Designing a simple photo frame. I wanted to make it so that it prints without supports. While printing, however, the back support part wobbles/vibrates with movement, and is printing terribly.
Ender 3v3 CoreXZ, PolyTerra matte black filament, using the CR-PLA Matte profile in Creality Print 7. Tried printing at slow speeds, the issue still persists.
I’ve tried making the back support with only 45 degree angles (previously it was an x diagonal), made it thicker (from 0.75mm to 1.5mm). Nothing is working. Please find the stl and 3mf here (https://www.thingiverse.com/thing:7281179).
Is this a printer issue, a model issue, or a slicer issue? Please advise. 
I would say this is a model issue. How about separating the model in 2 parts: the flat bottom part with the legs, and the ‘envelope’ part that stands upright. Both parts can then be printed flat fairly easy. You’ll only need to think of a way to join the two again after printing, perhaps by modeling two holes in the stand part, and adding two pins on the upright part
Printing the upright part flat on the bed is tricky, because it has a 0.75mm channel running through it for the photos. Wouldn’t that cause overhang issues, or is it too small to be a problem?
It would be an overhang problem. You could change the channel to a 45 degree notch. But it wouldn’t hold the photo as secure.
Simplest might be to fill in more of the back so you don’t have that thin rectangle going up high until it meets the other rectangle in the middle. Thickening those parts would also help. The back frame could be changed to a hollow square cross section. That would make it stable without significant material.