3D printing with flexible materials is not easy. Even if you manage to 3Dprint properly with it, you have to print very slowly (35% regular speed) which can be cumbersome on larger or high detail projects. In order to get around the speed factor, I used a 0.60mm nozzle and a 300 micron layer height (default is 200 microns). This cut the print time in half to under an hour for each prototype because each layer is 50% larger. 3D printing flexible material is almost impossible on a Bowden style 3D printer, but with a direct drive extruder like the Prusa i3 MK2, simply shut off retraction and let it run!
Since the design doesn't require high detail I was able to successfully use both a larger extrusion line and layer size. I found that the semiflex material printed much easier when compared to using the default 0.40 mm nozzle. In retrospect that's not surprising, as the material will have a larger hole to exit the nozzle requiring less force to push it through the nozzle. This helps with flexible materials because they can get stuck and jam in smaller nozzles.
Most of the time, the flexible material will fail because it will bend if too much force is trying to extrude from the nozzle. It becomes as difficult as pushing on a rope (which if you've ever tried, is impossible). You have to find the right balance of "pull" from the extruder gear and pushback from the filament squeezing through the nozzle. With a bigger nozzle, the same settings worked just fine to extrude the flexible material.
WHAT DID I LEARN?
What I learned from the first prototype was that the vertical guide did not need to be a solid stiff piece, but worked much better as a hollow tube with 2mm thick walls for strength and flexibility. This way the top piece wouldn't just snap off one day. It will have room to move before it breaks, meaning it should last longer and be more durable.