What is causing this ugly thing (PETG)?

I tried printing a coaster, and it turned out rather ugly. Using Bambu PETG, nothing special. Found a kind of similiar problem with Overhang speed, but do not believe this is that. So what could cause this (I normally have just fine looking prints with PETG)? any hint appreciated.

Have you dried your PETG before printing?


It’s in the AMS which is currently at 25% humidity… Is that already a problem for PETG?



Oh, and since i have that roll of PETG in the AMS i printed with it before just fine. So it’s not a wet roll (at least it was not one yesterday :)).


25% is a lot.

As soon as my hygrometer shows anything other than 10%, I know I should dry my desiccant and spools. Not so much due to absolute humidity but more importantly due to the fact that humidity is increasing despite desiccant being present.

What the AMS reports as humidity is only loosely correlated with filament moisture level. Consider that the desiccant pouches can actually soak up enough moisture that water starts to drip from them. So their humidity level would be approaching 100% while the AMS says something much, much lower. Same deal applies to the filament. It’s a sponge. The humidity of the air around it in the AMS is affected by how moist it is, but the humidity of that air is not a direct measure of the filament’s moistness.

The AMS doesn’t keep filament dry (it’d need to be hermetically sealed to do that, and it’s not), it only slows down how quickly dried filament will reabsorb moisture. For filament types that are extremely hydrophilic, I take 'em out and redry if they’ve been in the AMS more than a week or so.

In the world of “Humidity”, it would be said that the AMS “increases the Humidity time constant”. But it does not make that time constant infinite.

If the AMS was left powered on all the time, and it could track humidity levels, that’d be much more useful. Put something in the AMS and monitor the trend. When the humidity level starts creeping up, the filament should be re-dried.

Also worth keeping in mind, we’re dealing with “Relative” humidity, not “Absolute”. It’s dependent on ambient temperature. So the exact same RH from the AMS can have very different meanings depending on how warm or cool the AMS is.


@ EnoTheThracian - 25% is relative, i am unable in my area to get the hygrometer go below 25% inside the AMS.

My believe process was: get a new spool into the AMS with 25%. Spool coming from factory should be dry. Should take quiet a while to produce problems. (the spool is in there for 2 weeks).

But the symptoms show that i am wrong and you and @RocketSled are giving me good advise. I wish the sunlu s2 could be set to “dry to x and keep it dry”. (maybe that option exists, but i have not found it). The 6 hour cycle conflicts with my work day schedule :slight_smile:



I also used to believe a new roll is a dry roll. That’s simply not true anymore, unfortunately. If i open a new roll and it’s clear that the bag was still holding a vacuum, I’ll give it a shot. Otherwise the filament goes into the dryer before i waste any time with it. 3D filament is big business now. High volume production and global distribution mean more variability in the product.

You can’t over dry the filament. Put it in your Sunlu and turn off the timer, let it sit there with the heat on until you’re ready to use it.

That’s exactly what i do. I have 4 Sunlu 2 dryers. Turn them on, timer off, and load them day before (or more) i plan to print. Then the rolls go right in to the AMS and i run my print. I’ll leave the filament loaded and use it again for maybe a week at most (depends on ambient humidity). I print a lot of nylon and it always starts to degrade even with brand new desiccant. And unlike some unlucky few, my AMS seals tight, it’s not “leaky”. It’s just not a hermetic enclosure. Humidity is going to get in. Some of that will get sucked up by the desiccant, but some of it will get absorbed by the filament.


ANY new spool.get a min 4 hours dry.

No exceptions.

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When we talk about relative humidity, room temperature also matters.
The higher the room temperature, the higher impact relative humidity has.
Check this handy calculator that converts between relative humidity, and absolute humidity: Absolute Humidity Calculator

Might be printing too fast. Try setting volumetric speed to a value much lower and see if that helps.

It looks like it fails only at the upper layers, where layer time is low. This is symptomatic for the BL Basic PETG profile, where a max cooling value of 40% is set. Please set th8s value to 90% and give it a try. Or try printing two models at once, which doubles cooling time.

All of the replies are sound advice. I’d also add that the tops of your coaster are curved and depending on your nozzle size, layer height, amount of walls, line width, extrusion amounts, and if you’re using arachne or classic can affect how the layers are achieved by the slicer. It tries to keep your contour but it can only do so much with the variables at play. This can often lead to extra material being extruded (over extrusion) which is picked up by the nozzle on the next layer and especially if that layer is narrower due to the curve, it can drag it along and deposit it somewhere is not wanted. You can fix this by tuning your extrusion and flow rate, using a smaller nozzle, lowering the layer heights all over, or by using variable layer height and checking how the layers are going to be deposited after slicing when using arachne or classic lines. Oh and drying your filament :smirk:

calibrate flow rate un b studio each time you introduce a new filament

There are a few very helpful printable mods for the AMS regarding desiccant. They were among the first things that I printed since I do live in an lush green area. We pay for that with a lot of rain :joy:

The first one was this: Silica Dry Box for Bambu Lab AMS Hydra and Stock Version by Thomas | Printables.com
2 normal ones, 1 with a hygrometer holder. There are many others, but this one printed flawless with PETG as it avoids overly tricky overhangs.
I loaded the normal ones up with microwaveable desiccant but kept the hygrometer in the centre empty to ensure that I still get a meaningful reading from it.

