molding parts refer to those in direct contact with plastics to form the shape of plastic parts, wherein those constituting the contour of the plastic parts are called cavities and those constituting the internal shape of the plastic parts are called cores. Since the cavity and core directly contact the plastics of high temperature and pressure and rub with the plastic parts when protruding, it is thus required that they are provided with sufficient intensity, rigidity, hardness, abrasion resistance, corrosion assistance as well as low enough surface roughness.