The Rubber FAQ—Flexible Mold Materials Compared

Following is a short course on rubbers and rubberlike compounds for moldmaking—it does not deal with how to use them. Flexible molds are desirable because they allow much greater freedom for the sculptor, permitting undercuts (places where a rigid mold would catch and hold the casting) and much simpler mold configurations than the many-part molds (with many seam-lines) necessary with rigid mold materials.There are seven basic families of rubbery moldmaking materials that are available to you as a moldmaker (not including the more exotic thermoplastic compounds). Each of the seven is nothing more than a category grouping literally hundreds of different formulations manufactured by hundreds of different companies each one of which has certain features in common and yet other features that make it unique. That uniqueness is a pain in the ass, especially considering that there are hundreds of available sealers, separation agents and different substances your original pattern might be made of. This brings the possible chemical combinations, and the possible disastrous results, up into the tens of thousands. For example, most silicone rubber compounds that you can get are sensitive to the presence of the highly aromatic substance sulfur. What this causes is called “Cure Inhibition”, that is, the silicone never quite turns all the way into rubber, instead it becomes almost exactly like Bazooka bubble that has only been chewed for ten minutes. It just so happens that the most widely used sculptor’s mediums is a plastilene that contains sulfur, and that sulfur is aromatic enough that most sealers will not adequately contain it. As you may see, just the ignorance of these few facts contains the potential for a catastrophic mold failure that could well mean the loss of a delicate original sculpture and some very expensive rubber. It is impossible for any book to cover all the possible ways in which you might combine chemicals to successfully produce a mold, much less the novel and innovative ways in which you might screw up. Let me familiarize you with the seven classes of materials available.

  • Latex

    Latex is God’s own rubber, and is derived from tropical tree sap. It was the first widely used rubber and is still the rubber most often used by amateurs. There are several things about latex that make it a phenomenal material. It has unbelievable strength and tear resistance coupled with the ability to stretch over 500%. It lasts an incredibly long time in production. It is also inexpensive. However, there are many things that make latex a poor choice for moldmaking, chief among which is shrinkage. Depending on the formulation, cured latex can shrink as much as 12%. That’s pretty bad. Just touching the surface of a latex mold will prevent the next coat of rubber from bonding where you touched it. Not good. Latex molds and castings will deteriorate over time, losing elasticity and eventually crumbling. Latex also has another major flaw. It is an evaporative cure system.That means that latex has a vehicle, like paint does. In the case of latex this vehicle is usually water and ammonia. Like paint, you brush on a thin coat of latex and the ammonia and water evaporates into the air leaving cured rubber behind. The trouble is that you can not paint on a thick coat. The surface of the latex would cure and seal the outside, preventing the ammonia in the latex underneath it from evaporating. The latex must be brushed on thin layer by thin layer to build up a usable thickness. This makes latex molds very expensive in terms of labor, and ridiculous in terms of meeting production schedules. I never use it and recommend that you avoid it as well. What moldmakers the world over have been seeking for over a century is a rubber material that combines the very best of latex’s qualities, strength and stretch, with a characteristic called “deep section cure” This means that a three inch thick pool of poured rubber will cure uniformly all the way through. Virtually all man made rubber compounds are formulated to approach this ideal.

  • Gelatin

    Yes, this is the same stuff you find in Jello. Gelatin is a pourable, deep section cure compound that is derived from animal protein. It has been used in moldmaking for nearly a hundred years and was the first man made flexible mold material. It takes the form of a powder that is mixed with water and heated. Much like a thermoplastic rubber (though not in that category) it is poured hot and as it cools forms a rubbery mass. Although it will deep section cure, gelatin is not that stretchable and not very strong. The fact that it is poured hot makes it unsuitable for taking molds off of plastilene or wax models and that is bad. Like Jello, mold gelatin is also soluble in water. Since plaster is mixed with water this is quite some problem as well. In the past moldmakers have used surface preparations such as tanol that minimize these problems. Today, thankfully, we have alternatives that are much more usable and can be mixed and poured cold.

