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Stainless Steel Precision Casting Processes

Stainless steel precision casting

Precision casting is one of the casting methods. Precision casting is a special casting method compared with the traditional casting process. It can obtain relatively accurate shape and high casting accuracy.


The more common method is: first, according to the product requirements, design and manufacture the mold (with very small or no allowance) and cast the wax by casting to obtain the original wax mold; Repeat the coating and sanding process on the wax mold, harden the shell and dry it; Then melt the wax mold inside to dewaxing and obtain the cavity; Baking shell to obtain sufficient strength and air permeability; Metal materials required for pouring; Remove the sand after shelling, so as to obtain high-precision finished products. Heat treatment and cold working shall be carried out according to the needs of the product.


What is stainless steel precision casting?

Precision casting or investment casting, silica sol process. It is a casting process with little or no cutting. It is an excellent process technology in the casting industry. It is widely used. It is not only applicable to the casting of various types and alloys, but also produces castings with higher dimensional accuracy and surface quality than other casting methods, and even complex, high temperature resistant and difficult to process castings that are difficult to be cast by other casting methods, It can be cast by investment casting.

precision cast parts

The practical application of modern investment casting method in industrial production was in the 1940s. At that time, the development of Aerojet engine required the manufacture of heat-resistant alloy parts with complex shape, accurate size and smooth surface, such as blades, impellers and nozzles. Because the heat-resistant alloy material is difficult to machine and the shape of parts is complex, it is impossible or difficult to manufacture by other methods.


Therefore, it is necessary to find a new precision forming process. Therefore, learn from the wax loss precision casting handed down in ancient times. Through the improvement of materials and processes, modern precision casting methods have made important development on the basis of ancient processes. Therefore, the development of aviation industry promotes the application of precision casting, and the continuous improvement and perfection of precision casting also creates favorable conditions for the aviation industry to further improve its performance.


In the manufacturing process of stainless steel precision casting, the sulfuric acid oxide film is an important factor affecting the high purity and transparency of the oxide film of stainless steel casting. The porosity, purity, thickness and sealing quality of stainless steel precision casting will directly interfere with the mirror effect of the casting.


In the production of stainless steel precision cast parts, when some products have strange shapes and cannot be produced by elbow and other processes, in order to produce such products with irregular shapes (hollow or solid inside), a process of wax mold → sand mold → filling and forming with liquid materials is adopted.


Stainless steel precision casting processes

1. Make molds according to products of different shapes. The molds are divided into upper and lower concave molds, which are completed through comprehensive processes such as turning, planing, milling, etching, electric spark and so on. The shape and size of the pit are consistent with the half of the product. Because wax molds are mainly used for industrial wax molding, aluminum alloy materials with low melting point, low hardness, low requirements, low price and light weight are selected as molds.


2. A large number of industrial wax solid core models are produced by using aluminum alloy molds. Under normal circumstances, an industrial wax solid core model can only correspond to one blank product.


3. The margin around the wax mold shall be refined. After deburring, multiple single wax molds shall be pasted on the pre prepared die head(also known as group tree). This die head is also an industrial wax solid core model produced by wax mold(looks like a tree).


4. Apply industrial glue to multiple wax molds fixed on the die head and spray the first layer of fine sand (a kind of fire-resistant sand and high temperature resistant, usually silicon sand). The sand particles are very small and fine, which can ensure that the final blank surface is as smooth as possible.


5. The wax mold sprayed with the first layer of fine sand is allowed to air dry naturally at the set room temperature (or constant temperature), but the shape change of the internal wax mold cannot be affected. The time of natural air drying depends on the internal complexity of the product itself. Generally, the first air drying time of precision castings is about 5-8 hours.


6. After the first sand spraying and natural air drying, continue to apply industrial glue (silicon solution slurry) on the surface of the wax mold and spray the second layer of sand. The particle size of the second layer of sand is larger and coarser than that of the previous first layer of sand. After spraying the second layer of sand, let the wax mold dry naturally under the set constant temperature.


7. After the second sand blasting and natural air drying, the third sand blasting, the fourth sand blasting, the fifth sand blasting and other processes shall be carried out by analogy.


Requirements: adjust the number of sand blasting according to the surface requirements, volume and self weight of the product. Generally, the number of sand blasting is 3-7 times- The sand blasting grit size of each time is different. Generally, the sand in the later process is thicker than that in the previous process, and the air drying time is also different. Generally, the production cycle of sand on a complete wax mold is about 3 ~ 4 days.


