Living Hinges

Table of Contents

Introduction

From one-use food containers found at a deli to consumer electronic cases that are open and closed daily, living hinges are a cost-effective way to consolidate two parts with a thin piece of the same material bridging them together. Although the concept is a simple one, executing the design has some nuances to consider.

Additive manufacturing is a great way to create prototypes with living hinges, concept models and get design verification before moving into expensive machine-tooled injection molding or cast urethane for production. Because of the layer-by-layer build process, additive manufacturing has a limited number of open or closes, whereas injection molded and cast urethane can have 1000s of opens and closes.

Technology and Materials used for Creating Living Hinges

Additive manufacturing allows for cost-effective and speedy (can be as short as 24 hours) prototypes and concept models. The number of design iterations is also as easy as uploading the CAD files to our RFQ team. For quick quotes on an upcoming digitally manufactured or cast urethane project, please fill out this form or contact us directly at solutions@midwestproto.com or 608.437.1400.

We use laser sintering, multi-jet fusion, fused filament fabrication, and cast urethane for producing living hinges. We use Nylons and rubber-like materials. For best results, make sure the material has good elongation and flexibility.

Materials 

Generally, Nylon (PA 12)

Single extrusion print heads generally use Nylon (PA 12).

Dual extrusion print heads use PLA or ABS as the rigid portions and TPU as the flexible part.

Single Material – durable resin-like PP.

Multiple Materials – durable resin-like PP and ABS or rubber-like material for flexible portion.

Specifications 

.03 – .08 cm in thickness

.5 cm in length (min)

Minimum two layers thick

.04 – .08 cm recommended thickness

.04 -.08 cm thick

Number of cycles

30 to 50 until failure

25 -30 until failure

<10 until failure

Pros 

Not as likely to delaminate as parts made with Fused Filament Fabrication.

Dual extrusion print heads can print two materials.

MJ can print it in multiple materials.

More isotropic.

Smooth and aesthetically pleasing, like injection molded parts.

Cons 

They are printed vertically, meaning more supports which can be more cost and time (more material/more post-processing).

FFF can delaminate.

Materials are brittle, with fewer cycles until failure.

Cast urethane bridges the gap between prototyping and mass-producing parts. A wide range of materials is used in cast urethane, including polypropylene-like materials – the best option. The polypropylene family has a wide range of durometers, from more rigid (with short-lived hinges) to flexible (with softer part walls). For more information on materials, we offer for cast urethane or determining which materials might be the best choice for your application, contact one of our Solutions Managers directly at solutions@midwestproto.com or 608.437.1400.

When designing for cast urethane, pressure is not used like injection molding, where pressure is applied, and the material is forced through small areas. Hinges may need to be designed thicker.

Post-Processing

Once parts are completed, they can be annealed during post-processing.  Annealing the parts will increase the number of open and close cycles before failure.  To anneal a part, it is heated and then cooled down in the closed position. 

Types of Living Hinges

Flat Hinge

The most common living hinge and the simplest.  Also known as a straight hinge, they can be one long or short continuous or multiple segments and rotates 180 degrees.

Line Drawing of Living Hinges in an enclosures
Double Hinge

Two flat hinges separated by a narrow landing section that is good for creating a gap between two folding sections.  They can allow for 360-degree rotation.  Double hinges offer security and stability when opening and closing the part. 

Line Drawing for Living Hinges
Butterfly Hinge

Also known as flip-top caps for their flip action to open or closed.  

cap with living hinges
Bi-Stable Hinge

Bi-Stable is similar to butterfly hinges with three separate hinge actions creating an equilibrium in the open and closed position.

Line Drawing for Living Hinges 2

Design Guidelines for Living Hinges

line drawing box
Build Direction
Designing living hinges for additive manufactruing
Build Direction

When determining the build direction specifically for additive manufacturing, it is essential to know that the layer-by-layer nature of production will affect the mobility and strength of the hinge. The best practice is to build along the width of the hinge and not the length (the build orientation should be along the Z-axis). The thickness of the hinge should also be at least twice the resolution of the print. Following this advice could make the difference on your part, getting 10-25 open and close cycles or 100.

Length of Hinges

A hinge that is longer than six inches should be divided. This will lengthen the lifespan of the hinge.

Radii

Include Radii to reduce the stress concentration while the hinge is in the closed portion of the cycle.

Shoulders

Shoulders make sure the hinge’s bend is at the center to relieve stress where the hinge meets the rigid portions of the unit.

Thin and Flexible

Unlike many things, a hinge’s strength depends on a less thick design.  Thinner is more flexible. 

Are you ready to get your ideas made?

Or do you have questions about the use of living hinges in your parts?  Our Solutions Managers are standing by ready to help.

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