Maya Ellison
Plastic hinges, also known as living hinges, are a type of hinge design that uses a thin layer of plastic to connect two larger plastic bodies. This design allows for rotational movement of up to 180 degrees or more and is commonly found in consumer products such as bottle caps, containers, and electronic cases. Living hinges offer several advantages over conventional hinges, including reduced cost, improved user experience, and ease of manufacturing. However, they also have limitations, such as limited load-bearing capacity and the need for careful design to ensure durability. The thickness of the hinge and the orientation of the plastic molecules are critical factors in determining the strength and flexibility of the living hinge. This design is particularly important in seismic applications, where it can serve as a controlled weak point to prevent total collapse during an earthquake.
| Characteristics | Values |
|---|---|
| Material | Polypropylene (PP) is the best material for a living hinge. Polyethylene (PE) can be used in some cases. |
| Thickness | 0.007"–0.015". The thickness will determine how stiff or floppy the hinge feels. |
| Radius | Radius sharp corners to reduce stress and increase durability. |
| Load-bearing | Not suitable for bearing a load parallel to the hinge. |
| Durability | Can last well over a million cycles. |
| Ease of assembly | Uni-body design eliminates the need for assembly and reduces cost. |
| Design philosophy | Progressive failure: allow damage but prevent total collapse. |
| Use cases | Bridges, tall buildings, consumer products (e.g. bottle caps, containers), electronics. |
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What You'll Learn
Choose the correct plastic
When choosing the correct plastic for a plastic hinge, several factors must be considered. Firstly, the angle of rotation is important. Plastic hinges have varying angles of rotation, typically ranging from 0 to 70 degrees, but some offer a wider range of motion. Therefore, selecting a plastic hinge that meets the specific application's rotational requirements is essential.
The type of plastic used is another critical factor. Plastics such as polypropylene and polyethylene possess inherent elasticity and flexibility, making them ideal for hinges. These materials can endure repeated bending or flexing without permanent deformation or failure. Additionally, consider the environment in which the hinge will be used. Plastic hinges are corrosion-resistant and rust-proof, making them suitable for humid environments. However, certain plastics like polypropylene degrade with UV or sunlight exposure, so choosing a plastic that can withstand the specific environmental conditions is crucial.
The design of the plastic hinge also plays a significant role in material selection. Living hinges, for example, are thin sections of plastic that connect two bodies, allowing rotational movement without the need for a pin. They are durable and cost-effective, as they eliminate the need for assembly and reduce the number of parts required. To create a durable living hinge, it is essential to ensure proper molecular alignment during the injection moulding process, forcing the molten plastic through a small hinge area to achieve perpendicular molecule orientation to the flex direction.
Furthermore, the load-bearing capacity of the plastic hinge should be considered. Plastic hinges are flexible and can withstand heavy loads without breaking. They may flex slightly under weight but will maintain their structural integrity. This property makes them suitable for applications where flexibility and load-bearing capacity are essential, such as in automotive components or heavy objects.
In summary, when choosing the correct plastic for a plastic hinge, consider the angle of rotation, the specific type of plastic, the design of the hinge, and the load-bearing capacity. By carefully evaluating these factors, you can select the most suitable plastic material for your hinge application.
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Consider hinge thickness
When designing a plastic hinge, it is important to consider the thickness of the hinge. The thickness of the hinge will impact its flexibility, durability, and functionality.
The thickness of a plastic living hinge should typically range between 0.007" and 0.015". If the hinge is thinner than 0.007", the plastic may not fill the hinge section properly during the injection moulding process, leading to potential issues in the hinge's functionality. On the other hand, if the hinge is thicker than 0.015", the plastic molecules will not orient in the right direction, affecting the cycle durability of the hinge.
It is worth noting that the thickness of the hinge can also impact the range of motion of the hinge. For example, a thinner hinge may allow for a greater range of motion compared to a thicker hinge. Additionally, the thickness of the hinge should be considered in relation to the size and weight of the plastic bodies it connects. A thicker hinge may be more suitable for connecting larger or heavier plastic pieces, as it can provide more support and durability.
When designing a plastic living hinge, it is crucial to consider the specific application and requirements. For instance, if the hinge needs to fit within a small or compact space, a thinner hinge may be more appropriate. On the other hand, if the hinge needs to bear a heavier load, a thicker hinge with greater load-bearing capacity may be required.
In summary, the thickness of a plastic hinge is an important consideration in its design. By choosing an appropriate thickness within the recommended range, designers can ensure the hinge's proper functionality, flexibility, and durability while also meeting the specific requirements of the application.
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Design for prototyping
When designing a plastic hinge for prototyping, it is important to consider the desired functionality, material selection, and manufacturing process. Here are some key factors to keep in mind:
Functionality:
The design of the plastic hinge should be based on its intended function and performance requirements. Consider the load-bearing capacity, flexibility, and range of motion needed for the specific application. The hinge should be designed as the weakest area of the part, with its thickness and width influencing the force required to bend it.
