This dissertation addresses the field of four-dimensional (4D) printing based on programmed three-dimensional (3D) structures. The essential components of these programmed structures are elements capable of shape transformation (responsive elements) which react over time by changing their geometry upon thermal activation. The dissertation is based on three scientific publications and complementary research that significantly expand the study and advance the understanding of 4D printing.
The primary aim of the thesis was to investigate the effects of 3D printing parameters and thermal activation methods (hot water and hot air) on the responsiveness of programmed 3D structures. Various materials were analysed, among which polylactic acid (PLA) was selected as a responsive material, while acrylonitrile butadiene styrene (ABS), thermoplastic polyurethane (TPU), and modified PLA (PRO-PLA) served as non-responsive materials. These materials were combined with responsive PLA in programmed multi-material 3D structures. The research further examined the influence of geometric and structural features of programmed 3D structures on their responsiveness, performed experimental modelling and prediction, and developed three categories of programmed 3D structures to demonstrate practical applications of the findings.
It was found that both additive manufacturing parameters and thermal activation parameters play a critical role in ensuring the accuracy and reproducibility of the response in programmed 3D structures. Programmed 3D structures fabricated with optimised additive manufacturing parameters and activated in a more stable thermal activation medium demonstrated greater precision in response. Among material combinations, the pairing of responsive PLA and non-responsive PRO-PLA proved most suitable for producing multi-material programmed 3D structures. This compatibility results from strong inter-material adhesion, thermomechanical shrinkage between the two materials during thermal activation, and the low bending storage modulus of PRO-PLA within the activation temperature range. The significant difference in dimensional changes between the materials, combined with the low storage modulus, enables the programmed 3D structures to exhibit a higher response. This combination also uniquely supports good inter-material adhesion across various material print sequences. The study also determined that water is a more efficient thermal activation medium than air, enabling faster and more uniform reactions, and resulting in stronger shape transformation in the programmed 3D structures. The thesis contributes to the advancement of 4D printing by providing a deeper understanding of the key factors within the overall process model that influence the response of the programmed multi-material 3D structures for practical applications.
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