  • Alginates

    In the same family as gelatin, (collagen elastomers) although a different chemistry, is alginate. Alginate is derived from seaweed and comes as a powder that is mixed with water, forming a thick goopy material that will set in about 15 minutes. Because Alginate mixes with cold (or warm) water, and because it is fairly inert, it is ideal for taking molds from live models, or from patterns made of delicate materials or with delicate finishes. If it can stand being wet for 15 minutes, alginate will not harm it. But alginates are exceedingly fragile, tearing easily. They will not stand up to most plastics, and, if left exposed to the air, all the moisture will evaporate out of them, leaving them shrunken and stiff. Their best use is for producing a quick plaster or wax master that can then be used to create a proper mold. Alginates and gelatins seldom need any kind of separation agent.

  • Urethanes

    Now we’re getting modern. Urethanes are a huge family of compounds, the chemical structure of which allows for an amazing diversity of characteristics, from titanium filled tooling resins to an elastomer that mimics latex right down to the color. Urethanes are used in seat cushions, fabrics, and impact-resistant protective paint treatments. They represent the most widely used molding compounds around today at least partly because they are so varied and partly because they are inexpensive. Frankly they are not all that great. I guess you could call me a urethane hater. No matter how clever the chemists get, they will never overcome the limitations that the chemistry of urethanes imposes, the stretch-strength conflict. It is in the nature of urethanes that if you need a rubber that is highly flexible and stretchable you must necessarily sacrifice tear strength. If you need high tear strength you’ll have to live with a stiff unstretchable rubber. Therefore, urethanes will never achieve the Latex—With-A-Deep-Section-Cure ideal. That’s not the only problem. Urethanes have a frustrating tendency to want to stick permanently to practically everything, except other urethanes. This means that when brushing a layer of rubber on a mold you have severe limitations on how long you can wait before the next coat of rubber if you want the layers to stick to one another. It also makes for a lot of work painting everything that you don’t want it to stick to with separation agents. The unmixed material is susceptible to contamination from the moisture in the air, so shelf life tends to be short. Urethanes are also very toxic. When liquid they are flammable. When they burn they give off cyanide gas. Worst of all they contain chemicals called isocyanates. When you mix the two components together the urethane begins to vent off isocyanates and continues to do so for about fifteen or twenty minutes. Isocyanates are what is called a sensitizing agent that cause allergic bronchitis, can lead to emphysema and are suspected carcinogens. OSHA, the occupational health and safety administration does not consider organic filter respirators to be adequate protection against isocyanates. Quite a litany of nasty stuff, eh? That is why I never use them. Nor should you despite the cheap price.

  • Neoprenes

    Neoprenes are most familiar to you as the weatherstripping on your car and as the very stuff of your car tires. Neoprene is a thermoplastic compund that comes in gumstock sheets and I include it beacuse it is widely used to cast white metal and pewter, since it can easily withstand the 400 degree temperatures. To use neoprene you have to have solid masters in a material that won’t melt or deform at 350 degrees F, a vulcanizing press (to turn the gumstock into rubber by squeezing it over your pattern at several thousand PSI.) and a centrifugal caster—a washing-machine sized device that spins the mold to create pressure in the mold cavity. Neoprene is not really usable for most artists, although if you want to make lots of small, highly detailed metal parts it is a very quick process that should be considered.

  • Vinyls

    Vinyls are also thermoplastic. Most dildoes are made out of vinyl elastomer. The major problem with vinyls, other than the temperature at which you have to work them, is that Poly-Vinyl Chloride is one of the most carcinogenic substnces known to man. I can not emphasize strongly enough—PVCs can not be safely used by artists. Avoid them.

