrounding molding earth. In modern foundries, the knockout process is mechanized and carried out on various vibration machines, most often on shaking grates. The molding sand falls through the grate, enters the belt conveyor and is transported to the molding department for reuse.

After knocking out, the castings are trimmed and cleaned. Pruning consists in separating profits, upstreams and bays from castings.

Pruning is a heavy operation, difficult to mechanize. It is produced using pneumatic chisels, band and circular saws, presses, flame cutting.

Cleaning of castings, carried out after chipping, consists in removing molding earth burnt (crust), scale, and small burrs. The main purpose of cleaning is to reduce the complexity of subsequent machining and reduce wear intensity cutting tool. The cleaning of castings from burnt earth and scale is carried out in rotating (tumbling) drums, on sand-hydraulic and shot-blasting machines, as well as by chemical and electrochemical treatment of the internal surfaces of castings that are difficult to access with other cleaning methods.

Cleaning of small burrs, irregularities remaining after trimming is carried out on portable and stationary grinding machines with coarse-grained abrasive wheels.

Before being sent to mechanical workshops, steel castings are necessarily subjected to heat treatment - annealing or normalization - to relieve internal stresses and grind metal grains. In some cases, castings from other alloys are subjected to heat treatment.

Marriage can occur for various reasons at all stages of foundry production, while there is a defect that can be corrected and irreparable. The main types of defects in castings are: warpage; gas, shrinkage, earth and slag shells; cracks; underfilling of metal and junction; surface chill (For iron castings). Superficial shallow defects are eliminated by welding, pressing (with epoxy resins), metallization. Warping can sometimes be corrected by editing. The chill is eliminated by additional annealing of the castings.

With internal and deep external defects, the castings are sent for remelting. Suitable castings are sent to machine shops for further processing or to a warehouse. finished products.

3. Special casting methods

In recent years, special casting methods have been introduced everywhere in the foundry industry, which have a number of advantages compared to traditional casting in disposable sand-clay molds. The proportion of castings obtained by special methods is steadily increasing.

Special methods include casting: a) into permanent metal molds (chill mold), b) centrifugal, c) under pressure, d) into thin-walled one-time molds, e) by investment models, f) cortical, or shell, g) electroslag casting.

Special casting methods make it possible to obtain castings of more precise dimensions with good quality surfaces, which helps to reduce the consumption of metal and the complexity of machining; improve the mechanical properties of castings and reduce losses from marriage; significantly reduce or eliminate the consumption of molding materials; reduce production space; improve sanitary and hygienic conditions and increase labor productivity. ...

Most operations with special casting methods can be easily mechanized and automated.

The economic feasibility of replacing casting in disposable sand-clay molds by one or another special method depends on the scale of production, the shape and size of the castings, the casting alloys used, etc. It is determined on the basis of a thorough technical and economic analysis of all costs associated with a new technological process.

One of the most common is mold casting. A chill mold is a solid or split metal mold made of cast iron or steel.

Chill molds are designed to produce a large number of identical castings from non-ferrous or iron-carbon alloys. The resistance of molds depends on the material and dimensions of the casting and the mold itself, as well as on compliance with its operation mode. Approximately the resistance of cast iron molds is 200,000 tin-lead, 150,000 zinc, 50,000 aluminum or 100 ... 5000 cast iron castings. It is advisable to use chill molds both in mass and in mass production (with a batch of castings of at least 300 ... 500 pieces).

Before pouring the metal, the molds are heated to a temperature of 100 ... 300 ° C, and the working surfaces in contact with the molten metal are covered with protective coatings. The coating provides an increase in the service life of the mold, prevention of welding of metal to the walls of the mold and facilitating the extraction of castings. Heating protects the mold from cracking and facilitates the filling of the mold with metal. During operation, the required temperature of the mold is maintained due to the heat released by the poured metal. After hardening, the casting is removed by shaking or using an ejector.

Die casting makes it possible to reduce metal consumption for risers and risers, to obtain castings of higher accuracy and surface finish, and to improve their physical and mechanical properties. However, this casting method also has disadvantages. Rapid cooling of the metal makes it difficult to obtain thin-walled castings of complex shape, and causes the danger of the appearance of hard-to-cut surfaces in cast-iron castings.

Injection molding- one of the most productive methods for obtaining precise shaped castings from non-ferrous metals. The essence of the method lies in the fact that liquid or mushy metal fills the mold and crystallizes under excessive pressure, after which the mold is opened and the casting is removed.

According to the method of creating pressure, they are distinguished: casting under piston and gas pressure, vacuum suction, liquid stamping.

The most common shaping of castings under piston pressure is in machines with a hot or cold compression chamber. Alloys used for injection molding must have sufficient fluidity, a narrow temperature-time interval of crystallization, and not chemically interact with the mold material. To obtain castings by the considered method, zinc, magnesium, aluminum alloys and alloys based on copper (brass) are used.

Injection molding produces parts of devices: drums of counting machines, camera bodies and body parts weighing up to 50 kg, cylinder heads of motorcycle engines. Holes, inscriptions, external and internal threads can be obtained in castings.

Fig.5 Special casting methods

a - under pressure; b - centrifugal.

Figure 5, a shows the sequence of obtaining a casting on a piston machine (with a cold vertical compression chamber). The molten metal is fed in portions into the vertical pressing chamber 2. When moving down, the piston 1 presses on the metal, moves the heel 4 down, as a result of which the feed channel 3 opens and the metal enters the mold cavity 5. After filling the mold and holding for 3 ... 30 s the piston and heel rise, while the heel cuts off the sprue and pushes out the press residue b. The movable part of the mold 8 moves to the right, and the casting 7 is easily removed. Internal cavities and holes in castings are made using metal rods.

Before starting work, the mold is heated and lubricated. During operation, the required temperature is maintained and the mold is periodically lubricated.

Molds are made from alloyed tool steels (3Kh2V8, KhVG, Kh12M, etc.) and subjected to hardening with high tempering. The cost of the mold is 3...5 times higher than the cost of the mold.

The durability of molds, depending on the size and shape of the castings, is when casting from zinc alloys 300...500 thousand castings, aluminum - 30...50 thousand, copper - 5...20 thousand castings. The productivity of piston machines reaches 500 castings per hour.

In conditions of mass production, the use of injection molding is economically justified, since this method makes it possible to reduce the laboriousness of obtaining castings by 10–12 times, and the laboriousness of machining by 5–8 times.

Due to the high precision of manufacturing and the provision of increased mechanical properties of castings produced under pressure, savings of up to 30 ... 50% of metal are achieved compared to casting in one-time molds. It creates the possibility of complete automation of the process.

Centrifugal casting method It is mainly used to produce hollow castings such as bodies of revolution (bushings, shells for piston rings, pipes, liners) from non-ferrous and iron-carbon alloys, as well as bimetals. The essence of the method consists in pouring liquid metal into a rotating metal or ceramic mold (mould). Liquid metal due to centrifugal forces is thrown to the mold walls, spreads along them and hardens.

Long pipes and sleeves are cast on machines with a horizontal axis of rotation, short bushings, crowns of large diameter - on machines with a vertical axis of rotation.

