I made a 20mm beam which can be printed horizontal,

I made a 20mm beam which can be printed horizontal, this means I can print a max beam length of ~220mm (diagonal) on my reprap.
To archive “printing without support” , I had to split the beam in the middle and add an easy slide mechanism to connect both.

There are already some beam things available, but all are printed vertically which limits the beam length to 100mm on my reprap. Just printing the other models horizontally does not work well, because support material is needed which is very hard to remove.

Maybe the beams are good enough to build a MendelMax with PLA/ABS beams instead of aluminium beams. Any thoughts ?

It would be a T-slot based mendel, not really a mendelmax. 220mm is only 2/3 of the shortest standard MM extrusions.

these would likely be stronger than the vertical beams too.

Even if you find a way to make longer beams, a MM with plastic beams are not a very good idea. Aluminium beams is a lot stronger, more rigid and most likely both cheaper and faster to obtain as well. (But it is probably possible to build with plastic beams if you are careful)

it would be interesting to see just how much of the printer you can print.

I have printed som beams for prototyping before I order the actual beams. Saw this one earlier today. Will give it a shot as I need a few a bit longer. Good work.