Precision casting process

The precision casting method refers to the general term of the casting method using non-metal molds, and the dimensional accuracy of the product is higher than that of ordinary sand mold castings, including dewaxing orThere are three main types of plaster mold method and ceramic mold method.
2 Precision casting by dewaxing
2-1 Features and Benefits
(1) The maximum limit length of the casting is 700mm, and the easy-to-make length is less than 200mm. The maximum weight of the casting is about 100 kg.
It is less than 10 kg.
(2) The dimensional tolerance of castings is 20mm ± 0.13mm, 100mm ± 0.30mm, 200mm ± 0.43mm, and the dimensional accuracy of small parts is not easy to reach
Within ± 0.10mm. The angular tolerance is ± 0.5 ~ ± 2.0 degrees, and the minimum thickness of the casting is 0.5 ~ 1.5mm. The surface roughness of the casting is about Rmax 4S ~ 12S.
(3) There are almost no restrictions on casting materials, such as aluminum alloy, magnesium alloy, titanium alloy, copper alloy, various steels, cobalt-based and nickel-based heat-resistant
Alloy, hard material.
(4) For the production of complex-shaped workpieces, the dimensional accuracy is good, and the cutting process is less.
(5) Save material waste and mass production.
Dewaxing precision castings are widely used in jet engines, gas turbines, steam turbines, aircraft parts, internal combustion engines, vehicles, food machinery, printing machinery, paper making machinery, compressors, valves, pumps, measuring instruments, sewing machines, weapons, Office machines, and other machine parts.
2-2 Process
There are two major types of dewaxing casting methods: solid mold method and pottery shell mold method, the latter is an improvement of the former. The solid mold method takes into account the amount of condensation shrinkage of the wax mold, the amount of heating expansion of the mold, and the amount of condensation shrinkage of the molten metal, and then produces a mold that is approximately the same size as the final casting. The melted wax is poured into a mold made of metal or silica gel, and after the wax pattern is taken out, it is dipped into a slurry made of a mixture of fine powder refractory and a binder. After dripping, spread coarse-grained refractory material and allow it to dry. Put it into the casting frame, fill the refractory pellets mixed with the binder, and then dry. Then, the wax melts and flows out to form a casting mold. The mold is heated at a high temperature to burn away a small amount of residual wax and increase its strength, and then inject a molten metal. The method of pottery shell moulding up to the production of wax mold is the same as that of solid moulding, except that after one time of dipping and sanding (or floating sand), the overmolding operation is not performed, but the dipping and sanding is repeated repeatedly. Until a predetermined shell thickness is obtained. The wax is dried, heated and melted out, and then heated at high temperature before pouring. This method is widely used by current dewaxing precision casting industry because it has the following advantages:
(1) Better dimensional stability;
(2) Less use of refractory materials;
(3) Light weight, easy to handle, handle and make large castings;
(4) The process can be partially automated to save manpower and increase production rate;
(5) Low production cost;
(6) The ceramic shell mold is thin, and the cooling rate of the casting is high and uniform after pouring, so its mechanical properties are better.
(A) The main model
The shape of the main model is similar to that of the final product. The wax must be condensed and shrunk during the production, and the heating expansion of the mold and the condensation and shrinkage of the cast metal must be considered. For example, when the cold shrinkage rate of the wax mold is 1.2%, the thermal expansion rate of the casting mold is 0.7%, and the cold shrinkage rate of the cast metal is 1.7%, the size increase rate of the main model is 2.2%. Copper alloy and stainless steel.
(B) Wax mold
The wax-forming molds for mass production of mechanical parts are generally made of aluminum alloy by mechanical processing. At this time, they are processed according to the blueprint provided by the customer, without using the main model. The aluminum alloy mold is composed of several pieces. Before the wax injection, it is manually assembled, locked, and after the wax injection is completed, it is manually disassembled and the wax pattern is taken out. The master model is extruded into a superplastic sheet metal to form it.
(C) Shot wax
For those waxes with a wide solidification temperature range, they are pushed into the mold of the wax injection machine in a semi-solid state. The injection pressure is usually 7 ~ 12kg / cm2, sometimes 50 kg / cm2. When making wax models, pay attention to The temperature of the metal mold, the pressing temperature of the wax, the pressing pressure, the mold release time, and the temperature of the wax mold to obtain dimensional stability. Therefore, the wax injection chamber must be kept at a constant temperature. Sometimes plastics are used instead of wax. Polystyrene is used for small castings with large production volumes that do not require dimensional accuracy. Urea resins are used for large parts with thick sections (85% of urea particles, 10% of polyvinyl alcohol powder, and 5% of water). ) But the shot pressure must be as high as 140 kg / cm2.
(D) Heart shape
Hollow products with simple shapes, products with slender holes, narrow grooves, or products with complex hollow shapes that cannot be coated by the coating paste must use a heart shape. Heart types include metal heart, soluble wax heart and ceramic heart.
