Quantitatively describing a small concavity

AFAIK there's ways to digitize the cavity without taking a mold -- the result will be a 3D model ready to play CAD games.

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Tim Wescott 
Control systems, embedded software and circuit design 
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I've used products from Smooth-On successfully for duplicating tricky shapes. Their materials come in all sorts of hardness, and they have a good mold release spray that doesn't eat anything I've sprayed it on so far.

They are happy to advise you via email too.

John :-#)#

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I know nothing, but silicon? GH

I have a small (a few cubic inches) concave surface in (inflexible) plastic that I'd like to model, accurately (a few thousandths).

What can I use to coat the (plastic) surface (think mold release) that won't damage/mar the plastic but will allow a flowable material to be introduced to that concavity, allowed to "set", then removed?

What would be a good candidate for that "flowable material"? I'd like to be able to "machine" (some unspecified technology) the resulting "positive" extracted from the concavity. =====================================================

Go to

and browse and read. There are other companies selling similar products but I've used a few of theirs and was happy with product and service. It would be nice if you could find out just what kind of plastic you have, to be sure a mold release won't hurt it, but I bet their silicone release spray will do the job. I think one of their hard polyurethanes would be good. I've used their shore 70D material and it sets up nice and hard, like an old rigid plastic ashtray. Drillable and machinable. One issue you might run into is shrinkage on curing. Look at their data sheets and you can calculate how much there should be based on the size of your bowl, so you can see if it will be accurate enough. After you've spent some time on their site, give them a call for a recommendation.

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Regards, 
Carl Ijames

Check out the 3D scanner used in the video here:

Since I mentioned it, any idea what binding glue the 3D sand printer uses? I'm guessing it's CA, for the quick setting time.

Clifford Heath.

Silicon melts at 1414 degrees C. I suspect that'll damage the plastic.

Oh, did you mean silicone?

Clifford Heath.

Right, the stuff GE sells in tubes. (English is only my second language, I don't have a first, maybe pictures.)

George H.

Silicone rubber mold compound. There are quite a few videos on YouTube on how it's done: Smooth-On is the most common. I have some other stuff in the office that I use. Methinks it's Dow Corning RTV-3110 but I'm not sure: Some applications literature:

In general, if you want accuracy, then compositions that are almost rock hard are best. However, getting these to release from the original is tricky if there is any detail that might trap the silicone rubber. You may need to smear on some mold release compound if that's a problem. If your concave surface is quite smooth, that won't be a problem and you can use the hard stuff. However, if you have lots of detail and included shapes, you'll need to get something that can be stretched or compressed temporarily to release the mold. That also prevents the mold from being reusable as the rubber might tear when released. However, that's not fatal as it can be glued back together.

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elsatomers like silicone rubber tend to be hard to machine.

If the surface doesn't have any overhangs you could try Wood's metal or one of the less toxic alternatives. I'm not sure if a mould release will be needed.

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Then there's those beads that melt at low temperature

Silicone oil as the mould release and the blue rigid wax aka Trycut that they sell for student training on a lathe.

You melt it afterwards and cast back into cylinder blocks. It is more or less machineable and self lubricating if you don't cut too fast.

The only catch might be that the plastic original may not tolerate hot wax all that well. Plaster of Paris might be another cheap easy candidate for a copy but again it gets pretty warm as it sets.

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Regards, 
Martin Brown

Excellent! Looks like I've got a bunch of reading to do... Thanks!

Polyvinyl Alcohol release and polyester resin for mold.

You can get spray mold release compound. It's sold by the case to plastic injection molders. You can buy a single can from McMaster, for example.

--sp

Ask on rec.crafts.metalworking, and ignore the political crap. There's some pretty astute people on that group, some of whom are active or retired master machinists.

If your true aim is to make an accurate mold for the thing you want, or an accurate duplicate of it, say so -- let them tell you the best _way_ to achieve it.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com

Same here. I used to have problems with word association tests. They would say " Apple " and I would possibly visualize a Red Delicious apple with a bit of the branch still attached and maybe two leaves attached to the branch.

Dan

Liquid latex (used for making molds out of carvings) just paints on, and after a few layers you can put in something stiff (wire mesh?) and keep building layers atop that. It isn't really adhesive, except to itself, but you could oil the plastic beforehand if you want.

Available at Tap Plastics, and your neighborhood fetish supplier.

A latex mould won't hold its shape anything like well enough to the tolerances he asked for.

Rigid wax would or some of the tougher silicone thermoplastics. The big question really is how much heat can the original stand?

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Regards, 
Martin Brown

Whatever the material is, it will likely shrink as it hardens. Maybe a couple percent, maybe more. Even if it is only 1.5%, that's much more than his desired tolerance.

I was looking at some fake heatsinks ~12" x 4": that the Chinese put on an inverter box- along the sides of the box- just black plastic moldings that look like finned heatsinks. To keep the wall thickness roughly constant that means that blades have to go inside the plastic fins, and the molding shrinks down on those fins (the 'core') when the cavity is pulled off. They had to use almost 70 ejector pins per cavity to push them off properly.

--sp

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Best regards,  
Spehro Pefhany