DESIGN FOR PRECISION

Frequently a part will shrink unevenly causing warping.  Furthermore, parts may be heated during an assembly process, such as welding, or during curing following painting.  This can cause both temporary and permanent dimension changes.  Shrinkage also differs between in-flow and cross-flow.

Amorphous materials exhibit lower shrinkage than semi-crystalline. Amorphous are the hard plastics, such as ABS, Polycarbonate, etc. Semi-crystalline are softer such as Polyethylene and Polypropylene.

Fillers also affect shrinkage – tending to reduce it.

Wall thickness is a critical factor.  Thin sections have a shorter cooling time so less opportunity for correction within the mould or by jigging.  Varying thickness within a moulding also leads to distortion due to different shrinkages.  If possible a gradual transition between different sections should be used.

However, this differential shrinkage could also be used as a means of correcting warping by thickening sections at appropriate points.

Ribs can also be a problem, especially where they have been made thinner to prevent sink marks as once again there are variations in the section.  Gas injection or a blowing agent may be used to overcome the problem.

In addition to part design there are also a number of mould tool design and processing factors that need to be considered.

Tool design:

• Gate location
• Type and size of gate
• Cooling of the mould
• Ejection system

Processing:

• Melt temperatures
• Mould temperatures (each half of the mould can be a different temperature to help overcome warping)
• Filling, packing and holding pressures and times
• Post moulding jigs