Hot melt extrusion of carbon fiber reinforced thermoplastic composites for injection molding and fused deposition modeling

Abstract

Hot melt extrusion (HME) has been a well-established industrial manufacturing technology predominantly in the plastic processing. As an independent machine, the hot-melted extruder can also integrated with other manufacturing techniques including injection molding, thermoforming and blow molding to meet the manufacturing requirements. This thesis is mainly central in HME as the basic technology to realize the fabrication of carbon fiber (CF) reinforced thermoplastic composites by coupling with injection molding and fused deposition molding techniques. To improve the CF-matrix interfacial bonding, a simple and economical surface treatment method was developed for the first time to modify the surface roughness of CF by spray coating with carbon nanotubes (CNT). Then, The CNT-coated CF was then compounded, extruded and injection molded with high density polyethylene (HDPE) for hierarchical CNT-CF/HDPE composite fabrication. To evaluate the influence of CF contents and CNT coatings on the mechanical, thermal and morphological properties of the hierarchical composites, the CF content relative to the composites varying from 0-25 wt.% and 1 wt.% CNT coatings relative to CF were investigated in the research. The results showed that with the increase of CF content, all the evaluated properties including the tensile properties, flexural properties and hardness of CF/HDPE composites were effectively enhanced. Meanwhile, compared to uncoated composites, the CNT-spray coated counterparts have higher overall mechanical performance. The differential scanning calorimetry data indicated that the crystallization temperature and the crystallinity of HDPE was affected by the introduction of CF and CNT while the melting temperatures did not have apparent changes. An analysis of mechanical results combined with the morphological observation via scanning electron microscopy indicated that the interfacial bonding between CF and HDPE was improved by the incorporation of CNT. The FDM fabrication of CF/polylactic acid (PLA) composites was studied in this thesis by employing the HME-made PLA composite filaments as the feedstock. By varying the loading quantity, the effect of CF contents on the mechanical, thermal and morphological properties of these 3D printed composites was thoroughly investigated. Results showed that with the increase of CF contents, all assessed properties of CF/PLA composites including tensile properties, flexural properties, hardness and thermal conductivity were effectively improved compared to the neat PLA. Their performance exhibited the same upward-downward-upward trend with the addition of CF. It can be attributed to the mutual influence generated from inter-/intra filament porosities and high stiff CF. Meanwhile, a machine learning technique, Gaussian Process modelling was also introduced for the property prediction of the composites. In comparison with the experimental analysis, the optimal CF content of 6.7 wt% with best overall performance was predicted using this model, which was very close to the best experimental results at 5 wt% CF. According to the above research, the 5 wt% CF/PLA composite with the best mechanical properties was chosen as the feedstock in the FDM printing of a new class of chiral structures built upon the polynomial equation. Guided by experimental characterization and finite element analysis, the influences of the polynomial orders (n) and the incorporation of CF on the mechanical properties were systematically studied. Results show that the auxetic behavior can be tailored by varying the n value inside the polynomial equation. Moreover, the tensile modulus, strength and energy absorption at break of printed chiral samples were all enhanced with the addition of CF, especially for the improvement of modulus and absorbed energy at around 200%. Show more