I regularly make lost foam castings by just burying the foam in loose dry sand with a foam riser protruding above the surface. No need for greensand or venting.

Mike

When our factory (Willans& Robinson/English Electric/GEC/Alstom Victoria Works, Rugby) still had a foundry and made castings for EE and Ruston's Diesel engines up to the early '90s, they were all made from lost polystyrene foam into green sand. That was with SG cast iron.

Guy,

Like Michael, I have also made many lost-foam castings in dry sand, with no problems. I used the smallest grain sand I could find – almost like dust. I also used some quartz sand such as is sold for swimming bath filters. Make sure the pattern is well covered by the sand – if it is too close to the surface, then odd things happen. I assume the that as the foam evaporates in the heat, either the resulting gases 'explode' through the sand, or the sand collapses into the void. If it is buried a few centimetres under the sand, this doesn't happen.

I also use the rigid closed cell foam as mentioned by John Purdy. This stuff can be cut with a fine saw or hot wire, and can be sanded to a smooth finish. The white bead type foam is useless. You can make complex patterns by gluing together pieces of foam using hot-melt glue. No draft is necessary on the pattern.

The principle is described in many places online. This site gives a good description of commercial application: **LINK**, It mentions using a ceramic coating to form a sort of 'skin' to reduce the roughness caused by the granularity of the sand. Since I couldn't get hold of any kind of ceramic dip, I tried a suggestion I found online that coating the pattern with a thin coat of latex paint also gave satisfactory result – it does seem to work.

As John says – don't breath the fumes !