Part Details Review
Internal sharp corners or fillets are natural sites for stress concentration. They have the same effect on the impact strength of a plastic part as a notch on the Izod test bar. Figure 1 shows that the Izod impact strength of polycarbonate drastically reduces to one ninth (worst case) when the notch radius is just reduced to half of its original size.
Internal sharp corners (see areas "A" & "B" in Figure 2) also impede the flow of polymer melt during injection molding, produce localized high shear and can sometimes even cause shear degradation of the polymer. Too generous an outside radius with an internal sharp corner (see "B") creates flow restriction which can result in poor packing of thicker sections beyond (see "A").
For designing corners in a box-shaped part, as shown in Figure 2, a desirable fillet radius would be 50 to 75% of the wall thickness. When outside corner radius is equal to the sum of the inside radius and the wall thickness (see "C"), the bend maintains uniform wall thickness that helps provide smooth material flow through the corner.
Outside sharp corners don't create stress concentration points but they are harder to fill and may cause air entrapment. An outside sharp corner paired with too generous a fillet radius (see "D"), however, creates a thick section which may cause internal voids and/or sink marks and higher molded-in stress.
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