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Manufacturing Processes > Casting Processes > Sand casting

 

Sand casting

 

Process description

Moist bonding sand is packed around a pattern. The pattern is removed to create the mold, and molten metal poured into the cavity. Risers supply necessary molten material during solidification. The mold is then broken to remove the part.

Materials

Most metals, particularly ferrous and aluminum alloys. Some difficulty encountered in casting lead, tin and zinc alloys, also refractory alloys, beryllium, titanium and zirconia alloys.

SandCasting

Process variations

  • Green sand casting: the most common and the cheapest. Associated problems are that the mold has low strength and high moisture content.
  • Dry sand: core boxes are used instead of patterns, and an oven is used to cure the mold. Expensive and time consuming.
  • Skin-dried sand: the mold is dried to a certain depth. Used in the casting of steels.
  • Patterns: one-piece solid patterns are the cheapest to make; split patterns for moderate quantities; match plate patterns for high volume production.
  • Wooden patterns: for low-volume production only.
  • Metal patterns: for medium to high-volume production. Hard plastics are also being used increasingly.
  • Cosworth casting: low pressure filling of mold used for better integrity, accuracy and porosity of casting. Longer production times and higher tooling costs, however.

Economic considerations

  • Production rates of 1–50/h, but dependent on size.
  • Lead time typically days, but depending on complexity and size of casting.
  • Material utilization low to moderate. Twenty to fifty per cent of material lost in runners and risers.
  • Both mold material and runners and risers may be recycled.
  • Patterns easy to make and set, and reusable.
  • Pattern material dependent on the number of castings required.
  • Easy to change design during production.
  • Economical for low production runs of less than 100. Can be used for one-offs and high production volumes depending on degree of automation.
  • Tooling costs low.
  • Equipment costs low.
  • Direct labor costs high. Can be labor intensive.
  • Finishing costs can be high. Cleaning and fettling required to remove gates and risers before secondary processing. Parting lines may also need finishing by hand.

Typical applications

  • Engine blocks
  • Manifolds
  • Machine tool bases
  • Pump housings
  • Cylinder heads

Design aspects

  • High degree of shape complexity possible. Limited only by the pattern.
  • Loose piece patterns can be used for holes and protrusions.
  • All intersecting surfaces must be filleted: prevents shrinkage cracks and eliminates stress concentrations.
  • Design of gating system for delivery of molten metal into mold cavity important.
  • Placing of parting line important, i.e. avoid placement across critical dimensions.
  • Bosses, undercuts and inserts possible, but at added cost.
  • Steel inserts can be used as heat flow barriers.
  • Cored holes greater than ø6 mm.
  • Machining allowances usually in the range 1.5–6 mm.
  • Draft angle ranging 1–5°.
  • Minimum section typically 3mm for light alloys, 6mm for ferrous alloys.
  • Sizes ranging 25 g–400 t in weight.

Quality issues

  • Molding sand must be carefully conditioned and controlled.
  • Most casting defects can be traced to and rectified by sand content.
  • Casting shrinkage and distortion during cooling governed by shape, especially when one dimension is much larger than the other two.
  • Extensive flat surfaces prone to sand expansion defects.
  • Inspection of castings important.
  • High porosity and inclusion levels common in castings.
  • Defects in castings may be filled with weld material.
  • Castings generally have rough grainy surfaces.
  • Material strength inherently poor.
  • Castings have good bearing and damping properties.
  • If production volumes warrant the cost of a die.
  • Surface detail fair.
  • Surface roughness a function of the materials used in making the mold and ranging 3.2 –50 µm Ra.
  • Not suitable for close specification of tolerances without secondary processing.
  • Process capability charts showing the achievable dimensional tolerances using various materials provided . Allowances of ±0.5–±2mm should be added for dimensions across the parting line.

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