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

H. Noor, A. Duhduh, J. Coulter, A. Kundu, K. Alqosaibi
Lehigh University,
United States

Keywords: rheoprinting, additive manufacturing, PLA, crystallinity


Additive manufacturing has been presented as a new paradigm of fabricating products. The ability to build final parts 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 using Rheoprinting has been developed at Lehigh University. It is based on an extrusion deposition 3D printer. The mechanism is applied to the printer to allow a controlled temporal shear rate on the molten polymer strand as it is printed on a substrate. The controlled shear changes the melt rheology, which in turn controls the evolution of crystallinity in the printed parts. The temporal control of shear will translate 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. 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. Other factors of the results will be discussed in details in the presentation.