Top Layers not touching - UM2

[Nicolinux 288]

Hi,

 

Another week, another weird problem.

Lately my prints started to feature top layers where the filament strands won't touch each other. It looks something like this:

 

At first I thought it might be due to the bed leveling beeing off - because of the big gap between nozzle and the previous layer the filament strands would get streched too thin. I posted about it here (link).

 

In the process of troubleshooting this issue I noticed that my left and right y-rods did move a bit back and forth (backlash). Was easy to fix (loosened the top most pulley(s) and moved them far out so they would touch the ball bearings, then re-tightened them). Now there is zero backlash but I still get the patchy top surface:

 

It seems to get worse with increasing z-height. The first two layers are fine (ok this is because the first layer is squashed and fills up the gaps between the filament strands), but then it gets worse:

 

I found this old post (link) which seems to indicate underextrusion (UM2 underextrusion alarm goes off!!! - where is the <blink> tag?!?), but nah, I don't think it is underextrusion, because I would see it on vertical walls too.

Any ideas about the cause? This is on an UM2 with the standard extruder printing at 230° and around 40-50mm/s.

 

[mnis 11]

Hi Nico

Well, regularly it is sufficient to increase the material flow, as far to side by side strands again have to each other connection. Even with this material may be required a higher printing temperature.

Did it previously work better? Or is the material from a new role?

Markus

[Nicolinux 288]

Hi Markus,

I'll try increasing the flow although I don't like it. Fiddeling with the flow seems to be to be more of a bandaid solution because it whitewhashes all kinds of problems and does have a few side effects too (like overextrusion visible on vertical walls). Ideally I'd like to get to the bottom of the problem and not just patch the outcome. It worked before so it must be something else that either degraded over time or something that changed (filament, slicer, room temperature).

I tried different filament brands - that's not it. I'll try different Cura version next.

 

[mnis 11]

Did I understand correctly, the problem is with multiple filament variants?

Markus

[tlrt2701 0]

I have the same, I print slower and with a higher temperature. With 0.15mm Layerhigh.

Cura 14.03 V2Test

White PLA Innofill and 220°C-230°C

Thorsten

 

[Nicolinux 288]

Same here, happens with different filament colors and brands.

 

[mnis 11]

Strangely, today I have threaded new material and now I have apparently the same problem.

No kidding, I watch just the first object more closely and ...

come ... maybe negative vibrations from you to me?

Everything on the machine seems okay and unchanged.

If it is the next print even so, then I do a factory reset.

Markus

[Nicolinux 288]

Hehe, I hope not. I am a very positive person so no bad vibes here. I do live through some sh**ty times right now though, but 3D printing is my hobby and my joy and helps me to relax (except when my UM2 toy is broken - then big drama ensues...). That's why I am so sensitive to every bit and niggle. The upside though - I hope to help to improve the UM2 through my debugging

 

[mnis 11]

Similarly, it is the case with me, I think so Nico. :cool:

I still have a lot of print in that night anyway. :???:

So, engine OFF for at least five minutes, then re-leveling, and continue printing.

If the second printing just as flawed as the first, then I do a factory reset. :arrow:

It is currently PLA INNOFILL-Green, before everything went perfect with blue.

But it seems rather to be a problem with the machine.

Markus

 

[Nicolinux 288]

For me this issue happened gradually over time. I didn't notice it first because it is only visible on flat surfaces. But it now seems to get worse so I suspect there is something degrading somewhere.

Another thing I'll try. I'll rotate the calibration cube to see if the diagonal lines behave differently at different angles.

 

[illuminarti 18]

This print really doesn't look all that bad to me, I do thin kthat you might be over-thinking things a bit :-) That said, here are some observations on your thoughts so far:

1) Bed levelling only affects the first layer or two... once you're print on top of solid plastic that has been leveled by the passage of the nozzle on the previous layer, the bed level is irrelevant.

