Since starting to build a Dollo3D for myself, I've been interested in testing how far one can get with 3D printed parts. One of the problem areas with Dollo has been the Z-axis and for now I've resorted to using metallic lead screws. However, I didn't want to buy linear rails, I wanted to print them. Also I wanted the rail setup to be extensible, even though the lead screw isn't (yet). So came up with this setup where the slide has kind of spring mechanism so tolerances don't need to be 100%. Also it slides quite easily, making it a good option for Z-axis rail as it usually doesn't move that fast.
Source available in here: https://github.com/spegelius/Dollo/blob/master/NEW_long_ties/rail.scad
I've been running 4 ~50cm beams for something like 6 months now and the setup is still going on strong. Even with not so straight lead screws, the prints are coming great regards Z-axis. Lubricant strongly advised, I use PTFE based spray.
There are of course things to improve, like the joint between the rail parts and maybe making the parts smaller in diameter. Work in progress.
2 perimeters and not much infill needed in my experience. YMMV of course. Printed with PLA as I like to have the rails stiff.
Connecting multiple rails together is accomplished with the inner block. I also used soldering iron to melt the ends together while having them held in straight line. And sanded the junctions afterwards.
Lubricant strongly advised, I use PTFE based spray. Also some kind of silicone grease seems to work even better.
If the inner block doesn't fit or is too loose, use your slicer sw to scale it in X and Z dimensions.