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Dart foam is cracking, but the heads are not blowing off.... Help plea


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#1 Jumbo Pancake

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Posted 24 April 2019 - 11:28 AM

Hello all,

 

I recently modified a Firestrike. I put in a [k25] spring, put some e-tape under the o-ring, removed the air restrictor and dart post, and re-barreled with 1/2" CPVC. I'm hitting 90-100 FPS, which I don't have enough experience to know if it is high or low. However, one issue I'm seeing is that it is chewing trough my darts. I have heard of heads blowing off, but this is not the case. The foam is literally cracking near the head, which allows the air to escape, and I cant re-glue it so the dart is pretty much useless. Does anyone have any clue as to why this is happening, and how to fix it? I have been using somewhat old accustrike darts and a few brand new Adventure Force waffles. I do have to somewhat twist fit the dart in because the CPVC is quite tight.

 

 

I included images of the cracked darts, and of the barrel in case there's something wrong with it that I can't see?

 

 

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Thanks in advance.


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#2 Daniel Beaver

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Posted 25 April 2019 - 07:39 PM

Are they getting cut by something in the mechanism?

Foam splitting is typically caused by high peak pressures in the barrel, but I don't think you would necessarily see that in your setup. Possibly the barrel is too tight?


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#3 Jumbo Pancake

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Posted 25 April 2019 - 08:40 PM

Are they getting cut by something in the mechanism?

Foam splitting is typically caused by high peak pressures in the barrel, but I don't think you would necessarily see that in your setup. Possibly the barrel is too tight?


I cant see anything obvious that would be cutting it, but that doesn't necessarily mean its not there. By high peak pressure, do you mean too much pressure builds up overall, or there is a higher pressure spot in specific? The barrel is fairly tight that may be the issue. However, from the same piece of CPVC, I modded a clearshot with no dart splitting, or at least its rare. However the clearshot does not have a [k25] so its not getting the same power... I am coming to the war this Saturday, would you mind taking a look? Just because you have a lot more experience in this department than i do :)

Thanks for the ideas.
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#4 Daniel Beaver

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Posted 28 April 2019 - 09:48 PM

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To follow up on what we talked about at the war:

I think what's happening is a combination of a short, tight barrel and a small-radius plunger tube, combined with full length darts, means that when the dart is half-way out of the barrel, the pressure inside the hollow stem of the dart is quite high relative to atmospheric pressure. Cylinders that are experiencing stress due to positive pressure on it's inner surface will tend to split in the longitudinal direction. This is because axial stress is about twice as high as radial stress. The splitting you see is in the direction you would expect if the pressure inside the dart got high enough to rupture the foam. You normally only see this kind of splitting on air valve blasters with large volume pressure chambers, such as Big Blasts or Jobars. With those blasters, the pressure in the barrel remains quite high even when you're using a very long barrel - they're releasing so much high pressure air that even a very long barrel doesn't drop the pressure very much.

 

pressure_vessels.png

 

The solutions to this problem are:

  • Lower the peak pressure of your blaster. You can do that by using a weaker spring, or pumping up your air blaster less. This approach sucks because you lose range.
  • Use a longer barrel, so that the pressure inside the barrel (and dart stem) is lower when it exits the barrel. On a high volume air blaster, this sometimes requires you to use an impractically long barrel. On this pistol, you maybe only need a few more inches (maybe just 2-4 more inches?).
  • Use half-length darts, which will clear the barrel sooner and so there won't be as long of a section of the darts with a pressure gradient acting to split them. It's still possible for these to split, though.
  • Use solid foam darts. This is the most straight-forward solution, but most darts you can buy have hollow cores, so this is usually a no-go.

Edited by Daniel Beaver, 28 April 2019 - 09:59 PM.

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#5 hoongfu

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Posted 29 April 2019 - 10:20 AM

This isn't a direct answer to your issue, but you can fix your darts by dropping a little bit of super glue in the cracks. That should seal them up. If that doesn't work use clear packaging tape around the area and make sure the tape wraps all the way around and attaches to itself. I've revived many darts using those two techniques.
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