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IRobertI

Weird underextrusion problem

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Ok, this will be a long post so get comfortable or run away now while you still have the chance.

A few weeks ago I started having problems with my prints starting to look like they are underextruding after about 1-1.6mm of the print is done. The top of the pic below shows the print after about 3 layers where everything looks just fine (yes, it's overextruding a bit here but try to ignore that as I was trying to overcompensate here), looks like it has always looked. The bottom of the picture shows what starts to happen after 1-1.5mm into the print. As you can see it's showing signs of underextrusion for some reason. When it's doing the perimeter passes I can see that the inner pass (which goes a bit faster) is a fair bit thinner than the outer pass (that goes a bit slower). Before you say that I'm printing too fast, here's the thing, this print is actually done significantly slower than I normally print. This is at around 50-70mm/s which as we all know shouldn't be a problem at all for an Ultimaker (and hasn't been in the past).

print problem

I've also included a couple of vids showing what's happening. First vid is the first few layers (again, please ignore the slight overextrusion here) and the second shows when it starts messing up.

 

 

This patterns repeat on every print. I start a print, everything looks fine until it starts messing up. Stopping the print, popping off the bad print and immediately restarting everything again looks fine until it starts messing up again. illuminarti's suggestion was that there may be pressure building up that eventually causes the symptoms. To try and rule that out I paused the print when it started looking bad, lifted the nozzle for a few seconds, cleaned any dribble, lowered the nozzle and then unpaused the print. No change, if anything it got slightly worse since some of the plastic dribbled out of the reservoir.

gr5 proposed that the issue was backlash as one can see two lines tighter together, then a space and then two lines closer again. While this might be making the issue look worse it cannot be the primary issue here. If it was it would be consistent from the first couple of layers onwards, this is not what is happening. Also, when looking at the lines the top of them are not flat as you would expect, instead they are more round. This, to me, screams underextrusion. You can see in the first vid that it's really squashing the tops of the lines (because of an overcompensated diameter setting for this particular print) until it... doesn't.

Here's a list of some of the stuff I've been trying to fix this:

 

  • [*]Yes, I've tried more than one model. My main testmodel during all this has been the hollow pyramid that I've printed in the past without issues.

[*]I've tried "old" cura, cura with the new slicer and KISSlicer.

[*]Temperature: 190C - 230C (always printed just fine at 195-200 before, hell I've been down to 185C at times)

[*]Filament: Cheap crap from China, Faberdashery, Colorfabb

[*]New pullies (awesome job on those foehnsturm)

[*]New belts

[*]New blocks, namely the "banana blocks" by MoonCactus (my wooden ones where starting to break).

 

[*]New hotend, new bowden.

[*]Tried swapping the extruder driver with the z driver in case it was starting to overheat or something after a while.

[*]Tried both the old pressure plate and the new official Bertho-inspired pressure plate (yes, the bolt is clean).

[*]I'm direct driving and even tried replacing the couplers for that.

[*]The axes are as square as I can make them (I think I got down to 0.1-0.2mm difference).

[*]Belts looks to be about the same kind of tightness as in the instruction vid on the wiki. More importantly they feel as they did when my printer was working.

[*]Bed has been re-leveled several times (and shouldn't matter once past the first few layers regardless)

[*]During the retrofit of the hotend and new blocks everything was cleaned and ligthly oiled.

[*]Tried printing with and without the fans running (on the head that is) in case they were making the temperature drop suddenly.

[*]Speaking of temps, they seem stable enough, varies about 5C during the print.

[*]Firmware has been updated to the latest one that comes with the cura beta. I lowered the acceleration back to 3000 though, 5000 is a bit too aggressive IMHO.

[*]Measured my steps per E and it's spot on. I should mention that that's with the Bertho inspired pressure plate. Not so much with the old one as it puts much more pressure on the filament.

[*]Measured the Z accuracy with a digital caliper thinking that something went wonky at that magic 1.5mm spot. Moving in 0.1mm steps it's accurate down to a couple hundreths of a millimeter, as you would expect.

[*]Nozzle looks clean when I unscrew it and look through it.

[*]No leakage around the PEEK of significance. I can see a slight green tinge but that's it. I've run the old style hotend with more leakage and gotten better prints.

 

 

At this point I'm about ready to toss this thing out the nearest window because I'm just fed up with trying to get it to work. I never thought I would say that I miss having a popping bowden... Any help would be much appreciated.

 

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Take a look at your filament when it starts going wrong... and look at the teeth mark spacing (as discussed here...). Are you seeing reduced spacing, or other problems perhaps related to over-aggressive retraction? Whether that is the cause or the effect would still be open to debate, but it might at least give you a way to quantify the effect that you are seeing.

 

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I agree with all your conclusions. The backlash isn't the main problem.

The only thing that would get worse the higher you go seems like it would be z axis related. Other posters have had issues where the z axis kind of wobbles even with a slightly misconnected coupler. This would explain it printing find for the first 1mm or so but if this is the actually problem it should get good again around height of 3mm.

I know you said you tested this. But maybe you didn't test it at 1mm off the bed? Maybe it's difficult to measure?

1) To test for this I would print something taller than the height of the threads on the z axis. Print a 10mm on a side hollow cube. Print it fast and hot: 240C, 100mm/sec, .2mm layers. Make sure you print the full 10mm height. Does the problem come and go 2 or 3 times as you go up the side of the cube?

