Irregular quality from one layer to the next?

[rafael-matsunaga 0]

Hello friends, I'm having an issue that got me scratching my head for a several days now, I hope you can help me out. Some layers on my prints look less dense (?) than others.

 

On the first image, you can see some lighter layers. The second one looks more homogeneous, but it's also not perfect.

 

The prints are at 100% infill, 70mm/s, 225C, 0.1mm layers, PLA.

 

I assumed it was an extrusion issue at first, but I think I've gone through pretty much all topics and tips on extrusion, to no avail. It seems very random, some layers are fine, some are bad.

 

The filament is being pulled properly and the extruder teeth marks on the filament look very regular as well.

 

Could it be filament diameter variation? Moisture? I'm waiting for a new roll of PLA to run some tests, but any ideas are very welcome!

 

 

Thank you!

 

[edis 0]

i have similar problems with my printer as well. For me the difference in height varies with each part, not sure about you. I've got an advice from one nice forum member, that it could be that it's the spool, check if it can rotate freely, without any difficulties. Maybe it'l help you.

 

[nallath 1,120]

Moisture tends to result in bubbles in your print. If your filament is not quite round (or not consistent) you can get these results.

 

[gr5 2,167]

That is very good photography! I zoomed way in and could see quite good detail!

1) I'm pretty sure those lighter layers is white PLA. It's strange but sometimes when you change colors, the other color can stay in the print head for an hour before coming out. If you use higher temperature (240C-250C ) for a few minutes while changing filaments and pushing as much filament through as possible you might be able to get the other color out sooner.

2) How did you get the white ring inside the pink part? Was that printed separately and then assembled later?

3) How many line segments do you have for each circle? Wow. Very nice.

I had a similar issue to you where I printed an orange part (pumpkin) and then later printed these 3 white people and at their waist (half way up) some of the orange PLA came loose and changed the color:

 

Too zoom in, you need to do 3 things:

First click the image, then right click it and choose "view image" then click it again. This is what I had to do to see your image full size.

 

[rafael-matsunaga 0]

Hi everyone, thanks for the replies!

Oh, sorry gr5, the white ring on top is a silicone ring, not printed.

I used yellow PLA previously, but that was a while ago. I'm halfway through the purple spool and it has looked the same from the beginning.

I'll check the number of segments when I get home later.

I'll check the spool as well, although it seems to spin reasonably well on the holder. I'll print some hollow cubes or regular parts as well, might be easier to see the problems in detail.

 

[SandervG 1,521]

Where did you get the purple PLA from?

 

[illuminarti 18]

Make sure that there is always loose plastic on the spool - if you ever get to the point where the extruder has to actually 'pull' the spool there's a chance that the filament might get caught up briefly, and reduce the extrusion amount slightly.

I generally give the spool a few turns by hand every few minutes to make sure that there's always filament available that can feed in with minimum resistance.

 

[rafael-matsunaga 0]

Sander: It's a Chinese unbranded PLA from eBay, not the best quality, apparently. I won't be suprised if the filament is to blame.

Illuminati: I'll run some tests giving the spool some turns. Thank you!

 

[gr5 2,167]

I think you need to take a better picture of the first item. And put an arrow to the spot that you don't like. I don't see any under-extrusion - it's just a little blurry on the "worst spot". Under-extrusion can be caused by many things and a "tangle" is quite common - especially if the filament is falling off the reel. Especially on a new reel. Or if you don't push the reel against the back of the UM. But... there are 10 other causes for under-extrusion.

One being dust and your objects look very dusty.

But I'm not convinced you have any under-extrusion - so please zoom in or clip the photograph closer in to just the bad area and make sure it's clear. Part's of your photographs are amazing - I can see each .1mm layer like rice farms in China. Like steps in a stair case - so perfectly flat. And I can see a weird oscillation. Maybe caused from the original STL file.

 

[rafael-matsunaga 0]

Some of the oscillation happens when I go too fast, my linear bearings are not as smooth anymore. As for your previous question, each circle on that object has about 100 segments on the larger parts and 40 on the smaller parts on the top.

