PVC is fragile

[Technician1002]

I have been expecting this for quite some time. My Mouse Musket used to break PVC pistons on a regular schedule. I used a PVC pipe cap in my Marshmallow cannon to see if the stop assy and bumper would protect it. It took a while, but today, the Marshmallow Cannon died. The PVC broke. This is the exact same failure I had on the ABS test cannon before I replaced the PVC part with ABS.

Don't worry, due to the small size and the mass of the parts in relation to the projectile, I was not hurt as this came apart on a launch.

I was launching AA batteries at various metal targets. Even though the PVC broke, this shot still managed to put a hole in an old license plate.

We will rebuild and build it better.

This failure was expected. This is not a surprise. PVC is fragile and does not like repeated impacts. It will fail.

Never fear, repairs are already underway.

If you ever use a pin to attach something, make sure the hole goes all the way through, that way the pin can be driven back out. I used a piece of welding rod in the drill press to push the pin part way out so it could be pulled with pliers. This should be repaired by tomorrow night. Hardware store is closed already tonight.

[clemsonguy1125]

Well it lived a grand old life, are you replacing the PVC with ABS

[Technician1002]

I'm going with galvanized iron on it just like it's big brother. I have not seen ABS here in one inch sizes.

[deathbyDWV]

Aww... So sad...

At least you can replace it fairly easily!!!

[jackssmirkingrevenge]

Ouch, glad there was no associated injury.

[Technician1002]

The risk with the PVC breaking was a calculated risk. I expected it to break eventually and it did. Due to the small size of the chamber and resulting short duration pressure pulse the total energy is very limited. The projectile was on the move when the piston broke the PVC so by the time the mass of the assembly got a good push, most of the stored energy was already on the way out the front. Part of the mass to accelerate was my hand. I had a grip on the knob, so I felt a shove, but nothing more. I was not hit or hurt in the incident. The failure was very much like I expected it to be. By the time the piston made it out the breech, the chamber pressure was already gone, so there was not even a poped cork sound. It was silent and just poured out into my hand. (the muzzle end was still noisy as usual.) I didn't even really notice a power drop with the shot.

It made holes in stuff just like the prior shots. Most of the energy went forward. When you balance the forces on the masses, the battery moved the most, the piston and an stuff behind it was a huge mass to accelerate, so it accelerated little.

I would not want to even try the same thing with the 3 gallon 2 inch. It stores lots more energy. I remember the ABS cannon ejecting a piston on my foot. That was painful. It also has less mass between the piston and end cap, so when it did blow, there was less to keep the speed down.

Overall, I think the cannon is relatively safe, but powerful.

If you build PVC cannons with piston valves, even with a bumper, PVC weakens with time and impacts. Be safe.

[Moonbogg]

Would you expect aluminum to fail in a similar manner if placed under relatively similar conditions? I still don't know if aluminum is a brittle metal or not. I ask because one time I was trying to pry open an aluminum clamp with a screwdriver by hammering the tip of the screwdriver into the slit of the clamp, using it as a wedge, and the clamp split in half down the center. I don't know what kind of aluminum that was (likely cast 356) but it had me wondering if aluminum is brittle in its own right.
Sorry to hear about your failure. At least it was expected and you weren't clueless about the possibility.

[Gaderelguitarist]

Aluminum, I feel, would be more malleable than brittle, unless it has been conditioned to a high hardness of course. If I had the facilities, I would make an entire cannon out of aluminum.

[c11man]

pvc would of course break in this aplication but is perfectly safe as a pressure vessel. but if you have a bad bumper or none at all expect failures like tech said

[Technician1002]

Even with a good bumper be careful. This cannon did have a good bumper. My Mouse Musket used to break pistons at the rate of about 1 per year. True to form, this PVC part lasted about a year. The dense foam bumper is about half as long as the piston. The foam pad is cut from a garden knee pad.

[c11man]

Even with a good bumper be careful. This cannon did have a good bumper. .

i beg to differ, your foam is not very thick and once its compressed ( which doesnt take very much force) is a solid object transfering all the engery back to the cap. i like to use rubber or coiled rubber hose becuase the hose will colapse absorbing energy then the rubber will deform even more to absorb the rest of it

[Technician1002]

That is possible. The foam is dense as it is a garden knee pad. The 2 inch QDV is using a double thickness and is holding up well. The bumper in the Marshmallow Cannon is wearing out and is now about 1/2 it's original thickness. I am looking for a more durable material. I have some stuff in testing. I should have a video out this fall on air cannon bumpers. I need to finish some testing first.

The testing is for the eventual large 4 inch cannon. It will be high energy.

I do have another bumper design that I am working on that will be energy absorbing for an inelastic collision. Reducing the impact force and reducing piston rebound are the design goals.

[Technician1002]

I found a photo of another old PVC cannon of mine that broke the female PVC adapter. It is a cannon that used the piston that is now in my ABS cannon. I forgot about that beast. It was HEAVY. The ABS cannon replaced it and is much lighter. It is a 3 inch chamber 2 inch barrel coaxial.

I forgot that my ABS cannon is my 2nd QDV. This one didn't flow well as it used drilled holes for ports. They were too small. That cannon which had a pressure rated PVC female adapter. It belongs here in the fragile PVC thread. Enjoy.

I later scrapped the PVC QDV because I didn't trust it. The ABS cannon replaced it.