Creative Mechanisms has considerable expertise in machining polypropylene to produce living hinged prototypes. 

But no one believes us! 

This is because most engineers believe that, for numerous technical reasons, it is impossible to machine polypropylene.

To convince skeptical professionals, we created a Creative Mechanisms business card holder that has been machined out of a single piece of polypropylene. It has two living hinges, a flexible finger to hold the cards in place, and a series of snaps to complete the assembly of the two halves of the product.

If you don't believe your eyes, call me and I'll send you a free one with my compliments – no kidding.

In the meantime, this video offers some details: 

Machining polypropylene, while not impossible, is extremely difficult. It helps that this material is very soft, allowing a living hinge made from polypropylene to bend multiple times without breaking. The downside of this softness is that it makes the material very difficult to cut when the heat from a CNC endmill begins to melt the material.

In addition, polypropylene is notorious for not machining cleanly. It tends to gall up as it is being machined, which leaves a rough, burr-filled surface.

And that's why engineers think it's impossible to do what we are doing.

But we are doing it. In fact, we are able to machine polypropylene down to only five thousandths of an inch in thickness with a relatively clean surface. We are also able to produce several types of living hinges. The hinge for the lid has straps on the side, which help it to spring open. The hinge on the back side is a typical living hinge, as it is only used to allow the top half and the bottom half to close and snap together.