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Almost always missing layers / underextruding

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Posted · Almost always missing layers / underextruding

It matters a little bit for the quality of overhangs, or thin vertical parts - if you're printing quickly, and the lower layers don't have time to cool, then you can get poor quality vertical/overhanging surfaces because the plastic stays above it's glass temperature for too long, and flows out of the desired shape.

But I don't think it makes much difference in terms of laying down one line of plastic to the next within a layer, no, which was what the original comment referred to, and the fact that the heater is more than capable of providing sufficient heat to keep the head hot in normal circumstances. A different challenge is getting the extruded plastic itself up to temperature in the first place, when printing fast, because the plastic spends very little time in the hot zone before being extruded, and so doesn't get up to thermal equilibrium with the rest of the hot end.

 

Hmm

 

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Posted · Almost always missing layers / underextruding

Underextrusion strikes again :(

I printed the Blockbot (http://www.thingiverse.com/make:15641) scaled at 0.5 and one or two layers didn't print correctly. Later it went on flawlessly.

underextrusion

 

I thought it might be the filament but the previous model (Low Poly Stanford Bunny - http://www.thingiverse.com/thing:151081) worked perfectly.

poly bunny

What the hell?

George, if it is not too much to ask, would you print the blockbot (plate 1) at 0.5 with the orange filament? Then we'd have used the very same materials/hardware. Here are my print settings:

 

  • layer height: 0.2
  • shell: 0.8
  • bottom/top: 1.2
  • fill: 15
  • speed: 40
  • temperature: 230
  • heated bed: 70
  • retraction distance: 5.5
  • retraction speed: 35
  • expert settings - fan full on at height: 1mm

 

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Posted · Almost always missing layers / underextruding

Might it help to have the fan come on later? The original rationale for the staggered fan start up on the UM2 was that it blew too much air too close to the bed, and it basically bounced off the bed and cooled the nozzle - which could certainly cause under extrusion.

Also is combing turned off or on, in your settings?

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Posted · Almost always missing layers / underextruding

but the plastic extruded from the head cooling more quickly once it comes out, so it doesn't spread as easily.

 

aviphysics - I think you are going to be a good addition to this community. You express your thoughts well and think of things that not everyone thinks of.

 

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Posted · Almost always missing layers / underextruding

George, if it is not too much to ask, would you print the blockbot (plate 1) at 0.5 with the orange filament?

 

It's on my list for this weekend. I don't promise anything.

 

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Posted · Almost always missing layers / underextruding

Might it help to have the fan come on later? The original rationale for the staggered fan start up on the UM2 was that it blew too much air too close to the bed, and it basically bounced off the bed and cooled the nozzle - which could certainly cause under extrusion.

Also is combing turned off or on, in your settings?

 

The Blockbot has two sets that need to be printed. The first set (with underextrusion) was printed at 1mm full fan on. The second set at 5mm full fan on. I am not sure if it is related because I also changed filament for the second set. I will redo the test (and print another cool Blockbot set).

Combing was enabled.

 

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Posted · Almost always missing layers / underextruding

I've noticed under extrusion occurring completely randomly while printing flat base layers. It's always accompanied by a blob when it begins extruding again. The under extrusion lasts for only a few cm at most between perfectly extruded lines. This happens with a totally clear nozzle.

I'm wondering if there is something in the design of the UM2 hot end, that occasionally catches the material on its way through.

To support this, last night I was setting off a print, freshly cleaned nozzle by means of pulling it apart, manually inserting a stick of PLA at 230, cooling to 90, then manually pulling the stick out, taking any gunk with it.

I aborted the print because when the material began to flow, it balled up a bit on the nozzle, I decided to wipe it off and start again.

Then, no extrusion at all, not even heating to 260 helped... how could my newly cleaned nozzle be blocked??

I tried moving the material backwards via the click wheel, and it was stick in that direction also. I pulled the hot end apart, and found the nozzle was clear, but the material was caught on something just above there the bowden tube inserts. Bowden tube was properly seated at the time.

There were no other factors causing under extrusion here, once I cut off the bit that was caught, I fed the material back in, and the print completed flawlessly.

Is the feeder slightly under torqued perhaps?

 

 

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Posted · Almost always missing layers / underextruding

Where exactly was the filament caught? Photos or video of it failing in the manner you described would be really helpful for troubleshooting purposes.

The torque delivered by the extruder is dependent primarily on the current supplied to the motor; and that is software configurable on the UM2. The challenge in finding the right balance between applying torque, and not grinding away the filament when it doesn't move as much as you might like.

