Resin Casting on a Miserly Budget
Any modeler who ever opened a box to find a superbly rendered engine, ejection seat, or wheel; or anyone who has doled out twice the cost of the kit for an aftermarket “update”, has toyed with the notion of reproducing these parts. Resin casting, after all, is an uncomplicated affair. You simply dunk the master into some rubber glop. Wait till it hardens, pull it out and fill the resultant cavity with dime-store epoxy. Repeat until you have enough for all your other kits. Keep making them for all your friends. Continue pouring epoxy and go into business on the Internet. Get rich and buy our own injection molding company so you can make the planes you’ve always wanted. Get so rich that you buy and fly a fleet of REAL Messerschmitts and Mustangs with an occasional Tu-52 to boot!
OK, back down to Earth now. A bit of market research will pop that bubble. . Life is never as simple as at first it seems, and so is it with resin casting. The requisite materials are too expensive to make the endeavor cost effective. An 8 oz “starter set” of the most popular casting system (can we mention the name “Alumite”) retails for a whopping $60 US. That’s enough for a lifetime supply of aftermarket wheels and engines. Similarly, queries to wholesalers of artist or specialty plastic supplies invariably reveal that the compounds must be bought in minimum quantities ranging from multi-pound cans to 50-gallon drums.
Modelers are by nature tinkerers and experimenters determined to circumvent expenses through resort to household or “off the shelf” products. It is then that they have revealed to them the reasons for specifically compounded components. The rubber-based mold making substance is the limiting factor. It must be at once liquid enough to flow into and around the master form’s most intimate intricacies, such as a radial engine’s push rods and cooling fins, slippery, pliant, yet resilient enough to release from same without shredding; set within a reasonable time to a firmness capable of holding the original shape, and be both incompatible with and unaffected by any epoxy or resin reproducing material. The most logical, indeed the only readily available product is RTV (Room Temperature Vulcanizing) silicone rubber, sold in auto supply or hardware stores (“Ironmongeries” I recall from UK visit is the term there) as all-in-one caulks, sealants, adhesives, and gasket makers. They work passably well for copying simple shapes such as canopies and nose bowls, but fail dismally at attempts to capture the shape of engines, wheels, seats and such. RTV products are, after all, deliberately formulated to be viscous, viciously adhesive, resistant to compression, and utterly indifferent to greasy or oily surface contaminants. They will not flow into fine details, nor readily release their grip on the master form; no matter what type or quantity of mold release grease is used. Subsequently, any solvent strong enough to soften set silicone will dissolve resin; and even cast metal masters will suffer from the scrubbing required to remove all rubber residue.
These obstinacies prompted an exhaustive search of all the shelves in the kitchen, bathroom, and workshop-garage for mitigating admixtures. After futile fiddlings with silicone sprays, waterproofing liquids, and even cooking oils, a last gasp of alchemy led to the secret ingredient – the proverbial Philosopher’s Stone that transmutes base caulking compound into the gold of mold-making material. That Holy Grail is anti-seize compound - another inexpensive acquisition from the automotive supply section of Wal-mart. It is a grease-based suspension of metallic particles that may be powdered aluminum, or something more exotic, such as molybdenum disulfide lubricant. Sold for a few dollars in squeeze tubes or small tins, its actual raison d’etre is preventing the unwanted fusion of dissimilar metals in assemblies such as steel threaded spark plugs (Igniters, I believe they are called on the eastern side of the Atlantic) to aluminum engine cylinder heads. Whatever its intent or composition, it cures (no pun intended) all the shortcomings of RTV rubber, as relates to our own designs.
Not all by itself, though. Further experiments led to the following refined recipe: One part anti-seize compound, one part petroleum jelly (Vaseline), 3 parts RTV – the red “Hi Temp Gasket Maker” seems to work slightly better than others, though this may be imaginary, as the multiple RTV renderings are likely the same base with different pigments and prices.
Caveats and Cautions
Prior to launching into the details of the procedure, a few generalities bear mention. The rubber formula will likely need tweaking. If it sets up overly firm or sticky more anti-seize is called for. Conversely, if it refuses to harden more RTV is in order. This technique reproduces one-sided parts, such as engine facades, in a single mold. Wheels, landing gear spats, and similar components can be fashioned in mating halves, as they are in most short run low-pressure injected kits. Three-dimensional parts require a two-part split mold, with the additional complications of aligning pins and air evacuation channels. The rubber recipe is amenable to that process, but such parts are rarely needed in model airplaning.
