Horizontal banding defect, non-kinematic. Need some help!



  • Hey All -

    Long time happy Duet3D user, first time poster here. I need some help with the finishing calibration of a large-format 3D printer build I've been working on. I'm having some pretty severe horizontal banding, and I've run out of ideas on how to resolve it.

    First, some context: Hardware is a Duet 3 on a roughly 1m cube belt-driven CoreXY layout printer, with AC heated bed, E3D Hemera + Volcano hotend running direct drive. Printing PLA.

    Second, what I've tried:

    • My immediate thought with horizontal banding is Z stage imperfections, especially since this is a very large, very heavy (150lbs?) bed. However, after reviewing every known mechanical and electrical aspect of the bed, I wasn't able to show any meaningful impact to the banding - and, critically, I discovered when doing test printing that the banding got better/worse as the size of the test print changed smaller/larger. When I was printing large (in XY) test objects, I had intense, high frequency (in Z) banding, and small (in XY) test objects, I had moderate, low frequency (in Z) banding. This indicated to me that it was not likely tied to the Z stage mechanism but something else in the system that was linked either to filament consumption or a time-dependent factor.

    • Then, my thinking went to extrusion variation. I checked every possible source for filament thickness variation or filament friction, as well as all of the electrical settings around my extruder and hotend, but was, again, unable to see a direct influence on the horizontal banding. However, while doing these tests, I discovered that the banding was not related to filament consumption rate, but it was correlated with layer time - objects & regions with faster layer times showed less horizontal banding.

    Finally, where things stand now, and a picture: If I run the exact same test print gcode file, but use DWC to change the speed factor from 50% to 100% to 150%, I get drastically different amounts of horizontal banding in the print - where, somewhat unexpectedly - the faster prints exhibit far less banding. Attached below is a photo of the same print run three times back-to-back. The top one is 50%, middle is 100% (roughly 50mm/s print speed) and the bottom one is 150%.

    https://photos.app.goo.gl/HUCjTLtbtLopSzaK6
    PXL_20201116_000706495.jpg

    Any ideas on what might be causing this? It seems like there is some sort of time-dependent layer thickness or flow rate variation in my system.



  • Ugh, solved. The primitive bang-bang controller of the AC heated bed system was causing enough temperature oscillation to generate slow-moving thermal expansion waves that caused this time-dependent layer variation. I'd initially dismissed this as the cause because the temperature variation didn't seem high enough to explain things, but since there are two independent AC zones running bang-bang, the two cycles were probably going in and out of phase with each other to double the amplitude. I ran one of the same prints after unplugging the at-temperature heated bed, and the finish is basically flawless. Time to build a more sophisticated controller for the heated bed...


  • Moderator

    You're not the first to be bitten by z banding from thermal expansion of the bed.

    @nhfoley said in Horizontal banding defect, non-kinematic. Need some help!:

    Time to build a more sophisticated controller for the heated bed...

    Are you not able to control the bed heating loop via the Duet so you can PID tune it?



  • @Phaedrux said in Horizontal banding defect, non-kinematic. Need some help!:

    Are you not able to control the bed heating loop via the Duet so you can PID tune it?

    I have the parts for that in hand, but was holding out with the hope that I could use the off-the-shelf controller for a few months and learn firsthand whether something more sophisticated (like a system with independently controlled quadrants & super well isolated AC) was the better path. Seems like I at least need something of a middle ground to start.


Log in to reply