Shear Induced Crystallization through Altering Flow Area of Polymer Melt in Additive Manufacturing

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Additive manufacturing is a new paradigm of fabricating products. The ability to build near-net shapes by depositing layer by layer in additive manufacturing have provided a time and energy efficient processing strategy whose full potential is yet to be realized. A novel patent pending additive manufacturing technique called “Rheoprinting” has been developed at Lehigh University [1]. It is based on an extrusion deposition 3D printer. An innovative rotating nozzle is introduced to control temporal shear rate on the molten polymer strand as it is printed on a substrate. The controlled shear alters the melt rheology, which in turn controls the evolution of important parameters such as molecular orientation, crystallinity, and filler distribution/orientation in the printed parts. The temporal control of shear translates to spatial control of melt rheology during rheoprinting. Thus, the localized evolution of molecular orientation and nucleation/ crystallization kinetics as well as the mechanical and optical properties can be precisely controlled during the additive manufacturing process [2]. This research is focused on semi crystalline poly-lactic acid (PLA), a plant based biodegradable polymer used in many medical implants and other components. In this study, the effect of application of shear on the PLA is investigated analytically with primary focus on the role of the confinement of the PLA melt at the tip of extrusion nozzle. This is achieved by introducing a cone threaded to the tip of the extruder in a conical cavity whose diameter can be varied. It has been hypothesized that the confinement will induce an additional translational shear on the polymer the degree of which can be controlled by the gap between the conical cavity and the conical extruder tip. The analytical modeling results indicate that this strategy can increase the induced shear rate by a factor of four. Resulting in controllability of crystalline evolution [3].

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Journal: TechConnect Briefs
Volume: TechConnect Briefs 2019
Published: June 17, 2019
Pages: 131 - 134
Industry sector: Advanced Materials & Manufacturing
Topicss: 3D Printing, Advanced Manufacturing
ISBN: 978-0-9988782-8-7