Creating custom fuel tanks from HDPE sheet

Discussion in 'The Garage' started by DaBit, Jun 9, 2010.

  1. DaBit

    DaBit Been here awhile

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    I see people creating fuel tanks from shaped/welded aluminium sheet, steel sheet, carbon/glass/aramide composite, etc.

    But I don't see people creating fuel tanks from HD polyethene sheet material.

    So I wonder: why not? HDPE resists petrol including the ethanol, is easy to shape, not very expensive, a little flexible so it won't dent, and it's possible to weld it using an hot-air plastic welder. Sounds like a suitable material.

    What's the catch?
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  2. D.T.

    D.T. Difficult but useful

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    Try welding one up and let us know the results then.

    How you going to "weld up" the gas cap and filler neck?
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  3. Aussie Steve

    Aussie Steve Long timer

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    Might want to check on that, last I heard whilst it is possible the welds are weak and are not recommended, especially for fuel vessels.

    If theres a new process to sucessfully weld HDPE please let us know.
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  4. DaBit

    DaBit Been here awhile

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    Don't have a project which could use a custom fuel tank at the moment. I just had some weird ideas past night in the time slot between going to bed and actually falling asleep.

    Just bolt a metal one in with a gasket or sealant between metal and plastic?
    Or use the gas cap/filler neck from a junk PE tank. Old XR600 tanks can be found for little money, for example.

    OK, that might be the reason. I didn't expect those welds to be much weaker than the parent material.

    A plumber friend of mine used to do a lot of PE pipe construction where they weld the pipe ends together. No problems with joint integrity according to him.

    No new process that I know of. I have seen someone doing it with a hot air gun with special nozzle and decent temperature control. The trick was to slightly melt the base material and push the filler rod in with quite a bit of force. Personally I haven't welded a single inch of plastic so I can't comment from personal experience.
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  5. P B G

    P B G Long timer

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    Won't HDPE thermoform and stick to itself?

    I'll check the chart at work tomorrow, I do orthotics and prosthetics, so everything we do is pulling plastic.

    If you used a plug that you could disolve, you wouldn't weld the seams together, you'd dissolve the plug.
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  6. GSWayne

    GSWayne Old Guy nOOb

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    An advantage that the molded PE tanks have over welded ones is the strength from curved surfaces. By welding one from flat pieces it will not be as rigid, especially if you have any large flat surfaces that will tend to "oil can".

    It might take a bit of practice welding plastic before you want to make a tank holding gallons of flammable liquid between your legs:D
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  7. DaBit

    DaBit Been here awhile

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    Not sure what you mean by 'stick to itself', but yes, HDPE is a thermoplastic. It can be formed with heat, and heat is also used to 'melt' parts together. I assume one would need to heat it to at least a buttery consistency before fusion occurs.

    That plug should be dissolvable, and resistant to the heat involved in molding the HDPE. An aramide/glass composite would be easier when going that route, I suppose. Except for the ethanol-related issues.

    True, but a tank made of sheet metal or any other material with large flat surfaces suffers from the same issue. This is easily solved by either shaping the HDPE sheet or adding reinforcements.

    Again, no difference between HDPE and any other construction method :)
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  8. P B G

    P B G Long timer

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    Simply that if you take two hot peices of plastic, and stick them together they fuse.

    It leaves a seam, but that seam can be ground flat, like a weld.

    So if you had a plug you could dissolve, perhaps foam, you could vacuum the tank over a positive, using the tank bung as the vac. port perhaps? Or you could use the future location of a tank drain. although perhaps some variety of blow molding into a negative would be better, as the texture of the mold would remain on the surface.
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  9. Wreckchecker

    Wreckchecker Ungeneer to broked stuff.

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    Not only could be done, but has been many times for the desert sleds. It's custom and will cost $$$.

    Typically, the sheet is heated (about 385f) and pulled down over a pre-heated plug, baffles are added internally for support and slosh prevention, then the bottom is welded in place.

    Plastic welders will work, but plastic as a material does not weld at all like metal. Especially if you want a seam that is at least as strong as the parent material that it is joining. The trick for a good plastic weld is that you actually have to stir the plastic together.

    Watching a guy who is a good plastic welder is artwork in motion!

    For the welding itself, you might check out smaller plastic firms that make chemical storage tanks for lawn care, farm, and chemical companies.

    Don't forget to test it with a 4-5 psi positive pressure test. Do it with the tank filled with water unless you like shrapnel wounds!

    Sounds simple enough, right?
    :eek1
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  10. Chronus24

    Chronus24 Space cowboy

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    HDPE is really hard to weld. The strongest joint you can make with it is with a epoxy called poly-weld. That is specially made for HDPE and you need to use a torch on the edges before you weld to prepare the plastic to "accept" the epoxy.

    I have also been looking into making a tank out of HDPE. I have been thinking about thermoforming shells and then bonding them to a flat inner plate.

    Do a youtube search for welding or bonding HDPE and you'll see that its really tough to bond. Probably for the same chemistry that keeps gasoline from breaking it down, most glues and bonding techniques don't work.

    I have also thought about making a pinched seam with a sealer and numerous rivets. I think blowmolding a single piece is ultimately the best bet, the mold would be difficult but that would be all you'd really need to make.
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  11. Tylenol

    Tylenol Been here awhile

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    And good source of HDPE are those cutting Boards you can get from the hard ware store if you don't have a place to get it by the sheet/roll or what not.
    I have been thinking about this as well, I have a steel tank but it is really rusty and the fuel is starting to turn brown so I was thinking of using the metal tank as a mold and with a heat gun maybe heat mold a few cutting Boards to the tank shape and seal the seams, or weld them some how.

    Has anyone here molded HDPE to a pre-existing form?
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  12. DaBit

    DaBit Been here awhile

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    I did. Try to heat it even, and it will eventually almost drape itself around the form. It does shrink and deform during cooldown though.

    Also, I found that welding HDPE using hot air and the 'pendulum-technique' is fairly easy. Slighly harder to weld than polycarbonate, but not much.

    I did not yet create a fuel tank out of it yet.
    #12