Modeling and investigation of elastomeric properties in materials for additive manufacturing of mechanistic parts

Abstract

The absence of a design support system providing feature-specific information about Additive Manufacturing (AM) processes and materials has impeded the global acceptance of AM. AM offers designers more geometric complexity than ever before but as we start to use it to build mechanistic parts, we need to replace the conventional process constraints such as draft angles with new process constraints specific to AM to help the designers who want to use the new technology.

This project was initially an investigation into the viability of various AM processes and materials for the fabrication of interlinking structures like living hinges. The initial study focused on the mechanistic properties required for interlinking structures thereby classifying them into material related properties and design-process related properties. A theoretical model was developed to aid material and process selection for living hinges through a study of the elastomeric properties of AM materials and the kinematics of the bending mechanism. The initial analysis led to the hypothesis that it was possible to develop a set of quantifiable rules for living hinges that would allow designers to select the correct process and material from what is available. It predicted that the Objet material FullCure 720 would be a good candidate for the fabrication of living hinges. However, preliminary experimental results and a more detailed theoretical study proved otherwise. While FullCure 720 does exhibit elastomeric properties, it is not strong enough to withstand heavy use.

As a result, the initial hypothesis led to a modified one that it was possible to develop numerical models using Finite-Element Analysis (FEA) which would be able to predict feature behavior. Experiments were carried out to find out the exact material properties of specimens of FullCure 720 fabricated with Objet Eden 350. The results of the experiments were useful to select the most accurate FEA model to simulate the behavior of FullCure 720. After studying and trying numerous plasticity models, the original linear Drucker Pragar (DP) model was used in conjunction to the linear elastic model to model the behavior of FullCure 720. A detailed understanding of the living hinge concept as well as elastomeric properties was developed and the FE models were validated with experimental results. The numerical model was subsequently used to simulate the functioning of another mechanism which uses elastomeric properties for its functioning: snap fit mechanisms. The numerical results were in-line with expectations proving that the model could be used to understand the functioning of different mechanisms that use elastomeric properties and could be fabricated using FullCure 720.