With the casting method under consideration, castings are obtained dense, free of gases and non-metallic inclusions, with a fine-grained structure.

Centrifugal casting is highly productive (40...50 cast iron pipes with a diameter of 200...300 mm can be cast in 1 hour), makes it possible to obtain hollow castings without the use of cores and bimetallic castings by successively pouring two alloys (for example, steel and bronze).

As with die casting, metal molds are heated before pouring liquid metal and protective coatings are applied to them. After pouring, the molds are sometimes cooled with a water shower to increase the productivity of the machines and prevent them from overheating.

Along with high productivity and simplicity of the process, the centrifugal casting method, in comparison with casting into stationary sand-clay and metal molds, provides a higher quality of castings, almost eliminates metal consumption for risers and uplifts, and increases the yield of good casting by 20 ... 60%.

The disadvantages of the method include the high cost of molds and equipment and the limited range of castings.

Investment casting consists of the following. The metal is poured into a one-time thin-walled ceramic mold, made according to models (also one-time) from a low-melting model composition. In this way, precise castings from any alloys weighing from a few grams to 100 kg are obtained, which practically do not require machining.

The dimensional accuracy and surface finish of the resulting castings are such that they allow reducing the amount of machining or abandoning it, which is especially important in the manufacture of parts from hard-to-machine alloys;

The technology for the production of castings according to the models being performed includes the following stages: production of molds for models; obtaining wax models by pressing the model composition into molds; assembly of a block of models on a common feeder (in the case of small castings); applying a refractory coating to the surface of a single model or block; melting models from refractory (ceramic) mold shells; annealing molds; pouring metal into hot molds.

Split molds are made from steel or other alloys according to the drawing of the part or its standard, taking into account the shrinkage of the model mass and the casting metal.

The model composition (for example, from paraffin with additives of ceresin, petroleum bitumen, rosin, polyethylene) in a pasty state is pressed with a syringe or on a pressing machine.

The resulting models are removed from the molds and lined in several layers with a refractory coating, dipped several times in a binder and sprinkled with quartz sand. Each layer of the coating is dried. The model of small castings is assembled into blocks before coating, connecting them (soldering) to a common gating system, and then lining the block.

Melting models from ceramic shells is done with hot air or hot water. The model material is collected for reuse, and the resulting ceramic mold with a smooth working surface is fed to the calcination. The latter is necessary to give the form mechanical strength and the final removal of the model material. The mold is placed in a steel box, covered with quartz sand, leaving the gating cup available for metal pouring, and calcined at a temperature of 850...900 °C.

The metal is poured into a hot mold, which improves the fluidity of the metal and makes it possible to obtain the most complex thin-walled castings.

After cooling, the casting is cleaned from the layer of refractory coating by hand blows or on pneumatic vibrators. In cavities and holes, mold residues are removed by leaching in a boiling solution of caustic soda, then the casting is washed in warm water with the addition of soda.

Separation of the gating system from castings can be carried out on lathes and milling machines, vulcanite abrasive wheels and vibration machines.

Investment casting produces a variety of complex castings for automotive and tractor construction, instrument making, for the manufacture of aircraft parts, turbine blades, cutting and measuring tools.

The cost of 1 ton of investment castings is higher than those produced by other methods, and depends on many factors (serial production of parts, the level of mechanization and automation of foundry processes and casting machining processes).

In most cases, reducing the labor intensity of machining, the consumption of metal and metal-cutting tools when using precision castings instead of forgings or castings obtained by other methods, gives a significant economic effect. The greatest effect is achieved when converting to investment casting of parts, in the cost structure of which a large share is the cost of metal and milling, especially when using difficult-to-machine structural and tool materials.

The introduction of investment casting is given great attention, since most operations can be easily mechanized and automated. Through the joint efforts of employees of scientific research institutes and advanced factories, high-performance automatic lines and automated workshops for investment casting are being created.

Shell casting it is used to obtain castings weighing up to 100 kg from cast iron, steel and non-ferrous metals. Thin-walled (wall thickness 6 ... 10 mm) molds are made from a sand-resin mixture: fine-grained quartz sand and thermosetting synthetic resin (3 ... 7%). The sand-resin mixture is prepared by mixing sand and crushed powdered resin with the addition of a solvent (cold method) or at a temperature of 100 ... 120 ° C (hot method), as a result of which the resin envelops (clads) sand grains. Then the mixture is further crushed to obtain individual grains, clad with resin, and loaded into the hopper. Molding is made on metal models.

The model in the gating system is fixed on a model plate, heated to a temperature of 200 ... 250 ° C and a thin layer of a release agent is applied to their working surface. After that, the mouth of the bunker is closed with a model plate (the model is inside) and it is rotated by 180°. The mixture falls on the heated model, the resin melts and after 15...25 s a shell (half-mould) of the required thickness is formed on the model. The bunker is turned again by 180°, the remaining mixture falls to the bottom of the bunker, and the model board with a semi-solid shell is placed in an oven for final hardening at a temperature of 300...400 °C for 40...60 s. With the help of special ejectors, the half-mould can be easily removed from the model.

Fastening (assembly) of half-forms is carried out with metal brackets, clamps or quick-hardening glue. Sand-resin cores for hollow castings are produced in a similar way.

The assembled shell molds are placed in flasks to make them more rigid, they are covered with iron shot or dry sand from the outside and poured with metal. After the casting has hardened, the shell mold is easily destroyed.

Castings made in shell molds are distinguished by high accuracy and surface cleanliness, which makes it possible to reduce the mass of castings by 20...40% and the labor intensity of their machining by 40...60%. Compared to casting in sand-clay molds, the complexity of manufacturing castings is reduced by several times. In this way, critical machine parts are obtained - crankshafts and camshafts, connecting rods, ribbed cylinders, etc. Shell manufacturing processes can be easily automated.

Despite the high cost of the sand-resin mixture, in comparison with the sand-clay mixture, a significant economic effect is achieved in the mass and serial production of castings.

Shell mold casting is used for the manufacture of parts mainly from iron-based alloys (cast iron, carbon and of stainless steel), as well as from copper and special alloys.

At the Kiev Motorcycle Plant, ribbed cylinders are cast from modified chromium-nickel cast iron in this way; at the Gorky Automobile Plant, crankshaft halls from high-strength cast iron are obtained in shell molds.

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foundry technology

2. Main defects of castings

6. Die casting

7. Centrifugal casting

Literature

1. Technological concepts in the foundry

Foundry- a branch of mechanical engineering that manufactures workpieces by pouring molten metal of a given chemical composition into a mold, the cavity of which has the configuration of a casting. Upon cooling, the poured metal solidifies and takes on the shape of the mold cavity.

The workpiece obtained after solidification of the metal is called casting. The casting can be either a completely finished product, or subjected to further machining.

Casting molds that are used only once and are destroyed when castings are removed from them (sand-clay, shell with a resin binder, one-piece ceramic, etc.) are called one-time. Semi-permanent molds made of highly refractory materials (gypsum, cement, graphite, etc.) can withstand 3…100 or more pours of metal.

