Can you explain this? Ugly surface on ABS Top flat layer, 4 benchs w/photos

[s91m 0]

Hi everyone, I joined this forum particularly because this issue.

 

I'm new on 3D printing, and bought a CR-10s, 0.4 mm nozzle last week that runs beatyful. I managed to get an UP! ABS filament (theorically 270 °C nozzle temp. required) and I made some benchmarks to calibrate nozzle and bed temp., and extrusion flow rate (discribed below).

 

As you can see, the surface of the top layer is extrange; this effect happened me in another model I printed before, and I searched for it in common 3D printing problems lists but didn't find it (I dont even know this issue name!). I made some changes between each print, but as you can tell, the problem always remained. The explanation of some parameters are justified below.

 

 

 

 

I use Cura 3.2.1

Regular Print settings are:

Layer height: 0.2 mm, Wall thickness: 0.8 mm, Top/Bottom thickness: 0.6 mm, 20 % triangles infill, 121 % Flow rate, Travel speed: 120 mm/s, ABS Slurry as bed-adhesion, Retraction distance: 5 mm, 4 lines skirt, big plastic bag as enclosure.

 

Particular Print settings are, from left to right:

1) Nozzle temp.: 260 °C (max CR-10s capable temp.), Bed temp.: 80 °C, Fan: NO, Print speed: 60 mm/s

2) Nozzle temp.: 252 °C (obtained from temp. bridge bench), Bed temp.: 80 °C, Fan: YES (255 rpm), Print speed: 60 mm/s

3) Nozzle temp.: 252 °C, Bed temp.: 80 °C, Fan: NO, Print speed: 60 mm/s but decreased to 50 % in LCD screen (I guess it is percentage, it has no unit displayed)
4) Nozzle temp.: 260 °C, Bed temp.: 80 °C, Fan: NO, Print speed: 40 mm/s, Minimum layer time: 5 sec

(Notice that the 4th cube is turned 180° over the Z axis, the issues look exactly the same when facing the proper way)

 

About flow rate: After an extrusion lack in another print, I decided to make a simple 20 x 20 x 20 print, 0.8 mm thick wall, without bottom and top and use a caliper to measure the thickness. At the end I stablished 121 % flow rate to get a 0.8 ± 0.05 mm wall.

 

Do you know what could be so wrong, that the surface finish so horrible is? Should I try to print at lower nozzle temp., even further from the suggested 270 °C? The same problem was found in a model I printed for the lab, but I dont know if its clear in this image:

 

 

Thanks in advance! I hope we can solve this together