Depending on number of shots, plastic material used, size of the mold and needed surface finish, different mold materials can be used. The following basic mold material properties must be considered: strength and hardness, toughness, wear resistance, cleanliness, corrosion resistance and thermal conductivity.
Injection molding is a molding procedure whereby heated and plasticized thermoplastic or thermoset material is injected under high pressure into a relatively cool mold cavity for solidification. Injection molding is a high productivity method. However, the molds can be extremely complicated and expensive.
Performance of molds
The performance of the mold can be affected by the selection of mold material. Performance has different meanings to different people:
- Mold life
- Quality of the plastic part
The mold life may be determined by many different mechanisms:
- Surface defects
Wear may occur because of reinforced plastic or very long runs. Surface defects may already occur during mold manufacturing as a result of polishing problems, EDM defects or after some time in production because of attacks from the plastic resin. Deformation can sometimes be a result of too high a clamping force or because of uneven temperatures in the mold or under-dimensioning of the mold. Corrosion may of course be a problem when molding corrosive resins like PVC, but may also be result of aggressive cooling water or humid atmosphere.
Quality of the plastic part
The quality of the plastic part is based on the its appearance but of course, also the functionality of the part. The mold must keep the promised surface finish and that is also valid for the tolerances of the mold. Steel selection is important for high polished molds. The steel must be clean and have a very low amount of inclusions. Tolerances may be affected because of uneven mold temperatures which of course is very much dependent on the cooling channel sizes and position but also on the selection of mold material. Material like aluminum or copper alloys, with high thermal conductivity, may in some cases be used as mold material.
Even productivity can sometimes be affected by the selection of mold materials. The most obvious situation is the selection of high thermal conductivity materials. However, also selections aiming for lower maintenance of the mold i.e. less production interruptions, will lead to higher productivity. One example is to use corrosion resistant mold materials. This leads to less need for redrilling of cooling channels and repolishing of mold surfaces.