A small, one piece, easy to print carabiner with a good strength for a reasonable thickness (up to 70kg for a 6mm diameter).

Of course, never assume any strength from this thing if safety is at stake !

Like other existing designs, the gate is printed "outside", so that it closes by elasticity once inside. It wraps around the hook to increase strength and is slightly warped, just enough to lay flat on the print bed.

I did some tuning with the shape to improve the strength. The opening is one of the weak spots (I wanted the hook to have the same diameter as the body), so the D shape ensures that it doesn't carry as much load as the other side.

PLA provides excellent strength but it requires a good amount of force to open it. ABS is weaker, it starts to deform at lower load. The discoloration gives an indication of where the stress is the highest. I had mixed results with PETG, nylon and polycarbonate. It may require more tuning.