Re: [PSUBS-MAILIST] acrylic dome fabrication

Hi Emile,

Greg is the one with all the experience, but maybe you could use hydraulic clamps to

get an even pressure on the flange. When a NZ company blew a dome for me they

had cloth under the bottom of the acrylic sheet to disperse the air jet. This had another

effect in that the bottom of the sheet slid in to the dome while the top was gripped in

place. This was good because the acrylic has a longer distance to travel round the inner

heel of the dome. The flange of the dome was left with an angle from top to bottom as

evidence of what had happened.

Regards Alan

From: Emile <emile@airesearch.nl>
To: personal_submersibles@psubs.org
Sent: Tuesday, 19 June 2012 9:10 AM
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-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] Namens Greg
Verzonden: maandag 18 juni 2012 22:38
Aan: personal_submersibles@psubs.org
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- personal_submersibles@psubs.org [mailto:owner- personal_submersibles@psubs.org ] On Behalf Of Alan James
Sent: Monday, June 18, 2012 3:51 PM
To: personal_submersibles@psubs.org
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: personal_submersibles@psubs.org
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- personal_submersibles@psubs.org [mailto:owner- personal_submersibles@psubs.org ] On Behalf Of Smyth, Alec
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- personal_submersibles@psubs.org [mailto:owner- personal_submersibles@psubs.org ] On Behalf Of André Eriksen
Sent: Friday, June 15, 2012 4:16 PM
To: personal_submersibles@psubs.org
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