One-time and semi-permanent casting molds are made according to fixtures called models. The process of making such molds is called moulding.

Model according to its external configuration, it corresponds to the resulting casting and is distinguished by large dimensions, taking into account metal shrinkage and machining allowances. The model may have bar signs.

The configuration of the model should ensure that it is easy to remove it from the mold.; the surface of the models is carefully finished to ensure clean mold surfaces. The model must be strong, not change in size. Models are made of metals and alloys, wood, plaster, plastic, fusibleorganic materials.

rod called part of the mold, designed to obtain internal cavities in the casting.

Rod signscalled parts protruding along the model that do not form the configuration of the casting, but serve to form recesses in the mold, into which the rods are installed during mold assembly.

Gating systemserves to pour metal intoa mold cavity with a certain filling sequence and speed, as well as for feeding the casting during its solidification.

Metal preparation. Used in foundryliquid alloy (melt) and various melting units are used to prepare it.

For castings responsible appointment mainly electric furnaces of various types are used. Induction furnaces, electric arc furnaces and resistance furnaces are widely used. Vacuum melting and casting are widely used (for example, in the production of castings from titanium alloys).

2. Main defects of castings

shrink shells- closed cavities, mostly oxidized, in castings with a rough surface (Fig. 1). Shrinkage cavities are formed due to insufficient supply of the casting in places of accumulation of metal, improper design of the casting and the gating system. Shrinkage cavities are eliminated with the help of profits that harden last, as a result of which shrinkage cavities are displayed in the profit Then it is removed.

Rice. 1. Shrinkage cavity in the casting and a method for its elimination

hot cracks - through and non-through breaks in the body of the casting. They usually occur in places of transition from a thin section to a thick one, in places of sharp transitions of the section at a right or sharp angle (Fig. 2, a ), and also in the event that the mold or the core prevent the shrinkage of the casting (Fig. 2, b).

gas sinks- cavities in the casting of a rounded shape with a smooth surface, ranging in size from 1 to 10 mm, occur with low gas permeability of the mold, with an incorrectly constructed gating system.

Underfilling and sleep (Fig. 3) are formed from unblended metal flows that have lost fluidity and solidified before the mold is filled.

burnt - the interaction of the casting mold and the poured metal with its insufficient refractoriness and high chemical activity.

skew (Fig. 4) in the casting is formed during careless assembly of the mold.

3. Technology for producing castings in sand-clay molds

The method of casting in sandy-clay molds is one of the oldest methods. In a modernized form, due to the improvement of the composition of molding sands, this method is used in aircraft and shipbuilding.

Sandy-clay forms have a one-time purpose.

Casting sand-clay moldis a system of elements that form the working cavity (Fig. 4, a ) filled with molten metal. For the formation of holes and other complex shapes in the casting, casting cores are used, which are fixed in the mold with the help of signs included in the corresponding depressions in the mold cavity. Casting cores are made in core boxes (Figure 4, b ) from special sand core mixtures with the help of machines that perform the main operations in the core manufacturing process: compaction of the mixture and removal of the core from the box. To supply molten metal to the cavity of the mold and ensure its filling and feeding of the castings during solidification, a gating system is made. The process of making molds using a model is called molding.

b c

Rice. 5. General form sand-clay mold (a), rod (b) and model (c)

Models are made of metal or wood, with a parting plane (Figure 5, in ) The parting of the model coincides with the parting plane of the mold. With this method, the mold is basically split. (Figure 5, a ).

The mold must have:

a) strength - the ability to withstand power loads arising from pouring molten metal;

b) gas permeability - the ability to pass gases, steam, located and formed in the mold during pouring of molten metal;

c) pliability - the ability to decrease in volume under the action of casting shrinkage when it is cooled;

d) refractoriness - the ability not to melt under the action of the heat of the molten metal.

Molding mixtures are used to make casting molds.

Molding sands in the manufacture of molds adjointo the model and form a working layer of the mold in contact with the liquid metal. The properties of molding sands depend on their composition. The composition of the molding mixtures includes refractory materials - quartz Si O 2, or zircon ZrO 2 Si O 2 , sands, which are the basis of the form, clay asbinder and special additives that improve the characteristics of mixtures.

Forms can be made manually to obtain very complex single castings. At modern machine-building plants of mass and large-scale production, sand-clay molds are madeon molding machines in flasks on special pattern plates (Figure 5, which form the mold connector, carry various parts of the model (casting model 1 and gating system models 2, 3) and serve to stuff one of the paired flasks. Modern molding machines are usually mechanized according to two main operations in the process of making molds: compacting the molding sand in the flask and removing the model from the mold.According to the method of compacting the mixture, molding machines are divided into shaking, pressing, shaking with prepressing and sand throwers.According to the method of removing the model from the mold, they are divided into machines with rotary with a plate, with a pin lift with a flip groan and with a broaching plate.

The production of molds on press machines (Fig. 7) is carried out in the following sequence: on a pattern plate 4, attached to the table of the machine, a flask 5 is installed, and a filling frame 6 is installed on the flask. A flask with a filling frame is filled with molding sand. A press block 7 is installed on the traverse above the filling frame. Compressed air is supplied under pressure to the press cylinder 1. The press piston 2 rises up towards the press block 7, which enters the filling frame into the flask. After the pressure is released, the piston, together with the table and the flask, goes down. Then the flask is lifted up from the pattern plate by means of a removable mechanism 3.

Rice. 6. Special pattern plate

Rice. 7. Pressing machine for making sand-clay molds

Half molds with a height of not more than 200 mm are made on press machines, since at high heights a uniform
shape density. The half-moulds obtained by molding are paired, the rods are pre-installed, if necessary. The assembled molds are filled with liquid metal. A gating system is used to pour the alloy. In foundries of individual and small-scale production, molds are poured on a molding parade, placing them in a row. In large-scale and mass production, molds are poured on roller conveyors. Recently, automated lines have been used to make molds and pour metal. The preparation of casting alloys is associated with the process of melting various charge materials. For steelmaking, high-frequency induction furnaces are widely used, which allow heating the metal to high temperatures, creating a vacuum, and obtaining high-quality metal. It is practically possible to pour a wide range of alloys into sand-clay molds and obtain castings of unlimited mass and any size.

Crucible resistance furnaces are widely used for melting aluminum alloys, which can be rotary and stationary, as well as high-performance two-channel induction furnaces with a metal core (the metal core is the melt itself), in which the metal is obtained of a higher quality than when melting furnaces of another type. The melting of aluminum alloys has a number of difficulties due to their strong oxidation and saturation with gases. There are several methods of metal preparation that ensure the production of high-quality castings from aluminum alloys: melting under a layer of flux, refining the liquid melt with neutral gases or salts. During gas refining, after melting the aluminum alloy at a temperature of 660 ... 680 ° C, it is refined with chlorine. Refining is carried out by blowing chlorine through the alloy for 5...15 minutes.

In addition to chlorine, nitrogen and argon can be used for gas refining.

