Mould Finishing-Nearly all cavity and punch forms require a Finish to be applied to them. This may be a Polished finish, a photo-etched finish or a plated finish. However, for some technical Components the surface finish is not important and they are often left with a Fine EDM Finish, Where special finishes are required, there are several processes available including:
The correct steel must be selected if a very high surface finish is required on the core/cavity. It must be free of impurities and stresses and have low content of sulfur (often added to improve machinability).
Electro slag refining (ESR) is a process used by suppliers of Mold steels to reduce the amount of impurities in the steel. Vacuum de-gassed steels are also used in an effort to prevent the formation of localized impurities or pits in the surface of the steel, which can spoil a polished finish.
The polishing process is a slow laborious procedure, which involves three distinct stagers:
Coarse: using abrasive papers, carborundum paste or a mechanical reciprocating polishing device such as a Diprofile.
Intermediate: using fine abrasive stones
Finish: hand polishing with diamond paste and orange sticks or with a powered mechanical reciprocating hand tool
All molding surfaces should be highly polished in line of draw for cavities and punches unless an alternative finish is required. The sprue bush and runners should also be highly polished in line of flow to assist streamlined flow of the polymer and to prevent any tendency for these to ‘stick back’.
Chrome plating can be used to improve hardness and corrosion resistance and improve release of moldings- The degree of finish is very much dependent on amount of polishing prior to placing. Plating will never improve a surface, it will only accentuate a poor finish.
With very corrosive materials like PVC and certain types of thermoplastic rubbers. it is usual to use stainless steel to combat the corrosion. Unfortunately even the best stainless steel alloys have impurities and localized ferrous (iron) areas in them. These will soon be attacked by corrosive materials, resulting in pitting on the surface of the steel. With long- running tools it advisable to Chromium plate the cavity areas made from stainless steel to protect against this possibility. If a lion-stainless mold tool is to be used for molding corrosive materials, the molding surfaces must be chromed plated and the rest of the tool flash chrome plated to protect it.