Annealing PC as a forming process?

[gwr2]

We are manufacturing a curved face shield out of Polycarbonate. It is die cut from a flat sheet of Makrolon GP. The frame that it gets mounted in has a simple bend around 1 axis, forming a cylindrical shape. Easy enough shape to make. Is it possible to build a fixture which holds the flat blank in this simple curve and anneal the plastic to permanently set the curve? It appears to work, processing at 130C for 1.5 hours but I am concerned that this is an unconventional technique and problems will arise months later that we are missing today.
We have tried drape forming, but the process seems less reliable. The material cools so quickly, slight variations in handling time coming out of the oven cause varying results. We have limited resources for heating and forming.
Leaving the lens in the flat state and installing it stressed into the frame has shown much more sensitivity to environmental stress cracking. We need a way to effectively form.
Thank you for your help. I have enjoyed reading other posts on the subject of forming and ESC, you guys have the great knowledge base going.
Best regards,

 

[Pud]

Sounds ok to me. 130C is around the Tg of PC. Your comments re the better ESC would indicate that it is actually the correct way to do it! Sheet PC is extruded so residual stress free - you were adding residual stress by bending it.

PC is hygroscopic, so I would guess the only thing to watch for is outgassing if a lot of moisture in it and heated too quickly.

Additional info: PC pellets for moulding are dried at 120C, so you're not much above that.

H

www.tynevalleyplastics.co.uk

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

 

[gwr2]

Thanks for the feadback.
Today we are noticing a few pock marks, much like a fish eye mark on a spray painted surface. I'm guessing this is moisture related, as you indicated. Since we do have 2 ovens available, we are going to try to pre-dry at 120C for an extended period, then process for stress relieving in a curved form at 130C.

 

[Demon3]

You can actually see those stresses under polarized light. E.g. buy a polarizing film from Amazon or even using polarized sunglasses. That means you have a way to watch and know when the stresses are gone and you can stop the annealing process.

Chris DeArmitt - PhD FRSC

Plastics & Materials Consulting

http://www.phantomplastics.com

Plastic Training Seminars

http://www.plastictraining.com

 

[gwr2]

Good Point. We'll look into the polarize analysis tomorrow.
Do you have any idea when stress becomes an issue? If a part is effectively heated and formed to a 3" radius, but has to be installed in a 2.5" radius frame.... is this enough stress to have drastic ESC? Basically, How close is close enough?

 

[MikeHalloran]

The problem with PC is that it's subject to ESC even when annealed.

The worse, absolute killer, problem with PC is that even a short exposure to a mild solvent, even if immediately wiped off and washed, will leave it substantially weakened, so much that even a small stress applied after exposure will shatter it.

I suggest you buy a sheet of ~.020" PC, shear it into small rectangles, and experiment with rapidly dipping it into common chemicals and then bending it with your fingers.

That's why I think it's criminally irresponsible to use PC for, e.g. helmets, and for face shields.



Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

A comparative, subjective test used to be immersing in carbon tetrachloride. Watch parts moulded on a cold tool fall to bits.
Carbon tet probably banned now though...
H

www.tynevalleyplastics.co.uk

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

 

[gwr2]

What would be the best suited material for a safety shield? Everything is a give and take, other plastics will have their own inherent functional limitations.
Thoughts on transparant ABS, Impact modified PMMA, or Styrolux?
considering that inevitably the end user might disregard warnings, and choose to use harsh cleaners or disinfectants.

 

[Demon3]

Transparent ABS e.g. Terlux, would be a good option as it has chemical resistance from the SAN matrix.

Impact PMMA would be similar in mechanical performance. I suspect chemical resistance not as good as transparent ABS.

Styrolux is a block copolymer of styrene and butadience. Not as good chemical resistance as the transparent ABS.

Chris DeArmitt - PhD FRSC

Plastics & Materials Consulting

http://www.phantomplastics.com

Plastic Training Seminars

http://www.plastictraining.com

 

[Pud]

You could investigate PET or PETG sheeting. (I feel sure I have seen PC clad with PET somewhere, but I might had been dreaming...)



www.tynevalleyplastics.co.uk

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