Casting by smelted models
By casting according to smelted models (precision casting), castings weighing from several grams to tens of kilograms with a wall thickness of up to 0.5 mm are made. This method achieves the purity of the surface of grades 5-6 and accuracy for most parts of grades 5-7, and in some cases – Grade 3.
The essence of the casting process for the smelted models is as follows. Models of parts and a gate system are made from low-melting model compositions in special molds. They are connected and a “block of models” is obtained, on which a suspension-liquid facing coating consisting of a refractory base and a binder solution is applied in several layers. Each layer of the suspension is sprinkled with dry sand and dried, as a result of which a mold shell is formed on the model. This is followed by melting models, calcining the mold and pouring it with metal.
1. The dimensions and cleanliness of the surface of the mold cavity must ensure that castings are obtained with a given accuracy and surface cleanliness. At the same time, the dimensional accuracy and surface cleanliness of the working cavity of the mold should be 1-2 classes higher than the required accuracy and cleanliness of the casting surface.
2. Molds should have a minimum number of connectors, and convenient, fast and reliable extraction of models should be provided. Removing the model from the mold without damage and distortion is achieved by choosing the right connector, performing slopes and using ejector devices. The magnitude of the slopes is assumed to be at least 0.5 °. Ejectors are made local or solid. Local ejectors can leave traces on models, so they should be placed on treated or non-responsible surfaces, solid ejectors do not leave traces on the surface of the model.
To remove air from the mold cavity when filling it with a model composition, thin risks (0.1 mm) are made on the connectors, and thin ventilation holes should be provided in the blind recesses where air bags are formed.
4. The dimensions of the channels for the supply of the model composition should ensure that the model is obtained without bubbles, shrinkage and other defects. The place of supply should not spoil the surface of the model and should compensate for volumetric shrinkage. If the feeders of the casting cannot be used as the spigots of the model, it is necessary to bring the model mass into the thickest part of the model and to the treated surface of this casting.
5. The mold should be convenient to use. For ease of assembly and disassembly, handles, tides or finger depressions are provided on the mold, as well as clamping devices for fast and reliable locking of the mold.
6. The parts and assemblies of the mold must be strong, rigid and wear-resistant.
Steel molds can withstand up to 100 thousand model removals and more. Cast zinc-aluminum-copper alloys approach steel molds in terms of wear resistance. Molds made of other fusible alloys allow about 1 thousand removals. To increase the service life of the mold is often reinforced.
7. The design of the mold must be technologically advanced in manufacturing. Complex parts of the mold, manufactured by mechanical processing, it is advisable to divide into simpler elements.
The production of gypsum and cast metal molds is the same and is carried out according to the master model in metal clips.
Combined molds are also often used. Some of their parts are manufactured by machining, and the most complex parts are cast according to the master model by electroplating and other methods.
A properly constructed gating system should provide:
1) good form filling;
2) production of castings without shrinkage shells, looseness and porosity, without foreign inclusions and warping;
3) high mechanical properties of metal castings.
The model of the gate system is a load-bearing structure and must be durable. Usually the metal is supplied to the thick parts of the castings. All massive casting units are powered only from the riser or collector through feeders, or some casting units are powered from specially installed profits. This construction of the gate system ensures reliable power supply and directional solidification of castings. The dimensions of the gate system elements are determined by the factors characterizing the cooling rate of the castings. The criterion for the cooling rate of the casting is its reduced thickness, i.e. the ratio of the cross-sectional area of the casting body to its perimeter. The reduced thickness characterizes the cooling rate of the casting only at low heat exchange intensity.The dimensions of risers and feeders are determined depending on the thickness of the casting array, the weight of the casting and the length of the feeder.
1) the melting point should be within 60-100 °, and the temperature of the beginning of softening should be higher than the temperature of the working room.
