The Care and Coddling of Vacanopies
by Joel Hamm
Those of you who have the March 2005 Fine Scale Modeler will recognize Joel's name from an article "How to make your own CANOPIES". Much to Joel's dismay, FSM's editor's basically rewrote his original article to the point of where it was unrecognizable as Joel's work. He asked me to publish the original for those of you who may not have been able to follow FSM's rewrite. Unfortunately, there are no images to accompany this article as I believe that FSM now holds the copyright on those. However, if you have that issue, you can easily follow along.
Vacanopies [vah-can-oh́-peez]? What’s that, some fearsome fugitive from Jurassic Park? No, just a convenient concatenation of VACuum formed CANOPIES (and other transparencies). For all the angst these snippets of acetate inspire, they might as well be rapacious beasts. Many modelers would try their hands at a resin or short run kit, but for fear of fumbling an operation as delicate as transplanting a cornea, with a part just as precious. As FDR said, and countless others before and since: We have nothing to fear but fear itself. Vacu-formed clear parts are tame-able with only average dexterity. Once mastered, they open up a new world of modeling beyond Messerschmits and Mustangs. Even if bungled they can be replaced in the privacy of the home, without having to unearth the mythical Mattel Vacu-former, or re-mortgage the farm for a professional rigs.
Upon ripping open that one of a kind kit, the first sight of its vacanopy can be disappointing as well as daunting. They are often cloudy and considerably astigmatic, but both faults are easily cured without optical polishing. Two types of vac can be encountered. The male mold is most common, made by drawing a heated sheet down over a convex positive form. The thickness of the sheet, though measurable only in mils, creates a part slightly bigger than the master, which has to be correspondingly smaller than the finished part. More troublesome, all framing detail has to be pushed through from the inside and can be indistinct and inaccurate. Gaining market prominence is the more difficult to produce female mold. The plastic is sucked down into a concave negative form of the part. The outside dimensions of the vac now precisely match the master used to create the mold, as do all framing and surface detail. One problem plagues this primo process. In order to evacuate air from the concavity, holes must be drilled and these can appear on the finished part as raised nubbins. If situated along a framing line, they can be carefully filled from the inside and sanded off. If they pop out on a clear section, the only alternatives are to ignore them, or replicate the piece. The scientifically, or at least pedantically inclined would insist on clarifying that the part is formed not by suction from below, but by unopposed atmospheric weight above; though that correction adds nothing to the discussion.
Cutting and Fitting
Freeing the transparency from the excess sheet, and trimming to a perfect with the airframe, are the most crucial steps. Most advisors counsel a “score and snap with an #11 blade” technique. This can be disastrous to the fingertips as well as the part. A safer, more meticulous method is to fill the molding with a hardening but non-adhering compound, then grind the resulting solid to shape. The cockpit in question has to be devoid of anything poking above the rim, so seats and similar protuberances should be omitted until just prior to closure. If that isn’t possible, the technique can still be used but requires repeatedly removing and reattaching the canopy from its support for final fitting.
The best filling medium is “Durham’s Rock Hard Putty’. In its absence, combine patching plaster with a pinch of powdered resorcinol glue, which is probably just what Mr. Durham does in his factory. If you’ve ever a cogitated over cup of coffee to what use might be put those 1 oz plastical creamer cups the restaurant serves – they are perfect for putties, paints, and assorted aqueous alchemy. Solvents and epoxies will dissolve them. Mix the powdered Durham’s with surprisingly little water to the consistency of soft butter and spoon into the concavity. Most air bubbles can be forced to the surface by rapping sharply on the table; more stubborn vacuoles require jiggling out with a toothpick. Durham’s only fault is that it requires at least overnight to set, signaling readiness with a lightening color. When dry it has no attraction to plastic, so the canopy can easily be popped off. The next step is to repeat the process. This first casting is set aside to form a replacement in the event of a “Houston – we have a problem here…” catastrophe.
