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I've been playing around with PETG for the first time, and everything seemingly worked right just from the start - clean prints, no stringing, no bed adhesion problems, no warping or dimensional accuracy problems, etc. As expected it prints a lot like PLA, and as expected, it's less brittle/stands up much better to crushing/impact, except that it's really brittle when it comes to inter-layer adhesion. Vertical cylinders that were fairly strong in PLA just snap with no effort as PETG.

My particular PETG filament is Sunlu, with recommended print temperature 230-250 °C. I started out with 235 and am now using 250, which does somewhat better. I've used layer heights 0.125 - 0.2 mm.

Are these kind of results normal? Is there anything I should be doing to get better adhesion between layers?

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    $\begingroup$ Are you using part-cooling fans? If so, turn them off. $\endgroup$ – Mick May 26 '19 at 20:23
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    $\begingroup$ As @Mick notes, PETG doesn't like to be cooled too much, in that respect it is similar to ABS. A little cooling is necessary when printing sharp or thin objects like e.g. the nose of a rocket. $\endgroup$ – 0scar May 26 '19 at 21:04
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What you describe is usually the result of using a too high of a part cooling fan rotational speed. Like ABS, PETG doesn't require much cooling (if needed at all that is). If you do cool too much, layers and perimeters do not bond optimally (you can get string cheese like printed parts on failure).

Why should you use cooling for PETG? Cooling helps cool the deposited filament on small cross sectional parts. If un-cooled, the printed part picks up too much heat and will deform or sag out.

In such cases, reduce cooling to 40 % to start with (another option is to print more parts or increase minimal layer time). Note that there are so many print cooling fan constructions, some more effective than others, so you need to tune the print cooling fan speed to your setup. E.g. for an Ultimaker 3E I get good results at 50 % fan speed, for other self-build printers with effective part cooling solutions, 40 % works best (printed several kilometers of 2.85 mm PETG). First few layers don't need any cooling at all.

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    $\begingroup$ OK, that didn't make much of a difference on the dubious part I first tested that had delicate, precision 1-nozzle-width-thick walls involved, but on my M8 bolt printed parallel to Z-axis, it's amazing. Zero-clearance nut is so tight it can't be turned by hand, and I expected it to snap the bolt with tools, but it just makes a lot of noise and goes all the way on. I did it with 0% fan and did get some gunk stuck on some of the threads, so I'll try again with 40% or so. $\endgroup$ – R.. GitHub STOP HELPING ICE May 26 '19 at 22:07
  • $\begingroup$ And now I'm curious if I can get PLA to do this... I actually do have the PLA comparably strong with fancy geometry in the model, but going to try reducing fan speed with PLA to see if it can bond layers stronger too. $\endgroup$ – R.. GitHub STOP HELPING ICE May 27 '19 at 0:46
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PETG doesn't bond well if the layers aren't both at a fairly high temperature, as noted by the other answers. As mentioned, try reducing or simply turning off layer cooling. Additionally, try printing at a smaller layer height, or increased line width, to force the layers to bond more effectively. A final solution would possibly be to print a wall around the part, as many people without enclosures do for ABS prints, to keep the part nice and hot until the nozzle can deposit another layer on top. Works best when paired with higher bed temps. I personally had to print PETG around 25 degrees above the "max" recommended temperature to get good layer adhesion on my fleabay i3 clone.

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While 0scar was right that cooling fan hurts layer adhesion, I've continued to have problems with PETG even with no fan, regardless of temperature. I went looking for advice on the topic, and found a video by CNC Kitchen emphasizing the importance of tuning extrusion rate because of the compressibility of the material in the extruder gear. I'd already found this was a huge issue with TPU and other flexible filaments, so it seemed compelling, and sure enough I just measured that a nominal extrusion of 180 mm only moved the filament by 173.5 mm.

OK. Having extrusion rate off by about 3.5% is plenty to make prints brittle with PLA - I've experimented with this before just to see what would happen. A longer more precise extruder calibration showed more like 4% error. After correcting this, things were better, but I was still getting severe brittleness in some parts of the print but not others.

For a long time, I was able to mitigate most of the remaining problem with reduced speed. I had already reduced travel speeds down from 120 mm/s to 60 mm/s (my normal print speed) because PETG is sticky fast travel over it with the nozzle in contact will tear up the already-printed surface and inhibit adhesion of the next layer. (This seems to be soft PETG acting as a non-Newtonian fluid. Disabling combing, which I'd done for other reasons already with all materials, probably helped with this too.) After also reducing print speed to 40 mm/s, things seemed mostly ok. But I found recently I was still getting serious localized underextrusion in the form of entire lines nearly missing, especially after complex retractions.

I traced this problem down to some extreme extruder speed and jerk, which I'd allowed to mitigate the cost of lots of retractions and linear advance extruder moves. PLA and especially flex materials (where this matters most) can handle ridiculously high extruder speed (150 mm/s) and jerk (25 mm/s "instantaneous" velocity change), but PETG quickly starts slipping in the extruder gear when you do that, and making it easy to "lose" several mm of filament when unretracting. With this fixed (reverted to default 25 mm/s speed and 5 mm/s jerk; 10 mm/s seems to work ok too and performs a lot better), I finally have really strong PETG parts, comparable to PLA.

In the process I also tuned linear advance K factor for PETG, which could impact adhesion. I started with 2.0 which was too high, and dropped to 1.2 which was slightly too low; around 1.4 seems to be ideal. Having this too low could reduce layer adhesion right after acceleration due to localized underextrusion; having it too high could reintroduce extruder gear slippage by putting the filament under more pressure than the gear can reliably hold it to. (If a higher value is needed to get consistent extrusion, this would mean a limit on the speed would also be needed, and going at higher speeds would require an extruder upgrade. For reference, at 0.4 mm line width and 0.2 mm layer height, a K value of 1.2 requires the extruder to be able to compress the filament by about 2.4 mm to print at 60 mm/s.)

TL;DR: Fan completely off, tune extrusion rate to account for compression of PETG in the extruder gear, avoid travel over already-printed material especially at high speeds (limit travel speed to print speed), and keep extruder speed/acceleration/jerk profile conservative.

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