1 Topic by TickTock

  • TickTock
  • TickTock
  • Member
  • Offline
  • Registered: 2014-05-28
  • Posts: 263

Topic: Why do I have to reduce the flow when I reduce Z height?

I primarily use Slic3R and noticed that I get great prints at 100% flow rate when printing 0.25mm or larger Z height, but when I try 0.1mm I need to reduce to 85% to avoid bumps, smearing, and general badness.  My understanding is Slic3R should automatically adjust the flow based on the cross-sectional area of the bead.  Not a big deal but it would be nice if I didn't have to retune each time I change Zheight so I was wondering if there was some other tweak I need to do.

2 Reply by mdrVB6

  • mdrVB6
  • Member
  • Offline
  • Registered: 2013-12-18
  • Posts: 717

Re: Why do I have to reduce the flow when I reduce Z height?

IMO, You probably will never get a definitive answer on this.  Even if you did figure out why your printer does this, the next person with this issue might have different results.  The reason is that the overall calibration of the machine is affected by so many interconnected variables that it would be very hard to isolate the cause of these minor fluctuations.  For example, the combination of your extruder steps/mm, nozzle diameter, and flow multiplier might give good results and changing any one of these could make it worse.  However, there probably exists a combination of changing both that will also produce good results.  I.E., perhaps you could make up for too few e steps by bumping up the multiplier.

I have a similar issue whenever I swap out nozzles.  I have to adjust the multiplier when I change nozzles, or my single wall calibration cube comes out a little small when I go up a nozzle size to 0.60.  But I am dealing with changes in the 0.0X mm range, so I usually don't bother.  If I had a printer that I never wanted to change the nozzle on then maybe I would.

Anyway, what I'm trying to say is that while slic3r should automatically calculate the changes, there are so many variables that you may always have to make minor manual adjustments when any one is changed if you want near perfect prints.  After all, you are dealing with molten plastic pushed thru a hole, there will always be some distortions.  I hope that makes sense.  Just my opinion.

SD4 w/ RUMBA, E3D Volcano, all bearings, glass bed

3 Reply by redbarret (edited by redbarret 2015-01-26 18:47:15)

  • redbarret
  • Member
  • Offline
  • Registered: 2014-11-10
  • Posts: 520

Re: Why do I have to reduce the flow when I reduce Z height?

Yes, Slic3r calculates all that automatically.

And I think what mdrVB6 said is right that you still might need to manually tweak.

When you print at lower layer height, the nozzle is just closer to the layer so plastic coming out is squished more to a thinner layer. So
1) more squishing applied to the extruder plastic
2) the hot nozzle is closer to the print
3) slower extrusion, so plastic stays a bit longer in the hotend
Just few things.

Solidoodle 4

4 Reply by TickTock

  • TickTock
  • TickTock
  • Member
  • Offline
  • Registered: 2014-05-28
  • Posts: 263

Re: Why do I have to reduce the flow when I reduce Z height?

Thanks for the feedback.  So... Am I hearing that many of you are able to change Z height without re-tuning and therefore with some experimentation I might be able to reproduce?  Or re-tuning for thin layers is currently a reality we have to live with just because of the physics and the simplified slicer model?

5 Reply by mdrVB6 (edited by mdrVB6 2015-01-26 21:54:35)

  • mdrVB6
  • Member
  • Offline
  • Registered: 2013-12-18
  • Posts: 717

Re: Why do I have to reduce the flow when I reduce Z height?

Yes, I can change layer heights without doing an entire recalibration and get acceptable results but if you want the absolute best that your printer can do, I don't think adjusting flow rates between layer changes is too much to ask or out of the ordinary. 

Personally, I don't even have a 0.1 layer profile setup because I have yet to see a need to go below 0.2 layers and doing so would take twice as long.

Just curious, have you eliminated first layer issues (leveling, moving to fast, etc) and are you compensating for the Z lead screw rounding error by using the optimal layer heights from the prussa calculator?  Doing these things greatly reduced the instances of the nozzle running into the previously printed layer for me.

SD4 w/ RUMBA, E3D Volcano, all bearings, glass bed

6 Reply by IanJohnson

  • IanJohnson
  • Moderator
  • Offline
  • Registered: 2012-08-07
  • Posts: 2,815

Re: Why do I have to reduce the flow when I reduce Z height?

You could try running faster at the lower heights.  Low layer height means slower flow and less pressure in the nozzle.  Sometimes it seems like if the pressure is low, the plastic is more free to do what it wants, and possibly extrude a little less accurately.  You can compensate by running faster so the plastic extrudes faster to keep up.  Also the ratio of extrusion width to height is much higher, so you will probably get better results with lower layers by keeping extrusion width as small as possible, with the lower limit being the width of the nozzle.