Compression Molding from Holden Plastics Company

Compression Molding

Compression molding is the oldest plastic molding method. A compression mold consisters of two halves into which are machined the mold cavities. The mold temperature is maintained using electric heaters, and the mold is held shut with a hydraulic cylinder, or toggle clamp.

Material is placed in the mold, and it is closed under 2000 to 6000 pounds per square inch of molding area. Contact with the heated mold surface softens the material, allowing it to fill in the entire cavity and initiating the chemical reaction which cures the part. Cure time is determined by the thickest cross section, mold temperature, material type and grade. After curing, the mold opens and the part is ejected.

Compression Molding Advantages

There is a minimum amount of wasted material.
Internal stress and warping are minimized.
Dimensional accuracy is excellent.
Shrinkage is minimized and closely reprodcible.
Thick sections and large parts are practical.
Lower molding pressures allow molding of large parts on presses of lower tonnage.

Compression Molding Limitations

Compression molding is not suitable for fragile mold features, or small holds.
Uneven parting lines present a mold design problem.
High impact composites make flash removal difficult.
The depth of the molded holds is limited to 2 or 3 times their diameter.
Shot weight must be tightly controlled.
Dimension across the parting line may be difficult to hold but good accuracy may be obtained through tight process control.