Precision casting is a relatively delicate casting method, which uses carbon steel silica sol precision casting, also known as lost wax casting. Precision casting is a near-shape casting. The finished product is close to the actual size, and can be used directly with a small amount of machining or no machining. It is an advanced process of near-net shape and can save processing costs.
In the process of precision casting, there are also factors that affect the dimensional accuracy. Because the whole process of precision casting is relatively complicated, the unreasonable setting and operation of any link will change the shrinkage rate of the casting, resulting in the deviation of the dimensional accuracy of the casting from the requirements. The following are factors that can cause dimensional accuracy defects in precision castings:
- 1. The influence of the casting structure: the wall thickness of the casting has a large shrinkage rate, and the casting wall is thin and the shrinkage rate is small. The free shrinkage rate is large, and the hindered shrinkage rate is small.
- 2. The influence of casting material: the higher the carbon content in the material, the smaller the linear shrinkage rate, the lower the carbon content, the greater the linear shrinkage rate. The casting shrinkage rate of common materials is as follows: casting shrinkage rate K=(LM-LJ)/LJ×100%, LM is the cavity size, and LJ is the casting size. K is affected by the following factors: wax mold K1, casting structure K2, alloy type K3, pouring temperature K4.
- 3. The influence of mold making on the linear shrinkage rate of castings: a. The influence of wax injection temperature, wax injection pressure, and pressure holding time on the size of the investment mold is the most obvious with the wax injection temperature, followed by the wax injection pressure. After molding, the impact on the final size of the investment mold is minimal. b. The linear shrinkage of wax (mold) material is about 0.9-1.1%. c. When the investment mold is stored, it will further shrink, and the shrinkage value is about 10% of the total shrinkage, but after 12 hours of storage, the size of the investment mold is basically stable. d. The radial shrinkage rate of the wax mold is only 30-40% of the shrinkage rate in the length direction, and the influence of the wax injection temperature on the free shrinkage rate is much greater than that on the hindered shrinkage rate (the optimal wax injection temperature is 57-59 ℃, The higher the temperature, the greater the shrinkage).
- 4. Influence of shell-making materials: Zircon sand, zircon powder, Shangdian sand, and Shangdian powder are used because of their small expansion coefficient, which is only 4.6×10-6/℃, so they can be ignored.
- 5. The influence of shell roasting: due to the small expansion coefficient of the shell, when the shell temperature is 1150 ℃, it is only 0.053%, so it can be ignored.
- 6. The influence of casting temperature: the higher the casting temperature, the greater the shrinkage rate, the lower the casting temperature, the smaller the shrinkage rate, so the casting temperature should be appropriate.
Although precision castings are close-shaped castings, there will be certain uncertainties in each production link. These are factors that affect the dimensional accuracy of products. Therefore, if we want to obtain better products, we must strictly control the quality of each process. Quality, so as to cast high-quality products.