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Alternates to nichrome in the extruder

Posted by Frank Davies 
I would like to report on the results of one of my attempts to find alternatives to using nichrome wire to heat the tube and nozzle in the extruder.
Many years ago when I was a teenager I had one of the old fashioned solder guns. When I opened it up (by accidentally dropping it), I found that it had a curious cylindrical transformer in it. The tip was heated with current from a single turn secondary running through the center of the transformer. If the transformer has several hundred turns on the primary, then less than 1 amp of wall current would produce several hundred amps in the heating element.

Inspired by this, I set out to see how feasible this technique would be for the extruder. I got a common or garden steel M6 bolt and drilled a poorly centered hole down its center. I used a large battery to run about 60A through the bolt and measured about 30 millivolts across a 25 mm stretch of it. This meant that it had a resistance of about 0.5 milliohms per 25mm or a 20 micro-ohms per mm. I figured that the length that I would flow current through would be about 25 mm. To attain a heating power of about 20 Watts, I would need a current of about 64A. Of course there will have to be more in the circuit than just the bolt. I wanted the resistance of the rest of the loop to be about half that of the bolt. This worked out to be 6AWG wire.

I got a 60Hz transformer with a split bobbin (one where the primary and secondary are not wound on top of each other). The transformer showed about 400 millivolts per turn. I removed the secondary and replaced it with a 6 gauge wire about 100mm long. It had no insulation and two bog copper lugs crimped on to it. This was connected to a section of M6 threaded rod with a 3 mm hole drilled (rather poorly) through it. On lug was pressed against the bolt head by a split washer and a nut. The other is clamped between two nuts and a split washer 25 mm down the bolt. When I connected the primary to 120V 60Hz, the rod heated very quickly. A thermocouple inserted in the hole showed that the temperature rose to above 100C in less than 30 seconds.

The bolt heated satisfactorily, but it was interesting to note that the end where the current had to flow through the thread-to-thread nut-bolt connection got hotter than the end where the current could flow from the lug directly to the bolt head through a nice big flat surface. Unfortunately, the wire got unacceptably hot.

I then made a secondary out of a piece of fairly thick walled copper tubing, carefully banged into shape, with two holes drilled for the hollow bolt. This had half the resistance of the 6 AWG wire, but still got unacceptably hot.

The wire should not dissipate much to avoid heating the transformer and whatever insulation separates the wire from the transformer core and primary. Unfortunately, good current paths are pretty much always good conductors. I think that this is definitely going on in my experiments. Heat can be removed as it flows, though.

A 60Hz transformer with enough volts per turn (proportional to the cross-sectional area of the transformer iron core) will be rather large. Limitations on the wire resistance mean that the transformer must be very close to the bolt. This means that the transformer would have to be carried around right next to the extruder tip. The bolt or current path through the bolt must have a significantly (>10x) larger resistance than the secondary wiring to aviod excessive heating in the wire. This is not feasible with a M6 threaded rod or bolt.

Possible improvements etc.:
Different tubing material with higher resistance. Stainless steel or nickel?
Increase resistance of tubing by thinning it: This would allow heat to be concentrated
Increase heating at extruder end by passing current through the thread-to-thread connection between the extruder tip (acorn nut?) and the barrel.
Decrease transformer size by increasing operating frequency. If the transformer were driven by a 12V square wave, it would need about 120 turns to have 100mV per turn. Maybe there is a nicely sized toroidal inductor with this many turns and a big enough hole to stick a M6 through. If so, it would make a much smaller transformer than a big 60Hz one.
It may be possible to get a washer made of a material with a larger resistance (10 to 100 milliohms) and put it in the current path to focus the heat. The logical place would be right next to the nozzle.
If a high enough resistance washer were readily available or easy to make, maybe 12V DC could be used to heat it. This hypothetical washer would replace the nichrome wire directly.

Frank Davies
Re: Alternates to nichrome in the extruder
December 27, 2008 10:42AM
in short terms:
you made your own induction heater.

nice, IIRC we talked about something like that in the forums lately.
Maybe you'll find hints there to improve it.