I ran some more tests this morning, a 20x20x40mm hollow brick.

These are 5 tests, all at 70mm/s and 0.8mm walls, no infill:

225C 0.1mm (minimum layer time was on, oops, so probably printed at about 30mm/s)

225C 0.1mm

225C 0.1mm manually unwinding the spool, no noticeable change

235C 0.1mm

235C 0.2mm this one definitely underextruded on a couple of layers, but I could be going too fast

Here's an earlier piece, with several shades of purple (and some warping on the bottom :

And finally some close-ups on the troublesome layers. Some look "bubbly", some look slightly underextruded and some are just not like the others:

Dust could definitely be an issue here, I'll try putting some cleaning mechanism on the feeder.

Thanks again for taking your time to look at these!

 

[illuminarti 18]

What's the scale here? Looks like they are very small parts? The spacing of the lines is very regular and identical between most of the prints, so I'd almost certainly a z-axis issue.

Is the spacing of the bands 3mm - if so then it's related to the pitch of the z-screw. Probably the bed is wobbling slightly in Z as it moves down.

Make sure that the big brass nut in the back of the bed is free to move horizontally in the X-Y plane.

 

[gr5 2,167]

I don't think it's the Z screw - the spacing is about 40/5 or 8mm.

This is quite a puzzle!

I'm thinking something is happening on the filament maybe at regular intervals. This is a long shot but easy to test:

Please print one of those bricks one more time but after it finishes the base pause the printer (with ulticontroller you can pause and then continue - if you don't have ulticontroller just act quickly). Measure out an exact amount of filament (1 meter is good) from where it enters the feeder. Mark the filament with a pen. Then let the printer print 8mm and then stop the printer (just turn it off). Measure how far the filament moved.

Then figure out how much filament you used to print 8mm of your "brick". Is it exactly one turn on the spool?

Let's assume you used 30cm of filament. Measure the width of the filament with calipers in 10 or 20 different places along 30cm of filament.

 

[gr5 2,167]

To clean the filament as it feeds, take tissue paper. Wrap/wind the wrinkled/crumpled tissue paper around the filament near the base of the feeder. Use some blue tape to tape around the tissue to hold it tight. Now run another piece of blue tape from your "cleaner" to part of your ultimaker to keep the cleaner from getting eaten by the feeder.

After a week inspect it to see how much it filtered out.

 

[rafael-matsunaga 0]

Okay, so I ran a couple more tests:

The top left image is my reference at 0.1mm layers with a single wall (0.4mm), again at 70mm/s and 225C. It's pretty faint, but the irregularity is visible. Then I printed one with 1.2mm walls, effectively using three times as much filament per layer, that's the bottom right picture. Notice the spacing between the bad parts is still about the same, so it's probably not the filament, either.

Then two more single wall tests, at 0.15mm layers (top right) and 0.2mm (bottom left). The angle is different to make the irregularities easier to see. Speed was adjusted to keep the same flow rate as the first test.

The walls are also pretty wavy, but I'll deal with one issue at a time.

So maybe it's the Z-stage. My brass nut doesn't move in the X/Y plane at all. It may take some sanding to make that happen. What's the issue caused by a tight nut?

Thanks again, gentlemen!

 

[illuminarti 18]

Since it's impossible as a practical matter that the z-screw will be totally straight, and aligned exactly perpendicularly to the bed, and parallel to the motor shaft, the nut is designed to slide left and right, so that any eccentricity in the screw edge will move the nut alone, and not shift the entire bed.

The idea is that the z-screw sets only the vertical position of the bed, not the horizontal position. That is done by the two bars to the sides.

 

[gr5 2,167]

It's true that this is the design, but for me, my Z nut is locked in tight and will not move. This has worked very well for me as my Z screw itself is straight and doesn't wobble.

