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Fabricating plastic parts?

4/25/2000 5:35 PM
Fabricating plastic parts?
I guess this is sort of off-topic, but I don't know where else to ask...I need to replace a small plastic part on a mixer. It's a "tray" that the fader sits in, keeps the fader isolated electrically from the board and stabilizes it physically. It isn't available from the manufacturer. I got the bright idea to make one up, so I made a slightly oversized Play-Doh mold, greased it up with silicone spray, and filled it with epoxy. It made a perfect replica of the part, but it shrunk about 30%, so no go. I could buy some plastic and glue it up, but I'm not sure which plastic I need (nylon, polyester, polypropylene, etc) or what glue to use? Or should I try the mold again, using something different? Thanks for the help...
4/25/2000 6:02 PM
I have had some success molding parts from auto body  
Hobby shops have special plastics for molding.  
They also have styrene sheet, and glues for it,  
if you wanted to go that way.
4/25/2000 9:11 PM
Mike Burgundy

If you want to go all-out, styrene sheet (the hard kind) as well as any other medium-melting thermoplastic can be shaped in a (say, wooden for example) mold by heating it carefully with a paintstripper (wear gloves! wear gloves!). Works great on convex surfaces, concave is trickier but if you drill through the mold and take care not to enclose air anywhere it'll work. Oh, and remember, it also tends to shrink under heat a bit, so use generous overhang (nicer handling too). Maybe that'll do?
4/26/2000 4:10 AM
Mark Hammer

I'm not clear where the "shrinkage" is occurring. Does the molded piece itself shrink, or is it simply conforming to a shrunken mold? Having two kids who have taught me much about the properties of drying Play-Dough, my first inclination is to assume the latter, but then I know nothing of the properties of the filler material. If it is a "changing-mold" problem, then try using plasticine. It is easily shaped, relatively stiff (moreso if cooled), and won't shrink if left out uncovered. You can generally find it in art supply places, hobby stores, or better quality toy stores (look for places with no Barbie and lots of 's in the toy names).  
Finally, hobby stores that cater to the model train crowd have all kinds of plastic sheet material that is useful for more than just miniature train stations and barns.  
I commend your industry and diligence.
4/26/2000 4:26 AM
Eric H

This caught my eye, too 30 percent shrinkage with epoxy doesn't seem normal. The 2 part casting resins at home depot sound good --with the plasticene mold already mentioned.  
4/26/2000 2:33 PM

You're probably right about the mold shrinking. Living on the outskirts of Podunk, SC, plasticine would qualify as a controlled substance here, while Play-Doh is easily obtained by driving 30 miles to the nearest town.  
I'll give it another shot using the right stuff; thanks for the advice. BTW, Bubba's does sell toys (and guns, bait, and beer) but there aren't a lot of those funny "O"'s in Bubba...
4/27/2000 3:41 AM
Steve A.

    Look for a two-part epoxy putty that says "no shrinkage" on the label. You can find this stuff in the plumbing section of a hardware store or at a pool supply house. I've used it for various repairs and there was no noticeable shrinkage (even under water, George!)  
Steve Ahola

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