For this next molding, adherence is a virtue; so first coat the inside of the canopy with Micro Mask. If none is at hand, rummage through Mommy’s cosmetics case for a few drops of “facial mask”. An acceptable and more survivable second is a 50/50 mix of white glue and liquid detergent. After allowing some additional drying time, mark the separation line with a fine line permanent marker. Cut away the gross excess with an Exacto saw or toothed circular blade at slow speed in a Dremel tool. Approaching the marked line, switch to medium, then fine sanding discs in the grinder, finishing with a 220 grit sanding stick or emery board. Grind the curve of the fuselage by wrapping abrasive paper around a cylinder of similar radius; generally one of the Exacto knife handles will do. If the mating surface is flat, sand the putty-filled canopy against a sheet of 220 grit paper stuck to a glass plate or smooth counter top. Get a firm grip by sticking on a wad of re-useable rubber tacking adhesive (from the office supply or stationary store). Modeling clay or plumbers putty will also work but they leave an oily residue. Test fit the canopy while filled, or pry it off gently for the final trials and errors. Save the form to hold the canopy during the framing process, but if the sticky stuff is stubborn, toss the whole into warm water for an hour. Durham’s will turn to mush and the Micro Mask will wash away. Don’t expect an “injection molded” fit with the fuselage. Inevitable gaps and mismatches will be caulked with the fastening compound. The grinding process will leave a fuzz around the edges which can be scraped with a knife blade or smoothed with 600 grit paper.
Framing and Fastening
Framing transparencies is the touchiest and most tedious step in the make/break of a model. Techniques range from freehand painting to meticulous masking to commercial die cut appliqués. There’s no right or best way, but one that renders superior results with minimal effort and skill utilizes Pactra Trim Tape. This ultra thin adhesive vinyl is intended for flying models, but its ability to conform and stick to sharp compound curves earns it a spot in any scale enthusiast’s arsenal. It is available in a dozen or so colors. White, silver, black, yellow, and blue can be used right off the roll as they closely match aircraft hues. Other colors are pre-painted with any shade enamel or acrylic. Each roll contains several yards of various widths, but for sharp edges slit the wider ones with a steel straightedge and fresh #11 blade.
The cast putty form that served so well as a sanding aid now can be used like a shoemaker’s last to hold the canopy. With carpenter’s or melt glue attach a scrap of wood so the form can be clamped in a vice. Reattach the transparency with Micro Mask or whatever substitute was previously used. Lift lengths of Trim Tape between two tweezers and lay them in position, stretching slightly to help them conform. Start with horizontal sections then wrap around the verticals, leaving the excess hanging past the edge. Nudge the tape into final position with a toothpick. If any stickum gets on the clear pane remove it with a Microbrush just barely dampened with alcohol.
When the tape is properly and symmetrically aligned, everything is sealed with God’s gift to modelers (or at least Johnson& Johnson’s): Future Acrylic Floor Wax (a..k.a. overseas as Johnson’s Klear). To avoid its solvents loosening the tape, first dust on a few airbrush coats thinned 50/50 with isopropanol. Then treat the whole to repeated dunkings and dryings in full strength. All optical irregularities magically vanish, as do any suggestions that the tape is anything but real metal framing riveted into place. Allow the canopy to dry undisturbed until the odor can’t be sniffed, then give it an extra day sitting in someplace warm. When certain that the coating is tack free, trim the overhanging strips with a new double edged razor blade, which requires just a bare touch of pressure.
For fastening in place, many builders swear by “fog free” cyanoacrylate. Positioning a vac-form, however, demands several minutes of working time, so why risk instantaneous disaster when a sufficient bond is bestowed by white glue, carpenter’s glue, Testors Clear Parts Cement, and best of all – Future which has been allowed to thicken. Tack the canopy down with a drop at each end. When that sets, caulk all gaps with a bead of glue applied by a pin or piece of stretched sprue. As it dries it shrinks to a perfectly faired invisible joint. Touch it up with paint and the result is a transparency that rivals any high end injected kit.
Replacing the Irreplaceable
Suppose the part is ruined beyond repair. Some kit makers are including dual canopies. Some kit builders are boogering up both. No cause for suicidal ideation. Reproducing canopies and other parts on a homemade vacu-forming rig can be an emotionally and financially rewarding sideline. Numerous designs have been suggested in the modeling mags, but all require a complex device to hold the clear plastic sheet and flip-flop it between monoecious heat and vacuum devices. None of that is needed. Results rivaling the aftermarket vendors can be made on a setup cobbled together in an afternoon.
