DSM Engineering Plastics

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Radii and chamfers

When designing in plastics, applying radii or chamfers to sharp corners is of key importance for a parts manufacturability, dimensional accuracy and load-carrying capacity.

Radii and chamfers

Function

In the design of injection molded parts, sharp edges and corners should always be avoided. This is because they lead to the following results:

  • High molded-in stresses
  • Poor flow characteristics
  • Reduced mechanical properties
  • Increased tool wear
  • Surface appearance problems

Instead, by applying radii and fillets, the following results can be achieved:

  • Less warpage
  • Less flow resistance and easier filling
  • Reduced stress concentration
  • Less notch sensitivity
  • More uniform cooling
  • Better appearance

 

Radius and fillet dimensioning

  • Sharp corners should be rounded with a radius measuring between 25 and 60% of the nominal wall thickness. If the part has a load bearing function, then the upper end is recommended.
  • A minimum inner corner radius of 0.5 mm is suggested.
  • In order to keep the wall thickness uniform, the outside radius of a corner should be equal to the inner radius plus the wall thickness (see Figure 1).
  • All sharp outer part and rib edges should be broken with at least a 0.125 mm radius.
  • For a part with an inner corner radius half the nominal wall thickness, a stress concentration factor of 1.5 is a reasonable assumption (see Figure 2). For smaller radii, e.g. 10% of the nominal wall, this factor will increase to 3. Standard tables for stress concentration factors are available and should be consulted for critical applications.

Figure 1 - Corner radius.

Figure 2 - Stress concentration as a function of wall thickness and corner radius.

 

Chamfers

Like a radius or fillet, a chamfer can be used to soften sharp edges or make a gradual transition between two perpendicular faces. In view of minimizing stress concentration and optimizing flow, a radius is better than a chamfer. However, a chamfer can offer additional functionality like facilitating manual positioning of parts during assembly. A chamfer can also be used from an aesthetics point of view: where many parts are rounded-off, a chamfered edge creates a distinctive look. As with radii, a uniform wall thickness is preferred, so a chamfer on an outer edge should be copied on the inner corner.

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