How to A-Void that Sinking Feeling

In a recent blog we addressed mould shrinkage, recognising that mould shrinkage (the difference between the mould cavity dimensions and the final dimensions of the moulding) depends on the material (amorphous v semi-crystalline; filled v unfilled) and also on the specific process conditions (melt temperature, injection pressure, hold pressure, hold time etc).

However the real problems facing designers, toolmakers and moulders lie with non-uniform shrinkage.

In mouldings with thicker wall sections, particularly at the root of ribs and bosses and at corners, there is a larger volume of melt to cool and shrinkage will be greater at these features. For more flexible materials the shrinking melt at the core will pull the cooled skin down to produce a localised indentation, referred to as a ‘sink mark’, which detracts from the appearance of the moulding. If the cooling skin is more rigid, the contacting melt shrinks away from itself to leave little vacuum bubbles, referred to as ‘voids’ — not to be confused with vapour bubbles from air, moisture or decomposition volatiles.

For opaque materials, you may think that voids are harmless but they can lead to crack initiation under applied mechanical stress, particularly impact.

Minimising sink marks requires concerted effort from designers, toolmakers and moulders. At the design stage, sinks and voids can be minimised by avoiding thick sections and adhering to well published guidelines on dimensions of ribs, bosses and corners. Inevitable sink marks can be disguised with texturing and design features. Toolmakers can help by careful design of feed systems, gating in the thickest section and efficient cooling. Moulders can make a difference by changing process conditions: increasing hold pressure and hold time, reducing melt and mould temperatures, checking for mould cushion and leakage.

If in doubt, carry out simulation analyses.