The refined metal is poured into the prepared mold. After pouring and cooling the metal, the casting is removed (knocked out), and the mold is destroyed. The casting is removed from the mold either manually, mechanically or automatically, depending on the nature of the production.

Subsequently, the casting is cleaned in cleaning drums or shot-blasting devices of chamber or drum type. Chipping and cleaning of castings from the remains of feeders, burrs, fillings is carried out with abrasive wheels on abrasive presses.

4. The structure of the gating system

gating systemcalled a set of channels and reservoirs through which liquid metal from the ladle enters the mold cavity (Fig. 8).

Rice. 8. Scheme of the gating system

sprue bowl (2) - a reservoir designed to receive liquid metal and transfer it to riser 3.

Riser (3) - a vertical (sometimes inclined) channel of a round, oval or other section, designed to transfer metal from the bowl to other elements of the gating system.

Slag trap (1) - a channel in which slag and non-metallic inclusions are retained, entrained by liquid metal into the mold. To prevent slag from entering the mold cavity during pouring, its bowl must be constantly filled to the brim. This encourages the slag to float and prevent it from entering the mold cavity. However, part of the slag can still be carried away by liquid metal. To prevent it from getting into the mold, a slag trap is used. The slag, having a significantly smaller cavity than the metal, floats to the top of the slag trap and lingers in it, and pure metal from the bottom of the slag trap through the feeder enters the mold cavity. To keep the slag well, feeders are usually located below the slag trap.

The slag trap is used for heavy metals, which are characterized by a high slag floating rate. For light alloys, a collector-distributor is required, since the density of the poured metal is close to the density of slags and the slag floating rate is negligible.

Feeders (sprues)(4) - channels designed to transfer metal directly into the mold cavity.

Gating systems are divided into the following most common types (the designations in Fig. 9 correspond to Fig. 8):

Rice. 9. The most common types of gating systems

1) top (Fig. 9, a ) - feeders supply metal to the upper part of the casting;

2) lower or siphon - feeders supply metal to the lower part of the casting (Fig. 9, b);

3) slotted - feeders bring the metal along the height of the casting (Fig. 9, in );

4) tiered - feeders supply metal at several levels
(Fig. 9, G ).

The type of gating system is chosen depending on the type of metal, the design of the casting, its position during pouring, etc.

In addition to the choice of the type of gating system, the choice of the location for supplying feeders to the casting is of great importance. Depending on the properties of the alloy, the design of the casting (overall dimensions, wall thickness), when metal is supplied, they strive to ensure either directional solidification or simultaneous, uniform cooling of various parts of the casting.

Gating systems are calculated. The calculation is reduced to determining the area of ​​the smallest section of the gating system (riser or feeder), followed by determining the ratio of the cross-sectional areas of the remaining elements of the system.

The area of ​​the smallest section F ns find by formula

, (1)

where G is the mass of metal that has passed through the minimum section;

τ – filling duration, s: ;

γ is the density of the liquid metal, g/cm 3 ;

μ - flow rate coefficient of the gating system, taking into account speed losses, friction turns;

H p - design pressure, cm;δ - the predominant wall thickness of the casting, mm;

S - coefficient depending on the wall thickness and casting configuration: for titanium and magnesium alloys and steel - 0.91 ... 1.7; aluminum alloys - 1.7 ... 3.0.

Head H p depends on the method of pouring, the type of gating system, the position of the casting in the mold and other factors. For the case of metal supply through the mold part, which is very common in the foundry industry, H p can be calculated using the formula

, (2)

where H 0 - the initial maximum pressure of the poured metal;

R - the distance from the highest point of the casting to the level of metal supply;

With - height of the casting (according to the position when pouring the metal).

When calculating the areas of gating channels, the relations are used

Or 1:3:6

5. Casting in shell (crust, shell) molds

Shell mold casting is the process of obtaining castings by freely pouring molten metal into sand-resin shell molds made by molding according to a hot model.

There are many varieties of this casting method, the most common are as follows.

Shell molds are made from an unclad sand-resin mixture (quartz sand is the base, 3 ... 8% phenol-formaldehyde resin, 0.8% petroleum polymer) (Fig. 10, a ) or plated (Fig. 10, b ), for which the phenol-formaldehyde resin is pre-dissolved in acetone or alcohol, and then mixed with quartz. Clad mixtures contain resin in the form of a thin film covering the surface of quartz grains (Fig. 10, b ). Shell molds made of clad mixture have higher strength at minimum mixture consumption. The resin has the ability to melt when heated to 160 ... 200 ° C, go into a thermoplastic state, which helps to obtain a clear imprint of the model.

When heated to 290...350°C, the resin passes into a stable thermosetting (irreversible) state.

On Fig. 11 shows a process flow diagram for making a shell mold. On bunker 1 (Fig. 17, a ), in which the molding sand is located, a metal model plate Z with model 4 is fixed, heated to 160 ... 200 ° C. After that, the bunker overturns,molding sand 2 covers the hot pattern plate 3 and pattern 4 (Figure 17, b ). The hopper then rotates 180°. The molding sand layer remains on model 4 (Fig. 17, in ), and the model plate 3 is separated from the hopper 1 (fig. 17, G ) and placed in an electric oven for the final hardening of the shell. Then the finished half-mould is removed from the pattern plate 3 (Fig. 11, d ). The technological process is repeated to obtain the second half-mould. The two half-forms thus obtained are connected with brackets.

a b

Rice. 10. Unclad ( a ) and clad ( b ) sand-resin mixture

A B C D E

Rice. 11. The sequence of obtaining an ordinary half-form

Liquid metal is poured into the mold assembled and cooled to room temperature. After crystallization and cooling of the casting, the binder of the casting mold almost completely burns out, which makes it easier to knock out the casting from the mold.

Upon receipt of large castings, due to the danger of metal breakthrough, during pouring, shell moldsinterfere with the flask and fall asleep with cast-iron shot.

The shell form has 10-30 times greater gas permeability than the sandy-clayey form. The ductility of the shell mold is also increased, which reduces the appearance of internal stresses in the castings. Such molds have less crumbling of the crust and the release of weakly reducing gases at the time of pouring metals, which improves the cleanliness of the casting surface and reduces the amount of sand blockages.

Casting in shell molds makes it possible to increase the accuracy of the geometric dimensions of castings, to halve the allowances for machining; the consumption of molding materials is reduced by 5–10 times; the processes of mechanization and automation of casting production are simplified.

In this way, castings are made with a massup to 25...30 kg, and sometimes up to 100...150 kg with 6 mm holes and a minimum wall thickness of 3...4 mm.

Shell casting is used to produce crankshafts and camshafts, exhaust valves, gears, exhaust pipe flanges, cylinder block liners, cylinder block crankcase, ribbed cylinders, brackets, racks, covers, etc.

The limiting factors of casting into shell molds are:

1. Molds are detachable, which significantly affects the accuracy of the dimensions of the casting in directions perpendicular to the planes of the parting of the molds.

In the manufacture of massive castings, significant distortion of the molds is observed.

6. Die casting

Chill casting is the process of obtaining shaped castings by freely pouring molten metal into metal molds - chill molds.