For a vacuum source, any hose equipped Hoover will do. Construct the molding chamber by nailing and/or gluing together pieces of 1X3 lumber or plywood to form a 6 by 6 inch box. The bottom can be plywood, Masonite, or anything similar. The top should be a square of stiff metal mesh, various types of which can be found in any hardware store. Alternately, use a piece of aluminum perforated by numerous 1/8 inch holes in no particular pattern. Around the perimeter of the top add a gasket of foam weather stripping, or sacrifice a neoprene computer mouse pad. For the hose socket, in one side of the box drill a hole slightly smaller than the nozzle, then with a file or rasp taper ream it for a tight press fit.
The best heat source is a sealed (non-radiant) electric cook-top burner. The radiant coil types, or even gas on very low setting will do, but pose a risk of charring or igniting the acetate. If commandeering the kitchen stove will upset the domestic tranquility, a portable hotplate can invariably be found for next to nothing at a yard sale or Salvation Army store. The garage workshop may hold another excellent heater in the form of a paint stripping hot air gun. Mom’s hair dryer simply will not do, for insufficient output and other obvious reasons.
The frame for holding the clear sheet is simply another piece of aluminum, an inch or so wider and longer than the box, with a cutout large enough for reproducing the biggest part anticipated. The opening need not be rectangular. One way of cutting it is to use a hole saw designed for installing lock sets in metal clad doors; an approximately $3 investment. Another is to drill a small hole and enlarge it with a rod saw or jigsaw. Nothing need be neat or pretty – except the part that the vacu-rig produces; but a practical embellishment would be the addition of a non-conducting handle to keep the fingertips cool. No complex clamping devices are needed. The acetate is fixed in the fame by plain masking tape. Engineers call that approach a “quick and dirty solution”, but it saves time and effort building fancy machines when the object is to build fancy models. To answer question #1: No, heat will not loosen the tape. It sticks it tighter.
Question #2 – “What plastic should be used” – is just as easily answered: Anything. For convenience, the tag “acetate” has been applied, but it can also be PVC ( poly vinyl chloride), styrene, butylene, or any of the other “…ene’s” that bring us better living through chemistry. Hobby suppliers sell packets of chemically unspecified clear sheet for thermoforming; but a rummage through the cupboards will turn up piles of plastic that can be had gratis, with the additional advantage of saving the Earth’s landfills from overflow. Blister packs and pop bottles are perfect. A jug of 7-Up or Birch Beer provides green and amber for tinted transparencies. Surface scratches are insignificant because they will vanish when the plastic is semi-melted.
Set the apparatus up near the stove with ice water nearby for burned fingers, and an extinguisher even nearer for more serious singings. DON’T DOUSE AN ELECTRICAL DEVICE WITH WATER! Place the master form on the metal screen, holding it, if necessary, with a wad of re-useable rubber tack adhesive. Mount the plastic in the frame with one inch or wider masking tape so no air seeps under. Suspend it, tape side up, two to three inches above the burner, set on “high”, moving back and forth to get the temperature even. If a handle hasn’t been affixed, wearing a wet cotton work glove will add a measure of comfort. Different compounds and thicknesses will exhibit varying melting characteristics, so experiment and practice. Usually the plastic will first turn wet-look glossy and scratches, as promised, will disappear. It will start to sag but then re-tighten. As it gets hotter it will sag once again. This is the magic moment. Quickly transfer it to the vacuum box. Press the frame against the gasket, and trip the foot switch on the vacuum cleaner. With a gratifying hiss-whoosh-plop a new canopy emerges; like Venus from the sea and just as pretty. Incomplete forming means not enough heat; and the same piece can be re-heated and re-formed. Charring, bubbling, or clouding means too much.
Remove the newborn from its frame and back track to Step One – Cutting and Trimming. Label and save the form to help out a buddy who bungled his kit; or hang out a shingle on the Internet and peddle your services as a professional vacanopy tamer.