Living Hinge Design Guide for Injection Molding

A good designer is always looking for methods to reduce the cost and improve the user experience. The living hinge can use thin plastic sheets replace ordinary hinges, it can not only save cost but also improve the user experience.

What is a living hinge?

The living hinge is a thin sheet of plastic used to connect two plastic parts. It is surrounded by a thicker plastic support, the part can fold and bend at 180°. The living hinge cost is cheap and it is easy to be manufactured, it almost has no wear or friction during use.

When you need a structure that can connect two sides and need it can rotate, a living hinge is an ideal option.

This kind of flexible connector is usually applied in occasions that need open and close frequently, such as plastic caps, plastic containers, enclosures, and kinds of folding parts.

Benefits of using a living hinge

Cost efficiency

No need of assembly process and reducing the overall cost is the biggest advantage of the living hinge.

It spares the need for installing separate hinges or fasteners and the incoming process, reducing the material consumption and human labor steps.

Reducing the need for multi-cavity molds, if there is no living hinge, the two parts will be individual parts, and they need individual mold cavities, gates, and runners. Saving almost half of the mold price.

Durability

Compared with other original steel hinges, the living hinges have extreme durability. Well-designed and properly used living hinges can endure bend cycles over one million times.

Lightweight design

The living hinge uses plastic material rather than steel for action parts, which can reduce the product’s weight. It is a huge advantage in reducing product weight for packaging, consumer electronics and automotive industries.

Disadvantage of living hinge

The limitation of live hinges lies in their restricted material compatibility, with polypropylene (PP) and polyethylene (PE) being the optimal choices. Engineering plastics such as polycarbonate (PC) and acrylonitrile butadiene styrene (ABS) are unsuitable, resulting in comparatively lower load-bearing capacity. However, when using two components to form the hinge, material selection becomes unrestricted.

Common types of living hinge

Flat hinge

This is the most regular living hinge type, there are supporting surfaces for both ends, the middle area is a thin hinge.

Double hinge

It is combined with two flat living hinges; the middle section has a narrow supporting surface. It is suitable for two hinges need to have a gap between them, or the living hinge need rotate at 360° (not 180°)

Butterfly hinge

This hinge is usually used in dispensing bottle caps, or flip-top caps. Once the hinge opens or closes over a certain degree, it will automatically rotate and keep closed or fully opened. 

Bi-Stable Hinge

Essentially a butterfly hinge with three hinge sections rather than one, it is therefore the most flexible type of living hinge.

Hinge Material

Polyethylene and polypropylene are ideal materials for living hinge designs due to their high melt flow rates, which enable them to fill wall sections with different thicknesses while providing high tensile strength.

Polypropylene (PP)

Polypropylene can be divided into different types: homopolymer PP offers stable properties and can be formed into durable living hinges; random copolymer PP is the next best option and also performs well; impact-modified copolymer PP is not suitable for living hinges unless specific requirements exist for low-temperature performance or impact strength.

PP also possesses excellent chemical resistance, which is particularly important in packaging applications where products frequently come into contact with potentially corrosive chemicals.

Polyethylene PE

Polyethylene (PE) is the second choice after PP, particularly low-density polyethylene (LDPE).

However, PE usage is uncommon and is typically only considered when PP is unsuitable.

PE has extensive application in packaging and disposable products, where strength requirements are relatively low.

Nylon

For applications requiring higher strength and durability, live hinges can use nylon. Nylon is suitable for products that must withstand severe conditions but do not require a million-cycle bending life.

Design Considerations of Living Hinge

Hinge thickness

The thickness of living hinges should be between 0.008 and 0.015 inches. If it is less than the thickness, the melt flow may not be able to fill the living hinge area when injection molding. When parts bend, it will be tensile stresses, if the hinge is too thin, it will creak due to the lack of strength. If the living hinge thickness is too thick, it will have larger strength and elasticity, but the flexibility is reduced and leads to excessive stresses, it will crack eventually.

In injection molding, use a thin thickness at the beginning, because we can reduce the mold steel to make the thickness larger.

Avoid sharp corners – minimize stress concentration

In injection molding design, sharp corners should be rounded wherever possible, especially in the living hinge area and its surrounding area, which can reduce the stress concentration.

Avoid sharp corners, abrupt transitions, or thickness variations at the bending area or its surrounding area, as these will cause stress concentration.

Bend radius

Excessively small bending radii subject hinges to high stresses, increasing the risk of cracking or fracture. To prevent this, designs often employ larger radii to reduce stress and ensure smoother bending.

Gate location

The gate should be located in the middle of the living hinge, to make sure the melt flow is evenly distributed in the living hinge.

Ideal flow is when the melt flow passes linearly and evenly through the living hinge area, in even speed, the polymer chains are highly oriented and the material integrity is maintained maximally. This alignment enables the local material to resist cracks caused by bending stress and fatigue. It can also reduce knit lines.

For most snap-fit parts, the gate location should be placed at the thickest area. This will enable the melt folw from thick wall area to thick wall area. 

Ideally, gate placement should let the melt flow be parallel to the hinge length, to ensure straight-line flow, and prevent the weak points caused by transverse stresses.

Divide long hinges

If the live hinge length is more than 6 inches, it should be divided into two or more parts, to ensure the service life.

Flow simulation and stress analysis

Simulation tools can evaluate melt flow during the mold filling process, ensuring smooth and directionally consistent flow in hinge areas, and simulate the stresses and strains the hinge undergoes during use. Flow simulation and finite element analysis (FEA) can confirm flow paths and identify stress concentration points, thereby proposing recommendations for design optimisation.

Prototype and test

Before the massive production, we need to produce the prototype and test the living hinges thoroughly. Conduct fatigue testing and environmental testing (such as chemicals, ultraviolet radiation, and temperature), the testing should use the actual production materials.