CNC Machine Polypropylene Sheet Material Into Prototype Living Hinges with Accuracy up to 5/1000ths of an Inch
On the positive side, living hinges do not require maintenance or repair because they only fail when subjected to unnatural conditions. Because they are made from polypropylene plastic there is no possibility of rust as you would have with metal hinges. Additionally, the range of motion is much wider with living hinges than with other types of hinges.
On the negative side, living hinges can only be used for relatively small or lightweight applications. This is because their resistance to tensile and/or shear stresses is limited. The fact that designers must make sure the cross section of the living hinge doesn’t exceed a certain thickness (to allow proper function) limits the possible robustness of a living hinge, making it unsuitable for certain functions. Using a living hinge for a door, for example, would subject it to excessive weight and would quicken its path to failure.
Polypropylene (PP) is unique in that it is really the only material that can viably be used for living hinge applications. This is a limitation as the designer is restrained by the material properties and capabilities of the material itself. You can read all about Polypropylene here.
It may be possible to use Polyethylene (PE) in some situations. Additives and fillers that can be used are limited based on their propensity to change material properties that are important to the basic functionality of a living hinge. Other common types of engineering grade plastic like ABS or polycarbonate (PC) are unsuitable for living hinge applications. Traditional hinges would be a better fit if project requirements necessitate such a material.
For design verifications and prototyping, there are really two viable options to produce a prototype part with a living hinge:
#1: CNC machining a prototype out of a single piece of polypropylene (PP) sheet stock: For design verifications and prototyping it is quicker and cheaper to cut prototype parts with living hinges out of polypropylene stock material. Many people believe this to be impossible - for a number of reasons - one of which is the tendency of PP to gum up when being precisely machined. Fortunately for us these people are wrong. Polypropylene can in fact be machined into living hinge prototypes. Creative Mechanisms technicians can machine cut very small and precise living hinges of an infinite variety of designs down to 0.005 inch accuracy on our automated multi-tool CNC machine.
#2: Designing and manufacturing an injection mold tool (i.e. a block of aluminum or steel with an internal cavity in the shape of your prototype part): Getting the hinge right in an injection mold tool can be tricky and it is much more expensive to design and build an injection mold tool than it is to cut one or two prototypes on the CNC machine. We recommend this method when you need several hundred to a thousand prototypes for things such as pre-production testing purposes. On such a scale it is much more efficient to injection mold the parts out of the final material than it would be to CNC cut so many of them. Additionally, for sale samples where a couple of hundred prototypes are required, using injection molding is the better option, not only because of the quick turnaround, but also because the finish is better than a CNC cut prototype. This is vital in sale sampling where it is desirable that the prototype looks as close to the final product as possible. Note: even in such a case it is nice to first CNC cut prototypes to make sure the design is properly functional before making the investment in an injection mold tool.
For large prototype projects (100-1000 unique prototype units) the best option may be to create an injection molding tool and to produce the parts out of the actual material that will be used in production. For more on our capacity to provide this service read here.
For small prototype projects (1-10 unique prototypes units) and iterative design revisions there is really no substitute for our ability to CNC machine polypropylene living hinges out of sheet material. Other options such as 3D printing, making rubber molds and casting in low durometer urethane, cutting a small polypropylene hinge and screwing it into two separate parts, or simply using tape are all an order of magnitude less effective at best, or completely dysfunctional and subject to quick failure at worst.