Flexible polyester (FPE) is a generic label given to a 3D printer filament that combines rigid and soft polymers. Such filaments are comparable to PLA, but are softer and more flexible. The specific flexibility depends on the hard and soft polymers used, and on the ratio between them.
Two notable aspects of FPE include good layer-to-layer adhesion and a moderately high resistance to heat and a variety of chemical compounds. Given the wide range of FPE 3D printer filament that is available, perhaps the most useful way to differentiate between the wide range of FPE available is the Shore value (like 85A or 60D), where a higher number indicates less flexibility.
When Should I Use FPE 3D Printer Filament?
When flexibility in the print is desired, but ease of printing takes priority. Flexible filaments can be tricky to print, and FPEs go some way to offering an alternative that offers a little bit of everything. Easy to print, like PLA, but with the greater flexibility in the resulting print.