Welcome to the

White Paper: Forming Plastic Bubble Windows

About:

Contributor:	Ray Keefer/Ken Martindale/Richard Morrisson

Last update:	01/26/98

Synopsis:	This page contains a discusion on how to form plastic
		bubble windows out of Acrylic or Polycarbonate sheet
		plastic.

Disclaimer:	Be aware that due to the stresses induced while forming
		bubbles and the thinness of the material involved, bubble
		windows should only be considered for wet or ambient
		pressure submersibles. For use on 1 atmosphere submersibles
		the plastic must be considerbly thicker with a lot of 
		designing, testing and derating is needed. See the
		white paper on Window Considerations.

		You are resposible for your own safety. Even though
		this information may work or seem resonable in some 
		cases you need to approach the subject carefully and 
		even aquire the help of a Marine Architect before applying
		into one of your designs.

Body:

TEMPERATURE
			Water 	Droop	Melting	Bubbles	Burn 
			Vapor
			Removal
	Plexiglass
	Poly Carbonate

TIMING
WATER VAPOR

Plastic has left over water molecules imbedded in the plastic due to the
its manufacturing process. If the plastic is heated to too high a temperature
the water will boil into water vapor, steam, inside of the plastic and will
form bubbles inside of the plastic. Once the bubble are there you can't get
them out.

HOME OVENS

The problems associated with home ovens are:

	- oven temperature is not too even
	- air flow is poor
	- size of window is limited to size of oven
	- make a mess and its tough to clean off the interior 
	- gross temperature control

The reason for the temperature not too even and the air flow is poor is 
because of the sheet of plastic plastic blocks the air flow. The piece of
plastic needs to be smaller then the oven. Less the [x]" on each side is
good. As an alternative try using a fan or a convection oven with its
built in fan. Just be carefull you don't use a fan that can melt.

The size of the window is limited by the physical dimensions of the oven.
If it don't fit in the oven then you can't get it into the oven. However
as noted above, if the plastic is the maximum size that can fit into the
oven then there are air flow and temperature problems.

If some plastic flows onto and/or burns onto the interior of the oven
or oven racks it is very hard to remove. In some cases you must chip
or grind off. If the oven is dedicated to plastic forming the no problem.
If the oven is the family oven you use to bake then BIG problem. You
must clean it well or have your food taste like burnt plastic. [Health
hazard?].

The temperature controller in a typical oven is graduated in 25F incriments.
This may be too gross a gradiant. Especially if you need to dial in
something in between. Plus the heating elements are large so produce a
large input of heat while turned on which may heat one area of the
plastic to a melting point while leaveing the other areas relatively cool.

GRAVITY/DROOP FORMING

One forming technique is gravity or droop forming which lets the
avaliable gravity do the job (which elimenates the surface finish
problems associated with the mold, but not the amplification of surface
scratches upon stretching).  

Droop forming has the advantage that its inexpensive and does not require
the complication of building a form. 

Heat the plastic just hot enough than it takes to start
the droop (and then wait patiently!). Any hotter and bubble might form
since the droop temperature is near the boiling point of water. Once the 
water in the plastic boils you will get bubbles.

MOLDS/FORMS

If you can get the shape you want via gravity forming then great. If you 
need a form the simplest thing to do is find a a sutible shape you can
form over. 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


You can use plaster instead of cement.

Line your form with felt to minimize scratches. Still you will always 
get a few scratches and marks. Felt also insolates the plastic a bit so
it will stay maliable a longer.

A vent hole needs to be in the bottom of the mold to allow air to escape
as the plastic is formed into the mold. Not doing this will bulge, deform,
or destort the shape with trapped air under the plastic.

FORCE FORMING

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 problem with this method is that the force to pull the plastic over
the form comes at the edges of the plastic. This causes the plastic to
streatch and thin near the edges and does not force the platic to tightly
conform to the desired shape.

Also this method does not work for convex shapes.

PRESSURE FORMING

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      |
                          --------------


The compressed air should be regulated so that it doesn't just blow a hole
in the hot plastic but slowly pushes it into the form. However, it needs
to push fast enough that the plastic doesn't cool before the shape is 
formed. Also this allows the trapped air under the plastic in the form
chamber time to escape out the vent hole.



VACUUM FORMING

As an alternative to forcing the plastic into the the mold with compressed
air is to suck the plastic into the mold by a vacuum. Once the plastic is
clamped down start a vacuum pump and the plastic will get sucked down
into the mold. 
                        ------------------      plastic
                        ---            ---
                          |            | 
                          |form/vacuum | 
                          --------------
 
 
The advantage to vacuum forming is that it remove all the air in the
vacuum chamber so there is no distortion from trapped air. Another advantage
is that the air pressure differential is limited to ambient airpressure which
is 14.7psi at sea level.