My PETG prints keep clumping up and stringing. The print is otherwise fine, but there are so many imperfections because of this. It looks like the clumping comes from the infill.
I tried replacing the nozzle with a hardened steel one and fiddling with the settings, but the issue keeps happening.
How do I resolve this issue?
The color of these clumps suggests your PETG is burning. Dark clumps, heavy stringing, that totally feels like overheated material. Try to check if the nozzle really is 240 °C because it looks like quite a bit more. Possibly the thermistor lost some physical contact with the heater block and reads way less than it should.
PETG really likes to stick to nozzles, especially when the nozzle moves over already-printed material in the same layer, which can pull it back up and drag it around or leave it stuck to the nozzle for a long time until it eventually comes off somewhere it shouldn't be, often blackened from spending a long time in contact with heat and air, or from getting mixed with carbonized gunk that was already on the outside of the nozzle.
Your picture shows grid infill, which is a prime suspect for the root cause. Grid, triangles, and cubic are all self-intersecting infill patterns where the nozzle crosses back over lines that have already been printed. At the intersection points, the slicer does not make any attempt to compensate for double extrusion, and in fact there really isn't any good physical way to compensate without some fancy Z motion or stopping and starting that would be prohibitively slow on most printers.
Non-self-intersecting infill patterns like gyroid (any modern slicer) or full honeycomb (PrusaSlicer & derivatives) should go a long way to mitigate this problem.
Some users also find higher temperatures and/or lower infill speeds solve the problem by giving the needed time/heat to cleanly melt through the original extrusion line when crossing it the second time.
