Compression Molding of Polymers

a method for manufacture of articles from plastics and rubber stock in compression molds mounted in a press, most frequently a hydraulic press. A distinction is made between high-temperature (hot) and low-temperature (cold) compression molding.

In hot compression molding, the material, which may be in the form of a molding powder (usually pelletized or granulated) or in sheet form, is placed in an open mold that has been preheated to a specified temperature. As the plunger descends, the mold closes and the material is fluidized by heat and the pressure generated by the press, filling the mold cavity, thereby taking on the dimensions and configuration of the article being produced. Thermosets and rubber stock are held in the mold under pressure until the process of hardening or vulcanization is complete, after which the plunger is raised and the finished article is ejected from the mold. Hot compression molding of thermoplastics is used to a limited extent, since in this case the mold must be cooled before ejection of the article.

The temperature of compression molding of polymers ranges from 80° to 300°C, the pressure from 2 to 90 meganewtons per sq m (20-900 kilograms-force per sq cm), and the holding time under pressure from fractions of a minute to 30 min. Higher pressures are used for materials with lower fluidity and for articles of complex shape. The duration of compression molding is determined by the rates of heat transfer and hardening (vulcanization) of the material. The process is accelerated by such methods as charging into the mold material preheated by high-frequency current.

Cold molding is used mainly for processing thermoplasts that do not soften upon heating, such as fluoroplastics. In this case the material is pressed in cold molds and then, after ejection from the mold, it undergoes heat treatment (“sintering”).

Compression molding is a widely used method for manufacture of articles from polymers, particularly thermosets. (See alsoTRANSFER MOLDING.)