Improving Properties of Plastics Printed by Material Extrusion: Thermal History, Post-Processing, and the Design of Core-Shell Structured Filaments

Open Access
- Author:
- Ai, Jia Ruey
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 11, 2022
- Committee Members:
- Bryan Vogt, Chair & Dissertation Advisor
Hui Yang, Outside Unit & Field Member
Michael Hickner, Major Field Member
Seong Kim, Professor in Charge/Director of Graduate Studies
Amir Sheikhi, Major Field Member - Keywords:
- material extrusion
fused deposition modeling
coextrusion
polymer composite
fused filament fabrication
additive manufacturing
core-shell filament - Abstract:
- Material extrusion (MatEx) also commonly known as fused filament fabrication (FFF) is one of the most adopted additive manufacturing (AM) processes for thermoplastic and polymer composite materials due to its low cost for filament feedstock, filament formulation variousness, and 3D printer accessibility for users. Unlike traditional manufacturing processes such as injection molding (IM) which possesses mass production advantage, MatEx provides great potential applications for small batch production which required highly customized design or complicated structure. However, MatEx process inhere layer-to-layer deposition and non-isothermal printing conditions which generate welding interfaces, voids, and thermal stress within printed products. These defects deteriorate mechanical performance and dimensional accuracy of printed products which further limits AM for engineering applications. The first part of this dissertation provides an experimental approach to investigate the thermal history within MatEx process. The thermal history of MatEx process was examined and discussed based on the different printing sequences (series and parallel) and tensile specimen sizes (ASTM D638 type IV and type V) for both polycarbonate (PC, amorphous) and isotactic polypropylene (iPP, semi-crystalline). The results suggested different interfacial strengthening mechanisms in between printing lines interface for amorphous and semi-crystalline polymers. Moreover, the mechanical performance and failure mode of printed tensile specimens were thoroughly investigated which indicates elastic modulus is not always provide accurate mechanical performance prediction for the printed specimen. The results indicate the print path based on the combination of printing sequences, specimen size, and build orientation would have a significant influence on mechanical performance which provides information and guidelines for designing printed products. The second part presents a rapid microwaved post-processing on MatEx printed specimen to overcome weak interfacial adhesion and further enhance the overall mechanical performances. The rapid microwaved post-processing experiment of carbon fiber polyether ether ketone (cPEEK) was designed with different build orientations, microwave power, and exposure time. After rapid microwaved post-processing, the elastic modulus of tensile specimens increases 3 times compared with non-treated tensile specimens even with an increment of overall internal void size. This result confirmed that mechanical performance improvement is attributed to the enhancement of welding quality at print lines interface. To further understand this improvement, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were adopted to analyze the average crystal size and overall crystallinity. The average crystal size does not increase, nor does the overall crystallinity from this rapid microwaved post-processing. The improvement in mechanical properties is attributed to the improvements between adjacent printed roads from the microwave post-processing, which allows for some relaxation of chain orientation, entanglement of chains, and formation of crystals across the interface. The third part introduces a strategy for designing core-shell structured filament to overcome the drawbacks within MatEx process. The design of core-shell structured filament sperate the processing temperature of both amorphous core and semi-crystalline shell materials. The Tg of core material is elevated far from Tc of shell material which allows core-shell filament printed specimen to form stronger shell-shell bonding without any in-situ process or post-process. Moverover, the printed specimens contain embedded core fiber which act as continuous fiber reinforced polymer composite. The additional core-shell interface provides an additional frictional energy dissipation mechanism which further improves impact resistance in I-zod notch impact test. By further separating the Tg of core and Tm of the shell, the dimensional accuracy of printed specimen further improved. The last part of this dissertation discusses the interface interaction between core and shell interface. By introducing maleated isotactic polypropylene (miPP) to compare with isotactic polypropylene (iPP), two different core-shell structured filaments with same polycarbonate (PC) core but different shells are compared extensively. The coefficient of friction was confirmed with the reciprocal ball-on-flat tribology test which suggests the interfacial interaction is increased for miPP/PC compared to iPP/PC. By improving the interfacial interaction, miPP/PC printed specimens consistently possess better toughness within tensile tests as interface interaction determines the energy requirement for polymer composite fiber put-out mechanism.