Due to its toughness and ductility, polypropylene (PP) is uniquely suited for a fastening and joining concept that can't easily be reproduced with other engineering thermoplastics: the integral or living hinge. Molded-in hinges aren't new, but designers under the gun to consolidate parts and eliminate assembly steps are taking a closer look at molded-in hinges for everything from glove-box bins and tackle boxes to CD cases and electrical connectors.
Living hinges can be molded from any PP resin including homopolymers ( single monomer), copolymers (mixed monomer), impact copolymers ( toughness enhanced), and random (nonordered monomers). And while injection molding is the manufacturing option of choice, other processes, such as extrusion, blow molding, and stamping, can also form hinges that flex more than a million cycles without failure.
There are two basic types of integral hinges molded-in and die formed. Molded-in hinges are the most widely used because they can be made with conventional injection-molding techniques. Key to designing a living hinge that survives the long haul, however, requires proper polymer processing during molding. Optimum hinge performance, including high strength and good fatigue endurance, comes when PP molecules are oriented perpendicular to the center axis point of the hinge. The mold must be designed so the polymer flows across, and not along, the hinge length, and preferably at high velocities and melt temperatures. Filling too slowly, using low melt temperatures, and nonuniform flow through the hinge may cause premature hinge failure.
It is also important that the polymer's flow front crosses the thin section of the mold instantaneously. Gate locations must also provide balanced mold fill. Designers should be aware that a substantial pressure drop occurs as the flow front crosses the hinge. This increases the polymer's shrink rate and may require dimensions of the mold cavity be modified to ensure proper fit between the mating halves.
Another important molding feature that must be addressed is weld lines. These should be located away from the hinge, so gate location must be carefully designed. Another necessary step to further maximize hinge strength is to flex the hinge at least twice as it ejects from the mold. This helps optimize molecular orientation and boosts durability and strength.
Key hinge features include thickness, the radius on its “outside” surface, and the recessed, flat “land” element on top. Thin cross sections are needed to restrict the flow and orient the PP molecules perpendicular to the hinge. How thick the hinge is also controls stiffness and the amount of force needed for flexing. Optimum hinge thickness is often set at 0.015 in., but this may need to be tweaked when the hinge is tested because the closing force or “feel” is subject to individual interpretation.
The radius on the “outside” or tension side of the hinge provides the material needed for stretching. The radius should be 0.03 in. to allow the minimum thickness of the hinge at the center. A 0.03-in. radius also makes it easier to mold.
The flat land on the “inside” or compression side correctly orients the two hinged components and provides an opening for the deformed hinge to sit. Ideally, the flat section can range from 0.05 to 0.09 in., with 0.06 in. as an optimum. The flat must also be centered over the radius and recessed at least 0.005 in. from the surface to provide room for the hinge to close and ensure a smooth hinge bend. Notches must be avoided, but radii at the ends of the hinge help prevent stress risers and premature failure.
For larger parts or more complexdesigned parts, a hinge may be coldformed, or coined, into a surface mechanically. In this case, polymer flow may not be considered as critical as it is for molded-in hinges. A heated die compresses a slot into a flat surface between the two mating parts. Most of the polymer moves away from the die upon contact. The remaining thin wall cross section plastically deforms creating a living hinge. Hinge thickness is typically on the order of 0.01 to 0.02 in.
When forming the hinge it's important to keep the compression (die) force from exceeding the ultimate stress of the PP otherwise the hinge could fracture. The male section of the die needs to be heated between 250 and 290°F and the area supporting the part during die forming can be either rigid, such as steel, or flexible, such as a rigid elastomer or rubber. It takes about 10 sec to form the hinge.