Lest the lawyers get their advocacies in an uproar, the warning must be included that RTV and epoxies cause skin sensitivities and emit harmful vapors. Provide adequate ventilation and/or use a respirator mask. The 3-M Company sells a disposable surgical type mask that adds an adsorbant charcoal layer to the usual pressed paper dust filter. It is marketed as a protection only against “nuisance odors” such as latex paint fumes, but does an excellent job of blocking acrid and organic vapors. In the absence of an industrial respirator, such a mask is the absolute least protection that should be worn when sanding, spraying, or cooking up chemicals. The legal eagles will also insist upon a phrase or two regarding the reproduction of someone else’s enterprise. As with photocopying, as long as the duplication is for personal and non-commercial use, the “counterfeiter” is generally unconcerned with copyright issues, though a stiff upper lip should be maintained mated to a stiff lower one regarding the source of that superb P&W Wasp or MB MkVII “bang seat”.
Making the Mold
Before mixing up the mush the container consideration must be cogitated upon. The best and cheapest vessels are the half-ounce plastic tubs in which restaurants dispense butter, jelly, coffee creamer, and other condiments. Another alternative is the inexpensive plastic or metal paint pallets sold in artsy-craftsy stores for oil and watercolors but also suitable for mixing epoxy and forming shallow molds. At fifty cents apiece they can be discarded rather than cleaned. For larger parts, as in 1/48 scale, 2 ounce plastic bathroom cups, or those used for dispensing potions and pills for assorted ills are perfect.
The first step in the two-part process is selection of a master to copy, a radial engine for example. Superior results can be achieved, particularly in larger scales, by carving away pushrods or wiring harnesses, casting just the crankcase and cylinders, then replacing the details with wire or sprue. A generic part can often be made by combining the cylinders from one engine with the crankcase of another. Clean off any flash and seams, then give it a spritz of silicone lubricant and let that dry. The mold release oil will help even with all the greasy stuff in the rubber compound. Mix up the latter in a disposable cup. First melt the petroleum jelly in Mom’s microwave oven, then mix in the anti-seize compound. Add the RTV then STIR, STIR, STIR; because it is not very miscible with the greasy stuff (or much else on this planet). When the glop is uniform in color and consistency, spoon it into the proper size container, avoiding the formation of bubbles and voids. A few sharp raps on the table will bring air to the top. Carefully submerge the master form a millimeter or two beyond its midline. The flexibility of the cured mold will allow it to be removed. Save a spoonful of excess mold compound to judge its setting time.
Set the mold aside in a warm place for at least a full day, then if the test glob of rubber indicates setting, carefully cut away the plastic tub and let it sit another 24-48 hours. RTV cures by exposure to air so even if the outside is firm, the inside is likely still liquid-ish. The best clue to curing is the complete lack of any odor from the rubber. Carefully peel away a corner (or cylinder head) and see if the mold is ready to release the master. If the proper proportions have been used and adequate time allowed, it should pop out easily leaving a precise detailed female impression. Allow that still one more day to cure open to the air. If any rubber sticks to the master, the job has been boogered and a new mold must be tried with a greater proportion of slick stuff.
Casting the Duplicate
The master can be replicated using any hardening compound, but 2-part epoxies give the best results. Automotive polyester resin is difficult to use in such small quantities because the amount of hardener is impossible to judge. Water or solvent based putties, such as wood fillers or “Liquid Steel” shrink considerably as the carrier evaporates, and the acetone/toluene solvent may attack the mold. A variety of epoxies with various properties are available in any department store for only a few dollars, usually packaged in twin syringes that automatically dispense equal (more or less) amounts of parts A and B. Some harden in as little as a minute; some remain flexible; some are specifically formulated to bond plastic. For copying fine details the best choices are low viscosity and hard setting, which generally require a cure time of at least several hours. Any of them should be allowed to set overnight to assure a smooth tack-free surface. The liquidity and reaction time of all these glues can be improved by first warming the separate components in nearly boiling water, combining and pouring while still hot.
The bugaboo of any casting is bubbles, caused by stirring and air trapped in the mold. Professional pourers pop them with either vacuum or centrifugal force. I tried both, first building a vacuum chamber, which was ineffective, then converting an upturned bicycle to a centrifuge, which turned near deadly when the casting came loose from the wheel and became a missile. Both proved unnecessary, as bubbles can be minimized, if not eliminated, by everting and flexing the mold while pouring. Any that remain can be coaxed out by poking with a pin or toothpick.
Allow the pour to sit at warm room temperature at least overnight, which is the full curing time for even “instant” epoxies. By then the surface should lose all stickiness and easily pop free from the mold. The extra time is necessary also to produce a hardness which can be sanded, smoothed and drilled. Minor imperfections can then be corrected with additional epoxy or cyano-acrylate gel. The mold should last for about a half dozen pours, but by then tiny bits and pieces have worn away and the casting quality has deteriorated to the point that a new mold is in order.
This technique will not lead to fortune and fame; nor may it save much over the cost of ready-made replacement parts; but it will add an air of accomplishment in mastering a new niche in modeling.