Best Standards/Guidelines for Driving Screw into Plastic?

[RaySh]

Hi all,

My first post here, so I am not sure of forum etiquette, I will try to provide all necessary background to my question.

I am currently looking for a standard/guideline that I can use to determine the best driving torque for a screw. This is for an application where a hi-lo screw is being driven into a plastic stand-off. The two pieces being driven together are a sheet of metal, and a sheet of plastic which has the previously mentioned plastic stand-off molded onto it.

Not looking for a super specific solution, although if someone is willing to take a crack at it I can provide more details. I was more wondering if there exists some sort of literature that can be used to find a solution to my question.

Thank you,
Ray

 

[TugboatEng]

This varies a bit based on the type of plastic but I find, when installing screws directly in to plastic, you're not torquing it like you would a bolt where the tension holds it in place. For plastic, friction between the screw should hold it in place. Torque should only be applied to run the screw and tightening stops when the screw has bottomed out. This will help prevent cracking of the components as they age.

However, if the plastic is polycarbonate, the stress crested by the interference fit alone will cause the parts to crack with age. I suggest a different fastening method for polycarbonate.

 

[dgallup]

There are screw thread forms specifically designed for plastics. Most (all?) of these are proprietary. The one I have used is called Delta PT from a company called EJOT. They have all the engineering information you need. We screw them into glass filled nylon 6/6.

https://www.ejot.com/Industrial-Fasteners-Division/Products/DELTA-PT®/p/VBT_DELTA_PT

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[TheTick]

Always use Torx head whenever possible.

Maximize material on plastic standoff.

Each screw has its own spec'd hole diameter. 99% of the time, you want exactly that.

Room temperature is best.

Make sure you have adequate lead-in, enough thread engagement, and space for chips to fall.

Use the bigger screw. You won't regret it.