The compression molding process is not unlike making a waffle. A surplus of material must be placed in the cavity to ensure total cavity fill. Heat and pressure are applied, causing the compound to flow, filling the cavity and spilling out into overflow grooves. Compression molding is often chosen for medium durometer compounds - in high volume production, or applications requiring particularly expensive materials. The overflow, or flash, created by larger diameter parts is of particular concern when using the more expensive compounds.
Compression molding helps to minimize the amount of overflow.
The preload, however, can be difficult to insert in a compression mold of more complex design, and the compression molding process does not lend itself to the material flow requirements of high durometer compounds.