By Graeme Ogg
All text, illustrations and photographs by, and copyright of the Author.
NOTE: This may look like a long, complicated story, but rather than giving a very basic set of instructions (“1 – 2 – 3 – 4 – all done, easy peasy!”) and leaving people to struggle with unexpected little problems, I thought I should spell things out in more detail and mention various points to look out for. If you’re still interested, read on …]
I’ve done my share of trying to make new model windows from bits of thin, flat scrap plastic, which works fine if the curvature is slight, but if you get into restorations or conversions involving more heavily-curved glazing (including wraparound front or rear screens) that method can be inadequate. Trying to force flat plastic into a 3-dimensional shape can be a nightmare, and next morning you find the springiness in the plastic has gradually popped it loose from the adhesive. Some people resort to cutting suitably-curved sections from old plastic soft drink bottles, but if the rest of your model is looking pretty good, chances are that those makeshift windows will spoil the overall effect.
So eventually I bit the bullet and had a go at heat-moulding. Contrary to what serious modelling handbooks may tell you, a fancy vacuum-forming rig complete with hinges and clamps and an air pump isn’t really required for this kind of job. Just a hot-air blower, some balsa, scraps of old plywood and some suitable clear plastic. However it does take a bit of time, and trial and error, and in early attempts there were a lot of scraps of warped, over-cooked plastic in the waste bin before I got a decent result. So it’s not something that everyone would want to bother with. But for any enterprising soul who wants to give it a try, the following instructions may be useful. And if your eyes start glazing over (so to speak) just take a short break.
(Fig.1) Rough-cut a piece of balsa slightly too big to fit inside the model body. A useful first move is to cut the front face to roughly the angle the window makes with the car body (use one of those cheap see-through plastic protractors). It also helps if you trace the top and bottom edges of the screen opening on to tracing paper, cut out the tracings and mark them on to the top/bottom of the balsa block as a rough guide to shaping.
(Fig. 2) File and sand the balsa, test-fitting regularly, until it can be slipped up into the body and makes good contact all round the edges of the screen opening. (My rough sketch shows the top and sides of the balsa as flat, but of course you may need to curve the top surface, or round off the corners, or slope the sides inwards towards the top, to fit the inner body contour). Go very slowly in the final stages, continually checking the fit all round, because when the mould is “almost fitting in but not quite”, it is very easy to misjudge where it is binding and to sand away a little too much in the wrong place.. If that happens, and you end up with a visible a gap at along one edge or at a corner, cellulose stopping putty (knifing putty) is a useful remedy. Apply it where you’ve removed too much material. When hard, blend it to shape with 400/800 grade wet-and-dry. And if you have a particularly coarse-grained piece of balsa with cracks you think might show up in the moulding, thin down some cellulose putty with cellulose thinners to the consistency of thick primer and paint it over the entire face of your carved window.. When dry it will sand to a very fine smooth finish, and incidentally it can also reduce the problem of stray bits of balsa dust producing dimples on your cast screen.
(Fig.3) When you’re happy with the shape, hold it firmly in place inside the model and trace the window opening onto it with a fine-tip pigment marker pen, otherwise at a later stage it can sometimes be hard to see exactly where the actual window area is on the balsa. Take care not to dent the balsa by pressing too hard with a sharp pen point.
(Fig. 4) Slice off the back of the balsa roughly parallel with the front face of the screen. This helps to ensure you will be pushing the face of the screen flat into the heated plastic, rather than at an angle which could lead to one edge being forced too deep and maybe breaking through.
(Fig. 5). Glue the balsa block to a backing piece of plywood (half an inch or so bigger all round than the balsa) and add a block of wood or a piece of dowel for a handle
(Fig. 6) Use a scrap of plywood or similar for the female mould. In theory the opening should be the shape of the front face of your balsa moulding plus the thickness of the plastic all round. In practice you need to leave a little bit more clearance to avoid scuff marks or stress marks on the plastic. But if you are doing a fairly deep wrap-around shape, too big an opening may mean the sides don’t get wrapped round tight enough, so start with the minimum opening and increase the clearance a bit more if you find the heated plastic is getting scuffed as it is pushed in. It’s easiest to use some kind of coping saw to cut the opening, but failing that you can drill holes all round the outline, “join the dots” with a blade, remove the shape and sand the edges of the opening. At the hot-moulding stage there needs to be enough spare plastic around the opening to be drawn in smoothly during the push, so make your plywood big enough to allow you to pin down a piece of plastic with a border of about an inch all round the opening. Sand the top edges of the opening to a smooth curve to prevent marking the plastic on a sharp edge. When moulding, you can either hold the female mould in a vice, or make a couple of feet from scrap material, as illustrated, so it will sit on the workbench.
