The Resin FAQ—Casting Resins into Molds

Casting is the process of producing a part in a mold. Resins can be cast as a pourable liquid, or they can be “laid up”; laminated by hand with reinforcing cloth of glass or carbon fiber. Here we shall deal mostly with poured casting technique, since lay-up is rarely used by artists. Resin properly filled can cast solid figures 36 inches tall—most sculpture is within those parameters.

Most resins can be cast in properly-separating flexible molds made of polysulfide and urethane, but the best results are obtained from silicone molds, which require no separator coat. They can also be cast in rigid molds that have draft ( without undercuts, and slightly bigger at the bottom than the top) or composed of multiple drafted sections, if the molds are properly assembled. Rigid molds can be of plaster, fiberglass, metal, and even wood.

There are a few types of resin available for casting: acrylics, polyesters, urethanes, and epoxies are the materials most used by artists. To work with acrylics requires some equipment. Casting acrylic ‘water-clear’ means subjecting the polymer/monomer “dough” to high pressure and high temperature in an autoclave, using special molds. I know a company that has spent 4 years and untold thousands of dollars just trying to develop a reliable acrylic casting technique. Because of this, and the fact that uncured acrylic is highly toxic, most artists should avoid using it and it will not be covered here. Instead, we will concentrate on those resins easily cast in standard molds; the pourable resins that set at room temperature.

Polyester resin is the most common castable resin. It is a catalytic system wherein you add a small amount of MEK peroxide to accelerate the polymerization of the liquid resin (which would eventually cure on its own, particularly if left in the sun.) You can control the curing time by varying the amount of catalyst you add. Polyester resin is the resin component of most fiberglass layups and is normally glass clear. Understand that glass is not all that clear—it’s actually green, and so is polyester resin. It can be filled for thicker castings and to affect its appearance. It can be tinted colors both transparent and opaque using special pigments, and, for outdoor use, can have a UV inhibitor mixed in. Polyester resin comes in laminating [hot-promoted] and casting [slow-promoted] formulas. Do not attempt to cast with laminating resin, as it is meant to produce enough heat to cure in thin section. Also, its surface will not fully cure while exposed to air. In laminating fiberglass, this property, called “tack” ensures the next coat will bond well. So when laying up, the final coat needs wax mixed in it, to achieve a good, glossy cure.

Polyester resin is what most figurines you see in chain stores are cast in. Cast polyester resins become rubbery when subjected to temperatures over 200 degrees. They stink to high heaven; the resin stinks, as do the newly cast pieces and your workspace if you use them (although you won’t notice it after a while). Polyester is also quite toxic. The problem is not so much the polyester resin itself, but the crosslinking agents and catalysts, diluents, and accellerators which make them work. Cross-linkers include styrene, methyl methacylate, vinyl toluene, diluents include toluene and ketones, catalysts include methyl ethyl ketone peroxide, benzoyl peroxide, and cumene hydroperoxide. And there are accelerators and promotors like cobalt napthanate and dimethylanaline. The styrene which is the cross-linking agent in most polyester casting systems is “a highly toxic aromatic hydrocarbon solvent which can cause narcosis, respiratory system irritation, liver and nerve damage, and is a suspect carcinogen” (Monona Rossol, Artist’s Complete Health and Safety Guide). The initiator, usually an organic peroxide in a solvent base (like methyl ethyl ketone peroxide) is also nasty stuff, which can cause instant blindness if splashed in the eyes. These also convert to an explosive as they age, and support fires which cannot be quenched, as the material supplies its own oxygen. Many organic peroxides are potent allergic sensitizers as well. Their long-term health effects are largely unknown, but aren’t likely to be good—err on the side of caution. Use these materials only in positively ventilated rooms (that means with fresh air coming in behind you and an exhaust fan in front), and use a respirator equipped with organic vapors cartridges. Use Nitrile gloves (these solvents will eat regular rubber gloves), goggles, and protective clothing. Skin and nasal irritation and killer headaches are common effects of short-term exposure.