Material Selection:
Choose a material that suits your prototyping needs. Polypropylene (PP) is a popular choice for its flexibility, soft texture, and ability to allow true rotational mobility of 180 degrees without snapping, cracking, or ripping. However, it is challenging to machine and may produce rough edges. Polyethylene is another suitable material for its flexibility and soft texture. Consider using materials with high elongation before breaking and good tear resistance for 3D printing.
Manufacturing Process:
There are several prototyping methods available, each with its advantages and limitations.
- 3D Printing: 3D printing is a popular choice for prototyping plastic hinges due to its low cost, quick turnaround, and ease of design iteration. It allows you to test and adjust your design before investing in more expensive manufacturing methods. However, 3D-printed hinges may have limited durability and functionality due to their anisotropic nature, making them more susceptible to warping and breakage over time.
- Injection Molding: This method is commonly used for the final production of plastic hinges due to its ability to create extremely thin hinges with a long lifespan. It offers high durability and is cost-effective for large-scale production. However, it may require additional investment in expensive tooling and design modifications.
- CNC Machining: CNC machining provides precise control over the hinge's dimensions, allowing for machining down to a couple of thousandths of an inch. It is useful for prototyping, especially when working with challenging materials like polypropylene, as it can help avoid issues with incomplete filling of cavities and weld lines at the hinge region.
By considering these factors and selecting the appropriate prototyping method, you can effectively design and test your plastic hinge before moving on to the final production stage.
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Avoid weld lines
Weld lines are a common issue in plastic manufacturing, including in the creation of plastic hinges. They occur when two or more plastic streams meet at different temperatures as the plastic fills the mould cavity, and fail to mix together properly. This can be caused by non-uniform flow along the hinge, which increases the chance of weld lines or defects that could compromise the durability of the plastic hinge.
To avoid weld lines, it is important to ensure uniform flow along the hinge. One method to achieve this is by providing a separate gate on both cavities, which allows for multiple entry points for the molten plastic. When providing multiple gates, care must be taken to avoid positioning them at the hinges, as this can result in weld lines. The location and type of gate play a significant role in moulding a successful plastic hinge. The gate should be positioned so that the molten plastic flows across the hinge perpendicular to its length.
Another factor to consider is the thickness of the hinge. The thickness of the hinge determines the force required to operate it. It is generally recommended to start with a safe steel approach, which involves using minimum thickness and maximum recess depth. This allows for subsequent adjustments to increase the thickness of the hinge by reducing the recess depth.
Additionally, the flow direction of the plastic is crucial. By controlling the flow direction and increasing the speed, a molecular alignment along the hinge can be created. The molecules should be oriented perpendicular to the flex direction to extend the durability of the hinge. Proper mould design, including the elimination of weld lines, and process control fabricating procedures are essential to ensure efficient operation of plastic hinges.
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Flex the hinge post-moulding
When designing a plastic hinge, it is important to consider the application and the amount of movement required. Flexing the hinge post-moulding is an important aspect of the design process, and there are several factors to consider to ensure optimal performance.
Firstly, the choice of material is critical. Polypropylene (PP) and Polyethylene (PE) are excellent choices due to their high melt flow, allowing them to fill both thick and thin wall sections while providing high tensile strength. These materials also have the elasticity and memory to withstand the hinge's movement. It is important to note that the thickness of the hinge should be between 0.007" and 0.015" to ensure proper filling of the hinge section and correct molecular orientation for durability.
Secondly, gate placement plays a crucial role in hinge durability. Proper gate placement ensures that the material flows through the hinge at a consistent rate and in the same direction as the material flow, reducing knit lines. Additionally, the gate location is important when moulding a hinge. The material should flow evenly and perpendicularly to the hinge to avoid issues with thicker and thinner sections.
To enhance hinge life and strength, cooling is essential, especially in the hinge area. A parallel water circuit should be placed as close as possible to the hinge to minimise crystalline development and promote strength and longevity.
Furthermore, it is important to watch for blush (stress whitening) as the hinge is flexed during testing. Higher molecular weights and low melt-flow rates will improve performance but are more challenging to work with. Aim for low nucleation and proper molecular orientation, which depends on the resin, process, and gate location.
By carefully considering these factors and following the design strategies outlined, you can effectively flex a plastic hinge post-moulding and ensure optimal performance and durability.
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Frequently asked questions
A plastic living hinge is a thin section of plastic that connects two large plastic pieces.
The thickness of a plastic living hinge should range between 0.007"–0.015". The thickness of the hinge will determine how stiff or floppy it feels to operate.
Plastic living hinges are easy to manufacture, reduce production costs, and improve the end-user experience. They are adaptable to many designs and are durable, with good design being able to last for millions of cycles.
Plastic living hinges have limited load-bearing capabilities and are not suitable for applications that require significant weight placement on them, such as doors. They also have limited material selection, with polypropylene (PP) and polyethylene (PE) being the most commonly used materials.
Plastic living hinges can be found in various consumer products, including bottle caps, compact makeup containers, peppermint boxes, electronic cases, and condiment bottle caps.
