Part Details Review

A radius of 0.5 to 0.75 times the wall thickness, as suggested for a box or channel shaped part under the section on "Sharp Corners", might be too much for a part with T-junction, a rib or a cantilever as shown in the adjoining diagram. Excessive radius in such a case will create an excessive thick section resulting in internal voids or a sink mark on the supporting wall.

However, radii at such corners are important. The top fillet radius becomes even more important in case of a cantilever snap feature. When depressed for snapping action, the top fillet radius experiences flexural or tensile stress.

Figure 2 shows the effect of fillet radii on stress concentration factor. The shape of the curve is material independent. As you can see, the advantage of the "R/h" ratio starts to diminish as "R" increases.

The designer has to make a trade off here. Too much radius will increase the possibility of a visible sink mark and molded-in stress but too little will increase the stress concentration factor. Ratio of 0.15 to 0.3 often provides a reasonable compromise.

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