3 lb. Rigid Foam is a 2-part polyurethane expanding foam which expands as much as 18X to create rigid lightweight castings, parts, and backfill reinforcement of other materials. This is a lightweight 3 lb. density rigid foam, which means a cubic foot of expanded foam will weigh 3 lbs.
This product is mostly used as a filler, but works well as a casting medium if used properly. One problem is if you're trying to fill a mold, and you have no way to make it go into all of your detail, it will take the path of least resistance and just foam right out the top. In order to get it to fill that detail, it must be "packed". That is, it must be poured into an enclosed mold so that it builds some back pressure and is forced into all of the detail. In order to do this, the volume of the mold first needs to be estimated. Then, the mold must be capped and an excess amount of foam must be poured into that mold.
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