Living hinges offer a straightforward and economical method for integrating flexure bearings into plastic items such as containers and food packaging. Unlike most hinges that usually necessitate two distinct components, a living hinge is crafted from the identical material as the two sections it joins.

This article provides an overview of living hinges, focusing on the fundamental aspects. It explores guidelines for designing living hinges, encompassing both injection molding and the increasingly favored method of 3D printing. Furthermore, it delves into the optimal materials for constructing living hinges and presents various commonplace examples, ranging from shampoo bottles to medical devices.

What is a living hinge?

A living hinge refers to a slender and pliable portion of a substance that links two inflexible portions, typically composed of the same substance. Generally, the hinge and the two rigid sections are produced from a solitary mold (injection molding) or as a single printed object (3D printing).

The phrase “living hinge” was reportedly created by an engineer at Enjay (now ExxonMobil) during the 1950s. Enjay was exploring the use of the plastic polypropylene for injection molding at that time and found that, in extremely thin sections of a mold, the molecules of the material aligned themselves based on the flow direction. This discovery allowed the thin section of the molded component to be repeatedly bent without fracturing, effectively creating an ideal hinge.

Living hinges are distinctive due to their integration within the structure of two connecting sections. For instance, envision a scenario where a doorframe, a door, and its butt hinge are all crafted from a single interconnected piece of material – this would exemplify a living hinge. In practice, living hinges are commonly utilized in the production of small plastic items such as food containers.

Living hinges, when integrated into a component, have the potential to yield substantial savings in terms of costs and inventory reduction.

Living hinges are occasionally referred to as uninterrupted hinges or keyboard hinges.

Types of the living hinge

Simple tab-like joints are commonly found in most living hinges, although more elaborate designs offer the flexibility for diverse movements between sections.

• Flat hinge: A single thin tab that connects two rigid sections and can bend up to 180 degrees
• Double hinge: A two-part hinge that allows for a bend of up to 360 degrees
• Bi-stable / butterfly hinge: Bi-stable and butterfly hinges, seen in “flip-top” bottle caps, for example, can rest in either the open or closed position

Injection molding living hinges

For numerous decades, injection molding has served as the primary technique for manufacturing plastic components featuring living hinges.

Living hinges can be incorporated into a range of molded components; however, there are constraints on the design possibilities due to limited flexibility. It should be noted that living hinges are not suitable for oversized parts, as a thick hinge will easily snap under the pressure of bending. This is due to the fact that the molecules of molded polypropylene can only align themselves in confined areas.

The following guidelines must be followed when designing (flat) living hinges for injection molding, typically in polypropylene or polyethylene.

• The thickness of the living hinge should be around 0.3 mm
• The length of the living hinge should be around 1.5 mm
• A radius of 0.75 mm should be added to the outside bend

The success of a molded living hinge can also be influenced by factors such as gate placement and flow rate. However, the majority of injection molding companies have the capability to regulate these parameters in order to achieve the optimal result.

3D printing living hinges

Living hinges were made possible by the advent of injection molding, as the direction of flow plays a crucial role in ensuring the hinge’s strength and durability. However, 3D printing has emerged as a feasible method for prototyping living hinges.

The proper orientation of a part on the print bed is crucial when utilizing a 3D printer for creating living hinges. In order to ensure the durability of the hinge, it is important to align the width of the hinge along the Z-axis during the printing process, rather than the length which is typically shorter. If the hinge is bent repeatedly in the same direction as the layer lines, it will be more prone to breakage compared to when it is bent perpendicular to the layer lines.

Moreover, living hinges produced through 3D printing require greater thickness compared to those made through molding in order to ensure adequate durability, despite having inherently lower flexibility.

It is essential to follow the specified guidelines when creating living hinges for 3D printing.

• Parts should be oriented so the width of the hinge is built up layer by layer
• The thickness of the living hinge should be around 0.6 mm
• A radius of 1.5 mm should be added to the outside bend

Living hinge materials

Polypropylene is the most frequently used and optimal material for creating living hinges through injection molding. As an alternative, polyethylene can also be utilized.

Nylons (PA 11 or PA 12) are considered the optimal materials for creating living hinges through 3D printing, regardless of whether it is done using FDM or SLS technology. Nevertheless, with multi-material FDM printers, it is possible to print the hinge using a flexible material such as TPU, while the remaining portion of the object can be printed using a rigid material. In such cases, TPU hinges tend to have a longer lifespan compared to those made from nylon.

Living hinge examples and applications

Living hinges can be seen in many common plastic items, including:

• Shampoo bottle lids
• Dental floss boxes
• Condiment bottle lids
• Makeup compacts
• Storage containers
• Hemostats

Feel free to reach out to XinYang for a complimentary estimate on your upcoming injection molding or 3D printing endeavor involving living hinges.