The Urethane Rubber FAQ—Making Molds with Urethane Rubber

Urethanes are a huge family of rubbers as well as plastics which are easy, economical, and quick to work with. Rubber elastomers and hard-plastic urethanes are extremely similar chemically, but the hard types have a plasticizer in their formula. It is easy to find urethanes which can be painted on and have great tensile strength, exceeding that of silicone rubber. Urethane rubber typically costs around $35 per gallon, a third of silicone’s price, and used as a paint-on, goes a long way. But each material has advantages: silicones allow one to cast wax, polyester, and urethane resins without any release agent, while urethanes usually require a spray-on mold release of some sort, and never release as nicely. None is needed for plaster or concrete, (although it will extend the mold’s useful life) or for wax when the mold is new. After casting resin into a urethane mold a while, the mold’s surface grows harder and rougher. You’ll have to replace it if a slick surface is vital.

Also, be aware of heat buildup, the real enemy of this rubber; the sooner you demold, the longer your mold will last. Demolding too early may cause distortion, but this can be used to advantage—urethane plastic castings taken out at the “leather-hard” stage may be draped and twisted considerably, and soon take their final set retaining these deformations. These castings easily knit with armatures and inclusions, and with each other. So a half-hard, half-rubbery piece or a living hinge is easy to pull off. And speaking of pulling things off: don’t leave a urethane casting in a mold overnight. Spray release won’t prevent the masses from bonding over time.

Where the paint-on urethanes shine is in creating large, cheap, extremely flexible sheet molds. I like to make thin reinforced plaster or polyester backups, often in several pieces. Say you’re pouring waxes, and you have to work thin: to avoid distorting the part when demolding; the thin painted-on rubber mold is easy to peel from the cast wax. A thicker cast rubber mold won’t peel away like that, and this means whatever you cast in a heavy rubber mold has to attain great early strength before demolding. Concrete and plaster edition castings go much faster when the mold can be recycled sooner. The pourable urethanes are advantageous when brushstrokes would harm your pattern; then pouring the rubber on is safer. When molding a bas-relief, the labor is cut to almost nothing with a poured mold.

Another advantage of the urethanes over silicones is in incorporating fibers and foams. Some silicones won’t knit with these inclusions (some will) but I often place woven fiberglass cloth or cheesecloth in my paint-on urethane molds at the places I know are wont to tear. And I place foam rubber pieces (also urethane, ) cleverly cut and buttered with the rubber, into undercuts, so as to have easily-pulled-out collapsible areas.

I find that the affinity urethanes have for each other can be an advantage. I can pour a bas-relief mold, obtaining that nice flat back, and then paint-up another mold feature onto the front. (I can get perfect knitting between a clear amber pouring type and an ivory-colored paint-on type.) Smooth-On’s 724 system allows one to customize the softness of the rubber from gooey-soft to Shore D40 (fairly hard), as well as to vary the viscosity of the uncured stuff from the consistency of latex paint to that of peanut butter. They even tell you how to make it foam on purpose, so you can carefully paint on the first crucial coat, nice and thin, then pour foaming rubber over it for an extra-flexible but simple box-shaped mold. Other companies will sell rubber in different hardnesses and already thixotropic (non-flowing). If you have painted a brushon rubber evenly all over a pattern, you need a seam in order to take the pattern out and later to take your castings out. By casting bars of the same rubber like bars of margarine (maybe a foot or two long) and pulling them out of their molds when just barely cured, you can stick these to the paint-on mold where a seam is needed. As the rubber cures it take the shape of the new mold, not the straight one. You can cut right through the thick sections to form flanges or bosses which are easily held tight together by your support mold halves. This will work best when the painted-on rubber is very fresh. Pins may be used to keep the bars of soft rubber tight.

Nothing is perfect, though. Smooth-On 724 molds will degrade in seven years or so. Pour all the castings you will need when the mold is in its prime, and remember to cast and save a pattern part for remaking the mold. Also, once opened, one of the components of urethane will go bad in the can from the moisture in the air. A blanket of inert gas will help preserve unused supplies, but in general it is best to buy only what you plan to use soon. And most urethanes release toxic isocyanates into the air when mixed, so work with positive ventilation, with air being supplied as well as exhausted (a fan in front of you and an open door or window behind you.). Wear a respirator with organic vapors cartridges if you’re unsure about the air quality afforded by your ventilation arrangement. Isocyanates are heavy: they creep along the floor (or, most notably, the streets of Bhopal, India.)—so be careful not to let pets or children in the workspace when using this material. There are now a few types (like Smooth-On’s “Evergreen” line) which claim not to contain free isocyanates, but this type isn’t recommended for wax casting.

