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Solved: Poor hotend heating

Posted by JamesK 
Solved: Poor hotend heating
December 31, 2015 05:35PM
I've seen a few posts describing poor heating from standard ceramic heater cartridges which I've never understood as mine seems to heat like a champ. Then after a week of working fine I ran into something similar, so in the hope that it might be useful to someone I thought I'd describe what I found in this case.

I have a cheap MK8 extruder which uses a 40W 6x20mm heater cartridge. I haven't tested how high it will go, but it has no problem getting to 260C in a couple of minutes. Then I noticed that during a print it was having trouble maintaining the target temperature by just a few degrees. The next print, it wouldn't heat at all. I assumed the heater had failed which filled me with gloom as I have no spares. It was late, so I placed an order for a new heater and went to bed.

This morning I thought it was worth double checking what had failed. It seemed possible the problem was either with the Ramps, maybe with a dead mosfet, the wiring or the heater cartridge. I checked the voltage at the screw terminals of the Ramps and it behaved as expected (both terminals at 12V when off, the -ve dropping to near 0 when the heater was on). That suggested either wiring or the heater cartridge. A 12v 40W heater should have a resistance of roughly 3.3 Ohms, so I disconnected the heater wires at the ramps and measured. It was reading 180 Ohms which certainly explained the lack of heat, but also seemed a bit odd. I'd expect a heater element to either fail open circuit or maybe short, but somewhere in between maybe suggested a bad connection. The wiring runs through cable tow and has a join to extend the length, so to simplify things I cut the wires near the hotend and measured the heater resistance again. This time 4 Ohms - close enough! So the problem was obviously a bad wire running through the tow. To confirm before pulling everything apart, I measured each wire from the point where I'd cut it (near the hotend) back to the ramps. Both checked out fine. Confused, I jiggled the cables as much as possible to look for intermittent connections, but couldn't find any. So I went back to the heater and did the same there. One of the cables literally came off in my hand.

Because the working temperature of the heater is close or above the melting point of solder, they use crimps to attach the wires. The wire coming from the heater is solid core, the cable is multi-strand, which makes it a bit finicky to get the crimp pressure right. In this case the crimp wasn't tight and the connection was obviously poor.

I carefully re-crimped using side cutters and checked that the connection was snug

When I put the heater cartridge back in the block I added some Z5 thermal grease, and that seems to have helped a lot with the accuracy of the pid control, overshoots are now much reduced. Everything is now back to working as expected again, and I'm very relieved that I don't have to wait for a new heater to arrive from China!


Edited 1 time(s). Last edit at 12/31/2015 06:33PM by JamesK.
Re: Solved: Poor hotend heating
January 02, 2016 02:32AM
Wait until you see a thermistor-wire connection.
They are very thin and soldered. I had a few failures with them too.
The shrinktube seems to act as "crimp" connection there.
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