Compression Moulding

Compression Moulding

The technique is often used for moulding of glass fiber reinforced parts and high pressure is often used. Wear resistance, strength and hardness are required from the mould material.


Compression moulding is a technique mainly used for thermoset moulding in which the moulding compound is placed in the open mould cavity, the mold is closed and heat and pressure are applied until the material is cured. This technique is often used for moulding of glass fiber reinforced plastic.

Important methods for the automotive industry are:

  • SMC Sheet moulding compound (thermoset with 25-40% fiber)
  • BMC Bulk moulding compound (thermoset with 10-20% fiber)
  • GMT Glass mat themoplastic (thermoplastic with 25-40% fiber)

There are several advantages with compression moulding:

  • No waste of plastic material (no gate or runner system)
  • Minimum of internal stresses in the part
  • Process can be used for very heavy parts
  • Less expensive equipment

Limitations are:

  • Difficult to mould complex articles with undercuts or small holes
  • Fair tolerances
  • Flash can be a problem

Mould failure

Mould failures are often related to the high amounts of glass fiber and the high pressure used. The moulds wear in certain critical areas or show indentations.

Mould material properties

Important properties are:

  • Wear resistance
  • Strength and hardness