Feingussverfahren
Investment Casting Introduction
Investment casting(Lost-wax casting) is a metal forming process. A ceramic heat-resistant material is coated onto a wax model to form a ceramic mold. This ceramic mold is then dried, melted, and the wax model is removed to form a shell. Molten metal is then poured into the ceramic shell, and after the metal is formed, the shell is removed to obtain the casting. Some investment castings require further machining after casting. Investment casting is typically used to produce parts with complex shapes and high requirements for dimensional accuracy and surface quality.
Feingussverfahren
The core logic of investment casting is “using wax as a mold, the shell as a model, and casting gold using lost-wax casting.” The process demands both precision and a systematic approach, requiring strict control of process parameters at each step (such as temperature, humidity, and coating thickness) to ensure casting quality.
Suijin Machinery bietet Standard-Prozessabläufe für die industrielle Großserienfertigung und gleichzeitig optimierte Lösungen für die kundenspezifische Fertigung in Kleinserien.
Step 1: Preparatory Stage: Drawings and Process Design
1. Part Drawing Analysis and Optimization
Based on the 3D model (STEP/IGES format) or 2D drawings provided by the customer, analyze the part structure (wall thickness, internal cavity, fillets), dimensional tolerances (e.g., CT4-CT7 grade requirements), surface quality standards (Ra1.6-6.3μm), and material requirements (e.g., 316L stainless steel, TC4 titanium alloy);
Process Optimization:
- Add process thickness supplementation for thin-walled parts (≤1mm) to avoid incomplete pouring;
- Design “venting channels” for complex internal cavities to prevent gas retention in the mold shell, which can lead to porosity defects;
- Mark key dimensions (mating surfaces, sealing surfaces) for focused control in subsequent inspections.
2. Gating System and Shell Design
- Gating System: Design the gating gate, risers, and runner to ensure the molten metal smoothly fills the mold cavity while expelling gases and impurities (risers should be placed at the highest point or hot spot of the casting for feeding purposes);
- Shell Parameters: Based on the casting weight (≤50kg for standard parts) and the melting point of the material (high-temperature alloys require a thicker shell), determine the number of shell layers (3-7 layers), the thickness of each coating layer (0.5-1.5mm), and the type of refractory material (e.g., zircon sand for high-precision surfaces, corundum sand for high-strength requirements).
Step 2: Wax Model Forming and Assembly
1. Wax Preparation
- Common waxes: Paraffin wax-stearic acid alloy (ratio 7:3), medium-temperature wax (melting point 60-80℃), high-temperature wax (melting point 100-130℃, used for high-temperature alloy castings);
- Process Requirements: The wax must be heated to a molten state (temperature 10-20℃ higher than the melting point), stirred evenly, and air bubbles removed (vacuum degassing can be used).
2. Wax Model Injection
- Inject the molten wax into the metal mold at an injection pressure of 0.3-0.8 MPa, holding time 10-30 seconds (adjust according to the size of the wax model);
- Cooling Method: Natural cooling or water cooling. Ensure the wax model is completely solidified before opening the mold (to avoid deformation).
3. Wax Model Finishing and Inspection:
- Manually remove burrs and flash from the wax model surface, and repair minor defects (such as pinholes or missing material);
- Dimensional Inspection: Use a coordinate measuring machine to inspect key dimensions to ensure the wax model tolerances meet requirements (usually allowing 0.5-1.0mm of shrinkage allowance compared to the casting tolerance);
- Surface Quality: Visually inspect or use a magnifying glass to ensure there are no scratches, cracks, or bubbles (surface roughness).
4. Wax Model Assembly:
- Connect individual wax models to gates and risers via welding (wax welding) to form a “wax model assembly” (e.g., 5-20 wax models can be connected to one gate, adjusted according to the part size);
- Purpose: To improve production efficiency, reduce metal waste during casting, and ensure temperature uniformity across multiple castings.
Step 3: Shell Preparation (Key Core Step)
The shell is the “mold” for casting, requiring high precision, high strength, and breathability. It consists of 5 sub-steps:
1. Pre-coating (First Layer of Coating)
- Coating Formulation: Fine-particle refractory material (zircon powder, particle size 10-20μm) + binder (silica sol, SiO₂ content 30%-35%) + wetting agent (to prevent coating peeling);
- Operation: Immerse the wax mold in the coating, slowly rotate to ensure even surface coverage, then remove and drain excess coating;
- Core Function: Forms the final surface of the casting, directly determining its surface precision and roughness. The coating thickness (0.1-0.2mm) must be strictly controlled.
2. Sand Application (First Layer)
- Sand Material: Fine-grained zircon sand (40-70 mesh), consistent with the refractory material of the pre-coated paint;
- Operation: Place the coated wax template into the sand application equipment (rain-type or fluidized bed sand application) to ensure the sand particles adhere evenly to the paint surface;
- Purpose: To enhance the strength of the first coating layer and prevent cracking in subsequent coatings.
