The choice of many propmasters, polyurethane foam creates lightweight replicas suitable for Halloween, film projects and theater performances. You can make your own replicas out of foam at home with a polyurethane foam kit, which many companies produce. This kit usually includes a pair of bottles containing two different chemicals. When mixed together, these chemicals react and start expanding to many times their original volume. Eventually, they harden into lightweight foam.
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Consult several different companies for polyurethane foam kits. Many plastics companies produce foam kits, but the exact qualities of each one are slightly different. Some foams expand more than others, and some have a faster curing time than others. The most cost effective selection is to use a foam that expands a lot as it cures, effectively getting more use out of less material. However, go with the demands of your project. If you need a really fast-curing foam, opt for that instead.
Choose a Mold
Silicone molds are the best for casting foam replicas. Silicone's slick surface means that the foam will not stick to it, allowing you to easily demold your final piece without damaging the foam. However, if you have to use a foam made of another material be sure to coat it in liquid wax. This will keep it form sticking too much. Also make sure you have a lid with some holes in it to put on top of your mold. The lid will give your replica a flat bottom while allowing excess foam to bubble harmlessly out of the holes. These bubbles are easily sheered away. Be sure to coat the board in liquid wax as well, and bind it to the mold to keep it in place.
Invest in Safety
As with many molding and casting materials, uncured foam is very toxic. Always wear gloves and a respirator when you work with it. Work on top of several layers of newspaper or a tarp and make sure you do your project in a well-ventilated area. Crack a window open and keep children and pets well away from your work space.
You can color your foam in two ways. The first method is to dye the foam as you mix it up. Most companies sell dyes for their foam products. However, you may have to mix dyes together if you need a color that isn't standard. Still, this is an easy way to color your object all the way through. The other, cheaper option is to paint it once it's formed using acrylic paint. This method allows for more detail and color variation but it is also prone to peeling if it you use your object a lot.
Mixing and Measuring the Foam
Have your mold set up before you start mixing your foam. Once it is mixed, the chemicals begin to react very quickly and you won't have much time to pour it into the mold. The two chemicals are usually added in a 1:1 ratio. However, how much of each chemical you will need depends on the expansion rate of the foam. Check with your foam's packaging or the company to find out how much your foam will expand, and use that to calculate how much chemical you will use before you begin mixing.