What is cause of blob when layer area reduces by 50%?

Question

This is printing PETG extruding at 250 °C, 1st layer bed at 80 °C then 60 °C on other layers. Nozzle is 0.4 mm; printer is German Reprap X400. The all metal extruder is clean with no oozing. The print gets a blob when the wall width changes from 2 mm to 1 mm. The extruder squishes the blob out the sides of the wall with subsequent layers. Images below show both sides of wall. I'm looking for the cause to come up with a fix. I consistently get the same results in the same location when printing this file. The reduced area of the layer does cause reduced speed to increase the print time for the layer (speed reduced when layer would print in less than 30 seconds; minimum speed 15 mm/s).

When I printed two, there was only one blob on one of two. Below shows the speed settings. The perimeters are three vertical shells, so the walls are all perimeters. My understanding of the setting is the 2 mm wall printed at 20 mm/s and the 1 mm wall at 15 mm/s.

Printing similarly with 2 mm walls only and the same settings, the print is nearly perfect with only a few stringers. Printing with only 1 mm walls does not have the blob, but does have stringers.

See solution at the end.

Solution:

Speeding up the print speeds and using thicker walls not only eliminated the blogs, but also completely eliminated stringers, and gave a near perfect print. Apparently, the actual temperature that PETG extrudes is highly dependent on how fast it extrudes. Printing too fast clogs the nozzle because the filament doesn't have time to melt. Printing too slow causes the hotend to stop extruding from heat creep. Here, the actual temperature of the extruded PETG was decreased by speeding up the extrusion.

s://i.stack.imgur.com/Uhv61.png

Answer 1

Try lowering your extruder flow rate and maybe temperature as well.

I had this problem once and I thought it was just a bit of gunk that had been hanging onto the outside of the extruder. That is usually not the case and instead it is probably build-up from over-pressure inside the nozzle.

Also note that 250 °C will start degrading your PTFE tube unless you have an all-metal extruder. Lowering temperature can also reduce the stringing visible in the picture, however to fully, get rid of stringing, you may have to adjust your retraction and acceleration.

Answer 2

In my experience, when you see mysterious blobs on PETG (especially if they don't appear with other materials and comparable slicing settings), the cause is almost always this: PETG sticks really well, to itself and other surfaces. This means when you move the hot nozzle over already-printed PETG, especially if it's unretracted so that there's still molten PETG in the nozzle orifice, it will tend to "dig up" the material that's already there as a blob and deposit it somewhere it doesn't belong. Not only is this ugly and messy to clean up in postprocessing, but since the material was moved away from the place it actually belonged, it harms layer adhesion and makes your part brittle.

The best mitigations I know are:

• Disable combing (either entirely or effectively, e.g. by setting max combing move distance to 0.5 mm or so; the latter avoids slowing down printing of top/bottom skins). This will avoid unretracted moves over already printed material.

• Disable slicer features that omit retractions when there are "too many". In Cura this is "Minimum Extrusion Distance Window". Set it to 0. Otherwise retractions will start getting skipped at geometry-dependent points in the print, which may very well be what you're hitting.

• Keep travel speed reasonable. Above 80 mm/s is probably too fast and in my experience may dig up already-deposited PETG even if you retracted.

• Set "Minimum Wall Flow" (or your slicer's equivalent) to something like 20%, bounded away from 0. If the slicer emits near-zero-width wall lines because they're all that fit due to overlap, the move to print them is essentially a combing move (unretracted travel) because the flow is so low, and the same problems with digging up blobs arise. Better to just omit these lines and leave a small gap.