Tips to simplify designs
I think I know what you’re thinking. Living hinges are for take-out food trays and school lunch pails.
We are here to lift their status in your mind, because we know them to be a powerful design enabler that’s less used than it should be, precisely because of this cheap association.
A living hinge is a thin, flexible blade in a molding that connects two rigid regions, allowing them folding to allow the two rigid areas to reposition without the need for additional components or assembly. The geometry of this blade can be very simple or quite complex, depending on the application.
Found everywhere, from bottle caps to high-tech devices, living hinges offer cost savings, simplified assembly, intrinsic precision and enhanced durability. Here we will explore the fundamentals of living hinge design, material selection, manufacturing processes, and best practices to ensure a long functional life and a deep understanding of the benefits these clever, simple features afford the designer.
Shaked S
Purchasing Manager
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Jiga is the best way to get the parts you need, when you need them.
What is a Living Hinge?
A living hinge is a very thin web of material that is formed in the molding of a two (or more) part molded ‘assembly’ that is injected in one step, with the two parts joined by this fine web.
The flexible strip of plastic material allows two rigid sections to pivot/fold relative to each other, making a classic example of a 4D molding – a shape that is altered over time from it’s original molded form.
This flexible connector is typically found in; applications where a product needs to open and close repeatedly, such as plastic containers, enclosures, or flip-top lids; and in folding assemblies that allow a product component or enclosure to wrap around and form a more 3D structure without any need for assembly precision originating outside the component/assembly itself. The design needs of these two roles differ.
How does a Living Hinge work?
The success of a living hinge relies on its ability to precisely position two (or more) rigid elements from their as-molded form to their in-use position.
Two basic classifications of living hinge are common, differentiated by their resilience under cyclic and repeated folding. The more basic and less durable type is a simple strip connecting between parts.
By distributing the stress over a larger surface area, carefully shaping to help in stress moderation and in some cases, applying a coining process to toughen the bend zone, greater resilience can be induced in the hinge.
Application of these design/manufacture options to appropriate materials can deliver a living hinge that can flex thousands or even millions of times without breaking. The most common materials for high resilience hinfes are polypropylene (PP) and polyethylene (PE), which exhibit excellent fatigue resistance and durability.
Where are Living Hinges used?
Living hinges are widely used in everyday products, including:
- Plastic bottle caps (flip-top)
- Clamshell packaging
- Medical devices (e.g., pill dispensers)
- Consumer electronics enclosures
- Retainers, clips and mounting points
- Toys and games
These all benefit from the living hinge’s seamless integration into product, converging both functional motion and increased structural integrity, compared with separate hinges or non-moving fixing points.
Benefits of Living Hinge design
Living hinges offer several key advantages to product designers, rendering them an ideal choice in various contexts, for diverse motivations.
Cost efficiency
Living hinges are typically part of a single injection-molded component, eliminating the need for additional assembly of separate hinges/fasteners or secondary processes. This can markedly reduce manufacturing costs, requiring fewer materials and labor steps.
Durability
Compared to traditional metal hinges or other mechanical components, living hinges are remarkably durable. Effective compliance in the fine detail of design of a living hinge can offer flexion in millions of cycles without failing, great for frequent opening and closing. Typically fatigue-resistant materials ensure that the hinge can endure long-term mechanical motion without fracture.
Lightweight design
By using plastic rather than metal for the action, living hinges reduce the net weight of a product. This is particularly beneficial in industries like packaging, consumer electronics, and automotive, where minimizing weight is typically a high priority target, met by tens to thousands of microdecisions that shave off material.
Simplified manufacturing
Living hinges are typically created through a single stage injection molding, a manufacturing process that can produce such complex and fine-featured elements in a high-volume suitable mold. This allows for high-volume production with minimal setup costs. With no separate hinge assembly, no installation or fastener application is required, so the assembly and part-count is simplified, reducing potential points of failure.
Seamless aesthetics
In many products, the hinge can be rendered virtually invisible, hidden by the action of closure. This can facilitate a cleaner, more integrated appearance, in comparison with separate mechanical hinges. This seamless integration also enhances functionality by reducing the risk of snagging or detectable localized wear and stress marking.
