Sciencemadness Discussion Board - Blast caps

Blast caps

Turner - 18-2-2014 at 16:34

What is a suitable witness plate (or heavy piece of steel) for a blasting cap?

I pressed 3 caps earlier today and really pressed the ETN base charge heavy and hoping I did not press the ETN so much to be insensitive to 300mg SADS which I also pressed heavily, or press the ETN to the point that it now doesn't meet Critical Diameter.

So I want to see if all material in the cap detonates.

Ral123 - 18-2-2014 at 22:15

You could've used progressive loading for the ETN. Now you can't expect good reliability.
Use whatever you have, it must be consistent between tests, may be metal.

roXefeller - 19-2-2014 at 05:12

Hennig and Rosco just had a discussion about witness plates. (https://www.sciencemadness.org/whisper/viewthread.php?tid=43...) You can see in that thread that he used pretty much anything. Notice he also adhered the cap perpendicular to the plate so that he could see the end effect on the adhesion point of the plate and the splatter effect across the rest. If you look at this link (https://www.sciencemadness.org/whisper/viewthread.php?tid=43...) you can see him compare two tests. Like Ral123 said, consistency will allow this.

markx - 19-2-2014 at 05:28

Quote: Originally posted by Turner

What is a suitable witness plate (or heavy piece of steel) for a blasting cap?

I pressed 3 caps earlier today and really pressed the ETN base charge heavy and hoping I did not press the ETN so much to be insensitive to 300mg SADS which I also pressed heavily, or press the ETN to the point that it now doesn't meet Critical Diameter.

So I want to see if all material in the cap detonates.

Uggh....the barbaric practice of pressing primaries on top the base charge should really not be practiced by amateur experimenters (unless you have a remotely operated press with really heavy shielding). It is the industry standard because it is the least complicated method for producing a compound initiator, but industry does that in fully automated machines that have been designed with an accidental explosion in mind. If that should happen on a makeshift press with <1000mg ETN+300mg SADS in a metal casing...the consequences will be severe.

Hence consider this alternative: the primary compacted separately on a suitable pyrotechnic delay, a few milligrams of loose sensitive secondary (ETN, PETN, MHN) beneath that and the compacted base charge as the last link.
The pyrotechnic delays with the primary can be constructed and stored separately from the base charges until ready to use.

Something like this:

65/35 KNO3/sugar in a straw with 65mg of SADS sitting on one end+ 3-5mg loose ETN + 600mg compacted ETN 1g/cm3.

Hennig Brand - 19-2-2014 at 06:54

Quote: Originally posted by markx

If you keep a little distance between yourself and the blasting cap you are unlikely to lose body parts with only a 1g base charge. As far as high velocity particles from the fragmenting cap casing, it really depends on what the casing is made of and of course if shielding is present. The thin walled aluminum tubes I use normally fragment into very small pieces with poor aerodynamic qualities (velocity drops very quickly with distance traveled). The big thing is keeping some distance between yourself and the blasting cap. One should always be thinking, "what if this cap detonated right now?” Of course if you are holding the cap in your hand, when it detonates, you are going to lose fingers.

The big thing I worry about, other than holding a detonating cap, is permanent hearing loss from an accidental explosion. Along with good distance from cap when pressing, and shielding, very good hearing protection should be used.

Ral123 - 19-2-2014 at 08:39

Here's what exploding cap would do: http://youtu.be/luj3NdyBisA?t=37s
As can be seen a heavy metal block between you and the cap can suffice as a shield. I use a big hammer, hold a rod with pliers and hit the rod with something long. No body parts can be exposed to shrapnel Thick safety glasses, paper and earphones in the ears. Yes I invite accident, but I know exactly what can happen and the kinetic energy the hammer will have if the cap explodes.
Markx, your setup is cute but vulnerable to reliability issues.

markx - 19-2-2014 at 09:29

Quote: Originally posted by Ral123

Here's what exploding cap would do: http://youtu.be/luj3NdyBisA?t=37s
As can be seen a heavy metal block between you and the cap can suffice as a shield. I use a big hammer, hold a rod with pliers and hit the rod with something long. No body parts can be exposed to shrapnel Thick safety glasses, paper and earphones in the ears. Yes I invite accident, but I know exactly what can happen and the kinetic energy the hammer will have if the cap explodes.
Markx, your setup is cute but vulnerable to reliability issues.

If by reliability issues you mean misfires under normal operating conditions, then no, it is not prone to failure. I have used about a hundred of those with different fillers and charge amounts and have not had even a single misfire yet. And that is proof enough for me to keep doing things this way.
But if you meant that it will fail after being stepped on with a heavy boot, or soaked in rain for 2 days or being driven over with a car, then yes....sure it will fail. Then again, I do not intend to expose this device to such tortures. The setup fulfills the task that it was created for with excellent results and in my opininon exposes the operator to less risks than pressing sensitive primaries or hammering a charge into metal tubes if you will.
It's just one way of doing it that I have found to give good compromise between risk and functionality....I'm sure there are other and better ways, so let's hear and see them to gain knowledge and widen our scope...

testimento - 19-2-2014 at 09:44

One can easily construct a blast shield from wood or steel and use cartridge press or hydraulic press with lever. Upon detonation fragments will be shred around the shield and away from the charger. Even very thin plywood is more than enough to stop blasting cap fragments. Polycarbonate or other plastic sheet would also be a very good idea since it allows user to observe the pressing but it has a lot higher cost.

I would not want to find out what Al or worse steel casing would do with 1g PETN detonated few feet away. And I'm not the scariest type of guy.

[Edited on 19-2-2014 by testimento]