Vacuum forming
I
have been using the vacuum forming tecnique for many
years. It’s a very popular technique in the world of
aeromodelling, especially to make clear canopies, engine
cowls and wheel pants.
Personally I am not in love with the products made using
such method, especially for the wheel pants and the
engine cowls for internal combustion engines. The
materials used, ABS, PVC, Styrene etc, are not very
sturdy and tend to crack and break easily. For the
canopies, instead, the use of vacuum forming is basically
mandatory. I use such technique also to make instrument
panels , interiors and other details not subjected to
wear. A less common method is the making of vacuum formed
moulds for composite materials for small productions.
Basically, once the wodden or other material mockup is
made, rather than making a complicate fiberglass mould
it’s quicker to make it out of 2mm thick vacuum formed
plastic. The system I built works flawlessly and it’s
worthwhile to describe it.
The picture above
clearly shows the vacuum forming machine that I just
used to form one of the seven fairings for the
engine cowl of my Bucker, in this case the fairings
have been used on the Styrofoam matrix.
In the section below it’s clearly shown the functioning
of the vacuum box. It’s a 19mm plywood box which hosts
inside a divider containing a vacuum cleaner motor,
obviously the more powerful the motor the better. The
motor takes in air through some holes located on the top
side and expells it through 10mm holes in the back. A
strong and small hole mesh, which I recommend to be at
least 35x50cm, has to be positioned on the top side, you
can find a good selection at the hardware store.
The mesh prevents the plastic, softened by the heat, from
entering the 5mm holes and obstruct them. Alternatively
many 2-3mm holes can be drilled so to avoid the use of
the mesh, in this case it’s important to roughen the
surface a little so that the plastic does not stick too
much preventing the air from escaping. I cross milled,
with the cutting pantograph, several areas making a hole
at each intersection , but not everybody owns a CNC
pantograph. A rubber stripe has to be attached all along
the edge of the panel. Two frames will then be attached,
using 4 hinges, to the panel and the plastic sheets to be
vacuum formed will go in between. I mounted on my frames
two spring locks to keep still the plastic sheets, one on
the front would be enough though. The vacuum forming
consists of two stages, the heating of the sheet and the
vacuum forming itself. A 1600 watts oven resistor is used
to heat the sheet, the resistor has to be positioned
inside a sheet metal tub or a oven tray of appropriate
dimensions, bolting it to a side of the tub. Be careful
and make sure the electrical terminal clamps get well
insulated, needless to say you can’t fool around with the
electrical current.
The tub
must be located as per drawing, and the frame with the
plastic sheet has to be turned upside down on the tub
when the resistor is well heated. The matrix on which the
sheet will be formed has to be positioned at the center
of the vacuum table. Now the plastic sheet must be
observed carefully: the sheet will first curl up, then
will strech out, after the sheet will start to give up
caving in downwards. At this point the double frame has
to be immediately turned upside down on the matrix and
the aspitaror must be turned on , after the resistor is
turned off so to avoid electrical overloads and... the
miracle is done! ..., and with some practice it’s even
fun.
Suggestions
1) Keep in mind that the plastic will tend to round up
near the base of the matrix, so if you want a perfect
adhesion a spacer of about 1cm must be positioned under
the matrix lined up with the base.
2) To form small parts without having to use a 35x50
sheet, a frame reducer can be made with a piece of
plywood inserted in the frames with a rectangular opening
of about 15x15cm in the center to which small plastic
sheets can be stapled to. The pictures below show the
fairings of the struts, made with a frame reducer, of my
first Ro41.