## Wednesday, 31 December 2008

### Do we need nichrome?

While making a new heater I decided to try using stranded tinned copper tails rather than the solid tinned copper wire I used previously. The idea being to put less stress on the Cerastil covering.

I started with a standard piece of 7 x 0.2 stranded copper wire and removed the insulation. I found all seven strands too bulky so I decided to see how many strands I needed to carry 2A. I found that a single strand was cool to touch at 2A but very hot at 4A. I figured two strands would be sufficient for some margin.

The fact that a strand gets hot at 4A, and in fact red hot at about 5A, got me thinking that we could just use a single strand of copper for the heater. Nichrome is expensive, not that easy to obtain, and difficult to make connections to.

I measured the resistance of a strand 52cm long as about 0.3Ω (my meter only gives one digit). The strand measured 0.17mm diameter. Calculating its resistance from the resistivity of copper I get 1.72 x 10-8 x 0.52 / (π (0.00017/2)2) = 0.39Ω.

At 4A the voltage drop was 2.6V giving a resistance of 0.65Ω and a power of 10W. The thermal coefficient of resistance is 0.0039 for copper so the calculated temperature of the wire is 20 + (0.65/0.39 - 1) / 0.0039 = 191°C. It was certainly hot enough to cut through ABS.

10W and 190°C are not far from the operating conditions of an extruder. I tried winding it on the bobbin I had made for my heater but it was about twice as long as I could accommodate. I am trying to make a very short heater at the moment so I went back to using nichrome. Also 2.6V @ 4A is too much for my current drive circuit but it would be easy to come up with a switch mode converter to drive it, or simply use the 3.3V rail of a PC PSU.

So it has definite possibilities. Making the connections would be trivial. Just start with a piece of 7 strand wire and cut it down to one apart from at the ends. Some high temperature solder would keep it neat but would not be essential. A standard heater barrel with some insulation would be about 7mm diameter so 24 turns would be required. If you keep it taught and wind it in a lathe or drill chuck you can get about 2 turns per mm with some concentration. That would easily fit the space currently allocated for the heater.

1. That's a very good idea for the extruder. One more piece of "exotic" material done away with!

The winding method you suggested is pretty good but we would need smooth heater barrels and not this threaded stuff. Also, it is gonna take some more time and patience than the current design does.

I always get a little sketchy about using bare nichrome on my reprap because shorting it can really mess some stuff up. Bare copper is gonna be no different to me. Guess I'll have to get over it.

Demented

2. Yes if you use bare wire you have to put down a layer of insulation first. That makes it a two day job but it doesn't matter of the barrel is threaded or smooth.

I insulate the flying leads with PTFE tubing and since doing that I haven't had any shorts.

3. The major unknown is what would happen to the copper wire after a few temperature cycles. Would it harden and become brittle? In fact, would the expansion/contraction be sufficient to cause a problem?

Are any special provisions made in HT transformers to cope with the windings getting hot?

4. The killer here will be oxydation. A copper heating element won't last very long when compared to nichrome because any part of the element exposed to oxygen will break within hours at high temperatures.

5. If oxidation is a problem, what about bronze wire?

According to the chart at:
http://copper.org/applications/industrial/DesignGuide/oxidation04.html

and according to the discussion of tin alloys at:
http://www.copper.org/resources/properties/microstructure/cu_tin.html

tin and bronze have a higher resistance to oxidation than copper.

From the "Electrical Conductivity of Copper Alloys" article at:
http://www.keytometals.com/Article79.htm

The conductivity of bronze is about a third of that of copper, so you could use thicker strands of it. From that article it states that phosphor bronzes are used when resistance to corrosion and fatigue are needed.

6. True, there are other alloys with better oxidation resistance, but those probably won't be easy to come by, at least in wire form. Not to mention that few metals can match nickel and chrome in that department.

