Casting Foam?

[Robin Hewitt]

Originally Posted by schemo are you refering to polystyrene.

Love this site, there's another problem solved

I want to make a pattern for some axle boxes, basically a hollow cast iron cone. I was bogged down wondering what the heck to make them from and how to cut 15" tapers.

Now all I need is a 15" resistance wire and the foundry costs can only come down

[Thinwater]

I have used the blue foam and white styrofoam to make several castings in Al. I have had the foam's vapor flash (fire) and blow out chunks of liquid AL during the pour. Be carefull with new foams you have not used a lot.

I hack out the part close to size with the foam then mill it to size for a quick and cheap part.

My biggest home cast part was a bed for a horozontal mill weighing in at 22 lbs. It was done with a wooden pattern, petro sand and two people holding a bar hanging the crucible while I tipped it for the pour.

JIM

[3Dmap]

I've been looking into the same type of thing, and have found that there is a pourable marine foam that should do the trick. I havnt tried it yet but I believe this would be our best shot.

[awerby]

Originally Posted by 3Dmap I've been looking into the same type of thing, and have found that there is a pourable marine foam that should do the trick. I havnt tried it yet but I believe this would be our best shot.

The polyurethane foam products are pourable, but you can't use them for the lost foam process. Polystyrene-based foams will burn away (relatively) cleanly; the polyurethane foams generate extremely toxic combustion products. Remember Bhopal? The people there were poisoned by the same chemicals generated when polyurethane is burned - isocyanates and hydrogen cyanide. Don't even think about burning this stuff...

Andrew Werby
www.computersculpture.com

PS: Look up the MSDS for the product you're contemplating, and see what it says about burning it.

[phongshader]

Thanks for pointing that out. I was going down that road.
Is there any other pourable expandable foam that one could use other than polyurethane foam? I'm making molds for the positive plug on a 3d printer out of abs which I don't think will stand up to heat needed to expand polystyrene.
Another question...how much will plaster shrink if I were to build up enough plaster around the foam part, more than just dipping it, then disolve it out the polyurethane foam and pour aluminum with no foam in the mold?

[awerby]

Originally Posted by phongshader Thanks for pointing that out. I was going down that road. Is there any other pourable expandable foam that one could use other than polyurethane foam? [Not that I know of.] I'm making molds for the positive plug on a 3d printer out of abs which I don't think will stand up to heat needed to expand polystyrene. [I think you're correct there...] Another question...how much will plaster shrink if I were to build up enough plaster around the foam part, more than just dipping it, then disolve it out the polyurethane foam and pour aluminum with no foam in the mold?

[If you've got a 3d printer, why not make a positive model, take a rubber mold from it, cast wax into the mold, and treat the wax like a regular lost-wax part, casting it in aluminum via shell or investment casting? If you can't do this, any art foundry can. Also, some 3d printers (Perfactory, Solidscape) can produce models from a waxy material that can be burned out directly.

Trying to dissolve polyurethane foam out of plaster sounds like a nasty job to me; I certainly wouldn't want to be in the same room where that was going on (and PU is pretty resistant to most solvents; try getting it off your clothes sometime). Then you're left with plaster filled with solvent and PU; not something you'd want to pour metal into anyway. You're much better off relying on tried-and-true casting techniques than trying to invent new ones; people have been casting metals for thousands of years, and the processes, while they've improved somewhat, are basically the same as they ever were.
There are a few ways to do it right, and a whole lot of ways to do it wrong, with results ranging from bad castings to painful lingering death. Unless you're really an expert at this, I think you'll have better results by relying on people who are...]

[Thinwater]

I think there is some confusion with using foam. Most foam casting is by machining the foam to the part size needed, leaving a little extra foam where there are critical diminsions so it can be machined after casting. The foam is put in the mold box and sand rammed up around it to make the mold and the metal poured onto the foam, burning the foam away. The sand is the mold and will hold it's shape if properly tempered or petro bonded.

The foam machined to size because foam can be cut very fast and with almost no tool wear. Complicated shapes can be cast with no waisted metal. Think of milling out a custom L bracket. You end up with more chips than is left in your part.

I may be wrong but I have never seen foam burned out of a mold like wax has to be.

Most of the stuff I have read about as well as what I have cast with foam were peices starting at 1 pound and going up to engine blocks and cylinder heads. I don't think it would work as well with jewelry size parts unless you had a big sprue to force it in past the burning foam.

JIM

[phongshader]

[If you've got a 3d printer, why not make a positive model, take a rubber mold from it, cast wax into the mold, and treat the wax like a regular lost-wax part, casting it in aluminum via shell or investment casting? If you can't do this, any art foundry can. Also, some 3d printers (Perfactory, Solidscape) can produce models from a waxy material that can be burned out directly.]

That sounds fine to me. How hot does the wax have to be to melt? Would it be too hot to pour into an abs mold? I'm trying to build an automobile intake manifold. I've already compensated for the aluminum shrinkage how much more would I have to compensate for the plaster shrinkage in the mold?
There's a couple reasons I'm not printing a positive, one the printer isn't big enough for the whole part, although I suppose I could print it in parts, two I only have access to an abs or a powder printer so a wax part isn't in the cards for me and I'm trying to do this on a budget to see if the part will even work. If it does I'll spend the money for propper tooling until then I'm going as cheap as I can.

Trying to dissolve polyurethane foam out of plaster sounds like a nasty job to me; I certainly wouldn't want to be in the same room where that was going on (and PU is pretty resistant to most solvents; try getting it off your clothes sometime). Then you're left with plaster filled with solvent and PU; not something you'd want to pour metal into anyway. You're much better off relying on tried-and-true casting techniques than trying to invent new ones; people have been casting metals for thousands of years, and the processes, while they've improved somewhat, are basically the same as they ever were. There are a few ways to do it right, and a whole lot of ways to do it wrong, with results ranging from bad castings to painful lingering death. Unless you're really an expert at this, I think you'll have better results by relying on people who are...]

I agree, I don't want to mess around with any of that stuff, that's why I'm here asking questions. Part of my problem is I don't know enough to ask the right or relevant questions.
Thanks

[phongshader]

One more question. I've modeled a mold to cast a foam plug in that will be produced on an abs 3d printer. What if I were to make the mold out of plaster? That is print a mold for the mold to expand foam into. I could expand polystyrene in it and go with the tried and true lfc techniques...right? I've attached a SolidWorks edrawing html file so you can see what I'm talking about.
Thanks

[phongshader]

Any input or info about the dimensional stability of plaster? Does it shrink when it cures? If so is there a predictable rate of shrinkage? What would that be?
Thanks