Good morning,

I have to build a +- 2.5 liter CF air tank that hold 10 bars continuously.
My plan is to shape a cylinder (7.5 cm diameter and 15cm long) in a block of foam that need to resist epoxy resin under vacuum, wrap it with 6 layers of CF, after cooking I will drill two holes where I will later install some plugs, filling through them acetone to dissolve the foam and get a final clean inside piece.

My questions are :

- is my plan the easiest or do you have a better idea ?
- which foam to use (I want to avoid glue in the process and 10cm height is your only option) ?
- how many layers of 400gr CF would you recommend (my intention is 6) ?

Thanks for your kind advice.

               

You can think of it this way ..  Hand layup CF with vacuum has a slightly better strength than the same thickness in aluminium--

6 layers of standard 200gsm twill will give you roughly 1,5 mm thickness which will probably not be enough to have a safety margin ..

I would make such a "tank" by first laying up 2 layers of standard CF cloth, then ppel-ply, then vacuum ..

Once cured, I would wind on two layers of CF tow ... Hard .. while applying resin .. This will give the "tank" a very high "hoop-strength"  Start at one end, then go back over the first layer and keep the winding tight .. keep the tension on by clamping the "loose end" of the tow .. then let it cure to a tack, and apply an outer layer of Carbon/kevlar mix .. (to avoid sharp shards of CF IF the tank "breaks")

This way you will have a tank that has a good safety margin for the 10 Bar ...

You may want to think about making metal (stainless steel)  threaded adapters an integrated part of the layup.. it will be less prone to cracking along the strees points that wil always be present when drilling and gluing them in ..

Please post pics e.t.c.  here -- of your experiments. 

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

Thanks Dravis. Original tank is made of 5mm alloy... Do you know where I can purchase a 20x20x20 block of foam please ?

These types of thing are made as Dravis mentioned by winding tow onto a former, otherwise known as filament winding.

My first concern would be ensuring you take the appropriate engineering design processes very seriously so you don't end up with a carbon fibre "bomb" that explodes the first time you pressurise it.  Commonly tanks and pressure vessels are engineered to very high safety factors as well as having extensive testing in the design phase as well as batch testing and in many cases individual pressure tests.

Although 10bar isn't massive in terms of pressures cylinders can be made to, it is still plenty high enough to pose a serious hazard should a cylinder fail.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

This is definitely one of those projects where you will have to do extensive tests, and probably also a destructive test of at least one tank to be sure it will hold the 10 bar pressure..

I forgot to mention winding the tow on at an angle, but 45 deg. is too steep .. probably more like 10 deg ..

 I have made combination CF "tanks" for water propelled amateur rockets, that easily held 16 bar (The highest pressure the compressor would produce) ..   We made at test setup by drilling a hole in a sheet of 10 mm polycarbonate an mounting the "tank"  on one side, and keeping the testers and equipment on the other side .. We used a 160 Bar firefighters air tank to test, and the "rocket tank"  did not crack at 60 bar .. at which point we stopped .. any higher pressure would have made the tank a rather  too potent "cracker" --   
I was not worried by danger from the shards from the tank, but rather the metal fittings at either end ..

These "tanks" were made from a thin (0.6 mm) alu cylinder with a rocket nozzle at one end and a cone at the other .. Both were held in place by the CF wound on the outside.

Combined thickness was about 4 mm !!!    the outer layer was a kevlar braided hose .. 
Toray T700 12K Tow was used to wind on ... directly on the aluminium ..

So ... it can be done ... but at ones own risk, and it takes a lot of experimenting ..
Just sorting out the bonding in of the connectors is a job in it self ---

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

Thanks for your inputs. I will start testing in one week when the 25mm CF spread tow ribbon will be received.
My main concern is indeed about the inserts as I only have the oem ones so it should be perhaps a good idea to get some spares first. I will keep you posted if anything showable.

Do you know who is building that kind of thin 0.6mm alu cylinders as I need some for other tanks containing hot oil under pressure please ?

I think you can find extruded aluminium tubing around 1 mm thickness in many dimensions (Outside diameter)

The ones we made for the air/water rockets, were turned down in my lathe from 45 mm outside diameter extruded tube, with 1,5 mm wall thickness.

This allowed us to leave the ends at 1,5 mm, since we originally wanted to weld the end cap and rocket nozzle on ..  However the welder we had could not weld this thin material ..
So we ended up bonding it with structural adhesive first, then winding the CF tow over the whole thing .. + the Kevlar braided hose .

The rocket still flies -- now they are running a two stage water rocket, and working pressures are up to 30 Bar now .. --  Silly kids ..

They have joined the "Amateur Rocket Club" and are working on chemically propelled rockets as well  . They are trying to convince me to build a CF nosecone for one of them ..

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

Great story
The tank is now waiting all the air connectors before air pressure test. I have the feeling it is much stronger than necessary, and the challenge will be faced on connections... will see...