Living hinges: bending without metal parts

How a living hinge works

The hinge is really just a thinned spot in an otherwise rigid part. Two rigid parts, for example the lid and body of a box, are joined by a thin strip that bends when you open it. Lids made of polypropylene are classically made this way, and Wikipedia describes the principle. In 3D printing you use it where you do not want to assemble several parts and pins.

The strip must not be too thin, though. With a 0.4 mm nozzle a thickness of 0.4 to 0.6 mm can be printed, but such a strip is only one or two walls. The bend is then only as good as the layers hold together.

Mind the layer direction

Orientation decides. The hinge must lie so that it bends along the layers, that is the layers must run along its full length. If the hinge bends across the layers, it cracks the first time you open it. The safest approach is to print the lid and body flat and leave the hinge as a thin strip in the plane of the bed.

Also, a hinge bent to the same angle again and again tires sooner than you would expect. The bend should be long and gentle, with a transition to full thickness through a fillet, not a sharp notch.

For first tests leave the strip a bit thicker, for example 0.6 mm, and longer. A thin, short hinge cracks first, especially when opened abruptly.

PLA, PETG and other materials

A hinge in PLA often breaks after just a few bends, it is a brittle material. PETG lasts noticeably longer, but it is still not polypropylene, which is ideal for this. We do not print polypropylene, so rely on PETG and design the hinge with a margin.

Plan the hinge for occasional opening, such as the lid of a box for small items. If it is going to open a hundred times a day, use a classic two-part hinge instead, either assembled or printed as print in place.

When ordering

Nothing is finished after printing, so the hinge has to be complete in the model. Upload the model, choose PETG and order one piece first to try. You see the price and lead time straight away. Infill strength matters little here, what counts are the walls and the layer direction at the strip.