Design Optimization of Snap-Fit Integral Parts by FEA and Statistical Analysis

Snap-fit integral attachments are used widely for joining plastic parts. The proliferated use of integral attachment in the form of snap-fit features in designs is due to the ability to mould such parts of great complexity at little cost. The exceptional diversity of part geometry and integral snap-fit features has made it seem that design possibilities may be unlimited. Thus, attempts at optimization might be intractable. A design of experiments (DOE) approach coupled with three-dimensional, geometrical non-linear finite element analysis (FEA) was used to calculate the insertion and retention responses on such parts for various geometrical parameters like length, width and angles. A statistical technique was employed to formulate empirical relationships among the geometrical dimensions, to investigate the effect of these parameters on the design as well as to obtain optimal insertion and retention forces or strains. Design equations obtained from this methodology were verified within the DOE domain and it was observed that the predicted responses were ranged within 30% of the FEA results. During this investigation, it was observed that geometrical features of a block, which exert force on the snap-fit features, have a considerable effect on the results. Therefore, the effects of the block parameters on the various responses were also studied. An attempt was also made to understand the effect of the block parameters such as corner radius and thickness on the design formula, which depicts the geometrical parameters of the snap-fit part as a function of insertion and retention forces. It is expected that the results help to find optimal design parameters in order to enhance the performance of such snap-fit features.

Implementation of a Design Tool for Conceptual Design of Integrally Fastened Assemblies

The proper use of integral attachment features in mechanical assemblies has been more of an art than an engineering science. An organized set of design steps for generating conceptual integral attachment designs has recently been developed based on work begun by Bonenberger. These steps outline a formal design methodology for exploring the design space of possible alternative attachment concepts. This paper describes the development of a software tool that attempts to implement the integral attachment design methodology to assist a designer in developing attachment concepts. The tool is implemented using the Java programming language. A graphical interface is used to present the methodology as a series of options that approximate the design situation. This hides many of the details of the methodology in favor of ease of use. The end result is a set of suggestions for integral fasteners that are matched to the design situation. A discussion of how the hundreds of images are handled using Java is provided. A sample case study illustrates the approach of the program. The tool represents one of the few examples of a design tool aimed specifically at generating design concepts.

JAVA™-Based Design Calculator for Integral Snap-Fits

Many different types of snap-fits have been developed to replace conventional fasteners, and research efforts have been made to characterize their performance. It is often tedious to look for design equations for unique types of snap-fits to calculate the insertion and retention forces. If found, these equations tend to be long, complex, and difficult to use. For this reason, a snap-fit calculator has been created to help in designing integral attachment features. Studies of seven most commonly used snap-fits (annular snap, bayonet-and-finger, cantilever hook, cantilever-hole, compressive hook, L-shaped hook, and U-shaped, hook) were used to provide the equations implemented in this snap-fit calculator, more fasteners than any other snap-fit calculator available. This tool aids in designing snap-fits to meet specific loading requirements by allowing the designer to size the feature to obtain desired estimates for maximum insertion and retention forces. The software for this design tool was written in JAVA™ language that is independent of operating system platforms and can be distributed at a company site-wide over an intranet or worldwide over the Internet. This makes it easily accessible to a user, and universal upgrades can be achieved by simply updating the software at the server location. Designers will find this tool to be useful in the design process and the most convenient way to estimate the performance of snap-fits. This paper describes the development and operation of the IFP snap-fit calculator including several case studies comparing the calculated results to experimental data.

Methodology for Locking Feature Selection in Integral Snap-Fit Assembly

While every step in a systematic approach to the design of an assembly for integral attachment using snap-fit features is important, none is more important than the selection of locking features. After all, it is these features that hold the assembly together. Comprised of a latch and catch component, locking pairs must themselves be selected using a systematic approach. This paper presents that approach as a six-step methodology, after defining and describing latch and catch components. It then demonstrates the methodology using a case study taken from real life.