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Manufacturing Processes > Plastic processing > Compression Molding


Compression Molding


Process description

A measured quantity of raw, unpolymerized plastic material is introduced into a heated mold which is subsequently closed under pressure, forcing the material into all areas of the cavity as it melts. Analogous to closed die forging of metals.


  • Mainly thermosets, but also some composites, elastomers and a limited number of thermoplastics.
  • Raw material supplied in either powder or liquid resin form.
Compression Molding

Process variations

  • Flash-type: for shallow parts, but more material lost.
  • Semi-positive (partly positive, partly flash): used for closer tolerance work or when the design involves marked changes in section thickness.
  • Positive: high density parts involving composite Sheet Molding Compounds (SMC), Bulk Molding Compounds (BMC) or impact-thermosetting materials.
  • Cold-molding: powder or filler is mixed with a binder, compressed in a cold die and cured in an oven. Strictly for thermosets.

Economic considerations

  • Production rates are from 20 to 140/h.
  • Cycle time is restricted by material handling. Each cavity must be loaded individually.
  • The greater the thickness of the part, the longer the curing time.
  • Multiple cavity mold increases production rate.
  • Mold maintenance is minimal.
  • Certain amount of automation is possible.
  • Time required for polymerization (curing) depends mainly on the largest cross section of the product and the type of molding compound.
  • Lead times may be several weeks according to die complexity.
  • Material utilization is high. No sprues or runners.
  • Flexibility is low. Differences in shrinkage properties reduces the capability to change from one material to another.
  • Production volumes are typically 1000+, but can be as low as 100 for large parts.
  • Tooling costs are moderate to high.
  • Equipment costs are moderate.
  • Direct labor costs are low to moderate.
  • Finishing costs are generally low. Flash removal required.

Typical applications

  • Dishes
  • Housings
  • Automotive parts
  • Panels
  • Handles
  • Container caps
  • Electrical components and fittings

Design aspects

  • Shape complexity limited to relatively simple forms. Molding in one plane only.
  • Threads, ribs, inserts, lettering, holes and bosses possible.
  • When molding materials with reinforcing fibers, directionality maintained enabling high strength to be achieved.
  • Thin-walled parts with minimum warping and dimensional deviation may be molded.
  • Placing of parting line important, i.e. avoid placement across critical dimensions.
  • A draft angle of greater than 1° required.
  • Maximum section, typically =13 mm.
  • Minimum section =0.8 mm.
  • Maximum dimension, typically =450 mm.
  • Minimum area =3mm².
  • Maximum area =1.5m².
  • Sizes ranging from several grams to 16 kg in weight.

Quality issues

  • Variation in raw material charge weight results in variation of part thickness and scrap.
  • Air entrapment is possible.
  • Internal stresses are minimal.
  • Dimensions in the direction of the mold opening and the product density will tend to vary more than those perpendicular to the mold opening.
  • Flash molds do not require that the quantity of material is controlled.
  • Tumbling may be required as a finishing process to remove flash.
  • Surface detail is good.
  • Surface roughness is a function of the die condition. Typically, 0.8 µm Ra is obtained.
  • Process capability charts showing the achievable dimensional tolerances using various materials are provided . Allowances of approximately ±0.1mm should be added for dimensions across the parting line.