[bmurph]

I want to make reusable, and smooth molds, for aluminum casting. Would that fine cement be the cheapest way to go? Stainless steel or graphite would work good, but they cost a lot.

[paprjam]

Cement should work OK for a pattern. You'll have to seal it up with some paint so the surface won't stick to the sand. It may be a little heavy. I've used wood and bondo(2 part auto patch material). I seal it with a good high gloss paint, usually oil base but I don't know if that makes a difference.

[Geof]

It is not likely cement will make a long lasting mold because thermal shock is probably going to make the surface flake off. Permanent molds for aluminum casting are frequently made from ductile cast iron. It is easy to machine and is very stable to frequent heating and cooling.

[paprjam]

Sorry, I misread your post. I thought you were going to use it as a pattern in greensand or petrobond.

[bmurph]

I don't see anything ductal iron for sale from the small metal suppliers, so I'll just use steal, but I was worrying about the corrosion, I know that heat causes it. Is ductal iron blackend, or does coating in oil protect it without causing bubbles?

[Geof]

Mild steel works fine and corrosion is not a problem. Use hot rolled steel so there is less chance of distartion both from the machining and from the heat.

When steel is used for the mold you have to make sure to use a release coating such as powdered graphite otherwise the aluminum tends to stick.