Doctoral Dissertations

Date of Award

12-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Uday K. Vaidya

Committee Members

Ahmed A. Hassen, Chad E. Duty, Claudia J. Rawn

Abstract

Recent inventions in the ability of additive manufacturing (AM) to use carbon fiber (CF) reinforced pellets as a feedstock material to manufacture components has complemented its purpose from prototypes to structural load-bearing parts. In the first part of this research, we investigated the processability, microstructure, and mechanical performance of twin-screw compounded short CF reinforced polyphenylene sulfide (PPS) pellets as a feedstock material for extrusion deposition fabrication-additive manufacturing (EDF-AM) using big area AM (BAAM). The performance of the BAAM components was compared to that of traditional processing methods, namely injection molding (IM) and extrusion-compression molding (ECM). It was found that the AM composites exhibited 118% lower tensile strength and 55% lower tensile modulus when compared to traditional injection molding composite specimens; however, AM composites exhibited comparable properties to ECM composites.

In BAAM, CFs are usually aligned along the deposition direction, which result in anisotropic thermal properties as heat transfer and warpage. In this study, three male molds with different infill patterns were produced via the EDF-AM process. These include (a) 0°: infill pattern along the printing direction; (b) 90°: infill pattern perpendicular to the printing direction; (c) 0°/90°: alternate layers along and perpendicular directions. It was observed that the thermal conductivity had a direct relationship with the CF orientation, as the average top surface temperature for 0° > 0°/90° > 90° and inverse relation with warpage.

The third part of this work focuses on improvement of durability of AM composite molds. A new material system called hybrid tooling was developed, such that a thin layer of Invar working surface and composite supporting structure. The high cost and density of Invar makes it a challenging material for tooling applications. Therefore, in this research, a mold substrate of CF-PPS was prepared using AM, and a 0.1 mm layer of CF-BMI prepreg was attached as a skin layer. Durability of the mold was demonstrated from manufacturing seven (7) carbon fiber- epoxy hand lay-up parts using the mold. Also, it was observed that the mold held vacuum very precisely and no air leakage was observed.

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