The Latex and Urethane FAQ—Casting Positive Forms in Latex and Urethane

In order to make rubber masks, skins for puppets, and make-up “appliances” (the rubber parts that glue on to an actor’s skin to simulate age, injury or non-human characteristics), use what is called in the FX industries “foam latex” but is more properly described as “latex and foam” or another product called “self-skinning urethane foam”.

To make a part out of latex, you first must make a plaster mold from the original, which can be plastilene clay, wax, plaster, or something else. If you will be needing to make these latex items in large quantities, you will need to create a “positive” version of your master in a rubber material from which you can pull multiple plaster molds, because plaster molds wear rapidly. One way of doing so is to make a brushed-on or “blanket” mold of the cavity of your first plaster “negative” mold.

Use a good pottery plaster like Puritan from USG (Number One Pottery Plaster). You need the absorbancy. Use a brush and/or blow through a straw at the surface, wet with plaster, to ensure a perfect bubble-free surface. If the original has undercuts and fine details, you may have to destroy the original by digging it out of the plaster while the plaster is hot. Don’t worry—the flexible master you’re casting will replace it perfectly. A low-sulfur, waxy plastilene, or microcrystalline wax master will give you a the best plaster surface. The plaster mold will have to be dry before you cast latex into it.

If the latex piece you are trying to make is only one-sided and it’s okay for it to be very thin, then you are nearly done—buy a water-based latex and fill the mold with it. Allow it to sit for 20 minutes and pour off the excess. The plaster will suck the water out of the latex creating a “skin” on the inside of the mold about 1/8 inch thick. Leave the latex in the mold for half a day or longer, demolding only when the inner surface has cured. Before demolding, dust the inner surface with talcum powder—this will prevent the inner surfaces from sticking to themselves like contact cement. This is how your standard Halloween mask is made.

You can paint latex with a mixture of thinned latex and pure pigments, acrylic paints, or inks—do several tests first as the color of the mix changes dramatically as it dries. Use a cheap airbrush that can handle a thick material. This will give you a flexible, stretchable colored surface that will not crack or peel.

If you need to produce a piece that has dimension and needs to support itself, but still has only one side, then you must back the latex skin with foam rubber. In an open mold, you can literally paint the inside surface with freshly-mixed foam latex rubber compound and it will foam up to about ½ to an inch thick. However, if the mold is closed, or if you will be needing the inside of the latex part to follow some specific contour, (the negative contour of the wearer’s body, for example), you will need to create a plaster mold of at least 2 pieces. One piece captures the detail on the outside of the item, and the other mating piece determines the contour of the backside you wish to create.

The mold must be able to be strapped or clamped closed so that a foaming material will build pressure inside the mold. Cast the latex on the outer-side as above—but do NOT dust the surface with talc. (You need it to stick, not release) You can make the other surface latex if you wish, but it will just be unskinned foam in that case. If you want a tougher skinned surface on the back, you will need to shellac the back side mold surface and apply a separation agent like silicone or teflon spray release. The shellac seals the plaster so that it won’t simply absorb the release agent. Then, using Urethane Elastomer Foam (the best supplier of this is Burman Foam in the L.A. area, but there will probably be a supplier of similar materials in your area.) mix a little bit and pour it into your mold with the NON-DUSTED latex skin on one side. Clamp the mold closed. Move quickly as urethane foams go off fast. You may need to cut some tiny vents at the extremities of your mold to allow the foam to push the air out. When you see foam come squirming out of all the vents, you know you have a good casting. The foam will bond to the latex pretty well and you can paint the latex as above.

The other approach is to forego the latex entirely and use Self-Skinning Urethane Foam Elastomer. The “elastomer” part is important, since you want it to be rubbery, not rigid. You can cast this material into a plaster mold or a silicone rubber mold. If you use plaster, your mold must be sealed and coated with release agent (don’t use silicone-based release on a silicone mold) over the entire inner surface. You will again need a two (or more) part mold that can be clamped shut. You will again have to have small vents at the extremities. Mix a small quantity of self-skinning foam and pour it into your mold (again, moving quickly), clamp the mold shut, and, this time, when you see the foam come out, plug the vents with some plastilene or sticky wax.

For self-skinning foam to “skin”, the mold must build considerable internal pressure. (That’s why you plug the vents.) I design my molds for this use out of rubber with a partially occluded vent, so that when a vent pops I can just twist the clamp another turn to shut the vent off. Experiment with technique. You may have to rotate or tumble the shut mold to coat the inner surface—or even briefly brush the freshly-mixed urethane over the surface prior to shutting the mold—in order to get a good casting. Every mold requires you to discover the technique that get you the best casting from it. The resulting part will have a stiff rubbery skin with a foam-rubber core. To paint parts made of self-skinning foam, use Flexible Urethane Elastomer Paints. With this type of paint you can achieve a brightly colored surface that will not crack or peel off the rubber substrate, even when it is flexed. Once this latex, urethane, or combination “appliance” cures, use “spirit gum” to attach it to an actor’s face or body. It is then blended in with makeup to look natural.

Urethane elastomer foams, both self-skinning and not, come in various densities described as ‘weights’, usually in pounds, i.e. 2 pound, 4 pound, etc. This refers to the weight of a cubic foot of the foam. European formulators generally go by kilos per cubic meter or, sometimes, grams per cubic centimeter. Higher densities are progressively firmer, heavier, and less flexible. Foams also break down into the broader category of open-cell and closed-cell foams. Open-cell foams will absorb liquids like a sponge and are not suitable for anything that will go into the water, (except a sponge). Closed-cell foams (all self skinning foams are closed-cell) are waterproof and will float, but are also subject to significant changes in volume in response to large changes in ambient temperature. Urethane compounds are sensitive to the moisture in the air, which will degrade the material quickly once the can is opened. It’s best to use the whole quantity at once (don’t buy more than you need for the project at hand) but if you want to save some, fill the empty volume of the can with nitrogen, argon or some other inert gas. And put on your rubber gloves before you touch that can of urethane—the stuff is nasty and hard to remove.

Urethane Elastomers vent off Tri-isocyanates, the most dangerous of the iso-cyanates, when they are mixed. So wear an activated charcoal respirator with cartridges rated for organic vapor toxins when working with urethanes. The gases vent for approximately 20 minutes after mixing, after which the material is fairly safe. Cured urethanes typically “pyrolyze” (burn) around 320 degrees Fahrenheit. At this point they give off trace amounts of cyanide gas. This is an important consideration because cyanide is a cumulative poison—your exposure over years can build to the point of causing diffuse, difficult-to-diagnose systemic effects. Because of this, never use a hot wire to cut any kind of urethane foam. Do not throw too much foam material into a sealed mold—a little goes a long way—overfilling a sealed mold can cause a mold to explode, and/or the internal temperature to exceed 320 degrees.

by Christopher Pardell

With contributions from Andrew Werby