Flexible polyester filament is a thermoplastic elastomer; which basically means it is "printable rubber". It is the flexibility is indicated by the Shore Number, 45D in this case (we have multiple variants, 45D is our medium hardness). It can be processed in most current 3D printers, but it may take some trials to get it work exactly as you want it. Please check the guidelines below. FPE is very suitable for printing objects like phone cases and robot tyres.