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Design Considerations for Injection Moulding: Wall Thickness & Radii

When designing for plastic injection moulding, there are several design considerations to study, this article shines a spotlight on wall thickness and radii. Wall thickness refers to the thickness of the cross section of the plastic part. Radii signifies the radius of a corner rather than a sharp edge.  

 

Wall Thickness

We’ve touched on the importance of wall thickness in other design guideline articles but here we’re discussing just how important wall thickness is when it comes to design, product suitability and application.

Uneven wall thickness will present problems, therefore designing with uniformed walls and cross sections will simplify the manufacturing process.

 

A wall section that is too thin, could result in structural defects but a wall section that is too thick, could cause surface defects as well as a heavy, over-engineered product. It is also worth noting that thick walls cool slower with greater shrinkage compared to thin walls. Thicker walls have a longer moulding cycle time and are consequently more expensive than components with thin walls. Therefore, in most cases a thin uniformed wall is preferable to a thick wall.

 

When designing to achieve uniformed wall thickness consider the following:

  • Walls should be no less than 40-60% of the adjacent walls.
  • Guidelines recommend 0.75mm-3mm thickness for filled components and 0.5-5mm for unfilled (depending on your design and product).
  • Carefully select materials with wall thickness in mind.

 

Radii

Correct placement of corner radii in injection moulding design creates strong, high-quality and cost-effective plastic parts. Sharp external corners are ok and sometimes necessary to fulfill product requirements, such as triangular shaped items. However sharp corners can present challenges when designing for injection moulding, as they can cause stress resulting in a poor product, radii are key to reducing stress.

 

There are two types of radius, internal and external. When designing an internal radius, it should be a minimum of 50% of the wall thickness but ideally match the external radius to give a constant section and avoid creating a thick wall, which presents other challenges as discussed above.

 

When designing we aim to avoid sharp corners:

  • To reduce shear – the level of shear around sharp corners can damage the material and cause surface defects.
  • To ensure correct ejection from the mould – sharp corners can stick in the mould as the part shrinks.
  • To improve flow – melted polymers flow better around smooth radiused corners, reducing the likelihood of surface defects. 

In Summary

A well-radiused product design, combined with uniformed wall thickness ensures not only a robust product but one without surface defects. An experienced design team, such as ours at CJ Tool & Mouldings can best advise you when it comes to designing for plastic injection moulding, how to avoid pitfalls and product defects and how to design to improve performance whilst considering costings. Get in touch with our team of experts today! 

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