If the painting process gives you fits, it is pretty easy to form temporary casting shells using sulphur-free plastiline (sulphur can inhibit the setting of mold rubbers—test any combination of clay and rubber you’re not sure of) or wax sheets and then to fill your molds with the pourable rubber. Bigtime producers don’t stint on rubber, they just pour a boxful around the pattern and cut it open. These simple poured molds are the easiest rubber molds to make. Since no brush touches the pattern surface, this works well on soft plasticene bas-reliefs. To make them, you need a good flat board, preferably slick—Formica is good. It usually needs to be at least 2” wider all around than your pattern piece. After your pattern piece has been sealed with shellac or an acrylic lacquer, it should be glued or screwed to the middle of the board. Caulk the seam all around with plasticene (sans sulfur, remember?) or silicone (but that’s so strong, it could be glued forever.) All you want is to prevent rubber from running under the pattern.

Build walls around the pattern. Measure its highest point, using wood blocks and a straightedge. Your walls must be a little higher than this—at least half an inch. Their distance from the edge of the piece depends on how stiff your rubber will be: stiffer rubber permits thinner walls. How stiff you work depends on how delicate your details are and how hard your castings will be. Since wax is not very hard, and will flex slightly, you can use softer rubber. Concrete is very hard, and hard rubber is normally used, but if your detail is deep or highly undercut, use the soft. When using soft rubber (Shore 35 and below) leave room to get a square section at the edges, with the same depth as width. Then you won’t need a support mold. If your pattern is rectangular, cut wood strips and fasten them down to the board as well as to each other. With concrete plaques, it is common to have the walls directly against the pattern. Then the completed mold is flopped into a casting box just that size, so it lines the entire bottom. If the pattern is odd-shaped, plasticene walls may be easiest. Spray the whole interior area with a recommended release spray and set a fan blowing on it.

Mix your rubber thoroughly after measuring it carefully, either by weight or volume. Geniuses can compute cubic inches of rubber, the rest of us guess and get good at it eventually. Since it’s easy to mix more, guess low at first. Place the board on a truly level surface. I use a big level and shims to get it right. De-air the rubber if you have the equipment. Otherwise, pour it in a thin stream at the lowest point in the pattern or into the wall section if you can. Just pour it slowly, don’t slosh it. Blow it around with an air nozzle at low pressure if you suspect there are bubbles down there. When you just cover the highest point, stop and wait a minute. Lift and drop each side of the board an inch or two to jog bubbles free. If you see a steady stream of bubbles fom one point, that means the rubber is going under the pattern there—too bad…

Let it harden, usually overnight. Pull away the walls carefully. Slip a flexible knife between the rubber and the board and whisk it all around. Pull the mold up a little from each corner, and keep going around. It’s common to pull off poorly-adhered paint-jobs and shellac with the rubber. If rubber went under the edge, leave that edge for last and use a razorknife to slit the stretched rubber. (The easiest way to cut all rubbers is to use a sharp, wet, and soapy blade.) Flop your finished mold on a flat table and go read the manufacturer’s suggestions for post-curing, if necessary. If you have significant air bubbles, you want to fill them with new rubber now, before your first cast. If the walls seem too floppy, consider making a support mold, either of wood or reinforced plaster, but don’t spend too much time on it.

Two-part poured molds, like for a mushroom-shaped piece, start pretty much that way, but you pour up to the level where you want a parting line (the edge of the cap) and stop. As the rubber is jelling, press a marble into each corner of the edge. When it has set, pluck them out, and your second pour will have nice keys. Don’t pour the second part until you have sprayed the fresh rubber with a release agent. If your pattern piece, while basically flat and low, has a couple of upstanding spikes you need to mold, go ahead and pour the low part of the mold. Then cover the spikes with a thickened rubber, forming little “mountains”, or set some short tubes (like bottomless plastic cups, rim down) in the rubber just as it’s jelling, mix a small batch, and pour them full to cover the spikes. If you do this, you’ll need to make a support mold, probably from plaster. These extensions work best if they have taper (draft) like real mountains, and the tubes can’t go too tall or you’ll crack the spikes when removing the mold from the support mold.

by Dan Spector

With contributions from Andrew Werby