Materials for Living Hinges
The choice of material plays a pivotal role in the performance and durability of a living hinge execution. Not all materials offer the combination of flexibility and fatigue resistance required to ensure that the hinge can endure millions of flexing cycles without cracking or breaking. Material options are considerably wider for living hinges that either (a) operate over a small range of motion or (b) are only required to flex a very limited number of times, for installation and maintenance and not for long term, high intensity use.
Polypropylene (PP)
Polypropylene is the most commonly used material for living hinges due to its remarkable combination of fatigue resistance, flexibility, and durability. Even in the most basic design executions, PP living hinges can flex thousands of times without failing. With careful adherence to best practice in design, equivalent solutions can withstand millions of flexion cycles, making PP ideal for high-use applications ranging from flip-top bottle caps to vehicle dashboard switch elements.
PP also offers good chemical resistance, which is advantageous in packaging applications where exposure to diverse and potentially aggressive compounds is common.
Polyethylene (PE)
An alternative and highly suitable material for living hinges is polyethylene, particularly the lower-density forms grouped under low density polyethylene (LDPE). Like PP, LDPE offers great flexibility and fatigue resistance, though it may not be as stiff as polypropylene which can reduce its effectiveness as an ‘engineering’ material or use in structural or long-lived products.
It’s widely used in packaging and single-use disposable products in which lower strength capability is acceptable.
Nylon
For applications requiring greater strength and durability, nylon can be used for living hinges while delivering engineering-grade polymer properties. While not as flexible as PP or PE, nylon offers better mechanical strength and higher resistance to wear and tear, making it suitable for products that need to withstand harsh conditions but don’t require the million-cycle level of flexural resilience in the hinge.
Thermoplastic Elastomers (TPEs)
For specialized applications where a softer, more rubber-like feel is required, thermoplastic elastomers (TPEs) and thermoplastic rubbers (TPRs) may be used. These materials combine the flexibility of rubber with the processability of plastics, allowing for a more cushioned movement in the hinge and very high cycle-durability without design complexity.
A common usage context for TPE and TPR hinges is in overmolding, attaching rigid parts in ABS, for example, by molding the hinge feature along with other appliques such as closing holes with sealed rubber button actions.
Design considerations for Living Hinges
Designing a living hinge requires careful attention to geometric and processing factors that influence functionality and flexural durability. Designers and manufacturing ngineers must take into account optimal placement, section geometry, end-thinning, allowance for the full motion, and material properties – both intrinsic and resulting from nuanced processing. This can ensure the hinge performs as demanded over its planned lifespan.
Hinge Thickness
The thickness of the hinge element is the most critical design factor. A thinner hinge is intrinsically more flexible, but it will also be weaker and more prone to fracture/tearing. Equally, a thicker hinge provides greater strength and spring force, but reduces flexibility and increases the risk of failure due to stress concentration. The optimal thickness will depend on the material used and the specific application requirements. indicated in the table below.
Hinge flexural length and resultant bend radius
The available flex length of the hinge must be sufficient to dissipate the stress smoothly across the length during flexing. If it is too narrow, there is a risk of folding and rippling resulting in stress concentrations that will cause premature hinge failure. In general, wider hinges distribute the load more effectively, increasing the hinge’s lifespan but potentially destabilizing the closed position and risking twist-tearing in the open position
The resulting bend radius of the living hinge plays a significant role in preventing material fatigue. A small bend radius can lead to high stress at the hinge, increasing the likelihood of cracking or breaking. To avoid this, designers often use larger radii to minimize stress and allow for smoother flexing.
Gate location in and material flow
The gate location in the injection molding process defines the performance of the living hinge. The gate and fill pattern of the tool must be positioned such that the material flows evenly across the hinge during molding, avoiding any inconsistencies or voids that could weaken the result. The most effective flow has material feeding linearly across the hinge in a uniform propagation wave that creates intensive polymer chain alignment traversing the hinge area, for maximum material integrity. This molecular alignment is key to local material properties that are optimal in resisting fatigue and stress fracture in the bend.
Poor gate placement will lead to non uniform material flow that builds residual stress in the hinge and results in weaknesses or warping, negatively impacting durability.
Tony K
Senior Mechanical Engineer
"Fantastic platform for purchasing custom parts"
Jiga is the best way to get the parts you need, when you need them.
