With regard to acrylic dome fabrication,
there are two very important areas to address. One is obviously the mechanical
design such as shape, thickness, how it is sealed, making sure that movement
under compression is considered, etc.
But equally important when thermoforming
acrylic sheet is how it is heated and cooled (and the number of times it is
heated). For greatest ultimate strength, acrylic should be subjected to a
number of heat cycles. The first cycle is to more completely polymerize the
sheet than the casting process allows. This is a long process that also causes
the sheet to release, over time, any residual monomer left in the material. The
next cycle is a higher temperature stress relieving cycle, that releases
casting stresses and pre shrinks the material to reduce forming stresses later.
Then comes the actual forming cycle (the easy part!). After forming, a stress
relieving cycle is performed to remove internal stress from forming before any machining
or gluing (to prevent crazing). Another annealing cycle will be needed to
polymerize the glue joints (if there are any) and also to remove stress from
any machine operations that were performed. A thick part may stay in the oven
for a week to fully relieve stress. Also, even though there are published
tables for annealing acrylic, these are minimum
times required. Air flow across the surface of the sheet is also finely
controlled during these processes. After fabrication is complete, only very fine
wet sanding or hand rubbing is allowed.
George Kittrege was relayed a story to me
about a 1” thick acrylic dome (not annealed) that imploded at only 50 ft
and another, identical dome (properly annealed) that went to 823 ft! A 16 fold
increase in strength! Both domes looked exactly the same but were very
different from a strength perspective. I know it was true because he showed me
the test report.
Greg
From: owner-
Sent: Friday, June 15, 2012 5:00
PM
To:
Subject: RE: [PSUBS-MAILIST] how
to fit the acrylic dome
Hi Andre,
I’m not an
expert on domes, but I agree with David that the O-ring groove is in a
dangerous spot. You could, for instance, chamfer the outside edge and put the
O-ring there (see photo on the Snoopy project page). In addition, careful with
the drawing on the left and stress concentration at the flange. Domes are made
with a flange like that, but it should be machined off because during
fabrication it will have accumulated stress. When a shoulder is used, as in the
K250 domes, it is a bonded shoulder.
Viewports are an
area where one can very easily get something wrong with catastrophic
consequences. I would highly recommend purchasing a dome from someone like Greg
or Emile. It will come with free advice worth more than the dome itself. Of all
the things one has to build for a sub, this is one of the areas with the most
specialized fabrication requirements. I find viewport design is not a problem
if you get a copy of the PVHO, which should be considered mandatory reading and
is more or less a collection of recipes. But fabrication is something else
altogether, and to get to where the dome fabricators are has them years of
trial and error.
Thanks,
Alec
The
contents of this e-mail are intended for the named addressee only. It contains
information that may be confidential. Unless you are the named addressee or an
authorized designee, you may not copy or use it, or disclose it to anyone else.
If you received it in error please notify us immediately and then destroy it.
From: owner-
Sent: Friday, June 15, 2012 4:16
PM
To:
Subject: Re: [PSUBS-MAILIST] how
to fit the acrylic dome
From what you said I made this sketch (attached). Is this a good
way to fasten the dome?
-Andrè
2012/6/15 Emile <emile@airesearch.nl>
Hi Andre,
It is not done to drill / tap any holes in a
acrylic dome. Stressraiser!
For shallow divers you can glue .
Under high pressures the dome contracts more than
the ring
I use a o-ring on the edge and stainless steel
bands hold down the dome.
Flange also possible for shallow divers. In the
handbook of acrylics are some examples.
Emile
Van: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org]
Namens André Eriksen
Verzonden: vrijdag 15 juni 2012
17:25
Aan: personal_submersibles@psubs.org
Onderwerp: [PSUBS-MAILIST] how to
fit the acrylic dome
Hi,
I`m new to
this forum, and are planning to build a 3 person sub.
I can`t
seem to find any plans or pictures on how to fit the acrylic dome for
the hatch. Is it screwed down on flanges with an o-ring underneath, or liquid
gasket/glue? I`ve seen some domes without flanges, how do they attach them?
thank you.
-Andrè
Eriksen, Norway