The JKN Advance K2 parameter further corrects for over-pressurization in the extruder during the deceleration phase. This is because the pressurization during the acceleration phase and the depressurization during the deceleration phase are not symmetrical in effect.
JKN Advance K2 is used to reduce blobbing and splaying in the corners of boxes and at sharp angles between line segments.
To tune this value, begin by tuning JKN Advance K first as per Section 5.3. When tuning K2, leave K set to the value you arrived at from that step: do not set K to zero when tuning K2 unless you found zero to be the best choice for K.
Start by printing a calibration box with K2 set to 0 and 100% infill. Look at the corners: you will see they protrude in the direction of the filament travel. Increase K2 in units of 0.001 and check again. Keep increasing JKN Advance K2 in units of 0.001 until the corners are pulled in as much as possible.
When JKN Advance K2 is set too high, you will begin to see breakup in the top surface of the box.
Note that the firmware’s default value is 0.055 (Replicator-style printers) and 0.004 (Thing-o-Matic, Cupcake).
All four boxes shown in Figure 5.11 have K=0.00850, the default value for K on a Thing-o-Matic. The leftmost box, K2=0.0000, shows the corners when K2 is not applied. The second box with K2=0.0050 shows a little improvement with the two upper corners not projecting quite as much. The third box with K2=0.0100 is just beginning to show some of the infill plastic pulling away from the perimeter loops. This is quite noticeable for the final box with K2=0.0300. A good value of K2 is likely to be found between K2=0.0050 and K2=0.0100. Note that the differences will be more noticeable to you when you have actual boxes in your hands which you can look at from all angles. The pictures here do not fully illustrate the effect of K2 on the prints.