2) shrinkage during cooling and expansion during heating should be minimal and stable;
3) the specific gravity should be low (preferably less than 1;
4) the composition must have good fluidity;
5) the solidification time of the composition in the mold should be minimal;
6) the composition must accurately reproduce the configuration of the working cavity of the mold;
7) the composition should not stick to the surface of the mold and interact with the mold material;
8) the components of the composition should not dissolve in the binders of the lining;
9) in the solid state, the composition must have sufficient hardness and strength;
10) the composition must have good solderability,
11) the composition must be suitable for repeated use;
12) the composition should be well wetted with a lining composition;
13) the ash content of the composition should be minimal;
14) the composition must be harmless to the health of workers;
15) the components of the composition should be cheap;
16) the preparation of formulations should be simple.
a) depending on the main components and their ratio,
b) depending on the melting and softening temperatures, strength, etc.;
c) according to the condition when introduced into the mold (liquid compositions, pasty, heated to softening);
d) according to the method of removal from molds (melting, dissolution, burning).
Recently, new model compositions of paraffin-polyethylene, ceresin-polyethylene and paraffin-ceresin-polyethylene have been proposed.
Low-melting model compositions (paraffin, stearin, ceresin, etc.) are prepared in water, glycerin or oil baths with electric or gas heating; thermostats are also used.
Pasty model compositions are prepared manually at a small scale of production, at a larger scale — on special installations.
Model compositions with a high melting point (rosin, polystyrene, etc.) are manufactured in special rotary electric furnaces equipped with thermoregulators.
The methods of making models are diverse. The model composition is introduced into the mold cavity in the following ways: by free pouring, pressing in a paste-like state; by pouring under pressure; by pressing under high pressure heated to a state of softening powder or grit model compositions such as plastics.
The melting temperature before pouring must be constantly monitored (depending on the composition of the mass, the pouring temperature should be within 60-120 °). The use of hollow models makes it possible to increase the accuracy of castings, reduce the production time of models, reduce the consumption of the model composition and reduce the loss of the latter, reduce the weight of the model block, simplify and shorten the process of melting models. The method of introducing the model composition into the mold cavity in a paste-like state under pressure has found the most widespread in production practice.
The pressing of the paste—like model composition is carried out using a variety of devices – from the simplest manual syringe to complex automatic machines. The model composition is pressed from the reservoir cylinder into the mold cavity under piston pressure or compressed air pressing directly on the molten model composition.
Models made of thermoplastic materials such as polystyrene are manufactured on special presses used for stamping various plastic products. The principle of operation of the machines is that the heated plastic material loaded into the machine, usually in powdered form, is softened in the heating cylinder and pressed into the cavity of the mold through a special gating stroke under high piston pressure. To obtain a dense product, the pressure increases after filling the mold cavity. At the end of the curing time (a few seconds), the halves of the mold are pushed apart, and the finished product is pushed into the tray.
a) models of parts are soldered to the model of the gating system using a heated knife blade, an electric soldering iron or a molten model composition;
b) the models are connected in the conductor with the elements of the gate system mechanically or by gluing.
Assembly conductors are used for models from poorly mating model compositions. The mechanical connection of the models of parts and the gate system is used in cases where the models are made of a material that cannot be soldered (for example, polystyrene). For this purpose, a special metal riser with a petal clip is used. Assembling by gluing models is rarely used.
Foundry molds.
The process of manufacturing a mold consists of preparing materials, forming a shell on the surface of the model, removing the model from the shell, molding the shell into a filler and calcining the mold.
binding materials — ethyl silicate, liquid glass, alumina cement,
base materials — pulverized quartz, quartz sand, fused quartz, ground chamotte, pulverized talc, magnesite, zircon:
solvents and other materials—ethyl alcohol, acetone, ether-aldehyde fraction, hydrosite, hydrochloric acid, distilled water.