The above test would also check to see if something is non-linear. For example maybe the bed tilts funny at the 1.5mm mark. For example if something is hitting the bed or pushing on it or something. But as you keep moving the z axis it should get better again eventually.

Theory 2 is underextrusion. But what the hell? How could it work fine for 4 or more layers and then start to go bad?

Check teeth marks? (like illuminarti suggests)

Bad temp sensor? (maybe the temp sensor is off and your print head is actually at 150C and you don't know it?)

Stepper gets too hot? (maybe after the extruder stepper gets up to a certain temperature it gets too hot and starts missing steps???) You could test for this by swapping the stepper maybe? And swapping the driver for extruder with x or y driver.

Try pulling on the filament while printing both near the bottom when things are good and again when things are bad. What I mean is fight the extruder, try to keep it from extruding. When things are marginal you should be able to prevent it from working. When it's printing fine you should not be able to.

 

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Hey guys, thanks for the suggestions. I haven't really tested much since all it does is get me into a bad mood at this point but I did try a longer print. I printed a 30mmx30mm cube, solid to more easily see the problem and put a lot of plastic through the printer. I stopped it at a height of 12mm. I logged the temperature every 20 seconds during the print and the graph below shows the results. A few funky dips but we're only talking about a max difference of about 5-7C at those extremes

To check the temps I taped the temp probe of my multimeter to the top of the block. It's showing a lower temperature than the on board sensor but that doesn't surprise me since the multimeter probe is just touching its tip to the block while the on board sensor is embedded into the block. Regardless, 200(ish)C should still be enough.

I checked in on the print periodically and the only conclusion I could draw was that once it goes bad it doesn't seem to get any worse as the print goes on.

chart 1

I'll try to get around to testing your other suggestions eventually.

 

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Ok, a little update. I tried taking a pic of the filament after printing but it's pretty hard to get a good shot of it. It's two pictures side by side the top left went into the printer first.

filamentmarks

I checked the temperature of the extruder motor after the print and I'd say it was about body temp or so, certainly not overheating by any means. For longer prints I know that it gets hotter so I don't think it's an underpower problem.

I tried stopping the filament by squeezing it really hard between my fingers and I couldn't stop it fully, at least I can't see any signs of slippage on the filament. I squeezed it hard enough that I'm still feeling it in my fingertips the day after.

I decided to take the hotend apart again just to make sure the bowden wasn't restricting anything. The filament runs through smoothly. I checked the PEEK, still clean. Checked the nozzle opening, can see right through it. I tried feeding PLA straight into the hotend while it was all detached and it seems to work. I'm not sure how much force is needed here but it squirted out the other end at least.

I've already tried switching the drivers, z-axis <-> extruder

I don't have any spare steppers to try changing out the one on the extruder.

 

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I tried stopping the filament by squeezing it really hard between my fingers and I couldn't stop it fully, at least I can't see any signs of slippage on the filament. I squeezed it hard enough that I'm still feeling it in my fingertips the day after.

 

Ouch. Someone posted that the feeder can go up to 22 pounds of force on the filament when built properly. In other words if there is no other force (filament hasn't reached feeder) you could hang a 22 pound weight on the filament and it would lift it up.

What you wrote sounds like the feeder is working fine.

 

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Another theory (if you can't keep coming up with theories, you will never come up with the correct one):

One of the cura settings is to print the bottom layer slower. I've noticed that even though one would think only the bottom layer is at this low speed, that in fact what Cura really does is increase the speed a little bit on each layer until by the 5th layer or so it's finally up to full speed.

If you are even slightly musical hopefully you can hear pitch - musical pitch. Listen to the pitch on each layer and see if when the printer is up to full speed the layers are still looking good or if they get worse as pitch goes up. Also if you have a UC or Cura, lower the speed to 50% once it gets bad (underextrusion is obvious). See if that helps.

But you are thinking: But I used to print faster just fine so what's different.

I'm still thinking speed and nozzle hole size. Either your nozzle is now down to (for example) .38mm instead of .40mm due to a very very thin layer of crud. Or your temp probe is now permanently off by a mere 20C. I know you said you tried up to 230C but it didn't make *any* difference? Try 240C and 30mm/sec printing. See if either of those help and do a comparison to a print you know for sure wasn't working well.

 

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Measured my steps per E and it's spot on.

 

Another thought - somehow your steps/mm isn't what you think it is.

 

To be absolutely sure you are using the correct steps/mm I like to put it in Cura preferences which then puts it into the first line of very gcode file. This overrides any eeprom settings. Anything from 800 to 900 should be fine. I use 836. Even using 750 would be only off by 10% and it looks to me like you have more than 10% underextrusion.

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I use KISSlicer and have set the steps per E manually in there so it's added at the top of every file. I use 836 as well.

And a couple of pics of the prints I did today. First one is 20mm/s at 220C and the second is 20mm/s at 240C. It might be a bit hard to see due the colour but if you look at the far right you can see the same problem again. It's slightly better in the middle but I could see it was starting to mess up again.

The 240C one looks terrible :D I think it got hot enough to start bubbling the PLA hence the rough looking surface. When I paused the print and purged the nozzle a bit I think I could see slight bubbling.

20mm/s 220C

20mm/s 240C

 

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Well that print at 220C and 20mm/sec looks pretty much perfect. I don't think I can do any better than that. So does this mean you are all set? You are done now? Certainly if you print much faster (e.g. 100mm/sec) you can probably up the temp at least a little - maybe 225 - as the plastic will have less time to get up to temp.

 

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