Nozzle lifted by previously deposited filament?

I have a novel and interesting printing quality problem and need your assistance

It looks like a random mechanical nozzle lift. Upper part of photo shows ideal surface. Lower part of photo shows the problem.

Material is PLA, layer height is 200µm, nozzle diameter is 0.3mm, printing temperature is 190°C. Printer properties: Chinese Prusa i3 X by Geeetech with mk8 extruder. My Printer has a little backlash, a little jitter, and a little periodic z-wobble. The carriage can be pushed upwards easily by around 400µm due to the typically flexible smooth x-rods and a PLA x-carriage.

-tried 2 different 0.3mm nozzles with different size of the flat bottom area: ( 0 ) and (0) with same result
-occurs if external perimeter is printed last (lower part of picture)
-does not occur if external perimeter is printed first! (upper part of picture)
-does hardly occur if no infill is present
-does not occur on objects with a single perimeter and no infill
-occurs after 25mm-35mm of travel with a printing speed of 40mm/s
-effect occurrence is reduced by 40% if temperature is increased from 190°C to 210°C
-effect looks similar to this: [forums.reprap.org]

I assume the Nozzle is pushed up by previously deposited, neighboring and hardened filament. Can your carriage by pushed up slightly too? Do you have an idea how to solve this issue or have suggestions for further checking? Printing outer perimeter first is not a viable solution as it is bad for overhangs.

Edited 2 time(s). Last edit at 03/16/2016 11:33AM by lukie80.

Are you sure it's pushed up? looks more like being pushed out? Bit hard to see on the foto.

Despite the z-wobble it looks like it is either thinner or pushed up.

Edited 3 time(s). Last edit at 03/15/2016 05:10PM by lukie80.

Try hotter temps. Looks like periodical under extrusion to me. Try changing any and all of your settings to see if something may be slightly off. Sometimes you don't even know you have something set wrong when you can get a problem to go away by changing other settings.

Edited 1 time(s). Last edit at 03/16/2016 10:33AM by Montiey.

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Master Tinkerer

OK, I'll try changing various settings. I'm still puzzled by the fact, that printing outer perimeters first eliminates the problem fully.

I found a solution! But first I forgot to mention that the problem occurs after approximately 1mm of 1.75mm filament (50mm of print distance) and stops after approximately 3mm of filament (150mm print distance) so it does not occur on small parts. And second, using a sharpened nozzle (no flat area at the tip) did not help at all. So the nozzle lift theory was partially nonsense.

The ideal solution is to have identical printing speed for everything but I found it to be sufficient to have identical printing speed for external and internal perimeters. I did some testing:

Print speeds of:
Ext. Perim. - Int. Perim. - Infill - Quality
40 - 60 - 100 - bad
30 - 60 - 60 - bad
30 - 30 - 60 - good
30 - 30 - 40 - good

I am not sure about the reason but I think it is related to dynamic plug formation and under-extrusion. The diameter of my nozzle is 2mm and not 1.75mm which might be responsible for the problem. The regular print order is: 1. inner perimeter, 2. outer perimeter, 3. infill, 4. new layer, 5. inner perimeter, 6. outer perimeter etc. So print speed is fast for infill, fast for internal perimeter and slow for the subsequent outer perimeter. In such a way no plug is existent during printing of the infill and the internal perimeter due to the high extrusion speed. But the outer perimeter is printed slow (i.e. 0.6mm raw filament per second) and the heat melts more and more of the filament inside the nozzle. In such a way new filament is only partially extruded and partially widens the liquid filament diameter from 1.75mm to 2mm resulting in a under-extrusion. After some time at this speed an equilibrium state is reached and under-extrusion stops (after 150mm print distance in my case). See the attached illustration. So to summarize, changing print speed seems to have a deteriorating effect because it takes some time to reach the new equilibrium state which allows a good print quality.

What are your nozzle diameters? 1.75mm or more?

Edited 2 time(s). Last edit at 04/05/2016 02:50PM by lukie80.

I can tell you for sure that your nozle is not being lifted.
Also, interior diameter is around 2mm actually, so you are aok there.

Your problem, and Im really trying no to use "for sure", is temperature. You are not preinting fast but also not slow. 40mm/s is quit fast under some circunstances. Your thermistor could be giving you a bar reading, OR sometime the temperature reading is ok... but it is ok in the point where the thermistor is located. A lot of PLA filaments need to be printed at 200 and even at 215/220.
The temperature in your noozle could be a little smaller, also, you could have some air blowing to the noozle tip. All of these could make that the temperature is not enough and what you are seeing in your prints is just filament not being extruded, and that is why your problem somehow gets better when you increase the temperature. Try this: reduce speed to 35 and increase temp to 220. If it works, they experiment reducing temp by 5 degrees steps