Radii

Plastics One Injection Molding

In the design of injection molded parts, sharp corners should be avoided. Sharp
corners act as stress risers or concentrators, reducing part strength and causing
premature failures. Sharp corners may also effect plastic flow, producing parts with
objectionable surface flow patterns.

Insert Molding Tips

Another problem concerning high stress occurs with molded-in inserts. The plastic melt heats the metal of the inserts. During the cooling stage of injection molding, the plastic part cools, but the plastic boss surrounding the metal insert is reheated by the heat from the insert. This allows the plastic to continue to shrink around the insert, causing excessive hoop stress* that will eventually cause the boss to crack. The better design and process would be to use ultrasonic insertion or a hot probe (such as a heat staking unit) after the molded part has cooled throughout.

* Hoop stress: stress within the circumference of the boss

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