Injection Molded Living Hinges: Design, Material, and Performance Insights

| November 28, 2023

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Living hinges, the flexible webbing that connects adjacent wall sections in a wide array of applications, are a remarkable feat of engineering when designed and implemented correctly. As a common feature utilized across various industries, from consumer goods to industrial equipment, their reliability and longevity depend on factors such as material selection, design considerations, and molding techniques.


Material Matters

While several materials can be employed to create living hinges, polypropylene (PP) stands out as a favored choice due to its exceptional hinge performance. The material's ability to stretch (elongation at yield) is crucial, and incorporating fillers or reinforcements can adversely affect this characteristic. Lower aspect ratio fillers, like calcium carbonate, tend to maintain hinge performance better than others, considering factors such as hinge longevity and cycle requirements. Even Polyethylene (PE) can be used to create a living hinge if designed appropriately and the cycle requirements are limited.

Choosing the Right Material

For high-cycle polypropylene hinge applications, a few critical aspects come into play:

  • Homopolymers vs. Copolymers: Homopolymers provide the highest performance, followed by random copolymers and impact copolymers. Impact copolymers may be preferred for low-temperature applications.
  • Molecular Weight: Higher molecular weight contributes to hinge durability, although considerations about viscosity and overall processing are essential.
  • Nucleation (Additives used to increase the crystallinity of PP): Nucleation can improve hinge quality by freezing orientation at the hinge, but in scenarios with flow hesitation, it might be counterproductive.
  • Environmental Factors: Ultraviolet (UV) exposure, especially in outdoor settings, can degrade hinges. UV absorbers can mitigate this effect.

Design and Implementation

Living hinges' potential to endure thousands of cycles centers on proper design, processing, and implementation. While design variations are permissible, a typical flat hinge design for polypropylene might have a thickness of around 0.008 inches. Ensuring proper gate placement to facilitate symmetrical flow-front formation is crucial to minimize weld lines and hesitation. Utilizing mold filling simulations aids in determining optimal gate placement, geometry, and design. Additionally, proper cooling channels near the hinge minimize shear heating and enhance hinge durability.

Cold Drawing for Durability

For exceptionally durable hinges meant to endure thousands of cycles, post-molding flexing, or cold drawing, is recommended. Cold drawing is a process that uses tensile forces to elongate and thin out polymer materials, in this case, PP. This process enhances molecular orientation, thereby extending the hinge's long-term durability.

In summary, injection molded living hinges are a testament to precision engineering. With the right materials, careful design, strategic gate placement, and thoughtful cooling solutions, these hinges can thrive under rigorous cycles and provide solid performance in diverse applications. It is key to remember, the combination of material selection, design, and manufacturing is what brings these hinges to life, making them an indispensable feature in modern design and engineering.


About the Author

Cesar Alcantar | Senior Application Development Engineer

Cesar currently serves as Senior Application Development Engineer. Cesar has 30 years of experience in the plastics industry. During this time, he has held technical, sales, marketing, and management roles. Prior to joining Nexeo Plastics, Cesar was employed at General Motors, GE Plastics, and Celanese Engineered Materials where he served in several roles supporting their global customer base in most major markets and applications. Here he was responsible for developing new applications for manufactured resins, with a focus on injection molding thermoplastics in the global marketplace. Cesar has extensive knowledge in 6 Sigma processes, QS trained, DFMA, Lean manufacturing, Project Management and Change management, and acceleration and 3D printing processes. His role is to assist customers with material selection, processing challenges, part design, and providing general feedback on new product development. Cesar has a Bachelor of Science in Mechanical Engineering from the University of Texas at El Paso and many years of industry training in injection molding, extrusion, and 3D printing.

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