Solved Extruder heater problem

This printer has been working great for many months but has developed a problem with the extruder heater. Preheating he extruder gets it to the set temperature just fine and it stays there until the printing starts. After the printing starts, the temperature slowly begins to drop and over a period of about 2 minutes it is down 10 degrees and trips a hardware error. Reloading MatterControl enables me to preheat the extruder again and it climbs right back up to the set temperature.

Thinking it was a power problem, I measured the 12v supply when the system is idle, when it is preheating, and when it is printing. The voltage at the power supply never wavers from 12.14v by more than about 0.04v.

I then decided it might be the heater, so I replaced it with a new one but nothing changed.

Tightening the connections on the ramps card didn't help either.

All that's left that I can think of is a a worn wire somewhere because of the constant movement or a ramps card that can no longer supply enough power to the heater.

What confuses me is that it works just fine until it starts printing.

Any thoughts on what else this might be?

 

Since the ramps can hold the temp when it is preheated, I am guessing that when the movement starts, there is an intermittent in a heater wire somewhere. I will run a new pair of wires from the ramps to the heater and report back here.

 

Okay, changing the wires did not help. I am now going to reinstall the thermistor.

 

Well I'm stumped. I removed and reinstalled the thermistor, no help. The extruder and bed thermistors agree on about 20 degrees when idle and they both seem to climb normally when I preheat. It still holds at 270 degrees when idle and drops slowly when I start printing. When I measure the voltage at the supply, I get 12.14 +- .04. When I measure that the extruder, I get something I didn't expect. I figured the heater would be on, 12V or off , 0v but that is not what happens. From a cold start, preheat results in about 11.5v. When it reaches about 260 degrees, it tapers down and varies between 6v - 9v and holds the temp at 270. So it is variable, not just on or off. Now the interesting part is that when I start to print, it goes back up to about 11.4v or 11.5v and stays there, but the temperature is still gradually dropping.

I know 270 is kind of high, but I have printed a ton of PETG parts at that temperature and never had a problem until 2 days ago. Is the drop from the power supply to the extruder 0.6v - 0.7v too high?

This almost looks like the extrusion of the PETG is absorbing the heat faster than the heater can produce it. I am going to break out the oscilloscope to see if there is something weird going on with the PS or the Ramps to Extruder heater.

Any other thoughts are welcome.

 

The two question answers are:
The PETG clogs at 260 so 270 is what I have been printing with. I am using the default E3D heater so I guess it is 25w.

Finally making some progress on this problem. With the oscilloscope connected to the heater wires, I can now see that it is not variable voltage, but rather PWM. It switches between 12v and 0v and simply adjusts the on time to control the temp. When it is not printing it hovers around 55-60% duty cycle. When printing, it goes to 100% duty cycle.

I have found the difference between preheat and actual printing, it is the material cooling fan. If it starts printing and I stop the fan, it easily maintains the 270 degrees. As soon as I release the fan, it starts to drop. Why it worked for so long and suddenly became a problem is a mystery to me.

My next option is to take the advice of Mark Tomlinson and get a 40w heater. I feel confident that will fix this.

 

Thanks Mark. I really appreciate your help and the help of others on this site.

Goef, thank you also for the comments on PETG temp being too high. I am going to start a new thread asking for help configuring MatterControl with a Robo 3D for PETG. I may be extruding to fast, traveling too fast or something else.

 

I marked this one resolved.
I installed the 40w heater for the extruder and that resolved the temperature variation issue. I adjusted the firmware so that when it wants to extrude 100mm of filament, that is exactly what it does. I can now print at 260 degrees.

Thanks all for the help.