July 14, 2022

What is Mould Shrinkage?

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Mould shrinkage can be manipulated by nearly all process variations- melt temps, cooling temps, in. pressures and hold pressures, injection speeds and so on, as well as gate location and part geometry. All materials have a specific shrinkage rate range that provides a guideline to use that is based upon “best moulding practice’. The further you deviate from specified parameters, the further you will deviate in a normal shrinkage of the part. If you skirt the boundaries or exceed the boundaries of the material specification, the less likely you are to achieve a statistically reliable part. No one has more understanding of this than the material supplier technical group. So if looking for a more comprehensive understanding base upon a specific resin and a specific part, a comprehensive and complete mould flow analysis based upon the part and mould with runner and cooling will give you the best indication…… Which MF program is best— is up for debate and needs to be analyzed and quantified against actual real life results…… So far, I’ve never seen such an independent analysis.

Just remember a few things:

  • Semi-crystalline materials shrink more than amorphous ones.
  • When faced with a shrink window and the need to pick a number for the mould maker, choose towards the lower end. It’s a lot easier to make a part shrink more than to hold it to shrink less.

Depending upon the nature of the mould and the plastic being used we would often go the other way. For example with a laminate tool and semi-crystalline plastic we would apply slightly more shrinkage then expected in the flow direction (laminate stack direction) knowing if the part did not shrink as much as expected we could remove a few tenths from each laminate. Additionally when a high risk that shrinkage would be missed would make an effort to insert steel safe critical to function features such that true positions could be adjusted after initial samples. In such cases may even use a dummy insert (limited features) until such time shrinkage is understood and then replace with a fully featured tool insert which placed critical surfaces/features at the proper spacing. Hope for the best but always plan for the worst. Happy Moulding!

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