So my 3D printer uses two 10mm rods for the X axis with LM10UU bearings inside the toolhead carriage.

The regular metal bearings are quite noisy and have nasty resonance effects at 40mm/s speeds. They also have a lot of wiggle room.

In an attempt to solve this I bought the plastic variant which has no moving parts and just slides over the rod. They are too tight however and my printer struggles with moving in the X direction.

Does anyone have experience with these? I assume the plastic ones wear out over time so the friction should go down.

I don't have any personal experience, but there are some interesting experiments being made by spanish guy with fully printed linear bearings and ptfe tubes. Maybe his work will give you some ideas on improving yours.

Update: found him - printabot

Igus has tons of high quality plastic linear bearings and selection guides, lifetime calculator, etc. I’ve also got versions they didn’t have on their site by talking to a rep, like tighter tolerance for a specific bearing. Used them across many mechanism and products I’ve designed. Can get cheaper bearings elsewhere but without those resources so they tend to be my default.

Right. I'm sort of a cheapass so I ordered them via Aliexpress. Next time I will get the proper ones.

I did however find out that the metal bearings, despite the play in them, did not cause the resonances. Ever since I rebuilt the printer to a CoreXY machine some speeds were off-limits. Which is fine I guess but mildly annoying and I want to get it solved at some point.