hollow aluminum casting

[AKoch]

xxx
My foundry (in Italy) can't seem to make it with hollow aluminum casting- they do OK with solid casting, but the hollow version solidifies on them too soon. Is there a solution to this?
Arthur Koch

[iowasculptor]

the problem is that without the thickness to keep the metal hot it cools off too fast. Try heating up your mold and pouring your metal really hot. Its a tuff thing to deal with aluminimum doesn't hold heat very well.
matt

[fused]

You don't describe the methods you are using or forms to be cast.

I'm guessing lost wax in a ceramic shell or plaster silica investment.
How big is the sculpture you are casting and what thickness is the
wall/shell of these hollow forms?

I'm also curious why aluminum is your preferred metal for casting?

[AKoch]

Essentially, we're using the same tec as for bronze, i.e. plaster investment, lost wax. The pieces we've tried so far have been fairly small- about 40 cm high. Actually, we're experimenting.

[F.C. White]

Your cast needs to have resevoirs to hold extra metal near the cast cavity which would maintain the heat necessary to fill your mold. Otherwise, use a reverse "chill" inclusion into the mold, itself, nearest your cast piece that's difficult to fill. This inclusion could be a solid piece of steel that would retain its heat when the mold is pulled from the kiln and hold sufficient heat within the mold as long as you don't take too long to fill it. Also... use of powered chlorine (you know those blue toilet bowl cakes you sanitize your bowl with), try purging your aluminum with a small piece (about on inch in diameter), using some stainless steel rod appratus fashioned to hold that chlorine bit under the surface of the melt prior to pouring. Stirr the pot while it's doing its thing. It will bubble and boil dramatically at first and give off the odor of firecracker residue and show a beautiful aurora within the crucible, then you'll notice a crap load of slag raised up to the surface of the melt. That is the stuff in your aluminum that's causing your casts to be difficult to fill. Skim that then do your pour. You'll notice your melt having the consistency of thinned cream. The aluminum will flow much easier and leave less of a textured finish as well.

You should be purging your melt at near the temp your aluminum (zinc content) is about to flare. If you push it too long to where it does flare, leave it go.... open up the doors to air out the shop... then, when it creates its own surface skin and stops flaring, do your purge and commence to pour.

But seriously, aluminum is far easier to pour than bronze as long as you purge it first of its hydrogen molecule that it acquires off the propane during the firing process, as well as the additional alloys it's been given for whatever purpose the components you melted down was originally designed to endure.

Good luck... show us some pics!

[F.C. White]

Also.... you might consider putting a coil of copper tube (perforated in several locations along the length) inside the core investment within your cast cavity. This will allow for carbon gas to expell out of the core itself when the metal flows in around the core. Not venting your core can cause the appearance of melt not completely filling the cavity. It can also cause your core to crack, become dislodged and ruin a good piece of work effort.

[AKoch]

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Well, F.C. White, many thanks for your detailed suggestions. Now my big job is to translate all that! The people I work with are father and son, they've done some impressive stuff in the past. It's the son who broached the idea of trying aluminum, he's always fiddling around with new ideas. I myself am a carver in marble, wood, etc., and they make the bronzes from my carved pieces. So far we've only done a few small solid pieces (visible on my site www.ackoch.com), but we're hoping to get on with it, as it's a material with a lot of character, I think. Again, many thanks!

[ExNihiloStudio]

There are many, many variables that could be the source of the problem.
Two good books are Castings by John Campbell ISBN 0-4506-4790-6 and Casting Practice, the 10 Rules of Castings also by Campell, ISBN 0-7506-4791-4. Castings has all the detailed theory and The 10 Rules distills them down to rules of thumb. Both books focus on aluminum almost exclusively.

[AKoch]

Thanks for your book titles- I'll try right away to find them.
Arthur koch

[obseq]

Just out of curiosity, why do you want a hollow aluminum form?
What are you trying to cast?

Aluminum is so incredibly light, that casting solid doesn't present the same problems in casting solid iron or bronze.

[AKoch]

Obseq- Now that you mention it, I can't really say why hollow, but I have a feeling that a piece a meter tall would weigh some, even in aluminum. Still, it's a point that I'll raise at the foundry! Thanks!

[bobcoon]

You might consider the shrinkage in the cooling of the molten aluminum...or any other metal for that matter. Making a larger casting hollow is not just a matter of how much the final product will weigh. A more faithful reproduction is number one!!!

[obseq]

Akoch,

Any word back from the foundry? I'm really curious to hear their response.
Have they suggested another metal?

Let us know! Good luck! :)


Obseq- Now that you mention it, I can't really say why hollow, but I have a feeling that a piece a meter tall would weigh some, even in aluminum. Still, it's a point that I'll raise at the foundry! Thanks!

[AKoch]

obseq- They said: NO, no, hollow, its just a matter of practice!!! So, I leave it at that!

[fused]

I'm guessing that using the traditional approach to casting there is a sprue system surrounding the form with metal entering the bottom and vents at the top. If you want aluminum to flow consider doing a direct pour --sculpture form upside-down with the pouring cup centered on it's base-- while still employing vents at the top of the mold and shrink feeders -one inch or so solid blocks sprued to problem areas where shrinkage or chilling occur.