z-seam alignment and bloobs

[burtoogle 516]

Hi @gr5, following on from what you are saying there, I noticed recently something that quite surprised me. I was printing a first layer where some wall outlines were printed slower than others and I was repeatedly getting an underextrusion at the start of a wall that was printed quite a bit slower than the wall outline that preceded it. As we know, the extruder cannot do a step change in extrusion rate so this underextrusion could be expected but what surprised me was that there was a retraction and prime between the printing of the first wall and the printing of the second.

 

So that experience tells me that under/over extrusion due to step changes in extruder rate can occur even when a retract/prime occurs at the rate change point. Think what that means if, say, you are printing infill before walls and the infill print speed is a lot higher than the wall print speed. Even with a retract/prime between the infill and the first wall, it's going to overextrude when it starts that wall. If you want quality output there's a good argument to print all walls and infill at the same speed (probably skin as well).

[gr5 2,230]

Yes.  That makes sense to me.  When you retract you relieve the pressure and when you un-retract it goes exactly back where it was (if it is fast enough).  Retracting shouldn't actually suck filament up through the nozzle - that would add air inside the nozzle and be bad.

 

Feature X: So I was thinking the speed changes should be made more gradual in cura.  When it's doing one shell and it knows the speed of the outer shell it should do a fixed seconds gradual speed change.  Or maybe it should do a fixed distance gradual speed change.  Say either 10 seconds or 300 linear mm?  Or configurable?  This thought was over a year ago.

 

Then I was printing something with these multiple speeds (default profile) and it looked to me like it was gradually slowing down before printing the outer wall.  But I'm not sure if this is true.  I looked at the part in layer view with colors for speeds and didn't notice anything.  I didn't actually look at the gcode.  Could feature X already exist in Cura?

 

[burtoogle 516]

4 minutes ago, gr5 said:

Could feature X already exist in Cura?

 

No, it doesn't already exist. The new bridge code does some acceleration stuff to try and stop underextrusion after a bridge wall line but there's no general ability. Some printer firmwares (Marlin, RepRap) have the pressure advance feature that modifies the extrusion rate to try and avoid fat corners due to low acceleration but that probably doesn't work when there's travel moves and/or retractions between the print moves so maybe an experiment is worth trying.