I worked on a product that was a plastic enclosure mounted to a metal
cabinet. There was a gasket that went between the plastic housing and
metal enclosure to keep moisture and insects from getting in. Just after
a year we started getting field failures due to fire ants getting into
the plastic housing and shorting out circuits on the boards. We
confirmed the CM was torqueing the housing to the correct spec but the
field returns were coming back with loose plastic housings (We could fit
a feeler gauge between the plastic housing and metal cabinet). It was
found the plastic material we were calling out was going through a
process called "cold flow". How it was explained to me was that plastic
materials can and will continue to change shape (Flow) over time hence
the term cold flow. 

I am not saying this is what's going on in your situation because what
we saw happened over a longer period of time. In any case, the solution
for us was to change the plastic material with a higher glass content,
add a washer to help disburse the pressure from the screw and reduce the
torque requirements.   

Ted T


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gerry Gagnon
Sent: Wednesday, May 15, 2013 1:57 PM
To: [log in to unmask]
Subject: Re: [TN] Elastic/plastic deformation of thermoplastic
transistor housing

Hi Ed,
 
I believe the phenomenon you have is called "creep" or relaxation.
 
Good luck,
Gerry
 

 
> Date: Wed, 15 May 2013 15:57:28 +0000
> From: [log in to unmask]
> Subject: [TN] Elastic/plastic deformation of thermoplastic transistor 
> housing
> To: [log in to unmask]
> 
> I have run across an interesting phenomenon and was hoping someone has
done exhaustive research on it in Technet-ville.
> 
> I have a TO-3PF style package (plastic power transistor with plastic
tab) that has a specific torque requirement and thread-locker material
(microsphere type). When the subassembly is first assembled in a fixture
to align the transistors to the heatsink and hold the nut in rotational
place then torqued, they are verifiably tightened correctly. Once they
sit for a day or so, they are no longer tight and the screw/nut spins in
situ together. To my amateur eye, it seems the thermoplastic is
exhibiting elastic deformation at this point. If re-torqued again, it
stays put, which seems to possibly have crossed the threshold from
elastic deformation ("bending" the atomic structure) to plastic
deformation ("shearing" the atomic structure).
> 
> Is there any insight I can harvest before my 1:00 meeting today?
> 
> Ed Popielarski
> Engineering Manager
> 
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