It’s a year now that I’m enjoying my X1C nearly every day! I’m really amazed by its print quality. However, I print mostly small and mid sized parts. A few weeks ago I found the BaBo Organizer which has parts that fill the entire print plate. And printing such large parts I noticed a repeating and reproducible issue on top faces.
Interestingly the issue only happens in the rear left part of the print bed. Do you have any ideas what the cause of this problem could be?
I see something similar on my P1S. It happens at the front slightly off center.
Funny that this topic just came up because a month ago I purchased a smooth plate with a specular surface for the purposes of experimenting with gluing parts. Smooth parts glue better.
I recognize the single layer test pattern you’re using. It is definitely a pattern everyone should have in their 3D Printer toolbox precisely because it exposes every sin the printer, filament and printer plate may have.
I got the same results you’re seeing on my P1P. I probably tested about a dozen different filaments, some showed more of the striping like you’re showing and some showed less. I was able to mitigate this somewhat by turning down the max volumetric flow. In fact, on some PLAs it went away completely. I also noticed that once I expanded the one layer print in the Z axis using the slicer “scale” function, the problem went away 99%, but not completely mind you.
Although I do not have a specific answer to the question of why it is happening I do have a couple of theories and it would be a great discussion point to hear what others may think.
Here’s some of my theoretical causes in no particular order:
Precision Variation: Due to the bed’s inherent imprecision, a single-layer full plate print reveals positional and vibrational disparities between different plate regions, leading to repeatable artifacts. I can’t think of a way to test this theory.
Filament Drag Hypothesis: It’s possible that as the print head stretches the Bowden tube over distance, the tube’s angle might affect filament drag. Again, because the regions of striping happens in a consistent region
Thermal Disparities: Although I couldn’t test this theory, I noticed that the striping occurred in plate regions with significantly different temperatures (up to 5-10°C difference) compared to unaffected regions. A static thermal camera captured stills, but it would require me to hold the cheap camera I have since it doesn’t have a video recording mode.
In the end, I simply chalked this up to a curiosity that didn’t really impact my day to day use but I will confess, the OCD side of my brain desperately wants to tune this to make it disappear. 
Here’s the thermal camera I used. It’s only has a 32x24 pixel resolution upscaled to a 320x240 screen. It’s crude, yes, but I used it when trouble shooting electronic components on circuit cards when I’m doing system builds and for that, it’s good enough. You can do a search for MLX90640 camera and find many makers of the same item.
https://www.amazon.com/dp/B0BNQHB229/
Here’s an image of an empty plate warmed to 70°C. You can see the stark temp difference towards the outer regions. The camera only picked up 65°C
