The Silicone Rubber FAQ—Using Silicone Rubber for Mold-making

The best rubber for general-purpose sculptural mold-making is called 2-part liquid RTV (Room Temperature Vulcanizing) silicone. This comes in two basic varieties; tin-cure and platinum—cure. The tin cure type is cheaper and more forgiving of impurities in the model and release agents, the platinum cure type lasts longer and can be tougher. It also comes in various hardnesses, measured on the Shore D scale with a “durometer”, a hardness meter which pushes a tiny ball bearing into a surface, measuring the force required. A reading of 20 is very soft, 80 is very hard. The more undercuts you must mold, the softer the silicone you must use, to yank it out of difficult places. Being softer, it elongates and deforms easier. When softer silicone is used, it is less able to hold itself in place, and more likely to sag as gravity pulls. So softer rubber molds must be made somewhat thicker to avoid collapse. I prefer using a product called G.I. 1000 from Silicones Incorporated. If you’ve never done this before, I strongly recommend hiring an experienced moldmaker to assist you and walk you through the process. There are a lot of fine points to the moldmaking process that are hard to convey in a brief written article, which can nevertheless make the difference between success and failure.

Besides the silicone rubber and its standard catalyst, you will need to buy two additives: the first is a “thixotropic” agent to mix with the silicone. This agent makes the silicone thicken so that it can be laid up by hand in thicker coats without running off. The second is “ultrafast catalyst” which helps the action of the thixotrope and causes the rubber to cure rapidly enough to apply several coats in a day. You may also need to get a bag of glass microspheres, or silica-based thickeners called Cabosil or Aerosil. These are used to further thicken the rubber to a paste-like consistency. If you use them, be sure to wear a dust mask—the particles are very light and will float everywhere as you try to mix them in—and they aren’t something you want to breathe. Mix the rubber in HDPE plastic buckets, or use disposable plastic cups if they are flat on the bottom. For the thin material, stirring with a stick is sufficient; propeller mixers held in portable drills help with the thicker material. To avoid sticky spots in the mold, mix in one container, then scrape out the contents into a clean container and remix—this avoids pickup of uncatalyzed rubber. Weigh the rubber and catalyst carefully using a precision scale—the rubber needs every bit of its 5% or 10% by weight, but not too much.

Your sculpture may need to be sealed so that the silicone won’t sink into the surface. If in any doubt, do a test on a small section to determine if it tends to stick. If the answer is yes, then use either mold soap or a spray release that is not silicone-based—teflon spray will work. Again, if you are molding something precious, test a small area to check for bad effects—sometimes the release will discolor marble, or ruin the finish of a carved wood piece. To get the model out of the mold, as well as to release the finished castings, you either have to make your mold in sections, or cut it out afterwards. To do the former, set up oil-clay walls along the parting line, making sure to use a clay that you’ve proved compatible with the rubber you’re using. Carve some “keys” in the clay wall to insure registration, and apply the rubber to this side only, stripping off the clay when it is done, then release the edge with soap or another agent, and apply the next section. For a cut mold, paint the entire model with rubber at the same time, but build up some ridges at least ½″ wide and equivalently thick along the parting line. Cut down these ridges with a wiggling stroke when the mold is done (shell and all) to extract the model, but don’t cut any more than necessary—less seam is best. Design your shell to push the cut edges together when the mold is clamped shut.

It helps if you have a vacuum pump and chamber to eliminate air bubbles from the mixed rubber prior to applying the first coats. But if it is brushed on thin, the flowing material will tend to eliminate them without this. Be aware that high temperature and humidity accelerate the curing of rubber—do not use so much ultrafast catalyst that the material seizes up before you can get it smoothly applied.

Your first (face) coat needs to be finely applied and not very thick. Take the time necessary to eliminate air bubbles. You must wait for this coat to fully cure before proceeding, or you will find yourself pulling it off with your brush as you try to add rubber. The second (face) coat can be thickened as much as the agent will allow (usually 5% by weight is the maximum amount of liquid thixotrope you should use—any additional thickening is by adding powders). You must wait for this coat to fully cure as well.

After the first 2 face coats have set, it is time for the fill coats. You should try to do all the filling at once—it will take a lot of rubber. Measure out several buckets of rubber, so that each will require the same amount of catalyst. Measure out the catalyst and thixotrope for each bucket, but don’t mix them yet. Use the full amount of thixotropic agent but go light on the ultrafast for these coats. You want these coats to take a longer time to cure. Starting with your first bucket, add the catalyst and mix, adding aerosil all the time until the rubber is a stiff paste. (Wear that dust mask!) Smear this on, building up the average thickness and filling in undercuts. Move immediately to the next bucket and do the same thing—do not wait for it to cure—you want to do all the filling before the first fill coat cures.

