645 Nylon is an extremely strong 3D printing filament. It is one of the original Taulman nylons along with 618. Designed to be stronger and a bit more transparent for optical inspection in industrial applications.
Optimal printing temperature is 245C. Bed coated with PVA and at about 70C is useful. Or use a cold bed with Garolite LE surface.
More information at the Taulman site