Binders are a solution of ethyl silicate in organic solvents (alcohol, acetone) and aqueous solutions of liquid glass. The first is prepared by hydrolysis of ethyl silicate. The essence of ethyl silicate hydrolysis consists in the transfer of ethyl silicate esters into unstable silicic acids, which pass into a colloidal state. During hydrolysis, a gel of silicic acid of the specified composition and properties should be obtained.
Ethyl silicate and water do not dissolve in each other when mixed, but ethyl silicate and water dissolve well in alcohol, acetone, ether-aldehyde fraction and other liquids. Therefore, the hydrolysis of ethyl silicate is carried out in a pre-prepared water-alcohol or water-acetone solution. To accelerate the reaction, a catalyst is used – hydrochloric acid.
Depending on the quantitative ratio of the materials taken for hydrolysis, as well as their composition, colloids of different composition and properties can be obtained.
Liquid glass is used for the second and third layers of cladding, liquid glass dissolves to a given specific gravity, after which it enters the preparation of the suspension.
The preparation of the suspension consists in mixing the binder solution with a pulverized material (pre-washed, dried, calcined and sieved) until a homogeneous mass is obtained.
The formation of shells on the surfaces of the models consists in applying a suspension, sprinkling with dry sand and hardening of the shell layer, when forming the shell on liquid glass according to the method of P. S. Pershin, hardening occurs when the shell is wetted after sprinkling with sand with a 10% aqueous solution of ammonium chloride or nitric acid, chemical hardening occurs.
The application of the suspension on the surface of the models is carried out by the method of immersion of the model in the suspension. Other methods of application (spraying, dousing) are not widespread.
In the beginning, sand sprinkling was done manually in a continuously falling stream of sand. Currently, it is completely mechanized. The method of removing the model material depends on its properties and whether it is removed directly from the shell, before molding or after molding.
The melting of the model material can be carried out in cabinets with hot air, superheated water vapor; in baths with hot water, infrared rays; high frequency currents.
The most widespread methods of melting models in liquid and gaseous media. The burning and dissolution of models have not been widely used.
The shells are formed in flasks with a bottom for dry fillers or without a bottom for wet fillers, flasks are usually made welded from sheet material 4-5 mm thick, mainly cylindrical in shape, and also by casting into the ground.
Molding sand is used as fillers.
In the case when the models from the shells are melted after molding, the flask is turned over with a gating bowl down, it is necessary to keep the dry filler in the flask For this purpose, an end filler is prepared, for example, from sand with 4-6% liquid glass.The molding process is reduced to filling the filler into the flask around the shell manually or from a hopper with a small seal.
Forming with a wet filler consists in pouring the filler into the gap between the shell and the flask. The sealing is carried out on a vibrating machine. The hardening process takes place in the first hour after molding and lasts up to 2 days.
At the end of the molding and removal of the mold model, it is calcined, thereby achieving the removal of gaseous components from the shell, as well as better filling of the heated mold with liquid metal.
Calcination is carried out at a temperature of 850-900 ° in furnaces of various designs (electric, gas, etc.).
Metal melting and mold casting.
The same requirements are imposed on the quality of the metal of castings obtained by casting according to smelted models as for castings obtained by other methods. Therefore, metal from any melting unit can also be used for injection molding.
– free fill;
– filling with the application of air pressure or neutral gas to the surface of the liquid metal;
– vacuum filling with the creation of a vacuum in the form;
– centrifugal filling;
– combined centrifugal-vacuum filling.
In the case when the removal of the molds is carried out after pouring them, the risers with castings released from the filling mixture and knocked out of the flasks are processed on a precision machine where the remnants of ceramics and the stuck filling mixture are removed from the castings.
The castings are removed from the risers with a metal-cutting tool (circle), the remains of the feeder are milled for the face (the residue on the castings can vary up to 5 mm or more, as a rule is no more than 2 mm, depending on the technical process adopted at the enterprises).
The final processing of castings is carried out in a sandblasting or shot blasting chamber, a cooking drum, etc. Depending on the alloy grade of the castings, the casting is heat treated.