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How to Make a Hot Knife to Cut Styrofoam

Styrofoam can be cut into shapes such as a wig mannequin.
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Styrofoam is a very versatile material, used in many hobby applications ranging from remote-control airplanes to home remodeling. There are many different ways to cut and shape foam material. One effective way to cut styrofoam is by using a hot blade knife. This tool sends electricity through a high-resistance wire and because this wire does not conduct electricity efficiently it begins to heat. An insulated handle keeps the wire blade in place, which is used to sculpt the styrofoam.

Setting the Foundation

Locate a coping saw to serve as the frame of the cutting tool. The saw should have a wooden handle and a rigid metal frame.

Screw a metal screw into the wooden handle, in line with the blade. The screw should not touch the metal saw frame.

Remove the standard blade from the coping saw. This will be replaced with the resistance wire.

Getting Wired

Choose resistance wire to use for cutting. This will heat when powered and serve as the cutting surface. Select wire that is conductive but very resistant, such as guitar string or nichrome wire.

Install the resistance wire between the front blade mount and the newly installed screw. It should not, however, make contact with the blade mount closest to the handle.

Wrap the wire around the screw until it is firm, tight and straight. It does not need to be knotted. The screw, metal frame, and wire should now form a circuit that can be connected to power.

Feel the Power

Select a 12-volt DC power source. Adjustable power transformers work well. High-capacity lead acid batteries are common for mobile use. The ideal power supply should be 12-volt with high current, such as 15 amps.

Connect power wires to the screw point and anywhere on the metal frame, either with alligator clamps or by wrapping the exposed wire around them, in order to power the cutting wire.

Power the wires using the DC supply. The cutting wire should begin to heat very quickly. If an adjustable power supply is used, it can be fine-tuned to find the proper wire temperature.

Use the heated wire to melt styrofoam in a controlled, straight cut. Disconnect power immediately when finished.

Things You'll Need:

  • Coping saw
  • Screwdriver
  • Metal screw
  • DC power supply
  • Insulated handle
  • Resistance wire
  • Styrofoam


When fine-tuning for temperature, slowly raise the voltage on your adjustable power supply until the wire turns a reddish color. Be careful not to go too high though, as the clips might melt or the wire could break.


  • When dealing with electricity and heat, always use caution. Do not attempt to build or operate any electronic device until you understand its proper use. Be aware that fumes from hot styrofoam may be toxic if inhaled.
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