Many people have heard of “living wills.” And everybody knows what a “living room” is. But how many people, outside the plastic injection molding industry, have heard of a “living hinge”?
Unlike the aforementioned “living” examples, a living hinge is a thin flexible hinge fabricated from the same material as the two rigid pieces it connects. It is most often thinned or perforated to allow the rigid pieces to bend along the line of the hinge. In our industry, living hinges are most commonly manufactured from polyethylene and polypropylene because both those resins can be opened and closed literally thousands of times before it breaks off. Living-hinges are everywhere you look: dental floss boxes, pepper can plastic lids, clam-shell food service containers — the list is endless. Even high-tech medical devices like hemostats can benefit from living hinges, especially if cost and ease of manufacturing are issues.
While the range of products which incorporate living hinges is vast, the rules to mold them are rather simple. There are really only five:
- Hinge Design
- Resin selection
- Mold design and fabrication
- Injection molding process
- Quality Assurance testing
As simple as these rules are, chaos could ensue if not followed scrupulously. (In fact, you might say everything “hinges” on it!) Each step is intended to make a hinge which can take thousands, if not millions of openings-and-closings over the lifetime of the hinge without tearing, breaking off or otherwise making your life (and business) miserable. One more thing: one of the goals should always be producing hinges that are molded without flexing upon ejection from the plastic injection molding machine. .
Without belaboring the point, if you think you can skip any one of these steps, you’re sadly mistaken. Serious attention to each rule is a must. Again, if followed exactly, this process can relieve you of the common production issue of flexing the hinge immediately after ejection. While flexing can possibly help in certain circumstances, it’s unnecessary if you stick the five simple rules. Simply put, you are wasting time and losing money by flexing living hinges. Unfortunately, many molders just want to get the mold built and start making parts and don’t like to mess with the details.
But that would be a mistake: more on that in a later post.