Messy top layer with PETG filament?

Question

I'm having issues getting PETG to print nicely. I have encountered pretty much every issue because when I fix one thing another issue pops up. I see people saying that you should "just copy your PLA settings", but that definitely did not work for me, with issues from bed adhesion, stringing, globbing, and especially issues with the top layers. At the moment I am getting very decent quality prints from PETG, however the one issue that remains is the top of the print.

Infill looks fine, it is printing fast, so that is where I would expect the filament to glob to the extruder. But that happens on the very first top layer. The printer goes over the honeycomb, and the filament gets oozy and starts forming in globs on the edge of each inner wall. This takes a few layers to print over, and even after 5 layers at 0.25 mm layer height it has holes.

Software: I am using Slic3r PE 1.41.2, on Repetier Host 2.1.3.

Printer: Wanhao DI3 or Monoprice Maker Select Plus.

Filament: Amazon Basics branded Navy Blue PETG.

Settings:

• 40 mm/s print speed,
• 0.2 mm layer height,
• 70 °C heat bed,
• 235 °C hot end,
• 15 % honeycomb infill,
• 20 % - 50 % automatic fan speed.

I would like to solve this without increasing infill percentage because the parts are already quite dense with 15 % honeycomb and I don't want to waste material and time.

Answer 1

Five top layers should normally be more than enough to create a seamless top layer.

Indeed, PETG prints a little differently than PLA. It requires a higher hot end temperature, less part cooling (to improve sticking to previous layers), a higher build plate temperature and usually care in choosing the right initial layer height. Once the printer needs to create the top surface layers, too much temperature, too less cooling and too low of an infill percentage can cause the top bridging over the infill to fail. Your settings seem to be fine except for the low percentage of infill, 15% is very low.

Other possible causes could be under-extrusion and too fast printing, but in this case the low infill percentage is probably the main reason. To get a better top layer you could first try to increase the infill percentage. If you go to 20 or 30 % infill, you would only marginally "waste" filament. More filament and time is wasted when whole prints fail as of a bad top layer.

Answer 2

I have been having the same issue with PETG when printing but I have found that if I slow the printer down to 50% speed that left me with a perfect top layer. I have a 20% infill with 4 top layers and at 50% speed I am printing at roughly 32mm/m. I am sure that will help give you better top layers.

Answer 3

If you're using Slic3r, check what line widths it's using. By default it extrudes with dynamic widths up to considerably wider than the nozzle size. PETG is only borderline-printable at 40 mm/s with 0.4 mm line width and 0.2 mm layer height. If Slic3r increased some of the line widths to 0.55 mm or so like it typically does, you're likely getting underextrusion, and need to either go slower, constraint the line width narrower, use thinner layers, or find some way to increase the amount of material you can push through the nozzle per unit time (maybe going hotter or using a higher-power hotend).