What are safe T-Shirt Cannon tubes?

[pryland]

After reading numerous threads on T-shirt canons, we are VERY concerned about designing one with maximum safety.

We are looking at using polycarbonate (Lexan) for the barrels of our canon.
NOT for storage cylinders.

Would this be safe, or should we plan on aluminum tubes?

[cgmv123]

It would depend on how thick you make the polycarbonate, but I'd advise using aluminum tubes.

That said, if you do decide to use polycarbonate, make sure you use polycarbonate and not acrylic. Acrylic is a definite safety hazard in the presence of a rapidly expanding gas.

[FrankJ]

We use standard 2-1/2 copper tube, .080 wall, 2-5/8 OD. A little tight, but it also shoots stress balls.

[cbale2000]

Originally Posted by cgmv123

It would depend on how thick you make the polycarbonate, but I'd advise using aluminum tubes.

That said, if you do decide to use polycarbonate, make sure you use polycarbonate and not acrylic. Acrylic is a definite safety hazard in the presence of a rapidly expanding gas.

On a related note, if one's options were limited to Polycarbonate vs PVC, assuming both had the same wall thickness, which would be the better option?

[FrankJ]

Originally Posted by cbale2000

On a related note, if one's options were limited to Polycarbonate vs PVC, assuming both had the same wall thickness, which would be the better option?

Since my choice of PVC or nothing, would be nothing, PVC is out of the running completely. I have not researched polcarbonate enough to have an opinion.

[AdamHeard]

Originally Posted by cbale2000

On a related note, if one's options were limited to Polycarbonate vs PVC, assuming both had the same wall thickness, which would be the better option?

Where is someone located that they can't get metal but still have the resources to make a dangerous air cannon?

[Ryan_Todd]

For any material that isn't explicitly rated for pressure, you'll want to reinforce the outside of the tube in such a way that even a catastrophic failure of the tube would not produce flying shrapnel. Thin pieces of plastic are difficult to spot on an X-ray image (notice how only the sides of the PVC pipes are visible in this real-world example), so you really don't want shards of that stuff getting embedded in you!

If you take that thought and then go do a bit of research, however, you'll find that the only things that will reliably stop shrapnel from an exploding t-shirt cannon (essentially a pipe bomb) are going to be outright bulletproof-- sufficiently thick polycarbonate, metal, Kevlar, that sort of thing.

All told, then, you're probably best off if you make sure that the tube is rated for pressure just like the rest of the components in the system.

[GeeTwo]

I did a bit of googling, and found this.

While I haven't read it deeply enough to get a full understanding, the Statement of Problem section describes occasional polycarbonate failure in a shattering mode. On the other hand, it describes shattering pressures of around 1000psi. The scale drawing actually doesn't give scale; I don't know if 25.0 (outside diameter) is 25.0 mm, 25.0 cm, 25.0 in, or something even larger or smaller.

My takeaway here is that if the polycarbonate is thick enough that failure is not a possibility (and I do not know where that number is), it should work well. Otherwise, unless the specific composition and manufacture of your tubing is known to fail only through tearing or bubbling at the operational parameters, don't consider it safe.

[cbale2000]

Originally Posted by AdamHeard

Where is someone located that they can't get metal but still have the resources to make a dangerous air cannon?

Its more due to other design constraints that require the tube to be a certain thickness. I'd rather avoid using .25in thick aluminum tubing if I could help it.