Updated version: https://makerworld.com/en/models/114211

[Update after a few months of usage] Some refill coils may push the spool walls outwards and you may end up with a ruined spool. To prevent this, apply a bit of superglue to the grooves or ridges of the adapter

— Initial description —

This is my first attempt to create a minimum viable adapter from Prusament spools to refill coils using Reprap's MasterSpool standard. The goal was to use as little filament as possible and to make it usable without additional hardware.

Carefully remove the edges of the Prusament spool. To minimize the chances of breaking them, pull from the area that is closest to the cardboard core. Do this from multiple points while rotating the spool until the cardboard core is removed.

To install the new filament coil:

• Insert one edge into the printed adapter and the adapter into the new filament coil. DO NOT REMOVE the zip ties holding the new coil yet

• Take the end of the filament coil (no, that's not the end that goes into the extruder… it's the other end) :) and pass it through two of the spool holes to secure it in place and avoid the coil spinning freely on the spool). You should end up with something similar to the following picture (maybe in the future I'll edit the adapter to add filament mounting holes directly to it)

• Insert the second edge of the spool without pushing it all the way in so that there is enough room to remove the zip ties
• Cut and remove the zip ties
• Press both spool edges all the way in

To save filament, print with 3% cubic infill and a single wall perimeter. This way it takes less than 25 grams of filament and on a CoreXY printer it takes less than one hour to print. You can experiment with even less infill and see if it's still usable.

The height of the adapter is 50mm, and it fits 750g refills. The outside diameter is 102mm. A STEP file is included if you want to modify it to fit wider spools. Tested with FormFutura refills, but it should work with any refill that follows Reprap's MasterSpool standard.