A: The Railcore is designed around panels that provide rigidity and strength. The extrusion (1515) is mainly to give something to bolt the sides and parts to. Open frames can be made very strong, but boxing them in increases their rigidity by at least an order of magnitude.
Long A: The side panels increase rigidity over *inlet braces* (diagonal bracing) by an order of magnitude. It's nearly two orders of magnitude over an unbraced open frame.
To put this in perspective, going from 1515 to 2020 will likely increase your ultimate (failure) strength by ~1%, and your rigidity at the loads we're talking about by much less, probably .5-.25%. The strength and rigidity of the open frame go up linearly with the cross section of the frame members, so 2020 isn't quite twice (i.e, 225mm^2 vs 400mm^2) as rigid or strong in an open frame construction.
The only engineering math I can find about the relationship is in regard to constructing buildings.
The rigidity of the construct is the *in plane* rigidity of the panel material, with some reduction for mounting hole deformation. So the rigidity is the force it takes to make that rectangular panel of HDPE turn into a rhombus, with the ultimate strength limit being the tearout strength of the mounting holes and screws.
A: The Railcore is open source, so you can take anything useful you want and re-use it. That said, this is not a simple undertaking; many parts have 7.5mm specifications for the 1515 extrusion. If we had known originally that 1515 was so hard to source overseas, we might have started with 2020, but we had no idea. Sorry.
We've experimented with several means of additional support, with no change in print quality. Keep in mind that the rail is 1) hardened steel, and 2) 9x12mm bar of hardened steel. This is roughly equivalent to the standard 2x8mm smooth rods used in typical 3d printers from a while back.
We've tested with Bowden and direct configurations at our normal printing speeds (~60-100mm/s), and found no difference in print quality. We felt that for most purposes, the direct extruder was a less troublesome solution.
That said, if you prefer a Bowden configuration, we made it simple to switch - it takes about five minutes to remove the extruder from the carriage, bolt it to the chassis, and move the filament guide tube to the “Bowden” position.
Short A: yes, it's open, download files and go crazy!
Long A: Probably not. To maintain the same performance and precision as you scale up becomes progressively more expensive *per cc of build volume*. FEA in F360 suggests to me that building a 500^3 printer will be a significant engineering challenge, and much more expensive. CoreXY has long belts already - scaling up means either much wider belts - with the much higher tensions that entails - or abandoning the corexy mechanism.
You can do it with the same parts, but you'll sacrifice speed and precision and fundamental robustness. Also, remember, all other things constant, build time goes up as the cube of the linear expansion :D