The surfaces of the cavity and core are likely to be oxidized (rusted) and corroded by the volatile components from the molded material or from moisture content in the atmosphere.
Considering the mold release of the molded article, it is necessary that the surface of the cavity is carefully polished and is maintained in a shining condition.
Plating the surface of the cavity is used as a measure for preventing oxidization or corrosion. Hard chromium plating is a typical method of plating among the different varieties of plating.

Hard chromium plating has good wear resistance and corrosion resistance.
The hardness of the film on the surface is 700 to 1000 HV, and the surface has a white metallic glossiness.

To carry out the plating, the work to be plated is immersed in a plating solution made of chromic acid　anhydride, sulfuric acid, etc., and an electric current is passed to form an electroplated film.

The plated film is a robust passivated film of a base metal in air. Since the surface energy of the plated film is low, it has the property of being hard to be adhered to by other materials. Therefore, it can also be said that it has the property of making it easy for the plastic molded article to be released from the mold.

In order to carry out plating skillfully, it is recommended to carefully polish the part of the surface that is to be plated, thereby putting it in a state in which the plating can adhere to it easily.

In fields other than molds, this type of plating is used for preventing rust in the parts for food production machines, mill rolls, and for preventing rusting of machine parts.

In general, since the waste plating liquid treatment and management of plating liquid are important from the point of view of pollution prevention, in most cases, the work of hard chromium plating is entrusted to specialized companies.