While designing a cable clamp for an upcoming project, I ran an experiment to find the practical minimum size hinge my printer is capable of producing. I rarely print smaller than 2mm resolution, so that became my control. I still don't know what the lower size limit is, since I stopped when my test part got down to 1/4" square x 1" long. I realized that was as probably as small as I'll ever use.
If you ever find a need for a small printed hinge, this is a tight, clean design.
Step 1: Design Logic
Experience has taught me that at my printer's .2mm resolution, the gap between parts needs to be .5mm (.020"), no matter what the size or type of hinge is being made. Experience has also taught me that a 30% fill and a single shell makes for the best performing hinge. Common sense says the use of supports is a no-no. My initial designs were copied from those inexpensive plastic containers consisting of a ball (or dome) and bowl connection. They worked, but didn't hold up well under heavy use. I changed to a more traditional gudgeon and pintle design.
The g&p hinge turned out to be very robust, but unless the pins have a relatively large diameter, they're prone to breaking. One advantage of the g&p design is the ability to add metal washers when the print reaches the level of the hinge's gaps. Adding other materials to a print increases the printer's usefulness exponentially.
I think the design of the small hinge is a good compromise between strength and size. The cone shapes are extremely strong and their sloping sides don't taper and pull apart as ball & bowl hinges do. Cones are also less susceptible to overhang issues. The cones' sloping sides allow each successive layer to be partially supported by the one beneath it, internal material sag is eliminated.
This type of hinge has its drawbacks as well. The smaller the mechanics get, the more pressure is required to break the hinged halves free. Placing one end in a vice and gently taping the other works quite well. You only need to break the bond once, but the requirement of having to use tools is awkward.
The cone-shaped hinge is great for small items, but the combination of the .020" gap dimension and the lack of sagging material filling the gaps, results in the parts of larger items being loose. Added to the increased lever moment of the larger object, the hinge loses its precise feeling, becoming somewhat unstable laterally. The strength of the cone stile hinge seems to be on par with the p&g style however, so it ultimately comes down to a matter of preference.
Check out my fun chopstick project to get your hinge ideas flowing. Maybe it'll whet your appetite for more mechanical prints: