Compression Molding

Compression Molding

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

Material Recommendations


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

Important methods for the automotive industry are:

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

There are several advantages with compression molding:

  • 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 mold complex articles with undercuts or small holes
  • Fair tolerances
  • Flash can be a problem

Mold failure

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

Mold material properties

Important properties are:

  • Wear resistance
  • Strength and hardness