Very top of a round object not smooth - Silk PLA (Sunlu)

Bambu Lab X Series Bambu Lab X1C
1

So here is a bit of a confusing issue and I’m in an awkward spot since I can’t post proper photos due to the models being highly sensitive for this client (managed to get a small section below). Hopefully I can describe it as well as possible!

Imagine the model as a large bowl (10 cm in diameter). The sides of the bowl are rounded (in and out) and come up to a rounded point. The top rim of the bowl isn’t a flat surface as the inside and outside meet at the top point.

I’ve printed hundreds of these on my X1C and the absolute top edge is nice and smooth.

Over the last few months, about 30% of them have sort of a bubbled top edge. Over the last month this has increased to 70%. I have to completely discard the print when this happens.

top
top1920×1298 82.2 KB

Things I have tried:

  • dried filament
  • different printer (happens on both of my X1Cs, one is only 1 month old)
  • different versions of OrcaSlicer (thought the latest update may have changed something which increased the failure rate, although simply clicking re-print from an SD card file from 6 months ago still fails)
  • standard maintenance (tighten belts, changed nozzle, clean rods, lube screws, re-calibration etc)
  • different built plates (PEI and cool plates)
  • played with slicer settings (although I have preserved a file which printed perfectly at 100% so seems more hardware related?)
  • cut a section of the print to test different settings (always prints perfectly which indicates the issue only occurs across the full print - 6 hours)

An important note is that the ‘top surface’ is ‘internal solid infill’ due to how the model is sliced and various settings. I have also played with settings around internal solid infill with no success.

Screenshot 2024-11-24 102127
Screenshot 2024-11-24 102127787×430 73.4 KB

Physically watching the issue happen, I can see the bubbling happening BELOW the top ‘internal solid infill’ layer (almost seems like too much flow, but the rest of the model is perfect). Then this layer lays on top but doesn’t smoothen out the bubbling. The layers where this is happening is 7 solid layers of ‘inner wall’. This is due to the model having 4 walls so the last 7 layers have no infill.

Sorry for the info dump, I’m just hoping someone can see something that might be the issue here.

It’s just super annoying to get a perfect print, click re-print, and the next print has these issues. Makes me thing it’s hardware-related, but I’m really at a loss.

Thanks in advance!

2

A couple of thoughts - I’ve seen kind of similar issues when a model is trying to lift from the build plate. When the model finishes, is it really easy to separate like it might be partially free already?

The other question is how do you dry? Time/temperature or to a humidity in the filament dryer? Reason I ask is ambient humidity can affect how well filament dries and that can change significantly over weeks as weather changes. I’ve seen filament with just enough moisture do this but it was I think because a part of the print was lifting off the build plate.

The reason I come back to moisture is how you have used different printers and had prior success with the print. In my own experience I’ve seen prints gradually go from nice to issues before I was drying thoroughly.

Maybe it’s settings though but it sounds like you are using known good print files that have worked in the past. Maybe calibrations would pull it into spec?

3

Appreciate the detailed reply! Moisture was definitely my most likely issue, but no matter how many new/old spools I used and dried, I got the same result.

I have spent days cleaning, recalibrating, and changing settings one at a time and I think I found the solution.

Even though I have used the exact same filament brand and colour for over a year, I think there was a small formula change. I dropped the flow for the filament profile by around 3% and the issue is completely gone across 4 100% prints (2 on each X1C).

I will keep printing to ensure that I get this consistently (I have had streaks of 10+ prints with no issue then suddenly it comes back), but going from close to 100% of prints having the issue to 4 in a row of perfection, I think I have nailed it.

It makes sense too. Too much flow near the top of the print will create mini bulges where the upper layers are unable to hide them well enough. Silk in particular will highlight these issues more than other PLA.

Fingers crossed this was it!

3 Likes