A living hinge is a flexible piece of plastic that joins two rigid surfaces together, commonly used in disposable packaging. They are usually injection molded and made from materials like polyethylene and polypropylene. Living hinges are durable and can last longer than expected. They are used in various products, including microelectromechanical systems. Designing a living hinge requires careful calculation and understanding of physics.
A living hinge, sometimes called a flex pad or flex pad, is a segment of flexible material, usually made from some type of plastic, that joins two rigid surfaces together. The hinge is flexible, allowing it to bend. This type of hinge is most often used to join a lid to a container in disposable packaging.
While similar zippers can be made from materials such as fabric or leather, the term “living zipper” specifically applies to those made from some type of plastic. Such hinges are usually injection molded, which means the plastic is injected into a rigid mold, then removed when set. Injection molding is quick and cheap, and the amount of material needed for a living hinge is relatively small, so such hinges are usually an inexpensive option.
Despite the low cost, such hinges are unexpectedly robust and can last much longer than one might expect. Standard hinges generate friction between moving parts, leading to wear. Formed from a single piece of plastic, living hinges do not physically interact with any other part, so no friction is generated, increasing the life of the hinge.
Different types of plastics are used for different scenarios. Polyethylene and polypropylene resins are popular materials, as they have a low fatigue rate. This means they can be folded many times without breaking or breaking. Zippers that are unlikely to flex more than a few times are sometimes made of less fatigue-resistant materials, such as nylon resin.
Disposable packaging is the most common use for living zippers. Such zippers can be seen on the packaging of items ranging from cleaning supplies to baby wipes to food. Even plastic cases, such as soap containers or pencil cases, often use living hinges. Another common use, perhaps less apparent to the general public, is in the manufacture of microelectromechanical systems such as the circuits used in common electronic devices including telephones, computers and televisions.
A living hinge capable of flexing several thousand times before failure is known as a fully elastic hinge. All-plastic hinges can only flex a handful of times. Elastic plastic hinges, however, can flex hundreds of times or more.
Designing a living hinge that will last as long as needed requires careful calculation and a solid understanding of physics. The angle at which the zipper must open, together with the elasticity of the plastic fibers used, are critical factors in the life of the zipper. Designers must understand precisely how much stress will be placed on the hinge to determine the correct thickness and material for the requirement.
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