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Discussion of diving methods and equipment available prior to the development of BCDs beyond the horse collar. This forum is dedicated to the pre-1970 diving.
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Bronze06
Master Diver
Posts: 663
Joined: Tue Jul 01, 2014 5:33 am
First Name: Russell
Location: Tabuk, Saudi Arabia

Diaphragm for Draeger PA 61

Tue May 28, 2019 3:43 am

Hey Folks,

I am attempting to create my own diaphragm for the Draeger PA 60 series. It is 130mm in diameter, a real whopper! What I need to know from the "engineering GURUS, NABOBS and POTENTATES" is the best variety of self-setting silicon that I can use to mold it. Just asking. Thanks. :?
"Where'd ya get that ol' thang, don't cha' know them thare things ill kill ya!"

Live From the Red Sea,

Russ

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Fibonacci
Vintage Diver
Posts: 98
Joined: Wed Oct 19, 2016 5:08 pm
First Name: Graeme
Location: Melbourne, Australia

Re: Diaphragm for Draeger PA 61

Sun Jun 09, 2019 2:58 am

Dunno if I qualify as an "engineering GURU, NABOB and POTENTATE" but here's my 2psi :D

It is possible to accurately 3D CAD model the diaphragm and then generate a 3D printed SLA master that can be used to make up a silicone rubber mould for very low volume runs (10-30 depending on complexity).

I've used this process at work before, but only on prototyping relatively thick walled corrugated elastomer cable routing elbows to a powered liftgate.
https://www.3dsystems.com/on-demand-man ... graphy-sla

'From product development models and prototypes to low-volume production runs, this rapid prototyping process allows us to produce parts that accurately mimic the color, texture, feel and physical properties of injection molded parts.

Accurately mimic production colors and textures
Large material selection – rigid, flexible, durable, clear and high-temp
Offers similar physical properties to injection molded parts
Used for pre-production runs
Parts made for marketing samples and test prototypes
Low-volume manufacturing'

3D Systems now make a range of Urethane rapid prototyping materials in soft flexible 25-50 Shore A hardness.
https://www.3dsystems.com/on-demand-man ... /materials

Quite a few places now offer this service:
https://www.arrk.com/services/rapid-prototyping/
https://www.stratasysdirect.com/technol ... ne-casting
https://www.xometry.com/urethane-casting/
https://www.midwestproto.com/cast-urethane
http://www.rapidmade.com/silicone-and-urethane-casting
https://schmitprototypes.com/services/r ... g-casting/

HOWEVER, the wall thickness on a typical diaphragm is around 0.5mm and could be difficult to fill consistently with the vacuum cast urethane process.
The other consideration is that cast urethane has pretty much unknown performance under repeated flex cycles vs liquid silicone rubber (LSR) which modern injection moulded high performance diaphragms are made from... but also need to be heat cured as part of the process.

For SCUBA usage I'd also want any cast urethane material to be food grade to limit any issues in contact with air going into your lungs :shock:
'A man can never have too much red wine, too many books or too much ammunition' Rudyard Kipling

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ScubaLawyer
Master Diver
Posts: 1312
Joined: Thu Apr 08, 2010 11:25 am
First Name: Mark
Location: Laguna Beach, CA

Re: Diaphragm for Draeger PA 61

Sun Jun 09, 2019 10:12 am

Hey Russ,

I too do not fall into the category of luminary do-gooders that you seek. Here is a link to a thread I started a while back on silicone molding diaphragms. I experimented quite a bit and never got it exactly right where I wanted the consistency, strength, etc... It is still a work in progress. I know that several folks have had success painting silicone over pantyhose stretched over a mold but I have not tried that yet. Sorry I can't offer more. Mark

viewtopic.php?f=2&t=9799&hilit=Silicone+diaphragm
"The diver who collects specimens of underwater life has fun and becomes a keen underwater observer. .. seek slow-moving or attached organisms such as corals, starfish, or shelled creatures." (Golden Guide to Scuba Diving, 1968) :D

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Fibonacci
Vintage Diver
Posts: 98
Joined: Wed Oct 19, 2016 5:08 pm
First Name: Graeme
Location: Melbourne, Australia

Re: Diaphragm for Draeger PA 61

Sun Jun 09, 2019 5:48 pm

Had forgotten about the other thread, you have certainly made some great progress in your silicone moulding!

My biggest fear is that RTV type silicone rubbers do not have the strength of the original LSR which need to undergo a heat curing process before they reach their target performance. Compression set under clamp load could be an issue with diaphragms made from RTV silicones.

Seems the minimum viable LSR run is about 250 units... wonder what the global demand for Dräger PA61 diaphragms would be to help offset cost?

Hours of fun reading background info here:
https://www.simtec-silicone.com/capabil ... ne-rubber/

70 Page LSR Guide :mrgreen:
https://www.simtec-silicone.com/ebook/
'A man can never have too much red wine, too many books or too much ammunition' Rudyard Kipling

h2operations
Skin Diver
Posts: 44
Joined: Fri Jun 29, 2018 9:40 pm
First Name: Paul
Location: Coronado CA

Re: Diaphragm for Draeger PA 61

Wed Jun 12, 2019 9:01 pm

Russ,
I have heard tell of people finding ones in this size range in MOS. Medical off the shelf parts. Do you have a photo of the original one?

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