Hi Emile,
If you have unequal sliding of the flange:
How are the clamps spaced?
Is the male form up to temperature?
How fast does the form move?
How tight is the form when it reaches full
depth?
Do you have a radius on the forming ring?
Is the acrylic blank large enough?
Greg
From: owner-
Sent: Monday, June 18, 2012 5:10
PM
To:
Subject: RE: [PSUBS-MAILIST]
acrylic dome fabrication
Greg, Alan
How we did it:
-no pre annealing
-slow heating to 160-165 Deg.C
-slow pressing
-slow cooling to 100-110 Deg. C and
remove the male form. Silicone grease gives good results against sticking and
surface damage.
-slow cooling to room temp.
The press tool is intergrated with the
oven so there is no thermo shock for moving the dome in and outside.
Just turn of the power and it cools down
in 24 H.
-trim flange of
-24 hour annealing at 80 deg. C.
-machining edge
The press tool is intergrated with the
oven so there is no thermo shock for moving the dome in and outside.
Just turn of the power and it cools down
in 24 H.
of course unmanned pressure testing.
Not agree that forming is the easiest
part. What doe you do against the unequal sliding in of the flange??
The black magic book of acrylics...
Emile
Van:
owner-
Verzonden: maandag 18 juni 2012
22:38
Aan:
Onderwerp: RE: [PSUBS-MAILIST]
acrylic dome fabrication
Hi Alan,
I’m not familiar with Emile’s
methods so I can’t comment on what he does, but plug & ring forming
requires removing the male plug from the part before the part is completely
cool (usually at about 180 F) otherwise the part will try to shrink around the
plug and greatly increase the
internal stress (sometimes even cracking the part). Cast hemispheres are also
done this way- the male/female mold assembly is removed from the autoclave and
the male is pulled out while the acrylic is still hot and hasn’t had a
chance to shrink around the plug. Then, the acrylic hemisphere and female mold
are rolled back into the autoclave to completely harden and shrink. This
shrinkage occurs regardless of how slowly the mold/ part assembly is allowed to
cool.
Forming domes for PVHO is not a quick
process- it’s more like “massaging” the sheet into the shape
it needs to be. Careful heating and cooling is the key to a bullet-proof
acrylic bubble.
Greg
From: owner-
Sent: Monday, June 18, 2012 3:51
PM
To:
Subject: Re: [PSUBS-MAILIST]
acrylic dome fabrication
Hi Greg,
In Emiles case where he is pressing
the acrylic with a male mold inside an oven,
could all the
annealing processes (apart from post glue process) be rolled in to
one? IE, cut your circle, clamp in place, do the polymer
release & high temp stress
relieving cycle, then press at the correct temperature
followed by forming stress
relief while the mold is still in place.?
Regards Alan
From: Greg
<greg@precisionplastics.com>
To:
Sent: Tuesday, 19 June 2012 2:42
AM
Subject: RE: [PSUBS-MAILIST] acrylic dome fabrication
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:
personal_submersibles@psubs..org
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
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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