Epoxies are nearly twice the price of polyesters. They are a compounded system, two chemicals that produce a reaction, creating a third chemical. Epoxies have a wider temperature tolerance than polyester as well as greater strength, and have been formulated to achieve a wide variety of properties, from thin castables to peanut-butter thick trowel-on systems. Epoxies are mixed in ratios by exact weight or volume, and are made in different formulations for differing cure times. You can ask for the pot life you need. They too can be tinted and colored with special pigments and come both opaque and nearly clear. The amines in them are potent sensitizers, and can cause allergic reactions. Epoxy’s basic stickiness makes it good for adhesives and repair applications. Epoxies are available off the shelf with all manner of fillers—aluminum, titanium, steel, etc.-mixed in for different uses. Epoxies have the least objectionable odor of nearly all resins.

Urethanes are a huge family of resins with a wide variety of applications. Urethanes can be made to foam consistently, and can be rigid or flexible. Castable urethane resins are generally formulated to cure in one to five minutes. Ask about the cure time for any urethane you buy. They can be filled, and can be tinted with special pigments. Most urethane resins are a yellowish tan color to start, but are now widely available in white. There are a few companies offering a water clear that is truly colorless. Urethanes are the least brittle resin, offering some give before they break. The biggest reason urethanes are not more widely used in art and giftware is the smell. Not only do they smell badly and strongly—they never stop smelling. Ten years ago, Vagabond Corporation in Warner Springs, CA invented the best urethane for most artists’ uses, called “Odorless White.” Strangely enough, it really is odorless. It starts clear, but turns a creamy opaque white when it kicks. It doesn’t really stay white, though; sunlight and time will yellow it. Today many companies offer their own version of Odorless White urethane, such as MasterCast from Kindt-Collins.

One of the Holy Grails of rubber mold resin casting has always been the search for a method of casting an optically clear part. While many resins come in clear formulas, it seems impossible to cast them and have them come out clear enough to read through. As said before, resins always shrink and the last bit to cure is the surface—this conspires to prevent the reproduction of a glassy surface no matter how perfect the mold. (The one exception to this is a fully drafted vacuum-formed polyethylene mold, with a simple hemisphere shape—as the resin shrinks, the casting slides further down into the bowl, maintaining contact with the surface of the mold. Alas, more interesting shapes lack this quality.) While you can fill resins to minimize shrinkage, filling them will ruin the clear quality you’re after. For decades the only way to get an optical surface was to cast it close, and then laboriously hand polish it to a rouge finish. Today, several companies, such as Synair Corporation or BJB Enterprises, (see suppliers list at bottom) offer “water clear” urethane resins. They are tricky to work with. Some have inhibition problems with certain silicones, for example. All are tremendously toxic sensitizers. They achieve a far more faithful unfilled casting fidelity by taking a long time to cure, so that the surface is not soft when the core cures. Synair’s product takes 8-12 hours to set, and then requires a 2-4 hour “post-cure” at 275 degrees. These products are not perfect. There is nominal shrinkage, and the part comes out a little less than completely clear. However, they are close enough that adding one or two coats of clear gloss coat will achieve near-perfect clarity: a first in casting materials technology. Of course, being the holy grail, they cost an arm and a leg, but at least they’re available.

All resins shrink when they change from liquid to solid and all generate heat as they polymerize, (an “exothermic” reaction) and this causes problems. Because heat accelerates chemical reactions, all resins cure from the inside out. (As opposed to wax, for instance, which freezes from the mold surface inward.) As the setting resin generates heat, the center of the mass gets hot faster than the surface, which is transferring its heat to the mold. As a result, the center hardens and shrinks first, while the last area to harden is the area in contact with the mold. Unfortunately, by that time, the majority of the shrinkage has occurred, and there is no reservoir of uncured resin from which to draw. As the casting shrinks, its tacky surface is pulled out of contact with the surface of the mold, leaving a rough and randomized surface, distorted and unfaithful to the original. These spots are called ‘molding marks’ in the industry, but should be called ‘shrinkage flaws,’ because they are not caused by the mold.

Another consideration is that resins are poor conductors of heat, so they can generate heat far faster than they can radiate it. It does not take a very large mass of resin before the temperature it achieves exceeds the point of pyrolysis (or “burning”). You will know you’re there when the material starts smoking, giving off clouds of toxic fumes and cracking wildly. (Try to avoid doing this!) Heat also exacerbates the resin’s chemical attack on your molds—I have seen new molds reduced to a brittle, yellow-cheddar consistency with one resin casting that got too hot.

