IR annealing Polycarbonate

[RonJon1190]

I am looking to stress relieve injection molded lenses to reduce stress cracking issues. I have read that with a mid range IR heater, you should heat the PC to between 125-130 C and hold there for 60 s per 1/8in of wall thickness. I have done this, and have gotten no results. I am afraid to go hotter and deform my parts. My setup holds the parts 4 in. away from the heater. I am measuring temp. with a thermocouple attached to the part surface.

My part is an aviation lens, shaped like a taco. The walls are pretty consistently .165 thick. It is about 13" long, 5" tall and 2.25" thick.

Any suggestions on how I should proceed? I am completely new at this, so anything is helpful.

Thanks in advance

 

[MikeHalloran]

PC doesn't crack from stress. It can be formed like sheet metal at room temperature.

Its Achilles Heel is >>>Environmental<<< Stress Cracking, wherein a small applied stress cracks the material some time after it is subjected to some environmental agent. ... in fact, nearly any chemical, even diluted or otherwise nonaggressive.

The most likely aggressor in your case is your mold release. Check its documentation for PC compatibility, and try some others anyway.



As for annealing, you can detect locked in stress with crossed polarizers. It would probably be a good idea to set up some large Polaroid sheets in parallel apertures with a simple light source screen for routine inspection.

Most of my annealing experience is with acrylic in 1/4" thickness, which anneals nicely between polished plates in a computer controlled oven, ramping temperature up and then back down in a cycle that takes ~40 hours. All attempts at speeding the cycle, or even opening the oven door while the cycle was running, resulted in locked in stress.



Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

Have you tried moulding the parts at a high tool temperature? e.g. 90 - 100C. Most often it is moulded-in stress is the problem - high hold pressures and too low a mould surface temperature tool.

Of course, PC is about the worst possible choice for any part which you don't want to suffer from ESC!

As Mike noted, a polarized light viewer will give an indication of qualitative stress. Dipping the part in carbon tetrachloride and judging how long it takes to fall apart is another qualitative check.

H



www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.