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I am trying to build a ribbon cutter like this. I have completed the rest of the machine with the use of an Arduino and a couple of DC motors.

I'm stuck with the hot blade. I am not well versed with heating. Initially I tried to use a nichrome ribbon (5mm width 8cm long) with a transformer, but I got stuck as I could not figure out the resistance of it (I cannot measure it with a multimeter) to identify a suitable transformer.

I am thinking of using a soldering iron filament (40W) and fixing a brass blade to the tip.

This is how I have done the calculations:

  • c=375 J/Kg C for brass
  • weight of blade 10g (0.01Kg)
  • I need the blade to heat upto 300 degrees Celsius to cut through the polyester.

To heat the blade to 300 degrees Celsius the energy required is 1125J.

  • Energy = Watts * seconds
  • 1125J / 40W = 28.125 s

The blade should reach 300 degrees Celsius in less than 30 seconds. Is this correct? my instinct tells me it will not happen.

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  • \$\begingroup\$ The concern is not raising the material of the blade to temperature (if anything, thermal mass helps with stability in use) but rather should be the heat dissipated from it to the environment, and conducted through any mounting. In an open-loop soldering iron, the temperature achieved is the one where the rate of temperature-dependent heat loss equals the rate at which heat is added by the element. \$\endgroup\$ Commented Jun 8, 2016 at 7:23

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Your calculations are correct, but only if the blade is perfectly insulated thermally. Your biggest heat loss will be convection, and without knowing the blade dimensions there is no way to estimate that. With a large enough blade you won't be able ever to reach 300 C.

Your "best" bet would be to use a much larger temperature-controlled iron, but this will obviously cost you. If you use a non-regulated iron your final temperature will depend on your blade size and how easily air will flow over it.

From what you've shown I'm inclined to accept your intuition. 40 watts sounds pretty wimpy.

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