Polishing UHMW

[mingmongmang]

Hi,

Can anyone recommend a polish/technique to get a very smooth surface on a UHMW? Ideally something easily obtainable and not requiring a particular skill.

The part is a mold and I'm hoping a smoother surface will improve release of shore A 40 silicone parts. Currently the surface has machining and sanding marks in it and it's not releasing even with various mold release agents.

Thanks!
Jon C.

 

[btrueblood]

Wave a flame over it?

 

[CorBlimeyLimey]

Can you post an image of the cavity or at least it's profile sketch?
Is it an open (split) cavity?

The finishing process will be determined or restricted by accessibility to the cavity surface.

 

[1gibson]

High speed buffing wheel with a generous amount of pressure applied? Heat from friction should soften the material and help polish, especially at the rough surfaces. Flame is not a bad idea but would only gloss over the grooves, not necessarily eliminate them.

 

[MintJulep]

The very best way to get a smooth finish on UHMW is to not make it rough in the first place.

 

[MikeHalloran]

You can't polish UHMW.

You can waste a lot of money trying, of course.

Instead, spend the money polishing the tools that cut the cavity, bring their edges to razor sharpness, and never use them to cut anything else.

Then machine the cavity in a few passes, with the last/finish pass being not less than .003" (or maybe .005" or even .010", depending on the material particulars).

I.e., you can't sneak up on the size you want in UHMWPE, with skim cuts or abrasives; you have to cut right to size while producing a chip that is fairly strong and thick.



Mike Halloran
Pembroke Pines, FL, USA

 

[1gibson]

Just found this, first page of google results for "polishing UHMW"

http://www.connecticutplastics.com/qa/plastic-polishing-questions/

"I have a transfer mold for pulp & paper that requires a smooth finish to prevent sticking. The material is white UHMW. Is this material suitable for polishing? If not, can you recommend another material?

UHMW can be polished through buffing. Depending on the plastic part configuration, we can achieve a high finish."


Now how well it works is another issue and I am not making any claims about that, but this is your best bet. Before scrapping the mold and making a new one, it is worth a shot to take an angle grinder with a buffing wheel to it.

One more off the cuff suggestion, buff it with oil in an attempt to impregnate the plastic slightly. Probably won't work all that well but again, could be worth a shot.

 

[mingmongmang]

The mold's built up from 2" sheets and it's not split so not much access to internal surfaces (see attached).
I managed to get a nice shine on a small test area using 600 thru 1500 wet sanding then coarse and fine acrylic 'scratch remover' polishes followed by red rouge on a drimmel tool. Lots of labour :/
Has anyone actually tried flame polishing UHMW? I've heard it's quite easy to damage/oxidise the surface by going just a few degress beyond melting point.

 

[CorBlimeyLimey]

Is there no draft on those faces?

You wouldn't have to use an actual flame. Other heat sources would also work, and may be more controllable.

 

[mingmongmang]

Yup there's draft.
I guess I'll experiment with a heat gun on some samples..

 

[mingmongmang]

OK heat gun failed. On max temp it was putting way too much heat into the (sample) material and warping it before I could get the surface to melt. Melted surfave was nice and shiny tho..

 

[cadcoke5]

My idea would be to machine the orignal as a positive using a material you can polish, and then make a casting of it using melted UHMW. That casting would then become your mold.

I have no idea if UHMW can be melted and remolded, and then retain all of its original properties.

Joe Dunfee

 

[dgallup]

I thought you could only extrude UHMW. Why not use something more conventional for the mold like steel or aluminum?

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[mingmongmang]

I'm making prototypes in the mold and can't wait weeks for a replacement, I need to make this one functional.

FYI UHMW is machinable, about as slippery as teflon and one of the cheapest plastics out there (1/4 the cost of aluminum(?)).

So..it seems a flame is all that's left to try (or possibly a really hot heat gun?). Any ideas on what type of flame?

 

[Compositepro]

A propane torch is suitable for flame polishing. I'm not sure that it will work for UHMW because the high molecular weight means that the polymer never truly melts and becomes fluid. Flame polishing works because the surface will form a fluid film that levels-out imperfections due to surface tension. The heating has to be intense to melt the surface film without transferring significant heat into the bulk. You will get some oxidation. This same process is used to treat polymer surfaces to improve bonding.