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sipuli
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[*] posted on 7-11-2017 at 05:31
Working with old TNT


Disclaimer: i know that working with old military explosives is questionable at best.
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Hello, long time HE hobbyist and lurker here. Just recently i came up with a problem that has not been discussed in great detail before.

I have acquired some soviet 200 gram TNT blocks from World War II. As far as i know they have been exposed to elements for decades. I already cleaned them and dried in room temperature for several months.

I was little bit suspicious about the composition since i know the fact that at least end war explosive products may be far from pure TNT.

The material melts around expected 80-ish Celsius and when open flame is applied to molten material it burns with yellow to red flame and produces large amount of black soot. Also hammering it furiously does nothing as expected. So I think I do have safe product here.

Since some of blocks are damaged (crushed) i would love to make my own small TNT castings out of them (does TNT have critical diameter?)

Im looking for TNT casting methods and other information related to this project. Plan is to use ETN based homemade blasting cap/booster combo to make sure the TNT is set off properly. Old military initiators from same place and conditions are too damn scary to even handle.

And there is no way in hell im going to perform the initial melt indoors. I have a workshop in rural area and plan is to use modified electric stove (PID control to control the water bath temperature) and 50 meter extension cord.


[Edited on 7-11-2017 by sipuli]




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[*] posted on 7-11-2017 at 06:11


When they used to make TNT based bombs to be dropped from aircraft, manufacturers literally just poured in the molten TNT into the casings and left to solidify, this was even done for the Grand Slam bomb which contained a whopping 4 tonnes of TNT/RDX/aluminium powder. As for the detonating cap, you could place a steel rod in the TNT when it's cool enough to be mouldable then remove when solidified; a wooden dowel may also work but I don't know if it'll react with the TNT or not.

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[*] posted on 7-11-2017 at 07:12


If you cast TNT and allow for natural cooling you will end up with a concave surface due to strong contraction of TNT from the liquid state to the solidified state.

To avoid this, you will have to cool it from the bottom (bottom-up) that way the concavity will be less... you will have to put a little excess molten TNT after solidification on top to get a full loading.




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[*] posted on 7-11-2017 at 09:00


Nice pic learned amateur.

Here is a graph showing critical diameter for TNT from explosives engineering.
Maybe might be of some help. Seems for pure poured TNT, failure diameter is around 30-35 mm.
Much lower for the pressed grains though probably because there are more spaces in it (more porous than the melt cast).

[Edited on 7-11-2017 by greenlight]

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[*] posted on 7-11-2017 at 12:40


Quote: Originally posted by PHILOU Zrealone  
you will have to cool it from the bottom (bottom-up) that way the concavity will be less... you will have to put a little excess molten TNT after solidification on top to get a full loading.


Another method used is to top fill a case through a paper funnel, leaving excess melted TNT in the funnel above the desired fill height. As the lower levels are chilled, solidify and shrink, this additional still molten material flows downwards and prevents cavitation.

Ideally, just a bit more than exactly what will be needed to accommodate shrinkage is left in the funnel, then the slight excess is removed along with the funnel after solidification and shrinkage are finished.




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[*] posted on 7-11-2017 at 15:15


Quote: Originally posted by Bert  
Another method used is to top fill a case through a paper funnel, leaving excess melted TNT in the funnel above the desired fill height. As the lower levels are chilled, solidify and shrink, this additional still molten material flows downwards and prevents cavitation.

Precisely the same method used in metal casting, as metals also shrink on cooling.

The 'Sprue' into which the molten metal enters the mould is reasonably tall, creating Bert's 'paper funnel' for exactly the same reason.




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sipuli
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[*] posted on 7-11-2017 at 15:33


Very interesting read, thank you guys.

The melt and cast will happen next weekend :)




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[*] posted on 7-11-2017 at 19:12


That's pretty cool and slightly scary - though you've obviously proved fairly well the safety of the block's present composition. Any pictures of the initial blocks?



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[*] posted on 7-11-2017 at 21:40


I don't know ANYTHING about the casting of TNT, but I do know quite a lot about casting metals. (My other hobby being metalwork, such as casting, machining, blacksmithing, etc)
The method Bert mentioned of overfilling using sprue's and allowing the shrinking metal to fill the space is used all the time. That way you also get a bit of material left over.
If you do decide to use that method to fill the casings up, it might be beneficial to use a large sprue or preheat whatever funnel you plan to use. That way the material won't "freeze" on the way in - a problem that has plagued me when casting bronze.
In light of this, I might not use paper unless you can make the opening a fairly large size.
Or, perhaps i'm very wrong because most metals have a much higher thermal conductivity that TNT. I don't know enough, just wanted to add my thoughts.
Good luck.
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[*] posted on 8-11-2017 at 01:17


Quote: Originally posted by greenlight  
Nice pic learned amateur.

Here is a graph showing critical diameter for TNT from explosives engineering.
Maybe might be of some help. Seems for pure poured TNT, failure diameter is around 30-35 mm.
Much lower for the pressed grains though probably because there are more spaces in it (more porous than the melt cast).

[Edited on 7-11-2017 by greenlight]

The intergrain spaces induce hotspots when the detonation wave passes through... thus the all batch is sensitized... hence the smaller critical diameter.

Stangely in your very instructive diagram the cast, cast creamed and cast creamed with 10% powder are all at the same density...
==> I would have expected a visible slope with density into the sequency: cast > cast creamed > cast creamed+10% powder.

Funny to observe that all 3 curves will merge arround d= 1,6 with a critical diameter < 5 mm :o




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[*] posted on 8-11-2017 at 01:18


Quote: Originally posted by Bert  
Quote: Originally posted by PHILOU Zrealone  
you will have to cool it from the bottom (bottom-up) that way the concavity will be less... you will have to put a little excess molten TNT after solidification on top to get a full loading.


Another method used is to top fill a case through a paper funnel, leaving excess melted TNT in the funnel above the desired fill height. As the lower levels are chilled, solidify and shrink, this additional still molten material flows downwards and prevents cavitation.

Ideally, just a bit more than exactly what will be needed to accommodate shrinkage is left in the funnel, then the slight excess is removed along with the funnel after solidification and shrinkage are finished.

Nice method... didn't knew it ...
Thanks Bert for the information.




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[*] posted on 8-11-2017 at 05:36


It's worth mentioning that initiation sensitivity also changes (cast being harder to set of), so there are several good reasons for pressing rather than casting.



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[*] posted on 8-11-2017 at 17:52


Soy Lecithin or a little bit of beeswax will help avoid cast defects. add at like .5 percent.
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