Polycarbonate

[Subscuba@aol.com]

Good luck Paul,

Sounds like you are going through the same problems as I had when I formed 
the bow port on my ambient sub. Drying the poly was the most difficult for me 
along with accurate and even temperature control. I baked the poly at F 200 
for three days before forming. Even with the three day bake I got a few minor 
bubbles. I also had a problem with the temperature sensor in the oven. One of 
my trial runs the poly sheet 1/4 inch thick formed a barrier between the 
temperature sensor and the heating element. This caused uneven heating and 
some burning of the poly. I would suggest a large oven like a pizza oven if 
you don't have access to a better setup at your work site. I did mine in my 
wife's oven. Having a melted goo of poly around the oven grates didn't 
impress my wife.

Keep in touch on your process in forming your ports. My bow port is poly and 
my conning bubble is acrylic. People ask me about ramming something and the 
impact resistance of the poly was important to me. Taking about a foot square 
of the poly plastic and trying to break it with a large hammer on a concrete 
floor is difficult and is impressive when compared to acrylic which shatters 
easily.

As Gary and Vance points out most of the literature is on acrylic and is used 
on the deep subs. I haven't seen any testing done on the polycarbonate.  
Seems there is some sensitivity on use of polycarbonate, it would be useful 
to find concrete data on why acrylic is used for most sub ports. It may have 
something to do with the water absorption of the poly and/or processing 
problems. There was one posting on this site that discussed the mechanical 
properties of the two plastics with the conclusion that the acrylic was more 
suitable.

I went to a local vendor in this area and the FIT human powered subs. The 
human powered sub people definitely like the poly but the vendor didn't like 
to process and form the material. They said the process they use was 
proprietary and difficult. They also said they would not supply a port for me 
with the words "We don't do that anymore." Bear in mind my sub does not 
require the ports to withstand the ambient pressure,  it's a wet sub. If I 
were to build a 1 ATM sub, which I'm not going to do, I would get more data 
before using the poly. In my case if the port breaks I may not be in much 
trouble. I suspect your 1 ATM sub would be more of a problem. If the window 
broke and stunned you from the implosion, you probably would not make it back 
to the surface. 

I'm of the opinion that wet subs are lower risk. That's not zero risk.

Have fun,

Ken Martindale

In a message dated 8/12/99 10:33:07 PM Eastern Daylight Time, 
paul_suds@hotmail.com writes:

<< The only thing I don't like 
 about acrylic is that it has an Izod impact resistance of  0.5 ft-lb/in 
 compared to polycarbonate which has an Izod impact number of ~ 15.0 
 Otherwise, the basic plastics are very similar in strength and mechanical 
 properties. One of the problems with poly besides it's UV stability 
 (yellowing) is that it requires a higher temperature to process than 
 acrylic. Because of this, you have to "dry" it and minimize the absorbed 
 moisture before forming. Since my canopy is going to be 1/2 inch to 3/4 inch 
 thick, I'm going to have to form it myself. It would be easier if I used 
 acrylic, but I'd feel a lot safer with poly. It's overall strength is about 
 20 % higher than acrylic in most categories (Tensile, elongation,yield, 
 compressive, flexural). >>