atomickidstudios
Posts: 5
Joined: Sun Oct 18, 2015 11:25 pm

Bed shifts on Z during printing, forming blobs in layers.

Sat Jan 09, 2016 11:29 pm

Experiencing a very odd phenomenon, which I have not been able to isolate. It became very pronounced when printing a brim on a model today. During printing, it will consistently jerk / recoil the bed away from the print head during part of the layer causing a very wide blob to form. This gets compounded as each loop is made. When the next layer is applied these built up blobs now cause the print head to skip causing even more of a mess.

I've check the movement of the carriage manually after homing all axis, and nothing mechanically is causing the bump, e.g. a part of the carriage or side pulley mechanisms hitting. Adjustments in flow rate, diameter, etc... didn't seem to have an effect.

I was trying to see if there was some sort of forced retraction of the bed between the end of one outline and the start of a next, but I don't see anything like that. I also removed the brim and just did a 2 online skirt away from the model to clean the head. The problem persists, but doesn't cause as much of an issue because I'm not layering those "blobs" up to the edge of the model.

Lastly, tried printing in another part of my bed by moving the model. It's base is essentially an "X" at 120mm (sq). ... so I can't move it too much around, but the problem persisted.

Any thoughts?

KDan
Posts: 30
Joined: Fri Jun 19, 2015 11:57 pm

Re: Bed shifts on Z during printing, forming blobs in layers

Sun Jan 10, 2016 8:47 am

I believe this is fairly common first layer problem and I've talked with a few friends who've run into the same thing. Here are some my observations...

I've only seen it on hotends with pointy tips (very small flat area surrounding the hole)
It also seems to be more of a problem when the nozzle tip starts to wear.

The filament always seems to pile up in one direction only. If you are printing a first layer in a direction where the problem is occurring, printing the first layer in the opposite direction usually works fine. My theory is that if the nozzle tip wears unevenly more filament get extruded to one side more than the other. It also seems worst right after the hotend reverses direction at the perimeter of the first layer - when there is a brief surge in filament flowrate.

Things that seem to help...

Replace the nozzle.
Increase first layer height slightly - or reduce first layer extrusion width.
Try to level the bed more accurately.

I've only run into this with PLA. ABS is softer and seems more pliable and therefore doesn't deflect the hotend so much if it encounters an obstacle.

I'm very interested in hearing what other have to say about this problem too.

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jimc
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Location: mullica, nj
Contact: Website

Re: Bed shifts on Z during printing, forming blobs in layers

Sun Jan 10, 2016 9:00 am

If the bed is dropping during a retraction then that is in the layer tab. Retraction vertical lift. Generally you want that though. You keep it set around 0.2mm. This keeps the nozzle from dragging on your model. If your getting oozing during your travel move which is causing a blob at the beginning of your extrusion line then you need to adjust your ooze/retraction settings.

atomickidstudios
Posts: 5
Joined: Sun Oct 18, 2015 11:25 pm

Re: Bed shifts on Z during printing, forming blobs in layers

Sun Jan 10, 2016 1:55 pm

jimc wrote:If the bed is dropping during a retraction then that is in the layer tab. Retraction vertical lift. Generally you want that though. You keep it set around 0.2mm.
Oddly enough I don't have that set (in the Extruder tab), my settings are:

Retraction Distance: 1.2
Extra: 0.0
Retraction Vertical Lift: 0.0
Retraction Speed: 70mm/s
Coasting: n/a
Wipe Distance: n/a

It could be a worn nozzle, but as I'm physically seeing the bed twitch at certain points in the outline, I'm kind of stumped. I looked at the bed at eye level and manually moved the carriage around and it's definitely not hitting anything. Trying to follow closely when that happens. I should take a look at the G-code while it's printing to see if it's actually getting a signal to offset the Z. It's so strange.

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jimc
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Location: mullica, nj
Contact: Website

Re: Bed shifts on Z during printing, forming blobs in layers

Sun Jan 10, 2016 9:46 pm

if the vertical lift is set to zero like you have listed there then i'm 99.9% sure you wont find s3d putting any slight jogs in your z height. i have never seen that happen and in 3 years i have never seen that pop up on the forum. stranger things have happened but i would be really surprised.

