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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