I recently started getting into pen turning and needed a mold to make my own blanks. I initially intended to make a typical silicone mold but screwed that up. Being impatient, I decided to see if a TPU mold would work. Good news, it does! So far I've pulled 4 blanks out of the same mold and have another one curing right now!
You can find a full writeup on my instructable page here:https://www.instructables.com/id/Simple-Reusable-3D-Printed-Mold/
Print in flexible filament, TPU. Since this is a mold, it needs to be water tight. To accomplish this turn up the temp and increase the flow rate to make sure all the layers fully fuse. Extruder: 230*C Bed: 60*C Infill: 100%