H2D printhead freezes mid print causing blobs

Bambu Lab H2 Series
1

Sometimes my H2D just stopps the toolhead in the middle of a print for no visible reason, leaving a blob of filament in the model. After 30 seconds it continues normally without any error or warning.
I have no idea what this is, so I share this with all the information around that might be relevant to understand whats happening.

The filament is BambuLab PETG HF and everything on default settings.
I have several theories but no clue and I’m not willing to print midsized models for every possible tweak.
So maybe someone can help here.

My thoughts so far:

It could be related to the GCODE reading. Together with the speed it prints, it might not be able to keep up. Especially on (unneccessarily) high detailed models.
Or maybe it’s the USB Stick. If it is reading GCODE while simultaneously writing live video AND timelapse AND “restore on powerloss” information - that might be too much.
I carefully tested the new USB stick for sustained minimum write speed as the first mini USB drive got very slow and hot and clearly didn’t meet the requirements. So the USB speed should be no bottleneck here.
As you can see in my video above, the live ipcam video is still recording to the USB drive when the toolhead freezes.

The only option I can adjust is: no timelapse.
Maybe the option “Recover from STEPloss” writes to the drive as well - but I don’t know. But Steploss recovery can be done from volatile RAM and doesn’t neccessarily need to be written to the USB drive as powerloss recovery does, which I found no option to turn that off.

Or maybe it is something with the temperature.
If some sensor is defective or a cable is loose, the hotend temperature might drop to a certain point. The printer might wait until it’s up again.
But causing two times a delay of exactly 30 seconds makes not a lot of sense here.
Same for the model geometry causing the filament coolers airstream getting redirected to the hotend and cooling it down so that the printer needs to wait for stable temps… very unlikely - just in my mind.

It also stops for about half the time (15 sec) when cleaning the nozzle on the very beginning. But as I don’t have my requested metadata like temperatures along in the video, I cannot tell if it just waits to heat up.

It now happend several times on midsize or larger models - not on smaller ones.
Once I saw it happening and the printer display WAS responsive at that time, telling me the layer number where the problem occurs. Everything seems to be okay. But I didn’t check the temperature readings in that moment.

In every case I have exported STL files with a huge amount of triangles from freeCAD into BambuStudio. Several hundertThousand.
I tried the “simplify model” option in BS and got no blobs in that prints. Also I now exported STEP files, that are a lot smaller and printed one part without freezing.
But this is only correlation, not a prove of the cause…

Oh, and the printer is in LAN only mode with no connection to the internet (per router firewall rule). So no influence from the cloud here.

Any ideas?

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There is a thread here on how the wrong USB sticks can ruin your print.

But my first advice would be to toggle of live video recording on the printer itself ofr simpler, just remove the USB stick and see if the problem is gone.

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That is very strange indeed. You have some good theories but I don’t know sorry. Giving your thread a bump so maybe someone else can help.

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This looks like the USB stick issue. Get another USB drive with enough cooling

Screenshot_20250625_175322_Amazon Shopping
Screenshot_20250625_175322_Amazon Shopping968×2288 194 KB

The one I’m using

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I carefully checked the most obvious reason, I think.

The freezing issues on A1 and X1C were related to cloud issues, that are not the case here, as the printer is offline.

The USB drive was my first suspect.
So I checked it and indeed, the sustained write speed dropped to 7 MB/s when used for several hours. And that tiny thing got very hot.

So I got myself a new one (USB 3.1) with a sustained write speed far above the USB 2.0 specification that I completely wrote full while checking speed and temperature, as I said in the video.
In my opinion there is no headroom here to improve on the USB stick as the printer is USB 2.0 only.
That’s why I ruled that out.

Turning the video recording off will make it harder to examine what’s going on :confused:

And I’m not that much into printing the same bigger part over and over again to see if it’s the video recording, another USB stick, the amount of triangles of my exported model, etc.
But I agree that this modify-ONE-parameter-and-repeat testing would lead to the root cause.

