|SolidWorks Featured Author Blog - September 2012|
A living hinge is a small bridge of material connecting two other pieces. The bridging material is designed to fold, providing a hinge that doesn’t require any extra parts or assembly.
Components with living hinges are less expensive than their counterparts with mechanical hinges, and they are surprisingly reliable when made with the right material and design - they can be cycled millions of times.
Some useful guides for designing living hinges can be found in the following locations:
Unfortunately, components with living hinges pose a tough modeling challenge because the designer has to be constantly conscious of both the open and closed conditions of the part they are designing.
All models with living hinges are released for tooling in the open ‘as-molded’ condition. While modeling to this goal, the designer also has to consider the closed position of the component so the correct relationships can be added to insure the halves of the part mate and function as intended.
I wish there was one simple best practice that applies to all cases, but there really isn’t. Fortunately, the library of part reviewer sample files provides examples of several different viable approaches for modeling living hinges.
The ‘BlowMoldedStorageCase’ shows what is probably the closest thing to a ‘best practice’ because it is the easiest and most intuitive approach for a designer to create stable relations.
The storage case is modeled in the closed position as three separate bodies – the bottom, the top, and the latch. The last features in the closed configuration create representations of the two living hinges in their folded state. Though there is no harm in trying to make the closed hinge accurate, there is also no good in it; remember, the manufactured part is controlled by the flat ‘as molded’ configuration. In the closed position, the hinge really doesn’t need to be modeled at all (except maybe for calculating part volume/mass or for interference detection in an assembly).
A second configuration opens the part to the position it will be released for tooling. ‘Move bodies’ commands are used to reposition the bodies. ‘Move bodies’ commands are pretty tricky to use, but with a little work they will get the separate bodies into the correct position for molding. Then it is a simple matter of finishing everything up by modeling the correct, flat living hinge.
Though the above is the likely the easiest approach for the designer, it is NOT the best approach for making a drawing of the part because when the bodies move to the ‘as molded’ position the related dimensions don’t. “Insert model items” into the drawing will be useless on any elements that move during the unfolding process.
In this part, the hinge is modeled first. The hinge can be open and closed using sketch relations. Some creative use of sketch and feature relations are used to drive mating geometries so the box will close and latch correctly when the hinge is folded.
On the plus side, the model dimensions travel with the geometry no matter what position the lid is in.
Unfortunately, using a sketch relation to fold the hinge 180° is not entirely successful ; about half the time, shifting to the closed configuration will cause the sketch under the third feature to develop an unresolved conflict. The comment on the feature explains why this happens and how to fix it.
The third, most interesting approach is shown in the part ‘Strain relief.SLDPRT’.
The part is modeled in the open ‘as molded’ position. Like the previous sample, sketch relations and a ‘derived sketch’ are used to create relations that will provide proper seating of geometries when the part is folded.
What is interesting about this part is that it is folded exactly how it would be folded in the real world: by bending the hinge! The ‘Flex’ command is used to fold the part onto itself exactly like it will work in the manufactured article. Flex is extremely tricky to use and won’t work in some case, but in this part it does exactly the job that is required.
A fourth approach is not represented in the library of part reviewer sample files as of this writing. A designer could create the elements of a living hinge-box as separate parts in an assembly and then use the Join command to bring them together into a new part file. The downside is this requires a minimum of three files (two parts and one assembly) all to generate a single part number; this opens up the risk of some file management troubles. Also, managing relationships in an assembly is even trickier business than managing relations in a single part file!
The above is just a quick summary of what you can find in the part reviewer sample files relating to living hinge modeling. To really investigate these approaches and critically evaluate them I recommend that you download the linked parts and do a critical evaluation of which approach might work best for your next project. The parts are completely annotated with descriptions of the ‘hows’ and ‘whys’ for each feature and relation. Just fire up the part reviewer and enjoy the review.
Ed Eaton is a product designer with the DiMonte Group, a product development consultancy located outside of Chicago. www.dimontegroup.com