Die casting is widely used in serial and mass production of castings for a wide variety of products with a wall thickness of 3 ... 100 mm from copper, aluminum and magnesium alloys, as well as from cast iron and steel, the mass of which varies widely - from several grams, up to several tons; for example, large blades, heads and blocks of internal combustion engines, housings of superchargers of reactors, diffusers, etc.

Casting in a mold provides increased accuracy of geometric dimensions, reduces the surface roughness of castings, reduces machining allowances, improves the mechanical properties of castings in comparison with castings obtained in sand-clay molds.

The disadvantage of mold casting is the high manufacturing cost and high thermal conductivity of the mold, which leads to a decrease in its metal filling due to a rapid loss of fluidity.

The designs of molds are extremely diverse. The mold for simple castings is made of two parts, corresponding to the upper and lower flasks when casting in sand-clay molds. For complex castings, the mold is made from detachable parts, each of which forms part of the casting, while the parting surface of the mold is determined by the design of the casting; in this case, the parting surface of the mold is determined by the design of the casting. In addition, the thickness of the mold walls affects the rate of solidification and subsequent cooling of the casting, and, consequently, the formation of the structure of the casting.

To obtain the internal cavity of the casting, rods are used: for castings from low-melting alloys - mainly metal, for iron and steel castings - sand.

The gas in the mold is expelled through the vent and ventilation ducts located along the mold connector. To extract the casting from the mold, there are ejectors.

The die casting technology has a number of specific features due to the design of the metal mold and the requirements for the poured metal.

In order to obtain a high-quality casting and extend the life of the mold, it is covered with a refractory lining or paint. The operating temperature of the mold depends on the alloy being poured and is in the range of 150 - 300°C. By applying a thicker layer of paint to individual parts of the mold, rapid heat dissipation can be prevented at the metal-mould interface and thus in different parts of the casting.

Paints are often made from materials that release gas during pouring at the metal-to-mould interface; gas creates a reducing atmosphere that protects the metal from oxidation. The most commonly used zinc oxide, talc, graphite, aluminum oxide.

In mass and serial production, special mold casting machines with mechanized separation of individual parts are used. In this case, the poured metal must have good fluidity and low shrinkage.

7. Centrifugal casting

The use of centrifugal forces to fill and crystallize the metal in the mold cavity– distinctive feature of centrifugal casting. Centrifugal forces are generated as a result of the rotation of the mold.

This casting method is mainly used for the manufacture of hollow castings having the shape of a body of revolution (pipes, bushings, rings), from cast iron, steel, non-ferrous alloys (copper, aluminum, titanium, etc.), shaped castings with smallwall thickness, but increased density of the material (turbine blades, housings, parts of hydraulic equipment, etc.). To obtain castings, installations with a horizontal and vertical axis of mold rotation are used. Under the action of centrifugal forces, liquid metal 1 (Fig. 12) is pressed against the inner surface of the rotating mold 2, entrained by it, and crystallizes in this state. With centrifugal casting, it is possible to use not only a metal mold, but also a shell mold 1 (Fig. 13), a sand-clay mold, and a mold obtained from an investment model.

Rice. 1 Scheme of centrifugal casting

Centrifugal casting has a number of advantages over fixed mold casting:

1) castings have a high material density;

2) costs for the manufacture of rods to obtain a cavity in cylindrical castings are excluded;

3) the fillability of molds with metal is improved;

4) it is possible to obtain castings from alloys with low fluidity.

Rice. 13. Scheme of centrifugal casting in a shell mold

The centrifugal casting method has the following disadvantages:

1) contamination of the free surface of the casting with non-metallic inclusions (lighter than the casting alloy);

2) the presence of defects in the casting in the form of chemical heterogeneity in the radial direction due to segregation of the alloy components in terms of density. With an increase in the rotation speed, the segregation of elements in terms of densities in the section of the casting increases.

Mold rotation speed is an important parameter of centrifugal casting technology. At a low rotation speed, the inner surface is not smooth, and the castings are not sufficiently cleaned of non-metallic inclusions. At an overestimated speed, the internal pressure of the liquid metal increases greatly, which leads to the formation of cracks and the segregation of the alloy components in terms of density increases. The optimal rotation speed for each casting is determined by empirical formulas or nomograms.

8. Investment casting

Investment casting is a process production of castings in one-piece one-time refractory molds made using models from low-melting, burnable or soluble compositions. Both shell (ceramic) and monolithic (gypsum) forms are used. In this case, the working cavity of the mold is formed by melting, dissolving or burning out the model.

Model compositions used in investment casting must have minimum values shrinkage and thermal expansion coefficient, have high fluidity in the viscous-plastic state, are well wetted by ceramic or gypsum suspension applied to the model, but do not chemically interact with it, have a softening point exceeding 40 ° C.

The production of models is carried out by pouring or pressing the model composition in a pasty (heated) state into special molds 1 (Fig. 14). In particular, the injection method for producing polystyrene foam models on special injection molding machines includes plasticizing by heating (100 - 220 ° C) polystyrene granules, injecting it into a mold, followed by foaming and cooling the model. For the production of molds, both metal (steel, aluminum and lead-antimony alloys) and non-metal (gypsum, epoxy resins, formoplast, vixint, rubber, hardwood) materials are used. Molds used to obtain models must provide them with high parameters of dimensional accuracy and surface quality, be easy to manufacture and operate, and also have a service life corresponding to the level of serial production. So, in single, small-scale and serial production, mainly cast metal, gypsum, cement, plastic, wood, as well as molds obtained by metallization methods, manufactured by mechanical processing, are used.

Rice. 14. Investment casting: 1 - mold; 2 - model; 3 - model gate block; 4 - suspension; 5 - fluidized bed of granular refractory material; 6 - compressed air supply; 7 - melt of the model mass (or hot water); 8 – ceramic shell form; 9 – supporting filler (quartz sand); 10 - furnace; 11 - bucket

In the manufacture of gypsum molds, the standard model (standard model), made of any structural material, is poured with an aqueous suspension of high-strength gypsum grades 350 and higher. Such molds withstand the production of up to 50 pieces of models, but do not provide the latter with high levels of dimensional accuracy and surface quality.

For the manufacture of molds, the methods of electroforming, metallization and spraying are also used. So, a galvanic coating is applied to a reference model made of a polished alloy based on aluminum or zinc. When forming plasma coatings based on metal powders, metal alloys, graphite or gypsum are used as the material of the reference model. Pressing of model compositions is carried out on presses (pneumatic, lever, etc.) or manually. Mounting of model blocks is carried out by combining small models 2 into blocks 3(Fig. 14, b ) with a single gating system, which increases the manufacturability, productivity and efficiency of the casting process. The assembly of models into model blocks (i.e., the connection of casting models with a riser model) is carried out in different ways: a) by soldering with a heated tool (soldering iron, knife) or liquid model composition; b) connection of models in the jig with simultaneous casting of the model of the lantern system; c) connecting models into blocks on a metal riser (frame) using mechanical fastening (clamp); d) gluing casting models and gating system.

