Ask A Scientist© Environmental Earth Science Archive

name         Kenneth K. W.
status       other
age          30s

Question -   I want to build a hot wire tool for cutting expanded
POLYSTYRENE FOAM. If I am using NiChrome wire, does it matter if you use
AC or DC? I was thinking about using an old 12volt DC car battery
charger.  Do you recommend this or should I just use the transformer 
from the charger and use 12volts AC?  Thanks for your help.
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Either AC or DC will work fine.  A transformer is more convenient (doesn't
lose its charge), but is somewhat more dangerous.  You, of course, have to
calculate the gauge and length of wire that will come to the desired
temperature.  Probably cut and try will be needed.  I suppose 50 Watts
might be a good first try for a 3 foot length.  Using V**2/R = P,
R = V**2/P = (12V**2)/50W = 2.9 ohms might be a good first choice.

22 gauge nichrom at 0.94 ohms/foot might work.

Best, Richard J. Plano
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Hi, Kenneth !!

To cut polystyrene foam you need heat.
You can get it from DC or AC. Using DC,
from 12 V car battery, you must have just
the right length of wire.  As you know, if
the wire is too long or has a very small
diameter, than the current will be very small,
and no enough heat will be developed.
If you use a very short length of wire, or
if the diameter is too large, than the current
will be very large and also the temperature.
So you need find out the ideal conditions.
These laws may be useful to you :
V = R.I   .:   12 Volt = 6 ohm x  2 ampere
P = R.I^2   .:  24 Watt = 6 ohm x 4 ampere^2
R = r. L / A  .:  6 ohm =   0,000012  (ohm x m ) x [ 0.05 m / (10^-8m^2) ]
where 0.000012 is characteristic of material.
To know the area : you could weight the wire and
considering that the density of steel is approximately 7.8 g/cm3
having the length you can estimate the Area.

Best Regards

 Alcir Grohmann
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