So far in our Filament Fridays we’ve concentrated on rigid filaments that leave you with hard parts. This week, things are getting squishy with NinjaFlex – the most popular of the flexible filaments. NinjaFlex was originally created by Frenner Drives, a company that specializes in belt drive systems. The Frenner team discovered that one of the materials they used for making belts could be used for 3D printing also, and now we have NinjaFlex. The popularity of NinjaFlex helped Frenner split off the NinjaFlex brand into its own company: NinjaTek.

Flexible filaments fall into two classes of material: TPUs (Thermoplastic Polyurethanes) or TPEs (Thermoplastic Elastomers). These filaments need specific print requirements and more tuned-in settings to print correctly, as they will do everything they can to wiggle out of your extruder instead of printing.

While not giving a perfect surface quality, a tuned in machine can print NinjaFlex very well. Overhangs can be an issue along with stringing during jumps. Be careful printing flexibles on PEI build platforms, as you may end up with a hole in your coating. Using a little PVA based glue to coat your bed will provide a protective film. Bowden style extruders are also difficult to print flexibles with; if you are getting started, stick with direct drive.

Beyond just the fun of making rubbery models, NijaFlex can be thermally bonded to rigid materials like PLA or ABS. Dual extrusion prints can be made with both rigid and flexible elements. You can also create things like your own belt drives, tank treads, gaskets and more.

If there is one flexible filament to try, NinjaFlex is it.

Check back every Friday for weekly reviews on 3D printing filament.

If you have a filament you would like us to try out or are a producer of filament, email me at and we will try to make it an upcoming installment of Filament Friday!