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Lost Foam Casting: a Crankcase in Zinc Alloy

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vtsteam:
Unfortunately there were still flow lines and unvaporized polystyrene, though less and the casting flaws were shallower than yesterday's pour:

vtsteam:
The surface finish was better than yesterday with fewer flaws, but nothing like the pattern fidelity I'd got with aluminum pours in the past:

vtsteam:
Because the flaws didn't seem very deep, I put the casting on the mill and cleaned up a few surfaces with my carbide end mill. It looked good, so I will probably make an engine from it. I'll turn the front face and bearing boss on the lathe, but i just used the mill to see how deep the flaws were -- about .030" total out of nearly .250" so it's still a pretty heavy case.

For casting zinc alloy with lost foam, I'd say the patient survived, but the operation wasn't a success. I had hoped surface fidelity would be similar to the aluminum castings I'd done in the past. But the failure at high casting temperatures (for zinc) to vaporize all the foam meant flaws and poor surface finish. So I don't see it as a viable method for me in the future. Nevertheless, It looks like I'll get an engine out of it -- probably the only one I'll ever do this way.

I may yet try an aluminum version, just to check that the pattern and molding method weren't the problem for this particular casting:

mattinker:
Hi Steve,

ZA 12 machines beautifully. Looking good, even though it was semi successful. I think you'd get a beautiful finish in ZA12  if you dissolved the polystyrene out with a solvent. Originally, lost polystyrene didn't use any dry wall "mud" just the polystyrene replaced by the molten metal in dry sand, an amazing process, incredible that is works!! I think the mud in dry sand would work really well.

Regards, Matthew

vtsteam:
My feeling is that the mold would be too fragile if dissolved out, using a single coat of mud. I think Plaster of Paris would be  better choice since it can be built up yet still solidify through chemical means instead of drying, as mud requires. However then we get into the problem of more H20. The mud is so thin that water is negligible. Anyway, plenty more to explore. I'm allready leannng toward PoP rather than mud w/ zinc alloy.

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