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czucker

Decreasing quality on upper layers

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I was hoping someone could help me figure out what's going wring with the attached print.

I printed Nick Foley's motor count last night and in a few places towards the top of the print the quality decreased quite a bit.

You can see the banding that begins about 1/3 of the way up and the gaps in the wall above that.

This did not happen consistently across the layer. I don't have a photo handy at the moment, but the other side of the print looks just fine.

This was printed at 218º, 50mm/s, .2mm layer height.

Any thoughts?

Thanks,

Chris

Direct Driver Motor Mount

 

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There are two issues. You have underextrusion where there are holes and kind of dashed layers. The extruder probably slipped back a bit and it takes a little whie to start extruding again. Plus you have stringing where some strings are hanging out. For the stringing issue:

Do you have "enable combing" checked? That keeps the extruder inside the part if possible instead of jumping directly to the next location to print (and causing some stringing sometimes).

Also do you have "retraction" turned on? You want it on for this part. There are 6 retraction settings 2 of which are on the printer if you have UM2. You want it 1) checked 2) min distance: 0 3) min extrusion: 0 4) combing - checked I think - sometimes not checked is better though.

The last 2 are probably fine. On UM Original settings in cura, on UM2 settings on printer: set retraction distance to 6mm if your bowden wiggles much. Or at least 5.5. If your bowden is rock solid and tight then 4.5mm is good. Speed is probably fine as is. On UM2 I have it set to the max: 35mm/sec. On UM Original I have 40mm/sec but the max in the firmware is I think 25mm/sec.

 

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Underextrusion issue is more complicated. Not sure what is causing it. If this is UM2 then probably extruder is slipping backwards. Some people seem to have worse underextrusion right after retraction but I have no idea why. Maybe the retraction distance is too far? maybe the extruder slips a little when the retraction starts?

This can usually be made better by increasing the nozzle temp - 240C would be good. Or slowing down the print a bit. Try 50-80% of your current speed. The problem with 240C is you tend to get worse stringing at higher temps.

 

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Thanks for the advice.

I'm using the Chopmeister's H3 extruder, on my UM 1. It's been working flawlessly on smaller prints.

I'll check the spring tension and also make sure that the filament isn't snagging on the spool. That seems to happen here and there.

As for the temperature. . . . Is 240º okay for PLA? I've never gone above 220º.

 

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Yes, I print at 240C when I'm in a rush. If I'm doing .2mm layers and >50mm/sec (like 75 or 100mm/sec) I usually go 240C. But I get worse stringing. Other than stringing and some bumps it comes out fair quality - good enough for mechanical things.

I'm thinking your retraction distance might need tweaking if you have a different extruder. Maybe do a test print - two columns or something and for the first 20 layers try 2mm retraction, then 3mm retraction and so on. Mark the position where you change with a marker. Oh wait - UM Original - can't adjust on the fly. Well then print the same thing (two 10mm cubes side by side) at many different retraction distances from 2mm up to 6mm.

You want the exact right distance so the PLA goes from the top of the bowden to the bottom but doesn't actually pull back enough to let air into the nozzle.

 

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I've used my H3 extruder(with V2 bolt) at 210 C for 50mm/s (or even 65mm/s for infill) for a long time now, without any problems. I never go above 215-220.Do you have a printed bearing axle in the idler? If yes then this is probably one source of your problems (see info on thigiverse page). Use 4.5 mm retraction, like gr5 said.

Finding the right spring tension is a tricky job. If it's too tight, it will grind the filament slightly and the knurling can get all clogged up after a while, so check for that too. Also, check your temperature with a multimeter, just to see if what you have is correct. There have been reports over time of temp being way lower than what the printer reports, so that too is worth having a look at.

And finally, use an older version of Cura (12.7 or such) to check your E steps/mm.

 

 

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The horizontal banding lower down - where there isn't missing plastic, but the layers just don't line up right - is due to backlash most likely; e.g., when it is no longer printing the part on the left, the order of travel of the head changes on the subsequent layers, so that the head is approaching the printed part from a different direction. Due to backlash, it no longer lines up.

Make sure that your belts are nicely tightened - especially the short belts. But equally don't get the belts too tight - that can cause problems of its own.

 

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I'm pleased to announce that after futzing with the tension on my extruder and mounting the motors directly to the rods, my problems have disappeared.

My printer is pretty rock solid at the moment.

Thanks for all the excellent advice.

Here's a much improved print.

IMG 7338

Best,

Chris

 

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