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RE: Sub Window (Fwd)
>----------------Begin Forwarded Message----------------<
Date: Thu, 11 Dec 1997 18:00:17 -0800
From: "Morrisson, Richard D" <Richard.Morrisson@PSS.Boeing.com>
Subject: RE: Sub Window (Fwd)
To: "'Ray Keefer'" <Ray.Keefer@Ebay>
Ken,
I have some experience working with droop forming of plastics from
working on a human powered sub team. You are correct that it is
available to let gravity do the job (which elimenates the surface finish
problems associated with the mold, but not the amplification of surface
scratches upon stretching). I have had some moderate success with this
process after seeing another team member produce some nice samples. We
needed an eliptical cross section to match our sub nose so droop forming
worked out well (and in-expensively). I ran into the same problem with
bubbles forming. The best advice I can offer is to experiment with
small scraps and use an accurate temperature measurement and control
system. My best results were with 1/8" thick stock soaked at 210
degrees or so for 24 hours. I had only limited success with the 1/4"
stock (the soak time should probably be several days as you indicated
you were doing). I think you might have some error in your setpoint as
220 degrees would be causing bubbles normally (I understand the bubbles
to be water vapor still trapped when the plastic gets soft enough to let
them expand, boiling at 212 degrees). Since the boiling point is so
close to the softening point, you will find patience to be worth its
weight in gold! The less you have to heat the part, the less likely you
are to form bubbles. Don't heat it any hotter than it takes to start
the droop (and then wait patiently!). I have thought about making an
oven to do a larger piece and envisioned a window to watch through. I
used wires at the front and back of my forming stand to eyeball the
point of droop desired. However, if I do make an oven I would like to
look at electronic proximity devices and some semi-automated controls.
This will allow me to leave the process un-attended for the duration.
Also, you can change the shape of the bubble by adding a cover and
applying either vaccuum or pressure (very slight) while in the final
stages of forming. I have not done this but am planning to experiment
with the idea when and if I build the oven. I hope my thoughts are some
help to you. Good luck.
> ----------
> From: Ray Keefer[SMTP:Ray.Keefer@Ebay.Sun.COM]
> Sent: Thursday, December 11, 1997 3:06 PM
> To: Personal_Submersibles@psubs.org;
> MartindaleK@s1.keltec.sigtech.com
> Subject: Sub Window (Fwd)
>
> 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----------------<
>
>----------------End Forwarded Message----------------<