HDPE Cupping after Milling Pockets

[919RickR]

Hello All
I am milling 3/4 HDPE, and after pocketing, the material is cupping. I'm sure the pocketing is relieving stress in the material. The size of the finished blanks are 3 1/2 X 6 inches. In one piece I am pocketing 2 1/4 X 4 3/4 X 3/8" deep and the other piece is 2 3/8" X 4 7/8 X 1/2" deep.
So my question is ;
Is there any method of relieving the cupping after I do the milling?
I have 400 sets of these to do.
Thanks in advance

[knight_toolworks]

it is hard to find a material that does not warp when you remove only one side. the softer plastics are pretty bad I have pocketed most of them acrylic is about the only thing that won't warp in plastic.

[gundog]

Standard HDPE is not stress relieved try one of the marine boards like Seaboard or Starboard I machine pockets in the stuff all the time with no cupping. The marine polymers are stress relieved and cost just a little more but they machine better and have better properties.

Mike

[mkcfl]

I cut a lot of HDPE Starboard parts and it is a bit random which parts that cup/bend or warp. I try to store the parts on a flat surface and assemble them as quickly as possible. Some larger pocketed parts behave better with a depth less than 0.2" in my experience.

[BrandanS]

A common metal machining trick is to fly cut the opposing surfaces giving you issue. Are you able to do that prior to running your pockets?

Are you waiting for your mat'l to cool before unclamping?

There are a few tricks I've learned over the years, but it is always difficult to bring a part back once it's been machined and found to have a dish or bow.

Here's a little article, in case you're unfamiliar. Now it's intended audience is for metal workers, but maybe you could apply the theories :)

Chapter 11: Warping

https://books.google.ca/books?id=0ffbBqr2fr8C&pg=PA210&lpg=PA210&dq=Relieving+material+Stresses+machining+bow&source=bl&ots=7RLvKhHSOj&sig=YzYIyh1lttViUY3US-D8bftsg7o&hl=en&sa=X&ved=0ahUKEwjhwNWltafNAhVIxoMKHee3DO8Q6AEIKTAB#v=on epage&q=Relieving%20material%20Stresses%20machining%20bo w&f=false

[919RickR]

Thanks to all that replied to my issue with HDPE cupping.

BrandanS machining opposing surface is not an option. This is a prototype case that will be a slick finish, and or a pebble finish.

I'm just trying to fix an issue before it becomes one. I will check out the site you provided

Thanks

Rick

[Rick W]

Rick,

Why don't you try annealing the plastic by putting it in an oven on low temperature for a while.

Not sure this will work with HDPE or not, but we used to do this with lexan.

Just a suggestion!

Rik