blowing plexiglass canopies
Alic Beasley
Years ago I had an excellent booklet, source and title unknown,
which had everything you wanted to know about blowing Plexiglas but
didn't know enough to ask. This book is long gone, does it ring a
bell for anyone, I wouldn't mind getting another copy. Failing that,
any information on the construction of the "oven" including details
on the air exchange that has to take place inside the "oven" to form
a canopy would be appreciated
Al Beasley
Liquidbuck
I don't recall a book, it seems to me it was an article in a magazine like
Kitplanes. The plexi was clamped in an angle steel frame and then heated in an
oven laying flat until is just started to soften (not really precise). It was
quickly removed and laid over a vacuum box that had a hole in the top that
would yeild the desired shape when the plexi was drawn down thru the hole and
held in the desired shape as it cools. It seems to be more art than science.
You will need a precisly controlable vacuum source and some experimentation
with the shape of the form to get what you need. As long as the plexi was
heated evenly, the result will have pretty good optical quality. They also
didn't come right out and say it but it appears that not all of the resulting
shape was used (a bit of waste).
This is about all I remember of an article I read probably 15 years ago. Good
luck.
FYI There are devices available from hobby stores to make canopies for RC
aircraft that would give you a good idea of what is involved as well as a
cheaper way to experiment with the shape of the form. The kit I'm refering to
vacuum molds the plastic over a plug (models don't need optical quality glass)
but all the tool will be there.
Mark McCord
NC83526
Craig Wall
Alic Beasley <sanda...@sk.sympatico.ca> wrote in message
news:37BE2492...@sk.sympatico.ca...
You may be thinking of Ladislao Pazmany's "Light Aircraft Construction"
series that had an extensive section on canopy construction. I think it's
still available from the EAA. It is absolutely excellent.
Craig Wall
byron...@xtalwind.net
the library. (You are a dues paying member of the EAA aren't you?) Ask for
help.
There was a Budd Davisson article about manufacturing blown canopies also,
but more recent than the homebuilding article. I'm not sure if Budd's
article was in SA or some other magazine. Call Budd and ask.
BJC
Craig Wall wrote in message <7pqm3b$9...@journal.concentric.net>...
Jim Vandervort
watch while they make canopys. It is impressive.
They have an oven big enough to put a 4 X 8 sheet of plastic in,
and it looks like they get it red.
they have large pieces of plywood cut out to lay the hot plastic on,
and it assumes the shape from there.
JimV.
<byron...@xtalwind.net> wrote in message
news:7pslca$77t$1...@opal.xtalwind.net...
James M. Knox
>If you are ever near Enon, Ohio, the Airplane Factory will allow you to
>watch while they make canopys. It is impressive.
>They have an oven big enough to put a 4 X 8 sheet of plastic in,
>and it looks like they get it red.
>they have large pieces of plywood cut out to lay the hot plastic on,
>and it assumes the shape from there.
What do they have between the plywood and the plexi? When I have heated plexi
that hot (or even close to it) it will take the surface impression from the
mold (and hence have a wood grain finish! <G>).
jmk
Jim Vandervort
I wasn't really interested, I was picking up a flat piece for my
Tailwind<G>.
That is when I learned about the differences in plexiglas.
Like extruded or cast.
JimV.
James M. Knox <tri...@realtime.net> wrote in message
news:37c2a...@feed1.realtime.net...
highflyer
>
> In article <37c1...@news.siscom.net>, "Jim Vandervort" <ww2...@dragonbbs.com> wrote:
> >If you are ever near Enon, Ohio, the Airplane Factory will allow you to
> >watch while they make canopys. It is impressive.
> >They have an oven big enough to put a 4 X 8 sheet of plastic in,
> >and it looks like they get it red.
> >they have large pieces of plywood cut out to lay the hot plastic on,
> >and it assumes the shape from there.
>
> What do they have between the plywood and the plexi? When I have heated plexi
> that hot (or even close to it) it will take the surface impression from the
> mold (and hence have a wood grain finish! <G>).
>
> jmk
We cut the area that was to make the canopy from the plywood and then
bolted the plexiglas to the plywood with a strip of plywood around the
edge to spread the clamping pressure out. Then hung the whole shooting
match in the oven until we reached the forming temperature. I seem to
remember something slightly over 300 degrees or thereabouts. An oven
thermometer is highly recommended and Rohm and Haas gives the exact
temperatures for forming.
If you are forming over a plug mold you pull the whole mess out of the
oven and slap it down over the mold so that the frame nestles down to
the base of the mold. Have lots of guys with CLEAN ( brand new ) COTTON
gloves on to rub the surface down to the male mold. The mold is a solid
plug covered with a good grade of felt. The felt is glued on with the
seams carefully butted to avoid lines and marking off on the inside of
the canopy. You canopy is as accurate and as optical as your mold is
good.
If you are "free blowing" then you slap the plywood with the by now
saggy plexi over a large vacuum box and proceed to suck it down until
it takes the shape you want. Be prepared to modulate the vacuum and to
nudge the canopy here and there with some more new cotton gloves on, to
coax it into the shape you desire. Sometimes a little local
application
of heat with a good heat gun can help control the shape. It is one of
those things that I can TELL you about until we are all blue in the
face,
but it won't MEAN anything until you have actually ruined some
plexiglas! :-)
We made an oven out of plywood with two chambers. One entire side of
the
oven opened up. The Plexi sheet in its plywood carrier hung from hooks
inside the door. The wall that separated the two compartments of the
oven had a six inch wide opening from top to bottom at one end. At the
other end we had a bunch of box fans mounted in holes in the wall. They
circulated the air through the compartment the plexiglas was hanging in
from the back compartment. The back compartment had the heating
elements
from a couple of electric clothes dryers mounted on standoffs. The air
circulated through the back compartment of the oven over those heating
elements and out through the opening at the end of the compartment into
the front half of the oven where it passed lenghtwise along both sides
of the plexiglas hanging in the center of the oven compartment. We had
an oven thermometer that we could stick through various holes in the
oven door to check the temperature in different parts of the oven. It
took us several heats to get the temperatures reasonably consistent.
The thermometers are sold for heating and airconditioning service people
to measure the temperature inside furnace ducts and cover a useful
range.
The are made to stick into a duct through a small hole and read the
inside
temperature. They are quite inexpensive. I have had mine for years, so
the
price I paid would only irritate you now! :-)
The door was the entire side of the compartment and the plexi frame was
hung
on the inside of the door. The "door" had casters under it and clamped
to
the oven with lunchbox catchs on both ends. We just unlatched the door
and rolled it right over next to the mold or vacuum box. Then we
unhooked
the plexi in its plywood frame and set it down over the mold/vacuum box
and went to work on it while a helper rolled the "door" out of the way.
It heated rather quickly and the plexi got a good "rubbery" consistency
and molded quite nicely. We made some scrap plastic before we started
to get optical quality stuff, but it made some nice canopies.
My suggestion is to start on a small scale making landing light covers
and
various small plastic fairings in the kitchen oven until you get a feel
for stretching it into shape and for the particular degree of "rubbery"
that you need. Then you can fire up the big oven an go for that full
canopy! :-)
--
HighFlyer
Highflight Aviation Services
Jerry J
My complements on your excellent post.
Jerry J
----------------
Big Snips
James M. Knox
>It is one of those things that I can TELL you about until we are all blue in the
>face, but it won't MEAN anything until you have actually ruined some
>plexiglas! :-)
Yeah... I have some wonderful Object d'Art that I will sell you cheap, results
of some early attempts! <G>
Anyway, great post... thanks for the firsthand info.
jmk