delrin/acetal remelting

Howdy guys,
hope this question is not too far out of the scope of this newsgroup, but it
appears to be the best newsgroup for working with delrin discussion.
I have a pile of used acetal/delrin scrap that I've collected. All the same
item, from the same manufacturer, so no blends of brands/types.
I've googled acetal/delrin to death and cannot find any mention of
recycling it into new items. Is it safe/easy to melt and pour into a simple
cast? I want to make it into 4" x 4" x 2" blocks to then machine
I'm aware that melting starts at 180 deg c and Formaldehyde is released at
230deg c.
Any suggestions on making a simple mold? Will a machined out block of
aluminium with a bolted on bottom (think of a tube with a cap at one end)
that can be disassembled be good enough for mold release?
Any help is greatly appreciated,
Regards,
Mark
Reply to
Mark Hathaway
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Acetal is a thermoplastic amorphous polymer. It melts around 350F but melted amorphous polymers do not flow well enough to be cast. You will not be able to get it into one homogeneous mass. Acetal castings are made from resin.
Reply to
Glenn Ashmore
I googled "injection molding acetal conditions" because I'm pretty sure you're not going to melt and pour it. The first hit is this site:
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Scroll down and you'll find acetal. It might reprocess o.k., don't know, but the scrap would probably have to be shredded and dried as a minimum. Drying conditions are listed on the site, and are probably specified for 1/8" or so pellets.
Pete Keillor
Reply to
Pete Keillor
"Acetal is a thermoplastic amorphous polymer. It melts around 350F but
melted amorphous polymers do not flow well enough to be cast. You will not be able to get it into one homogeneous mass. Acetal castings are made from resin. "
Acetyl is not an amorphous polymer, it is crystalline. It will turn thin once melted
I think that will release from the mold fine because it will pull away from the walls by about .020" per inch.
I think you may not like the end product. Most thermoplastics are not just "cast" but are formed under some pressure. Roto molded parts, "roto cast" the plastic isn't actually melted all the way but is more of a sintered powder.
Your part will cool and solidify on the outside first, which will want to arrest further shrinkage. So the inside will tend to create voids and porosity. The outside skin ends up under compression and the inside is under tension. When you go to cut it, it will "draw" towards the cut. The thicker, the worse this problem.
Some plastics that are cast, such as some PVC, some nylon, some acrylic, are actually polymerized in the mold, so do not rely on cooling to solidify, there is less shrinkage so they avoid this problem.
Delrin (POM) is not very castable, but you should try it and tell us how it works for you.
Unlike nylon acrylic, abs etc, delrin does not absorb moisture and will not need to be dried in your case. Heating it to melt it will dry the surface moisture away
Reply to
Polymer Man
Thinking about your project a little more. I'd said self polymerizing castable plastic don't shrink much. That isn't accurate. But they do shrink through out the part uniformly because they solidify through chemical means, not by shedding off heat, which would work from the outside in, creating problems.
My point is, without the specialized high pressure equipment used to process thermoplastics, you're going to have some difficulties casting this plastic. Uncontrolled shrinkage is the root of the problem.
I believe you will be able to create a casting with some usable properties, so I hope you'll try. You should encourage the melt to cool as evenly as possible. Do this by cooling very slowly. Some plastics suffer badly due to prolonged exposure to high heat. POM isn't one of them. Preheat your mold before pouring the plastic in. Then set them in an oven at 150 F to cool overnight. Your finally casting will be more even than if you let it cool quickly.
Sorry to be long winded. This happens to be an interesting subject to me.
Reply to
Polymer Man
I wonder if it would be at all feasible (largely depending on how fluid the stuff is, and thus how good the seals would need to be) to make the mold as a cylinder/piston arrangement, and either drop it in a shop press or toss a clamp on it to provide some pressure. Obviously not the same sort of magnitude as a high pressure injection mold system, but a good shop press wailing on a small-area cylinder can get some reasonable pressures going. ie, if the OP is making his mold 4x4 inches (by 2, or however long - might be less fussy to make a longer block and cut slices off of it), a mere 20-ton shop press could apply 2500 PSI, if the plastic didn't all squeeze out past the piston.
Reply to
Ecnerwal
That's an interesting thought. If it's tried, better take some safety precautions. Full body welding leathers, and a face shield at the minimum.
I used to work with a guy that was working on a molding machine. The nozzle was still hot, and the plastic inside was too wet. It created a pocket of steam that sprayed liquid plastic all over his legs. It had to be surgically removed. It wasn't pretty even 20 years later.
Reply to
Dave Lyon
Howdy guys,
thanks for such a quick response. I've seen the Delrin melt before, and it actually drips like a candle, so I'd just assumed it could be molded easily. I'm waiting for the original parts manufacturer to send me a MSDS so I can confirm which variety of acetal it is. Shrinkage of the part overall dimensions is not a problem, the block's I was after are to be machined up anyway If I can avoid casting a hollow piece I'm in luck, but its not worth spending weeks on recycling what is probably 50 bucks of acetal, though I've had trouble finding any stock here in Australia that fits my 4" x 2" requirements, hence my idea.
I don't have a press in the workshop (my employers workshop) so pressurising my mold is not a possibility, but bringing the mold down from flow temp to room temp overnight could be done.
I'll have a go and post back, thanks so much for the help here guys.
Mark
Reply to
Mark Hathaway
Be careful. A tiny whiff of burning polyacetal will have you in pain for half an hour. A good snort will have you in the ER with chemical pneumonia. Formaldehyde is nasty.
Reply to
Richard J Kinch
Howdy Richard and everyone else,
after thinking about my plans over the weekend, it's all too hard (and dangerous) to save a few bucks recycling acetal. Scrap copper from the metal recycler works out cheaper then any Acetal supplier here.
Thanks everyone for the advice.
Mark
Reply to
Mark Hathaway

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