When you have all the undercuts eliminated, and the high spots uniformly covered with at least ¼ inch—mix up one last bucket with no thixotropic agent but with ultrafast catalyst. Do not add aerosil, you want this “gloss” coat to be thin. Using a brush, wet it with silicone and brush it over the fill coats, smoothing the fill coats down . The idea is to leave as smooth and uncomplicated a surface as possible on the outside of the rubber. Let it cure overnight, or longer if the space they’re in gets below 55 degrees F. at night.

You will not need to reinforce the silicone with cloth; it has a tear strength of 100 pounds per square inch. But a layer of cheesecloth can act as a “ripstop”, since an existing tear in silicone rubber tends to propagate. Also, I find that my hands work better than any brush in applying the fill coats—wear disposable gloves. If you do use brushes, count them as expendable items, since the rubber is difficult to remove, and tends to set up in the hairs.

When making the shell to support the floppy rubber mold. you do not need to put a separation agent on the exterior of the rubber—virtually nothing will stick to cured silicone. You can make a casing (mother mold) out of fiberglass-reinforced polyester resin or fiberglass-reinforced epoxy resin if you don’t plan to cast hot wax in your molds (it will stick). You can also make a casing faster, cheaper, and better out of sisal-reinforced plaster. Sisal is the raw fiber from which manila rope is made. Look for it at construction supply houses. Other materials, like chopped strand or scrim-weight fiberglass, can also be used instead of sisal to reinforce the plaster. If you are casting hot wax, soak the plaster, and it will release accidental spills. For a large mold, build a wooden or steel frame that you can tie the plaster into for added strength. The plaster casing itself should only be about a half inch thick on average—more than this adds needless weight, although long sections might benefit from rigid reinforcement to avoid cracking.

A mold made as described above should produce 1 to 2 dozen castings in polyester resin before failing. (Other materials, like wax, and plaster, are less stressful to the mold, which will last longer if they are used exclusively.) There is a Mann/SmoothOn product for restoring the surfaces of silicone molds, extending their useful lives. Paint it on a cleaned mold, wait 2 hours and the castings should show improved gloss.

If you load the polyester gel coat with ground alabaster and talc you can closely approximate the look of marble; use pecan shell flour and it will resemble wood, or use flakes and dyes designed for polyester if you want to experiment with flashy effects. For casting glass-reinforced resin, a process called “lay-up”, I would suggest contacting a fiberglass boat builder and getting a quote. You can do it yourself, but fiberglass casting is tricky and you will have a long learning curve, with lots of bad castings and frustration along the way. If you want to learn it, see if you can hire an experienced lay-up person to make one with you assisting.

For some patterns, especially flattish reliefs, it may be feasible, if somewhat less thrifty, to simply pour the rubber over the piece. Construct (or find) an enclosure of some sort that will hold your model, with half an inch or so of space all around. (If, for example, you’re doing a round piece in a square pan, you can fill in the corners with a compatible plastilene clay, to save material.) Adhere the pattern firmly to the bottom of the vessel (if it floats, the mold is trash.) Then mix the silicone without any thickener, de-air if possible, then pour a thin stream of rubber onto the bottom of the mold, letting it fill from the bottom. Tap it a few times, or tilt it before it’s full, to free any trapped air bubbles. Watch for leaks—small ones can be plugged with the plastilene. Then let it set overnight before unmolding.

The preceding technique is referred to as “slab” molding, since it generates a mold that is essentially a large flat slab of self supporting rubber. It works fine for small or relatively flat patterns, or for patterns that are very much the shape of the box you build around it, however, it is not suitable for more organic forms above 2 or three inches in height—nor for any pattern which will require cutting the rubber to demold. The reasons are first, that silicone and other rubbers are very expensive to pour in large sections, and second, that while rubber compounds do not shrink a lot, they do shrink somewhat—and significantly thicker sections will shrink out of proportion to the thinner sections, causing warpage and distortion of the cavity.

Here is a better technique for more complex patterns, which will work with silicone or low-viscosity urethane rubbers. Mount the pattern to a formica sink cut-out or similar non-porous surface. Spray the pattern with the appropriate release—(for a plastilene pattern: teflon spray for urethanes, semi-gloss acrylic lacquer for silicone) Determine if the mold will need to be cut open to get the castings out, and where that separation line will run. Along this separation line, look for areas that might catch air in casting. Visualize the sculpture upside-down as if it was made of hollow glass, full of water. As the imaginary water drains out the bottom (now the topmost point of the pattern), any spot that might hold a little puddle is an area that will catch air when the mold is inverted for casting. You can vent these spots by cutting a length of wire or welding rod to length and sticking one end into the point you wish to vent and the other into a hole in the formica board.

