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Anonymous Poster

tightening torque values for metal screws into plastics???????

09/26/2008 1:07 AM

hi, i need to calculate the tightening torque values for metal screws in plastics.

is there any theoretical method to calculate torque values????

If any, please guide me......

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#1

Re: tightening torque values for metal screws into plastics???????

09/27/2008 3:08 AM

I would get me a piece of the plastic you want to screw them in a test it.

fastest way to figure it out.

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#3
In reply to #1

Re: tightening torque values for metal screws into plastics???????

09/27/2008 7:36 AM

To expand on this: I'd torque a sample to failure, then spec 85% value.

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#2

Re: tightening torque values for metal screws into plastics???????

09/27/2008 4:33 AM

Hi Guest,

Contact the producion company that makes the plast type you are using with this question, they will give you all the data you need.

Spencer.

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#4

Re: tightening torque values for metal screws into plastics???????

09/27/2008 9:48 AM

Is hard to find a table for steels...There are so many plastics and changes along time.I'd start look for in plastic cases material suppliers and ask them like SAE when you look for the same in steels: IDES.-

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Anonymous Poster
#5

Re: tightening torque values for metal screws into plastics???????

09/27/2008 3:32 PM

Does this help you out?

http://www2.dupont.com/Plastics/en_US/assets/downloads/design/DCI283.pdf

If you need a value for the tensile yield stress - just post what material(s) you are dealing with.

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#6

Re: tightening torque values for metal screws into plastics???????

09/29/2008 9:40 AM

There are so many varieties of plastics, probably 100,000+ grades, if you include all the variations of fillers and additives, that there are no simple calculations possible. The torque value also depends as much on the part design, method of manufacture, and the type of screw, as the plastic. Will it be an injection molded, extruded, or machined part? Will it be a self-tapping screw? If so, will it be thread cutting or thread rolling? Will the hole be tapped before hand? If so, will the threads be cut or molded in? Will the hole be into a solid wall or a raised boss? What type of plastic will be used? Will there be any fillers in the plastic (glass, carbon, aramid fibers, minerals, etc.)? Will the threaded hole be formed by a brass or stainless steel threaded insert? What is the screw size? All of these things will affect the torque value and there are no calculations that I know of that take into account all these factors. A good staring point would be to look at the material manufacturer's information (see the link posted by Guest for DuPont), and also the screw manufacturer's torque tables. But remember, they don't take into account all the variables and are most likely tested as close to ideal conditions as possible. They want their products to look better than their competitors, so they won't publish the "worst case" condition. I would use no more than 85% of their published data. Before casting anything in stone, I would get samples of actual finished parts and a variety of screws and torque test them. Even then, don't necessarily go with your tested result, because lots of screws and plastic parts all vary within certain tolerances. I would use something like 85% of your tested result. It seems like a lot of work, but it is less work than having to go back a year or two later and try to diagnose why the screws are suddenly coming loose or the plastic is cracking when it has always worked fine before.

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#7

Re: tightening torque values for metal screws into plastics???????

10/04/2008 7:40 PM

The first question is : what type of screw you use?

Does it cut or form the thread?

The tightening torque should consider as well the prevailing torque as the ruin torque. there are several recommended equations but the best is to make a series of measurements for both torques and take a value some where in the middle.

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#10
In reply to #7

Re: tightening torque values for metal screws into plastics???????

01/11/2021 11:38 AM

That's right - is it a self-tapping screw into a drilled pilot hole, or a machine screw into a threaded hole, or what?

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#8

Re: tightening torque values for metal screws into plastics???????

01/06/2021 7:36 AM

Any <...theoretical method...> needs to be based upon the temperatures that the <...plastics...> will experience in service.

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#9

Re: tightening torque values for metal screws into plastics???????

01/06/2021 10:08 AM

First you need to determine the holding force needed of the part...then it's all about the size of the screws, and the number used, necessary to meet that qualification...It might be easier and cheaper to make a snap together connection rather than using screws...

https://www.reddit.com/r/AskEngineers/comments/3n6dp1/how_to_calculate_max_bolt_torque_for_metal_bolt/

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Anonymous Poster #1
#11

Re: tightening torque values for metal screws into plastics???????

01/18/2021 9:45 AM

Flog it up until it strips, then back it off 1/4 of a turn.

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