Sand Casting Process
Sand Casting Introduction
Sand casting is one of the most widely used casting processes, accounting for over 70% of global casting production. Its core principle involves creating a mold using molding sand (a mixture of quartz sand, bentonite, etc.), pouring molten metal into the mold cavity, and then breaking the mold to remove the casting after the metal cools and solidifies.
This process is flexible, cost-effective, and can produce various metal parts ranging from a few grams to hundreds of tons, finding wide application in machinery manufacturing, automotive, construction machinery, agricultural machinery, and hardware industries.
Sand Casting Process
The sand casting process can be summarized into seven core steps: “Mold preparation → Molding sand preparation → Molding and core making → Mold assembly → Pouring → Cooling and sand removal → Cleaning and post-processing.” Each step is interconnected and directly affects the quality of the casting.
Step 1: Mold Preparation
To create the mold used to form the sand mold cavity and sand core, serving as the “replica base” for the casting’s external shape/internal cavity.
“Pattern”: Corresponds to the external shape of the casting, used to press the sand mold cavity (for example, to cast an engine block, the pattern is an “enlarged version” of the block);
“Core Box”: Corresponds to the internal cavity and channels of the casting (such as the oil passages and air passages of the block), used to create the “sand core” (after the sand core is placed in the mold cavity, molten metal fills the gap between the sand core and the mold cavity, forming the internal cavity after cooling).
The pattern and core box dimensions must allow for shrinkage (metal shrinks during cooling; allow 1%-2% for cast iron, 2%-3% for cast steel, and 3%-5% for aluminum alloy) to avoid castings being too small.
Step 2: Molding Sand Preparation
To produce molding sand with “forming + support + venting” functions, forming the “skeleton” of the sand mold.
Aggregate (85%-95%): Quartz sand (widely available, high-temperature resistant, the core raw material);
Binder (3%-10%): Bentonite + water (traditional wet sand), resin (resin sand), water glass (water glass sand), used to bind the quartz sand into shape;
Additives (1%-3%): Coal powder (to prevent sand from sticking), sawdust (to improve permeability), disintegrating agent (easily breaks down after cooling, facilitating cleaning).
Step 3: Molding + Core Making
To create the “cavity” (the outer shape of the casting) and the “sand core” (the inner cavity of the casting) using molding sand and molds is the core step in sand casting.
Fix the pattern in the sand box, fill and compact the molding sand, and then smoothly remove the pattern. A “cavity” consistent with the outer shape of the casting is formed inside the sand box.
Fill the core box with molding sand (often with added resin for strength), compact it, and then remove it to create a “sand core” (such as the oil passage core for a cylinder).
The sand core needs to be reinforced with “core bones” (iron wire/steel bars) to increase its strength. A “coating” should be applied to the surface (to prevent sand adhesion and improve the smoothness of the inner cavity). A “core head” should also be provided (to fix the sand core in the mold cavity and prevent displacement during pouring).
Step 4: Mold Assembly
To assemble the upper and lower sand boxes and the sand core into place, ensuring a sealed mold cavity and preventing molten metal leakage or displacement during pouring.
1. Place the sand core into the positioning slot of the lower mold cavity (core head fixed), ensuring accurate sand core positioning (determining the inner cavity size);
2. Align the upper and lower molds (using positioning pins) and clamp them securely (to prevent molten metal from blowing open the sand box during pouring);
3. Arrange the “gating and riser system”:
4. Gating: Includes the pouring cup (receiving molten metal), sprue, runner, and ingate (guiding the molten metal smoothly into the mold cavity);
5. Riser: Placed above thicker parts of the casting (to store excess molten metal) to compensate for shrinkage during solidification (preventing shrinkage cavities and porosity);
6. Check the seal: Seal any gaps in the sand box to prevent leakage or sand ingress during pouring.
Step 5: Pouring
To inject molten metal into the mold cavity to fill and form the shape.
Metal Melting:Raw materials such as cast iron, cast steel, and aluminum alloys are added to the furnace (cupball furnace, electric arc furnace, or induction furnace) and heated to a molten state (cast iron 1300-1400℃, cast steel 1500-1600℃, aluminum alloy 650-700℃).
Temperature:Requires stability (too high a temperature can cause shrinkage cavities, too low a temperature can lead to incomplete pouring).
Speed:Maintain a steady speed (too fast a speed can cause sand erosion and porosity, too slow a speed can lead to cold shuts).
Method:Slowly pour the molten metal through the pouring cup, ensuring that the molten metal fills every corner of the mold cavity.
Step 6: Cooling + Sand Removal
To allow the molten metal to cool and solidify in the mold cavity, then break the sand mold to remove the casting.
Cooling: After the molten metal is poured in, it needs to cool naturally in the sand mold (small parts: minutes to hours; large parts such as machine tool beds: several days of heat preservation and cooling) to avoid rapid cooling that could cause cracking.
Sand Removal: After cooling to room temperature, the sand box is broken and the molding sand is removed by manual tapping or a vibrating sand removal machine. The casting is then taken out (at this point, the casting surface is covered with adhering sand, and also has gating gates, risers, and flash).
Step 7: Cleaning + Post-processing
To remove impurities and inspect quality to ensure the casting meets usage requirements.
Cleaning Impurities:
- Removing Adhering Sand: Sandblasting, shot blasting (high-pressure sand/steel shot impact on the surface);
- Removing Excess Parts: Grinding (removing risers, flash, burrs);
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Removing Oxide Scale: Acid pickling, alkaline washing (for cast steel and aluminum alloys);
Quality Inspection:
- Appearance Inspection: Check for sand holes, porosity, and cracks;
- Dimensional Inspection: Verify dimensional accuracy using calipers and a coordinate measuring machine (sand casting accuracy is typically CT10-CT13);
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Internal Inspection: X-ray flaw detection, ultrasonic flaw detection (to check for internal shrinkage cavities and sand inclusions);
Subsequent Processing (as needed):
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Machining: Turning, milling, and grinding of surfaces requiring high precision (such as bearing holes);
- Heat Treatment: Annealing (to relieve internal stress), quenching (to increase hardness), etc., to improve mechanical properties.
Suijin Casting Principle
Sand Casting Process Flow Summary:
Mold Preparation → Molding Sand Preparation → Molding (Making the Cavity) + Core Making (Making the Inner Cavity) → Mold Assembly (Installing the Sand Core + Sealing the Sand Box) → Pouring (Pouring in Molten Metal) → Cooling → Shaking Off (Removing the Casting) → Cleaning (Removing Impurities) → Inspection + Post-Processing → Finished Casting
The core principles of each step are: “Accurate cavity forming, stable molten metal filling, uniform cooling and shrinkage, and rigorous cleaning and inspection,” to produce qualified castings.