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Severe shrinkback

7 years 11 months ago #701 by Archived Forum Admin
I am having a severe shrinkback problem on some wire. This wire is a 20 AWG stranded wire with Tefzel insulation. After the part they were installed on was thermal cycled over a large temperature range (-112C to 132C) some wires showed shrinkback on both ends of the wire as large as 3/4 inch. Other wires showed nothing. Does anyone have any idea what could cause such large shrinkback (either wire manufacturing or wire stripping, etc...) Your help/ideas wouldbe helpful.

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7 years 11 months ago #702 by Archived Forum Admin
Hello there,

As you no doubt know, shrinkback is generally the result of inbred stress in the insulation due to shock (cooling) or sudden cooling of some form or another.

Assuming that that your polymer properties are constant and your wires were all made from the same polymer manufacturing run, the usual methods of curing shrinkback are;

- by making sure that the conductor is clean,
- by lowering the insulation melt temperature,
- by using gradient cooling and,
- by using conductor preheating.

Likewise make sure that your cooling water is not subject to huge fluctuations in temperature (repeatability) and that the air cooling distance (crosshead to cooling trough) is always the same (repeatability).

If you are drawing the Tefzel down with a vacuum or just by pulling the cone down, make sure your drawdown ratio is not too large (polymer manufacturer's recommendation) and that your tip and die are always the same size (repeatability). Likewise, make sure that the distance from the exit of the tip to the exit of the die is always the same (repeatability). Finally, make sure that the crosshead is not too hot, that the heaters are all working properely and that there are no hotspots.

Then make sure that your process is standardized and fully repeatable by quick experimentation and longer term monitoring.

If you were to strech the Tefzel on the wire sample during preparation, it would also result in inbred stress and shrinkback so be careful.

Kindest regards,

Peter Stewart-Hay
Stewart-Hay Associates

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