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Layer shifts sometimes

Posted: Tue Jul 31, 2018 9:10 am
by SteveNZ
<r>My printer was working really well for a long time. I'm running a 0.5mm nozzle and the prints were nice and I was getting very few problems with the filament.<br/>
I then started to print a project which has a wide base and thin walls rising from the base. I was getting contraction problems with parts of the print. So I built an enclosure to keep the whole print warmer. I don't have an extra heater, I just rely on the hot end and the bed heater to keep the interior warm. The temperature gets up to about 45 deg C, I also have a temperature sensor on the outside of the X stepper motor and it gets up to about 58 deg C. The contraction problems are much reduced with the enclosure.<br/>
However, almost immediately I am got x layer shifting, some of it is major with a layer shifting 5mm gradually over about 10 layers then suddenly shifting back to almost the correct location.
I have tried a lot of different things and I have had no consistent improvement.<br/>
I have reduced the maximum current on the stepper motors in the firmware - to reduce the stepper temperature.<br/>
I have cleaned and lubricated the x and y rails and the z shafts.<br/>
I have reduced the filament multiplier in case the nozzle was getting bumped by an overfill of plastic <br/>
the last thing I did was reduce the printing speed. I'm using Slic3r and I halved all speeds.<br/>

After slowing the printing I printed a tower that for me was almost perfect.
So I tried to print my wide and thin walled project with the same slow speeds - and I am back to x-shifts only about 1mm and some very slight y-shifts and lots of stringing.<br/>
So I slowed it down further and the print is better in that the x and y shifts are smaller but still not acceptable but stringing was worse.<br/>
I'm wasting a lot of filament and time and could really use some help to fault find this problem.<br/>
By the way I can print a perfect 20 x 20 x 20 spiral vase calibration model. and a 5 x 5 x 50 solid tower which is very nice. <br/>
I only have problems with prints that run for more than about an hour.<br/>

Some other thoughts, I am printing with 1.75mm Black ABS.
I am using Octoprint - and I only started using Octoprint after I built the enclosure.
I wanted to add some images, but I can't figure out how to add an image to this post.

Re: Layer shifts sometimes

Posted: Tue Jul 31, 2018 2:10 pm
by ednisley
SteveNZ wrote:
Tue Jul 31, 2018 9:10 am
a wide base and thin walls
The corners of the walls warp upward, the nozzle smacks into the bulges, the XY stepper motors stall, and the next layer is offset. A few layers later, another bulge, another whack, a different displacement. It's essentially random, although specific model features tend to accumulate warps and blobs: acute angles and thin walls in particular.

There are as many recommendations as you can imagine, much contradictory advice, and little resolution, because it's really a fundamental problem of building objects from threads of molten plastic with a high thermal coefficient of expansion.

Slower works better, because it gives the nozzle time to melt its way through the blob, but very slow causes other problems.

In some cases, you must change the model geometry to work with the "manufacturing process" by eliminating the features triggering the problem.

Larger objects provide more opportunity for problems to arise, so assembling smaller sections into a completed part tends to work much better than trying to build one heroic object.

Good luck ...

Re: Layer shifts sometimes

Posted: Tue Jul 31, 2018 8:35 pm
by SteveNZ
Ed, at first I thought no that couldn't be the cause. But thinking more about what is different from when I was getting good prints until now, maybe it is.

I also have a new stock of filament. I did some quick measurements and found that the diameter varies quite a bit, from 1.70mm to 1.80mm I guess extrusion through a 1.8mm section could cause a bigger blob.

I've also reduced the maximum stepper current, (in case the steppers were overheating) but maybe this is the wrong thing to do, maybe the stepper needs to drive harder to overcome blobs, basically to break through rather than stall.

I'll try setting the current limit back to default and then I'll try reducing my filament multiplier and see what happens.

The thing that doesn't fit is why is the very bad shifting in the X-axis only?

Re: Layer shifts sometimes

Posted: Wed Aug 01, 2018 4:09 pm
by ednisley
SteveNZ wrote:
Tue Jul 31, 2018 8:35 pm
the diameter varies quite a bit
Typical filament can vary by 0.05 mm either way from the nominal 1.75 mm, so it's within spec.

However, the diameter should vary slowly along the length; if it's ripply enough to see and measure that easily, I'd call it junk.
maybe the stepper needs to drive harder to overcome blobs
The stepper motor current is determined by the allowed power dissipation in the windings. The power varies as the square of the current, so you must set the current to the correct value and leave it alone.

The correct solution is to eliminate the blobs causing the stalls. There's no upper limit to the force applied to the nozzle, because it can't plow through solid plastic.
why is the very bad shifting in the X-axis only?
Most likely it's the object's shape, with corners / edges / whatever aimed such that the next motion along the X axis rams the nozzle into the uplifted plastic?

Or maybe coincidence; we're really good at making up plausible stories from random events.

Re: Layer shifts sometimes

Posted: Fri Sep 14, 2018 10:56 am
by JLeithauser
If the layer shifts are inconsistent, check and make sure the print bed is secure and the clips that hold the glass down are tight. I had similar issues where the offsets were inconsistent (mainly on the X axis but also sometimes on the Y) and got better if I slowed the printer down but never really went away. I tried a whole bunch of things and messed up my profile settings quite a bit trying different things but it ended up being that the clips had backed off from removing the bed and were no longer holding it in place and it was shifting during the print. This may or may not help but it's something simple to check and after I tightened mine back up the problem went away.

Re: Layer shifts sometimes

Posted: Fri Sep 14, 2018 4:02 pm
by dsharp02
Does the enclosure include your electronics? You might be getting sporadic thermal cutoff on your stepper drivers due to the high ambient temp caused by the enclosure.

Re: Layer shifts sometimes

Posted: Thu Nov 08, 2018 9:28 pm
by SteveNZ
It's been a long time since I started looking into this problem. I really couldn't find a fix so I left it for a while.
Last weekend I decided to strip the whole machine down and to thoroughly clean everything.
I was hopeful, but my problems continued.
Then, yesterday, as I was observing a print that was going wrong I saw the extruder twist back and forth in the mount. A close inspection of the mount showed a crack in the mount (and an easily twistable extruder motor).
I see at some point Makergear changed to a metal extruder mount and a cable box to the side.
I ordered the metal mount, as I am in New Zealand it will probably take a couple of weeks before it arrives. In the meantime I have glued the mount with epoxy, I'll see if that can get me printing again.
I'll report back once I have some news.