For polymeric materials, shear rates are known to have significant effect on crystallization development during processing and performance of the parts fabricated therefrom. The primary objective of this research was to gain fundamental insight on the effect of dynamic melt modulation during injection molding on the crystallinity development of PLA and the associated properties. The dynamic melt modulation was achieved by a Dynamic &amp; Intelligent Shear-Controlled Injection Molding (DISC-IM) instrument that utilized an in-house developed control system capable of oscillating the injection screw. The frequency, duration and onset of the vibration that ensued from the oscillation of the injection screw was controlled individually to impose a controlled shear rate onto the polymer melt. The melt modulation process proved to be an efficient processing technique for enhanced properties in a particular commercial grade of PLA that contained various additives. The cycle time was reduced by 40&nbsp;% and total crystallinity was increased by ~50% during DISC-IM as compared to the conventional injection molding under similar conditions. This study was focused on the synergistic effects between nucleating agent, a common additive in commercial grade polymeric materials, and the melt modulation conditions on the molecular alignment in the polymer melt and the subsequent effect of that on the crystallinity development.The primary polymeric material of interest in this study was polylactic acid (PLA). PLA is a bio-degradable thermoplastic polymer derived from renewable resources. It is viewed as a favorable substitute to other polymeric materials derived from petroleum based non-renewable sources. PLA is widely used for various bio-medical applications such as implants for bone fixation and tissue scaffolds. In such applications, the mechanical properties and biodegradability need to be controlled precisely. Those properties are highly affected by crystallinity of the product. Orotic acid (OA) was utilized as a nucleating agent for PLA.The key DISC-IM parameters investigated in this research included vibration frequency, vibration duration and onset of the vibration period as well as the mold temperature. The crystallization behavior of neat PLA was dramatically enhanced by 1wt.% of orotic acid. Introduction of DISC-IM enhanced the crystallinity of PLA-OA blends from 25% to 56% at a mold temperature of 70℃. Although, the overall crystallinity was relatively similar for different DISC-IM conditions, the crystalline phases that were formed was different. Both α and ά phases were observed to be present in the molded parts. A larger fraction of ά phase was formed as the vibration frequency increased from 1Hz to 8Hz. Increase in vibration duration resulted in the formation of more ordered crystalline domains. A delay in onset of vibration of during the injection cycle introduced different modes of melt modulation and offered another approach for fine tuning the nucleation and growth of the crystalline domains.

Enhanced Crystallinity Development of Poly-Lactic Acid by Dynamic Melt Manipulation

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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