Injection Moulded Part Radiusing and
Draft Guidelines

Proper use of draft and reinforcing fillets will aid in ejection, add rigidity to part ribs and strengthen the mould.

Rounding corners

Avoid unnecessary stresses by rounding out corners

It is also a good idea to avoid the creation of stress risers in sharp corners. This can be done through the use of a radius to distribute the stresses and also streamline the flow of the molten plastic.
Sharp corners have high stress concentrations and plastic flow is hindered.

Rounded corners have reduced stress concentrations and plastic flow is enhanced.


Include sufficient draft

We strongly advise using at least 0.5 degrees on all "vertical" faces.
  • 2 degrees works very well in most situations.
  • 3 degrees is minimum for a shutoff (metal sliding on metal).
  • 3 degrees is required for light texture (PM-T1).
  • 5 or more degrees is required for heavy texture (PM-T2).

Use drafted shutoff surfaces

A “telescoping shutoff” is when the shutoff surfaces slide with respect to each other as the mould closes and opens. The more sliding that is done, the greater the wear and the more potential for mould problems (e.g. breakage) and part problems (e.g. flash).

So the “steepness” of a telescoping shutoff surface is ideally a few degrees off vertical to avoid these problems.

This example shows a part feature that is much more easily manufactured with drafted walls than without them.
No draft results in sliding parallel mould surfaces
Draft results in improved mould shutoff surfaces

Sliding Shutoffs Video