After multiple iterations and trial and error, I'm proud to present this foldable ruler to you all!

Model specifications:

• Size - 200x30x3 mm when folded; and 400x15x3 mm when unfolded. The maximum length that can be measured is 40 cm;
• Weight - 18 grams;

The design uses a locking mechanism and a one-piece printed hinge that meet the requirements. In addition, the two halves of the ruler fit together perfectly to prevent deformation under light loads

It was printed on the Ender 3 v2 neo using standard PLA material. I used the default settings of the Cura slicer, with a layer quality setting of 0.2 mm. The only change was that I used 100% infill during printing

I also tried printing with a layer height of 0.12 mm, but my printer wasn't calibrated well enough to maintain consistent quality at such a high level of detail. If you want to give it a try, you can give it a shot, though as far as I can see, the difference isn't very noticeable.

After trying it out, I can confidently say that drawing lines across the hinge won't cause any problems, and the lines will remain perfectly straight.

Note: The locking mechanism may get stuck a bit during printing, but you just need to press it gently with a sharp object in the gap until it loosens up, and then you can proceed with the next step.

Wish you a pleasant printing experience!