Working with Polyurethane Foams

This is a topic that I have wanted to write about for some time.

Foams.

There are 2 types of foam that prop builders use when building props.

There are 2 part polyurethane foams: Which are closed cell, and cure via a chemical reaction when the 2 parts are mixed together. This is the type of foam I use when casting items in my studio. They come in various densities and depending on the density you get different yields from the kit. As Density of the foam increases yield of the kit decreases. All expansion rates and times are temperature critical. Temperatures below 75 degrees F will lower the expansion rate therefore requiring more foam. Ideal working temperature is 75 to 80 degrees F or above.

There is foam in a can: Some prop builders use Great Stuff foam or other “foam in a can” products. I highly discourage use of these products since they are very temperamental and can be frustrating to use. They are one part foams , they need moisture in the air to cure. If you were to fill a skull mold for instance all at once with Great Stuff the center may not ever cure since it would not be exposed to air or moisture. It is not designed to fill large voids.

Safety: Polyurethane foams will off gas carbon dioxide when expanding so for safety you should always work outdoors or in a well ventilated area. You can use a solvent such as acetone, denatured alcohol or lacquer thinner to clean up when the product is still liquid, however polyurethane foams will stick to almost everything except slick plastics so you should cover-up anything you do not want the foam to damage.