I just dont like the great stuff, mostly because I have worked with the real thing. 2 part urethane foam. Rigid or soft {nerf like}
Neat stuff is essentially the same sort of dimethalisocyanate polyamine chemical. How they keep the iso, separate from the poly until you press the go button is beyond me.. that is likely most of what you are paying for. And its lower density, lower quality than what you would mix yourself. about 2lb density.
With the 2 part polyurethane foam from smoothon, polytec, or a fiberglass place... line the mask with a garbage bag, have some sort of support shell scheme... premix the B component well before adding A component. after combining A & B....Mix until the chemical goes "Amber", {about 1 minute} continue mixing until it starts foaming, and feels like whipping cream. You get better results {less surface weirdness, on the part where you pour it into the mold} if you pour during the "cream stage". It should rise slowly for about 1 -2 minutes. If it rises too far, you simply hold up the garbage bag liner to keep it from spilling over. Once it stops foaming, and sets up firm. {it will be about 300F at this point} you can peel off the latex mask.
While working with great stuff, or polyurethane foam you should wear rubber gloves, and expect your clothes to be ruined. You could wear a tyvek "Devo" suit.. they cost like $4.. Airgas welding and safety supply places should have them.
Be that as it may.... Foam Wig heads might be your best choice. OR you could custom carve/cut some thick mattress type foam. Foaming is one of those projects... IF you have to ask too many questions... it may not be the project for you!!!!!!
Another option if you dont mind wieght..... "lightwieght" concrete. 'Lightwieght concrete' weighs only 80-90 lbs per cubic foot, instead of 148... The portland cement has a silly property that cant be avoided... a specific gravity of 3.14. Perlite {expanded sand, available at garden centers} makes a good lightwieght concrete. Dont even ask me to provide a mix design, as I cant figure out the specific gravity of perlite.. at any rate a basic volume recipe of 1 part portland cement, 4-5 parts perlite should work, with just enough water to make it "plastic", flowable, workable. A support shell scheme is definitely required.. and a garbage bag liner. A rebar, allthread, J-bolt etc could be embedded into the head, which helps when securing to frame "cross" later.
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