Manufacturing/Prototyping Living Hinges
Living hinges are typically manufactured using injection molding for complex parts or vacuum forming for simple products like food clamshells, but there are also prototyping methods that can help, depending on the complexity of the design and the materials needed. These are the details of manufacturing processes for creating or simulating living hinge products.
Injection molding
Injection molding is the only method for mass producing living hinges integrated into parts with complex geometry and divergent wall sections. Molten plastic is injected into a mold cavity whose profile the material retains, as it cools and solidifies, including the critical living hinge aspects. The precision of the process allows for accurate reproduction, intricate designs and consistent quality, making it ideal for producing fine and process-critical living hinges.
Additive manufacturing
With progress in additive manufacture (3D printing), living hinges can now be created in prototype parts. They will reflect the desired function of the hinge, although they will have a short operational life – typically only a few cycles. However, that’s enough to validate most operating principles and gross-geometry. While 3D printing is slow and unsuited to mass production, it allows for rapid evaluation of designs, before committing to tooling.
Certain materials – nylons and highly flexible elastomer filaments are available for extrusion based printing, and some limited success can also be achieved using nylon powders in selective laser sintering (SLS) and other powder bed processes.
Thermoforming
The overwhelming majority of living hinges are used in packaging products – take-out clamshells, egg packaging and Consumer display packaging. These are generally made by thermoforming of flat sheets, with length/curvature/local flexibility imposed in the shape to enable the relative motion of stiffer sections in a narrow flexure zone. In this process, a plastic sheet is heated and formed over a mold. Thermoforming is often used for larger and/or low complexity designs and single use articles. It’s not as precise as injection molding in reproducing fine details like living hinges, but the process delivers good outcomes for low CAPEX burdens.
Testing and validating Living Hinges
Validating that a living hinge will perform as intended over time requires intensive flexure and often environmental testing. Several methods are used to assess the strength, flexibility, and durability of a hinge before it can be signed off as a final, market ready product.
Fatigue testing
The primary issue with high-intensity use living hinges is their ability to tolerate repeated cycles of bending through the boundary conditions of operation. Fatigue testing simulates real-world use by repeatedly flexing the hinge until failure. This facilitates precise assessment of the feature longevity and identifies any design flaws or material performance issues.
Flow modeling and stress analysis
Simulation tools can assess the flow in tool filling to ensure smooth, unidirectional flow in the hinge area and to simulate the stresses and strains a hinge will experience during use. Flow modeling and finite element analysis (FEA) can help optimize the hinge design by confirming flow geometry and identifying stress concentration points and suggesting modifications to improve tool and component performance.
Environmental testing
In some applications, living hinges will be exposed to harsh environments, so environmental testing is essential to evaluate product behaviors under adverse usage circumstances. This may include testing the hinge’s resistance to UV radiation, moisture, aggressive chemicals, and temperature extremes to ensure it will hold up under real-world conditions.
Applications of Living Hinges
Living hinges are utilized across a huge variety of industries and products due to their simplicity, durability, and cost-effectiveness. From single use packaging to consumer products and highly specialized industrial and medical applications, living hinges are exploited for their ability to provide flexibility without sacrificing the overall integrity of the design.
Packaging and single use goods
By far the most common application of living hinges is in packaging, particularly in products like flip-top caps for bottles, containers, and clamshell packaging. The hinge inclusion allows consumers to easily open and close a package multiple times without compromising the seal or structure of the packaging. Cycle lives are typically tens to hundreds of operations, making these technically simple applications.
Living hinges greatly simplify manufacturing by integrating the cap or lid into a single piece, reducing assembly time and costs. Equally, the stacked simplicity if clamshell packages and the easy utility in their closure and opening are valuable.
In addition to single use packaging, living hinges are often found in consumer electronics such as protective cases for phones and tablets. Their seamless, unobtrusive design ensures that the product remains minimal, low cost and functional, while the flexibility of the hinge provides users with ease of access. Again, relatively short cycle life requirements commonly apply, other than in flip covers and stands for phone/tablet cases, which require greater resilience and fold-count lifespan.
Medical devices
Living hinges are frequently used in medical devices where sterile, single-use or reusable parts are essential. They are incorporated into drug-delivery devices, pill dispensers, and surgical tools. These benefit from living hinges due to their ability to repeatedly cycle without mechanical failure while maintaining the integrity and precision of the device. Additionally, the one-piece design helps in reducing potential contamination points in sterile usage scenarios.
