NInjaFlex adheres exceedingly well to PEI print surface. We strongly recommend applying PVA glue (white school glue) directly on the print surface using a glue stick (such as Elmer's® brand) before printing to ensure easy part removal.
From Ninjaflex website:
The Market Leading Flexible Filament
NinjaFlex® flexible filament leads the industry thanks to its superior flexibility and longevity compared to non-polyurethane materials. Its consistency in diameter and ovality (roundness) outpaces other polyurethane materials. Made from a specially formulated thermoplastic polyurethane (TPU) material, this patented technology boasts a low-tack, easy-to-feed texture. The result is uniquely flexible, strong prints ideal for direct-drive extruders.
Truly flexible (no fill or layer manipulation required to achieve performance)
660% elongation allows for repeated movement and impact without wear or cracking
Polyurethane composition allows for excellent vibration reduction
Patented low friction exterior allows for smooth feeding
Abrasion resistance 20% better than ABS and 68% better than PLA
Consistent diameter and material properties providing reliable, high quality prints
Chemical resistant to many industrial materials, including naphtha, ASTM Oils #1-3, petroleum and freon
Adheres to all standard build platforms without glue/tape