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 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 at 21:04

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.

  • $\begingroup$ 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.. May 26 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.. May 27 at 0:46

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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