Technique for flattening bowed HDPE plastic piece?

Hi all,

I milled a very specific vacuum box cover out of HDPE (originally about 12mm thick, milled down to 4mm with holes throughout except for 4 pegs that rise 3mm above). It was pretty flat when the job finished, but now it’s bowing on both major axes:

I observed this problem much worse in a thicker job that ultimately failed due to the issue having a large enough effect on the middle of the job that the Z-height no longer worked. Is this a shortcoming of HDPE?

The more pressing question is whether there is a way to fix this. I know relatively little about plastics. I have a custom PCB reflow oven with fine temperature control that I can easily “bake” at any temperature from 30 C up to about 250 C for any length of time. I read that the melting point of HDPE is around 130 C, and it seems like I should probably be able to bake it for a while at some temperature lower than this and just let gravity flatten it out. Or maybe that won’t work for some reason.

Anyone else had to solve this problem before?

Yes. They make stress relieved HDPE, but it’s not as readily available.
Generally, when you thin out a sheet enough, it will warp due to those internal stresses (like many materials).
Try machining both sides to your final thickness rather than just one. Could you take off a tiny bit of the other side at this point?
The heating might work. Might need more than gravity, though. I’d do some research on thermoforming HDPE.

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If it will fit in the PCB oven heat it up with weights holding it flat until it cools. You won’t need to reach the melting point to flatten it. Just make sure you don’t deform the nubs sticking up. You could probably do it on an asphalt parking lot mid day. Lay a sheet of aluminum foil down to protect the plastic and weight it flat and the heat from the parking lot will work too.

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Or the dashboard in a hot car.

These folks have a helpful looking list of what temperatures different plastics would need to see for how long

https://www.boedeker.com/Technical-Resources/Technical-Library/Plastic-Annealing-Guidelines

They also indicate that it’s preferable to have an inert gas or oil but that’s probably less feasible for home use, although the PCB oven may be easier to gas purge if that’s really necessary.

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I work with solid surface (acrylic based countertop material) at my regular job and we do thermal form it. In the oven at 350 and then to a fixture or mold for cooking. I have placed warped material in the oven and let it flatten out, then shut it off and open the door to let it slowly cool. It would probably work with that but as the other gentleman stated I’d probably put some kind of weight on it during cooling. That piece seems a little thinner and may curl during cooling. And remember whatever you use for weight should be flat as possible because I’ve found it will get indentions when hot.

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I’ve had it happen too. The ticker the material, the more pronounced the warpage. If I can I will cut both sides to combat it.

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Damn voice to text….

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