3. Drying (First Layer)
- Drying Conditions: Temperature 20-25℃, humidity 40%-60%, drying time 4-8 hours (natural drying) or 1-2 hours (forced ventilation drying);
- Quality Requirements: The coating should be completely dry, without drips or bubbles, and the sand particles should be firmly bonded to the paint.
4. Repeated Coating and Sanding (Subsequent Layers)
- Second to penultimate layer: Use coarse-grained refractory material (corundum sand, 20-40 mesh) + silica sol. After coating, sprinkle corundum sand of the corresponding particle size. Each layer should dry for 6-12 hours.
- Final layer (cover layer): Apply only the coating material, without sprinkling sand. This enhances the mold’s sealing properties and prevents molten metal from seeping in during pouring.
- Total number of layers: 3-7 layers (3-4 layers for small parts, 5-7 layers for large parts). Total mold thickness: 3-10mm (adjusted according to casting weight and material).
5. Final Drying and Pre-Firing Inspection of the Mold Shell
- Final Drying: Temperature 25-30℃, humidity 30%-50%, drying time 12-24 hours, ensuring the moisture content of the mold shell is ≤0.5% (excessive moisture will cause the mold shell to crack during casting);
- Inspection: Visually inspect the surface of the mold shell for cracks, missing sand, and peeling. Perform non-destructive testing (such as penetrant testing) if necessary.
Step 4: Dewaxing (Loss-Wax Process)
1. Dewaxing Method Selection
- Steam Dewaxing (Common): Place the mold shell in a high-pressure steam autoclave, introduce saturated steam at 0.6-1.0 MPa, maintain a temperature of 160-180℃, and wait 10-30 minutes. The wax pattern melts and flows out from the gate, which can be collected and reused (recovery rate ≥80%).
- High-Temperature Dewaxing (Suitable for high-temperature wax or complex mold shells): Place the mold shell in a roasting furnace, slowly raise the temperature to 400-600℃, and hold for 2-4 hours. The wax pattern completely burns and decomposes, preventing residual wax from affecting the casting quality.
2. Post-Dewaxing Inspection
- No wax residue in the mold shell cavity (can be checked with an endoscope), and the mold shell has no cracks or deformation.
- If wax residue is found, the dewaxing time needs to be extended or the temperature increased; otherwise, wax carbonization during pouring will cause porosity and inclusion defects in the casting.
Step 5: Shell Firing
1. Firing Temperature:
- 900-1100℃ (Adjust according to casting material: Stainless steel, carbon steel 900-950℃, high-temperature alloys, titanium alloys 1000-1100℃);
- Firing Time: 2-4 hours, heating rate 5-10℃/minute (to avoid shell cracking due to thermal shock);
2. Core Functions:
- Removing residual wax, moisture, and organic matter from the shell;
- Improving shell strength (shell strength at room temperature ≥20MPa after firing) and permeability;
- Forming a dense structure on the shell surface, reducing chemical reactions between the molten metal and the shell.
Step 6: Metal Melting and Casting
1. Metal Material Preparation
- Select pure metals or alloys (e.g., 304 stainless steel, Inconel 718 high-temperature alloy) according to casting requirements, ensuring the material composition meets standards (e.g., ASTM, EN standards);
- Material Pretreatment: Remove surface oil and oxide scale to prevent impurities from entering the molten metal.
2. Melting
- Melting Equipment: Medium-frequency induction furnace (commonly used, suitable for carbon steel, stainless steel, aluminum alloys), vacuum induction furnace (used for easily oxidized materials such as titanium alloys and high-temperature alloys);
- Melting Temperature: 50-150℃ higher than the metal’s melting point (e.g., 304 stainless steel melting point 1450℃, melting temperature 1500-1550℃);
- Refining Treatment: Add deoxidizers (e.g., silicon-calcium alloy, aluminum wire) to remove gases and impurities from the molten metal, ensuring the purity of the molten metal.
Step 8: Post-processing and Quality Inspection
1. Removal of the Gating System
- Use an abrasive wheel cutter, plasma cutter, or wire EDM (for high precision requirements) to remove the gates and risers;
- Grind the cut surfaces smooth to ensure a smooth transition with the casting surface.
2. Surface Cleaning
- Remove burrs and flash, and grind away residual sand and oxide scale from the casting surface;
- Perform shot blasting (using steel shot or glass shot) if necessary to improve surface finish (Ra can be reduced to below 1.6μm).
3. Heat Treatment (as needed)
- According to the casting performance requirements, perform annealing, normalizing, quenching + tempering, solution treatment, etc.;
- Example: Solution treatment of 304 stainless steel castings (holding at 1050-1100℃ for 1-2 hours, water cooling) to improve corrosion resistance; quenching (850-900℃) + tempering (200-300℃) of tool steel castings to improve hardness and wear resistance.
4. Quality Inspection & Packaging and Shipment
Suijin Casting Principle
Investment Casting Process Flow Summary:
The core principles of investment casting revolve around “precision forming, controllable quality, and a balance between efficiency and cost.” These principles form the underlying logic that guides process design, parameter settings, and quality control, and directly determine the precision, performance, yield rate, and economic efficiency of the castings.