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Hi Ken,
Welcome.
I am sending this reply to both you and the list sever discussion group as
some of those members may be able to give better advice then me.
In my college days I worked for a plastics fabricator. Being more focused
on college then on the tricks of the trade I did glean a few techniques in
plastic forming. It was also ten years ago so I may be remembering everything
wrong.
Here goes.
The ovens we used to heat plastic tended to be larger than your typical home
oven. They were flat, electrically powered, and had a fan to stir the air up.
The space was larger than the plastic to heat to allow for air circulation
and had a typical thermostate. One oven was big enough to walk in but that
one was rarely used due to the power consumption. You may be able to
build one with sheetmetal, heating elements and fire brick.
Sorry to say I don't remember the heating temperature but bubbles in the
plastic indicate the plastic got too hot. I was bending plexiglass one day
with bar heaters to form clear display shelves and a few got bubbles when
left on the heater too long.
If you can find the shape you want for your form, fine. If not then you will
have to make it. One idea is to make a positive or concave form with
cement and mesh wire. If you need it symetrical then you can cut a
piece of plywood to the cross section shape you need and rotate it on
a center pivot over the cement and wire shape. Ascii picture:
center pivot bar
|
----------|---------
| form - |- form |
| / - |- \ |
| / / | \ \ |
| / / | \ \ |
---- | | | ----
---------------- base
^ ^
| |
---------------- cement and wire form
We lined our forms with felt. Still was always got a few scratches and marks.
You can use plaster instead of cement.
At this point you have two ways to go. You can sandwich the plastic to be
formed in a picture frame like frame. With the a frame on both sides and
over lapping the plastic and bolted together. When the plastic is heated
force it down over positive mold and let cool.
The other way is to make a negative, female, or convex mold by forming over
the positive form. Then heat the plastic and lay over the form and use
vacume to suck the plastic down into the form. The vacume is applied to
the bottom inside of the form. More compicated still is to build an air
chamber that can be clamped to the plasic and negative form and apply
compressed air to force the plastic into the form. Keep in mind that
the compressed air may cool the plastic. Ascii:
--------------
| air |
| chamber |
--- ---
------------------ plastic
--- ---
| |
| form |
--------------
We lined our forms with felt. Still was always got a few scratches and marks.
If you heat the plastic hot enough to use gravity, then the plastic is
too hot and hard to control.
If you use your wife's oven again then get your own grate and line the
bottom with aluminum foil. Stir the air with a fan.
Good luck,
Ray
>----------------Begin Forwarded Message----------------<
Date: Thu, 11 Dec 1997 15:53:34 -0600
From: "Ken Martindale" <MartindaleK@s1.keltec.sigtech.com>
Subject: Sub Window
To: Ray@psubs.org
------- Forwarded Message Follows -------
From: Self <martindk>
To: Peterc@microsys.com
Subject: Window
Hello,
I got your name and address off your Submarine Web site
which I enjoyed reviewing. I noticed it is fairly new. I wish there
was more interest in this field.
The reason I am writing is that I am building a battery powered wet
Sub based on the work done by the human powered submarine
researchers. I have finished the hull and controls. I am having a
problem making the port or window for the Sub.The Sub is 28.5 inches
in diameter and 10 feet long. I have tried making the front port by
heating up a piece of 1/4 inch polycarbonate in my wife's oven and
allowing the plastic to flow over a Wok to get the shape I need. The
problem I am having is that the oven temperature is not too even with
the sheet blocking the flow of air in the oven. The result is that,
even though I heated the plastic for three days at about 220 F to
remove the water before heating to the flowing point, the surface
finish is poor and I still have some bubbles forming in the
polycarbonate. Also the port is small since the area I have in my
wife's oven is also small. I did not impress my wife when some of the
plastic flowed around her cooking grates which I had to remove with a
grinder. She is now relunctant to have me try her oven again.
I read in the papers on the Web site and learned one way is to allow
the plastic to flow by gravity into a bubble shape. The questions I
have is where do I find an oven and what is the correct temperatures
and times. Could you help me or direct me to an information source.
I would appreciate your time and help.
Thank you,
Ken Martindale
2315 Weber Road
Malabar, Fl
407-723-1176
PS I am an experienced EE with a background in Power Electronics and
an active diver.
>----------------End Forwarded Message----------------<