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Designing to Reduce Plastic Shrinkage and Warpage

In this article, we will be exploring what causes shrink and warp in plastic injection moulding. All injection moulded parts shrink, but they do this by varying degrees. We’ll highlight how to reduce and control shrinkage and warping and why it is important to do so. To understand the causes of shrink and warp it is first important to explain shrinkage and warpage.

  

Shrinkage

Shrinkage is a reduction in size of the molded part as it cools. Different plastics shrink more than others and some plastics shrink differently in one direction than in the other. If a part shrinks perfectly uniformly in all directions then it simply becomes smaller, retaining its correct shape. However, if any element of the part shrinks at a different rate, then it creates internal stress, if the stress exceeds the part’s structural integrity then the part will warp.

There are 4 types of shrinkage:

  1. Regional – one area or region shrinks more than the other
  2. Thickness – this happens when one side of the cross-section is different to the other and therefore one side can shrink more.
  3. Directional – shrinkage can happen parallel and perpendicular to the material’s orientation or direction of flow.
  4. In-plane Vs. Thickness – Polymers typically shrink in the thickness direction rather than the plane of the surface.

  

Warpage

Warping is a distortion in the intended shape of the molded part that occurs during cooling, it can make the parts twist, fold, bend and bow.

 

What Causes Shrinkage

Why parts shrink depends can depend on a number of factors:

  • Material and whether any filler or fiber reinforcement has been used. It is important to note that amorphous materials shrink less than semi-crystalline materials.
  • The part or wall thickness
  • The injection pressure, low pressure would be more likely to cause shrinkage
  • A short pack-hold time or cooling time
  • High melt temperature
  • High mould temperature

  

What Causes Warping

Warping is a direct result of shrinkage, but variations in wall thickness and improperly gated parts can also cause a component to warp.

 

Problems caused by Shrinkage

Shrinkage can lead to surface defects such as sink marks or voids, we have covered these in a previous article. And as we’ve already identified warping is directly linked to shrinkage, so how do we reduce and control these issues?

  

How to reduce and control shrinkage and warping

  • Avoid non-uniform wall thickness
  • Pack the cavity after fillingand use a proper packing pressure level
  • Balance the filling pattern
  • The cooling system should apply uniform cooling across both the thickness and throughout the part

 

When designing for plastic injection moulding, the amount of shrinkage needs to be accurately predicted and this can be determined by part size and resin selection in some cases. It is also important to consider the shrink of the resin when designing the mold.

Our in-house team of specialists can help you design and fine-tune the shrinkage of your parts by adjusting the density of the material and considering the above factors. Get in touch today to discuss your requirements, our team will be more than happy to help, advise and quote.

 

 

 

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Our team will be more than happy to help, advise and quote for your requirements.

Looking for more information? Check out our blog, News & Views for useful articles, tips and tricks on plastic injection moulding.