Compression Molding – manufacturing process guide

Car tyres manufactured by compression molding
Car tyres manufactured by compression molding

Compression molding is one of the oldest and most widely used processes for the production of thermosetting plastic parts. Compression molding machines consist of platens to which the two halves of a mold is attached. The name given to the raw material used for the production of the parts is known as a charge.

  • A calculated amount of charge (which can be in the form of liquid, pellets or powder) is heated and then placed into the lower half of the mold.
  • Both halves of the mold are preheated. Then with the help of a hydraulic system the mold is closed, forming the material into the shape of the mold cavity.
  • The mold is left for some time so that polymerization can take place, converting the raw material into the solid final product.
  • The mold is opened and the part is removed.

Compression molding process diagram

compression moulding, compression molding
Process diagram of compression moulding Image © 2018 EngineeringClicks
Rubber boots manufactured by compression molding
Rubber boots manufactured by compression molding

Compression molding products

  • Car tyres / rubber tyres
  • Rubber gaskets or seals
  • Handles
  • Electrical components
  • Automotive parts
  • Other various polymer matrix composite parts

Materials for compression molding

Compression molding is mostly used for the production of thermosetting polymers, but other materials including elastomers, some composites and a limited range of thermoplastics can also be processed using this method.

Design Considerations

  • The process typically has a single plane mold, so the complexity level of the part geometry is limited.
  • Parts are limited to an area of 3 mm2 to 1.5 m2.
  • Section thickness is typically limited to between 0.8mm and 13mm.
  • Features like holes, ribs, lettering, threads and bosses can be added.

Process Variations

  • In compression molding there are three type of molds, used for the production of different types of parts: Flash type, Semi positive type, Positive type.
  • Shallow parts are manufactured using flash type molds. These produce a lot of waste material as flash, which requires post processing and flash removal.
  • Parts with tight tolerances or variable sectional thicknesses are manufactured using semi-positive type molds.
  • High density parts are processed using positive type molds. This includes Bulk Molding Components (BMC) and Sheet Molding Components (SMC).

Economic Considerations

  • Compression molding can product roughly a hundred parts per hour, depending on the size and shape of the part.
  • Parts with thin walls will require less curing time compared to parts with thicker walls, so their production rate will be faster.
  • To increase the production rate, a multi-cavity mold can be used to produce multiple parts in a single cycle. Some degree of automation can also be applied to increase productivity.
  • Polymerization time for a part depends on the thickness section.
  • Depending on die complexity, lead time can be several weeks to produce mold tooling.
  • Labor costs are moderate, while equipment costs are high.

Quality Considerations

  • Good surface finish.
  • Minimum internal stresses.
  • Entrapped air can cause problems during compression (voids).

Compression molding advantages and disadvantages

Advantages: Low material waste, little or no post processing required.
Disadvantages: Limited materials can be used with this process, limited producible geometry.



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