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I came across several issues which seem to have been lowered. Firstly, I changed from a 0.4 to a 0.5 mm nozzle. Because of the backpressure I was not able to print my PETG (Colorfabb XT filament) below 270°C which caused unresolvable oozing. After that I was able to extrude till 230°C.

The left print below shows the result. I disassembled the hotend, there was no leak or whatsoever. Maybe it was too cold for printing. However, the temperature displayed was 250°C. Then I replaced the cheap aluminum heat block with a copper alloy based one. After that my PIDs did not work anymore. I had to greatly enhance the d-term, otherwise there was a big overshoot. Guess there was a serious heat conducting issue with the old BQ hotend.

Prints using BQ Alu Heatblock (left) and Copper alloy Heatblock (right)

Anyway, from there it became better. However, I noticed that I still have severe underextrusion after travel moves (second piece, first picture, first piece, second picture). I use Cura, so I activated retraction with the feature to prime after travel moves. I got the wall closed just after 0.35 mm³.

Prints using Copper alloy Heatblock (left)

My Question: Is this underextrusion after travel moves normal for PETG/XT? I did not discover such behavior with PLA or ABS in the past.

Current Site Advice: Despite the weight, copper heat blocks seem to be worth the upgrade.

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  • $\begingroup$ What is your retraction length? $\endgroup$
    – 0scar
    Jul 7, 2018 at 21:57
  • $\begingroup$ I need 2.5 mm for PETG. But the underextrusion happens also if no retraction is active. $\endgroup$
    – dgrat
    Jul 8, 2018 at 11:25

2 Answers 2

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Have you tried adjusting the Extra Restart Distance in Simplify3D or Retraction Extra Prime in Ultimaker Cura? Most slicers should have a similar setting, perhaps named just slightly differently. A small positive value can help prime the nozzle after a long travel when you've lost back pressure or oozed a little.

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Zero retraction is just a special (worst) case of under-retraction. You'll pretty much always lose material to oozing if you don't retract before travel. At best this ends up hidden inside the print (but can still affect weight and weight balance); usually it'll also harm the surface.

Ensure that retraction is set to always happen (not skipping short travel), and at least 5-6 mm for bowden extruders. Direct extruders can get away with less but I'm not sure exactly how much less. Less-rigid materials need more retraction to make up for compression of the filament between the extruder gear and the nozzle.

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    $\begingroup$ The second part of your answer implies Bowden style printers considering the rather large retraction, I think it is best to state that explicitly. $\endgroup$
    – 0scar
    Jun 7, 2019 at 9:05
  • $\begingroup$ Yes, will do. Thanks. $\endgroup$ Jun 7, 2019 at 12:20

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