Boss Cracking on a Polysulfone Part

A recent question was received via e-mail…

We are encountering cracking at the bosses on a polysulfone part. We press fit inserts on the bosses but we don’t see cracking during insertion. It is not until a day or two later that we see the cracking. Any thoughts?
My Response
The occurrence of cracking and crazing in this case is most likely due to the factors of molded-in stresses and strain.
Since polysulfone has a high degree of semi-crystallinity and rigidity, the resulting parts have many molded-in stresses… especially in the case of a boss. Many injection molders will anneal the parts with a heat cycle to reduce these molded-in stresses. Doing this before and after the insertion would significantly reduce these stresses.
Reducing the strain during insertion will also reduce the likelihood of cracking and crazing. Using a barbed, threaded, or trilobal insert may reduce this strain. The use of ultrasonic insertion or in-mold insertion will also reduce this.
Additional Thoughts
It may also be a good idea to mold the part in a translucent material so that you can see where the stresses are present under polarized light.

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