11/06/2023
By Danielle Fretwell

The Francis College of Engineering, Department of Plastics Engineering, invites you to attend a Doctoral Dissertation Proposal defense by Nicholas Bowen on: The Effect of Wall Slip Controlled Injection Molding of Recycled Materials on Crystalline and Mechanical Properties.

Candidate Name: Nicholas Bowen
Degree: Doctoral
Defense Date: Monday, Nov. 20, 2023
Time: 10 a.m. to noon
Location: Perry Hall 215

Committee:

  • Advisor: Davide Masato, Asst. Professor, Plastics Engineering, University of Massachusetts Lowell
  • Margaret Sobkowicz-Kline, Professor, Department of Plastics Engineering, University of Massachusetts Lowell
  • Amir Ameli, Assistant Professor, Department of Plastics Engineering .University of Massachusetts Lowell
  • Peng Gao, Assistant Professor, Department of Engineering and Design, Western Washington University

Brief Abstract:

Recycled materials have seen a rise in demand as the effort for sustainable and environmentally conscious practices continues to grow. However, the reprocessing of these materials causes inherent process and product quality inconsistencies that also lead to a loss in mechanical properties.

Additionally, due to the presence of contaminates and alterations in shear history, these recycled materials present with more variable rheological behavior. As such, there has been an effort to find ways to increase the mechanical performance of recycled materials to better resemble those of their virgin material counterparts.

To address this issue, this work first investigates the injection molding of recycled and virgin polypropylene. Using a multi-cavity hot runner mold, correlations between processing parameters, such as melt temperature, mold temperature, pack pressure, soak time, and mechanical properties, were systematically investigated. Results indicated significant correlations between processing, mechanical properties, and morphology. In particular, for the PP, the melt temperature affected the formation of β-crystals and their transition to α-crystals upon tensile testing. Different morphology was observed for the rPP, in which crystallization was affected by polyethylene contamination.

To further investigate the effects of processing on material morphology and mechanical performance, the discussion can be further expanded to look at deformational flows. It has been seen that wall slip, a rheological phenomenon which has proven difficult to fully characterize, occurs in polymer melts that exceed a critical shear stress during processing. The presence of wall slip can alter the shear profile and generate nucleation sites which ultimately can change material morphology.

Additionally, fibers and fillers are commonly added to recycled materials to increase mechanical properties or material stability. As such, the alteration in shear profile from wall slip can result in a change in filler distribution and orientation. As such, the goal of this work is to investigate the fundamental phenomena that create the onset of wall slip during the molding process and the effect of the rheological change on flow-induced crystallization, fiber orientation and, ultimately, mechanical properties. Variables which influence the materials affinity for wall slip includes shear stress, polymer-wall interactions, and polymer-bulk interactions, which can be more directly influenced by changing factors such as mold geometry, melt temperature, injection velocity, and material (branching, molecular weight, etc.). Additionally, investigation to see how wall slip behavior may change with different polymeric families (i.e., polyolefins vs polyesters) as well as how recycling may change a material’s ability to slip and crystallize will be investigated.