Screws into Plastic

[drawoh]

I am investigating a thermoplastic plastic housing in which some hex socket cap screws are known to come loose. I have some existing units to play with, and I have found that it takes minimal torque to remove the screws. I am recommending a torque wrench, and a change to hex socket button head cap screws or Torx pan head machine screws, which will clamp a larger area. The plastic very likely is creeping under the screw heads.

I have observed that while it takes very little torque to undo the screws, I feel a sharp snap or break when they come loose. We can see no sign of threadlocker on the screws. Could anybody out there have an explanation for this?

 

[TugboatEng]

If it's creeping then the plastic may be conforming to the surface roughness of the screw head. When you break it loose it has to shear off any interferences.

If it's polycarbonate, PC is very sensitive to cracking and the material itself may well be fracturing when you loosen the bolt.

Are the screw threads in plastic?

 

[drawoh]

Tugboat,

The parts have metal thread inserts. The plastic is a UV resistant grade of nylon.

 

[TugboatEng]

Nylon isn't very dimensionally stable, especially with regards to water absorption. This presents a challenge for maintaining screw tension. I suggest considering the use of Belleville spring under the head of the fastener.