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Old 1st January 2008, 12:37 AM   #1
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Default I try to learn something new every now and then!

Many times I've wished I could get some part that isn't make anymore, or that costs so much that it might as well not be made. This includes things like funny plastic knobs on a GR decade transformer, and the cracked gears in my eBay HP signal generator.

It turns out that if you have one good part, you can duplicate it using a silicone rubber mold and casting resins. Maybe everybody but me knew about this, but it's pretty cool. The "entry fee", however, turns out to be about $40 or more.

You suspend or support your part in something like a plastic cup (there are many methods), then mix up a 2-part RTV and pour it around the part. If you want absolute flawless perfection, it helps to degas the RTV under vacuum, if you have a pump available.
After the RTV hardens, you typically cut the mold in half (again, many different methods exist) and remove the part.

Then you cut a channel for the resin and put the mold back together. There are dozens of different urethane resins, plus coloring agents. You can also use epoxies. The resin gets poured into the mold and cures. Because the mold is flexible, you can then pull it off, even if the part has undercuts and complex features, and reuse the mold to make more parts.

I'm making gears right now, and will post some pictures when they're done. I wanted to post this just to say, don't give up on some old piece of gear with missing or broken parts, because you might be able to make very high quality copies with surprisingly little trouble. Google silicone+mold for more info.

I've also seen the technique used for chocolates- anybody up for a dark chocolate V15 Type III- hey, Happy New Year!
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Old 1st January 2008, 01:09 AM   #2
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Got any black resin? Lucrative venture for you to cast some 2SK389's and 2SJ109's.
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Old 1st January 2008, 02:11 PM   #3
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AudioXpress magazine just did an article:

audioXpress June 2007
"Casting Replacement Parts"
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Old 1st January 2008, 06:56 PM   #4
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Amazing- I haven't subscribed to that in a few years, though I'll sometimes see an issue on the newsstand. Not to go too far OT, but a few years ago I thought their articles were too focused on selling things and not enough on educating people. If that's changed, I might resubscribe. Anyway, here's a picture of my gears for an HP8640B, the mold, and the original gear glued to a piece of Delrin. Gear diameter is about 1". Next step is to install new brass bushings and trim the excess material. They could have been cast to finished size, but I wanted a nice open cavity mold to insure full degassing and no bubbles.

Click the image to open in full size.
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Old 2nd January 2008, 12:02 AM   #5
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I think the actual name of the compound is Poly Vinyl Siloxane.
Dentists use it for taking impressions. It sets fast, low shrinkage and blends well without many bubbles.
I made a very similar looking gear years ago for a timer on an old water softener/deioniser. Saved me having to replace the whole unit.
Magic stuff there.

A tip... if using resin or epoxy for casting material, add some chopped cotton fiber to the mix for added strength. It looks like powdered sugar, and is sometimes used as a thickening agent too. You can find it at body shops, boat repair supplies etc. I think I bought mine at Aircraft Spruce.
Money well spent.
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