Oh AFAIR it makes sense to improve the frequncy as well professional induction heaters welders and such use something closer to 250Hz than 60Hz winking smiley

Re: Alternates to nichrome in the extruder
December 27, 2008 03:03PM
Good stuff Frank! Nice to see people actually doing the experiment.

Any chance you could take some photos of your setup and copy this into the Builders Wiki? Even though it didn't necessarily work out exactly as intended, it's very valuable to know what doesn't work so we can get on to what does work.

Here's the discussion Sid mentioned:

Interestingly, I think Nophead was talking recently about trying to minimize the size of the heated zone in the heater. Something along these lines might help there; instead of heating a barrel, just heat the nozzle directly with an induction coil. Of course, there are problems with size of the coil, currents, etc, but it is interesting.

A more robust heater would be a good thing.

Re: Alternates to nichrome in the extruder
December 27, 2008 04:14PM
I am not sure why you would want to replace something as cheap and simple as nichrome with a big transformer but you could put say ten turns of thick copper wire around your steel bolt and then the current would be ten times less. It would then be manageable to take wires back to a transformer with ten times as many turns on its secondary.

Most inductive heating systems use a high frequency to make use of the skin effect that makes all the current flow in the surface of the conductor. That gives it a much higher effective resistance so the current becomes manageable.

Re: Alternates to nichrome in the extruder
December 27, 2008 05:41PM
I was just thinking that something more reliable would be good. Instead of red hot tiny nichrome wires, decomposing potting compound, and problematic connectors, a coil that stays relatively cool around a nozzle that gets hot might be better. I wonder if you could get similar effects with many turns of wire with less current? Could you do 1000 turns of wire with 1/100 times the current? I don't know much about transformers though. It's no good if it doubles the weight of the extruder.

Of course, some Cerastil would probably be perfect. I have been getting good results with some fire cement potted nichrome wire on my current extruder, but I expect it'll give out sooner or later.

Mostly I was just impressed that Frank took the time to test something and post results, even if they were negative. We don't want to waste time by running down avenues that are doomed to failure, but it doesn't hurt to do the experiment sometimes. I know that I've been surprised a few times by things that shouldn't work but actually do when you get around to trying them.

Re: Alternates to nichrome in the extruder
December 27, 2008 07:14PM
With proper potting compound like Cerastil, or the stuff Forrest found, the heater will last pretty much forever. It is a good thermal conductor so the nichrome doesn't get anywhere near red hot, it will be only slightly hotter than the barrel. It doesn't decompose either as it is good for much higher temperatures. The first heater I made with it is still working nine months later.

I certainly don't want to discourage people experimenting, that is why I suggested a way to make Frank's idea more practical.

1000 turns would give 1000th of the current at 1000 times the voltage. But unless the wire is thick, and hence bulky, then it will dissipate significant power in its resistance and get hot itself. It could easily create more heat than the one turn bolt.

That is why induction heaters use high frequencies to get any sort of efficiency. If you think about it, to get 90% efficiency the I2R loss in the coil would have to be ten times less than the bolt. So for a ten turn coil with 1/10 of the current the wire would need to have no more than 10 times the resistance of the bolt. A tall ask for ten turns of copper wire compared to one very fat turn of steel.

You might get close with a short thin walled stainless steel tube and some very thick copper wire.

You can heat steel with very high currents. I remember as a child putting a pair of scissors across a massive 6.3V transformer's terminals and making the blades orange hot. The transformer didn't like it much though. Tar bubbled out of it and caught fire when it met the scissors. The transformer was about twice the size of a PC power supply.

Re: Alternates to nichrome in the extruder
December 27, 2008 07:27PM
Actually the de-soldering iron heater I blogged here: [hydraraptor.blogspot.com] has a thin walled stainless steel tube running through the middle of it. I just connected it to a 10A current limited PSU and it got to over 100C. So with 15-20A it could get hot enough to be an extruder heater.

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