2) Having the rods be slipping slightly in the bearings isn't, and doesn't really cause, backlash. Backlash is when the head doesn't move as much as the steppers have commanded it to, and that is usually because some of that movement gets lost in slack belts that shift or stretch, rather than moving the head fully. The axial component of the rod isn't part of that drive system, it just holds the head up in the air. When the motor moves the pulley and the pulley moves the belt, then the belt pulls the head into place. If the head can, and does, drag the rod with it as it moves, rather than merely sliding over the rod, that doesn't necessarily cause the head to not end up in the right place. The looseness needs to be fixed because its annoying, and because it could ultimately cause the rods to fall out of their bearings, and because it potentially twists the belts for the other axis out of line. But it doesn't usually directly affect print quality.

3) That said - It's possible that the problem is indeed backlash related... you have two passes close together and then a small gap which http://umforum.ultimaker.com/index.php?/topic/1872-some-calibration-photographs/?p=14474 is a symptom of backlash. So possibly tightening your short belts a little might help with that.

4) Finally, it could just be routine under-extrusion. There's a big difference between a simple mis-match between the necessary and actual extrusion amounts, and the acute extruder slip issues that have plagued you in the past. But both are under-extrusion. Have you checked that your filament diameter is still consistent with what you have set in the printer? Now that filament diameter is hidden in the printer, rather than in Cura, it's easy to overlook it.

 

[valcrow 144]

I have this problem as well, It seems to have become more noticeable over time.

The first few layers will actually be over extruded a bit, (kinda squishing up the sides) by the time it gets to the top layer, there's roughly a 0.25 mm gap between lines worse than nicolinux's picture. sometimes you can see 3 layers down. Really annoying for painting stuff.

It would be good to have cura increase the flow by % the higher it goes or something...

 

[Nicolinux 288]

This print really doesn't look all that bad to me, I do thin kthat you might be over-thinking things a bit :smile:

 

Well if you mean this image, then yes, it ok. But this is just the begining of the cube where only 2 layers have been laid down:

When the cube is finished it looks more like this and that's not ok:

 

2) Having the rods be slipping slightly in the bearings isn't, and doesn't really cause, backlash. Backlash is when the head doesn't move as much as the steppers have commanded it to, and that is usually because some of that movement gets lost in slack belts that shift or stretch, rather than moving the head fully. The axial component of the rod isn't part of that drive system, it just holds the head up in the air. When the motor moves the pulley and the pulley moves the belt, then the belt pulls the head into place. If the head can, and does, drag the rod with it as it moves, rather than merely sliding over the rod, that doesn't necessarily cause the head to not end up in the right place. The looseness needs to be fixed because its annoying, and because it could ultimately cause the rods to fall out of their bearings, and because it potentially twists the belts for the other axis out of line. But it doesn't usually directly affect print quality.

 

Ok, I understand thx.

 

3) That said - It's possible that the problem is indeed backlash related... you have two passes close together and then a small gap which we have seen before is a symptom of backlash. So possibly tightening your short belts a little might help with that.

 

Of course! I shouldn't throw my UM1 experience over the board now that I own an UM2... I didn't think of it, sorry.

Now after tightening the short belts (loosening the motor, pushing it down and tightening the screws), my cube looks like this - only a wee little bit better:

 

4) Finally, it could just be routine under-extrusion. There's a big difference between a simple mis-match between the necessary and actual extrusion amounts, and the acute extruder slip issues that have plagued you in the past. But both are under-extrusion. Have you checked that your filament diameter is still consistent with what you have set in the printer? Now that filament diameter is hidden in the printer, rather than in Cura, it's easy to overlook it.

 

Ha, routine under-extrusion sounds so cute - like holydays, ice cream and singing campfire songs in the midsummer night. Not to be confused with monster under-extrusion with extruder skipping that drives one mad..

I tried that too. Normally I print with a filament diameter of 2.9mm (and the orange filament has 2.86mm). If I set the exact diameter, the cube looks like this:

 

Again, a wee bit better, but not gone. Isn't it possible to have no gaps on the top surface? Am I imagining things? I went through some of my old prints, and sure enough there are many with tight top surfaces.

