Gravity Die Casting Process
Gravity Die Casting Introduction
Gravity Die Casting (GDC) is a classic metal forming process. Its core principle is to use gravity to inject molten metal into a pre-made metal mold (usually made of cast iron, steel, or copper alloy). After the molten metal cools and solidifies within the mold, the mold is opened and the casting is removed.
This process is widely used in the automotive, machinery, aerospace, and home appliance industries, and is particularly suitable for the mass production of medium to large-sized metal parts with complex shapes and high dimensional accuracy requirements.
Gravity Die Casting Process
Gravity Die Casting (GD/Mold Casting) is based on the principle of using gravity to guide molten metal into a metal mold, which then cools and solidifies to obtain high-precision castings. The entire process requires strict control over the mold condition, molten metal quality, pouring parameters, and post-processing techniques to ensure casting consistency and yield.
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Step 1: Mold Design and Manufacturing / Inspection
Core Operations:
- Design molds (including cavities, runners, risers, venting channels, and ejector mechanisms) specifically for the casting structure to ensure smooth molten metal flow, sufficient feeding, and thorough venting (avoiding porosity and shrinkage cavities);
- Use hot-work die steel (e.g., H13) as the mold material, heat-treated (quenching + tempering) to improve high-temperature resistance and wear resistance. Polish the cavity surface to Ra 3.2-6.3μm (to ensure a smooth casting surface);
- Pre-production mold inspection: Clean residual impurities from the cavity, check for cavity wear/deformation, verify the flexibility of the ejector mechanism, and confirm the unobstructed nature of the cooling channels (if a cooling system is included).
Quality Control:
Mold dimensional tolerances must be 1-2 grades higher than the casting requirements. Venting channel width is 0.2-0.5mm, depth is 0.1-0.3mm, to prevent molten metal overflow while ensuring effective venting.
Step 2: Raw Material Preparation and Metal Smelting
Core Operations:
- Select suitable alloy materials (e.g., aluminum alloy A356, zinc alloy ZAMAK 5, copper alloy brass H62). Raw materials must conform to industry standards (e.g., ASTM, GB/T), and remove surface oil, oxide scale, and other impurities.
- Prepare materials according to the formula ratio and heat them to a molten state in a furnace (e.g., resistance furnace, induction furnace): aluminum alloy 650-700℃, zinc alloy 380-420℃, copper alloy 900-1100℃.
- Metal refining treatment: Add refining agents (e.g., hexachloroethane for aluminum alloys) to remove gases and impurities. Let stand for 5-10 minutes to ensure uniform composition. Analyze the alloy composition using a spectrometer to ensure it meets standards.
Quality Control:
Gas content in the molten metal ≤ 0.15ml/100g (aluminum alloy), impurity content ≤ 0.5%, to avoid porosity and inclusion defects in the castings caused by gases or impurities.
Step 3: Mold Pretreatment
Core Operations:
- Mold Preheating: Use electric or gas heating to raise the mold temperature to 150-300℃ (aluminum alloy castings), 100-200℃ (zinc alloy castings). This prevents the low-temperature mold from contacting the high-temperature molten metal, which could lead to cold shuts or incomplete pouring.
- Applying Release Agent: Apply a thin layer of high-temperature resistant release agent (such as graphite-based or water glass-based release agent) evenly to the cavity, runner, and ejector mechanism surfaces. The thickness should be 0.05-0.1mm to ensure smooth demolding of the casting and protect the mold cavity.
Quality Control:
The mold temperature must be uniform (temperature difference ≤20℃). There should be no accumulation or missed application of the release agent to avoid affecting the surface quality of the casting and the demolding effect.
Step 4: Mold Closure and Positioning
Core Operation:
- Close the upper and lower molds or multi-segment molds using a mold closing device (manual, hydraulic, or mechanical transmission) to ensure accurate mold positioning (the clearance between the locating pin and the locating hole should be ≤0.03mm) to prevent misalignment and excessive flash on the casting.
- Check the mold closing pressure: Adjust the pressure according to the mold size (generally 0.5-2MPa) to ensure a tight mold closure and prevent molten metal overflow.
Quality Control:
After mold closing, the cavity should have good sealing with no obvious gaps, and the positioning deviation should be ≤0.05mm.
Step 5: Metal Pouring
Core Operations:
- Pouring Method: Manual pouring (small batches, simple castings) or automated mechanical pouring (large batches, high-precision castings, such as robotic arms + quantitative pouring machines);
- Pouring Parameter Control: Uniform metal flow rate (avoiding turbulence), pouring speed 0.5-2 L/s (adjusted according to casting size), pouring temperature 30-50℃ higher than the alloy melting point (ensuring fluidity);
- Pouring Angle: Smoothly pour along the runner, avoiding direct impact on the cavity wall (preventing air entrapment and cavity wear), ensuring the metal slowly fills the entire cavity.