Be sure that only those vents that are along the separation line you intend to cut run to the surface of the board. These wires will be visible on the bottom surface of the finished mold and will act as guidelines for you to follow when you cut the mold open, guiding the point of your knife to the precise points on the pattern you want the cut to run through. Any areas needing vents that don’t lie on the parting line can be vented back into the body of the sculpture.

Next you will make a contoured form to pour the rubber into, using a compatible plastilene clay. Get a quantity of plastilene (it need not be the finest grade—some with plaster chips in it will work fine) and either melt it and pour it into a slab mold to produce half inch thick slabs, or use a slab roller to roll out the plastilene into half inch thick slabs. Cut the finished slabs into long strips about 3 to 4 inches wide. Use these plastilene slabs to build a customized form around the pattern, maintaining a half to three quarter inch gap between the plastilene slab and the surface of the pattern. You can bend and contour the plastilene wall to follow the pattern closely. Go all the way around with one 3 to 4 inch width—leaving plenty of clearance for the separation line to be cut with a key knife. Then add the next layer of plastilene slabs on top of the first. Use tools to thoroughly smear the two slabs together where they abut.

Try to keep in mind that the hollow space you are creating will be solid rubber, and that the inside surface of the plastilene form will become the outside surface of the rubber. Consequently, be aware that this surface will need to be drafted for however many casing parts you will make to hold the rubber. You should be able to determine at this point how many will be necessary, and design the outside of the wall accordingly.

Keep building up your wall of slabs like building a coil pot, making sure there are no potential leak points. Once your form is 2 inches taller than the highest point on the pattern, cut a slab of plastilene to cover the open top of the wall temporarily. Mix a small bucket of plaster and splash a thin coat of plaster all over the plastilene form, but not on the temporary top. Don’t make it too strong—this isn’t your permanent mold casing; it just helps ensure against the rubber leaking out. Be sure to let the plaster glop over the edges of the formica board you are working on—you want a good mechanical attachment so the whole shebang won’t levitate when you pour the rubber in.

The plaster will seal and reinforce the plastilene form. Take off the temporary top. Mix your rubber (de-gas it under vacuum if you can; otherwise, pour in a pencil-lead thin stream) and pour it into the form. With a little experience, you will be able to do all of this in about 3 hours for a figure, say, 12 inches tall. When the rubber has fully cured, chisel off the plaster splash coat, exposing the entire outside of the plastilene form.

With an eye toward draft, cut the plastilene along the separation line for the first casing part, and peel the plastilene off the rubber beneath, exposing only the section of rubber mold that the first casing part will mate to. Leave the rest of the plastilene stuck to the rubber for now—you will use it to stick more slabs to, in order to form the separation line for the plaster casing or “mother mold”. Make the first casing part, using plaster reinforced with sisal or fiberglass. Then cut off another section of plastilene, exposing the next section of rubber for the next casing part. (if this was a two-piece casing, you will have exposed all the remaining rubber) Apply a separation agent to the exposed edge of the first plaster casing part and make the second casing part. If all the piece required was a two-piece casing, you can now pry the casing open and remove the rubber mold and master from the formica board. (If not, continue to make plaster casing pieces until you’re done.) Rip the mold up the separation line with a key knife, then, take a fine point X-Acto or similar sharp little knife and continue the key cut to the end of the first vent along the separation line. Follow the vent wire with the point of the knife until you come to the pattern, extending the cut as you go to each successive vent wire.

This technique will give you a very good long-lived mold with a fairly uniform thickness from a fragile master pattern. It will use far less rubber than a slab mold, and will be more flexible and accurate as well. Remember that for small items, particularly low reliefs, coins, and refrigerator-magnet type patterns, a slab mold is perfect. This technique is better for figurative or wildlife subjects in the 4 to 18 inch size range; beyond that, a brush-on mold is more useful and cost-effective.

Another simple technique for small molds that must be cut open is to find a waxed paper cup that will fit over the pattern, then stick it down around the rim with hot wax or warm plastilene. Cut the bottom of the cup out to allow the rubber to be poured in, and fill it with rubber. When the rubber is cured, peel the paper cup off, cut it open and remove the pattern. Now any waxed paper cup like the first one will act as a perfect casing (mother mold) AND will generate castings with NO flash (leakage along the separation seam) Be sure to poke a hole in the cup bottom so that the mold will slide back out easily without suction.

by Christopher Pardell

With contributions from Andrew Werby and Dan Spector.