Making Carbon Composite Tanks!

14 Sep 2022 by Finn in

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(Edit 19 Sep 2022)

Both Tanks we build so far are made out of carbon and glass fibre composite with an epoxy matrix.
The tanks consist of an inner composite liner, two composite bulkheads with a threaded aluminium adapter embedded, and a composite sleeve over the whole thing.
We used a 6K 70mm carbon fibre sleeve and an 80g/m2 plain weave glass fibre fabric, combined with Resoltech 1050/1056S epoxy resin as the matrix.

Tank Alpha

Alpha’s liner is made out of the carbon fibre sleeve, hand laminated over a 80mm diameter tube as mandril. We used some backing paper over the tube to prevent the laminate from sticking to it, heavily lubed with vaseline so that we can remove it at all. Wrapped over the laminate is a peel-ply, and the whole thing rotates slowly during curing to prevent dripping and sagging.

The bulkheads are made out of carbon fibre as well. Some glass fibre is between the carbon and the aluminium to mitigate a bit of galvanic corrosion. The bulkhead is laminated by hand over a 3D-printed mold, again with vaseline and a rubber ballon for easy removal. For the same reason we sanded the mold smooth to 800. Again, peel-ply over the laminate and rotating slowly. After the bulkheads cured, I laminated some additional glass fibre inside at the front to ensure a good seal with the adapter. Sanding inside proved very tedious, but I’d rather not risk an unfixable leak.

When both bulkheads and the liner are cured and well (!) sanded at the contact area, the bulkeads are glued inside the liner with a more viscous epoxy glue.
After that has cured, the whole tank is sanded (again, very well, has to pass water break test).
The carbon fibre sleeve overwrap is laminated over the whole tank.

Tank Alpha is very stiff but had some leakage issues, so I had to sand and apply a thin laver of epoxy over the whole tank multiple times.
The tank passed hydrostatic pressure testing up to 36bar.

Tank Bravo

Bravo’s liner is made out of 2 layers of glass fibre. This is an (succesful) attempt to prevent most of the leaks that Alpha was plagued with. The manufacturing of the liner is otherwise the same as with Apha. The resulting liner is not very stiff and flexes easily, but is watertight.

The bulkheads consist only of glass fibre over the aluminium adapter. I laminated multiple overlapping gores on the same mold as before, and after curing on the inside as well.
These bulkheads fit more loosely inside the liner, as htis glass fabric is thiner than the crabon sleeve, so I had to laminate two additional layers of glass fibre over the bulkead before gluing them into the liner.

The bulkheads are glued into the liner, now fitting snuggly.
The whole tank is sanded and a carbon sleeve overwrap is laminated on. The flexing of the glass liner made it a bit difficult to apply enough pressure on the sleeve, but it worked.
After the sleeve cured, I sanded the tank and applied one thin layer of epoxy.

Tank Bravo passed hydrostatic pressure testing up to 25bar. As Bravo is going to be the water tank, it doesn’t recieve pressures much over 20bar, unlike the air tank Alpha.

The glass fibre liner seems to be successful, as Bravo had no leak issues.

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