Re: bubbles in plastic

[Nathanael Henderson <jude@pconline.com>]

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.)
    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