While using variable height future with PETG-CF Bambu. I’m getting clumping on the nozzle. I don’t know if this is an error on this function or if it is acting normal. Is there anyway to avoid nozzle clumping using this feature? It only happens on the last few layers of my print because of the chamfering. I don’t want to waste anymore filament troubleshooting this and the variable height cleans up prints so well. Thanks
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Nozzle clumping is because the filament is sticky, and your nozzle unfortunately doesn’t have enough coating to repel it. It can happen even if your filament is perfectly dried and your nozzle is relatively new.
A lot of filaments are not suitable for extra low layer heights, because the lower the layer height the deeper the nozzle presses itself into the molten filament. And also, the extrusion isn’t a linear function anymore for those very sticky filaments.
Linear extrusion means that, the slicer can have an accurate prediction of given a certain flow rate and a range of extrusion speed, how the filament would be pressed by the nozzle, into a consistent line width, with a consistent layer height.
Not linear anymore means that for some filaments, with a given flow rate, within a range of print speed (e.g., 50mm-300mm/s, then you can convert it to volumetric flow rate but i’m too lazy to do the calculation), the slicer cannot get an accurate calculation. Basically means that now everything changes with the extrusion speed.
Now because it’s almost nonlinear, this means the extrusion given by the slicer would be inaccurate, and it would cause over extrusion or underextrusion depending on the filament. You can try to reduce the flow rate, but unfortunately flow rate is a fixed number for current slicers, so you would have other underextrusion issues if you do change that.
When the filament is too sticky, you’re essentially dipping the nozzle into a peanut jam.
In short: I avoid to use anysort of variable layer height for filaments that I know might be bad for it, including PETG.
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I’ve printed many kilos of PETG, PETG HF, and PETG-CF, and I would say this behavior is more or less typical for these types of filaments. PETG is inherently sticky and will tend to adhere even to coated nozzles. Make sure the nozzle is clean at the start of the print, and avoid printing with layers that are too low.
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Pretty sure this is the issue with variable layer. It’s making the layer height too small for the PETG.
Calibrate your flow and temperature and it will just get couple specks instead of clumping. I’ve been using vlh for most of my prints , and I use fiber infused petg and abs all the time. I wonder what some guy do to think that clumping is normal… even after 20hours at .12 my nozzles are clean 
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That’s a cute take, but unfortunately not how material science works.
With over 8000 print hours — including fiber-reinforced PETG and well-tuned flow and temps — I can tell you: some materials will leave residue, period. PETG is naturally sticky, and even with ideal settings, you’ll get gradual buildup over time. Especially on long prints and with abrasive blends.
If you’re getting 20-hour prints at 0.12 mm without a speck, maybe your nozzle is running divine firmware. But claiming clumping shouldn’t happen with PETG? That’s just misrepresenting reality.
What’s next — saying TPU doesn’t string if you just calibrate it right?
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I totally agree with you. Some PETGs maybe are not as sticky as others, 0.12mm maybe okish for those, but that’s definitely not the common situation for most other petgs.
Personally I wouldn’t go below 0.16 for most petgs on 0.4mm nozzle
Ive found that tpu doesnt string when dried correctly. Petg aswell. That and the lowest possible nozzle temps
Right …ain’t gonna argue with hobbyists .My experience in industrial manufacturing and prototyping with over 150 machines over the last 15+years says otherwise …but… You do you. Even though stringing will NOT happen nor clumping if you fine tune and keep your materials in optimal condition…last time I had stringing with any material, ender was the hype and you can’t really say the deposition process was ideal in those machines. Here though you have perfect pressure, hardware and conditions for 0 stringing and yes 0 clumping . Petg will stick to the nozzle yes, but that will only happen excessively if your filament settings are wrong even slightly. And infused filaments actually tend to be less sticky exactly due to the fibers inside.you won’t melt the cf or gf at printing temps . And no lower height doesn’t make the nozzle “dig deeper” that’s a completely wrong understanding of how layer height works… from 0.2 lh and ~0.4 lw you go to 0.12 lh and ~0.4 lw . The lower height comes from a less material coming out essentially . Not because the nozzle somehow compresses the same material amounts into smaller dimensions…if that was true a print that is 100grams on .2 would be weighting more at lower layers …
Everyone in this forum is [insert double digits or triple digits here] years experienced in using over [insert a phone number here] machines.
And yet we don’t have a general agreement on everything lol, that’s why I like 3d printing, it’s magic
I have already stated above, that due to difference in viscosity, the model that the slicer uses to calculate the extrusion doesn’t always work. You’re assuming that the slicer’s prediction of extrusion is 100% accurate, which in real world it’s not the case. Yes, the slicer indeed tells the extruder to make less extrusion, but that doesn’t mean that over or under extrusion doesn’t come into play when you modify the layer height too much.
What you think the relationship is like:
What in reality, the relationship is like:
Thermoplastic is a complex thing. Real world isn’t always that ideal. At least not for the affordable filaments that we use everyday.
I’ve got a few models I’d love to see anyone print without stringing—even with perfectly dried filament and a beautifully dialed-in profile 
You can definitely minimize it, but not in every situation. I’d say about 98% of my prints come out just fine without tweaks, but for those annoying 2%, I do need to fine-tune things if I want to keep my TPU from going full spiderweb.
PETG, on the other hand, doesn’t really string if your settings are spot on—but it does love to slowly build up on the nozzle during long prints. Not big blobs, just enough to make you frown halfway through a job.
Also, in my experience, color makes a difference too. My red PETG-CF? Starts smearing the moment you look away. But the gray and black ones? Clean as a whistle every time. Go figure.