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 cut, weighed or extruded into a preform of greater volume than the finished part will be.
The preforms, at ambient temperature, are then loaded into each cavity of the open compression mold which has been preheated. The tool is closed and placed under the clamping pressure of the press as the preform is squeezed into the cavity shape.
Heated platens on the press help maintain mold temperature as the molded parts are given time to crosslink. The molded parts are removed from the tool and allowed to cool to ambient temperature before deflashing or final inspection.
Like injection and transfer, compression molded rubber parts and components are used in a variety of markets and applications. Some of these parts have a metallic insert that have the rubber bonded to a metallic insert that adds dimensional stability to the friction, sealing, or flexibility of the rubber surface. Compression lends itself well to small production runs and prototyping.
Compression molds and equipment are generally simpler in design and less expensive than those for either injection or transfer, and have correspondingly lower overheads.
Because the rubber preform is at ambient temperature when placed in the mold and because the raw rubber compound is worked less than in the injection and transfer molding processes, the compression process generally requires longer time in the heated mold for complete crosslinking to occur. The results are longer in-mold cycle times.
It is therefore slower and best suitable for smaller parts, lower volume production quantities and prototyping.
Most rubber compounds can be compression molded without process difficulty or special consideration. The end usage determines the design requirements and when an existing formula is not available that meets the design criteria, the chemist develops a compound to do so.