You should do this test:

loosen the Z screw from the coupler so that it can be removed. now slide the bed up and down (the Z screw will also go up and down). Check for binding when the bed is low and when the bed is high.

Rotate the Z screw until the bed is near the bottom. Raise and lower the bed/zscrew together. Does the bottom of the Z screw slip into the coupler easily? Or is it aligned badly.

Also consider removing your z screw completely and putting it on a flat clean surface and rolling it to see if it is straight.

If everything seems perfect then I recommend you *don't* loosen the Z screw. The design is to have a loose Z screw but I find a tight Z screw works even better if everything else is aligned well.

 

[gr5 2,167]

The quality issues you are having with these cubes is near the limit of what the UM can do. I think your particular filament is showing up errors more than most filaments so they are more visible. For example white filament tends to hide these kinds of errors.

However it is definitely possible to improve the quality more than what you are seeing now.

I think Z issues are a good place to start, but not just the screw - are there other things that might affect this? Something that the bed bounces against? Maybe if you add a weight to the bed to keep it from vibrating? Try printing at half normal speed just to see if this helps quality - also try dropping your XY acceleration value in half - this is controlled easily with the ulticontroller - if you don't have one let me know and I'll explain how to do it. When you change acceleration, the values go back to default when you power cycle the Ultimaker. So if you prefer different acceleration values you need to "save" them.

 

[gr5 2,167]

Looking again at the photos - the changes in color are always at the same heights. And the heights are more than 3mm apart (spacing of lead screw threads). This makes me think the leadscrew has some non-linearities. I would consder taking it out and cleaning it. Maybe use WD40 and a rag and clean all the cracks with a tooth brush. Then dry it and re-apply grease and put it back together.

 

[rafael-matsunaga 0]

I'll be out of town for a couple of days, but I'll clean and check the Z screw when I get back, since that's reasonably easy to do. If that doesn't work, then I'll try adding some weight to the bed and if none of those work, I guess I'll have to disassemble the stage and work on that brass nut.

Thanks again for the feedback!

 

[rafael-matsunaga 0]

Quick update: no luck.

- tried a new PLA, same effect

- added weight to the printbed, no difference

- cleaned the leadscrew and nut, same thing

- disassembled the stage, sanded some of the nut casing to allow some X/Y play, no difference

Here's a closer look at the issue. It still happens every ~8mm, doesn't matter which layer height I choose in Cura.

Most layers look fine and have the expected thickness (0.2mm in the image). Then every ~8mm I get a bunch of thinner layers followed by a bunch of thicker layers, so that in the end the part has the expected final height. I painted some dark lines with the expected layer height in the image.

I even checked the G-code, but the layer heights there are all correct.

I find it really odd that it happens at regular intervals that don't seem to be related to the Z screw pitch. The mystery annoys me more than the print quality.

 

[gr5 2,167]

Amazing photo. It could be other things but it sure looks like a Z issue. I would disassemble the Z screw. Check to see if it is straight. Lay the z screw on a flat table and check it carefully by eye.Slide the bed up and down without the z screw. Also see if the z screw lines up perfectly with the coupler.

 

[rafael-matsunaga 0]

Welp, after some more poking and disassembling the z-stage a couple more times, I think I can safely discard the leadscrew and nut as the sources of the issue.

I checked the leadscrew for straightness and alignment, no issue.

I then switched to a new leadscrew and new nut, and the problem remained exactly the same!

My only theory now is that there's something wrong with one of the 12mm linear bearings (dislodged balls or something?), causing a slightly bumpy ride in regular intervals.

Somehow I have an extra pair laying around. This is the final attempt.

 

[xeno 108]

You might also try to put some more weight on the Z-stage, this helped for me

 

[rafael-matsunaga 0]

Changing bearing did not help, oddly enough.

How much more weight do you suggest? I tried putting exactly one iPhone between the wood and the acrylic and that didn't help.

 

[gr5 2,167]

About 10 iphones worth of weight. I have a 1/2 inch aluminum 8X8 inch plate. It weighs about a kilogram. Z works well.