  • Polysulfides

    Now we’re talking! Polysulfides are a marvelous moldmaking compound. They are the only rubber mold materials around with NO shrinkage. This is important in moldmaking where dimensional accuracy is a must.They can be mixed and poured and will cure in a deep section. They can also be brushed on like latex and filled with powders to make a stiff paste for filling and building thickness. You can wait a month to put on the next coat and it will stick like a single piece of rubber. Polysulfides can be vacuumed to remove air bubbles. You can speed the setting time by adding extra hardener. Polysulfide molds also store well, remaining usable for years. Blak-Tufy and Gra-Tufy are the most popular brands of polysulfide. The major flaw of polysulfides is that they are inherently weak as far as tear strength goes. This can be compensated for, though, by reinforcing the rubber with a light weight glass cloth called Veil Cloth. Another flaw in polysulfides is that they stink. I mean really smell. Like rotten eggs. Apparently one of the sulfides in polysulfide rubber is hydrogen sulfide. Once the rubber has cured the smell ceases and after about fifteen minutes around the stuff you don’t really notice it anyhow. A big drawback is that polysulfides have “Alzhiemer’s”. You can not leave the rubber mold out of its casing, laying crumpled on the table for 15 minutes or it will “forget” its original shape and want to adopt the crumpled shape. Poor “memory” for its cured shape is a major flaw. Polysulfides also contain lead in the catalyst, do not get it on your skin. This is difficult to avoid, as the cured molds continually exude a nasty oil. Another thing to watch out for is styrene. You know, like those packing peanuts. Styrene MELTS polysifide rubber. Turns it to goo. Not dramatically, but slowly, over a period of hours. You do not want your polysulfide mold to come into contact with any form of styrene.

  • Silicone

    Oh yes, this is the good stuff. Silicones are the cream of the moldmaking crop. They are the prayer answered. Silicones embody the very best of all worlds, they cure in a deep section, can be vacuum-degassed, and have extremely low shrinkage. They have virtually no surface tension, a property that allows them to flow even into the pores of wood. Once cured they have a high temperature tolerance and a tear strength of one hundred pounds per square inch with a 300% stretch. (or more) While remaining soft and pliable, they retain a perfect memory of the shape they are supposed to hold. You can cast virtually anything into a silicone mold and know the mold will release from the casting without fail. They are also just about the least toxic of all. Their principal flaw is that they cost an arm and a leg, from $100 to $180 per gallon! Another problem is most of them are easily inhibited by sulfur, which as you now know is present in most sculptor’s plastilenes.Silicones are often referred to as RTV silicones. This stands for Room Temperature Vulcanizing and refers to the fact that these products do not require any kind of heating to stabilize and strengthen their properties. There are two basic types of silicone, tin cure and platinum cure. Can you guess which one costs more? That’s right, platinum cure systems are the most costly. In addition they are the most finicky, inhibited by nearly everything. They do have much wider temperature tolerance and are far more resistant to chemical attack than tin systems and the molds they make store far longer, too. Some platinum cure silicones are “food grade” material, meaning that you could, for example, cast chocolate bunnies in them safely; however, not all platinum systems are food grade—if that’s what you need, ask for it. Tin cure systems are the more common and affordable of the silicones, though they vary quite widely in properties and price. What makes silicone worth the price is production. Although a silicone mold can easily cost three times as much as an identical urethane mold, a silicone mold will easily produce five to ten times the number of castings as a urethane mold provided you cast it non-stop until the mold fails. If you are going to use moldmaking to produce a product in anything like large numbers then silicone is the material your molds should be made of.

Due to the myriad chemical interactions briefly mentioned above it will be your responsibility, Bucko, to exhaustively test any new materials you may come across in some harmless manner to ensure that your precious mold project doesn’t pull a Titanic on you. What you need to do whenever you get some new, unfamiliar material like, say, a new silicone rubber, is to rig up a controlled test on every other kind of material and combination of materials you could conceivably want to pour that rubber on. Like a swatch of pure plastilene and one of plastilene sealed with lacquer and plastilene sealed with shellac and anything else you might have in the cupboard. Keep a log that records the results since it may be a long time till you actually use the stuff and memories often fail. I cannot stress too strongly the importance of testing your materials! Think of it as the “look before you leap” doctrine with a wee bit of lab work thrown in.

by Christopher Pardell