Investment casting, with its advantages of high precision and near-net shape, is widely used in the production of precision parts. The surface finish and smoothness directly determine the subsequent processing costs and performance of the casting. Precise control of key stages throughout the entire process is necessary, achieving improved surface quality through source optimization and strict process control. The core technical points are as follows:
Wax pattern preparation is the fundamental prerequisite. Prioritize the use of waxes with low shrinkage rates and high surface quality, such as modified paraffin-stearic acid composite wax or precision casting-specific microcrystalline wax. The shrinkage rate should be controlled within 0.8% to avoid surface defects caused by wax pattern shrinkage and deformation. During the molding process, injection molding parameters must be strictly controlled: injection temperature maintained at 55-65℃, pressure at 0.3-0.5 MPa, and holding time of 5-10 seconds. This ensures that the wax material fully fills the mold cavity, while also ensuring a mold cavity surface finish of Ra≤0.8μm, preventing mold defects from being transferred to the wax pattern. After molding, the wax pattern requires 12-24 hours of constant temperature aging treatment to eliminate internal stress and reduce subsequent deformation.
Shelling process is the core guarantee. The face coat slurry should use fine-grained refractory aggregate, such as 320-400 mesh zircon powder, combined with high-purity silica sol binder. The slurry viscosity should be controlled at 18-22s (Ford Cup No. 4) to ensure a uniform and dense coating. During dipping, a slow lifting method should be used, with a speed of 5-10 cm/s, to avoid bubbles or sagging. For sand application, 200-300 mesh zircon sand should be used, employing an electrostatic sand application process to ensure uniform coverage and firm adhesion of the sand particles. Face coat drying should be carried out in a constant temperature and humidity environment of 20-25℃ and 45%-55% relative humidity for at least 6 hours to ensure sufficient curing of the coating without cracks. For the backing layers, coarser-grained aggregates can be used to balance strength and permeability, preventing damage to the face coat under pressure.
Pouring and post-processing require precise control. The dewaxing process utilizes steam dewaxing at a temperature of 100-110℃ and a pressure of 0.6-0.8 MPa to prevent rapid dewaxing that could lead to shell surface spalling. The firing temperature is controlled at 900-1050℃, with a holding time of 1-2 hours, to thoroughly remove residual wax and organic matter and improve the cleanliness of the shell surface. During casting, the molten metal temperature must be precisely matched to avoid overheating and erosion of the shell surface. The pouring speed should be uniform and stable to prevent the molten metal from scouring the inner cavity of the shell. After the casting cools, mechanical vibration is used for sand removal to avoid damage to the surface from forceful sandblasting. If necessary, light shot blasting is performed to further improve surface finish.
Through precise parameter control and optimized material matching throughout the entire investment casting process, from mold making to shell making and casting, the surface roughness of the investment castings can consistently reach Ra1.6-Ra3.2μm, effectively reducing subsequent processing steps and ensuring product performance and market competitiveness.
