UHMW PE trouble

[jdolan707]

I an a bit of a thermoforming novice, but have had some luck with forming 1/8" ABS. I was given the advice that Ultra-high-molecular wt. Polyethelene would work well at a fraction of the cost. I bought some (it is much cheaper), but am having difficulty forming it. It takes a long time to sag (get hot), and then seems to twist and turn as it cools. The parts are pretty much deformed by the time I have them off the mold. Ideas? What am I doing wrong? Thank you.

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[rugburn]

You have two totally different materials there.
The ABS has one direction of shrinkage, whereas the UHMWPE has biaxial shrinkage, meaning it shrinks in the extrusion direction as well as across the width of the sheet.

In order to combat this excessive shrinkage and twist, you need to understand HOW much the material shrinks as well as how to control this shrinkage.

First off, most polyolefins shrink about .020" per inch in BOTH directions, so your mold must be built to accomodate this.

Secondly, the mold must be cooled to reduce the shrinkage either by cast in water lines or by cooling the part off of the tool with a mist of water on both sides of the part. Clamping the part along all four sides to reduce the twist you are seeing can help as well.

what are you trying to make??????

john

[jdolan707]

Thanks for the reply John. I'd be glad to send you some pictures if you'd like. My e-mail address is jdolan707@aol.com. I am trying to form a cover, or protector for a trailer fender. They come out pretty well in ABS. I am using a thicker material in the HMWPE than the ABS though. The ABS is .093", and the PE is .125". My tool is primitive. It's a fender I have wiped it with a damp cloth to cool in between pulls. What would be the ideal tool temp? Even when the PE looks decent after molding, when I trim it out, they "spring" all over as compared to the ABS. Again, thanks for your time. Jim

[rugburn]

Cooling the part and trying to control the deformation after rmoving from the mold is the target.

If you can clamp all four edges down after removing from the mold and try to cool ie evenly with two fans; one from below and one above, you may eliminate SOME of the warpage.

I have even sprayed water on HUGE parts to cool them suitably.

Either way you go, you will still see deformation.

Hope this helps you out.

[jdolan707]

John,
I took your advice and focused on cooling the parts completely, and have seen huge improvments. Thank you. Jim

[rugburn]

awesome to hear.