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lars86

Feature Requests

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A couple things that I've noticed so far in printing:

  • [*]In areas with very quick reversals (<5mm or so) the machine overextrudes. This is made even worse with lower acceleration values. I could be wrong, but it seems like it doesn't take XY acceleration into account when deciding extrusion speed, so for fast XY speeds wiht low XY acceleration, it's pretty bad.

  • [*]The feed rate override on the main display modifies both extrusion feed rate, as well as rapid movements. I can't imagine why you would want these two tied together. Having the main override control only extrusion moves would be much more useful. This would let you tune printing speeds for quality, while maintaining fast rapids for shorter print time and less stringing.

  • [*]Most of my stringing happens on the appproach, rather than the departure. I am not using the "extra length on start" feature in Cura. Is there some sort of lookahead that preemtively starts extrusion?

Thanks, I'm having a blast with this thing so far!

Lars

 

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I think the over-extrusion is related to excess pressure and ooze, rather than acceleration. The move planner does keep the x,y,x and e axes in step as acceleration occurs, so I'm pretty sure it's not related to that.

I'm struggling to understand how stringing can be occurring on approach. It doesn't pre-emptively start extruding and even if it did, there's nothing for the plastic to grab on to, so worst case you're likely to end up with a blobby mess at the point where the head reaches the printed object, as any extrusion gets wiped off onto the object.

There is an optional feature called 'Advance' I think which does do some sort of 'intelligent' extrusion management - but it should be off by default. You might want to check the config files of your custom build. Maybe that got turned on?

Can you perhaps shoot some video showing the problem, so we can better understand the effect?

 

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  • [*]In areas with very quick reversals (<5mm or so) the machine overextrudes. This is made even worse with lower acceleration values. I could be wrong, but it seems like it doesn't take XY acceleration into account when deciding extrusion speed, so for fast XY speeds wiht low XY acceleration, it's pretty bad.

 

 

 

Marlin tries to move X,Y,E (and occasionally Z) together. the gcode sets the desired speed in X,Y,E and marlin splits the move into 3 sections: accel, constant speed, decel. Often it never gets to the desired speed so there is just 2 sections: accel, decel. Marlin looks at things like jerk and max acceleration and picks the limiting acceleration (usually XY, but possibly E) and limits acceleration on *all* axes so that as you might think of it, it travels in a *straight* line from one point in 4 dimensional space to the next point. If it didn't make it a straight line, then you would get overextrusion in one section and underextrusion in another section of a given segement.

So I don't think the problem is with the software.

However, there is a lag or delay in when you tell the extruder to move and when the filament comes out or stops coming out. The extruder controls the pressure in the head almost more than the actual amount of extrusion.

Maybe someday Marlin will model this but it's complicated as it depends on the type of filament, pressure, hole size, etc.

 

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  • [*]The feed rate override on the main display modifies both extrusion feed rate, as well as rapid movements. I can't imagine why you would want these two tied together. Having the main override control only extrusion moves would be much more useful. This would let you tune printing speeds for quality, while maintaining fast rapids for shorter print time and less stringing.

 

 

 

Having rapid movements doesn't reduce stringing much and there are so many other things to control - it's easier to just speed up and slow down everything. If you print from Cura you can control 3 or 4 separate speeds I believe (walls only, fill only, etc.).

You also can control "flow" live as you print. This is handy e.g. if you want the bottom layer perfect (no gaps, no overlap).

 

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I think the over-extrusion is related to excess pressure and ooze, rather than acceleration. The move planner does keep the x,y,x and e axes in step as acceleration occurs, so I'm pretty sure it's not related to that.

I'm struggling to understand how stringing can be occurring on approach. It doesn't pre-emptively start extruding and even if it did, there's nothing for the plastic to grab on to, so worst case you're likely to end up with a blobby mess at the point where the head reaches the printed object, as any extrusion gets wiped off onto the object.

