Snap-fit hinge wallet / card case

I wanted to switch from the classic thick leather wallet to a cards-only slim wallet, and I thought this would be a great thing to 3D print. Having tried many of the models already on Thingiverse, I found them all lacking. They all struggle with the same problem: how do you keep the cards securely in place, but also easy to remove?

Leather is a great material for this. The high friction keeps the cards from moving, while the flexibility allows you to easily push the leather aside so you can firmly grab the card and pull it out with enough force to overcome the friction. PLA as a material has very little friction and is very stiff, so this approach doesn't work.

Some designs get around this by requiring you to add a non-3D printed part that has these properties, such as a piece of cloth. Others try to clamp the cards in place by making the inside ever-so-slightly smaller than the size of a card, so you can force the cards inside and they will stay there. This requires very tight tolerances - something 3D printers aren't very good at, plus everyone's printer is tuned a little differently. It also puts the plastic under constant strain, which (at least in the case of PLA) will deform it over time and make it lose its grip.

This design solves the problem by using a lid with a snap-fit joint. The plastic has to flex a little when opening or closing the lid, but there is no strain when fully closed or fully open. By allowing the entire lid to flex, even a stiff material like PLA does not flex beyond the point of no return. This means the snap-fit joint will not fatigue with repeated use, and so will last a long time.

After a few iterations, I've been using this version of the design as my daily driver for over a year now. It shows no signs of fatigue - and I love fiddling with the snap-fit hinge all day long!

The reason the lid is slightly curved is so that it has some space to flex. A previous version where the lid sat flush against the body broke from fatigue, since only a short section of the lid near the snap-fit joint was able to flex.

The lid hinge is printed in-place and should break free with some gentle wiggling. This is the only part that is sensitive to your printer's tolerances. It prints just right on my Ender 3. If it comes out fused solid for you, let me know and I'll see about uploading a version with looser tolerances. Alternatively, try setting a slightly negative horizontal expansion in your slicer.

The number of cards it holds is easily adjustable, so I've uploaded a number of STL files that each holds a different number of cards. Or you can change it yourself easily in the OnShape document, all it takes is changing a variable: https://cad.onshape.com/documents/3b90c37d91cfc08159a670da/w/1eb86d447dafe104e1b7539f/e/7cff152818f2e56687d99954?renderMode=0&uiState=621d3f907732ee156dac47cd

I recommend tuning the horizontal expansion and horizontal hole expansion settings in Cura to print the hinge so it's tight but not fused together. For my printer, -0.2 horizontal expansion and 0.1 hole expansion works, but it will probably be different for your printer. I also recommend printing with 4 walls for extra strength.