Living Hinges 101 — What You Need To Know
6 minutes | 25 Oct 2019
Living hinges are durable and can be used in an infinite number of applications, so it’s worth understanding a little more about them.
What is a living hinge?
A living hinge is a thin, flexible section of (usually) plastic that’s used to connect two rigid sections on an item or appliance.
Often, the two sections and the living hinge itself are fashioned from one continuous, moulded piece of plastic. But, you can also purchase individual living hinges that can be attached to the two sections you’re attempting to connect using mechanical fixings.
A hinge by any other name
It’s worth noting that living hinges don’t always go by that name.
How does a living hinge differ from a conventional hinge?
A conventional hinge typically needs some kind of central pin or rod in order for it to be able to turn, as seen in the examples here:
|Screw-on concealed hinges
||Stainless steel lift off hinges, flag
A living hinge, on the other hand, is fashioned from a single piece of material. Its design simplicity is shown here, in Essentra Component’s continuous hinge:
Types of living hinge design
There are four main kinds of living hinge designs, each with its own advantages:
|Flat / straight hinge||By far the most widely used living hinge. It consists of two straight strips with a narrow, flexible hinge section.|
|Double hinge||Comprises two straight living hinges separated by a narrow landing strip. Double hinges are useful for applications where space is needed between the two connected sections, or the hinge needs to rotate 360˚ instead of 180˚.|
|Butterfly hinge||Commonly used on dispensing caps, butterfly living hinges are what we often refer to as flip-top caps. Once the hinge is opened or closed past a certain angle, the hinge will complete the rotation then stay in the full open/closed position.|
||Basically a butterfly hinge with added “oomph.” Where the butterfly living hinge has a single hinge section, the bi-stable living hinge has three. Like the butterfly hinge, the bi-stable hinge will snap open or closed once it reaches a certain angle; the added hinge sections result in a stronger action and added stability in both end positions.|
If it’s a custom living hinge you need, it’s worth knowing about the various manufacturing methods.
Until recently, the best way to produce a prototype living hinge was through CAD and CNC machining.
But 3D printing has grown enormously in prototype manufacturing: it’s quick, cost-effective and, often, entirely automated. 3D printing is best suited to creating single prototypes for demonstration purposes, as printed hinges don’t have the same durability as those manufactured in other ways.
When you think of CNC machined hinges, your mind probably jumps straight to something like heavy duty steel hinges rather than plastic.
But there’s still a lot to be said for CNC machined plastic — as long as you work with a company with skilled machinists, you can get great results. CNC machining is perfect for producing prototype and test batches of products with plastic living hinges.
It’s quicker and far more cost-effective than moulding, particularly if it transpires that the design needs to be changed.
For prototype batches of around 100 units or more, you’ll want to consider paying for custom tooling, and going down the moulding route.
If you’ve perfected the design of your living hinge, and you’re ready to mass produce, it’s time to move on to injection moulding.
Once you’ve made the initial lay-out for the design and manufacture of bespoke tooling, injection moulding is by far the fastest and cheapest method of producing items with living hinges.
Not your typical living hinge, but still worth a mention due to its growing popularity. You can use a laser cutter to create living hinges in thicker sheet material such as MDF.
This kind of living hinge is still weighted towards aesthetics rather than durability and is often used for luxury commercial items like notebook covers, folders and laptop cases.
Extrusion is used when creating plastic continuous hinges that are designed to be attached to other items, so it’s far more relevant to hinge manufacturers than product engineers.
Like injection moulding, extrusion uses heat to mould an item into a particular shape – In this case, a cross section. The heated plastic is forced through a die and, when it cools, it solidifies.
- Living hinge vs conventional hinge: which to use?
It can be hard to know when to use a particular kind of hinge, so we’ve broken down the main characteristics of living hinges here. The primary advantages include:
Living hinges are compact, simple and versatile, which makes them suitable for a vast range of applications, both commercial and industrial, such as light-duty HVAC components.
With their unibody design, moulded living hinges place high on the durability scale. There’s nothing to fiddle with and they operate with zero friction, so they tend not to suffer from the same wear and tear that conventional hinges endure, even after hundreds of thousands – or millions – of cycles. This makes them a popular clip-close solution for things like enclosure door latches.
Assuming they’re manufactured correctly, and not damaged or subjected to unnatural conditions, living hinges can be expected to last the whole lifespan of the item they’re part of.
Made from a single part, living hinges are far less expensive to produce en masse than conventional hinges, which require a variety of tooling, multiple components and manual assembly.
If you’re opting for a custom living hinge, keep this in mind. While the initial outlay for moulds and bespoke tooling can be quite expensive, the time and money recouped during mass production easily offsets this cost.
- Chemical resistance
Living hinges are typically made from plastic, which won’t rust like conventional metal hinges. Because plastic living hinges won’t corrode like stainless steel hinges, they’re often favoured for use on, for example, an outdoor utility cabinet, outdoor electrical enclosure or an outdoor telecom equipment cabinet.
Living hinges offer brilliant flexibility around their point of rotation, typically offering more than 180˚ in standard straight living hinges.
Living hinge design can be tailored to offer 360˚ rotation, or to limit rotation at a particular angle if needed.
This added flexibility is useful in applications where the lid needs to rotate by 180˚+ and move completely out of the way, such as electrical enclosure door hinges and data centre server cabinets, where unrestricted access is needed.
Long plastic continuous hinges can be used to provide uniformly distributed support and stability along the rotating edge of your product, reducing the weight and stress placed on the both the connected sections and the hinge during each cycle.
This added stability is well suited to compact, wall-mounted plastic cabinets, and gives living hinges an edge over different types of cabinet hinges.
Living hinge limitations
Like any kind of hinge, living hinges have their weak points. You’ll want to consider conventional hinges if you’re prioritising any of the following characteristics in your design:
Living hinges aren’t favoured for their strength, nor are they suited to applications where two (or more) weighty sections are being connected.
So while they’re suited to some industrial applications, they’re just not strong enough for others. They’d work fine as part of a heavy duty door latch, for example, but they’d never match up to the usual heavy door hinge types. With their highly flexible middle section, living hinges have very limited load-bearing ability and can easily be damaged by excessive force, particularly if it’s applied unevenly to the hinge.
- Thermal stability
Plastic has a relatively low melting point, which means that your typical living hinge is completely unsuited to applications in which it’ll be subjected to extreme heat.
Likewise, polymers tend to become brittle and easily breakable when exposed to extremely low temperatures. It’s best to use living hinges only for items that will be used in moderate temperature environments.
- Lack of material choice
Living hinges are almost always manufactured from plastic, usually polypropylene PP or polyethylene PE.
And while you can use additives to enhance or reduce certain characteristics inherent within the polymer, you’re still limited as to how much you can alter its make-up before you risk compromising the effectiveness of the hinge.
What applications can a living hinge be used for?
Living hinges are everywhere you look in the consumer goods market. They’re flexible, resilient, impervious to moisture, and cheap to mass produce, which makes them the perfect solution for:
|Consumer applications||Industrial applications|
|Bottle caps for food, drink and personal hygiene products||Outdoor enclosure door hinges|
|Cosmetic compacts||Weatherproof outdoor cabinets|
|‘Click-lock’ closures on food storage tubs||Different types of electrical junction boxes|
|Tool boxes||Hinged parts of an HVAC system|
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