8. Before the baking process, the wax mold that has completed the sand blasting process shall be evenly coated with a layer of white industrial emulsion (silicon slurry) to bond and solidify the sand mold and seal the wax mold, so as to prepare for the subsequent baking process. At the same time, after the baking process, the brittleness of the sand mold can be improved, which is convenient to break the sand layer and take out the blank.


9. In the baking process, the wax mold fixed on the mold head and completed the sand blasting and air drying process is put into a metal closed special oven for heating (usually a steam furnace burning kerosene). Because the melting point of industrial wax is not high, the temperature is about 150 and the wax mold is heated and melted to form wax. The water flows out along the gate. This process is called dewaxing.


The wax mold after wax removal is just an empty sand shell. The key to precision casting is to use this empty sand shell. Generally, this kind of wax can be used repeatedly, but these waxes must be re filtered, otherwise the unclean wax will affect the surface quality of the blank, such as surface sand holes and pits, and also affect the shrinkage of precision casting products.

precision casting processes

10. Baking sand shell in order to make the dewaxed sand shell more firm and stable, the sand shell must be baked before pouring stainless steel water. It is usually baked in a furnace with high temperature (about 1000).


11. Pour the stainless steel water that has been dissolved into liquid at high temperature into the dewaxed sand shell, and the liquid stainless steel water fills the space of the wax mold until it is completely filled, including the mold head in the middle.


12. Since there will be mixed materials of different components in the boiler dissolved in stainless steel, the factory must detect the percentage of materials. Then adjust and interpret according to the required proportion, such as adding those elements to achieve the desired effect.


13. After the liquid stainless steel is cooled and solidified, the outermost sand shell is broken with the help of mechanical tools or manpower to expose the solid stainless steel product, which is the shape of the original wax mold, that is, the blank needed in the end. Then it will be cut and separated one by one, and then rough ground to become a single blank


14. Inspection blank: the blank with sand holes and pores on the surface must be repaired by argon arc welding. If it is serious, it shall be returned to the furnace after cleaning.


15. Cleaning blank: the qualified blank must go through the cleaning process.


16. Carry out other processes until the finished product.


Advantages of precision casting

The so-called precision casting process is simply to use fusible materials (such as wax or plastic) to make fusible models (hereinafter referred to as investment models or models), coat them with several layers of special fire-resistant coatings, form an integral mold shell after drying and hardening, melt the model from the mold shell with steam or hot water, and then place the mold shell in a sand box and fill dry sand around it for modeling, Finally, put the mold into the roasting furnace for high-temperature roasting (for example, when using high-strength mold shell, the mold shell after demoulding can be directly roasted without modeling). After roasting, the mold or mold shell is poured with molten metal to obtain the casting.


The biggest advantage of precision casting is that because investment castings have high dimensional accuracy and surface finish, it can reduce machining work, only in parts A little machining allowance can be reserved for the parts with high requirements, and even some castings only have grinding and polishing allowance, which can be used without machining. It can be seen that the use of precision casting method can greatly save machine tool equipment and processing hours, and greatly save metal raw materials.


The dimensional accuracy of investment casting is high, generally up to ct4-6 (CT10 ~ 13 for sand mold precision casting and CT5 ~ 7 for die casting). Of course, due to the complex process of precision casting, there are many factors affecting the dimensional accuracy of casting, such as shrinkage of mold, deformation of investment mold, linear change of mold shell during heating and cooling, shrinkage rate of alloy and deformation of casting during solidification, Therefore, although the dimensional accuracy of ordinary investment castings is high, its consistency still needs to be improved (the consistency of castings with medium and high temperature wax materials needs to be improved a lot).


When pressing the investment mold, the pressure mold with high cavity surface finish is adopted, so the surface finish of the investment mold is also relatively high. In addition, the mold shell is made by coating the refractory coating prepared by high temperature resistant special binder and refractory materials on the investment mold, and the surface finish of the mold cavity in direct contact with the molten metal is high. Therefore, the surface finish of investment castings is higher than that of general precision castings, -- generally up to RA 1.6~3.2um.


Another advantage of precision casting method is that it can precision cast complex castings of various alloys, especially high-temperature alloy castings. For example, the streamlined outline and cooling inner cavity of the blade of jet engine can hardly be formed by machining process. Using precision casting technology can not only achieve mass production and ensure the consistency of castings, but also avoid the stress concentration of residual knife marks after machining.


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