A living hinge is a thin band that allows a plastic part to be bent without breaking. Applications include parts that bend once for assembly—a flat part that folds into a cube for example (see Figure 1)—or a cover that may open and close hundreds, even thousands of times (see Figure 2).
Figure 1: Protomold Sample Cube
Figure 2: Typical living hinge application
We’ve often pointed out the consequences of forcing melted resin through a thin area of a part into a thicker area. The most serious of those consequences is the possibility of voids in the thick area beyond the “bottleneck.” To create a living hinge, you must do exactly what we’ve warned you against: force resin through a very thin area into a thicker section of the part (see Figure 3).
Figure 3: The arrows above point to the very small section where resin would be forced through to fill the other side of the body of the part. The resulting web of resin is thin enough to allow repeated bending.
The only alternative would be to have resin enter through two gates and meet at the hinge. Unfortunately, this would create a knit line (a weak area in any part but especially problematic at a thin area) precisely where you want the part to be strong enough to bend without breaking. So, since living hinges are so very useful, the question is how to avoid—or at least anticipate—problems if we choose to include them in a design.
Rule One: Scrupulously follow the guidelines for hinge design. These can be found in the earlier design tip, Night of the Living Hinge.
Rule Two: Follow all the other guidelines for molded part design; just because you are bending one rule—no pun intended—doesn’t mean you can push your luck on others.
Rule Three: Choose an appropriate resin. Choose one that flows well through thin areas and that, assuming a properly designed hinge, can handle bending without weakening or breaking. The ideal resin for living hinges is polypropylene; second choice is polyethylene. Depending on your application, there may be other resins with limited applicability, but if you absolutely require the strength of a glass-filled nylon or the durability of acetal, for example, you probably shouldn’t plan on using a living hinge.
Rule Four: Prepare for possible cosmetic consequences. A living hinge is like a construction bottleneck on a freeway; during injection it’s going to increase pressure on the upstream side of the hinge and decrease pressure on the downstream side. Also, because of the mold’s increased surface-to-volume ratio at the hinge, resin is going to cool significantly at that point. Consequences downstream of the hinge may include sink, flow marks, and bubbles. (Bubbles may not be apparent in an opaque resin, though they may affect strength. If you use a clear resin, bubbles will be visible.) Consequences upstream of the hinge may include flash caused by the increased pressure needed to force resin through the thin area.
Gate location is critical in a part with a living hinge. Protomold will propose a gating plan for you to approve. The plan will take into consideration the presence of a living hinge, but final approval will be yours. As with all submitted designs, your ProtoQuote will include a design analysis that will identify other potential issues to consider.
Finally, if you need a hinge and the living kind just won’t work, there are some very clever alternatives. For example, our previous design tip Matched Mating Parts shows a two-part box with molded-in hook-and-axle hinges. Got questions about hinges, living or otherwise? Proto Labs Customer Service Engineers are standing by to help at 877.479.3680.