July 14, 2022

What is Mould Shrinkage?

blog aco mold

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!

Related Blogs

blog injection molding
Injection moulding simulation tools

Whether the injection moulding simulations software (not only Moldflow) are 90% or 95% or 99% accurate or not: we should remember that these tools (or many of these tools) have …

blog aco mold
Galvanic corrosion in a steel mold base

Q: We have a mold with severe corrosion around the water line holes in the aluminum cavity blocks. There is an o’ring that seals the water channel in the cavity …

blog aco mold
Reducing sink marks

Let’s face it, those of us in the plastics molding industry typically have to deal with sink marks with most parts. We all understand that the Geometry, Variations in Wall …

blog aco mold
Injection molding gate types

Injection Molding Gate Design A gate is a designed small opening to allow the melted plastics into the mold cavity, a successful gate design is determined by gate type, dimensions, …

Tell us your request right now and contact us today about getting started on your next project together!


Or Fill Out The Contact Form Below:

Support Your Business with Better Molding Solution

Contact Info
Copyright © 2023, ACO Mold. Terms of Service and Privacy Policy. All rights reserved.  Powered by ACO MOLD.
1 1.png

Join Our Network

Please email to sales@acomold.com
or fill out the contact form below: