The Mechanics and Optimization of Cantilever Snap Joints

Market pressures of a world economy demand that corporations continually develop more efficient and cost effective product designs in a shorter period of time. Prior to the development of modern age plastics and molding techniques, components for products had to be fabricated from expensive casting and or machining processes. Once fabricated, multiple component assemblies were secured together using numerous fasteners or rivets. Plastic molding technology combined with innovative snap joint designs now allows for complex net shape parts to be fabricated and snapped together cost effectively. The design of plastic components utilizing snap fits that are easy to snap together, provide a secure joint, and don’t break when snapped together, presents a major challenge for the product design engineers. Since mold die tooling represents a major capital investment with long lead times, the snap joint designed must work right the first time with minor tooling modifications. Classical analysis of snap joints utilizing beam formulas are often only crude approximations and usually do not address the secureness or holding strength of the snap joint. Non-linear contact analysis capabilities found in finite element analysis programs like ANSYS, combined with fast personal computers, now allow engineers to rapidly evaluate and optimize snap joint designs prior to committing to costly prototypes and tooling. Presented is the non-linear contact analysis of a tapered cantilevered snap joint commonly used to join multiple plastic molded components. Primary focus of this investigation is the optimization of joint secureness as influenced by snap engagement, rake angle, coefficient of friction, material modulus, and cantilever taper ratio. Snap joint mechanics and snap joint cam-out phenomenon during pullout are discussed in detail. Technologies like finite element analysis play an important role in allowing product design engineers to develop and optimize robust snap joint designs up front prior to fabricating costly mold die tooling.
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