Natural or synthetic rubbers (elastomers) are compounded with additives to meet design use specifications and/or ease of processing. The compounded rubber, more technically a thermal set elastomer (TSE) is generally fed in strips into the screw and barrel where it is warmed to temperatures approaching but below that of the mold.

From there it is injected through channels in the tool into the cavities of the closed hot mold. The combination of pressure, temperature and time facilitates the crosslinking of chemical bonds commonly referred to as vulcanization. Process parameters are usually programmed into the press to control temperature, volume and rate that compound is injected into the mold cavities.

The process engineer also programs the press to provide adequate time for complete crosslinking. When the press opens, parts are removed and allowed to cool to ambient temperature before deflashing (if necessary), inspection and preparation for shipment.