| Popis: |
This research investigates the theoretical basis of a new photo-fabrication system. By this system, optical and thermal effects are used, together or separately, to locally induce a phase change in a liquid resin. This phase change phenomena is used to "write" three-dimensional shapes. In addition, a thermal-kinetic model has been developed to correctly simulate the physical and chemical changes that occur in the bulk (and surroundings) of the material directly exposed to radiation and/or heat, and the rates at which these changes occur. Through this model, the law of conservation of energy describing the heat transfer phenomena is coupled with a kinetic model describing in detail the cure kinetics in both chemical and diffusion-controlled regimes. The thermal-kinetic model has been implemented using the finite element method. Linear rectangular elements have been considered and the concept of isoparametric formulation used. The Cranck-Nicolson algorithm has been used to integrate the system of equations, resulting from the finite element discretisation, with respect to time. Three different photo-fabrication processes were investigated. The first process uses ultraviolet radiation to cure a thermosetting polymer containing a certain amount of photoinitiator. The radiation generates free radicals by cleavage the initiator molecules, starting the chemical reaction. The second one uses ultraviolet radiation to start the curing reaction of a liquid thermosetting polymer containing a certain amount of photo-initiator. In this case, a heat source is also used to increase the temperature, and consequently, to increase the rate of gel formation and the fractional conversion, decreasing the necessary exposure time. Finally, the third system uses a thermosetting material containing small amounts of both thermal and photo-initiators. In this case ultraviolet radiation and heat are used to simultaneously start two types of chemical reactions: thermal-initiated and photo-initiated curing reactions. Moreover, the heat source is used to increase the rate of gel formation of the photo-initiated curing reaction. This third process has been found to be advantageous because: the generation of radicals is more efficient, small concentrations of initiators can be used and consequently light can penetrate deeply inside the polymer, the curing reaction is more localised and the system has more tunability. Finally, in order to test the proposed photo-fabrication principles, a device has been constructed. This uses the mask-based writing method, in which an image is transferred to a liquid polymer by irradiating through a patterned mask. Various polymer shapes have been produced. The initial conditions to produce these shapes have been established from the understanding of the physical and chemical transformations of the selected resin under thermal and photo-initiated curing reactions and from simulation. In addition, the device has been used as a platform to test the thermal-kinetic model in real situations. The correlation between the experimental and predicted results is excellent. |
