Nichrome Wire foam cutter/heater

[hikarusteinitz]

I am planning to build a foam cutter by heating up a piece of nichrome wire. I found that nichrome has a relatively high resistance compared to other conductors and it is the reason why it is used as a heating element. That is what confuses me because I think it only applies for constant voltage sources, but I've seen DIYers online using a battery directly with the wire.

If I use a constant voltage source , then P=V^2/R and the smaller the resistance the higher the output power and the wire will heat up faster. A high resistance wire like nichrome doesn't seem to be the ideal type to be used for a constant voltage supply. I've seen diy foam cutters online powered directly by 9V batteries. How does it work? If a nichrome wire works for a battery, then a lower resitance copper wire should work too, as long as it isn't lower than the battery's internal resistance.

A constant current source is more suited for a high resistance wire, since P=I^2R.

 

[oz93666]

I've used these wire cutters a lot . Nichrome is used for a reason , just because of this high resistance , it's a good match for a 12 V source .(car battery).

If you used copper wire the resistance is very low , so you need a 1 or 2 volt source to deliver about 4 Amps not easy to find ..., also copper is not strong when hot (or cold) it will break !

Don't worry about the theory too much ... first you need a power supply , Ideal is a bench , mains powered , variable supply at least 4A 12V , you can vary the output to get the right temperature.

Tell me what you have already ...have you bought the wire? what about power? how long does the wire have to be to cut the size of foam ? what foam , expanded polystyrene ?

 

[hikarusteinitz]

Thanks for the reply. I haven't bought anything yet. I wanted to read and ask about it first.

If you used copper wire the resistance is very low , so you need a 1 or 2 volt source to deliver about 4 Amps not easy to find ..., also copper is not strong when hot (or cold) it will break !

What if I used a 9V battery and a copper wire then use a potentiometer to regulate the current to 4A. Will the copper wire heat up? I'm asking this just to satisfy my curiosity, so let's neglect the possibility of the copper wire breaking for a wile.

 

[CWatters]

You don't necessarily want the wire to heat up quickly. What matters is the temperature is the right temperature. Too cold and it takes a lot of force to pull the wire through the foam and you need a lot of tension to keep the wire straight. Too hot and you can easily burn grooves in the foam and the cut is wider.

I once used a 12V battery and the speed controller from an electric model plane to control the temperature of the wire. I've also used combinations of car light bulbs as resistors to drop the voltage if necessary. Some people have used PC power supplies.

Lots of good videos on youtube.

 

[oz93666]

very true CWatters heating up quickly is not an issue ... we 're talking 1or 2 secs , once the cutting starts the temp is constant ... It's important you tell us what you have at hand ... it could be done with iron wire ... I have some very cheap galvanised iron wire , about .. 0,5 mm from a local shop , works fine , and as Cwatters said if you don't have a variable supply you can put many things in series to reduce the current (from a car battery) ..

Number one question how big are the cuts you want to make ... how long will the hot wire be , this will determine everything else.

 

[dlgoff]

This Staco 3PN1010 autotransformer would be your best choice for a power supply IMO. I have one of these that came from a business that did foam cutting.

 

[Sparky]

Basically you cannot successfully use anything other nichrome wire (in my experience anyway) Thats how a heater works, because it has resistance it resists the flow of current (amps) and heats up, if you use ordinary wire it will not impede the flow and just heat up and melt exactly like a wire fuse. Sparky of over 50 years experience here so I do know a little bit of what I am writing here, if any one can post there experiences on foam cutters I would be interested as I have been experimenting with a variac (variable transformer) and a transformer with about 16 volts output, and while it works OK I find the nichrome gets a bit tired after some use and just breaks, as it does need to be quite tight to be any use at all.

 

[DaveE]

Nichrome wire is a good choice for this application. First, as you said, it has high resistivity, which will keep the voltage and current within reasonable ranges. But, the real reason it's used in industry is that it has a low TCR (temperature coefficient of resistance), which means that the resistance doesn't change much as it heats up compared to common conductors like Cu and Al. There are better alloys, but they are much more expensive.

https://eepower.com/resistor-guide/resistor-fundamentals/temperature-coefficient-of-resistance/

 

[TonyStewart]

There are reasons in Physics why Nichrome is preferred;

1. 66x more resistance than copper thus less material at the same power.
- save material for same surface area for same junction to air thermal resistance.

2. ~ 10% of the positive tempco. or PTC of Cu (more stable resistance-temp. coefficient)

3. - much low oxidation rates than Cu which increase with temperature in air. (more reliable)

Toaster heating elements are typically made of a metal alloy, such as Nichrome or Kanthal.

Nichrome = alloy nickel and chromium with various ratios, 80% Ni is optimal.
Kanthal = alloy of iron, chromium, and aluminum.
It is also corrosion-resistant but has a higher melting point than Nichrome, but can be a bad taste for vape heaters. (never tried)

But can you raise the temperature 100'C with the same power in Copper vs Nichrome. Absolutely! The only difference is the efficiency of your voltage source must be much lower effective series resistance (ESR) than the heater resistance. Preferably <= 1% R Ratios and you need 66x longer wire if using the same gauge, then 10% of the voltage and 10x more current (roughly) or I^2R.

As temperatures get up, tungsten heaters in halogen with quartz glass such as below or other more expensive alloys for kilns.