Btw. I also printed with a different orientation and the gaps are still there:

 

But I noticed that the long belts do not have the same tension. The sliding blocks are supposed also work as a belt tensioner. But how would one adjust them?

 

[illuminarti 18]

The long belts have built-in spring tensioners, but they aren't adjustable. You could just add any of the UM1 belt tensioner designs to the belts however, and see if that helps.

How many layers of solid top are you printing? Have you tried just making the top surface thicker? Or adding more infill? You may just need more layers to properly bridge the infill.

 

[illuminarti 18]

Valcrow - you say there's a 0.25mm gap visible? The threads are being printed 0.4mm apart. That means that for there to be a 0.25mm gap between each thread, the threads would only by 0.15 mm wide - in other words the threads are significantly thinner than the gaps between them? Is that really what you're seeing.

More likely, I think is that you're getting the gap every other line, as in Stefan's photos above - two close together (overlappin, in fact) and then a gap? In which case, I'd check for backlash - try tightening the belts etc.

 

[Nicolinux 288]

@Illuminarti: Printing at 0.2mm layer height and 1.2mm top/bottom -> 6 layers (without infill). The 20mm cube is fairly small, I think the UM2 should handle it properly.

I'll try some belt tensioners. I have a wide array of failed and still good tensioners from my UM1 trials

 

[valcrow 144]

Valcrow - you say there's a 0.25mm gap visible? The threads are being printed 0.4mm apart. That means that for there to be a 0.25mm gap between each thread, the threads would only by 0.15 mm wide - in other words the threads are significantly thinner than the gaps between them? Is that really what you're seeing.

More likely, I think is that you're getting the gap every other line, as in Stefan's photos above - two close together (overlappin, in fact) and then a gap? In which case, I'd check for backlash - try tightening the belts etc.

 

Hmm, I might be overestimating .25mm in that case, my brain ruler's not that good. But in the worst cases, the gap is roughly 1/3 the thickness of the line of PLA and you can just see a bit of the 3rd layer.

It's also not like a double line and missing line like Nicolinux's pictures, mine look quite accurately spaced apart. I usually end up doing like 9 layers of top/bottom fill to minimize the look of the problem but it still persists on big flat pieces.

I'll get some pics up a bit later.

 

[Nicolinux 288]

Simon,

Could you test your belts and see if then have matching tension?

thx.

 

[illuminarti 18]

They do, more or less, yes.

 

[Nicolinux 288]

Ok, thanks.

Dang! None of my old belt tensioners fit so I'll need to scout for a very thin one that won't fall off and destroy the respective axis.

 

[mnis 11]

Ha ha, I probably enjoyed myself a little too early... :???:

Because only the objects with layers of 0.1 mm look absolutely correct. :cool:

 

Objects with layers greater than 0.1 mm (0.15 mm ... with me currently) still have problems as described above.

I'm not sure, but:

It looks almost as if the machine outputs only material for 0.1 mm layer. And that is clearly visible when on the infill new surfaces arise. The increase of the material flow does nothing, because it then comes to irregularities, such as Nico mentioned.

Markus

 

[Nicolinux 288]

Just a little update. I was unable to find a belt tensioner that fits the UM2. The clearing between the (long) belt and the sliding block is just too small...

 

[SandervG 1,521]

Hi Stefan,

What about using the iron part of a clothespin?

I always thought that worked the best, and you can put it on upwards or downwards.

In one of the first posts I read about a suggestion of trying an older version of Cura.

Anyone tried it already?

Did you update it recently?

Or FW?

 

[Nicolinux 288]

Clothespin... nice. I'll try it. Gotta nab one from my neighbours

I did try older versions of Cura, no difference (other then better surface quality with Cura 14.01). But I did update the firmware lately. Another thing to try. Can I downgrade the firmware safely?

Thanks.

 

[Nicolinux 288]

I haven't been able to find a proper belt tensioner. The clothespins I found do not fit. The space between belt and sliding block is too small (tried both orientations) :/