One of the more interesting new filament types that have come out in the recent months is FlexiFil, a Thermoplastic Co-Polyester sold by FormFutura. They were kind enough to provide us with a sample so we could try it out on our Prusa i3.
As the name suggests, this filament is flexible. Very flexible. It is so flexible that normal extruders have trouble handling it, as it is likely to buckle and fold between the hobbed bolt and the hotend. We already saw this happening with LayWood, which is a lot less bendy than FlexiFil. Luckily, the people at LulzBot have designed a new extruder specifically addressing this problem: the FlexyStruder. We built one ourselves (LulzBot open source everything they develop, how cool is that!), more about that here.
With the Flexystruder, printing with FlexiFill is a breeze. FormFutura recommends printing very slowly with this filament (10-20 mm/s), presumably to prevent the filament from jamming, but we were able to print at our regular speed (up to 70 mm/s!) and we never had a single jam.
At a nozzle temperature of 200ºC, extrusion was smooth and the inter-layer adhesion excellent. The bed surface we used was BuildTak, which is turning into our go-to surface for printing just about anything. However, at a bed temperature of 60ºC, the bed adhesion of FlexiFil left something to be desired, especially for long, narrow prints such as the iPhone bumper we printed. After a few layers, we saw serious corner warping and we had to cancel the print. We increased the bed temperature to 110ºC and added a 4mm brim for good measure and the bumper stuck just fine. Removing the brim is a bit difficult without tearing the material so make sure you use a very sharp knife.
As you can see, the iPhone bumper can be scrunched up completely and it will immediately return to its original shape. This property makes this material perfect for anything where you need flexibilty, such as hinges or custom seals/gaskets. The flexibility/stiffness of the print can be tuned by varying the infill percentage, which means that you could even have different sections of the printed parts have a different flexibility.
All in all, we are very happy with the results that FlexiFil gives us, especially in combination with the FlexyStruder. It opens a whole new range of possibilities!