Overmolding is the process of taking a part and then using plastic injection molding to add additional layers over the current part. With overmolding, you essentially take two different plastics and combine them into one part. Traditionally, overmolding starts with injection molding a hard plastic resin that has a higher melting point. Then, once that part is cooled, another plastic resin is injection molded over the first part. Usually this is to create a grip or accents on the original part.
