Polytek offers three free rise densities of Rigid polyurethane foam and two free rise densities of Flexible polyurethane foam. From these few products you can create virtually any density you need by simply “packing” or over filling the mold.
Foams are normally poured at 2 or 3 lbs. above their free rise density in order to achieve good surface details.
Polytek also offers a 15-lb density platinum-cured silicone foam.
Learn more here: https://www.polytek.com/product-type/casting-foam