(Fig. 7) Test the two halves of the mould together. The male mould only needs to go in just far enough to ensure the shape is properly formed. To avoid the risk of pushing too far (which could over-stretch the glazing) you might need to fit spacers of scrap material to the underside of the backing piece as shown. That then allows you to push firmly down “on to the bump stops”, which helps to avoid any wobbling before the plastic has hardened, which could distort the screen.
You can use various types of clear plastic, but some clear packaging material has a ”scoured” finish which can look bad in close-up on a small windscreen. And some kinds of plastic sheet that looks quite promising will crinkle and curl at the merest touch of heat. (Hard to tell the good from the bad till you experiment!) Too thin and you can easily break though when pushing a deep shape. Too thick and the result can look clumsy, and sharp curves may come out somewhat blunted. I use 20 thou clear polystyrene sheet and simply fix it to the female mould with 4 drawing pins at the corners. You may need a pliers or small hammer to get them into the wood the first time. For repeat attempts just push them into the same holes each time.
Having once burned out a domestic hair drier I now use a hot air stripper on its low setting at a distance of 5-6 inches. Don’t go too close or the plastic can “fry” or bubble. Play the heat back and forth over the whole piece of plastic, not just the central area. That way the surrounding material will also be softened and will pull in smoothly. After maybe 20-30 seconds it will sag, then start to bulge up again. Press the male mould in gently. If there is too much resistance, don’t force it or you’ll get scuffing or stretch marks. Apply a bit more heat and try again. When the mould sinks in nicely, hold it steady for 20-30 seconds to let it cool. Prise up the drawing pins with a small screwdriver and remove the glazing piece. If there are scuff marks/stress marks in the plastic, ease the female mould a bit more before trying again. A smear of mould release (liquid vaseline) over everything can also help avoid this problem. If the casting looks OK, slip it back onto the male mould and trace the window outline from the balsa to the glazing so that you can see exactly where the window starts and ends on the plastic “blister”. Also mark any tabs or borders required for concealed gluing inside the body shell. Remove the pigment ink with methylated spirits. Trim carefully. A small craft scissors is recommended. NIbble carefully around any sharp corners to prevent the thinned plastic tearing or stressing.
Fig. 8) For a relatively simple curved screen (or for curved side glazing on flush-glazed vehicles) just shape a single piece of balsa to the required curve, fix one end of a strip of glazing to it using double-sided sticky tape, apply heat (keeping it away from the sticky tape as far as possible) and use a scrap of smooth wood to press the glazing down on to the curve and hold till it cools. Use white spirit on a cotton bud to remove the sticky tape afterwards. NOTE: If you don’t apply the heat for long enough, the glazing will soften enough to bend but won’t be heated right through, so the residual elasticity will tend to unbend it a little as it cools. Give it a bit longer. If you keep getting an inadequate bend, try exaggerating the curve of the mould just a little to compensate for this.
(Fig. 9) You can also use the heat-and-press technique to form tail-light or headlamp units with a more severe wraparound. Once again, if you don’t get the heat right through the plastic, it will tend to uncurl. Give it another dose of heat and press it down again (decent plastic can be re-heated and re-bent a few times before it starts to permanently distort).
Even if you aren’t into making your own models or doing conversions,, you can also use this technique to replace cracked or discoloured glazing on precious older models, or add it to models which didn’t have any in the first place but would look the better for it. You are bound to have quite a few experiments that don’t turn out quite right, but eventually (with any luck) you should get a decent result.
Male and female moulds for a couple of models. In the left-hand one the balsa was left “raw”. The one on the right got a thin smoothing coat of cellulose putty, hence the dark appearance.
A selection of successful attempts at window moulding are show below
Panoramic rear screen for 1960 Chevrolet (modified Brooklin convertible)
Side glazing panels for Pontiac Type K ( conversion from Yat Ming Firebird)
Compound-curvature rear window for 1960 Edsel (modified Brooklin ’60 Ford)
Or maybe something a little more outrageous
Rolls Silver Seraph glass-top Ceremonial limo (fictitious)
Compound-curved side and rear glasses for Bentley K2 SUV (fictitious)
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