Automotive components
In the automotive industry, living hinges are used widely in interior and exterior components. They provide lightweight, durable solutions for small parts such as glove compartments, cup holders, and storage enclosures. By integrating living hinges into a vehicle’s plastic components, manufacturers reduce the complexity of assembly and part/process count while minimizing weight, which helps in improving fuel efficiency and reducing production costs.
Industrial and aerospace applications
Living hinges are also employed in industrial and aerospace applications, where high precision and long term performance are crucial. For instance, equipment housings, access panels, and cable management systems can all benefit from living hinges due to their operational simplicity, reliability and flexibility. In aerospace applications, reducing weight without sacrificing strength is a primary design target, and the use of living hinges in non-critical parts can help deliver this in thousands of micro-savings.
Toys and games
The toy industry extensively uses living hinges in the design of products such as action figures, board game components, and plastic cases. The ability to cheaply provide repeated bending action without fracture and generating splinters is crucial in products designed for children, who often handle toys roughly. The use of living hinges also eliminates numerous small parts, thereby improving child safety AND assembly costs.
Javier L
Principal Systems R&D Mechanical Engineer
"Game changing in the online manufacturing space"
Jiga is the best way to get the parts you need, when you need them.
Best practice for design of Living Hinges
Designing a robust living hinge requires a convergence of material knowledge, structural and molecular structural insight, and an understanding of how the part will be used. The following best practices can help engineers ensure that their living hinge designs are reliable and long-lasting:
Material selection
The choice of material is central to ensuring that a living hinge can endure the required number of bending cycles without fracture. Materials such as polypropylene (PP) and polyethylene (PE) are ideal for their flexibility and fatigue resistance. In some cases, choosing a higher-grade material can deliver the enhanced durability necessary in applications that require long-term performance and greater dimensional stability.
Optimize thickness and geometry
Balancing the pressures on thickness and thinness of the living hinge is central to product performance. A hinge that is too thin will lack the integrity to withstand repeated cycles, while a hinge that is too thick will not handle the stress distribution. The differential stretch/compression resulting from greater thickness/rigidity induces large internal stresses/shear that disrupt intra-molecular bonds, instigating fracture/tearing.
The use of finite element analysis (FEA) during the design process can determine the optimal dimensions for the hinge based on the expected load and frequency of use – although in most products, approximately guidelines will deliver good results in moderate use products.
Minimize stress concentration
Avoid sharp corners and sudden junctions and thickness transitions in the flex area or flex-adjacent features, which will lead to adverse stress concentrations. Smooth, gradual transition between the hinge and the surrounding parts of the product helps distribute the stress uniformly/smoothly, reducing the risk of localization and fracture.
Consider manufacturing constraints
Designers must work closely with manufacturers to ensure the design is feasible from a production standpoint. Factors such as gate location, mold design, flow simulation and part cooling greatly affect the quality of outcome, and must be considered thoroughly to ensure the hinge performance and minimize defects.
Prototype and test
Before mass production, it is crucial to prototype and thoroughly test living hinge designs. Simple thermoformed parts can easily be prototyped in near mass-production conditions. However, prototype performance of complex products that are to be injection molded is notoriously difficult to interpret in terms of mass production performance.
As a rule of thumb, if a carefully executed rapid prototype from high quality, high resolution filament based additive manufacturing functions through even a handful if cycles, the design is fairly close to right!
Prototyping of thermoformed parts allows designers to identify potential issues and refine the design based on real-world testing. Fatigue testing and environmental testing, such as exposure to chemicals, UV radiation, and temperature fluctuations, should be conducted to validate the performance of the hinge but can only be valid when performed on real-production materials.
Conclusion
Living hinges are a valuable aspect of modern product engineering, offering unique advantages in terms of flexibility, durability, and cost-reduction. By understanding the principles behind living hinge functionality, material selection, and best practices in design, engineers can create robust and reliable products that meet the needs of various industries.
Across diverse applications in packaging, consumer goods, medical devices, automotive applications, and more, the living hinge is a versatile and powerful tool in reducing part complexity, improving performance, creating smart 4D parts and achieving long-lasting flexibility.
As maas manufacturing and additive manufacturing technologies continue to evolve, living hinge design will increasingly take a key role in developing innovative, efficient products.