I think there's fairly clear evidence that there is less effective driving torque on the filament in the UM2 than there is in the UM1; simply because it is designed to skip steps, rather than grind the filament. But it might be possible to fine tune the settings on the UM2 a bit, so deliver additional force without crossing the limit to the point where it can tear up the filament.

 

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Posted · Almost always missing layers / underextruding

Sorry, I didnt take any. I'm wonder if there's a slight edge where the bowden tube seats, such as the thickness of teh bowden tube wall itself, I may be wrong as to where it was actually caught.

 

 

Where exactly was the filament caught? Photos or video of it failing in the manner you described would be really helpful for troubleshooting purposes.

The torque delivered by the extruder is dependent primarily on the current supplied to the motor; and that is software configurable on the UM2. The challenge in finding the right balance between applying torque, and not grinding away the filament when it doesn't move as much as you might like.

I think there's fairly clear evidence that there is less effective driving torque on the filament in the UM2 than there is in the UM1; simply because it is designed to skip steps, rather than grind the filament. But it might be possible to fine tune the settings on the UM2 a bit, so deliver additional force without crossing the limit to the point where it can tear up the filament.

 

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Posted · Almost always missing layers / underextruding

Sometimes when I load a new filament it gets stuck somewhere inside the head - not sure where. I retract with the menu a cm or so and then reinsert a few times and it usually gets past the sticky spot. But then everything is fine. Not sure what the filament gets hung up on but it could be the same thing.

 

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Posted · Almost always missing layers / underextruding

Is it possible then, if retraction length is too long, it might catch occasionally during a retraction? All the under extrusions I saw while watching my last print (happened 3 times printing the base layer) happened during retraction.

 

Sometimes when I load a new filament it gets stuck somewhere inside the head - not sure where. I retract with the menu a cm or so and then reinsert a few times and it usually gets past the sticky spot. But then everything is fine. Not sure what the filament gets hung up on but it could be the same thing.

 

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Posted · Almost always missing layers / underextruding

Ok, I have a failing print in progress, pics attached... all bottom layers are perfect, until retraction begins, why then does everything begin to under extrude?

Also, why does cura print the areas indicated in such a direction, hence needing many retractions?

Uex 02

Uex 01

Uex 03

 

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Posted · Almost always missing layers / underextruding

@gr5 & @braddock

Have either of you pulled the filament out of the extruder when it was 90C and let it cool without touching it? Is the exterior smooth (aside from feeder knurling) or does it have a texture to it or a ring? I was watching some of the Ultimaker Q&A presentations about the hot end V2 by Martijn, and after seeing your two comments it may be that there is some abrasion inside the section of the white teflon section the bowden tube mates with.

eArxXvx.png

If there is an abrasion in this suppose to be cylindrical walled section of the head then it seems to reason that if a bit of material is retracted it could cool up there and get snagged in the abrasion with the snag being hardened plastic in the shape of the abrasion. It may not be an abrasion either, but a small deformation at either end of the bowen/printing head interface. This could also explain why others can do so many retractions with out any problems while only some have problems, gr5's abrasion may be shaped differently so it only snags the tip of the filament while it's being fed through the bowden tube.

@gale posted this image in this thread:

Filament_stucked_annotated2.jpg

http://umforum.ultimaker.com/index.php?/topic/649-very-high-friction-in-the-bowden-tube/?p=5039

If there is significant knurling on the filament this may negatively interact with an abrasion inside the white teflon piece and attenuate movement of the filament through the hot end, and cause moments of under extrusion.

I understand the low friction coefficient of teflon and such, but if there is a physical abrasion as a function of a cut or chunk in the material this may be enough to distort the inside diameter of the hot end to cause all types of problems.

 

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Posted · Almost always missing layers / underextruding

Thanks Aaron, I think it has to be something like this. Here's the white PLA I removed before doing the print I posted above in orange, there's a definite ring above an expanded stump on the end.

I'm not sure what all of this means? do I need to change my Bowden tube? or the Nozzle? something else?

 

filament

 

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Posted · Almost always missing layers / underextruding

Interesting ideas. Perhaps making a sort of hook by bending a needle could be a good way to detect if there's a ridge in there? The sharp point of the needle would be likely to catch on any irregularities.

Also check the teflon piece. On the original some people have seen the end of it become deformed with time so that the hole becomes smaller.

