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Manufacturing Processes > Casting Processes > Plaster Mold casting

 

Plaster Mold casting

 

Process description

A precision metal pattern (usually brass) generates the two part mold which is made of a gypsum slurry material. The mold is removed from the pattern and baked to remove the moisture. The molten metal is poured into the mold and allowed to cool. The mold is broken to remove the part.

Materials

  • Limited to low melting temperature metals, i.e. aluminum, copper, zinc and magnesium alloys due to degradation of the plaster mold at elevated temperatures.
  • Tin and lead alloys are sometimes processed.
Plaster Mold Casting

Process variations

  • Patterns can be made from: metal, plaster, wood or thermosetting plastic. Wood has a limited life due to water absorption from the plaster slurry.
  • Composition of plaster slurry varies. Additives are sometimes used to control mold expansion and fibers added to improve mold strength.

Economic considerations

  • Production rates of up to 10/h typical.
  • Lead times can be several days to weeks.
  • Material utilization high.
  • Low scrap losses. Waste recycled.
  • Mold destroyed in removing casting.
  • Easy to change design during production.
  • Suitable for small batches of 100 and medium-volume production.
  • Tooling costs low to moderate.
  • Equipment costs moderate.
  • Direct labor costs moderate to high. Some skilled operations ecessary.
  • Finishing costs low. Little finishing required except grinding for gate removal and sanding of parting line.

Typical applications

  • Pump impellers
  • Waveguide components (for use in microwave applications)
  • Lock components
  • Gear blanks
  • Valve parts
  • Molds for plastic and rubber processing, i.e. tyre molds

Design aspects

  • Moderate to high complexity possible.
  • Possible to make mold from several pieces.
  • Deep holes not recommended.
  • Sharp corners and features can be cast easily.
  • Bosses and undercuts possible with little added cost.
  • Placing of parting line important, i.e. avoid placement across critical dimensions.
  • Cored holes greater than Ø13 mm.
  • Where machining required, allowances of up to 0.8mm should be observed.
  • Draft angles ranging 0.5–2° preferred, but can be zero.
  • Minimum section ranging 0.8–1.8 mm, depending on material used.
  • Sizes ranging 25 g–50 kg in weight. However, castings up to 100 kg have been made.

Quality issues

  • Little or no distortion on thin sections.
  • Plaster mold has low permeability and can create gas evolution problems.
  • Moderate to high porosity obtained.
  • Mechanical properties fair.
  • Surface detail good.
  • Surface roughness ranging 0.8–3.2 µm Ra.
  • A process capability chart showing the achievable dimensional tolerances is provided . An allowance of approximately ±0.25mm should be added for dimensions across the parting line.

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