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I've tried printing the Basic Stringing Test on my CR-10S using PETG (250 °C nozzle, 80 °C bed) with different retraction distances leaving all the rest unchanged.

In the image below you can see 0 mm, 5 mm and 10 mm retractions (in order left-to-right).

Basic Stringing Test on CR-10S with 0 mm, 5 mm and 10 mm retractions

The result stunned me because I would have imagined that raising retraction distance would have diminished the stringing.

Why is that?

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  • $\begingroup$ I know that having too high of a retraction can lead to other problems (namely under extrusion and possibly heat creep). The stringing surprises me too. $\endgroup$ Feb 7 '19 at 20:17
  • $\begingroup$ Yes, it's very counterintuitive! Should I go beyond 10mm to see what happens? $\endgroup$ Feb 7 '19 at 20:23
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    $\begingroup$ @FredoCorleone No, but lowering the temperature also reduces strings, PETG can be printed at a lower temperature. $\endgroup$
    – 0scar
    Feb 7 '19 at 20:59
  • $\begingroup$ Toy around with values around the 5 mm $\endgroup$
    – Trish
    Feb 7 '19 at 22:35
  • $\begingroup$ So I could pick the best result so far (retraction distance of 5mm) and progressively lower nozzle temperature to see if quality further improves. What about increasing rectraction speed? What about increasing cooling fan speed? $\endgroup$ Feb 8 '19 at 0:22
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You cannot endlessly increase the retraction distance, doing so leads to different problems as you encountered. As a rule of thumb, the retraction distance should not exceed the length of your nozzle. Depending on the type of extruder, many printers use a value between 2 and 7 mm (e.g. the Ultimaker Cura retraction length is 6.5 mm at 25 mm/s, this is for a Bowden style extruder).

In fighting stringing (in which PETG is very subjected to happening) retraction length is only one of the parameters that you need to adjust to optimize the printing process. Other important parameters (besides retraction length) for stringing are:

  • retraction speed
  • travel speed
  • minimum travel distance
  • flow modifier
    -> From experience with printing kilometers of 2.85 mm PETG: if PETG is over-extruded by little, it causes the excess of material to form "curtains" like in the left image.
  • combing
    -> added by comments: PETG does not like to be "combed" over; moving back over the already printed material without retracting will "pull" it back up (due to adhesion to the molten material in the orifice) as well as pulling more material out of the nozzle; this results in junk all over the place which can manifest as "stringing" even though it's different from what "stringing" usually is.

Furthermore, the type of extruder (direct or Bowden) is also important and other parameters that may reduce the stringing are:

  • coasting (stop extruding before the end of the print line is reached using the residual pressure in the nozzle to extrude the last filament)
    -> Please note that coasting is a last resort as the amount of material needed for the whole print is not what is being extruded, i.e. stopping earlier means that the overall amount of material is less than what should be put in...
  • pre-loading/priming filament after a move

Usually it suffices to tune the first parameters. As you see in your results, the retraction of 5 mm is close to the result you would expect, increasing it, may lead to clogs and in your case leads to a sort of stringing. This latter result is probably due to the fact that the filament does not reach the print in time (you definitely see under-extruded cylinder columns), while leftover pressure after printing the column layer oozes some of the filament creating strings.

Just play with the retraction length in steps of 0.25 mm (around the 5 mm) and you may try to change the speed with 5 mm/s increments if necessary. Also experiment with the flow modifier.

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