Lost Foam Casting 2 - Foam, Surfaces, and Sand

What foam to use? Almost all foam sold in building supply stores contains fire retardant. The right property for home insulation but the wrong property for casting. The fire retardant makes it hard to cut with a foam cutter and increases the thermal mass which in turn increases the risk of a short pour; i.e. a pour where the aluminum freezes before vaporizing all the foam.

Styrofoam pieces sold at craft stores work fine however the grain is coarse and the price is wrong. The best foam is from packing materials such as boxes that contain appliances, electronics, or computers - fine grained foam, no fire retardant, free.

Surfaces and sand: I like foam casting because it is quick. Make the piece, plonk it in some loose sand, pour. This, however, leaves a rough surface with embedded particles of sand. I like this surface for some of my art but it is terrible for anything that will be machined.

The topic of surfaces depends upon the sand. Using fine sand such as petrobond or green sand will produce a surface that matches the foam surface.

When you use a tool such as a grinder, rasp, or even sandpaper not only does it leave foam dust everywhere but the surface is coarse. A hot wire tool such as a foam cutter or an inexpensive soldering iron seals the surface as it melts foam. Other techniques I have read about, but never tried, are dipping the piece in molten wax and coating with latex paint. Reportedly these leave a smoother surface however as these materials get exposed to molten aluminum they burn.