Can I cast alloy in silicon

[chola]

Hi,

I have a lot of left-over scraps from metal machining (aluminum), and would like to cast off a few very small parts (suspension arms and the like for 1/8 scale cars / RC car size).

Is there an alternative way of mould making than sand casting, like using a suitable hi-temp silicon (eg: Wacker M4470), or is sand casting the only real option for this.

Would really like to use these alloy scraps as seems a real waste to throw them out.

Any information greatly appreciated.

Thanks,

[Geof]

Have you considered lost wax casting? Also known as investment casting.

I have heard of silicone rubbers that can withstand the melting temperature of some zinc based alloys but I think aluminum is much too high.

[Harryman]

Sand casting is fine for parts that can have a rougher surface finish, investment for more precision parts with a fine finish.

High temp silicones are marginal even for zinc alloys, they don't hold up too long, but you can get some parts out before they croak. AL is too high a temp for rubber molds.

[chola]

Thanks Geof & Harryman, so it looks like investment is the way to go (I do need a good finish on these parts). I'll stick to using the silicone with low melt alloys.

Another thought from left field, what about plaster of paris for the mould? I've been reading a little about this and can't seem to come up with a definitive 'yes a good idea', or 'no, forget it'?

Thanks again,

[Harryman]

That's essentially what jewelry investment is, just a different formulation. I wouldn't bother trying plaster though, get the real stuff, it's pretty cheap.

Thats the route I'd suggest unless you're planning on big pieces, then ceramic shell is the way to go.

There's plenty of info on the web and probably in your library too on casting using poured jewelry investment, you can garage engineer all the bits you need.

[Geof]

Even if you use plaster of paris you still have to bake the mold. Maybe not as hot as for the ceramic molds for proper investment casting but still hot enough to drive all the water out.

I will second Harryman's suggestion to go for the ceramic shell. I think the amount of effort and mess may even be less with wax patterns and proper baked investment molds.

Using silicone to get the best surface finish and the most durable mold you have to get involved in vacuum degassing of the silicone prior to pouring the mold. I can tell you this can get very, very messy and the silicone that gets all over the countryside when something comes adrift during the degassing is much more difficult to clean up than a little bit of clay slurry.

[rowbare]

Do a search on Google or You Tube for Investment Casting and you will get a pretty good idea of what is involved.

http://www.ganoksin.com/borisat/dire...y/subject/33/1 has some pretty good jewelry-oriented articles on casting.

[chola]

Thanks guys, this information is great.....

Ok, ionvestment it is (I do need a good finish and these are going to be small parts where detail is important). The epieces I have in mind range in size from an inch square to about 2 inches square, so not big by any scale.

I've read that a mix of around 50/50 of normal plaster of Paris and silica sand / river sand (the very fine wite variety) is a good mix.

My next problem is I don't have access to a furnace or anything like that for the 'burn-out', though I would be happy to use the 'ahem' wifes oven, or even the receptacle I'll use for the melting of the alloy. Even if it doesn't get as hot as a furnace, can I assume that, as the plaster moulds are relatively small, I could get away with just leaving them in here a little longer (a good few hours), to drive out the wax and any moisture?

Thanks again guys, been a great help.

[Harryman]

To do it right, use casting investment, it's only @ $1 a lb. Unless your time is really worthless and you don't care about remaking your waxes. : ) I've seen pieces cast using unconventional materials like clay and although you can successfully cast into it, your castings will end up looking like doo. If you want a nice surface, use the real deal. Any investment will also come with instructions on how to mix and fire it as well, follow them for the best results.

Your oven will get hot enough to melt out most of the wax, but you can accomplish the same thing without fuming up the house and annoying the wife by steaming it out in a scrap cooking pot in the backyard. Neither is good enough at that point for casting though, you need to get it to 1325 degrees F and hold it there long enough (@ hr for a small flask) to both change the chemical structure of the plaster and burn away all the wax and carbon that's soaked in. If it's not completely gone, it'll cause problems with the metal as it fills the mold. It's got to look bone white after firing.

You could probably rig up a little kiln by stacking fire bricks around the flask and pointing a propane burner inside, I've seen similar ones at work in Mexico. Not optimal, but it'd work.

[chola]

Thanks Harryman,

I've been searching for a while here in OZ for a proprietary mix for casting and have not yet found anything off the shelf, is proving difficult to come by...of anyone has any links to suppliers in Australian feel free to throw them up.

I read about steaming the wax out and I think you're right with this being the way to go, that way I can also tilt the mould as necessary to help remove the wax from any undercuts.

My next problem is going to be the furnace, I was thinking of modifying an old webber barbeque...I remember years ago I had one going and made the mistake of using a barbecue meat fork to shift the coals around, within minutes it melted the rivets out the held the forks onto the handle. I'm sure with a bit more though gone into it it would get up to temp, ice and solid those thigs are too.

The next thing to consider is if gravity casting will be enough to get the metal into the mould, or do I need to rig up a centrifical machine. I've got an old drill press motor and various pulleys laying around that I am sure I could gear down sufficiently. I guess I just need to experiment with this.

Would love a complete step-by-step on setting up a home-based casting system.

Thanks again...

[Harryman]

Try these guys for investment:

http://www.ajsonline.com.au/

The webber would work, especially if you could stick some fire brick inside or line it with refractory cement.

Gravity casting AL would be tough because it's so light, especially a small amount. Rigging up a centrifugal would be a good idea, or vacuum casting. You could also try slinging it by hand, that would be especially exciting. : )

[chola]

....I checked out the link, will be giving them a call next week!! Brilliant.

Slinging by hand...hmm..sounds interesting....and if it fails...well...I never did like the neighbours cat, wondering if they would like a still-life alloy sculpture of it

Thanks again guys for all the input.