A living hinge is not like a regular hinge as you’d normally see on a door, however, there are a surprisingly large number of products that use plastic living hinges today and fit into the category of plastic mechanical parts.
In this guide, we’ll look at what a hinge is so we can see how a plastic living hinge fits into the ‘hinge’ portfolio and some of the common engineering plastics used for living hinges and their properties.
What’s a hinge?
A hinge is a movable joint or a mechanism that holds two pieces together while allowing them to move independently about the central spine. A typical hinge can be found on a door mounted in a frame, a gate, a shutter, a lid, and any other product or item that has two parts joined that can swing. The hinge is made up of three parts, two plates and a central pin connecting that the plates pivot around. The plates are fixed to the two separate components that need to articulate around the central axis of the hinge.
Here’s a common hinge seen on a door so you can visualize this:
The difference between plastic living hinges and conventional three-part metal hinges
Living hinges made from plastic differ from the conventional three-part metal hinge (seen above) in that the hinge is created within the plastic component as a single part. You will see these types of hinges in everyday items such as a shampoo lid, sauce squeezy bottle, or the lid of a toothpaste tube, for example.
Due to the material properties, the living hinge is capable of lasting millions of cycles.
Which plastic is suitable for living hinges?
There is one plastic that stands out when it comes to the suitability of use for living hinges, polypropylene. Polypropylene (PP) has excellent injection molding capabilities as well as being a very flexible yet strong plastic.
The feature of a living hinge is that the flexible hinge area is designed to be thinner than the corresponding areas adjacent to the hinge. During the injection molding process, the PP molecules tend to orient themselves in the direction of flow. This means that the bending motion of the hinge is perpendicular to the molecule orientation resulting in a strong flexible section that can withstand millions of flexes without breaking, tearing or fracturing.
The image below shows a living hinge design of a container with a lid, as you can see, the hinge thickness is small compared to the wall thickness of the container. This narrowing within the mold tool is what allows the molecules to orientate across the hinge.
Image source: plasticstoday.com
PP features and benefits
Polypropylene has excellent fatigue resistance as well as great elasticity. It also has high insulating properties making it suitable for insulating cases and electrical enclosures and excellent chemical resistance. It’s widely used in the medical industry for various containers and items with living hinges.
Its low density means that it is a lightweight plastic making it an ideal material where weight-saving is a design consideration.
It also has excellent resistance to water absorption and can be used in applications that call for a waterproof design.
As for the working temperature, it is not recommended for environments above 180℉ (82℃) as above this temperature the performance will start to degrade and the product may suffer operational faults and potential failure of the system.
Finally, the cost of PP is relatively low at $1.16/lb (in summer ’22) which makes it a very cost-effective plastic solution for many applications.
Summary of Plastic Living Hinges
A living hinge is an integrated mechanism designed into a product that has two distinct parts that can flex 180 degrees or more about the central point of the hinge. The hinge design is a thin section of plastic between the two main components.
The predominant plastic used for living hinges is polypropylene (PP) because of its excellent flexural properties, toughness, and ease of moldability. Some of the other key properties of polypropylene which make it the ‘go-to’ plastic for living hinges include: low cost, good chemical resistance, excellent electrical resistance, and its low density that means it’s one of the lighter weight engineering plastics.