My research lead me to "roto molding".  You build a cheap, sheet metal form, that the molder fills with a calculated
amount of powdered plastic, and then puts on the end of a rotating arm that goes in an oven.  As the form is heated, it
is rotated on all axis.

I was willing to make the form, but couldn't find a molder.  I ended up just welding the seems to make a tank.  The
number of Dyke Delta rotary builders is an even smaller subset than the Cozy rotary builders 8*).  I used .050" for my
tank since it has to actually hold the weight of the fuel.  If I were going to be burying it in a wing where it would be
supported on all sides, I would have used the thinnest material I could have reasonably worked with (.020" maybe?)

Chad Robinson wrote:

I looked into what it would take to get these blow-molded, maybe with a
group buy from other builders. The complication is that the strakes are
a lifting body and are structural. The ribs are part of that structure.
They also serve as anti-slosh plates inside the tanks. Any solution
needs to include both of them. It's do-able: you just need a tank
blow-molded for each of the individual tank sections, then you
plastic-weld them together around the strake ribs. But it means you need
a bunch of molds...

At the time, I had already bought a batch of ProSeal, so I went with
that. It looks pretty good - guess we'll see how it holds up (it sure
wasn't cheap.) So I never finished the research.

The issue is there's a large group of rotary builders but a smaller
group of Cozy MKIV builders. Cozy builders with aircraft engines don't
need to deal with this - so it's an even smaller group. I'd be surprised
if you could get even 25 people together on an order, and you'd need
several hundred to make it economical to build the blow-molds.

One alternate plan I had tinkered with involved making a very
rudimentary mold in the shop, then heating a standard 5-gallon red
plastic gas can and using compressed air to "inflate/reshape" it. A very
thin wall is just fine (preferable, even). The structure comes from the
Cozy's strake structure - you're just making a liner. But it's a lot of
work and finicky to get it right. I also looked into bladders, but there
are maintenance and reliability concerns with them filling/deflating
properly in odd-shaped spaces.

Regards,
Chad

On 12/2/2011 10:48 AM, Ernest Christley wrote:

I reluctantly made a decision to not trust sealing a gas tank to any
petrochemical that is shipped to me in a liquid
form.  You never know what all those petrochemical engineers in
Congress will decide mandate to be put into our fuel
next.  So I welded a fuel tank and glassed that into the place that
was designed for a fiberglass tank.  I would have
preferred buying a premade poly-ethylene or aluminum racing tank, but
I couldn't find one that even came close to fitting.

Aren't the Cozy tanks just big cubes sitting inside the wing roots?
Would it be safer, and possibly easier, to buy a
tank, sit it in the, and glass it into place?

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