Strange snagging problem in infill when printing

Hi everyone,

I'm having a strange issue on my Prusa i3 Rework where the print does not come out and has snagged strings and blobs of filament in the infill, as you can see below:



The bottom layer goes on just fine, but all the layers above it have some little part where it starts to string and snag. Then the layer above it strings and snags in the area where it was messed up before, and makes additional mistakes, and soon the whole infill part is messed up.

Here are some more images:







0.2mm layer height with 3mm PLA (0.4mm nozzle). Bed temp is 70 degrees, and extruder is at 183 degrees.

Thanks so much for helping me out.

Edited 3 time(s). Last edit at 11/16/2014 06:18PM by Blue Ice.

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Blue Ice

blog at [contemporarymaker.com]

4 possibilities:

1. Extrusion too hot.
2. Nozzle obstruction.
3. Metal hotend not been actively cooled.
4. something loose in the extruder.

Blobs and strings are symptoms of high temperatures (thermistor loose, or wrong selection in marlin) once too hot, filament might jam inside the hotend, but you don't mention this? can you post pictures of your extruder and hotend.

Hi ggherbaz,

Here it is:



I don't believe it is too hot- filament does not drip off the extruder, it sticks. I also don't suspect anything in the nozzle.

Thanks for helping me out.

This looks like a retraction issue

Is Retraction enabled? If not enable it.

Have you checked your vref stepper driver setting for the E axis? If not, make sure you set the vref on the stepper driver correctly....then do steps below to bring retraction performance to within safe (albeit slow and unoptimised) working parameters

Retraction settings for a safe safe setting should be as follows (remember you can always turn these settings back up or down for improved performance but we want a set of safe settings first to make sure retract is working correctly)

1. In firmware set retract acceleration to 50mms (this is very slow but will make sure your e motor is not skipping steps on the retract and you can increase later) - you can do this using printerface command line with the following command "M204 T50" (without quotation marks)
2. Set retract speed in the slicer software (KISS CURA Slic3r) to 10mms
3. Set the retract distance to 3mm (again this is a long retract for a direct drive extruder, but its a safe setting and is set in the slicer software)

Run the print again, watch the driven gear (large gear) on the extruder, it should reverse positively at least 1/8th of a turn and the sound should be "normal" during the retract phase.

If it looks like the extruder gear wheel just "twitches" a small amount whilst the motor makes a bit of an unpleasant "graunching or grinding" sound then i would be checking that the extruder moves freely and is not binding mechanically

Maybe take a short video and youtube it, and post the link here

good luck

Edited 1 time(s). Last edit at 11/16/2014 08:12PM by Mutley3D.

Thanks for the advice, Mutley3D.

I have a strong feeling that there is something wrong with my cooling system, because the first layer is fine, and then they get progressively worse. I also tried printing with no extruder fan when I first built the printer and I got similar results to this. I think it's some sort of heater tube jam... ?

I'll try printing with a house fan right next to the extruder and see if that helps. I think I want to get to printing a mount for a more powerful (larger) extruder fan.

Stringing wasn't this bad before, so I'm not sure what is happening. If the house fan doesn't work, I'll try what you mentioned, Mutley3D, and report back here. Getting back to the printer in a couple of days.

Thanks so much.

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Blue Ice

blog at [contemporarymaker.com]

You have what it looks like a clone J-head, if so need to opened and see if there is any gaps between the Teflon liner and the heater block barrel. If there is a gap, most likely it's building a plug and will create the issue you are experiencing.