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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.

It turned out the last remaining problem was speed. PETG does not like to be printed fast, and apparently part of that is not about needing a slow-moving nozzle to bond with the previous layer, because it wasn't the fast-printed part that was brittle, but the part printed immediately after fast-printed support structure. (One plausible theory for the mechanism of the failure here seems to be filament slipping in the extruder due to trying to extrude too fast, resulting in underextrusion.)

Reducing all the speeds (including support!) down from 60 mm/s to 40-45 mm/s seems to have done the trick. This is for 0.2 mm layer height. Presumably I could go higher with thinner layers, but I've heard PETG also doesn't like thin layers, so this is probably the sweet spot. Thicker layers probably don't make sense because they'd just require proportional slowdown, at least without a larger nozzle.

In the process I also tuned linear advance K factor for PETG, which seems like it might have been a contributing factor. 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 far off in either direction could/should impact layer adhesion as well, but only with prints that use widely varying speeds or if acceleration limits are very low.

TL;DR: Fan completely off, tune extrusion rate to account for compression of PETG in the extruder gear and avoid speeds over 45 mm/s at 0.2 mm layer height and 0.4 mm nozzle diameter.

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