Moulding Your Own Cockpit Canopies
When you start building from plans rather than kits, one of the problems you will
eventually come across is the necessity to produce your own cockpit canopies.
Those in kits are usually vacuum formed, but as not many of us are lucky enough
to have a vac-form machine, we have to use another method. To aid those attemping
a home made canopy for the first time, here is a step by step illustrated guide to
the simple "plug moulding" method I use.
Step 1
Take a block of medium to hard balsa and carve to the shape of the required canopy.
This is the hardest part of the whole process - it takes time, and you make lots
of mess! Test fit the part constantly against the the model, and
cut a template from a copy of the plan (stuck to cardboard if you like) to make
sure you get the correct outer profile. Really take your time to get as good a
fit as possible to the model - this will pay dividends later on. Finish off the
mould with progressively finer grades of sandpaper, down to about 600 grit or
so. I do not apply anything else to finish the mould - just bare balsa (you
wouldn't get away with this if using a vac-former, as the plastic would be
sucked into all the imperfections)
The picture below shows the finished mould for the canopy of my 14" span
Saab J29 placed on the model. This canopy is a relatively simple one, because
it fits on a flat plate, and is not particularly deep (why do you think I chose it?)
Step 2
Next we must glue a 1/8" thick plate onto the bottom of the canopy mould. If the
canopy has any vertical or steep edges that also mate with the fuselage, these
need to be extended as well. For a very steep edge on a deep canopy, you may
need to add a thicker lump of balsa, and round it off towards the top, in order
to stop excessive thinning of the clear sheet over the sharp corner during
moulding. The addition of this extra material is important, because it ensures
we have a clean, sharp edge when the canopy is trimmed. The join between the
mould and the extension acts as a cutting guide when trimming.
Here you can see the 1/8" plate glued to the bottom of the mould before it has
been trimmed and sanded flush with the mould contours. Also note a handle
added to the underside made from scrap 1/4" square balsa (you can use anything
suitable)
Step 3
With the extra material sanded flush with the canopy form, we now need to make a
simple female jig to push the mould into. This is just a hole cut in a piece of
spare 3/16" or 1/4" balsa sheet, so that there is about 1/16" clearance to the
male mould. This ensures that the plastic sheet stays close to the sides of the
male mould when it is pushed through. Note the entry to the hole is rounded off
on one side of the sheet and smoothed to give the clear material a nice gradual
lead-in.
Step 4
Now we need to find some plastic sheet to use for moulding. I never go out and buy
material, but collect pieces of blister packaging whenever I see something that
might be suitable. This is mostly PET and PVC I think, but don't quote me on that!
My favourite material is vac-formed Easter egg packaging, and you can see some of
my collection below (eating the contents is a necessary chore that just has to be
done....) If you cut a piece to use, and place it on a tray under a grill, it
will soon revert to its original flat shape.
Step 5
Cut a piece of sheet to generously fit over your female mould, and attach it as
shown. Note the highly sophisticated (!) method used, involving lots of pins.
You can use little clamps or clips if you like, but I find pins work fine. Also
note that the piece of sheet is not perfectly flat - it really does not matter
at all if it came off the grill pan with ripples all over it.
Step 6
Now for the fun part. Put on some oven gloves, and grasping the far end of the
balsa sheet, hold the pinned clear sheet under the grill - you can rest it on
the tray at a suitable height. Here is where trial and error come into play -
you have to pick the right moment to remove the sheet. Peering under the grill,
you should see the clear material tighten up, then just begin to sag a little.
At the same time the first wisps of smoke may just be seen coming from it. At this stage
quickly remove it and plunge the male mould into the clear sheet, through the
female mould. The photo below shows how far to push it in.
A couple of things can go wrong at this point. Firstly, if the sheet is not
hot enough, you will not be able to push the male mould all the way in. If
this is the case, then simply put it back under the grill again. The sheet
will flatten out again, and you can leave it a bit longer next time. Repeat
this procedure as many times as you like!
If you have overheated the material, it may be discoloured, or gas bubbles may
have formed inside. If so, don't heat it so much next time. In extreme cases,
it may catch fire of course, so BE CAREFUL and NEVER leave it unattended under
the grill.
I had some sheet once that went milky when you heated it up. Some
clear material just does does not work very well, in which case, bin it, and
try some different stuff.
You may need several attempts to get a good result, but that doesn't
matter - the mould will survive quite happily. For deep drawn parts, the
thickness of the sheet can be critical. Ideally you want the thinnest canopy
you can get, to save weight, but a deep moulding will need a thicker sheet
than a shallow one if it is not to going be so thin at the sides that it will
be unusable. Some experimentation will probably be needed. These mouldings
tend to be thickest at the top (where the mould first touches the plastic sheet)
and thinnest at the lower sides, where the sheet gets most stretched.
Step 7
Remove all the pins and admire the finished canopy. The first photo below shows
the canopy with mould still inside, and the second with the mould removed.
Step 8
The canopy is trimmed with the male mould reinserted, so you can cut through the
canopy with a sharp knife into the join in the mould where the canopy block meets
the extra 1/8" sheet we added earlier. Test fit the canopy to the model, and do
final adjustments using nail scissors and/or emery paper.
Step 9
Now the canopy must be attached to the model - a process which often gives
problems. I have quite a few models where smears of glue on the canopy or
windscreen spoil their appearance.
Until recently I favoured clear UHU glue - the stuff in the yellow tubes. This
has the advantage of being transparent, strong and does not give off any nasty
fumes to "bloom" the inside of the canopy (this happens with most Cyano adhesives,
and unless you can get at the inside of the canopy to wipe it off afterwards, you
are stuffed!) The main problem with the UHU is that it is very difficult to apply
without getting strings of glue everywhere, or unsightly blobs.
I have attached canopies with normal PVA wood glue before, which allows you to
do an almost invisible join (it dries crystal clear and does not attack the canopy
at all), but the join is not very strong, and you can lose your canopy in a heavy
landing!
My latest method, the favourite so far, is as follows:
Add any cockpit detail and paint as required.
Hold canopy in position with a couple of pieces of masking tape, or a weak rubber
band or two - fiddle around until you are sure it is as perfect as possible.
Retrim if necessary.
Apply a tiny bead of PVA wood glue around the canopy/fuselage join with a pin.
Work round carefully, a bit at a time, and smooth the outside if necessary with
your finger.
When dry, remove the tape or rubber bands, and complete the join with PVA where
you couldn't reach before. Check for any gaps and fill with more PVA if you find
any.
Apply a thin layer of cyano adhesive over the top of the PVA using a pin,
spreading it slightly onto both the canopy and fuselage. You will be painting
frames around the base of the canopy anyway, so you probably have 2 to 3mm to
play with before you get into a visible clear area. The Cyano will not fog the
inside of the canopy because the PVA has sealed the joint first.
Below is the finished canopy glued in position on the Saab J29. When I come to
paint a model, I mask off the transparent bits, leaving the framing uncovered,
and spray it with the fuselage. This makes the canopy seem more an integral part
of the airframe, rather than having that "tacked on afterwards" look.
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