The metal core method is to first fix the metal core in a metal mold, solidify it after injection of wax, and extract the metal core from the wax mold. This is limited to products with simple shapes in the hollow part. The shape of the hollow part is more complex or the inner hole is tortuous. The soluble wax can be used as a heart shape and fixed in a metal mold. After the wax is solidified, the wax shape is soaked in water to dissolve the heart shape. However, the heart shape made of soluble wax has poor strength and is easy to break. Therefore, for the slender hollow part, alumina powder, spinel powder, barium titanate powder and ethyl silicate, colloidal silica or Resin and other binders are mixed to make a pottery heart shape, which is poured in the finished product. After being suspected of solidification, the solvent is used to dissolve the pottery heart shape. The hollow part of the turbine blade is made using this method.
(E) Group tree
The wax pattern of the small workpiece must be welded and bonded to the pouring guide piece by piece in an artificial way to assemble it into a wax tree. This is the most time-consuming step in the dewaxing casting process. The flat-head type is used when the tree is assembled. An electric soldering iron is used to fuse the trunk or the base plate and the wax pattern, and then adhere them. The matters needing attention when assembling are as follows:
(1) Assemble multiple wax patterns as much as possible;
(2) When assembling, the wax pattern should easily flow out along the trunk or base when melting;
(3) The finished product part should be solidified first, and the pouring pond should be solidified last;
(4) After pouring and solidification, the workpiece is easy to be cut off from the trunk or base;
(F) Sludge and sand.
The group of trees first washed the release agent, grease, etc. on the wax surface with acetone or alcohol, then immersed it in the slurry of the adhesive and the fine powder refractory material, dripped the excess adherent slurry, and then sprinkled the refractory particles. Dry it. The main point of this operation is to uniformly coat the wax surface without causing bubbles to adhere and remain.
This process is repeated many times until the predetermined thickness is obtained. The number of operations and the thickness of the ceramic shell depend on the size and weight of the workpiece. The thickness must be able to support the weight and working pressure of the molten metal during pouring without cracking.
The main ingredients of the slurry are mostly silicic acid glue. The filler uses zirconium powder. Its weight ratio is about 1:35. The slurry must be added with an appropriate amount of wetting agent (to make the slurry easily adhere to the wax surface) and defoaming. After the agent is foamed, it must be stirred for twelve hours before it can be used. The management items of the slurry include viscosity, specific gravity and pH value.
The first layer of the slurry uses a high viscosity with a large amount of filler, and the second layer and subsequent slurry use a low viscosity and a better permeability. The vicinity of the slurry tank must be maintained at 22 ~ 25 ° C to prevent the slurry from aging quickly. . Before the slurry drops to dry, spread coarse refractory particles. The method of spreading generally uses a simple and effective clean sand bucket. A few use sanding machines. The inner part of the coating layer uses fine particles of 50 to 100 mesh. It is thickened to 20 to 40 meshes to the outside, and is dried in a conveyor belt, forced air drying room that controls temperature and humidity.
(G) Dewaxing and firing
The method used in industry is to put the coated wax group tree into a high-pressure container and lock it, then heat it at 150 ° C to make the wax melt and flow quickly. The purpose of firing is to make the surface of the residual mold. A small amount of wax is completely burned, removed, and the strength of the mold is increased. The firing temperature of the mold is between 700 ~ 1100 ° C, depending on the material of the casting metal. The time is 20 ~ 60 minutes. Take the mold out of the heating furnace and immediately The casting is performed so that the casting has a good metallographic structure.
3 plaster mold method
The chemical composition of gypsum is mainly calcium sulfate, which is divided into three types: anhydrous gypsum (CaSO4), hemihydrate gypsum (CaSO4, 1 / 2H2O, burned gypsum) and dihydrate gypsum (CaSO4, 2H2O). The die method is limited to the production of non-ferrous alloy castings and has the following characteristics:
(1) Suitable for precision casting for non-mass production, high mold cost;
(2) The surface of the casting is smooth, the dimensional accuracy is high, the shape is complicated, and the surface has fine unevenness (for example, 12 μm).
(3) Due to the small cooling rate, thin-walled castings can be cast, such as 0.6 ~ 1.5mm;
(4) The casting strain is small, and the microstructure and mechanical properties are uniform;
(5) The manufacturing cost is high, which is 5 to 10 times that of sand mold castings. It takes 2 to 3 days from mold drying to pouring;
(6) The thermal conductivity of the plaster mold is not good, the castings are prone to shrinkage defects, and the yield is low;
(7) The air permeability of the plaster mold is poor, and it needs to be pressurized or decompressed during pouring.
The gypsum for casting mold is made of gypsum as a binder and mixed with various additives to improve its fire resistance, reduce shrinkage stress during drying and expansion stress during heating and firing.
1 Job Engineering
(1) Model and release agent
The material of the model can be metal, rubber, wood, etc. When using the wooden model, it must be painted to prevent the phenomenon of stickiness. The release agents include stearic acid dissolved in kerosene, petroleum jelly, silicon grease, etc.
