Vacuum-Forming Cowls-2011/07

46 MODEL AVIATION
IF YOU BUILD your own models, you often
need to construct cowls and perhaps canopies.
Vacuum-forming is one way to produce a
lightweight plastic cowl and is ideally suited
for making canopies.
To vacuum-form parts, you need the
following equipment:
• A vacuum source. I use my shop vacuum.
• A vacuum box. You can build or purchase
this. I’ll cover this more later in the article.
• A mold (called a plug), which is the shape of
the desired cowl or canopy.
• A place to heat the plastic sheet. This is
usually your kitchen oven.
An excellent source of information about
vacuum-forming for hobby projects is the book
Vacuum Forming for Hobbyists by Douglas E.
Walsh ($12.95). He also sells plans for
building a vacuum box and has much other
information that is of interest to the hobbyist.
Douglas does business as Workshop
Publishing. See the “Sources” listing for
contact information.
Plastics: There are several plastics you can use
for vacuum-forming operations. I have
employed two, which have been satisfactory
for my needs. Check the Internet or the Yellow
Pages for plastic suppliers.
PETG (sometimes referred to by the brand
name Vivak) is a clear plastic that is available
in thicknesses of 0.020, 0.040, and 0.060 inch.
It forms readily after heating at 270°-310°.
Warm the material in your oven until it sags
approximately 11/2-2 inches before you pull it
out and quickly press it down over your plug
on the vacuum box.
Your vacuum source determines what
thickness of plastic you can form successfully;
you will have to experiment. I typically use
sheets that are 0.020 or 0.040 inch thick.
ABS plastic is normally white and
therefore unsuitable for canopies. It is available
in similar thicknesses as PETG is.
For ABS, set your oven at 350°-360°.
Allow roughly the same amount of sag as for
PETG. ABS cools slightly slower than PETG,
which gives you more time to work, and it
forms well.
Plugs: Molds can be made from several
materials, but remember that they get
considerable heat when hot plastic is formed
over them. The best material to use is wood.
Balsa works fine and is easy to shape.
You can make plugs from blue or pink
foam, but you must coat them with something;
otherwise, the heat will distort them. I have
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covered foam plugs with successive layers of
auto-bonding putty, which is available at any
auto-parts store. It can be sanded to a nice,
smooth finish.
If you do that, you will need a putty
thickness of roughly 1/8-3/16 inch on the foam
plug to properly insulate it from the hot plastic.
Or you could use foam and face it with pieces
of balsa, which you would then shape to form
your plug.
Vacuum-Forming a Cowl: Follow along with
the photos to learn this process.
Photo 1 shows four plugs made with different
materials. At the bottom is a canopy plug made
from pink foam and coated with auto-body
bonding putty. Narrow strips of 1/64 plywood
are glued to the model to simulate canopy
framing.
On the left is a plug made from pine
wood. It was turned on a lathe to form a
radial cowl for my Cessna 195.
At the top is a canopy plug made from
balsa, and to the right of it is a large
canopy plug made from pink foam covered
with auto-body bonding putty. In the center
is a PET-G canopy made using that plug.
Photo 2: Glue together balsa blocks from
your scrap box that are big enough for the
cowl plug.
Photo 3: Use templates from your drawing
to outline the top, side, and back
dimensions of the cowl you plan to
produce.
Photo 4: Roughly cut out the plug with a
band saw and sand to shape.
Photo 5: Check the outside curvature using
a template of light plywood.
Photo 6: When you are satisfied with the
shape and finish, seal the balsa. I use waterbased
polyurethane.
Photo 7: Use fine sandpaper to achieve the
final finish. Any imperfections in the plug
will be faithfully reproduced when you
vacuum-form the cowl.
The plug is sitting on a spacer that can
be made from foam or wood. That spacer
helps prevent “webbing” from forming on
the part of the cowl you want to use.
Before you vacuum-form the cowl,
spray the plug with mold release or rub a
light coat of Vaseline on it. This will make
it easier to remove the plastic cowl from
the plug.
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48 MODEL AVIATION
Photo 8: You are ready to vacuum-form the
cowl. For this example I am using PETG and
the smaller of two vacuum boxes I have. This
box requires a 9 x 12-inch sheet of plastic.
The larger version I have uses 12 x 18-inch
plastic.
Hook up your shop vacuum to the vacuum
box, and place it close to the oven where you
plan to heat the plastic. Set the proper oven
temperature and warm the plastic, keeping an
eye on the sag of the hot material.
When it has sagged 11/2-2 inches, turn on
the shop vacuum. Be sure to wear gloves
when you handle the plastic in its frame.
Quickly remove the hot plastic in its frame
and press it down over the plug, sealing it to
the frame of the vacuum box. It may take a
couple tries before you are successful.
Photo 9: This is a repeat of the process I just
described, except this time I used ABS. Both
cowls turned out well.
Photo 10:Measure the required depth of the
cowl, and carefully trim the plastic.
Photo 11: Check the fit of the part on the
airplane.
Photo 12: I manufactured this cowl from
0.020-inch-thick PETG, which was thin after
being stretched over the plug. So I reinforced
it on the inside with 0.75 ounce/square yard
fiberglass cloth. I framed the back edge of the
cowl so I could secure it on the model with
magnets.
Photo 13: The finished and painted cowl is
shown installed on my WACO biplane. MA
Roy Day
[email protected]
Sources:
Workshop Publishing
(248) 391-2974
www.build-stuff.com
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Photos by the author
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