 

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Posted · Almost always missing layers / underextruding

Ok... I think I'm figuring this out. I'm guessing the ring / back pressure bulge is caused simply by play in the bowden tube. There is atleast a few mm of play in mine, it simply doesn't stay seated flush with the white teflon hot end.

This causes a gap, when the filament is retracted back into the tube, over time, the outer surface of the material gets caught on the edge of the bowden tube causing friction, this shaved off material gets pushed down towards the hot end, forming the shape I posted above. From then on, it's a knock on effect, firstly under extrusion occurs, and eventually the filament grinds down at the feeder.

This explains under extrusion even though the nozzle it completely clear, it also explains why some people aren't having the issue, I'd say their bowden tubes are seated properly without any play.

So... how to get around this? how do you make sure your bowden tube is totally seated all the way down, and how do you eliminate any play?

Another pic attached after pulling the head apart... again!

 

swell 02

 

 

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Posted · Almost always missing layers / underextruding

Lol, we might have a winner! Well at least we may be on the right track :smile:. So in the screen shot I posted before, it looks like the bowden tube is suppose to fit concentrically within the top portion of the white teflon piece; I understand the screenshot is for the V2 hotend for the UM1, I think it stands to reason yours should fit inside there as well. Is there a way to mark the bowen tube to see how far it is going below the aluminum plate? This could help aid in confirming the bowden tube is seated in the inner-inner diameter of the white teflon piece.

 

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Posted · Almost always missing layers / underextruding

How do you go about re-assembling your hotend braddock? If you're not already doing it this might be a step that helps.

Re-assemble the head so that it is basically all together but don't fully tighten the four screws, leave maybe 2-3 turns on those. It's hard for me to say exactly how much because i've only ripped the hotend apart once so far. Then push down firmly on your bowden so that it bottoms out in the teflon piece. Once you're sure that it has gone down as far as it can, tighten the four screws. This will put more upwards pressure on the tube so that it pushes firmly against the locking ring thus making it dig in nice and tight into the bowden.

A trick to make sure your bowden is really bottomed out is to insert it while the hotend is taken apart (when you can see things clearly) and put a mark on it that lines up with the top of the teflon piece.

I'll admit this is "old school" stuff from the UM1 but I doubt it's not applicable here.

edit: Also check the metal "teeth" inside the white locking ring and make sure they're clean and check the tube for damage that might prevent them from getting a good bite.

 

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Posted · Almost always missing layers / underextruding

That's correct Aaron, it's meant to seat directly in the inner diameter of the white teflon part.

Robert, the main issue doing that are the 3 black plastic stacked spacers between the hot end, and where the bowden tube enters the head, it's a juggling act keeping it all together while you force the bowden tube into place. I actually tape them together to keep them as once piece once I've seated the tube. If the bottom piece were clear plastic, you could at least see a little better.

Anyway, it's not seating it that's the problem, it's keeping it there. There is too much play in the the clips keeping the bowden tube seated. Just 1mm of play is enough to cause the issue here.

I'm going to turn retraction off and run the same print again. I know it'll be horrible, bu tmight go to proving this is the problem.

 

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Posted · Almost always missing layers / underextruding

Could you take a photo of the spacers? I'm interested in what you are describing.

Edit: When you get a chance, I didn't know you had put it back together.

 

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Posted · Almost always missing layers / underextruding

What I suggested was to prevent that play from happening by putting upwards force on it and thus pressing it firmly against the "roof" of the topmost black housing piece.

Let's try it this way then. Assemble the head fully as you normally would. Now, undo the four bolts so that you get a mm or two of play, now press down on the bowden so that it moves down that same distance (the locking "horse shoe" should still be on the white ring so that it can only move downwards), then tighten up the screws again. That should force the teflon to push up on the bowden, the white ring should be forced up with the bowden as well and eventually hit the "roof" so that it is locked in place and can't move up and down anymore.

 

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Posted · Almost always missing layers / underextruding

Thanks Robert, I'll give that a go in the morning, just set off a 2 hour no retraction test. It's after midnight here, time to sleep.

Aaron these are what i meant, spacers is probably the wrong word.

 

spacers

 

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Posted · Almost always missing layers / underextruding

BTW braddock the explanation of your hypothesis above is really good. If you think the interior of the bowden tube is sharp enough to shave away little bits of filament, do you think the tube could cut into the white teflon interface if misaligned in assembly? I don't know how easy it would be to misalign the bowden tube during installation. They are like materials are they not, both teflon?

 

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