How To make a polystyrene hot wire cutter.


This project was born of a one-time need to cut up some polystyrene blocks in a neat way, without leaving little white balls stuck to everything.

I had originally planned to use mains AC, via a variable transformer and an isolation/step-down transformer for safety. So I quickly screwed a few bits of scrap wood together to form the frame. The cutting wire is 0.5mm diameter pure nickel wire. The kind often used by people who install and maintain neon lighting. I wasn't sure if it would be possible to use standard electronics solder to join copper to nickel, so out came the crimp tool for that part of the assembly.

As this was a cheap project, the variable transformer was a household dimmer switch. I wired up the electrics, made sure I wasn't touching any part of the assembly, and turned it on. It made an odd, quiet buzzing noise but the wire did not heat up sufficiently to melt the polystyrene.

Ok, time for Plan B. I bypassed the isolation transformer. Making doubly sure I wasn't touching any part of the assembly, and turned it on. It made an odd, less quiet buzzing noise. The wire remained at sub-polystyrene-melting-point. After about thirty seconds the dimmer gave a muffled bang! and it released the magic smoke that previously made it work.

At this point I was about ready to throw the whole lot in the bin and pretend I had never started the project. I had a cup of tea and a think. Then it occurred to me I had a (relatively, compared to mains AC) safer source of current. The battery for the (kick-ass) bike lighting system I bought from Lumicycle some years ago. A quick raid of my "bits box" produced a 2.5mm socket and a on/off switch rated at about 8A. A little more crimping (I'm not getting the soldering iron out now!) and I was set.

With the switch off, I placed the polystyrene block on the wire. Switched on, it took less than a second to reach melting point. With a little practise of flicking the switch off and on (because it is, effectively, short-circuiting a high-ish Amp/Hour NiMH battery pack) I was able to cut up the polystyrene neatly and with no messy little white balls.

I would speculate that Lumicycle would not endorse the use of their battery pack for this application. So be aware of the risk of killing your battery pack. Perhaps an old ATX PC power supply could be used to supply 12V DC to the wire, although you might have to plug in an old motherboard to make the PSU start up.

I have been told that doing this with the Lithium-ion batteries is a bad idea, because the current regulator board attached to the batteries will not allow what is effectively a short-circuit. At the time I was still on the Ni-MH battery which didn't have such a device to prevent abuse.

There are a few photos of the cutter, to give you a starting point from which to work.

The complete wire cutter
Close up of how the nickel cutting wire is attached Close up of power connector and on/off switch The electrical part of the wire cutter