Huge carriage sag (>>0.2mm)

[namronb]

I got a FF creator pro last November. It's generally pretty easy to use with PLA, and I can do ABS with some extra attention. I've recently tried to use PETG, though, and was pulling my hair out trying to get the bed level due to adhesion issues. I hadn't leveled off the center of the bed before (because the bed is very flat), but when I did, I was surprised to see that the center was "high" by over twice the thickness of the provided leveling card. So I stuck a magnetic-base dial indicator to the floor of the print area (not the bed) and watched the droop of the X and Y axis rails as the carriage moved toward the center. It turns out that the carriage is drooping about 0.004" in the shorter Y axis and 0.007" in the longer X axis, for a total of about 0.28mm. No wonder I'm having miserable luck with larger prints or extrusion-sensitive material like PETG.

Does anybody have any advice on how to either reduce the droop or compensate for it? Are the 8mm x/y axis rails solid or hollow? Is there a 10mm upgrade possible?

Thanks!

[wirlybird]

What is the bed surface? Some droop is normal and what you have is pretty tiny in the FDM realm. Leveling should be done over the leveling adjusters and it is not uncommon to have high and low spots on the bed also. Especially if it is the stock bed without glass.
PETG does not like to be squished like PLA does. I use a glass bed and glue stick for PETG and it has worked nicely. I give a good level with paper and use a first layer height of 100-110% I think.

[Sebastian Finke]

Does anybody have any advice on how to either reduce the droop or compensate for it?

Removing a stepper (for the left extruder) is pretty much all you can do. That'll reduce the overall mass of the carriage by about 40%.

Are the 8mm x/y axis rails solid or hollow?

I assume they are hollow for reason of cost and weight. That's not a bad thing as hollow rods are more rigid too.

Is there a 10mm upgrade possible?

Not to my knowledge. Its a massive upgrade though. New bearings, carriage, X end rod mounts, Y end rod mounts, pulleys. Would theoretically reduce you X and Y travel by 2mm too. Doable but a ton of work. Is it worth it? Probably not as you would be adding to the moving mass and get even worse prints.

If you're using glass with clips they can warp the glass ever so slightly adding to the discrepancy.