Manufacturing Processes, Full Mould Evaporative Pattern Casting

Full Mould Casting

Full mould casting, otherwise known as full mould evaporative pattern casting, comes under the general category of casting as a manufacturing process and is an expendable mould and pattern process. Typically a pattern is formed by injecting polystyrene beads into a heated die usually made from aluminium and then passing steam through it so that the polystyrene expands and fuses together. The pattern becomes a solid mass of polystyrene that can be removed by splitting the die along a split line after cooling, ejectors are normally used to aid removal.

Once removed from the die, runners and risers can be glued onto the pattern as required and then the complete pattern with runners and risers in place is coated with a refractory slurry to form a shell around the polystyrene pattern when it is dried.

The pattern is then placed into a flask and loose sand is poured in around it using vibration to ensure it is well compacted around the pattern. A vacuum is pulled at the base of the flask and then the molten metal is poured into the runner. The polystyrene pattern is destroyed by the molten metal as the metal takes on the form of the pattern.

Once poured the casting is allowed to cool and is then extracted from the flask. The sand can be cleaned and recycled. The runners and risers will need to be removed from the casting to leave the base component, but it is normal to recycle this excess material for further castings.

Pros

  1. All equipment is rudimentary and relatively cheap
  2. A very flexible process suited to prototype manufacture or one off production
  3. Can be used for highly complex shapes

Cons

  1. High casting temperature required to vaporize pattern
  2. Poor surface finish and texture
  3. High potential for inclusions and porosity
  4. Pattern material destroyed
  5. Long process time due to multiple stages

Note: there are 2 types of evaporative pattern casting:

  • lost foam which uses unbonded sand
  • full mould casting which uses a bonded green sand

There are only subtle differences between the two methods and they are often considered to be the same process.

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