The lost wax casting method has found wide application in industry (especially in the aircraft industry) due to the use of one-piece ceramic shell molds. having a set of necessary operational properties (gas permeability, heat resistance, rigidity, surface smoothness, dimensional accuracy, lack of gas generation, high operating temperature, etc.).

Typically, a ceramic shell consists of 3–8 successively applied layers (in principle, the number of layers can reach 20 or more), resulting in a total mold wall thickness of 2 to 5 mm. In some cases, smaller wall thicknesses (0.5–1.5 mm) of the ceramic shell are also allowed. Layers of suspension 4 are applied by immersing a model block into it (Fig. 20, b ). After excess suspension drains from the models, they are sprinkled with refractory material (for example, quartz sand, fireclay crumbs, electrocorundum with a grain size for different layers in the range of 0.1 - 1.5 mm) in a fluidized layer 5 (Fig. 14, G ) and dried. In this case, each layer of the shell is dried until the content of the liquid phase in it is no more than 20%.

The advantages of this casting method are: the possibility of obtaining castings of complex configuration; use of almost any alloys; high surface quality and dimensional accuracy of castings; minimum allowances for machining; providing high-quality equilibrium, columnar and single-crystal structure with high level performance properties.

The disadvantages of the casting method include: multi-operation, laboriousness and duration of the process, the variety of materials used to make the mold.

Investment casting is used to produce complex, high-quality castings, such as turbine blades from heat-resistant alloys, permanent magnets with a certain crystallographic orientation of the structure, art products, etc.

9. Injection molding and squeezing method

Injection molding is a method of producing shaped castings in metal molds, in which the mold is forcibly filled with metal under pressure exceeding atmospheric pressure. Cast under pressure ensures high accuracy of geometric dimensions and low surface roughness, significantly reduces the amount of machining of castings and in some cases completely eliminates it, provides high mechanical properties of castings, and makes it possible to obtain castings with complex configurations with small wall thicknesses.

This method produces castings from aluminum, magnesium, zinc and copper alloys with a wall thickness of 0.7 to 6.0 mm, weighing from a few grams to 50 kg. It is used for the manufacture of parts for electronic calculating machines, optical instruments, cylinder blocks, brake discs, etc.

In die casting, metal molds have a more complex design and are made more accurately and carefully than in chill casting. Injection molds are made of steel with steel rods. The use of sand cores is excluded, since a metal jet under pressure can erode the sand core.

To create pressure when filling the form of metals, special very complex machines are used. There are machines of compressor action and piston. The pressure on the metal in different designs of machines varies widely (from 60 to 2000 Pa).

Squeeze casting is used to obtain thin-walled large-sized panel-type castings with dimensions up to 1000-2500 mm with a wall thickness of 2.5 ... 5 mm. The method also makes it possible to produce castings of the type of thin-walled cylindrical shells. The accuracy of castings approaches the accuracy of castings obtained by free casting into metal molds, yielding to them due to the inaccuracy of joining half-moulds. characteristic feature squeeze casting is the absence of a gating system and the possibility of pouring metal at lower temperatures (in a suspension state, i.e. in the initial stage of crystallization).

10. Casting properties of alloys

Not all known alloys are equally suitable for castings. From some alloys (tin bronze, silumin, gray cast iron, etc.) it is possible to obtain shaped casting of a given configuration with the corresponding properties by any casting methods, from other alloys (titanium, alloy steels) the production of castings is associated with great technological difficulties (vacuum protection is required, high pressures and etc.).

The possibilities and difficulties of obtaining high-quality castings from metals and alloys are to a large extent predetermined by their casting properties. Casting properties - properties that characterize the behavior of metals and alloys in the manufacture of castings from them.

Thus, casting properties are such technological properties of metals and alloys that directly and directly affect the production of high-quality castings of a given design with the necessary performance indicators: accuracy and surface finish.

The casting properties of alloys must necessarily be taken into account in the specific development of a casting technology, as well as in the process of creating and designing cast structures. The reliability and durability of products are largely determined by the casting properties of the alloy used for their manufacture.

The range of casting properties depending on the level of production of casting alloys and general development technology may change over time. At present, the nomenclature of casting properties consists of the following indicators: fluidity; shrinkage; tendency to absorb gases and form gas inclusions; tendency to form non-metallic inclusions; structural features during primary and secondary crystallization of macro- and microstructure; crack resistance; formation of foundry stresses; propensity to liquidate; activity of interaction of alloys with the medium and the casting mold.

Fluidity is understood as the ability of metals and alloys in a liquid state to fill molds in which a casting is formed.

Good fluidity is necessary not only to reproduce the shape of the mold in the casting, but also to improve the withdrawal of shrinkage cavities outside the casting, to reduce the risk of all types of porosity and cracks. Filling a casting mold with liquid metal is a complex physical, chemical and hydromechanical process.

Fluidity depends on the nature of the movement of the alloy, and with turbulent movement it will be less than with laminar. The loss of the ability of the melt to laminar motion, ceteris paribus, depends on the Reynolds number Re : the lower the value of the Reynolds number for a casting alloy, the easier it is to change from laminar to turbulent motion. Number R e for steel, twice the number R e for cast iron. It follows that steel can go from laminar to turbulent motion more easily than cast iron.

Fluidity is dependent on the position of the alloy on the state diagram. Pure metals and glories of eutectic composition have the highest fluidity (Fig. 21); the smallest - alloys that form solid solutions. This is due to the fact that during the solidification of pure metals and eutectic alloys, crystals of constant composition are formed, which grow from the surface of the casting in a continuous front, and the liquid melt has the ability to move freely inside the casting. In alloys of the solid solution type, crystallization proceeds with the formation of whiskers, which penetrate far into the volume of the casting in the form of thin branched dendrites, which leads to a strong decrease in fluidity. Fluidity depends to a large extent on the crystallization range of the alloy.

Rice. 15. State Diagrams ( a ) and fluidity ( b ) system alloys Rv - Sn

Fluidity is a function of a large number of variables and its analytical determination is very difficult, therefore, in practice, technological samples are used to establish fluidity. Test results are usually plotted in terms of fluidity - pouring temperature or fluidity - chemical composition, etc. The resulting curves are used when choosing the pouring temperature or the composition of the casting alloy.

Shrinkage - the property of metals and alloys to reduce the linear dimensions and volume of the casting during cooling. When the casting is cooled, its linear dimensions begin to change from the moment when a strong hard crust forms on the surface.

In the foundry, the shrinkage of castings, associated only with the properties of alloys, is commonly called free shrinkage. If shrinkage is determined not only by the physical properties of the alloy, but also by the size and design of the mold, then such shrinkage is called difficult.

In table. Table 1 gives guide values ​​for free and difficult linear shrinkage for the most common alloys. The shrinkage of alloys changes due to changes in their composition.

