An additional mold piece that is used to form features that cannot be formed by the two mold halves, such as an external undercut. A side-core slides into the mold along one of its four sides and forms the shape of the undercut. A single side-core can be used to form multiple undercuts if they are accessible along the same side of the mold and are close together. An internal undercut can be formed by a side-core if it is accessible from the side of the part.