Quality Control:
No splashing or interruption during pouring, cavity filling time controlled within 5-30 seconds (depending on casting complexity), avoiding excessively fast filling leading to porosity, and excessively slow filling leading to cold shuts.
Step 6: Cooling and Solidification
Core Operations:
- Natural cooling (simple castings) or forced cooling (complex, high-precision castings): Cooling water or compressed air is introduced through the mold’s built-in cooling channels to control the cooling rate.
- Cooling Time: Adjusted according to the casting thickness and alloy type (e.g., 10-20 seconds for 2-5mm thick aluminum alloy castings, 30-60 seconds for 10-20mm thick castings) to ensure complete solidification and a dense internal structure.
Quality Control:
Premature mold opening is prohibited during the cooling process (to avoid casting deformation and cracking). Demolding is only permitted after the casting has solidified and the temperature has dropped below 200℃ (aluminum alloys) or below 150℃ (zinc alloys).
Step 7: Mold Opening and Casting Removal
Core Operations:
- After the set cooling time is reached, the casting is ejected from the mold cavity using an ejection mechanism (hydraulic ejector pin, mechanical ejector rod). The ejection speed is uniform (to avoid impact damage to the casting).
- The casting is removed manually or by a robotic arm, while simultaneously cleaning burrs from the gating system, risers, and connection points with the casting (preliminary cleaning).
Quality Control:
During the removal process, collisions and scratches on the casting are avoided. Ejection marks are ≤0.1mm deep and do not affect the appearance or performance of the casting.
Step 8: Gating and Riser Removal & Flash
Core Operation:
- Remove gating, risers, and flash from the casting using mechanical cutting (e.g., abrasive wheel cutting, plasma cutting), punching, or grinding methods;
- Grind the cut edges after cutting to ensure a smooth surface free of sharp burrs (burr height ≤ 0.1mm).
Quality Control:
Avoid damaging the casting body during the removal process; cut smoothness ≤ 0.2mm/m.
Step 9: Casting Cleaning and Inspection
Core Operations:
- Surface Cleaning: Remove oxide scale, release agent residue, and impurities from the casting surface through shot blasting (0.2-0.5mm diameter steel shot), sandblasting, or chemical cleaning.
- Visual Inspection: Visually inspect the casting surface for defects such as porosity, cracks, cold shuts, inclusions, and shrinkage cavities (surface porosity diameter ≤ 0.5mm, and no more than 2 per square centimeter).
- Dimensional Inspection: Use calipers, micrometers, and coordinate measuring machines to inspect critical dimensions of the casting (tolerances must meet design requirements, such as ±0.1-0.3mm).
- Internal Quality Inspection (Critical Castings): Use X-ray flaw detection, ultrasonic flaw detection, or penetrant testing to check for internal defects such as shrinkage cavities and cracks (internal shrinkage cavity area ≤ 0.5%).
Quality Control:
Appearance defect rate ≤ 3%, dimensional pass rate ≥ 98%, internal defects in critical castings must conform to ASTM E446 or GB/T 6402 standards.
Step 10: Subsequent Processing (As Needed)
Core Operations:
- Machining: Turning, milling, drilling, and grinding are performed on key parts of the castings (such as mounting holes and mating surfaces) to improve dimensional accuracy and surface finish (e.g., mating surface roughness Ra 1.6-3.2μm);
- Heat Treatment: Aluminum alloy castings undergo aging treatment (150-200℃, holding for 2-4 hours) to improve strength and hardness; copper alloy castings undergo annealing treatment to eliminate internal stress;
- Surface Treatment: Painting, anodizing (aluminum alloy), electroplating (zinc alloy/copper alloy), and passivation are performed according to application requirements to improve corrosion resistance and appearance.
Quality Control:
After machining, dimensional tolerances are ≤±0.05mm (key parts); after heat treatment, hardness meets design requirements (e.g., aluminum alloy HB 80-100); surface treatment layer adhesion meets standards (no peeling during cross-cut adhesion testing).
Step 11: Finished Product Packaging and Warehousing
Core Operations:
- Classify and label qualified castings (indicating model, batch, and production date);
- Use moisture-proof and impact-resistant packaging (such as bubble wrap, pearl cotton + cardboard box) to avoid damage during transportation;
- Conduct a final random inspection before warehousing (sampling rate ≥ 5%), and register the castings into the warehouse after confirming there are no quality issues.
Suijin-Gießlösung
Gravity Die Casting is a high-precision, high-efficiency, and high-volume metal forming process. Its core advantages lie in stable casting quality, high production efficiency, and high material utilization, making it particularly suitable for the production of castings from alloys with good fluidity, such as aluminum and zinc alloys. Although the initial investment in molds is relatively high, it can significantly reduce the cost per piece in mass production scenarios, making it a mainstream casting process in industries such as automotive, machinery, and home appliances.