Injection Moulding

Depending on the number of shots, plastic material used, size of the mould and needed surface finish, different mould materials can be used. The following basic mould material properties must be considered: strength and hardness, toughness, wear resistance, cleanliness, corrosion resistance and thermal conductivity.

General

Injection moulding is a moulding procedure whereby heated and plasticized thermoplastic or thermoset material is injected under high pressure into a relatively cool mould cavity for solidification. Injection moulding is a high productivity method. However, the moulds can be extremely complicated and expensive.

Performance of moulds

The performance of the mould can be affected by the selection of mould material. Performance has different meanings to different people:

Mould life
Quality of the plastic part
Productivity

Mould life

The mould life may be determined by many different mechanisms:

  • Wear
  • Surface defects
  • Deformation
  • Corrosion

Wear may occur because of reinforced plastic or very long runs. Surface defects may already occur during mould 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 mould or under-dimensioning of the mould. Corrosion may of course be a problem when moulding 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 mould must keep the promised surface finish and that is also valid for the tolerances of the mould. Steel selection is important for high polished moulds. The steel must be clean and have a very low amount of inclusions. Tolerances may be affected because of uneven mould temperatures which of course is very much dependent on the cooling channel sizes and position but also on the selection of mould material. Material like aluminum or copper alloys, with high thermal conductivity, may in some cases be used as mould material.

Productivity

Even productivity can sometimes be affected by the selection of mould materials. The most obvious situation is the selection of high thermal conductivity materials. However, selections aiming for lower maintenance of the mould i.e. less production interruptions, will lead to higher productivity. One example is to use corrosion resistant mould materials. This leads to less need for redrilling of cooling channels and repolishing of mould surfaces.