I'm in the process of making a plastic forming "tool".

It has a hot wire inlaid into the top.

I have not come up with the power supply yet but have a few iron core transformers laying around.

The hot wire is 24ga Kanthal A1 wire.

I don't remember the exact length off hand but the resistance is 3.01 ohms at room temperature.

I would like the temperature to be around 250 degrees F but anywhere between 230 and 300 will work. My largest transformer has a secondary voltage of 24V @ 60Va, not sure if that would be "overkill" or not.

  • \$\begingroup\$ As @JRE suggests in their answer, you have to work out the mechanical/thermal part of the calculation (off topic here) before you can work out the electrical part. \$\endgroup\$
    – Theodore
    Commented Apr 16, 2021 at 15:35
  • \$\begingroup\$ Draw a sketch of this tool. Plastic forming tool is a too broad description. It's almost impossible to guess/calculate what voltage is needed to produce a desired temperature. The most probable is that it won't reach the temperature or it is going to burn. So you do need a kind of closed loop control, so make a depiction or better description of this tool. \$\endgroup\$ Commented Apr 16, 2021 at 19:42

1 Answer 1


This is from some of the folks who make Kanthal wire.

It goes into some detail about designing things with Kanthal heating wires and elements.

At no point does it tell you how to calculate temperature from voltage and current or voltage and current from temperature because you can't do that.

  • You have to have the ambient temperature.
  • You have to have values for the heat conductivity from the Kanthal to the surroundings.
  • You have to have the contact area where the wire touches whatever your wire is touching.
  • You have to have the temperature of the stuff you are cutting.
  • You have to know the heat conductivity of the stuff you are cutting.

It does tell you how to calculate the power consumed by the wire from the wire diameter and length.

It also tells you how to calculate the resistance of the wire from the applied voltage and the resulting current.

From the resistance of the wire, you can estimate its temperature - there are equations in there that tell you how.

From the estimated temperature of the wire, you can adjust the current or voltage to make the wire heat up to your desired temperature.

With appropriate controls (a proportional, integral, derivate (PID) controller and a dimmer type circuit to control the current and voltage,) you could make a regulated "hot knife" that will maintain a particular temperature.

Alternatively, you can do what most folks do who make this kind of thing:

Hook your wire to a variable output transformer (variac or a regular tranformer with a dimmer) and adjust it until it seems hot enough to cut without burning.

Then, cuss when it cools off and gets stuck halfway through or cuss because you turned it up to get it unstuck and now it is too hot and burns your material instead of cutting.

These folks built one, and added a third reason to cuss: The wire got too hot and parted (broke) under the tension needed to use it as a knife.

  • \$\begingroup\$ I created my design from the best parts of several other designs from about 10 YouTube videos. The lower portion of my "tool" is just a flat press. 2 pieces of plywood on hinges. I'll make a single cut, length wise, thru a section of PVC pipe. I'll use a heat gun to warm up the PVC enough to flatten it out then lay it in the flat press with some weights on top to let it cool. Giving me a flat sheet of PVC plastic. \$\endgroup\$ Commented Apr 17, 2021 at 23:02
  • \$\begingroup\$ The upper portion of the "tool" is basically an acrylic bender. 2 pieces of plywood on either side of an aluminum channel with a hot wire running through the center of the channel. I'll lay the flat PVC over the top, to heat a straight line across where I want the bend. So the heating element will be a single strand (not coiled) and not touching anything other than the mounts at either end. I did use much of the design from youtu.be/mt4qJ96Pxj8 (sorry, I don't know how to make a "link") \$\endgroup\$ Commented Apr 17, 2021 at 23:23
  • \$\begingroup\$ Oh, hey look, it's a link now! Thats cool! \$\endgroup\$ Commented Apr 17, 2021 at 23:24
  • \$\begingroup\$ @MaintenanceMike: The same problems apply. The temperature of the wire depends on what it is contact with and how good the contact is. It will take more current during the bending process. With the aluminum channel in there, it'll take more current than if it were just the wire - you have to heat the aluminum, and the aluminum is also conducting (and radiating) the heat away. \$\endgroup\$
    – JRE
    Commented Apr 18, 2021 at 7:36
  • \$\begingroup\$ the only thing in contact with the wire is air. The aluminum channel is only there to keep the plywood from catching fire. The hot wire is also recessed so it will not contact the plastic. There is a minimum of. 375" (3/8") air gap around the wire. \$\endgroup\$ Commented Apr 19, 2021 at 12:47

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