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?