Layer shifts sometimes

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SteveNZ
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Joined: Fri Jan 05, 2018 1:51 am

Layer shifts sometimes

Post by SteveNZ » Tue Jul 31, 2018 9:10 am

<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/>
<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.
<br/>
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.
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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/>
<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/>
<br/>
I'm wasting a lot of filament and time and could really use some help to fault find this problem.<br/>
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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.

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ednisley
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Re: Layer shifts sometimes

Post by ednisley » Tue Jul 31, 2018 2:10 pm

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 ...

SteveNZ
Posts: 3
Joined: Fri Jan 05, 2018 1:51 am

Re: Layer shifts sometimes

Post by SteveNZ » Tue Jul 31, 2018 8:35 pm

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?

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ednisley
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Re: Layer shifts sometimes

Post by ednisley » Wed Aug 01, 2018 4:09 pm

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.

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