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A hot wire foam cutter is a great tool for cutting polystyrene (such as pink foam and styrofoam). This is useful for craft-work and for making prototypes and models.

Craft stores sell low-cost foam cutters (consisting of two D-cell batteries, a metal arm, and a piece of nichrome wire), or you can make your own.

The principle behind a hot wire foam cutter is to run current through resistive nichrome wire, rapidly heating the wire. The hot wire will then cut through foam like a knife through butter.

Making your own hot wire foam cutter

To make your own, you'll need a power supply, some nichrome wire, and frame to put the wire under tension. You want to build the frame to stretch out the nichrome wire while leaving enough space on all sides of the wire to give you plenty of room to maneuver the foam you're cutting.

You'll need to put the wire under tension (with a spring or through some aspect of the frame's design) as the nichrome wire will stretch some when it gets hot. You want the wire to remain taut so that it cuts a smooth line and doesn't bow out when you pull the foam through it.

To heat the wire, you'll want to connect a power source to it. For a few inches of wire, two D-cell batteries make a good power source. You'll need more voltage if you're using a longer wire You'll also want a simple switch so that you can turn the heating element on and off easily. The nichrome wire doesn't solder easily so you'll probably need to tie it to the frame both ends.

Make sure to use reasonably high-gauge wire (such as 14 AWG) or a metal part of the frame for connecting the nichrome wire to the power source. You want the nichrome element to heat up, not what you're connecting it with.

When you start running current through it, the nichrome wire will heat up within a few seconds. The wire will get hotter (and sometimes excessively hot) when not in-contact with the foam than when in-contact (as the foam absorbs the heat faster than the air does), so you'll want to turn it off when not in use. The heat of the wire will determine how quickly you can move it through the foam.

If using batteries as a power source, they will stop being effective after a few minutes. If you turn the cutter off for a few hours, the batteries may be useful again.

To determine the voltage you need to heat the wire, you'll want to take into account the length of nichrome wire, the resistance of the wire (in Ohms per unit length), and the amount of power / heat you want the wire to generate (also as a function of the length of the heating element. The resistance of the wire will depend on the gauge of the wire (for example 32 AWG nichrome wire has a resistance of about 10 ohms per foot). You'll want about 1 watt of power per inch of length. Some things that are useful in calculating this include:

  • The total resistance of the nichrome wire is the length of the wire multiplied by the the resistance per unit length of the wire.
  • The total power dissipated by the wire is equal to the square of the voltage across the wire divided by the resistance of the wire.
For example, if your wire is 1 ohm/inch and 3 inches long, the total resistance will be 3 ohms and you'll want 3 watts to power it. This will require voltage = sqrt(p*r) = sqrt(3 ohms * 3 watts) = 3 volts and current = power / voltage = 3 W / 3 V = 1 amp. Two D-cell batteries will give you the 3 volts and should be able to provide 1 amp. Note that smaller batteries may not be able to output enough current.

You may make a hand-held cutter fairly easily with a small frame, or for larger projects you can have the frame stretch the wire upwards from the surface of a small table. The latter configuration can be used similar to a jigsaw where the cutter remains stationary and you run the wire through it, using the table surface to help make perpendicular cuts.

Important safety tips:

  • The nichrome wire heating element gets HOT. Be careful not to burn yourself on it! The cut surface of the foam may also be hot after passing through the wire.
  • Unless you really know what you're doing, don't use a power supply other than D-cell batteries. Using a transformer in combination with AC power from an outlet is possible, but is extremely dangerous, especially if you don't know exactly what you're doing and don't follow electrical safety precautions.
  • Do not use wire other than nichrome wire. Standard copper wire has much less resistance than nichrome wire and will mean that you're effectively shorting out your power supply. This may cause the battery to explode or may otherwise start a fire.
  • Make sure to have good ventilation as the hot foam will give off fumes that you almost certainly don't want to breathe. Inhalation of fumes could result in lung damage or other nasty injuries.
  • Make sure to wear safety goggles when building and using this apparatus. It's better to look dorky than to get blinded.
  • If you're a kid, make sure to have adult supervision at all times when constructing and using this apparatus.

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