Compression molding is the process of using pressure to mold plastics and rubber into the desired shape. The process is used to manufacture composites that are used in brake liners and clutch liners in automobiles, and reinforced fiberglass components, etc. Suitable reinforcements can be used in the molding process. Compression molding is the most common method of forming thermosetting materials in the industrial sector.
The process of compression molding works well with thermosetting plastics and rubber in a partially cured stage. Usually granules, performs or putty is used as the raw material. The raw material is preheated and then placed in a preheated die. The mold is closed and pressure is slowly applied while the required temperature is maintained. The pressure forces the material to squeeze itself into all areas of the mold. When the heated mold is closed the thermosetting material goes through a chemical change that permanently hardens it into the shape of the mold.
The three compression molding factors of pressure, temperature and time will vary with the design of the finished product and the material that is being molded. Sufficient curing time is given, with constant pressure and heat, before the component is removed from the mold and cooled down. When the item is removed form the mold, any extra rubber or plastic needs to be cut off of the item.
Because the process of compression molding is automated, the need for an operator to cut and lay charge/blank is done away with and part-to-part reproduction is increased. Compression molding is a high volume process that is ideal for molding complex, high-strength fiberglass reinforcements. The investment in the molding equipment is minimal and there is very little waste of raw material. The downside of compression molding is that it generates a lot of flash material that needs to be trimmed, which leads to extra costs.
Another advantage of compression molding is that you can mold large, fairly intricate parts by this method. Compression molding also produces fewer knit lines and less fiber-length degradation than injection molding. Unfortunately, the consistency is not uniform and small intricate parts cannot be compression molded.