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Polystyrene cutting

Tool for cutting polystyrene

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paul sims30/07/2017 11:03:16
20 forum posts
5 photos

I will be having a house extension that will be built with ICF Insulated Concrete Forms, when the electrician wants to run conduit in the 50mm deep polystyrene insulation they use a hot knife cutter that can cut-out U channels of various depths, these cost from £140 to £400 they are of 150 watts to 200 watts AC mains or battery supplies, the blade would have to be 10 guage or more and heat to 600 degrees, I have investigated and found that NiChrome wire of 10 guage will require 23.3 Amps to heat to 600 degrees and if of 6 inches in length will have resistance of 0.03124 Ohms.This to me means a very low voltage is required. The cutters on the market are quite small and I can not see how they can make them so small.

My questions are what metal do they use in the heating cutter, replacements appear to cost £20 to £40. I want to make one that is sturdy enough to survive a building site and not be dangerous.

Many thanks to anyone who may have experience of these.

Paul Sims.

Russell Eberhardt30/07/2017 11:29:37
2600 forum posts
85 photos

You don't need 600° even if that's °F. Polystyrene melts at about 240 C but it's glass transition temperature (the temperature above which it softens and flows) is only about 100 C.

Have a look at how the instant soldering guns are made. They use a mains transformer with very few turns of thick wire as the secondary.


Phil Whitley30/07/2017 15:06:00
1257 forum posts
147 photos

Are you after making a hand tool, or bench mounted? for a hand tool you could use a weller type soldering iron with a piece of thick copper wire bent to the profile you need to cut. Funnily enough, I built a bench mount one on my startrite sawbench last week, to cut some 75mm polystyrene down to 50mm. I used an old garage lead lamp transformer which was 240v in and 12v out at 10 amps . for the element I used some nichrome wire from an old storage heater element, and also scavenged some porcelain insulator discs and porcelain beads from the same source. I used an arbitrary length of nichrome, and left about a foot of spare wire so I could shorten the wire to increase the temperature, and lengthen it to reduce it. Mocked it up with the wire stretched between two bench vises, and it worked perfectly, so I built this.

The springs keep the wire in tension as it expands when it warms up. Cut the sizes I needed from 1200x2400x75mm sheets, then pushed them through to reduce them to 50mm thichkness. It draws 6 Amps and remains at black heat. Perfic!

Phil Whitley30/07/2017 15:16:49
1257 forum posts
147 photos

Just notice how bad the sawbench isblush, I bought it from a guy in the village, and it was stored under an open lean to(o?) shed. Trouble is, Hammerite light green is a discontinued colour. It will have to be the darker one on this and my Qualters and Smith hacksaw. Very soon I will be doing jobs like this! Bliss! My workshop rebuild and refurbish, scheduled to take a year, is now in its fifth year due to other commitments, consisting of a wife and two daughters, who insist on ridiculous things like holidays! Ho hum.

vintagengineer30/07/2017 18:29:51
468 forum posts
6 photos

I used to make big polystyrene cutters for the industry. We used single phase arc welders as the power source and stainless steel mig wire as the cutting wire.

Sam Longley 130/07/2017 18:45:01
794 forum posts
28 photos

I cut polystyrene for model aeroplane wings using heavy gauge fishing wire trace hooked up to a standard dimmer switch & a 120V site transformer. I am sure that one could rig up a piece of wire to a transformer on a wooden handle to form the "U" shape . The lower voltage of the transformer should be better than 240 V

Neil Wyatt30/07/2017 18:52:51
18235 forum posts
714 photos
77 articles

We used to use these in school. Plastic frame.

You need a bit of old fire element or you can buy thinner nichrome which needs less current of ebay.

Make a H-shape out of three bits of wood, with the wire across the bottom and a 'spanish windlass' of twisted string and another stick across the top. Wind on plenty of tension as the wire will slack slightly as it heats up. Don't run it anywhere near 600C or F!

~12V should be plenty unless you have a very long wire.


Edited By Neil Wyatt on 30/07/2017 18:53:05

Engine Builder30/07/2017 21:41:10
228 forum posts

I have one of **LINK** , works off a 12 volt supply and the probe could be bent to a U shape.

paul sims07/08/2017 18:23:26
20 forum posts
5 photos

I have been doing some testing using my Arc Welder as suggested and a "H" frame. I started with a 2mm copper wire of 150mm length with The Welder set at 30Amps, result it would stick to the polystyrene but not cut, I increased to 50 Amps and it cut slowly but still sticking a bit. I then tried 60 Amps and that was just right the Voltage at 60 Amps was 0.23 volts.

I then tried 2mm Brazing rod 150mm length, it cut sticky at 30 Amps and cut right at 50 Amps and Voltage 0.24 Volts.

I then tried a steel Bicycle spoke of 2.5mm 150mm length. it cut right at 30 Amps (my lowest available setting) and the Voltage was 1.3 Volts.

I then tried 2mm NiChrome wire 150mm length, it cut possibly too well at 30 Amps the Voltage was 1.7 Volts and after several seconds started glowing red!

I have been modifying a Transformer from a 4 Amp 12 Volt battery charger,I had ealier tried to remove the laminations from a larger Transformer but they were stuck too fast and I turned to the Battery charger. I have removed the laminations and the secondry coil and intend to wind 2 turns of a heavy guage wire to it. as demonstrated on a web site, I am trying to obtain some fiberglass sleeving. I thought I must make some sort of report back before you all loose interest, I will report on my final result when finished.

Thank you all again for your help.