Table 1

Free and hindered linear shrinkage of casting alloys

Alloy	Linear shrinkage, %
		free	difficult
Gray cast iron	1,1…1,3	0,6…1,2
White cast iron	1,8…2,0	1,5…2,0
Carbon steel	2,0…2,4	1,5…2,0
Special steel	2,5…3,0	2,0…2,5
Brass	1,5…1,9	1,3…1,6
Tin bronzes	1,2…1,4	0,9…1,0
Tinless bronzes	1,6…2,2	1,1…1,8
magnesium alloys	1,3…1,9	1,0…1,6

Shrinkage is one of the most important casting properties of alloys, since it is associated with the main technological difficulties in obtaining high-quality castings. Shrinkage can cause stresses in the metal, deformation of the castings and, in some cases, the formation of cracks in them. The reasons for the stress state of the casting material can be: mold resistance, metal shrinkage and non-simultaneous cooling of various parts of the castings, an incorrectly chosen casting method. When different sections of the casting are cooled at different rates, the shrinkage of these sections of the metal proceeds differently, as a result, foundry stresses develop.

In order to obtain dense castings from alloys with high shrinkage, profits are provided in the development of gating systems. The profit is installed in the upper part of the casting in such a way that, due to the accelerated cooling of the bottom and the tendency of the liquid metal to move to lower levels, all shrinkage cavities would be inside the profit, which is then separated from the casting.

When choosing a metal for cast parts, the designer must beaware of its fluidity, castingshrinkage, the technology of obtaining this casting and its influence on the strength characteristics of the developed unit.

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Casting is one of the most important and widespread methods of manufacturing blanks and machine parts. The mass of cast parts is about 60% of the mass of tractors and agricultural machines, (70 ... 85)% of the mass of rolling mills and metal-cutting machines.

The essence of the casting process essentially consists in the fact that molten metal of a certain chemical composition is poured into a pre-prepared casting mold, the cavity of which, in size and configuration, corresponds to the shape and dimensions of the required workpiece. After cooling, the workpiece or finished part, called castings, is removed from the mold.

To obtain high quality castings, casting alloys must have certain casting properties: good fluidity, low shrinkage, chemical structure uniformity, low melting point, etc.

Most of the iron and steel castings are obtained by casting into sand-clay molds (up to 60% of the total volume). To obtain castings with high accuracy (minimum machining allowances) and surface roughness, a homogeneous metal structure, special casting methods are used: casting into metal molds (chill molds), centrifugal casting, pressure casting, investment casting, casting into shell molds and etc.

Main casting advantages before other methods of obtaining blanks and parts are:

a) the possibility of obtaining blanks and parts of various configurations, from various metals and alloys;

b) the possibility of obtaining shaped products of complex configuration (hollow, voluminous, etc.), which are impossible and economically impractical to manufacture by other methods (for example, cutting - a large consumption of metal into chips, significant time, etc.);

c) universality of technologies - the possibility of manufacturing blanks from a few grams to hundreds of tons;

d) the possibility of processing waste products and rejects:

e) relative ease of obtaining and low cost of castings.

Along with the advantages of casting, it also has limitations:

a) the difficulty of obtaining a homogeneous chemical composition of the casting;

b) the accuracy and quality of the surface of the part is lower than when it is processed by cutting or plastic deformation;

c) inhomogeneity of the composition and reduced density of the material of the workpieces, and, consequently, their lower strength characteristics than the workpieces obtained by pressure treatment.

Main directions of development of foundry production are: reconstruction and modernization of existing equipment; replacement of obsolete equipment with high-performance casting machines and semi-automatic machines, robotic complexes; reducing the material consumption of products of the machine-building complex by increasing the share of casting from alloyed steels and high-strength cast iron, as well as precision casting.

The main technical and economic indicators of the work of foundries are: annual production of castings in tons; production of castings per production worker; we will eat castings from one square meter of the production area of ​​the workshop; yield of suitable metal; percentage of casting rejects; level of mechanization and automation; proportion of castings obtained by special methods; cost of one ton of casting.

A) Casting in sand-clay molds

A casting mold having a cavity into which molten metal is poured is made from a molding sand according to a model. A model is a device for obtaining a future casting in the form of a working cavity. Models can be made of wood, plastic or metal, their dimensions must be larger than the dimensions of the castings by the amount of shrinkage of the metal and the amount of allowance for subsequent machining.

Molding mixtures for casting molds and cores consist of quartz sand, special clay, water and a number of additives (linseed oil, rosin, dextrin, liquid glass, wood chips or peat chips) that provide gas permeability and plasticity of the mixture. When making a mold, the molding sand, moistened and thoroughly mixed, is poured into the lower flask, after setting the casting model (Fig. 1). Next, the mixture is compacted manually with various devices or on special molding machines. After compaction of the mixture, the model is removed from the lower flask. Similarly, the mixture is compacted in the upper flask, having previously installed in it, in addition to the casting model, a gating system model that forms channels for pouring liquid metal into the mold cavity. The gating system consists of a gating bowl, a vertical riser, a slag trap, a feeder and a riser. The gating system must ensure the smooth flow of molten metal into the mold and the removal of gases from the mold.

Then, having installed the rods in the form, they assemble it: the upper flask is installed on the lower one and the flasks are fixed with pins. In this form (Fig. 1), the mold is ready for pouring with melt.

Metal melting is carried out in various melting devices. Cast iron is melted in cupolas, steel is melted in converters and electric furnaces, non-ferrous metals and their alloys are melted in electric furnaces and crucibles. The temperature of the molten metal is brought to the pouring temperature, ᴛ.ᴇ. 100 ... 150 C higher than the melting point of the alloy.

After the melt is poured into the mold and cooled, the castings are knocked out of the mold and cleaned of the molding sand manually, on vibrating grates or shot blasting machines. The trimming of the elements of the gating system is carried out with disk cutters, band saws, on trimming presses, flame or plasma cutters. Cleaning of castings from burrs and bays is carried out with abrasive wheels.

Before being sent to mechanical workshops, steel castings are necessarily subjected to heat treatment - annealing or normalization - to relieve internal stresses and grind the metal grain. In some cases, castings from other alloys are subjected to heat treatment.

The advantage of casting in sand-clay molds is the low cost of molding materials and pattern equipment. Moreover, this method of casting is more labor-intensive in comparison with others. At the same time, casting in sand-clay molds provides low dimensional accuracy and high surface roughness.

B) Special casting methods

Special casting methods compared to casting in sand-clay molds make it possible to obtain castings of more accurate dimensions with good surface quality, which contributes to: reducing metal consumption and laboriousness of machining; improving the mechanical properties of castings and reducing losses from marriage; a significant reduction or elimination of the consumption of molding materials; reduction of production space; improving sanitary and hygienic conditions and increasing labor productivity.

These include casting: in permanent metal molds (chill mold); centrifugal; under pressure; into thin-walled one-time forms; investment models; cortical or shell; electroslag casting.

Casting in shell molds. With this casting method, special shell molds are used, made from quartz sand (92–95%) and thermosetting synthetic resin (5–8%). The sand-resin mixture is prepared by mixing sand and crushed powdered resin with the addition of a solvent (cold method) or at a temperature of 100 ... 120 C (hot method), due to which the resin envelops (clads) sand grains. Further, the mixture is additionally crushed to obtain individual grains clad with resin, and loaded into the bunker.

