Your friend is attempting a method of
forming called “vacuum assisted plug and ring forming”. You can
find descriptions of this on a few of the acrylic manufacturer’s
websites. If done properly it can produce a more consistent thickness on a
formed plastic part. Producing an acrylic hemisphere this way without excessive
distortion and mark-off is difficult.
Using air alone to pressure form acrylic sheet
into a dome will produce a part with varying thickness, but it also has some
advantages. Many people focus on the thinning at the apex as a downfall- but
another way of looking at it is to think of the periphery of the dome as having
EXTRA thickness to compensate for the fact that it is (1) an opening in the
part, and (2) the high pressure face that must resist stress crazing/ cracking.
The ultimate thickness at the apex may be limited, but the real question is, is
it sufficient for the design depth?
Another thing to keep in mind is that both
the thinning of the dome and the overall roundness of the dome degrade rapidly
when you try to go all the way to a 180 degree segment. The apex both thins and
flattens while the thicker sides start to bulge. I found that it is possible to
produce a dome that has a true radius on both inner and outer surfaces
(although they are different radii) by restricting the segment to about 170
degrees. This produces a thicker, rounder part with less surface area.
From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Alan James
Sent: Wednesday, January 06, 2010
6:55 PM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST]
Am wondering what the thickest acrylic was that you blew
domes from
or were they vacuum formed?
I have a guy here in NZ vacuum forming a 35mm x 550mm
dome, he says
he is going to press the apex up to aleviate thinness in
that area any thoughts on this?
----- Original Message -----
Sent: Thursday, January
07, 2010 12:22 PM
Subject: RE:
[PSUBS-MAILIST]
Polyesters have excellent resistance to
acids but are readily attacked by strong alkalis (such as soda lime). Battery acid mixed with seawater can produce chlorine
gas. AGM batteries are the safe bet these days.
Greg
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org]
On Behalf Of landnseawelding
Sent: Wednesday, January 06, 2010
5:25 PM
To: personal_submersibles@psubs.org
Subject: [PSUBS-MAILIST]
Thanks for the great tips guy's. I like the stainless steel
battery tray idea but should also treat the bottom below it as mentioned. Does
fiberglass resin react to battery acid?