In investment casting, the shell serves as a mold for casting metal parts. The shell-making process typically includes:
1. Wax Model Cleaning
Purpose: To remove mold release agents, grease, and impurities from the surface of the wax model.
Method: Immersion in a neutral cleaning agent or ultrasonic cleaning to ensure coating adhesion.
2. Sand Coating Process (Layer-by-Layer Shell Making)
▶ First Layer: Surface Layer (Direct Coating)
Slurry Preparation:
• Refractory Material: Zircon sand (ZrSiO₄) or corundum sand (Al₂O₃), with a particle size of 200–325 mesh (fine sand) to ensure surface precision.
• Binder: Silica sol (SiO₂・nH₂O) or sodium silicate (Na₂O・mSiO₂). Silica sol is often used for high-precision castings (e.g., aerospace parts), while sodium silicate is less expensive.
• Slurry Viscosity: Controlled at 18–30 seconds (silica sol system) using a viscometer (e.g., Ford cup 4) to ensure uniform coating.
Operating Procedure:
i. Immerse the wax tree in the slurry, agitating it up and down for 10–15 seconds to ensure complete coverage of the wax model surface, avoiding air bubbles.
ii. Lift the wax tree vertically, draining excess slurry (draining time: 30–60 seconds), then immediately apply sand.
• Sand Material: Fine sand (200–325 mesh) of the same material as the slurry, evenly sprayed through a vibrating screen, ensuring sand particles are embedded in the slurry layer.
▶ Backing Layer (Final Backing Layer)
Function: Forms the main structure of the shell to withstand the pressure of the molten metal during casting.
Material Selection: Coarse-grained corundum sand (40–80 mesh) or quartz sand, slurry viscosity increased to 30–40 seconds (silica sol system).
Number of Layers: Determined according to casting size. Small castings (<50mm) require 3-5 backing layers, while large castings (>100mm) require 5-8 layers, with a total thickness of 5-15mm.
3. Dewaxing
Method: Steam dewaxing (pressure 0.3-0.6 MPa) or hot water dewaxing (90-98°C), dewaxing time 10-30 minutes.
Key point: Ensure rapid and thorough wax removal to avoid shell cracking.
4. Shell Sintering
Temperature: 800-1000°C (higher temperatures are required for silica sol shells), holding time 1-3 hours.
Function: Removes residual wax, improves shell strength, and reduces gas release.
