All the valves we produce are made through precision casting with silica sol. This issue will provide a detailed explanation of the process flow of silica sol casting.
Silica sol precision casting (also known as investment casting or lost-wax casting) is a high-precision casting process with excellent surface finish, widely used in aerospace, medical, automotive, and other industries. Below is a detailed breakdown of the process flow and key control points:
1. Mold Design & Manufacturing
– 3D Modeling: Create a 3D model based on part drawings, accounting for shrinkage (typically 0.5%~2.0%).
– Master Pattern Fabrication: Produce a master pattern (prototype) from metal or resin to make the die (mold).
2. Wax Pattern Production
– Wax Injection: Molten wax (e.g., paraffin, beeswax, synthetic wax) is injected into the die under pressure (typically 0.3~1.5 MPa).
– Wax Assembly: Multiple wax patterns are welded onto a wax gating system (sprue, runners, etc.) to form a “wax tree.”
– Key Controls:
– Wax temperature: 60~70°C (avoid overheating to prevent deformation).
– Dimensional accuracy: Must account for subsequent shrinkage.
3. Silica Sol Shell Building
Steps:
1. Silica Sol Coating: The wax tree is dipped into a silica sol (aqueous suspension of SiO₂ nanoparticles, 10~20 nm in size).
2. Stuccoing: Immediately after coating, refractory grains (e.g., zircon sand, mullite, 50~100 mesh) are applied.
3. Drying: Cured in a controlled environment (20~25°C, 40~60% humidity) for 4~8 hours.
4. Layer Repetition: 6~9 layers are applied, with coarser sand (e.g., aluminosilicate) used in outer layers.
Key Parameters:
– Silica sol viscosity: 25~30 sec (Ford Cup #4).
– Each layer must dry thoroughly to prevent cracking.
4. Dewaxing
Steam Dewaxing, The shell is placed in a high-pressure autoclave (150~160°C, 0.6~0.8 MPa) to melt and remove the wax.
– Wax Recovery: Filtered and reused, achieving 80%~90% recovery.
5. Shell Firing
– High-Temperature Sintering: Fired at 900~1100°C for 2~4 hours to eliminate residual wax and strengthen the shell.
– Key Controls:
– Heating rate: ≤100°C/hour (to prevent cracking).
– Final shell porosity: ~30%~40%.
6. Melting & Pouring
– Alloy Melting: Performed under vacuum or protective atmosphere (for materials like stainless steel or superalloys).
– Pouring Methods:
– Gravity casting: For simple parts.
– Centrifugal casting: For thin-walled complex parts (200~400 rpm).
– Temperature control: 50~100°C above alloy liquidus (e.g., ~1500°C for 316 stainless steel).
7. Post-Processing
– Shell Removal: Vibration or high-pressure water jetting.
– Cutting: Gating system removal (via grinding or wire EDM).
– Heat Treatment: Solution treatment (e.g., 1050°C water quenching for stainless steel) or aging.
– Surface Finishing: Sandblasting, acid pickling, or electropolishing.
8. Quality Inspection
– Dimensional Check: CMM or optical scanning.
-Non-Destructive Testing (NDT): X-ray (porosity, inclusions), fluorescent penetrant inspection (surface cracks).
– Mechanical Testing: Tensile, hardness tests.
Key Process Challenges
1. Shell Cracking: Insufficient drying or excessive heating rate.
2. Surface Defects: Impurities in silica sol or uneven stuccoing.
3. Dimensional Deviation: Incorrect wax shrinkage compensation.
Application Examples
– Aerospace Turbine Blades: Wall thickness ≤1 mm, surface roughness Ra 1.6 μm.
– Medical Implants: Titanium casting, compliant with ASTM F67.
By strictly controlling process parameters, silica sol casting achieves dimensional accuracy of ±0.1 mm/m, making it ideal for high-complexity, low-volume precision components.