The manufacture of shell molds is carried out as follows (Fig. 2.). A metal model, heated to 200...300 C, is covered with a layer of heat-resistant lubricant (silicone liquid) and placed in a hopper, then covered with molding sand and held for 10...30 s. During this time, the shell is pre-sintered on the model. Then, excess loose molding sand is removed from the model and kept together with the shell in the oven for 1 ... 3 minutes. at a temperature of 300 ... 375 C. In this case, the final sintering of the shell with a thickness of 7 ... 15 mm occurs. After cooling, thanks to the separating layer of heat-resistant lubricant, the shell can be easily removed from the model. The individual parts of the mold and the gating system made in this way are assembled by gluing along the planes of the connectors and fastening them with brackets or clamps. The manufacture and assembly of shell molds is easily mechanized and automated.

In contrast to casting in sand-clay molds, casting in shell molds provides greater dimensional accuracy and less roughness. The machining allowance is 0.5…3 mm. At the same time, the limited mass of castings (up to 250...300 kg) and more complex technological equipment are disadvantages of this casting method. For this reason, casting into shell molds is used in serial and mass production of castings of small and medium sizes.

Investment casting. The casting process is as follows. In a mold, a casting model and elements of the gating system are cast from a low-melting mixture of stearin (50%) and paraffin (50%). The pressing temperature of the mixture is 42...45 C. The model and the gating system are assembled into a block, covered with a ceramic shell (2...8 mm thick). The ceramic coating consists of 60...70% powdered quartz or finely ground quartz sand and 30...40% binder (ethyl silicate solution). Next, a model is smelted from a ceramic mold with water, steam or hot air. The molds freed from the model are placed in flasks with sand, compacted and calcined at 900–950 C for 3–5 hours. In this case, the residues of the model composition burn out and the ceramic mold is annealed. After calcination, the finished molds are sent for pouring with metal.

Lost-wax casting provides greater accuracy in casting dimensions. This method can be used to obtain castings of the most complex shape with a wall thickness of up to 0.3 ... 0.8 mm with a minimum allowance for machining (up to 0.7 mm).

Disadvantages - the cost of castings obtained by investment patterns is higher than those made by other casting methods.

Casting in metal molds. Metal casting molds (chill molds) are made split and one-piece, mainly from steel and cast iron. To obtain complex cavities, metal and sand rods are used.

The mold casting process includes the following operations: mold cleaning, applying a refractory coating (from quartz, graphite, asbestos and liquid glass) to its inner surface, heating the mold to 150 ... 450 C, pouring molten metal. The application of a refractory coating provides an increase in the service life of the mold, prevention of welding of metal to the walls of the mold and facilitating the extraction of castings. Heating protects the mold from cracking and facilitates the filling of the mold with metal. After hardening, the casting is removed from the mold using a pusher.

The advantages of mold casting in comparison with casting in disposable sand-clay molds are: obtaining castings of more accurate sizes and shapes; fine-grained structure of the metal and, accordingly, with the best physical and mechanical properties; ensuring high labor productivity; lower cost of castings; improving the working conditions of the foundry worker.

The disadvantages of the method - the high cost of kikili; low gas permeability and ductility of the metal mold, leading to the formation of gas shells and cracks in castings; rapid cooling of the metal makes it difficult to obtain castings of complex shape, causes the danger of the appearance of hard-to-cut surfaces in cast iron castings.

Injection molding. The essence of the casting process is essentially that the molten metal fills the mold under the pressure of the piston (Fig. 3a). After the metal hardens, the mold opens and the casting is removed.

Before starting work, the mold is heated to 150 ... 400 C based on the poured alloy and lubricated with a lubricant based on mineral oils with graphite.

The productivity of piston machines reaches 500 castings per hour. In conditions of mass production, the use of injection molding makes it possible to reduce the complexity of obtaining castings by 10–12 times, and the labor intensity of machining by 5–8 times. Due to the high precision of manufacturing and the provision of increased mechanical properties of castings obtained under pressure, savings of up to 30 ... 50% of metal are achieved compared to casting in single molds. It creates the possibility of complete automation of the process.

Centrifugal casting method - a high-performance method for manufacturing hollow castings such as bodies of revolution (bushings, pipes, sleeves) from non-ferrous and iron-carbon alloys, as well as bimetals. The essence of the method consists in pouring liquid metal into a rotating metal or ceramic mold (mould). Liquid metal due to centrifugal forces is thrown to the mold walls, spreads along them and hardens. Non-metallic inclusions are collected on the inside of the casting and removed during further machining (Fig. 3b). After cooling, the finished casting is removed from the mold with the help of special tools.

Castings are obtained with an exact configuration, with low surface roughness and have a dense fine-grained metal structure.

As with die casting, metal molds are heated before pouring liquid metal and protective coatings are applied to them.

Centrifugal casting is highly productive (40…50 cast iron pipes with a diameter of 200…300 mm can be cast in 1 hour), makes it possible to obtain hollow castings without the use of cores and bimetallic castings by sequential pouring of two alloys (for example, steel and bronze), compared to casting in stationary sand-clay and metal molds provides a higher quality of castings, almost eliminates the consumption of metal for profits and uplift, increases the yield of suitable castings by 20 ... 60%.

The disadvantages of the method include the high cost of molds and equipment, the limited range of castings.

continuous casting - this is a method of obtaining broaching castings of a constant cross section by continuously supplying the melt into the mold and pulling the hardened part of the casting out of it. Given the dependence on the direction of stretching, a distinction is made between vertical and horizontal continuous casting. Vertical casting is commonly used to produce ingots and pipes.

The scheme of horizontal casting is shown in Fig.4. The mold 2, installed in the metal receiver 1, is made of copper, graphite and, less commonly, steel. It has an internal cavity, the profile of which corresponds to cross section castings. A water-cooling jacket 3 is installed at the outlet part of the mold. Ingot 6 is pulled out of the mold by pulling rollers 5 and divided into measured pieces using a saw 7 or plasma cutting. The central part of the ingot after its exit from the mold remains liquid, and therefore, in order to accelerate solidification and prevent the breakthrough of the melt through the hard metal shell, a shower device for cooling with water 4 is installed.

Continuous casting produces blanks of constant cross section in the form of a circle, strip or more complex profile. The disadvantage of this casting method is the limited range of castings associated with the impossibility of obtaining blanks with complex shapes.

Vacuum suction casting - this method produces castings such as bushings, rings, gear blanks, sleeves, etc. On the surface of the melt in the metal receiver 3, a flat ring of refractory material 2 is placed, a metal water-cooled mold, a mold 1, is lowered onto the ĸᴏᴛᴏᴩᴏᴇ from above. By removing the vacuum in the mold, it is possible to remove the melt from it and obtain hollow castings. Due to the directed crystallization from the surface to the center and the feeding of the hardening casting from the metal receiver, it is possible to obtain a dense casting without shrinkage defects and gas porosity. A feature of this process is a high yield of usable metal, since there is no extreme importance in the gating system and risers.

Casting defects- due to incorrect design of castings, violation of casting technology or errors in its development. The main defects include shells, cracks, surface defects and non-compliance of the configuration and dimensions with the requirements of the drawing.