Fiber-polymer composites show tremendous promise due to their unique mechanical/electrical/thermal properties. Such composites usually consist of polymer matrix embedded with high-strength synthetic/natural fibers. By printing fiber-polymer composites using additive manufacturing (AM) techniques, the three-dimensional models can be fabricated with arbitrary free-form geometry, improved mechanical properties, and even multiple functionalities. Yet grand challenges such as limited material choices, weak fiber-matrix interface, and little control over filler distributions within the matrix still exist in the major additive manufacturing processes for fiber-polymer composites. 
 
Among the many additive manufacturing techniques that have been investigated for the productions of fiber-polymer composites, direct ink writing (DIW) is an extrusion-based AM approach. It has a relatively diverse choice of printable filler and matrix materials for the composite structure. Applications, including energy storage, mechanical reinforcements, medical implants, and biomimetic materials, have been presented through the DIW of fiber composites. However, challenges, including the complicated process planning, low geometrical accuracy, filament shape instabilities, and minimal fiber extrusion capability, are still existing in the DIW of fiber composites. 
 
This dissertation aims to address these challenges in DIW of fiber-polymer composites and to develop a fundamental understanding of the complex interplay of ink properties, printing process, and functionalities by answering the following research questions: 
Q1 How does the incorporation of an electric field between the printing nozzle and the substrate affect the shape-stability and the geometrical accuracy of the printed filament? 
Q2 Can the addition of hydrogel change the storage modulus and loss modulus of the synthetic fiber suspensions to suit the extrusion-based additive manufacturing process? 
Q3 What are the influence of the probe sonication process on the natural fiber structure and their dimensional variations under a polymer-based composite configuration? 
 
Following the research questions, I conducted fundamental studies in material printability evaluation and process/design enhancement. A DIW setup equipped with an electric field is developed. The DIW process without and with the electric field is modeled analytically and validated experimentally. Synthetic-fiber-polymer composite inks and natural-fiber-polymer composite inks are prepared and characterized for DIW printing. Functional parts are printed using the formulated composite inks, and the printed structures are characterized. The scientific contributions can be summarized in the following perspectives: 
C1 Developing generalized mappings between feature geometries and process parameters 
C2 Understanding the influence of liquid rheological properties on printing stability 
C3 Establishing approaches for modifying synthetic/natural fiber for printable composites 
C4 Incorporating field-assisted and temperature-controlled process planning strategies 
 
These findings enable significant improvements in applied techniques for fabricating next-generation energy storage devices, high-performance structural materials, real-time optimizations in manufacturing, and novel liquid/droplet control approaches. The outcomes serve as one of the first efforts that advance the processes, materials, and designs in future manufacturing technologies

Direct Ink Writing of Functional Fiber Composites

Paper Proceedings: https://asmedigitalcollection.asme.org/MSEC/MSEC2021/volume/85062

2021 International Manufacturing Science and Engineering Conference (MSEC2021)

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