I have a strange thing going on when I print flat pieces with ABS. For instance, a 2.0 mm flat base for an electronics enclosure gets printed in this order on my rig (10" i3v):

  1. Base layer, 0.4 mm thick, printed on 45 degree diagonals
  2. Layer 2, 0.2 mm, on the other 45 diagonal.
  3. Layer 3, 0.2 mm, back to the first 45 diagonal
  4. Layers 4, 5 and 6 are honeycomb infill
  5. Layer 7, on 45 degree diagonal, has to "bridge" the openings in the infill. This is where the excitement begins...


Instead of sagging down into the honeycomb voids, the layer 7 threads bow upward. My first impression was that heated air was getting trapped and puffing up. But now I don't think this is the explanation because the worst threads for bowing occur before an individual honeycomb cell is even starting to be closed up.

This bowing up is worse towards the middle of an object, where the diagonals are longer, than nearer the corners where the diagonals are short. This layer ends up having the texture of alligator skin, with lots of little sharp peaks.

The ABS cools in the upward bowed position. When layer 8, which swaps diagonals and crosses at 90 degrees, goes over the layer 7 threads, the nozzle hits the hardened plastic. I can hear and feel the collisions. Layer 9 swaps diagonals again, and is not quite as rough as the previous two. But the finished surface does not look very good.

My present working theory is that hot end (0.4 mm hex) is becoming cooler over the course of placing the long diagonal threads. This is based on the observation that the short diagonal threads don't have the issue. There is a brief stop at the end of each diagonal where there is no filament pulling heat out of the hot end. (I have printed the ABS up to 250F and still had the problem, FWIW.)

I'll also mention that the problem is much worse with ABS than PLA. My PLA bases are a little rough, too, but much more acceptable than the ABS.

No doubt someone else has seen this. What is causing it? How do I fix it?

Jim