[Tom Kay]

Hi Gents;

I know what lost wax casting is, and I've had some parts done for me (never done the whole thing myself). I know that once the ceramic shell is made, the wax is melted out, or LOST in the process.

But is lost foam different? Is the foam left in a sand mold to simply be burned away once you pour molten aluminum into the mold? Or have I just not understood the lost foam process?

My concern is that if there's a big mass of foam blocking the entry of the melted aluminum, wouldn't this slow down the entry of the metal into the mold, and lead to premature freezing of the metal before it reaches all the intricate corners of the mold? Or, wouldn't it lead to so much off-gassing that you'd have tons of extra gas to vent, plus maybe porosity problems?

I don't get it. Anyone explain how this works? And can you do a very detailed casting, such as an engine block complete with internal webs, etc? Is lost foam usually done with sand (not ceramic shell)?

Thanks, Tom.

[paprjam]

The simplest way is to bury a piece in loose sand and pour molten Aluminum in or I've used green-sand packed lightly around the part. Here's a couple shots of an experiment I did last week with different foams. The green piece was a failure and the rest were about equal.





There is a lot of smoke and fire but you just have to hang in there and keep pouring. I have the metal hotter than usual and try to pour fast and steady or the sand will try to collapse.

[Tom Kay]

Hi Paprjam;

Thanks for the quick reply. The blue and pink foams just look like house insulation. Correct? They're easy to get in bulk, if they are from Home Depot.

Second, if I'm doing a pretty intricate part, would it work to use lost foam and some kind of sand binder, everything from green sand to furan, petrobond, mollasses, etc? I'd hate for the foam to burn away too quickly and have a mold collapse.

Cheers, Tom.

[paprjam]

Some have used a plaster of paris mix for a coating. I haven't tried it, it's supposed to help with surface quality too. I think they mix POP with some fine sand and maybe some silica flour. I believe it would require more venting, or it may crack from the pressure. I'll try to check and get back to you.
FrankG has done some pulleys in lost foam, I can't find a thread about it, but here is his website. www.theworkshop.ca

[Geof]

I have not done this but did a lot of reading about it one time. I remember it was stated to use only the white expanded styrofoam with no additives of any type. The hot metal does burn out the foam and pure styrofoam when it gets hot enough only produces water vapor and CO2 when it burns. Sometimes if the burning is not hot enoguh you do get quite a lot of soot which is just unburnt carbon.

Some foams have fire retardants in them so of course these will not work and the colored foams may have metal compounds for the color so these will not burn away in the same manner.

[Tom Kay]

Thanks Geof;

That makes sense. It still surprises me that a large volume of foam can be burned away fully so that aluminum has time to fill the mold before freezing. But, I guess it's just a case of try some experiments and see. I suppose the foam could also be made hollow in thicker areas, as long as ramming sand around it doesn't cause it to collapse.

Tom.