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I have seen some people use plastic bottles as air tanks for pneumatic chambers, but they often switch to a PVC or metal tank because they want more airflow, more pressure, or structural integrity. By addressing the latter two problems, PET can become a cheaper, lighter, and safer alternative to PVC tanks.
PVC is a heavy and brittle plastic, which is why manufacturers don't recommend it for use with gas: when PVC fails, the release of the compressed gas can send fragments of plastic flying everywhere at high speed, and that is a very dangerous failure mode.
PET bottle, on the other hand, are much more lightweight, and they fail in a much more controlled manner: the plastic "tears" along the side, rather than producing many fragments of plastic. (examples here and here)
Air Command Rockets, a group who deals mainly with water rockets, has investigated reinforcing soda bottles with fiberglass cloth and epoxy. In their quest for maximum performance and altitude, they have managed to create bottles with a maximum pressure of 300PSI; very significant for a plastic bottle like that.
PET bottles can either be reinforced with fiberglass cloth and epoxy, or a similarly strong and easier to obtain method, fiberglass strapping tape. The use of fibers to reinforce bottles also makes them safer: the fibers reduce the tearing that occurs when bottles fail, and they hold shrapnel together to an extent, making them safer in comparison to PVC failures.
The main downside to regular plastic bottles comes from their opening: it is relatively small (0.87" inner diameter, slightly larger than 3/4" PVC) and it requires a nonstandard thread to connect to it. There are some bottles with much larger openings (1.2" ID) but they are more uncommon. To attach the bottle, you can either glue a piece of 1/2" into the bottle opening, or make an adapter that threads onto the bottle to go to a standard NPT-type fitting. As of right now no such adapter exists but I'm hoping something will come of this thread.
To summarize, PET bottles can be a good pressure vessel for cannons that don't need a lot of airflow because they are cheap, lightweight, and safer in some ways versus PVC tanks. I'm hoping this inspires some people to try using reinforced bottles as alternative tanks as a way to save money or to be a little safer. When I have time I will post some pictures of an example tank that I'm working on.
Why go to all that effort when you could just use a piece of PVC?
I mean its fairly versatile you can sleeve it or you could just use some galvanized pipe (for higher pressures), its also relatively cheap.
And adding fiberglass, epoxy etc are going to increase the weight.
It was the plastic bottle along with the failure mode of PVC that got me looking into another alternative for larger cannons.
After looking at the double wall (think bottle inside a bottle construction) of ABS DWV pipe, and noting the two surfaces were both thicker than a PET bottle, I wondered if ABS DWV pipe would be up to the task.
My test results are below in my sig if you wish to see the result of the test.
Last edited by Technician1002 on Wed Jul 27, 2011 3:32 am, edited 1 time in total.
The point of using something other than PVC is for the lower weight and cost, and most importantly, increased safety.
If you care about weight: if you were to build a 2L PVC chamber (4" PVC, ~9.5" long), it would weigh at least 4x as much as one made from a plastic bottle and some fiberglass reinforcement. But, the PVC would still have the more dangerous failure mode. Adding a sleeve or using galvanized steel pipe only increases weight and cost.
But hey, if you want to keep using PVC, feel free; I'm only presenting an alternative.
or you could get some abs witch is used for pressure and has a very safe failure mode, its much easier to make and come on if a 2l sized peace of pvc is to heavy for you, go to the gym or lose weight somewhere else
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The last time I checked you cannot drive down to Home Depot or Lowes and pick up some pressure rated ABS pipe. Most likely you would have to order it online or from a catalog, which isn't always possible and can be more expensive. I agree that pressure rated ABS is a very good alternative to PVC but sometimes it's not the best option.
Please do not misunderstand; I am not saying a small PVC chamber is too heavy, I am saying there is a safer choice that is also lighter. Maybe if someone was trying to reduce the weight of their cannon, they could use a bottle instead. I'm just trying to give more options.
Last edited by aEx155 on Wed Jul 27, 2011 3:59 am, edited 1 time in total.
What I want to see is a big RC plane with a fuselage made out of such bottles containing air for a BBMG with a barrel poking out of the side as suggested here
Say you made it out of three 2 litre bottles, 6 litres of air at 300 psi would be enough to shred pretty much any target...
Hum I never thought about reinforce bottles with fiberglass
looks good, but you could use some tape for that too like those bottle airguns on youtube, and much cheaper than fiberglass
You can actually get small sections of fiberglass cloth for surprisingly low cost, you just have to look.
I did mention that there is fiberglass strapping tape, and that it's easier to buy and use. It's just not as strong.
Something else I just thought of: AirCommandRockets only tested 2 layers of fiberglass, and that got them to 300PSI. If used in air tank applications rather than rockets, one could increase the layers more for even higher pressure.
I´d like to see a 800 psi plastic bottle would be spectacular
Thanks for the informative post, I'm sure it will be useful to some members who wish to follow that route of cannon building. It does seem difficult to reinforce a bottle with fibreglass cloth though, does it have to be cut up in to many pieces to cover non-uniform parts like the bottom of PET bottles?
Goes on like papier-mâché, piece of cake, then your wrap it all in clingfilm and let it cure.
So the entire cloth has to be soaked with an epoxy or is it just applied to the underside? For those (everyone but JSR) who don't bother to buy epoxy by the litre, it sounds like it might use a fair amount.
Reason I'm asking is because I'm considering doing the same thing to my ABS barrel on the piston hybrid. I have a 15cm x 30cm rectangle piece of kevlar from an America's Cup yacht sail at the base of the barrel to protect my arm from potential burns if the barrel happened to tear or such.
Probably wont happen, probably wont do much if it did happen but... may as well
You don't need to dip it in, paint some epoxy on with a brush (disposable, forget ever using it again...) lay the cloth then paint on another layer of epoxy.
I had reinforced a bottle with cupper wires and epoxy, but I used it for a clear combustion
I also made a gun stock out of polystyrene, and covered it with epoxy it took me 1 week to cure and 40 euro of epoxy
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