Plastic Fuel Tank Design 1

I don't have any standards, and even if I did, I would not know if they apply to your market area as you give no indication of where you are on the WWW.

Some pitfalls re material selection are:-
It must be antistatic
It probably should be treated with fluorine to improve it's resistance to vapour transmission
It probably should be UV stabilised to a high standard
It should have very good resistance to environmental stress crack
It should of course have excellent resistance to all the fuel and possible additives it will be exposed to.
It should have excellent fatigue resistance
The thread for the filler cap, should be designed so that the cap will not be blown off by vapour pressure at the highest temperature likely to be reached under hot soak conditions after a long run at full load, at the maximum ambient temperatures likely to be encountered
The tank, should be designed so that it will not be excessivly distorted nor stretched by vapour pressure at the highest temperature likely to be reached under hot soak conditions after a long run at full load, at the maximum ambient temperatures likely to be encountered



Regards
pat

You'll find that (as of now) plastic fuel tanks fall into two categories, but they both use the same bulk material - High Density Polyethylene (HDPE).
The two categories are:
1) single layer (or monolayer as they're called)
2) multilayer

The monolayer tanks are blow-moulded to shape and then flushed with fluorine gas to convert the inside surface of the HDPE into a form which resists the absorption of gasoline-type materials. It forms a 'barrier layer'. Fluorine is dangerous stuff, of course.

The multilayer tanks are also blow-moulded but using a specialised moulding head to the machine which produces a layered 'parison' (the tube of almost molten plastic) which is composed of HDPE and nylon. This is usually in 5 layers: Outer HDPE with extra carbon black and regrind, virgin HDPE, nylon, virgin virgin HDPE, nylon - or similar.
The structure of the nylon offers much more of a barrier to the absoption of gasoline molecules than the HDPE does and also performs much better than the flurinated HDPE in the monolayer.

There was some activity a few years ago with injection moulding techniqes and vacuum forming. In these, the plastic tank is made in two halves - just like a steel tank, and then welded together. The welds have to be covered to provide a barrier to gas molecule transmission.

For a vehicle tank (car, light truck) the moulding machines are huge and the associated finishing processes take u a lot of space. Needless to say the tool costs are HUGE.

Something else to consider:

This is outside of my normal line of interest, but two instances of concern come to mind. This past summer, I had two gasoline powered items fail due to fuel tank defects. Coincidentally, both were powered by the same engine type/brand/series. Both had approximately 20 hours of previous run time without fault. One was a power washer, the other a lawn mower.

When I refueled the units, gasoline began spilling uncontrollably from what appeared to be a seam in the tank. The mower had been running about 2 hours when it ran out of fuel, the engine was hot, and the leak was dripping on the hot engine, but on the opposite side of the exhaust.

Tipping the units on their side to place the leak at the highest point, then carefully removing the tanks showed what appeared to be a thermal weld or adhesive weld joint that had no apparent medium in the parting joint. I could actually open the joint my manual pressure.

When I attempted to purchase a replacement for each unit (they failed on the same weekend!), I was told that they were not available for sale, that the manufacturer had posted a recall on their units and were replacing them across the board. Both were replaced free of charge. Later I was informed that the tanks were more than $60.00 USD each.

Lesson learned: A single blow-molded form is safer than a two piece/welded form, especially if the weld joint is below the level of gasoline.

Also, if this situation happens to you, immediately contact your small equipment implement dealer and enquire if there is a recall on the fuel tank. If the tank is removed, they will not warranty the unit since there is no serial number trace on the tank.

Franz

Good point - I was relating what I know to be the case for car/truck tanks, and there are plenty of them in service.
Even the blow-moulded tanks have a 'welded' area at each end of the tank where the semi-molten tube that is extruded from the moulding head is pinched together to form the final tank envelope as the blow process starts.

A well designed and executed weld in HDPE can be as strong as the original material

Regards
pat

Pat
I agree that a properly executed weld joint (be it metal or plastic) is or should be stronger than the parent meterial. The key point here is the comment "properly executed". This engine manufacturer (the US largest small engine manufacturer) had apparently slipped somewhere on QC.
Both tanks had their failure on EXACTLY the same location. I was able to section open one of the tanks and verified that there was nothing there which could have been used as a fusing agent. This is just my instance. I purchased these units about one year apart from different sources, in different cities. Sort of takes the probability factor of a "lot defect" out of the picture!

Franz

The subject seems to have drifted off a little.

In Europe, the major supplier of raw material for plastic tanks is BASF. I would suggest that getting into contact with them would be useful.
If you're in the US, then perhaps getting in touch with Walbro Corp and Kautex may help. In case you don't know they are international manufacturers of plastic fuel tanks. Kautex also makes plastic containers and drums for chemicals.

I assume you'll be using some form of cap tether. Instead of something which restricts the opening and is, therefore, in the way when trying to fill the tank, may I suggest a "figure 8" design, similar to that used to close soda bottles after the snap-off cap has been removed. This design was almost released by Chrysler, back in the fifties, before, at the "last moment," a VP saw it and said, "You're not putting that on MY Imperial." This was, of course, before many of the stringent design constraints presently in place, so it had merely a snap-on design, but I see no reason why the same scheme couldn't be used with a screw-on cap.