May 2004 Design Tip

Living with hinges

At Protomold we don't design parts - that's your job. But if you happen to be unfamiliar with the technique called "living hinges", this Design Tip may come in handy someday.

Figure 1A, 1B. Without a living
hinge, this box would require
two moulds, two moulding
operations, and assembly.

As described in detail by Dr. Glenn Beall in his August 2002 Injection Moulding Magazine article, "By Design: Polypropylene part design, Part 2 - Living hinges", in the late 1950s it was discovered that below a certain thickness, polypropylene molecules oriented in the direction of flow. And repeated bending perpendicular to that orientation was possible without breakage due to the increased strength that resulted. The name "living hinge" was given to this technique and has been used ever since.

Living hinges are very useful in certain designs for injection moulded parts because you can combine two or three parts into one. And as noted on (an excellent online engineering fundamentals resource) in a page dedicated specifically to living hinges, a well designed hinge in these materials can last for millions of cycles. Additional materials with somewhat less of a life expectancy are Nylon and Acetal.

Check out both of these excellent articles for additional technical design details and illustrations.

Here's a recent example designed by our customers and manufactured via Rapid Injection Moulding:

You can visit the Protomold Design Guide for other helpful Rapid Injection Moulding design information.