Thanks! Cold flow also happened on wire wrap (back in the day).
Your situation with the fire ants sounds to me like a serious debugging problem (I beat Dewy to that one!)
Ed Popielarski
Engineering Manager
970 NE 21st Ct.
Oak Harbor, Wa. 98277
Ph: 360-675-1322
Fx: 206-624-0965
Cl: 949-581-6601
https://maps.google.com/maps/myplaces?hl=en&ll=48.315753,-122.643578&spn=0.011188,0.033023&ctz=420&t=m&z=16&iwloc=A
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tontis, Theodore
Sent: Wednesday, May 15, 2013 12:53 PM
To: [log in to unmask]
Subject: Re: [TN] Elastic/plastic deformation of thermoplastic transistor housing
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
>
> [Description: FullLogo]
> 970 NE 21st Ct.
> Oak Harbor, Wa. 98277
>
> Ph: 360-675-1322
> Fx: 206-624-0965
> Cl: 949-581-6601
>
> https://maps.google.com/maps/myplaces?hl=en&ll=48.315753,-122.643578&s
> pn=0.011188,0.033023&ctz=420&t=m&z=16&iwloc=A
>
>
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