Compression molding is a molding process during which resin is placed between two dies of a mold and subjected to pressure and heat to create a finished part. The process uses thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses or preforms.
According to About.com, it is a method of molding in which the molding material, largely preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas. Heat and pressure are maintained until the molding material has cured. Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiberglass reinforcements. The advantage of compression molding is its ability to mold large, fairly elaborate parts. Also, it is one of the least expensive molding methods compared to other methods such as transfer molding and injection molding. Compression molding produces fewer knit lines and less fiber-length degradation than injection molding. It is also suitable for ultra-large basic shape production in sizes beyond the capacity of extrusion in techniques. It was first developed to manufacture composite parts for metal replacement applications.