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UHMW Polyethylene

John King

John King

Joined
Aug 31, 2006
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330
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Location
Montgomery, TX
Website
www.gulfcoastwoodturners.org
I have a couple of sheets (0.5" x 12" x 12") ultra high molecular weight (UHMW) polyethylene that have warped/twisted. They are no longer suitable for making jigs because they do not lay flat.

Any ideas/thoughts on how to flatten the sheets? - John
 
John,
Polyethylene is a thermoplastic, which means you should be able to thermally creep flatten it. However other household member(s) may not appreciate this use of the oven, even though you only need (maybe) 130-135F. Heat lamps or hair dryer might work. The idea is to place the material on a sturdy flat surface and heat it very gently, just enough to relax the internal stress so it will sag onto the flat surface. Surely others will have some good suggestions.

For ref. my experience is based on vacuum creep flattening 4' x 8' sheets of 1/4" titanium, so may not apply exactly to UHMW PE.
 
Richard purty much nails it

I've done it by placing the UHMW on plywood in hot (summer) sun and watched it flatten out. It can take anything from a few hours to several days
 
Assuming they warped from improper storage, e.g. subject to gravitational sag, and not internal flaws, they can creep back to flat by reversing the orientation. Of course, that would take the same amount of time.

The oven trick could work, but I think it needs a higher temperature. I've laminated HDPE (poor man's UHMW, and almost as slick) from flat milk bottle sides, into solid sheets at about 350F. I place the pile between two sheets of 1/4-inch aluminum, with a stack of washers or coins at each corner. The stacks are about 10% greater than the target thickness, to account for cooling shrinkage. I put 20 pounds of lead on the top sheet (because I had that much on hand), heat the whole affair for a couple hours, and let it cool overnight. My niece, a chemical engineer, assures me the increased opaque appearance is not a chemical change.

I wouldn't be able to get away with this if I were married. A married friend of mine gets unlimited grief from washing car parts in the kitchen sink.
 
Joe Greiner said:
Assuming they warped from improper storage, e.g. subject to gravitational sag, and not internal flaws, they can creep back to flat by reversing the orientation. Of course, that would take the same amount of time.

The oven trick could work, but I think it needs a higher temperature. I've laminated HDPE (poor man's UHMW, and almost as slick) from flat milk bottle sides, into solid sheets at about 350F. I place the pile between two sheets of 1/4-inch aluminum, with a stack of washers or coins at each corner. The stacks are about 10% greater than the target thickness, to account for cooling shrinkage. I put 20 pounds of lead on the top sheet (because I had that much on hand), heat the whole affair for a couple hours, and let it cool overnight. My niece, a chemical engineer, assures me the increased opaque appearance is not a chemical change.

I wouldn't be able to get away with this if I were married. A married friend of mine gets unlimited grief from washing car parts in the kitchen sink.
Click to expand...
Yea, they frown on washing shotgun shells in the clothes washer also.😱
 
I used to work in a sign factory and we heated the plexiglass to 350 degrees before forming it. I've done the same thing with thinner plexiglass in the oven but it doesn't take that high a temperature. I would put it in the oven on a flat sheet and heat it gradually to different temperatures. You will probably find a heat setting that will flatten it. If it starts to curl put a weight on top of it. Once it's flat let it cool completely.
 
I have decided that UHMW plastic is not very good for jigs anyway because of the fact that it can gradually deform after repeatedly being subjected to loads. It is OK to use it where it is mainly providing a slick surface, but not a structural member of the jig or fixture. For example, I have an aluminum fence on my tablesaw that has an UHMW strip on its face to reduce friction. In this instance, the UHMW is not providing structural support to anything. I also haved a router fixture that is made mainly of UHMW and has a toggle clamp mounted to a strip of UHMW -- this is NOT a good use of UHMW. A material that that I now prefer for jigs is black bakelite sheets. It comes in various thicknesses and can be bought from McMaster-Carr as well as various other sources.
 
Bill, Is the material you buy reinforced? Most unreinforced plastic resins will "creep" under continuous or sustained load. The amount of creep varies widely with the type of plastic and applied load, of course.
 
Anything will creep under sustained load, even steel and concrete. It's part of the design process for really big structures, especially prestressed concrete bridges, where we try to balance the two opposing creeps.

Joe, PE/SE
 
Ok,

I'll admit that I'm a chemist with 20 years of PE manufacturing experience (HPDE included). The sheet is deforming because it has strain formed into it. Throw some saran wrap in the fire next time you're camping and see it in fast-motion. It won't creep if it doesn't have any internal strain (unlikely) or if it's mucho thick, or if it never gets hot.

You can try strain relieve by heating it somewhere between the softening point and melting point http://www.sdplastics.com/hdpesheet.html
(Look at the heat deflection temperature and the vicat softening point) Note this is not UHMW, the melt index is 0.70.

THen you'd have to cool it in such a way that it won't have any strain left (slowly). Sounds like too much trouble to me. I'd use a thicker piece or switch to a different material.
 
balance the two opposing creeps.
Some days that's what we do on many of the message boards. Fortunately that rarely happens here. 🙂
 
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