My $2 Hot Wire (Nichrome) Foam Cutter

[Alex Franke]

Here's another microproject based on an idea I saw in Make magazine a couple years ago. I use it to cut custom packaging material from foam insulation sheets, and it works perfectly. I had everything on hand except the nichrome wire, which I ordered from ebay for $2.

More details and photos on my site here: http://www.thefrankes.com/wp/?p=2244

[leehljp]

One of these days, I am going to make one too. But for now, I have to get my shop in shape. Nice looking!

[pelligrini]

Looks good, handy contraptions.

Keep the area well ventilated, I had a flashback smell hit me just looking at it. We used to do a lot of rough models with a hot wire & those foam sheets. The architecture school finally banned them for indoor use.

[LCHIEN]

so, looks like you only needed about 8" -10" of wire?
Was getting enough heat a problem?
Nichrome 60 (looks to be the most popular type) is 6.75 ohms per foot, then your wire would have about 5 ohms, it would draw about 2.4 amps from your 12V supply and provide about 25W of heat. Was that sufficient, or more or less would have been nice?

More power would cut/feed faster but At some point it would burn, I'm guessing. So what temperatures does foam insulation melt, burn?

Possibly a good contraption for cutting nylon and polypropylene rope for fray-free ends?

[Alex Franke]

so, looks like you only needed about 8" -10" of wire?
Was getting enough heat a problem?
Nichrome 60 (looks to be the most popular type) is 6.75 ohms per foot, then your wire would have about 5 ohms, it would draw about 2.4 amps from your 12V supply and provide about 25W of heat. Was that sufficient, or more or less would have been nice?

It's probably too hot, actually, because it starts to glow. The wire itself is about 7.5 inches, but is probably actually 31 gauge because I measure a higher resistance -- around 5.5 ohm over that length for the wire. With 12 volts the whole thing pulls about 2 amps.

I have an old camcorder battery charger that I want to try instead. It's 7.5V/1.6A. That should bring it down under 10 watts.

...in fact I think I'll go try it now.

[LCHIEN]

THis website:

http://www.nichromewire.biz/

has a table listing current vs. temperature
for various gauges and currents.
It says for wire in free air, for 2.1 and 2.5 amps and 30 gauge wire, that you will get 1200 to 1400F.

to lower the current you would have to increase the resistance (smaller dia. wire) or decrease the voltage which reduces the current. Website suggests 26 ga. wire for foam cutters, if you go a whole lot smaller than 30 ga. you might break the wire.

Highly doubtful your camcorder charger will provide 1.6 Amps! Be careful you don't melt it down or worse set it on fire.

just curious about how the heat levels and wire was chosen. I'm reading in Wikipedia that 200C (~400F) would be sufficient for cutting foam. That would be about an amp for your wire. So you need about 5-6 volts.

[Alex Franke]

It says for wire in free air, for 2.1 and 2.5 amps and 30 gauge wire, that you will get 1200 to 1400F.
...
to lower the current you would have to increase the resistance (smaller dia. wire) or decrease the voltage which reduces the current.
...
just curious about how the heat levels and wire was chosen.

1200-1400 might have been about right at 12V. It's hot, but not hot enough to melt the wire.

Another option would be to use a longer wire or even run a couple of wires in parallel. I actually considered running a couple of wires because it would make it easy to cut the specific width I need. But that would also make the tool less useful for freehand stuff.

I chose the wire gauge because I thought it would be a good balance between resistance and strength for the length I was considering (6 inches). I made it a bit longer to get it down to around 2A. The power supply I had on hand was 12V. I didn't really choose the heat; it chose me. 30-31 gauge seems plenty strong enough for cutting 1/2" styrofoam.

I did try it with the camcorder charger last night (It does say 7.5V/1.6A) and it seems to do fine with about 1.1A. Whatever the new temperature is, it doesn't begin to glow, and cutting is a bit slower, but I actually kind of like the slower speed. According to that link you posted (Great link, btw!) that should get me to about 600 deg F.

The next thing I need to figure out is how to cut a groove into the styrofoam. A short, thick wire that will hold its form while bent would require a huge current at a tiny voltage on paper...

[LCHIEN]

...
Another option would be to use a longer wire or even run a couple of wires in parallel....

longer wire would work to increase the resistance; but paralleling the wires would reduce the resistance and make it twice as hot.

P.S. Nichrome melting pt. is 2550F

[Alex Franke]

longer wire would work to increase the resistance; but paralleling the wires would reduce the resistance and make it twice as hot.

Oh right -- or would it draw twice the current and heat each wire the same? I guess it's a good thing I didn't try that. :lol: I know just enough to be dangerous

[LCHIEN]

Oh right -- or would it draw twice the current and heat each wire the same? I guess it's a good thing I didn't try that. :lol: I know just enough to be dangerous

if they were separated, they would draw (jointly) twice the current and each wire would be the same temp.

But together side by side or twisted, they would draw twice the current and probably get close to twice as hot.

Temperature is an eqilibrium state. With no loss of heat it would get to an infinate temperature, when the loss rate of heat equals the input energy rate (power) then the temperature stabilizes.

A single wire radiates heat into 360 degrees of space thats cooler. But a pair of wires side by side loses no heat to an object at the same temperature, therefore it will only lose heat into 180 degrees or half the heat loss rate and the equilibrium point will be twice as high.

[Alex Franke]

But together side by side or twisted, they would draw twice the current and probably get close to twice as hot.

Yeah, that makes sense.

Hey do you have any hints on how I might do a short, rigid cutter? When I did the math it looked like I would need something along the lines of 6 amps at 1/2 volt to make it work on paper, and I just don't see that working out...

Thanks for all your helpful input on this!

[Alex Franke]

Here's a name I cut out. It's far from perfect, but you can see the potential.

Oh yeah -- and pelligrini is right: Good ventilation is a must!

[LCHIEN]

temperature is not a function of length... just current and resistance.
You might look at a stiff wire, about 14-16 gauge. so you are correct, you're going to need 6-8 amps with Nichrome wire. from a low voltage, high current source.
Probably the most practical source would be a AC transformer with a low voltage, high current output - you don't need a real large one, just about 5W, but it will have relatively heavy wiring. (AC will heat the wire just as effectively as DC).
Your other option is to find a resistance heating wire with significantly higher resistivity than nichrome, which offhand, I don't know of one.

P.S. Inconel would be another wire with good high temperature properties, but alas, its resistivity is very similar to nichrome.
Stainless steel wire is also used for similar purposes, but its resistivity is about 3/4 that of Nichrome so it would have less resistance for an equal length.

[LCHIEN]

Here's a name I cut out. It's far from perfect, but you can see the potential.

Oh yeah -- and pelligrini is right: Good ventilation is a must!

what PC font did you use as a stencil?

[Alex Franke]

what PC font did you use as a stencil?

It's called "Ravie" -- I think it came with MS Office, but I'm not entirely sure. I did it in Inkscape and nudged all the letters together so they overlapped a bit.