There is an optional feature called 'Advance' I think which does do some sort of 'intelligent' extrusion management - but it should be off by default. You might want to check the config files of your custom build. Maybe that got turned on?

Can you perhaps shoot some video showing the problem, so we can better understand the effect?

 

If it was just an issue of ooze, I don't understand why it could be cutting back and forth, laying down infill in a 20mm wide area perfectly. Then when that width shrinks to say 5mm, massive overextrusion.

On the next print that I notice the weird stringing issue, I'll record it. It's not a huge deal, but since those rapids occur mainly in the same XY position, I'll get a series of warts on that wall.

 

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Marlin tries to move X,Y,E (and occasionally Z) together. the gcode sets the desired speed in X,Y,E and marlin splits the move into 3 sections: accel, constant speed, decel. Often it never gets to the desired speed so there is just 2 sections: accel, decel. Marlin looks at things like jerk and max acceleration and picks the limiting acceleration (usually XY, but possibly E) and limits acceleration on *all* axes so that as you might think of it, it travels in a *straight* line from one point in 4 dimensional space to the next point. If it didn't make it a straight line, then you would get overextrusion in one section and underextrusion in another section of a given segement.

So I don't think the problem is with the software.

However, there is a lag or delay in when you tell the extruder to move and when the filament comes out or stops coming out. The extruder controls the pressure in the head almost more than the actual amount of extrusion.

Maybe someday Marlin will model this but it's complicated as it depends on the type of filament, pressure, hole size, etc.

 

I am definitely noticing that disconnect between extruder drive movement and physical extrusion. It's definitely making me want to suspend the extruder drive above the machine, cut the bowden tube length in half, and have a much more direct, lower friction, filament path.

 

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If you want to get rid of stringing alltogether, around 5mm is about right for retraction. Don't tell it to "extra length on start" and keep your temperature low. See this:

http://umforum.ultimaker.com/index.php?/topic/1872-some-calibration-photographs/

 

I am finding that even with very low fan, or no fan, my temps need to be in the 225-235 range. Anything less and I see a lack of proper fusion and/or underextrusion.

 

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Having rapid movements doesn't reduce stringing much and there are so many other things to control - it's easier to just speed up and slow down everything. If you print from Cura you can control 3 or 4 separate speeds I believe (walls only, fill only, etc.).

You also can control "flow" live as you print. This is handy e.g. if you want the bottom layer perfect (no gaps, no overlap).

 

I notice a big difference in stringing in slow vs faster rapids. It makes perfect conceptual sense too: the faster you pull on that dwindling filament strand, the more likely it is to shear away. If you pull it slowly, you will get plastic deformationas you stretch it out.

 

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One would think. But I can print very slow and still get no stringing easily by lowering my temp to 190C.

I wish you would post a picture. I assume you are talking about very minor stringing at 225C. I get absolutely zero stringing at 190C with 4.5mm retraction enabled. At 200C sometimes I get stringing, sometimes I don't and it's very minor. I can print at 200mm/sec or 20mm/sec and it makes no difference.

 

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Let me clarify that - I can't print 190C at 200mm/sec. But the speed doesn't affect stringing for me - just the temp. The retraction feature is the key thing and then secondarily, lowering the temp keeps the nozzle from leaking - even a little.

 

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I am definitely noticing that disconnect between extruder drive movement and physical extrusion. It's definitely making me want to suspend the extruder drive above the machine, cut the bowden tube length in half, and have a much more direct, lower friction, filament path.

 

Yes, well this is the weakness of the ultimaker. Moving the feeder farther away means the head can move much faster. I've seen photos of other people who did just what you say - suspended the feeder above the ultimaker with string. But I still think you can get good results if you keep experimenting. Maybe your spring should be tightened on your feeder so you can get down to 200C or so? Make sure you print at lower speeds - e.g. 50mm/sec with .2 layer or 100mm/sec with .1 layer when going under 200C.

 

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