Editor's note: In the final chapter of our series, Robert Cramer, senior development scientist with Dow Plastics, tackles the issue of living hinge design. He explains how to modify the general guidelines and apply engineering principles for living hinges to polypropylene, polyethylene, and even such nontraditional hinge materials as nylon and PET.

Studies have shown that assembling two individual components is one of the most expensive manufacturing operations, particularly if separate fasteners are used. Snapfits, ultrasonic welding, and other assembly methods reduce costs by eliminating the fasteners. Living hinges take the cost reduction process even further by combining two separate components into a single part. For example, any kind of box requiring a lid could be made in one piece using a living hinge.

Most hinges are added to an assembly after the parts have been made, while living hinges are molded directly into the part. A living hinge is a thin portion of plastic material that connects the thicker walls of two portions of a part. It allows the part to flex or open and close without the use of a separate hinge assembly. Polypropylene and polyethylene are commonly used for living hinges because these materials can be flexed many times without breaking. Often the suppliers of PP or PE will recommend a generic hinge design that is conservative enough to fit a wide range of applications.

The designer faces a problem, however, if these generic designs do not meet a particular requirement of an application or if the choice is made to use a different material family. How should a living hinge for nylon or PET differ from the recommendations for PP and PE?

Living Hinge Design

A typical polypropylene living hinge design is shown in Figure 1. The hinge land length is typically about .060 inch (1.5 mm) with a thickness of .008 to .015 inch (.2 to .4 mm). These proportions will usually give an acceptable balance of flexibility and stiffness. All corners in the hinge area are radiused to reduce stress concentrations and promote flow through the hinge during the molding operation. The radii are typically .030 inch (.75 mm). Finally, the upper surface of the hinge should be recessed .005 to .010 inch (.13 to .25 mm) to allow the mating parts to fit properly and to prevent a sharp fold in the hinge when closed.

It is important that the mold cooling in the hinge area is sufficient. The shear rate through the hinge is high during filling, which generates additional frictional heat, and the tool must be able to remove it. The part must also be gated so that during filling, the flow front reaches the entire width of the hinge as uniformly as possible. This sets up the most favorable orientation of the polymer molecules for increased flexural strength. If there are multiple hinges, it is critical that the polymer melt does not hesitate at one or more of the hinges while other parts of the mold fill. This will cause the material to cool rapidly in the hinge area, making it difficult to continue flowing across the hinge or set up high residual stresses that could cause failure.

The first time the hinge is flexed, the hinge land will neck down and undergo plastic yielding. If this is done a few times immediately after molding while the part is still warm,
the material in the hinge will orient in the proper
direction and increase the
fatigue life.

Going Beyond the Guidelines

As with inserts and ribs, guidelines exist for hinge design. However, the designer may not be able to use the generic proportions shown in Figure 1. Or he may want to use a material other than polypropylene. How can the typical design be modified with some assurance of success? Basic elasticity theory can be used to analyze living hinges to determine whether the strain is purely elastic or whether there is also plastic strain. If there is plastic strain, it can be caused purely by bending or have an additional tensile strain component.

For the pure elastic case, the hinge can be considered as a beam undergoing pure bending. The hinge bends in a circle with the neutral axis through the hinge center. The hinge failure criterion is considered to be when the stress in the outer fibers reaches the yield strength of the material. Limiting the hinge to elastic strain requires a very large radius at the neutral axis, which is generally too large to be practical.

Most living hinges undergo some plastic strain, with the outer portion of the hinge undergoing plastic deformation while the inner remains elastic. Failure occurs when the strain reaches the ultimate strain of the material. If the upper hinge surface is recessed as shown in Figure 1, the hinge can stretch when the mating parts are assembled. This can cause plastic strain in both the outer and inner portions of the hinge, with a portion of the hinge around the neutral axis remaining elastic. If the maximum stress in the hinge reaches the ultimate strain of the material, hinge failure occurs.

The critical parameter in determining whether or not failure will occur is the ratio of the hinge thickness to hinge width. The sidebar at left gives a simple procedure that allows the hinge design to be evaluated. Examining the equations shows that the smaller the value of this ratio, the less likely failure is to occur. This ratio for the typical hinge design shown in Figure 1 is between .13 and .25, depending upon the thickness chosen. Given polypropylene's high ultimate elongation, this recommended design will be suitable for many applications.

Applying the Principle

The manufacturer of storage boxes for small fasteners designed a new series of products using a living hinge to consolidate the lid and box body into a single part. Having chosen polypropylene for the material and knowing it is commonly used for living hinges, the product engineer was surprised when the hinge cracked after a only a few cycles. The polypropylene had a yield stress of 5000 psi (34.5 MPa), modulus of elasticity of 200,000 psi (1379 MPa), and an ultimate elongation of 100 percent. The original hinge was .024 inch (.6 mm) thick and .060 inch (1.5 mm) wide. The hinge recess was .020 inch (.5 mm).

Checking the hinge design using the equations in the sidebar revealed these proportions resulted in plastic bending and tension that exceeded the material's ultimate elongation, resulting in hinge failure. A successful hinge design with this material would require changing the hinge proportions to reduce the value of b/L. Obviously, there were two options: decrease the hinge thickness or increase the hinge width.

Checking the effect of maintaining the hinge width and reducing the thickness to .020 inch (.5 mm) showed that plastic bending and tension would still occur, but the ultimate elongation of the material would not be exceeded. Unfortunately, reducing the hinge thickness would require welding the mold and recutting the hinge area.

Next, the effect of maintaining the original hinge thickness but increasing the hinge width to .080 inch (2.0 mm) was examined. Again, plastic bending and tension would exist but the ultimate elongation would not be exceeded. This option did not require welding the mold and was chosen by the manufacturer. The mold was modified and sample parts showed excellent fatigue life.

The manufacturer could have considered another grade of polypropylene with a higher elongation. The tradeoff would have been reduced stiffness with the higher elongation material.

References

1. "Designing Hinges that Live," Machine Design, July 23, 1987, pp. 103-106.

2. "Design Tips: The Living Hinge," Plastics Design Forum, May/June 1989, p. 96

3. Designing Plastic Parts for Assembly, by Paul A. Tres, Hanser Gardner Inc., pp. 139-171.

Design Method for Living Hinges

The ratio of the hinge thickness to hinge width is used as the design parameter (see Figure 2).