Re: [TCML] Black printing can be *very* conductive!

[Mddeming@xxxxxxx]

Hi Phil,
 
I guess the consensus would have to be: SOME black printing is  conductive, 
so when in doubt, peel it out.
 
Matt D.
 
 
 
In a message dated 3/24/08 4:34:44 P.M. Eastern Standard Time,  
FIFTYGUY@xxxxxxx writes:

Folks-

Well, I was sitting in my office today, and  I  recalled our discussion 
about the conductivity of black printing  on  Sonotube cardboard forms.
So I went to one corner,  and picked up a  corrugated cardboard box with 
black printing on it  and in the other corner  I got my megger.
My megohmmeter  is a slightly older AEMC Model 1015,  but it's been just 
fine for my  purposes. I used the 1000 V setting.
The corrugated cardboard  box had black printing on  the side. The part I 
tested had a black  rectangle, 1" by 3", with an "unprinted"  5-digit number 
in 
the  middle. Call it 60-70% black coverage in that  rectangle. And not the  
greatest printing job, as it wasn't as solid a color  as it could've  been. 
As I had posted earlier, I knew the black  pigment  in the ink was 
basically soot, but I was astounded at how  much a difference in  
conductivity it made!
>From one end  of the rectangle to the other, I read  1 Meg of resistance. 
Enough  that the 1 mA of current sparked form the probe to  the surface of 
the  
box, and I could instantly, and strongly, smell burnt paper!  If I  touched 
the 
probe down just a coupla millimeters to the side of the  black  rectangle, 
the 
resistance read infinite - or at least over  1000 Megs, according  to my 
megger. Interestingly, the smallest  reading I could attain was 300K, with  
the 
probes 1/4" apart on the  black rectangle. So it doesn't seem linear with  
distance.
I was surprised that the basic cardboard box itself  was  "infinite" 
resistance - the corrugated layers are held together with   water-based 
starch 
adhesive, and the paper layers themselves have  some  moisture content.
So there you have it! Black  printing on cardboard  can be *dangerously* 
conductive - enough to  pass a few mA at a few thousand  volts, or enough to 
ignite the  underlying cardboard! And this was a rather weak  print job.

-Phil  LaBudde
Center for the Advanced Study of Ballistic   Improbabilities







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