The Prusa i3 MK2 is a fantastic printer, and the Multi-Material upgrade makes the printer even better. Except in one way. You can no longer print flexible material, specifically Ninjaflex. This project is my investigation into rectifying this problem with the ultimate goal of printing Ninjaflex with a support material.
Another option for modifying the current extruder is to replace the 4-way multiplex and bowden setup with a direct extruder. This is like converting the MM setup back to the older single-extruder setup. However, I don't really want to revert my printer; instead I want an easily swappable direct extruder for when it makes sense (e.g. printing flexible material). Besides, the original Prusa extruder was never great for flexible materials anyway.
The above design is based on the previous bowden extruders I created, but adapted to fit into the slot left when the multi-material splitter is removed. The extruder slides into place and is secured using the same bolt. Short pieces of PTFE tubing are inserted into the top and bottom to provide a smooth path into the extruder and out to the hotend. The picture below shows the extruder mounted on the printer.
For printing single materials, this extruder has a couple of advantages: first, it will print flexible filament (such as NinjaFlex) at full speed, something I never would do before; and second, for normal filament I can greatly decrease the retraction and maintain or improve print quality over the bowden setup. Because of that, this has actually become my standard print setup when I don't need to use support material.
Of course, I did need to sacrifice one of the stepper motors to this design, leaving only 3 for the multi-material setup. However, because I mainly use the printer for either single or dual material printing, I don't find this a big loss.
In order to simplify the extruder path, today I used a different piece of PTFE tubing to run the filament straight from the extruder into the hotend (replacing the usual hotend PTFE liner). This gave me a much more direct path from extruder to heat block. The result (no photos - sorry) was much improved with the NinjaFlex being extruded far more reliably. Not *quite* print quality yet, but a little fussing with temperatures and extruder speed might be enough to fix that. This gives me hope that, if I can straighten the pathway for the flexible material, this still might work.
Reading about how to print NinjaFlex successfully, and after a few attempts to print it with the standard printer setup, my first goal was to replace the current MultiMaterial (MM) extruder with something with better filament support. Filament support, as best I can tell, simply means that the filament is given less freedom to deviate from the desirable path. I used the BondTech extruders as a baseline for this, I designing a different extruder part utilizing the same BondTech drive gears.
The extruder above is made from three pieces: The main body (green) onto which the stepper motor is attached, the idler (blue) to apply pressure to the drive gears and the clip (orange) to mount the extruder to the frame. I designed left and right handed version to match the rotation of the stepper motors on the Prusa.
In tests, the extruder performed very well, moving filament through the bowden tubes more effectively than the original design, especially softer filament. However, because the filament path is still very long, and not particular straight due to the MM multiplexer, this extruder still cannot push the NinjaFlex reliably into the hotend as it compresses too much. So, not the magic solution to printing NinjaFlex on the MM Prusa.