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How do you design a living hinge in a moulded part?

A moulded living hinge is a thin web, usually about 0.2 to 0.5 mm (roughly 0.008 to 0.020 in) thick, joining two thicker sections, almost always moulded in polypropylene. Published starting points include a thickness of about 0.012 to 0.015 in with a 0.030 in radius on the outside of the bend. The plastic must flow across the hinge, not along it, and the hinge should be bent several times as soon as the part is ejected.

Living hinges are used on flip-top caps, boxes and clips. Made well, they can last for the life of the product; made badly, they crack within a few flexes.

Published dimensions

The figures agree closely. Machine Design explains the logic: the thin section restricts flow and lines the polymer molecules up across the hinge, which is what gives it fatigue life, while the thickness also controls how stiff the hinge feels.

Material

Fictiv describes polypropylene as the best material for a durable living hinge because of its toughness, ductility and ability to survive millions of flexes, with polyethylene as the usual alternative. eFunda notes that polypropylene and polyethylene hinges can survive over a million flex cycles. Stiffer, less ductile resins are rarely used for moulded-in hinges.

Moulding rules that decide hinge life

• Flow across the hinge. eFunda advises placing the gate so the plastic flows across the hinge; avoid layouts where two flow fronts meet in it, because a weld line in a hinge is a weak point.
• Flex it while warm. eFunda advises flexing a moulded hinge at least twice as it comes out of the mould, while it is still hot. Fictiv says this cold-draws the plastic and greatly increases service life.
• Start thin, adjust later. Fictiv quotes a toolmaker's trick of starting with a deep flat land and a thin hinge, because steel can be removed to thicken the hinge after testing.
• Avoid notches. Machine Design recommends rounding the ends of the hinge rather than leaving sharp corners, which concentrate stress.

Coined hinges

A hinge can also be pressed into a flat moulded part with a heated die, called coining. Machine Design gives a die temperature of about 250 to 290 °F, about 10 seconds to form, and a finished thickness of about 0.01 to 0.02 in; eFunda gives 0.25 to 0.5 mm after coining.

Prototyping a hinge

3D printed hinges are weaker than moulded ones but useful for checking fit and feel. Fictiv suggests a printed hinge at least twice the printer's resolution, for example starting at 0.5 mm in printed nylon. Final life testing needs moulded parts in the production resin.

Dimensions here are published starting points. Hinge feel and life are confirmed by testing moulded parts and adjusting the steel.

Questions buyers ask

How many flexes will a living hinge survive?

A well-designed polypropylene hinge can survive a very large number, which is why they are used on caps opened every day. The exact life depends on design, resin grade and moulding.

Can I use ABS for a living hinge?

Generally not for repeated use. ABS lacks the fatigue resistance of polypropylene; consider a separate hinge pin or a polypropylene part.

Why did my hinge crack at one end?

Common causes are a weld line in the hinge, a sharp notch at the ends, a hinge that is too thick, or flow running along rather than across the hinge.

Should the hinge be on the parting line?

It depends on the part and mould layout. Agree the hinge position and gate location with the toolmaker early, as both affect hinge life.

Sources

• Protolabs, Living hinge basics for injection molding
• Machine Design, Care and feeding of living hinges
• Fictiv, How to design a living hinge
• eFunda, Living hinge