Les Jones 107/08/2017 21:43:09
2162 forum posts
149 photos

I made a device similar to the one you describe for cutting grooves in expanded polystyrene to hold small cutters etc.
I uses the stainless steel strip removed from a windscreen wiper blade to form the U shaped blade. I did not use a temperature as high as 600 Deg. C but it may survive long enough to do the job. I just used a few turns of thick wire as the secondary on a toroidal transformer. (The transformer I used was one that the original primary had failed on. I removed the original primary and fed the original secondary from another low voltage transformer. (About 24 volts.) There would be enough room to wind an extra secondary on a good toroidal transformer to avoid using two transformers as I did.


paul sims08/08/2017 18:23:32
20 forum posts
5 photos

Thanks for this Les, I have re-assembled my transformer, I have used ten 1 mm strands of copper wire, wound flat one and three quarter turns, and I will join the strands at each end forming a flat single wire, I will test another day, I just thought that I would respond to your message, The Windsreen wiper Stainles steel was a brilliant idea, I could even of used it to make the secondry winding, if my ten strands do not work I may do that.

I will give a up-date after testing. ( but Wednesday is Housework day and that must come first ! ).


Les Jones 108/08/2017 18:36:01
2162 forum posts
149 photos

Hi Paul,
The stainless steel would not be suitable for the winding as it would have too high a resistance. It would get just as hot as the blade part. The wire I used for the low voltage high current secondary was something like 6mm (6 square mm. not 6mm diameter.) mains wire. Using a number of strands of thinner wire in parallel should work just as well.


paul sims09/08/2017 19:33:07
20 forum posts
5 photos

Dear Les,

Thanks for this, I have tested my transformer and the output was 0.4 volts which heated my 2mm Ni-Chrome wire but only stuck to the polystyrene and not cutting it, I then altered my 10 strands by putting half of them in series with the other five , this gave me 0.8 volts which did cut the polystyrene but did stick a bit, I have pulled the TX to bits again and will wind 4 turns of 4mm ish diam wire, your 2.8mm dia wire would probably not suit my 2mm Ni-Chrome wire that I have chosen because of its rigidity and use in a builders envoronment, thanks again and I will report back later.


paul sims10/08/2017 19:56:40
20 forum posts
5 photos

Paul Here,

I have wound some cable 4 times round this time, I wanted 4mm dia but settled for 3mm dia the 4 mm insulation was too thick and touched the laminations, it gives me one volt off load and 0.95V with the 2mm Ni-Chrome, it cuts just right even with 6" of wire bent into a "U" shape, even leaving it switched on for five minutes the TX does not get warm.

That is the easy bit done, now I must make it into a safe handheld tool, it will also require a sledge to maintain the depth of cut.

Many thanks to you all, each reply to my plea for help helped me to think sideways a bit and understand a lot more, with regards to the temperature I should imagine it is about 400* F because it just melts solder.

Thanks and Bye.

Ian Skeldon 210/08/2017 20:06:28
489 forum posts
41 photos

For future reference, I have just finished cutting large 50mm thick sheets down to size to insulate my garage as it's going to become my workshop very soon. I used the flat blade heating element of an old toaster with a battery charger, worked a treat and cost nothing.face 23

Michael Gilligan26/08/2017 00:17:42
16372 forum posts
714 photos

I have just found this useful page about nichrome wire: **LINK**

General properties, and some detail about hot-wire foam cutters.


paul sims26/08/2017 10:22:47
20 forum posts
5 photos

Dear Michael,

Thanks for this site info, it is very helpful, I have chosen to use Ni-Chrome 14 Guage wire I assume it is "C" type the supplier did not state, the site that you supplied sugested that there was very little difference. I am making progress with the Sled that will guide the wire, but I am a very slow worker, but I expect that I will get there in the end!

Best regards, Paul.

paul sims11/10/2017 20:07:31
20 forum posts
5 photos

p1060995.jpgp1060996.jpgp1060997.jpgp1060998.jpgp1060999.jpgDear All,

I have finished my Polystyrene cutter and tested it on some scrap pieces, The building extension has been postponed until the spring of 2018 so it has not done any proper jobs as yet.

It has two handed operation to ensure accuracy of cuts and arm fatigue, the thumb of either hand can hold down the ON button, it heats and cools very quickly, there is a red neon to show when it is heating, the width and depth of cut is adjustable, it has two cutting wires one for the fitting of conduit and the other for the box for the sockets.

The Voltage output is 1.4 volts, the cutting wire is 14 guage Ni-Chrome, I found this the best for cleaning after using, the residue scraped off with a finger nail, other metals required the use of a tool. The secondry winding was eventually 7 turns of 60mm square wire that is normally used as earth cabling ie green/yellow.

A Tip is that after dismantling the original TX secondry windings and reassembled the laminations it is possible to feed and wrap the new wire round the former, this makes it easy to experiment with different turns and you can get more on.

Hopefully some pictures will now appear!

Jon Cameron11/10/2017 20:25:40
342 forum posts
113 photos

I haven't read all of this thread so forgive me if I'm repeating, but when I used to make Christmas grottos for shopping centers. They were built up of Poly blocks fixed together with expanding foam and steel rods as bracing while it all set.

The blocks were formed to shape with a device very much like what's been posted above. It used a wooden frame with a 0.75mm wire from a length of 3core electrical cable, bolted at each end. It was powered by a mains transformer output was 12V DC and just 30amp. The wire glowed red, but would cut poly like a hot knife through butter. At times it would snap and the transformer was turned off. A switch was mounted on the handle eventually so that it could be switched on/off without turning the transformer on and off.

For an insulating material, look up vidaflex sleeving, it's heat resistant, and insulated against leakage. Perfect for what you need.

Emgee11/10/2017 20:47:39
1717 forum posts
231 photos


Anyone cutting foam blocks with a hot wire should be aware of the noxious fumes created and take the necessary precautions.


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