Friction-reducing Spool Sleeve

A simple sleeve to reduce friction between the spool holder and cardboard spools, without the complexity of many bearing/spring/retraction designs.

I found that cardboard spools induce a lot more friction against the spool holder than almost any type of plastic spool. I've tried various bearing-based spool-holder designs in the past, but I find many of them are too smooth rolling and can cause the spool to unwind in bursts, possibly leading to a tangle.

So this simple sleeve, when printed from a harder material like PLA, will act as an smooth-running intermediary between the cardboard spool and the spool holder. Its internal ribs provide a very minimal contact surface with the spool holder, so it's turns easily (but not completely freely, like bearings) even with a full spool on it, and the outer surface of the sleeve provides lots of surface area for the cardboard spool to grip without having to slide across it. The concave outer surface tends to keep the spool centred on it, so the spool doesn't get pulled tight against the centre strut of the spool holder.