He, Yinfeng
(2016)
An investigation of inkjet printing of polycaprolactone based inks.
PhD thesis, University of Nottingham.
Abstract
Traditional manufacturing methods like moulding or subtractive manufacturing place significant limitations on structures which could be manufactured in a single process. These limitations can now be overcome by a new manufacturing technology—Additive Manufacturing (AM), which provides the users much more freedom to design and produce structures in one piece. Additive manufacturing refers to a range of processing technologies, which fabricate 3D parts by adding successive layers. With this technology, complex 3D structures can be produced directly following the production of a geometric data. Additive manufacturing also enables production without the need of tooling, which brings the prospect of a revolution in the manufacturing industry.
Material jetting is one of the additive manufacturing techniques, which generates material layers through inkjet printing. This technology also allows the user to build structures consisting of more than one material, which further expands the capability of additive manufacturing to include the production of multi-functional products. However, due to the strict requirements on the rheology of usable inks, there is a limited number of materials available for use in this technology.
This research aims to develop a novel polycaprolactone based ink which is suitable for material jetting and could be potentially used for fabricating scaffolds. The bespoke nature of these devices often require a complex structures, customized design and small batch sizes, which all together make the product costly when using the traditional manufacturing methods. Additive manufacturing technology can reduce these costs, in the main due to the nil marginal cost (e.g. tooling cost, mould design etc.) when changing product design. In addition, material jetting can also incorporate multi-materials or multi-functional devices, mixing several materials at micron level, potentially enabling more advanced and intelligent functions to be incorporated into the final devices.
In this project, Polycaprolactone (PCL), commonly used for its biodegradable properties, was investigated as a candidate for material jetting. Both solvent based and UV reaction based jetting techniques were attempted to build up an understanding of the aspects and parameters involved in material jetting ink development and jetting parameter optimization.
For solvent based PCL ink, PCL flakes were dissolved into various solvents with different concentrations to prepare a low viscosity ink which could be printed. Volatility, viscosity and surface tension were investigated to confirm that the prepared ink was suitable for jetting. PCL with 5wt% in 1,4-dioxane was successfully jetted by using a Dimatix material printer. A range of experiments were carried out to investigate the ink’s printability under different conditions. During the study, efficiency limitation for solvent based ink was also realized. In order to meet the printing viscosity limit of the inkjet printheads, the loading level of a solute in a solvent ink as well as the efficiency of stacking precipitated layers were both restricted. This curbed the possibility of solvent based ink be applied in making large 3D parts.
For UV reaction based inks, the printed ink can fully solidify to form structures after UV illumination, which overcame the processing efficiency limitation of the initial solvent based inks. Pure PCL is not UV curable and therefore chemical modifications were made to graft UV curable functional groups into the PCL structure. The rheology of synthesized UV curable PCL polymers were studied and modified to make them suitable for material jetting. Different photoinitiators were also investigated to work out the suitable composition to achieve real-time curing. Oxygen inhibition was found to be the main side effect which inhibited the curing reaction in an air environment. Type II photoinitiators can help overcome this effect and 3D structures were able to be obtained in both air and nitrogen. It was also found that a nitrogen environment can improve the properties of the printed specimens and the printed samples showed better hardness and modulus than those in printed in air. It was also noted that the increasing concentration of the photoinitiator can improve the curing speed of the ink printed in air. However the samples with higher concentration of photoinitiators manifested a reduction of hardness and modulus. A post-curing procedure, carried out using further UV illumination, was shown to help improve both the hardness and the modulus, but this improvement was limited to the directly illuminated surface.
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