Of the many reasons designers choose molded plastic, part consolidation is perhaps at the top of the list. And falling under this category are integrally molded fasteners, also known as snapfits.
Designing the various types of snapfits to adequately withstand stress, however, can be tricky due to the nonlinear nature of plastics in which stiffness changes with applied loads. Does this mean you need to perform nonlinear finite-element analysis every time you design a snapfit, or is there another solution?
|Figure 1. Developers predict that this will be the most popular use for Snap Designânamely, to design cantilever snap tabs. The left window lets users input parameters and shows calculation results, while the right window shows the 3-D view at any angle specified.||Figure 2. Annular snapfits, commonly seen in pen caps and bottle closures, are analyzed with red used to represent the elastic element and blue to model the rigid one.|
Dennis Que of Closed Loop Solutions (Troy, MI) tells IMM that the answer for snapfits is not so black-and-white. "We recommend using closed form equations widely, backed up by nonlinear FEA when required," says Que. "In fact, designers often learn what's needed during FEA by applying closed loop solutions first." Furthermore, he adds, to avoid overstressed designs, each snapfit feature should be analyzed in some way.
|Figure 3. Torsional snapfit features are not common, but can be used for releasable drawer stops molded in the line of draw.|
|Figure 4. In addition to its generic materials database of common thermoplastics, Snap Design lets users add their own material data. A nonlinear material model is used to accurately predict the stiffness of the snapfit.|
Que's newly formed company has developed a software module called Snap Design, which is based on closed form equations and parametric geometries. With it, users can bypass some of the iterations that FEA requires. To use FEA, you need to build a model; apply boundary conditions, loads, or displacements; specify material properties; and then wait for the computer to solve for deflections and stresses.
These values must be compared to the desired values, and inputs must be adjusted iteratively until all of them are acceptable. With Snap Design, if you know any two of the three inputsâloads, material, or dimensionsâyou can solve for the third unknown without building a model or adjusting inputs.
This isn't the first time plastics designers have seen software and guidelines for easing the task of snapfit design, Que admits. AlliedSignal, Bayer, BASF, GE Plastics, and Dow have all released their own versions in the past. But the new package builds on the existing knowledge base, and relies on some heavy-duty contributing technical advisors, including Paul Tres of Montell, a noted plastics design authority.
Accurate results in a short span of time appear to be this software's strong point. For example, using the software to solve for a material, users select the snap typeâcantilever, annular, or torsional as in Figures 1 to 3âthen enter dimensions, allowable deflection, and deflection force. After crunching numbers through the closed form equations, materials that satisfy the inputs are displayed in a summary window. To narrow the materials choices to a manageable size, users can fill in known criteria such as the material's generic name or operating temperature range. The package also includes an integral materials database with tensile stress-strain curves; users can expand the database easily with their own material properties, as seen in Figure 4.
Snap Design sells for $349. The package runs on Windows95, NT, 3.0 or 3.1.