If you have unequal
sliding of the flange:
How are the clamps spaced?
>>6 or 8 clamps . we tighten/loosen the clamps to adjust the unequal sliding
Is the male form up to temperature?
>> Jep, all in
oven
How fast does the form move?
>> pressing some 10 mm with
hand hydraulic pump an the wait 5 mins.
How tight is the form when it reaches full depth?
>>?? just
fit..
Do you have a radius on the forming ring?
>> Yes, R 15
mm
Is the acrylic blank large enough?
>> thinks so: 900 mm for 610
m 180 Deg. dome and 1300 mm for 900mm, 130 deg.
dome
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