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Recently, I've been noticing what looks like under extrusion. My first thought was a clogged nozzle, but it only occurs in some places:

(Photo of the issue)

My printer is a PrintrBot Simple Metal, using MH Build Series 1.75 mm PLA and the model was sliced with Cura 5.1.0. What could be causing this, and how should I fix it?

Slicer settings:

  • infill density 10 % (pattern: cubic)
  • layer height: 0.2 mm
  • print speed: 50 mm/s (travel: 150 mm/s)
  • print temp: 200 °C (bed: 0 °C)
  • retraction: enabled (Z-hop: disabled)
  • cooling: enabled (fan speed: 100 %)

I'm not printing faster than usual, and I watched some of this print print, and there were no obvious issues like a clicking extruder. I find it strange that it occurred regularly along the edge of one piece, but the piece next to it was perfect. It seems to occur around two sharp corners. The other piece didn't have those sharp corners.

Another doubtful theory by me:

I've had some issues with a loose plug recently. Could this be causing a drop in temperature?

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  • $\begingroup$ Are you printing fast? What temperature? Please share some settings. $\endgroup$
    – 0scar
    Jun 12, 2023 at 10:28
  • $\begingroup$ @Oscar I added the print settings: I'm not printing extra fast, and my print temp seems fine for PLA. $\endgroup$ Jun 12, 2023 at 15:09
  • $\begingroup$ Is the extruder skipping? Should be easy to observe, look at the gear that pushes the filament. Do you hear a click every time the gear rotates backwards? $\endgroup$
    – anttix
    Jun 13, 2023 at 9:44
  • $\begingroup$ @anttix I had the honor of watching the shown part print. The extruder gear wasn't skipping. $\endgroup$ Jun 14, 2023 at 22:18
  • $\begingroup$ There is one topic discussing the under extrusion of this printer. This printer uses an old (PEEK) hotend, which underperforms. From what I read, the original hotend is not the best around and the stepper is weak. Replace the hotend for a more modern design, or buy a new printer to save the hassle and frustration. For the price of the Simple you can get two entry printers with better extruders/hotends. $\endgroup$
    – 0scar
    Jun 20, 2023 at 18:45

3 Answers 3

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Ultimately, your printer is pushing out less filament than the gcode is telling it to (which you know, but its worth focusing on the exact problem).

If your stepper motor isn't missing steps, then that leaves just a few possibilities:

  1. The extruder's hobbed gear is slipping against the filament
  2. The extruder's hobbed gear (or whatever is being driven by the extruder motor) is slipping on the motor shaft.
  3. You got a bad spool with short lengths where the filament diameter is substantially out of tolerance (in the negative direction).

#3 seems extremely unlikely though is still possible. The filament need not be noticeably under spec, it just needs to narrow enough that the hobbed gear can't grab onto it well enough and you get slippage.

#2 is also unlikely but its so easy to check, might as well.

#1 is what my money is on.

Now we have to think of what could be causing filament slippage. This occurs when the hobbed gear's ability to grab the filament is overpowered by the resistance to flow through the nozzle.

This can either be from too little grip, or something causing unusual and intermittent resistance to flow that causes the hobbed gear to shred the filament.

  • If there is too little grip, you simply need to tighten the screw that has the spring that pushes the idler arm and idler pulley against the filament more pressure means better grip for the hobbed gear).

  • Your hobbed gear might have gunk between the teeth preventing it from gripping well and merely needs to be cleaned with something sharp.

  • Your idler pulley might have be broken, check for any play or unsmooth rotation on the shaft

Once you're certain grip is nice and strong, that leaves something causing resistance to flow:

  • There might be a spec of debris with an irregular high aspect ratio shape in your hotend. It can't exit through the nozzle, but it also can't completely block it either, and is constantly moving around and only becomes a problem some of the time.
  • One of your hotend's thermistor wires might have an intermittent short, or the thermistor itself has cracked. Low/short circuit resistance ends up getting interpreted as really high temperature, so no power is sent to the heater until a good enough thermistor connection returns. This can result in the hotend getting too cold for short periods. This option seems plausible since there seems to be a rough correlation between the print head position and when it happens, which is consistent with a broken conductor issue. The connection goes bad when the wires are positioned in certain ways, basically.

And it could of course be any combination of these things cooccurring and all contributing.

Unfortunately, under extrusion has so many potential causes that you just have to start going down the list from easiest to hardest thing to fix, and hope the problem is more towards the easy end.

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  • $\begingroup$ The odd thing here is that it's only happening in specific spots. If it was a random fluke, It would happen all over the print instead of just on one side. $\endgroup$ Jun 22, 2023 at 18:15
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    $\begingroup$ @TheTridentGuysupportsUkraine: When it only happens in certain spots, normally that means the condition causing underextrusion is dependent on the sliced geometry. It could be that there's a long move just before the underextrusion where the motion has enough length to accelerate to a higher speed than you have the flow for. It could be that there's a retraction/unretract just before, and your retraction speed or acceleration is bad, causing filament to slip or the E motor to skip steps while adjusting the filament position, offsetting the amount extruded. Etc. $\endgroup$ Jun 22, 2023 at 23:35
  • $\begingroup$ Another very possible cause is oozing during a travel move just prior to the line that's underextruded. In this case, the missing material was extruded, just in the wrong place. This can be a result of poorly calibrated retraction settings and/or "combing"/skipping-retractions. $\endgroup$ Jun 22, 2023 at 23:36
  • $\begingroup$ @R..GitHubSTOPHELPINGICE Thank you! This is the point I've been trying to make. Turns out condition causing underextrusion was the filament resisting coming off the spool and the extruder moving quickly away from the spool. $\endgroup$ Jun 24, 2023 at 15:43
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I would strongly follow Teach Tech's calibration guide. It's entirely possible that there are a number of things out of whack, contributing to the overall print quality. In this case, I'd look at doing a PID tune and seeing if your print temps are consistent.

Also, as the filament is fed into the nozzle, that lowers the temp, and if your temperature is not consistent, it might be lowered to a point where it starts to solidify, this would account for the results you get but again, could be a number of things. Even wet filament.

The filament you are using has a temp range of 205 +/-15. That means anything from 190 to 220. Even in your pic you can see the issue starting at lower layers.

Calibration is clearly the first thing to do before anything else.

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Solved! Finally...

So after having this issue off and on, an extreme case appeared. It turns out that filament was resisting coming off the spool, and when the extruder and hotend (the Simple is a direct-drive) pulled away from the spool it created tension in the filament, effectively pulling it out of the hotend, though only just enough to cause this issue. In this specific case, it seems that the long edge before the side of the piece was pulling the filament.

Shoutout to @metacollin and @DragonDon for their answers. I'm still calibrating my machine, so they'll useful.

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    $\begingroup$ If the motion of the printhead (on a direct drive printer) is pulling the filament to turn the spool, and this is affecting its ability to push the filament through the hotend, this can be mitigated with a reverse bowden tube running from a fixed point near the spool to the filament intake on the toolhead. If you don't want to do that, adding bearings to reduce the rolling friction of the spool may suffice. $\endgroup$ Jun 24, 2023 at 19:54
  • $\begingroup$ Thank you so much for posting this, this was my issue too! What a silly problem that I never would have considered. Thank you so much! $\endgroup$
    – Ampp3
    Feb 12 at 5:19

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