Uneven extrusion depending on travel direction

[gaduffl 0]

Hi all,

I am kind of at the end with my ideas here. My printer cannot create a smooth surface somehow.

In the attached detail picture you can see how my first layer looks photographed from below. But the issue is also appearant on other layers.

It seems if the head moves in one direction, it extrudes a very thin line. Then, one the way back, it extrudes a normal line

but almost on top of the old one. Since my circles are also not really round, I tightened my belts as suggested in this very

good article wich also metiones the "pairing" of lines:

https://support.3dverkstan.se/article/23-a-visual-ultimaker-troubleshooting-guide#circles

Tightening the belts however did not solve the issue. The print bed is leveling is quite accurate

so I also do not really get where the differences along the lines comes from you can see in the in the overview picture.

I am already too close to the bed and still get the gaps. They kind of equal the distance of the bolt holes of the linear guide rail?!

 

Do you have any ideas what I could check? Thanks guys.

 

Nozzle: 0.4mm plated copper

Print bed: Filaprint on a precision milled aluminuim plate

Printer: Migo3D with auto mesh leveling

 

Edited November 7, 2020 by gaduffl

[gr5 2,266]

90% of your issue on the bottom layer is what we call "underextrusion".  If it occurs mostly on the first layer then it means you haven't levelled very well.  Typically the bottom layer is 0.3mm thick (it's a cura setting - initial layer height) and it prints with the Z value at the same value (in this case 0.3) which means the nozzle is 0.3mm above the glass in this example.  Except you are supposed to level things a bit tight so it's really only say 0.2mm above the glass so that you overextrude the bottom layer.  If you don't overextrude the bottom layer and get some good squish going then the part will almost surely come loose by the end of the print.

 

So answer #1: leveling problem.

 

However you said you get underextrusion on other layers so you have a more serious underextrusion issue and maybe not a leveling problem.  A typical printer maxes out at about 5-20 cubic mm per second.  So keeping it below 5 should be easy for any printer.  To calculate the number multiply your line width X layer height X print speed.  Yikes Math!  Don't panic.  It's just 3 numbers you probably already know memorized.  Line width is probably 0.4mm if you have a .4 nozzle and layer height is probably 0.1mm or 0.2mm and print speed you should know.  So for example 0.4mm line width and 0.1mm line height and 40mm/sec multiplies out to 1.6mm^3/sec.  Well under 5.  So check that first.

 

Answer #2: underextrusion.

 

Regarding some lines being thicker and the line going the other way being thinner - it's probably a little bit of backlash/play as stated in the 3dverkstan guide.  But it looks very minor in the above photos.  Fix underextrusion first.

 

Answer #3: don't worry about backlash/play - the amount you have appears minor.

[gaduffl 0]

Hello gr5,

 

thank you for the detailed reply.

 

I lowered the nozzle a bit like you suggested and got better results. Still some gaps however depsite being really close and the top layer looks the same.

 

I suspected that I have an underextrusion issue as well.

To check that, I printed a hollow cube in vase mode. So there is only one wall which should be 0.4mm thick. At 101% flow rate, I got to almost exactly 0.4mm. The caliper is a factory calibrated Mitutoyo, so it should be precise.

Also, as you can see in the overview image, the nozzle impacts the already printed first layer (0.15mm) quite severly. So that should be an indication that too much material is deposited or the nozzle is to close.

 

 

The too point above indicate that it is not an underextrusion issue. Or do you think it still could be underextrusion despite that?

What I also do not get: If it is underextruision, why only in one direction?

 

The volume I am extruding is 3mm³/s, on the initial layer and top skin even 1,5mm³/s.

 

I uploaded a picture of the top layer and also the first layer after infill as well.

The top layer is actually printed with 110% flow rate.

 

I found it interesting that on the first layer after infill, what looks like 8 lines are actually only 4 (watched closely during printing). The start and endpoint of the bridges do not seem to be aligned somehow.

 

Printing Material is FormFutura PLA at 205°C.

Edited November 8, 2020 by gaduffl

[gr5 2,266]

In the first picture just above (2 pictures up) you can see where it slows down for the end point it fills in nicely but when at full speed it is under extruding by about 30% (one third is air gap).

 

This is pretty bad underextrusion.  I don't know what is causing it.  I'd try cutting the speed in half again (I think you can do that in TUNE menu live while printing).  There may be something wrong with the printer.

 

The lower picture (picture just above) is typical of the first layer after infill.  You can see the infill pattern below.  Usually you want 6 layers above infill to get back to perfect top layer.  At a glance it looks fine.  I'd have to see the 6th layer above this to be sure.

[gr5 2,266]

On 11/8/2020 at 5:35 AM, gaduffl said:

What I also do not get: If it is underextruision, why only in one direction?

I don't know what you are talking about.  I am seeing both directions are exactly equally underextruded.  Those gaps are an indication of underextrusion and go away at the edge of the part where the print head is slowing down.

 

I have indeed seen underextrusion in the past only happen in one direction on printers with a bowden where when you tighten/loosen the curvature of the bowden it over/under extrudes but I don't see that in your photos.

[gr5 2,266]

You may be right that it is extruding more in one direction than the other but I have no idea why it would do that.  Something to do with where the filament spool enters the machine and it gets pinched or something as the head moves back and forth.  I would turn off power and remove the filament.  Once head is cool slide the filament in and out of the print head and feel the friction (ideally with nozzle removed) and then move the print head to the 4 corners of your print area and see if that friction changes.

[gaduffl 0]

Hi gr5,

 

OK I will try reducing the print speed even further and report back.

I just checked in Cura. Since the test cube consisted only of an outer wall, it was printed at only about 10mm/s. So that might explain why the wall measured 0.4mm like it was supposed to!!

 

My printer is a direct drive one and the spool is exactly in the middle above the extruder. So perfectly symmetrical. The filament is fed completely freely without any strain.  That should not affect anythin

Quote

I don't know what you are talking about. 

That was referring to the very first picture. But I think you saw it as well.

 

Thanks!

[gr5 2,266]

In the very first picture in the first post it looks like there is some surface tension between two traces that make a line between.  I suppose it *could* be a line going the other direction but that seems very unlikely.  I've never seen that problem before.

[gaduffl 0]

So I checked again by deliberately setting up the nozzle gap to high in order to see the single lines more clearly.

One can definately see that there are always two lines paired up. It seem like at one end of the line the print head does move over to the next line and on the other end it kind of stays in almost the same track it came from.

Could that be an belt tension issue?

 

What I also noted: If I print a square around my print bed fot leveling purposes, the nozzle height seems to change after a 90° corner. Looking at the motion of the axes this means:

Print head still, bed moves in Y-direction --> Layer height ~0,15mm

Then at the 90° corner:

Print head starts moving along X, bed stops moving --> Layer height ~0,1mm

 

 

[gaduffl 0]

I solved it! Or better: I found a workaround!

So obviously there was some kind of slack/hysteresis on one of the axis.

To get the influence of the axes down to one axis at a time only, I changed the "Top/bottom line directions" parameter in Cura to [90,180]. This means lines of the top and bottom layers are always parallel to the x-axis, not 45.

Therefore only one axis moves during the print of one line and tadaaa, problem vanished.