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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