Re: bubbles in plastic

[William Sewell <cstmwrks@xyz.net>]

Nathanael Henderson wrote:

> On Fri, 10 Dec 1999, William Sewell wrote:
>
> > I have read again the psub white pages and have been surfing the list
> > archive for details on working with plastic and notice that the heat
> > temp is 300F and water boils at 212F.
> > does the fact that the sheet was formed under pressure at the factory
> > account for the water not "boiuling" out when heating it to 300F??
>
>     As a purely practical matter, you can get acrylic hot enough to form
> without getting bubbles in it.  I can't explain the physics of it for sure
> (perhaps the vapor pressure of the moisture in the plastic only becomes
> great enough to displace the plastic and 'bubble' well above the actual
> boiling point, ie. the plastic itself is too strong for the bubbles to
> expand into below a certain temp.)

I have heat formed cast acrylic with great succes but it was in a non
pressuriezed application ( meter block for a rebreather ).. I would really
like to find a few more clues
on time and temps for after the first de stressing and the forming.

Bill

P.S. thanks for your input!

>
>     But at any rate, with a little care and practice, heat forming acrylic
> without gettings bubbles isn't too difficult.   (Emphasis on
> practice...play with scrap until you're comfortable with the process.)
>     I've never tried forming bubble domes...vaccuum or air pressure
> forming seems like the simple aproach.  (The web site has a little bit on
> the process.)   What I'm wondering is how 1 ATM sub domes are normally
> produced....   Casting from resin?   Lathing from a block?  Forming?
>
> 'Than