CompoundCarl
Posts: 2005
Joined: Wed Aug 05, 2015 7:23 am

Re: Bed shifts on Z during printing, forming blobs in layers

Mon Jan 11, 2016 6:27 am

Just look at the preview. It will be very easy to see if there are random Z-axis movements in the print. Otherwise, it's probably a mechanical/electrical issue

MichaelHerron
Posts: 112
Joined: Mon Mar 16, 2015 2:36 pm

Re: Bed shifts on Z during printing, forming blobs in layers

Mon Jan 11, 2016 4:42 pm

atomickidstudios wrote:Experiencing a very odd phenomenon, which I have not been able to isolate. It became very pronounced when printing a brim on a model today. During printing, it will consistently jerk / recoil the bed away from the print head during part of the layer causing a very wide blob to form. This gets compounded as each loop is made. When the next layer is applied these built up blobs now cause the print head to skip causing even more of a mess.

I've check the movement of the carriage manually after homing all axis, and nothing mechanically is causing the bump, e.g. a part of the carriage or side pulley mechanisms hitting. Adjustments in flow rate, diameter, etc... didn't seem to have an effect.

I was trying to see if there was some sort of forced retraction of the bed between the end of one outline and the start of a next, but I don't see anything like that. I also removed the brim and just did a 2 online skirt away from the model to clean the head. The problem persists, but doesn't cause as much of an issue because I'm not layering those "blobs" up to the edge of the model.

Lastly, tried printing in another part of my bed by moving the model. It's base is essentially an "X" at 120mm (sq). ... so I can't move it too much around, but the problem persisted.

Any thoughts?
If you have dual nozzles, you might try removing the one not-in-use. if it is hitting part of your print, it will lift the head as it goes by creating a blob. It may create another blob as the unused nozzle hits the new blob too.

If it is a first-layer phenomenon, you may also try adding .05-.10 mm of Z offset. It COULD be "blobbing" because of too much backpressure in your hot-end.

As always, let us know your printer type/material and other relevant stats for better help...

atomickidstudios
Posts: 5
Joined: Sun Oct 18, 2015 11:25 pm

Re: Bed shifts on Z during printing, forming blobs in layers

Mon Jan 11, 2016 6:27 pm

If it is a first-layer phenomenon, you may also try adding .05-.10 mm of Z offset. It COULD be "blobbing" because of too much backpressure in your hot-end.

As always, let us know your printer type/material and other relevant stats for better help...
I have a first-gen DITTO, comparable to the DITTO+, by Tinkerine. It's a .4mm, single extruder.

It is definitely more of a 1st /2nd layer phenomenon. And I did start to mess with the Z offset, but maybe I need to keep experimenting with that. I does behave like there is serious back pressure issue, any my first layers are very tight to the bed.

I'll keep testing to see if I can sort it out.

Johnm
Posts: 18
Joined: Mon Nov 19, 2018 1:31 pm

Re: Bed shifts on Z during printing, forming blobs in layers.

Sat Jan 09, 2021 12:22 pm

I am having a similar issue. I am printing a cap design (6 each) which is basically round. I am getting blobs. Mostly at the edge. The first layer looks pretty good, though. I think it starts after the first few layers. Then around 4-8 layers I get a slight layer shift. I have the E3D V6 gold edition hotend and Zesty Nimble Direct drive extruder. I am still trying to figure out how to stop stringing. So back pressure might be my issue with blobing. I have Retraction set to 2mm and 80mm/s. This still does not help with stringing. I get very little stringing with solid parts like the current one I am printing. Using other printing techniques. I did tighten my X Axis Belt which helped at first then it went back to the layer shift issue when printing the next part. So maybe my issue is that the blobs are having an issue on shifting the layer. I am not sure. Even the parts that did not show a layer shift issue, The layer or 2 on the bottom of the top cap which is 76.2mm or 3". Before it prints the other round part/section that seats into a hole is 25.4mm or 2".

Any thoughts,

Thank You
John

gwhite
Posts: 156
Joined: Wed Jul 19, 2017 9:37 am

Re: Bed shifts on Z during printing, forming blobs in layers.

Sat Jan 09, 2021 7:06 pm

Just a thought: If the blobs are sticking up a little, the layer shift can be caused if the extruder smacks into one. Try a little Z lift & see if that helps.

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