PETG filament doesn't stick reliably to fake Buildtak surface I just got my first spool of PETG and tried to calibrate an Ultimaker Cura profile for it on my Ender 3 today with limited success. 

I've had severe issues with the filament not sticking to the build surface, instead balling up around the edges of the nozzle tip. (rather like this post) Sometimes it sticks alright, but that happens properly > 50 % of the time. I've never seen this behavior with my PLA materials.

So far, I've only been trying to print this calibration part.

Relevant Profile Params: (let me know if I need to post more, I think these are the relevant ones)


Layer Height: 0.2 mm
Temp


Nozzle: 240 &deg;C
Bed: 60 &deg;C

Print Speed: 60 mm/s (more specific params left to auto calculation)
Travel Speed: 300 mm/s (max)
Cooling fan


Initial: 0 %
Fan Speed: 100 %

Retraction


Distance: 5 mm
Speed: 50 mm/s

Skirt min length: 250 mm


I was initially going by advice from Thomas Sanlanderer's video on PETG, starting with 230 &deg;C/70 &deg;C, but when that really didn't work, I then tried the advice of a Reddit user (can't relocate thread) that said to try a lower bed temp. I then experimented with different bed temps in the 50's to mixed avail. Most other threads are talking about PETG + glass, which seems to work well with a PVA glue stick surface finish.

I'm getting a glass build surface soon (as soon as GearBest can ship it from across the pond), and I hear that will help, but in the meantime, I want to find a way to make it work with the fake Buildtak. What can I do to try to make this work better? My thinking is that the issue is with the bed config (temp, surface, etc.) and not with the nozzle temp, but I could be wrong.
I have printed kilometers of PETG and found the sweet-spot for my brand to be 240 &deg;C for the hotend and 70 &deg;C for the build plate (for my Ultimaker 3 that is, the extruder temp is 5 &deg;C higher for my home build HyperCube Evolution). The reason for the 70 &deg;C is that the glass temperature of PETG is around 70 &deg;C. The PETG is flexible at that temperature such that there are no stresses because of shrinkage causing the PETG to keep attached to the heat bed surface (aluminium, glass, Buildtak, etc.). A little PVA based glue (stick) or spray (hair or specific print sprays) can even further improve the adhesion. A slow first layer also helps adhering better.

Note that the hotend temperature should be calibrated to the speed you are printing. If you print faster, a higher hotend tempearture is required. To determine the sweet spot for your filament you can print typical calibration towers that can be found on e.g. Thingiverse. Note that you need to manually change the G-code file after slicing of the tower or use plugins of your slicer to change the temperature at a certain level.

Furthermore, PETG does not like to be cooled by the print fan, so keep cooling fan rpm low to prevent layers not to bond (else you get a sort of string cheese print).



Edit:

I use parametric stair case style calibration prints that include the slicer print settings that are to determine the best settings for temperature, print cooling, layer size and print speed.