A previous knowledge base article addressed mould shrinkage. We recognised 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).
Avoid a void!
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. 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. This is referred to as a ‘sink mark’. It 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. These are referred to as ‘voids’ — not to be confused with vapour bubbles from air, moisture or also decomposition volatiles.
For opaque materials, you may think that voids are harmless. However, 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 also 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 also make a difference by changing process conditions. These include increasing hold pressure and hold time, reducing melt and mould temperatures, checking for mould cushion and leakage.
If in doubt, carry out simulation analyses.
This Avoid a void article was written by Dr.Charlie Geddes for Hardie Polymers.
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