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I can print a reasonably high quality print, but then there will be a millimetre or two of rubbish.

Bad layer Bad layer

This can happen at the start of a print, in the middle or at the end.

It doesn't seem to be bad G-code as I can reprint with good results.

There seems to be a correlation between the bad layers and walls. They seem to occurs when there is a transition to printing a horizontal wall.

The latter if there two images if the tail end of a thirteen hour print.

  1. What's causing this?
  2. Is it something I can fix or is it that the (new) printer is failing?
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  • $\begingroup$ that looks like 3 or 4 bad layers before recovery and then again... can you do a 2 cm diameter, 10 cm high pillar test print and check if that error always happens at the same height? $\endgroup$ – Trish Jan 7 '19 at 13:00
  • $\begingroup$ @Trish I did some narrow cylindrical prints, 0.5 cm by 10 cm with a curved base for support. The first failed the first millimetre then reduced, the second no failures (both had wobbly profiles but good adhesion). $\endgroup$ – BanksySan Jan 7 '19 at 13:34
  • $\begingroup$ @Trish I'll print your recommendation, any specific settings that are good for a test print? The prints above were 0.1mm layers. $\endgroup$ – BanksySan Jan 7 '19 at 13:35
  • $\begingroup$ Thanks for the test. a larger diameter would allow to rule out thermal troubles with printing narrow objects (at some point they just blob). Just the diameter and some arbitrary hight that is larger than where you get errors is needed - if it always appears in the same hight on prints one after another it hints to the movement while random placement hints to extrusion. I suggest no infill and 2 or 3 walls - and no top layers $\endgroup$ – Trish Jan 7 '19 at 13:38
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This can be caused by various problems, of which an aforementioned filament diameter is only one of multiple possible causes.

The observed phenomenon is called under-extrusion. Temporary under-extrusion can also be caused by filament friction on the spool, too low or too high filament extruder gear friction (pressure on the filament by the roller), or friction in a Bowden tube (dust particles).

Please ensure your spool unrolls freely (not entangled; entangled filament is audible like "ker-chunk" when the filament is rearranged on the spool by force of the extruder pulling) and check the extruder for friction or slip. If the extruder gear slips, this is also accompanied by an audible "click" that is called clicking. This generally implies that there is too much friction in the hot end, or an obstruction/clog.

Considering that your print recovers from the problem, and that it occurs randomly, the best guess is that your filament in entangled now and then.

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  • $\begingroup$ Thanks Oscar. How do I check the extruder for friction and slip? Is there a calibration process or similar for it? $\endgroup$ – BanksySan Jan 7 '19 at 16:53
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    $\begingroup$ I just realised, I occasionally hear a loud "ker-chunk" which I'd attributed to the spool turning ungracefully around the holder. I'm wondering not if that's when the bad layers appear. $\endgroup$ – BanksySan Jan 7 '19 at 17:05
  • $\begingroup$ @BanksySan If you hear that, it means your filament is entangled on the spool, this causes extra friction, it is most probably your actual problem. Checking the filament that comes out if the extruder will reveal if you have too much or too less tension on the roller, you see that by the indentation the gear leaves in the filament. There are many pictures to be found on that topic. If you have a direct drive extruder you need to un-mount the hotend. $\endgroup$ – 0scar Jan 7 '19 at 18:22
  • $\begingroup$ @BanksySan Ker-chunk can also be a skipping extruder. It hints to a clogged nozzle often $\endgroup$ – Trish Jan 7 '19 at 18:38
  • $\begingroup$ I've had similar problems related to friction in my bowden tube. The extruder will make frequent clicking sounds as it skips. Certain filaments seem more prone to this as well. I've found that lubricating the tube and/or filament fixes it. There are several sponge holders on Thingiverse; add a little cooking oil to the sponge and install it where the filament enters your extruder. $\endgroup$ – mbmcavoy Jan 7 '19 at 18:46
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One of the reasons for such a defect may be a local decrease in the diameter of the filament. Carefully check your filament.

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  • $\begingroup$ Thanks Dmitry, what do you mean by check the filament? It looks OK. It's gray and long and thin. $\endgroup$ – BanksySan Jan 7 '19 at 17:04
  • $\begingroup$ @BanksySan, measure the diameter of the filament with the micrometer or caliper in several points (>=10) on length about 5-10 meter $\endgroup$ – Dmitry Kasatsky Jan 7 '19 at 17:56
  • $\begingroup$ @BanksySan, can you disassemble the extruder? The extruder gear can be loosely fixed to the motor shaft. $\endgroup$ – Dmitry Kasatsky Jan 7 '19 at 18:02

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