I’m observing carefully and will try things like even another USB drive.

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The idea for disabling the video was purely for root cause analysis. If you have a good USB drive, you can enable it again.

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you don’t have to disable the video, leave it on otherwise it’s impossible to diagnose. I think it’s the usb drive overheating issue.

You don’t need a fast drive. You only need a stable and reasonably fast drive.

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My advice was a bit further back to first figure out if the USB would even be the cause.

We are already beyond that point.

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Thanks for all your feedback.

I think the point “temperature of the USB drive” is worth digging a little deeper.
Since I already realized the thermal throtteling of the first USB drive, that was clearly too slow.
And I did my sustained write speed test on my PC - not in the printer.

What makes me wonder is the fact, that it records 30 FPS video in FullHD while freezing the print. Clearly visible in my video: some strings are moving in the wind of the cooling fan, that’s also still active.
And when I once saw it happening live, the printers display was still very responsive while the printhead was standing still.

I think it’s also not extruding during that time, because 30 seconds would result in a much bigger blob of filament according to the fast speed with PETG HF.

I might put a thermocouple to the USB drive or record a video of a thermal camera pointing on it. The latter method is tricky on reflective surfaces, but at least I can compare that again to a new testrun on the PC if that keeps the USB drive much cooler.

12

New theory: as the H2D printer is heavily vibrating during prints, maybe the USB connection contacts got interrupted?!
But it wouldn’t record video then, I think…
Just wanted to share my thoughts, because this issue is annoying.
Yet another blob on a big, plane surface.
When the print is finished I will have a look, if it paused again for 30 seconds…

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Except it’s not heavily vibrating. It’s just subtle movement, damped mild shaking.

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I know it’s difficult to describe in an enthusiast forum where there are lab standard describing all the sine wave forms by amplitude an frequency.

I just observe the USB stick standing out quite bit and having a mass with it’s center of gravity somewhere outside the connector. So it ‘might’ resonate at some point.

Just sharing this thought. Even if my highest bet is the temperature related write speed of the USB drive.

But no prove for any of the theories so far…

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This can’t be simply related to the USB stick because I have removed the USB drive from my H2D yet the pausing, restarting and blobs issue persists. It happens so frequently that it has mad my brand new printer unusable for me.

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Welcome to the forum.

Suggest creating a ticket and having support look at the log files.

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Thank you. I submitted a ticket early this morning so I hope to hear back soon as I rely on my Bambu Lab printers for business.

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A litte update:

I try to not alter more than one parameter at once to find the root cause - but that’s difficult, as I don’t want to do a lot of testprints…

I switched from hi-resolution .stl files to .step files, because they are smaller in size.
Sidenote: I’m still on the same firmware version, which I keep to make the ONE comparison, when I found my benchmark - as there is no way back once I update :confused:

So far I didn’t have the pausing issue since then.
But I didn’t print big parts either.

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Welcome and sad to hear that.

So your printer paused even without any USB drive?

That would foster my theory of the amount of data beeing too high.
I had the issue only at ‘bigger’ parts that print more than 4…5h with a very high polygon count.

So I tried reducing the data by using the “simplify model” function from the right click context menu in BambuStudio. Now I switched from .stl to .step files and let BS determine the right amount of polygons.

It’s strange, as even the slow USB2 drive can provide 20 Megabytes per second and that should be plenty enough if a printjob takes 4 hours.

But it might come to a bottleneck when the printhead has to be moved quite often to new positions on very fine trajectories at a very high traveling speed.
That’s my current assumption of the rootcause…

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I have had the ‘pause then blob’ problem with 3 different prints. 2 were complex / large prints (large cat bust models), one was a plate with many objects. What all 3 have in common is the use of fuzzy skin. When I print the same cat bust models with fuzzy skin off, no pause/blobs. It seems the complexity of fuzzy skin on a large model is just too much to process and it need to pause to catch up before continuing.