Back in the '60's I worked with a couple of sailplane homebuilders trying to
make canopies. Yes, we filled a dumpster with trashed Plexiglas before we
climbed the learning curve.

We decided we didn't like the results from molds. The optics just weren't
great no matter how good the mold. Fortunately, there's another way.

If your canopy is close to a section of an ellipsoid, you can free blow the
canopy and get perfect optics. The way we proceeded was a 'cold bend-hot
blow-cold bend' process. The plexy was bent cold into a forming tool and
loosely clamped. The tool and plexy was then heated to forming temp in a
large plywood oven like you describe. The clamps were then tightened and
the canopy was blown to a curve slightly greater than the finished shape.
(Actually, we used a shop vac to suck the canopy into the desired shape - no
cold air introduced.)

When we had the 'suck' step where we wanted it, we shut off the heat and
kept the vacuum on until the plastic cooled. Then we trimmed the flashing
and cold bent the plastic the last bit to fit the frame. The folks using
this method have gone on to make a lot of canopies with perfect shape and
optics.

The trick to make this work is to plan on wasting a lot of plastic in the
trimming step. In other words, start with a much larger plastic bubble than
you really need. That way you can select the part of the bubble that best
fits the curve you want. When you do the last cold bend, the curve flattens
a bit which is why you want to 'over blow' it a little.

Bill Daniels


"c hinds" wrote in message
om...
I know this has been beat to death over the years, but, I am ready to
proceed with canopy fabrication. I am fortunate enough to have a
canopy tool (female mold) but am unsure how to proceed. Use the
female to make a male plug and try stretch forming over the plug? If I
use this method, how do I compensate for the thickness of the plexi
when making the plug? Or I could use the female and attempt to vaccum
it down?
I have an oven started, a huge plywood shipping crate about 6'x4'x4'.
My plan is to heat the plexi and plug together in the crate and upon
reaching proper temp, slide it out and stretch it down.
I have already sent photos of the tool to a canopy house and they
estimate $3k to make one. I figure I can burn up a lot of plexi for
that.
Words of wisdom appreciated.
Thanks,
Clark