Because they shrink, cast resins cannot be easily reinforced with internal armatures like steel rod. The resin shrinks around the unyielding steel—and is split as if by a wedge. Resins can also tear themselves apart. Polyester resin, for example, goes through a jelly-like state before becoming hard. In thicker castings or those catalyzed too hot, the core is already hard when the outer areas gel. The gelled material must shrink as it becomes a solid, but when it cannot compress the hard core, it splits on the outside instead. When you see spiral fractures running from the surface into the casting, but not all the way through, you know that you are casting too large or too hot.

There are ways to manage this problem in order to achieve larger castings. The techniques fall into 3 strategies: 1) minimize the heat generated, allowing the entire casting to cure without the internal stresses, 2) reduce the shrinkage, which lessens internal stresses and keeps the resin in contact with the mold to get a better surface, or 3) cast the surface first, with a resin that coats the inside of the mold and is then poured out, building up layers until a structural shell is achieved. (These castings can be left hollow, or filled after the surface is set.)

One way to minimize the exothermic reaction is to slow down the polymerization. A given volume of resin will generate a given number of calories of heat. Polyester resin ordinarily comes “promoted”, or dosed with a cobalt dryer which allows the catalyst to cure it slowly. Mixing polyester resin with the minimum amount of catalyst causes it to generate its heat over a day or two rather than 15 minutes. This time function allows the heat generated to be radiated away, lowering and equalizing the temperature of the resin. The ratio of catalyst to resin is not, unfortunately, a simple formula capable of being universally applied. Each mold configuration will require some adjustment of the ratio to optimize the set time without overheating and cracking on the one hand or, on the other, failing to set because sufficient heat wasn’t achieved. Epoxies and urethanes can similarly be ordered in slower-setting formulas, although their exothermic reactions are not as dramatic as polyester’s.

Filling the resin also helps produce castings with fewer shrinkage and heat problems. This works in two ways: first, it reduces the quantity of the resin in the total volume, which reduces shrinkage and exotherm. Fillers also act as a ‘heat sink’—basically giving the resin molecules a place nearby to absorb their heat. While the material still gets hot, that heat is expressed predominantly in the molecular motion of the filler; the filler’s capacity to absorb heat will significantly limit the peak temperature attained. The filler expands as it heats, while the resin shrinks as it polymerizes, so these tendencies cancel one another to some extent. The filler also creates a rigid matrix in the mass, mechanically preventing excessive shrinkage.

The specific gravity of the filler will also affect the density and weight of the casting. In giftware, calcium carbonate and talc are often used to fill resin to give the casting the look of porcelain or marble and greater heft, which equals higher perceived value. Glass beads will add significant weight, while aerosil will make it feather-light. High abrasion applications would indicate a ceramic filler, whereas cast parts that will be subjected to high temperature suggest a metal filler, which will absorb some heat. Dried and ground pecan shell is commonly used as a filler to create parts almost indistinguishable from wood. Finely chopped fiberglass or Kevlar strand is sometimes used to give high resistance to breakage. Bronze or aluminum powder gives castings the look of metal. Any opaque color protects resin from UV, which slowly destroys castings. (Epoxy is more vulnerable to this than polyester, but any resin will degrade in sunlight given enough time and exposure.) Whatever you use, your filler needs to be bone-dry. Moisture in the filler can cause all kinds of problems with the chemical reactions of resins. For example, urethane resins will foam up when contaminated with water.

Fillers cause two problems of their own. First, they carry air with them into the mixed material. Second, in the amounts that will eliminate the shrinkage and heat concerns, they turn a liquid resin into a viscous porridge, in some cases too thick to vacuum. Both cause or exacerbate air-bubble problems in the casting.

