Nozzle Dragging

I am getting very minor nozzle dragging. It is noticeable in that at the edges of infill, it pushes a tiny wall of a minute amount of material into the perimeters, and it also creates small channels along the infill where the nozzle drags. This manifests itself in final prints as a slightly inaccurate perimeter wall with some small bulges sticking out, and in the final layer not being flat but ridged. As a print gets higher, the pile of molten plastic the nozzle pushes around gets worse. You can see in the attached images a tower calibration piece that shows these issues well - at the bottom the perimeter is perfect, but the higher it goes the worse the buildup that is pushed to the edges becomes larger. It's odd because the shorter square I also attached a picture of doesn't exhibit these issues as prominently, maybe because it's horizontal area is larger and hides them better - but the top still has minor ridges when it should be flat. Note that these prints were made immediately one after the other.

It is my understanding that nozzle dragging (while printing with PLA) could be due to:
- The printer extruding more material than it thinks (adjust extruder steps in firmware, adjust filament diameter in slic3r, or adjust layer height properly with respect to nozzle diameter).
- The printer is not moving as high up the z-axis as it thinks (adjust z-axis steps in firmware).

I believe I have all of the settings related to the above correct, so what else could be the cause?

Thanks for the help!

Edited 3 time(s). Last edit at 02/05/2015 07:03PM by Jonny Five.

If it is getting worse as the z goes up, I would suspect missed steps or the height adjustment ( steps per mm) is slightly off. I would grab a tall calibration piece and see if you print as tall as it should be

That could also be cooling issue. Do you have a fan blowing on the print? It may not be on or set too low

I believe people in the past have had this issue when their max Z speed is set too high. The movement of these motors is time-dependent, and if your motors aren't going as fast at the firmware thinks they can, these types of errors can accumulate as the height grows.

You might want to check your value in the firmware.

Thanks for the feedback so far.

I was under the impression that PLA did not require active cooling. I have one fan directed constantly at the hotend. The thought occurred to me that the plastic could be slightly expanding after being deposited, but after reading many other posts, I found that a lot of people print PLA without a fan with no issues. Is it really worth adding a print fan?

As far as the z-axis settings in the firmware, I have a prusa i3 rework and my settings are below. I can't see much I can decrease further other than perhaps max z acceleration to 10. Stepper motor Vref was set at 0.8V and I've tweaked it before to what I thought was the right value. Any thoughts?

I also did a quick test and commanded a 100mm z-axis movement, and measured from the bed as my datum. It measured 100mm on the money, so I think my step settings are correct.

#define DEFAULT_AXIS_STEPS_PER_UNIT {80,80,4000,455}
#define DEFAULT_MAX_FEEDRATE {500, 500, 2, 25} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {2000,2000,20,1000} // X, Y, Z, E maximum start speed for accelerated moves.

#define DEFAULT_ACCELERATION 2000 // X, Y, Z and E max acceleration in mm/s^2 for printing moves
#define DEFAULT_RETRACT_ACCELERATION 2000 // X, Y, Z and E max acceleration in mm/s^2 for retracts

#define DEFAULT_ZJERK 0.4 // (mm/sec)

A fan on PLA is almost a must in my opinion. ABS does not need one, but PLA is sooo much better with a fan.

Here's a perhaps vital clue: it only seems to happen when the infill density is 1 (rectilinear pattern).

I mean, look at the attached near perfect prototype handle for a plane door I printed. I feel so close, but it's not quite perfect yet. For honeycomb fill and perimeters it looks great, but for the top of the narrow parts where it used solid infill it was obviously over depositing/dragging. I'm tempted to use an extruder multiplier, but that feels like cheating.

Towards the end of the print where I knew the dragging issue was going to rear its head, I just pointed the fan directly at the print. I didn't notice any difference. I also printed that whole piece without a fan until that point, so I'm not sure what benefit a fan would offer me really?

*Fan*

See my above comment about the fan.

What would a fan accomplish? The only thing I can think of, is that it would cool solid infill and cause it to contract. Is this really that substantial a difference? Otherwise, it only seems helpful for bridges and overhangs which are printed without support, which I'm not struggling with (yet).

Without a fan the material stays in a molten state. When you then drag more filament over it the molten filament gets pulled and some goes upward, nozzle then is at the same or lower height which causes collision and since the nozzle is hot it reheats the molten part again and it then gets pulled around by the nozzle even more until you have a mess.

You can easily test this by placing any fan next to the bed as you print (after first layer). Just a small desk fan or old computer fan blowing across it. Once you see the difference then you can mount a proper fan near the hotend with a funnel to aim it down to the print. There is a reason most any printer that does PLA these days comes with a fan. Rather than argue against it, just try it.

I'll definitely try using a fan, I appreciate the help. Your explanation makes logical sense as to why it would help. In thinking about it, it does explain why very small layers and those with solid infill exhibit this the worst. Not everybody's advice online is legitimate, and I've found it's usually prudent to ask for a reason and use some common sense before just trying what everyone says.

You can also try using the slow-down settings of your slicer, building a sacrificial column at the same time to kill time, or building another part along with it.

The fan fixed it! Thanks for the help and advice everyone, you're all very helpful.