By Laurence Claus, NNI Training & Consulting
Why is fastening into plastics such a challenge? After all, since it’s softer than the steel screw used for the joint, it should be pretty easy to form threads. On one level, this is an accurate observation. But when you look deeper you will find that the ease of joint failure through stripping and loss of clamping loads are commonplace.
The second of these two challenges – the loss of clamp load over time – is particularly vexing for designers of plastic joints. It stems from the material’s innate visco-elastic properties. This means that the material behaves both like an elastic solid, and a viscous fluid.
In everyday parlance, it means that when stressed, the plastic material will want to quickly shed this stress, and thus undergoes rapid stress relaxation. If the magnitude of this relaxation is great, the designer is left with a joint that no longer has much clamp load. And a joint without clamp load is a loose joint. Read More here.