The second one was this: Better Knurled Cap, Better Cap Threads, & Thinner Walled Silica Gel Container by ironjohnhenry - Thingiverse
Printed quite a few of those but am using microwaveable bags of desiccant rather than only the pearls as the holes are a bit too large. I always have one ready loaded with nuts as a spool weight when needed.

The last set replaced the original Bambu desiccant bags when they ran out. Again loaded with desiccant pearls rather than bags though.
Bambu Lab AMS Desiccant Tray by Eldon | Printables.com

Since my last microwave session, we have had about 4 weeks of high humidity (always 70%+ humidity in the shed, most of the time 80%+). The AMS, well sealed, is still sitting at 10% as this is the minimum my hygrometer shows. Desiccant still fully bright orange.
Once the hygrometer shows anything other than 10%, I know that I have to dry the desiccant again (or did not close the AMS properly :wink:).

Same. And I active-dry them as I print. Wet filament can lead to so many problems and mess up your calibration.

Hard to tell for sure. Looks like oozing but that would usually show up along the seam line of each island. Try these steps.

  • Clean your nozzle, PETG likes to glob
  • Dry your spool, It can be a night or day difference with PETG
  • If the print is solid, it could be overextrusion, calibrate the flow ratio (only do this after you dry the filament)
  • Check for thin walls on the tops of those cylinders, it could be extruding scratchy lines and smearing them as it prints.
  • Check the slicing preview on the problem layers and see if you can spot anything odd. Switch view modes to see line widths, speed, and flow rate. A sudden change in any of those can sometimes cause issues.

After drying the PETG for 9 hours, a new print looked better (no more flocking or loose parts), so water was a big factor.

@EnoTheThracian - beside the inspool desiccant mod, i have everything else. I feel my AMS is closing pretty tight (it makes a very satisfying pop when i open it, beats me how else to figure out how tight it is?), still i do not get more than 25% shown by the hygrometer in the middle. I use desiccant from bags (open the bags, poor it in the containers), primarily because they were easy to get here in the area. Maybe they need to be baked a bit before i put them in there.

Now the PETG is just showing a very “shiny” and more flat surface on top, which is probably related to flowspeed, but not worth concerning myself about for this print. I printed the same with PLA matte, and that looks much better for a coaster.

But i would like to understand what parameters to tweak to make this better in PETG. Is this just fan speed/flowrate? (top one PLA, bottom PETG). In non pixel peeping mode they are both quiet usable (considering there is a glass on top anyway). But still, one likes to learn.

I think the main reason for your high AMS humidity may be the use of the small, pre-used desiccant bags. If the spools are not dry, then the desiccant bags themselves are most certainly also not dry when coming out of the bag. So they would, at best, be ineffective in the AMS. At worst, you’d actually add moisture to the AMS. Also, I have seen very different desiccant materials in use. From clay to silica pearls, all of them single use (i.e. drying will probably not do anything helpful).

I use this:
# 1 kg Silica Gel Orange Trockenmittel Indikator regenerierbar Entfeuchter (1 KG)
And this:
Wisedry 50 Gramm [6 PACKS] Silicagel Regenerierbar Beutel Kieselgel Trockenmittel mit Orangenperlen Feuchtigkeits indikator Luftentfeuchter zur Entfernung von Lufttrockner, Lebensmittelqualität : Amazon.de: Gewerbe, Industrie & Wissenschaft

I do not use an oven for drying though. Up to 3 1/2 Minutes at 80% power in our 800W microwave and a good warm, uncovered rest.
I have also used my filament dryer to dry desiccant bags. Just fooling around really, but it does work. :sweat_smile:

Regarding PETG, that is much more tricky than I originally thought. There are a lot of threads here for that reason. From my experience printing the Digital Sundial by Mojoptix in PETG where defects originally looked very much like yours, I would try to increase the number of top layers. In my prints at the time, I noticed the inter-infill drooping of the top layers to be responsible for that. So giving the top layer a better foundation helped.
Since then, I have done more reading here in the forum on PETG and there may be better thought out solutions though. Tailoring cooling, going slow for PETG in general but fast for overhangs,…

Hope this helps and happy printing :grinning:

From the picture, it looks like it is clumping - usually = wet roll.
Also check your settings and do a manual calibration just to verify.
25% is too high…
change out all desicant and nuke it in micro to dry out… while you are doing that, replace with new desicant.
Then I would wait a day before printing…
Now we are all hoping that you made the desicant holders for between the feeders in the AMS… and also made holders for the compartmants under the rolls… That’s a min of 5 desicant holders and will last you about 2-3weeks before it starts to climb.
I usually change it before I hit 20%.
Also you may want to slow down the print speed a wee bit and maybe add more top layers &/or check “ironing” - takes longer but smooths it out