Compression molding is most commonly used to make rubber and silicone parts but parts made by this process also consist of either thermosets or thermoplastics.  The molds are the combination of upper and lower mold halves which create a space cavity in the middle.  The molds are typically made of aluminum or steel with the decision to use either alloy based on part volume and tonnage requirements from the mold.  In the simplest description, material is then placed into the mold, which is then heated, before the mold halves squeeze down onto the cavity with pressure, transforming the material into a geometric part.  Once the part cools in the mold the part is then removed with excess material and flashing removed manually.  The process is then repeated.