Several of the 3mm plastic filament spools we stock at the office lack internal cores but instead came loosely packed with zip ties. Having loose spools was never a problem with my old CupCake, I usually ended up hovering around the workshop while it was working and feeding it more plastic was convenient enough. However, once we got our Thing-O-Matic we have been printing much more hands off, the printer is in a separate room for noise reasons and we often just leave it alone for several hours. Long prints and intensive coding sessions turned out to be a bad match, in the best case someone had to interrupt their work every now and then to untwist the spool or alternatively we would lose prints when the plastic feed jammed. Not a good thing.
To fix this I hacked together a solution using 50mm plastic sewage pipe, some cardboard and a two 608 skate bearings. To assemble it all together I designed a simple coupler that press fits the bearings and sewage pipe.
The contraption worked but still had problems. The filament under tension cut through the loosely packed layers and jammed when it tightened around the core. We solved this by respooling the filament on the new core but the internal diameter of the spool is too small at 50mm and the few innermost layers have a lot of tension in them. As print segments of filament occasionally unwinds and there is even now a few turns of loose plastic around the spool.
The solution is good enough for now but I probably need to revisit it later when we get an Ultimaker and can print a coupler for pipe segments with larger diameters.
Here are links to the Thingiverse page and directly to the editable coupler.