PETG layer adhesion
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?
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.
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.