Creative Mechanisms has considerable expertise in machining polypropylene for 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 one with my compliments. No kidding.
In the meantime, this video offers some details:
Machining polypropylene is extremely difficult. The good news is that this material is very soft, which allows a living hinge made in this material to bend multiple times without breaking. But this softness creates a very difficult material 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 and leave a rough burr filled surface. As you can see we are able to get a relatively clean surface considering the material we are using.
That's why engineers think it's impossible to do what we are doing.
In fact, we are able to machine polypropylene down to only five thousands of an inch in thickness.
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, once the lid has passed a certain point. The hinge on the back side is a typical living hinge, because it is only used to allow the top half and the bottom half to close and snap together.