A living hinge is a thin section of plastic moulded as an integral part of a component so it can flex repeatedly without a separate pivot. Done properly in polypropylene it survives an enormous number of cycles. Done casually in the wrong resin or with the wrong gate it fails within days, and the difference between those outcomes is a small number of specific rules.
The geometry
The hinge is a thin web between two thicker sections, with generous radii blending into each side so there is no stress concentration where the thin section starts. The web must be thin enough to flex without yielding and long enough that the bend radius does not exceed the material's limit. The transition matters as much as the thickness, and it is where most failures start.
The resin decides whether it works
Polypropylene is the material for living hinges and nothing else comes close. Its semi crystalline structure orients under the flow and the flex, producing a hinge that fatigues extremely well. Polyethylene is a distant second. Amorphous resins such as ABS and polycarbonate crack at a hinge after a modest number of cycles, so a living hinge in those materials is a design error rather than a compromise.
Gating and the first flex
The melt must flow across the hinge rather than along it, so the polymer chains orient across the flex axis. That is a gate position decision made in tool design. The part should then be flexed once immediately after moulding while it is still warm, which sets the orientation permanently. A hinge left unflexed for hours never reaches its full life.
Printed living hinges
A printed hinge cannot orient the polymer the way moulding does, so it will not match a moulded one. Sintered nylon gives the best result and survives a useful number of cycles. Filament printed hinges fail along layer boundaries unless the layers run across the hinge rather than along it. Use a printed hinge to check geometry and expect a moulded one to behave differently.
Questions people ask about 3d printed living hinge
Which plastic makes a good living hinge?
Polypropylene, by a wide margin, because it orients under flow and flex and fatigues extremely well. Polyethylene is second and amorphous resins are unsuitable.
Why must the gate be positioned carefully?
Because the melt must flow across the hinge so polymer chains orient across the flex axis. Flow along the hinge gives a hinge that fails early.
Should a moulded hinge be flexed immediately?
Yes, once, while still warm from the tool. That sets the molecular orientation and is the difference between a hinge that lasts and one that does not.
Can a living hinge be 3D printed?
It can be approximated, best in sintered nylon. It will not match a moulded polypropylene hinge, so treat a printed one as a geometry check.