Let’s deal with the bubbles first. What you can do about them requires identifying their cause. The most basic idea you need to understand is that you are not pouring resin into an empty mold. You will be pouring resin into a mold that is already full to the brim with air. A good moldmaker knows that for the resin to go in, the air must get out. Except for molds with large openings, never ask the air to come out the hole the resin is going in. Engineer the mold with vent channels, and run those vents to the uppermost surface of the mold so that you won’t have to try and plug vents as the mold fills. Orient the position of the pattern and the separation planes of the mold so as to reduce the occurrence of air traps, and to facilitate the placement of gates (where materials go in) and vents (where air comes out). For small pieces that will require the thickened resin to flow through tight spaces, gate the mold so that the introduced resin enters the casting cavity at the bottom, displacing air upward. Think also about what you will be using: some epoxies takes 4 hours to set, but urethanes may go off in 2 minutes. Filling a big horse mold with filled urethane resin through a thin leg may present a problem. Make sure you design your mold so that you can get the resin you intend to use into the cavity before it starts to set. Proper mold design will eliminate all large bubbles and most medium-sized ones.

By far the biggest source of air bubbles is the process of mixing the resin. Careful mixing of pure resins can yield good results, but you will want to fill most castings, and all powdered materials are full of air. You can subject most freshly-mixed resins to a vacuum to eliminate air from the mix, placing the material in a bell jar and evacuating the air to 29 hg (inches of mercury) or higher. Realize that the mix will rise like bread under vacuum, easily tripling its initial volume, so don’t overfill the container or you will overflow it. Urethanes present a problem due to their fast cure time. Even with a fast vacuum system, they will aready be setting by the time you begin to pour. So the use of mixing paddles which minimize air entrainment is important, as is the sizing of a batch in relation to the mixer and mixing vessel. Pouring degassed material into a well-designed mold will eliminate 95% of small air bubbles.

Finally: you are down to that air which was not displaced as the mold filled, or was introduced into the material by turbulence as you poured. This is particularly noticeable in materials with high surface tension, as they can skate right over detailed mold surfaces, trapping air bubbles that will cling in place. This problem is most significant with highly detailed mold surfaces such as fine hair textures or sharp protruding points.

There are two solutions to this: The first is mold rotation and/or agitation. By sloshing, banging, vibrating and, where possible, sticking your gloved hand or a brush in there and physically displacing the bubbles, you can get the air to rise out of the gates or vents of a well designed mold, if the resin isn’t so heavily filled that air won’t rise through it. The other solution is to apply pressure, and there are 2 ways to do that. For slow-setting materials, you can craft a vacuum chamber, big enough to contain your average mold and hook it up to your pump. Do not forget to pre-vacuum your resin before filling the mold, or nearly all your resin will boil out of the mold and down the sides when under vacuum. Be aware that some resins have volatile components that vaporize under vacuum and will never fully stop bubbling, so even with pre-vacuumed material, you may have to design your mold with a reservoir feature above the gate to contain any resin that boils out and funnel it back into the gate as the resin settles back down.

What will happen under vacuum is this: any air bubbles entrapped in undercuts or mixed into the material will expand hugely. The bubbles will become so buoyant that they will rise and burst while most of the air in undercut areas will bleed off as the bubbles expand beyond the undercut area entrapping them. You still have air bubbles in your casting at this point; in fact, the undercut entrapments are still the same size as they were before. However, the remaining air is at nearly zero air pressure. It is crucial in vacuum casting that you release the vacuum while the resin is still liquid. This slams 14 pounds per square inch (atmospheric pressure) against the resin and squeezes the remaining air bubbles either into solution or to a microscopic size.

This technique does not work well with urethane resins because there is inadequate time to evacuate a chamber big enough to hold an average mold. For urethanes, you have to skip the vacuum part and cut straight to the pressure. You want to pressure-cast urethanes at 50 to 80 pounds per square inch. This is too bad, really, because it is far easier to build something to hold 14 pounds psi OUT than to build something that holds 80 pounds psi IN. (Pressure vessels can be extremely dangerous, so don’t try building one unless you really know what you’re doing.) You can use an autoclave (one with no heating system) and a big enough compressor to bring up the pressure in the autoclave within 20 seconds or so. For smaller castings, some shops use painter’s pressure pots. To do pressure casting, you fill the mold, pop it into the chamber, lock it, slam the pressure on and hold it there until the resin has cured. This technique should eliminate 98.5% of all air-caused flaws in cast resin parts.

by Christopher Pardell

With contributions from Dan Spector and Andrew Werby