Also keep in mind that oxidation speed increases exponentially with temperature; make sure any reference data you use takes your target operating temperature into account.

There are other reasons as well why I would stick to specially-designed materials like nichrome wire in a heater. Any variation in composition or thickness in the wire can lead to the formation of hot spots which are prone to failure.

Finally, when it comes to joining copper and nichrome wires, I generally use small crimp-able brass sleeves. If none are available, try to cut off the crimp portion of a standard crimp-on lug (whole lugs are oft too bulky to use). Also, do use copper wire that is much heavier than the nichrome so that it will act as a heat sink and prevent the junction from heating up and thus rapidly oxidizing.

7. Hi Guys,
Some interesting points I hadn't considered.

I don't think copper expands any more than nichrome does and that does not seem to cause a problem underneath cerastil.

Oxidation may be an issue. I guess I will find out because I have just made a heater with two strands of 0.2mm copper tails.

I don't think the wire gets any hotter than about 300C, otherwise the solder would melt. It did when I used lead free solder and it all oxidized away but not with HMP solder.

The problem with crimps is that I haven't found any that weren't way too big.

8. The smallest crimp sleeves I found were 14 to 22 gauge connectors at:
http://www.shop3m.com/80610063002.html?WT.mc_ev=clickthrough&WT.mc_id=shop3m-AtoZ-3M-Scotchlok-Crimp-Sleeve-Connector

and 14 to 16 gauge sleeves at:
http://www.kencove.com/fence/Crimp+Sleeves_detail_C12.php

and 10 to 18 gauge connectors at:
http://www.idealtruevalue.com/servlet/the-16961/Detail

9. McMaster-Carr carries suitably small crimp ferrules (#9681K11) that are only ~5mm long and made for 26ga. wire. Those are quite cheap (5\$/100), but McMaster have been a pain with export sales lately, so you might require a U.S.-based friend to purchase those.

I have used similar ones that were sold for Nitinol wire terminations, but I can't remember where I got those from.

Again, they are all made of tinned copper, so they can be affected by corrosion in the long term (nickel-plated steel would be better). It may very well be that the slip-knot & high-temp solder be the best solution after all.

10. Instead of bare copper, have you considered using magnet wire or some other wire with a light enamal coating? Could connection points be protected with an anti-oxidation agent (No-Ox and Ox-Guard in the States or something similar). May help solve some of the insulating problem (if there is one) and also leads towards eliminating the Nichrome...

-Rick

11. Hi Rick,
I think most enamelled wire only has a working temperature of about 200C. There are some higher but getting a bit exotic.

I hadn't come across Ox-guard before. Thanks for pointing it out. I haven't been able to find out whether that works at high temperatures.

Nichrome is beginning to look a lot more practical.

12. May do some testing with the mag wire on my own, for kicks. But that 200C kinda knocks down another idea for an anti-oxidizer. The silicon based heat sink compounds (grease) usually have some anti-ox properties, but they are only guaranteed to 200C. What is the target temperature? I know a couple of Chem E's that may be able to offer some insight...

-Rick

13. I usually run the extruder at about 240C and try to use components and materials that handle 300C.

I use some heatsink compound between the heater and the barrel in my latest design. I hadn't realised that had a 200C temperature limit.

I wonder if it is better to not use it at all than it is to use it above its limit?

14. Sorry for the double post. Didn't mean to startle you, I should probably clarify. In what I have seen (admittedly my information may be lacking), silicone based grease compounds usually have a guaranteed limit of 200C as in "guaranteed to stay wet for 1000 hours at 200C". Not all compounds are created equal, but if you are running consistent temperatures of 240C or better, might want to do a little research. I think silicone starts to become unstable at 250C (somebody correct me if I'm wrong). Maybe a quick email to the manufacturer's tech division would get an answer.

-Rick

15. That could be tricky, I bought a big syringe of it more than 30 years ago and have been using it ever since. Might be time to get some more up to date stuff!