UHMW and humidity absorption

[PatCouture]

HI,

I have used UHMW-PE for the manufacturing of three parts, one that is 7 inch wide by 24 inch long and 3/8 thick. Inside of this parts I have two holes of 5"1/2 diameter. then I have two other identical parts, who looks like donuts that inserts in this hole with the same thicknesses than the first one. They have a 0.010 press fit tolerance when inserted in the hole.

Since UHMW-PE is reasonably malleable we have no problem inserting the parts manually. The problem is that these parts are used in a tempered environment at 4°C with a lot of humidity. And after a couple of weeks we are no longer able to insert the parts manually. So I wonder if it's because the material absorb some humidity and then he changes dimensions.

What do you think about it? Am I right? what type of plastic material who is FDA approved that would be much stable?

Thanks in advance for any help.

Patrick

 

[diamondjim]

How far apart are the 5 1/2 diameter
holes? Are the donuts also plastic
and attached to a plastic part?
Why are you pressfitting these .010
tight? Could one be a pressfit and
the other to size? I do think humidity
would change the parts. It might be
cheaper to have a second jig for the
extreme temperature range.

 

[PatCouture]

The donut is 5"1/2 ext and 4"1/2 int. All the parts are in UHMW. I am pressfitting them to be sure that they can't move inside of the holes. If I'm clear there is two donut indentical of say 5.5100" that are inserted in the sheet that have a hole of 5.500". the problem is not having one for the extreme temperature but to be able to know the exact dimensions when we manufacture at normal temperature to be able to have the correct fit once we put the parts in the tempered environment.

Patrick

 

[EdDanzer]

UHMW does not absorb water, but it does stress relieve and change shape, and has a high coefficient of expansion. You shape could be causing thermal distorsion. Try rough machining the parts, cooling them to operating temperature and finish machining them when cold.