Instructions to construct a hot-wire polystyrene foam cutter
These instructions are intended to make a cutter for expanded
polystyrene foam sheet of 5cm thickness. The cutter is suitable for
cutting wind turbine blade sections designed using the club cycom software. Only use to
cut polystyrene foam since other materials (such as polyurethane foam)
will give off toxic fumes.
Buy 1 standard 100 watt soft soldering gun . The cheapest (maybe 10
Euros), most basic model should be purchased with no controls other
than the on-off trigger.
This gun will contain a transformer which transforms the mains
supply voltage down to a very safe 0.3 volts AC. This very safe low
voltage can then cause a high current (100s of amps) to flow in a low
resistance copper loop.
The gun is supplied with a copper loop, plated to minimise corrosion,
and will get hot enough to melt solder. Less power is required to melt
polystyrene foam and so we can replace the loop intended for soldering
with another loop with higher resistance (still copper wire but with
less cross-sectional area, and/or a longer length). The higher
resistance loop will cause less power to be dissipated both in the loop
and inside the transformer and so this modification will not damage or
overload the soldering gun.
Buy a couple of meters of mains wire, intended to carry around 40 Amps
in normal use (e.g. European Electric shower cable) which has a bundle
of 7 wires per conductor such that the copper bundle of 7 wires will
have a diameter of around 3 millimeters and will fit into the low
voltage terminal posts of the soldering gun.
If present, remove the white outer sheath that binds the
live-neutral-earth together so that you can separate the individually
colored insulated conductors. You will use just one of the individually
colored insulated conductors.
The 2 raw ingredients, (i.e. the solder gun, and the individually
colored insulated conductor) are shown below.
The example above uses a 40cm length of wire. Strip the insulation from
the ends for 2 cm to allow insertion into the solder gun terminal
posts. Strip the insulation from the centre 10 cm and cut the outer 6
of the 7 wires leaving just the centre wire intact. The conductor will
now have a high resistance section for the centre 10 cm and low
resistance elsewhere. Ensure that each end of the conductor still has 7
wires and that you have not accidentally pulled the centre wire out.
The conductor should now appear as shown below.
Cut off the unused wires from the centre section, then bend the
conductor into an appropriate shape for connection to the solder gun
The final conductor loop will now look as shown below.
Connect the loop to the solder gun and tighten the screws of the
terminal posts. The hot-wire cutter is now ready for use and will look
as shown below.
If you are concerned, you can compare the AC voltage (with trigger
pressed) at the terminal posts with the new loop fitted, with the
voltage when the original soldering loop was fitted. If the voltage
with the new loop is the same or higher that the voltage with the
original loop then the power taken from the gun is not more that its
design value and it will not overheat.
You can also construct a deeper loop (using 1m of conductor) for
reducing a large polystrene sheet to more workable rectangles.
This loop is shown below.
You can download an mpg movie of the small
loop in operation (348kb) or an mpg movie
of the big loop in operation (481kb).
You can obtain an alternative source of high current/low voltage by
threading the 40Amp cable once though the magnetic circuit of any >
100 watt transformer. (e.g. through the central hole of a toroidal
transformer). If attempting to use a 12V car battery you will need to
use a much longer (3 meters!) hot wire section, or switch the wire
to a higher resistance material (e.g. nicrome wire from an
electric fire or hairdryer).