Radii & corners

In the design of injection molded parts sharp corners should always be avoided;generous radii should be included in the design to reduce stress concentrations.

Fillet radii should be 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 radius of 0.5mm is suggested and all sharp corners should be broken with at least a 0.125 mm radius.

Sharp corners, particularly internal corners introduce:
High molded in stresses
Poor flow characteristics
Reduced mechanical properties
Increased tool wear
Surface appearance problems, (especially with blends).

The inclusion of a radius will give:
Uniform cooling
Less warpage
Less flow resistance
Easier filling
Lower stress concentration
Less notch sensitivity.

The outside corner radius should be equal to the inside radius plus the wall thickness as this will keep a uniform wall thickness and reduce stress concentrations.

Corner radius.

For a part with an internal radius half the nominal wall thickness a stress concentration factor of 1.5 is a reasonable assumption. 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.

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

In addition, from a molding view point, it is important is to avoid sharp internal corners. Due to the difference in area/volume-ratio of the polymer at the outside and the inside of the corner, the cooling at the outside is better than the cooling at the inside. As a result the material at the inside shows more shrinkage and so the corner tends to deflect (see figure below).

Sharp corners.