(2) Water mixing and stirring
Water is pure water or one with similar properties. The mixed amount of semi-aqueous gypsum powder is between 30 and 35%. Spiral blades are stirred in the container to form a creamy slurry, which is poured during the period when the slurry has sufficient fluidity. In the forming frame with the model, apply a little vibration or decompression with 10 ~ 10-1 torr to defoam to avoid air bubbles from sticking to the surface of the model. At this time, hemihydrate gypsum generates water and reaction, and the viscosity of the slurry It rises, loses fluidity, coagulates and hardens, and forms dihydrate gypsum. As the crystal grows, it generates heat and causes condensation to swell. It only takes tens of hours to complete the expansion.
(3) Drying
Dry plaster method is the most time-consuming and most important step in the whole project. The rapid heating will cause the mold to break. Therefore, several drying temperatures should be set, and the temperature should be gradually increased and heated. The final drying temperature is 200 ~ 230 ° C. The mold reaction , Anhydrous gypsum made by removing water. The drying device preferably uses a hot air forced circulation exhaust air type electric drying furnace with automatic temperature control. The temperature distribution in the furnace is uniform to shorten the drying time, that is, the final drying temperature is directly set, and the mold is rarely cracked.
2 Dewaxing casting method using plaster mold
Dental methods, rings, jewelry, decorations and small crafts are all made using this method. Gypsum is used as a binder and the main body is refractory such as silica.
The original model of the finished product made of metal is placed between two pieces of rubber, and heated and pressed on a hot press to permanently deform the rubber to form an empty cavity of the model. The two pieces of rubber are aligned and locked so that the injection port is close to the injection port of the wax injection machine, and the molten wax is filled into the rubber mold with a force of 0.2 to 0.5 kg / cm2.
After condensing, open the rubber mold, take out the wax pattern, use a low-power soldering iron to locally melt the wax pattern, weld it to the wax stem that acts as a flow channel, and perform the group tree operation. Put the group tree into the stainless steel ring and pour the plaster. , Then shake and vacuum defoam. After hardening the gypsum mold, place it at room temperature for more than 12 hours. Use an electric furnace (120 ~ 150 ℃) or pressure sodium steam to dewax. The dewaxed mold is slowly heated to 650 ~ 800 ℃ and fired. Quality completely burned off. When the residual waxy combustion is incomplete, it becomes a carbon residue which will cause defects on the casting surface. The firing temperature is maintained for a period of time to make the temperature inside and outside of the mold uniform, and slowly cold-chilled in the furnace to a temperature suitable for pouring, and then the plaster mold is taken out and placed in a vacuum casting machine or a centrifugal casting machine for pouring. Jewelry such as rings and brooches are very complex in shape, and the finished products with subtle bumps are made by centrifugal casting. The machine is equipped with a high-frequency heating device to melt the precious metal in the crucible, and then the molten metal is filled with the gypsum mold by centrifugal force (400 ~ 1000rpm). After condensing, the stainless steel ring is hammered, the gypsum mold is taken out, and the gypsum is scattered with a water gun to remove the casting. Deal with it. Such as Buddha statue vases or other larger crafts are made using a vacuum casting machine.
4 Pottery shell mold method
Pottery shell mold castings are mostly produced in small quantities, which is not easy to mechanize, automation, and high cost. Therefore, their scope of application is quite limited. The main objects are:
(1) Those originally welded can be cast in one piece
(2) Material with poor machinability can greatly reduce the amount of fine machining
(3) Those with complicated arts and crafts
Compared with dewaxing casting, it can be made into large castings or thick meat pieces or there are too few dewaxing casting methods in quantity. Applicable products are:
(1) Various metal molds: die-casting, plastic molding (injection molding, hot-press molding) for glass products, rubber products, forging;
(2) Blades: large steam turbines, various moving impellers;
(3) Parts: electronic wave guide tube, aircraft stabilizer, valve body, etc .;
The ceramic shell mold method uses a silicic acid colloid solution as a binding agent, and mixes a powder filler of a refractory material and a gel.
Add agent to make a slurry. The slurry with sufficient fluidity is poured on the surface of the model, the slurry is allowed to condense, and then the mold is demolded, dehydrated at 700 ° C, dried, and fired at 1000 ° C to improve the strength, and the ceramic shell mold is made. The thickness of the ceramic shell mold is 3 ~ 20mm. When casting large castings, the back of the mold must be cushioned with supporting material to support the weight of the molten metal.
When the amount of the model material is small, surface-treated materials are used. In mass production, metal or resin is used. The filler uses high-purity, high-melting refractory materials to prevent reaction with molten metal. The material depends on the type of casting metal. The particle size distribution of the refractory is a mixture of large particles and small particles, which can obtain moderate strength and surface degree. The purpose of the gelation accelerator is to accelerate the curing of the binder. Generally, alkaline substances such as ammonium carbonate are used. Adjusting the weight percentage of filler and binding agent can control the fluidity of the slurry, the strength and air permeability of the ceramic shell mold. The amount of filler is used to make the volume of foaming slurry 1.5 to 2.5 times the volume of cement.
Pottery molds can be used as dry molds, oil sand molds, carbon dioxide molds, or permanent metal molds. The ceramic shell mold must be preheated before casting. The iron and steel materials are cast at 500 ° C and the low melting point metal is 200 ° C. During or after casting, it is best to perform in an inert gas to prevent oxidation or decarburization. www.wxyszj.cn

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