Fix:under extrusion after long move

Round one

2018  Jan/07

Story beigined almost one years ago. i print a box for electronics, it contian thin wall.  at the layer starting point, there is obvoius gaps, mean under extrusion occured.

Many uers running into this problem, it seems seldom resolved, or disappeared without find the real cause.  there is many factors could cause under extrusion. with pring things without thin wall, under extrusion disappeared, this indicating such a situation: the extruder itself might good enough, it's something else cause under extrusion at that starting point of layer diameter.

fighting this problem is long history and misleading by such a analysis. other situation is good enough for extrusion, only in this special scenario, problem existed.

to relief this problem, following things should be done:

1. make sure the extrude is smooth.

2. make sure the e-steps is calibrated.(for length)

3. checking the 3d Filament diameters, use flow-rate to fix diameter problem or use good filament., use single layer testing cube, the cube thickness should be the expected thickness by slicing tools (like slic3r, this will be commented in the gcode that it generated).  use flow-rate to compensate the thickness.(may be change filament diameter.)

4. lower the E-steps accelerating set, i use 100-200 mm/s^2 for MK8 direct extruder.

5. retraction speed 60, but restarting speed set to 20  [this is not true]

6. use restart extra, 0.002 to 0.005

( default retraction 2 mm, less than 2 mm will cause oozing)

Round two

@7/9  2018

this problem always come back.  recently i re-assemble the extrude several times, trying everything to make it working, but failed every time, only set restart extra seems work. this does not make sense, consideration my old i3 use wade extrude and never behave like this MK8 extrude.

2 days ago i replace total MK8 extrude with new one (the step motor is same one),  this problem  ease a little, but still there.  i had done everything i can to make sure the extrude is smooth and the nozzle is in good condition, but still can not fix it.

the suggestion retraction speed is 40 ~ 60 mm/s, i exploring a lot to find a workable solution, but it seem never get a better result.  today i increase the retraction speed to 70mm/s ,  good news is, this problem is almost disappear without bother the restart extra. 

i decide to exploring  high speed retraction.   this problem annoying me for almost 1 year.

Round Three

7/12/2008

i found it back again some how. ohh dam.

i suppose i need extra on restart again. but after a detail look to the retraction i found the reason is not the retraction itself,  extrude not retraction while move in same layer at all.  this would leaking filaments from noze while it move a long distance.

i finnally remember the option in slic3r: retraction only on layer change. i need to close it..

and b.t.w, prusa edition slict3r had different de-retraction speed option, which is key to solution to cure 'long distance movement  under extrusion' problem.  5mm length, 60mm/s retraction speed,  85mm/ de-retraction speed is also work well.  in testing 2 object separately place on a distance 18cm,  quality is still acceptable.

after a long moment, retraction still be a little bit buggy, only you try to find the thin wall thickness, it's 0.4mm instead of 0.5 mm which it should be, a extra length on restart is good, but seems not necessary, it's ok for me.