UHMW Warping

[knowlzy0791]

Could not find a truly appropriate place to post this issue (my apologies, the google has failed me for the first time).

I bring in pallets of UHMW (1/4"X48"X120") in quantities of 40-50 per pallet and 3-4 pallets at a time. I bring one pallet into my shop and store it on a rack that we have built to be perfectly (eye balled and leveled) flat with 1/4" steel.

I have tried both leaving the pallet on the shelf and hand unloading the sheets directly onto the flat steel. Over time, and sometimes not very long at all the UHMW begins warping. Sometimes it creates a wave, sometimes the corners warp up (I could understand gravity pulling them down but the go opposite, almost as if the center is being pulled down.

There seems to be a pattern with it. Once one sheet begins warping, they all follow suit. It is never in the same spot or to the same degree.

Does anyone have any idea how to stop this or minimize this from happening?

[weber765]

Many plastics warp from internal forces over time, I would suggest contacting the manufacturer. I have had this problem with uhmw, MC 901 nylon and polypropylene, and several others. I have never found a way around it and it can get MUCH WORSE after machining it.

Glenn

[Gundawg]

I find the thinner the material (UHMW) the more it warps. I have not had the same problem with the thicker material 1" thick or thicker. Good luck you can heat it to try and get it flat.

Mike

[knowlzy0791]

Yeah I have noticed my 1/2" and 1" don't warp nearly as bad. It becomes quite a pain when it can ruin 2+ parts at over $20 per part on a sheet. Every sheet. 10+ sheets a day

The new pallet that was brought probably didn't even take 2 hours and it is warped and wavy already.

[BernieNUFC]

youre not having temperature change related issues are you?

[JimmyD]

I use it, but have not had any of the issues you're having. I don't use 4 x 10 sheets either.

I am an engineer in the aerospace world so perhaps I can help you find some of the causes.

What are the ambient temperatures outside and inside your shop?
Does the issue happen at all times of year?
How long was it in transit before you received it?
How long was the pallet outside before being moved inside?
How long are you letting the pallet sit inside your shop before unbanding it?
Is it appropriately flat on the shipping pallet?

I suspect the issue is the change in temperatures as it goes from the manufacturer/warehouse to truck to your yard to your shop to your rack. A 50 sheet pallet would have quite a thermal mass and the changes in shape as it is shipped/stored may be occurring because the induced stresses from the temperature changes (manufacturer/warehouse to truck to your yard to your shop to your rack) haven't had time to equalize prior to you putting it on the shelf where it has minimal thermal mass and minimal weight on it to keep it flat. Hence it warps.

Without knowing any of the above data, I would recommend bringing a pallet inside and leaving it banded, stacked, untouched for about a week and then see what happens. I recognize that you may not have the space nor the luxury of time to try this, just a suggestion.

Jim

[Gundawg]

Do you use vacuum hold down to hold the material while you machine it? Are the parts not good because the finished part is warped? How big are the finished parts? I am just curious I have not cut any 1/4" but I would think my vacuum table would suck it flat if it where 1/4"

Mike

[knowlzy0791]

Sorry for the delayed response.

Temperatures outside vary, I'm in the midwest so season depending it's anywhere from -30s to upper 80s F. Inside at night is 50 and working time is 65 F.
The issue seems to have become a norm of sorts. Kind of just always there, worse now to weather changes I imagine.
It takes me about an hour to pick it up from manufacturer and bring it back here.
How long it sits outside depends on sales, the more we sell, the more I make, the faster it comes in. It might be outside a few days, might be a month.
Usually when I have to bring a pallet inside it means I need parts, so I do not have the luxury of letting it sit inside while still banded. Unless it happens to be on a Friday.
It's not terribly warped on the shipping pallet, I did see an improvement when I started removing the sheets form those onto my flat rack.

I do use a vacuum hold down, but it just is not enough for this stuff. It's a regen blower too, just spent a few grand on it hoping to never have any issues with this or sliding parts again. Naive I know. The parts vary in size, from a few inches by a few inches to h a half or whole sheet. The finished parts don't show the warp nearly as much, but I also bend most of them so that kicks it out. I am really surprised at how long it takes some of these sheets to actually be held down by the vacuum. A few times there was no changing it, I had to use my pressure foot, but still lost a few parts.

These are always going to be issues when we are pushing the max on machine speeds, trying to keep up with orders in the hundreds, if not thousands, and only getting a MAX of 24 per sheet. It can get crazy.