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Title: Demolding mechanics of micro-cast UV cured thermosets
Authors: Yeo, Lip Pin
Keywords: DRNTU::Engineering::Manufacturing
Issue Date: 2007
Abstract: UV micro-casting of polymeric micro-patterns is an emerging replication method with potential for large scale manufacturing. The setup requirements for UV micro-casting are non stringent since polymer replication takes place under room temperature and pressure. An additional merit of UV micro-casting is the fast cycle time since the polymer usually cures in matters of seconds. The main technical challenge of UV micro-casting is to achieve repeatable and successful demolding. Hence, the motivation of this thesis is to investigate the mechanics of demolding. This involves understanding the changes in the material properties of the polymer, the evolution of the stresses during the cure and demolding processes and the importance of the interfacial fracture energy between the polymer and the mold. Polyurethane diacrylate thermoset (EBECRYL 270 as oligomer) compound was selected as the polymer of interest for patterning due to its good bio-compatibility for bio-medical purposes and its high structural rigidity. The rigid nickel micro-arrayed mold is chosen for its high mechanical strength and low surface energy. Shrinkage is an aid to demolding since the polymer tends to “shrink” away from the mold after UV irradiation. Shrinkage can be directly correlated to the degree of cure (or otherwise known as C = C double bond conversion). As the cross-linking density increases, the rigidity of the network formed also increases, resulting in higher polymer strength. However, higher elastic modulus comes at a price of increased brittleness and too much shrinkage also results in the loss of dimensional fidelity during replication. As such, it is important to understand the effects of shrinkage on demolding. Hence, there was a need to investigate the changes in material properties of the polymer during cure and this was carried out by changing the amount of UV irradiation time while keeping the UV intensity constant. In this way, the amount of shrinkage for the polymer could be controlled. Polymer fracture strength at varying shrinkage levels was determined. Thermoset polyurethane materials belong to the family of elastomeric polymers that exhibit stress softening and hysteresis effects. The behavior of polyurethane diacrylate was characterized by cyclic uploading and downloading mechanical tests at different strain rates. Its time dependent behavior was described by the Bergström-Boyce material model. The mold interface fracture toughness values, which directly impacts on the ease or otherwise of demolding were rather low and difficult to be measured. As such, a special test jig was designed to conduct low force peel testing of polyurethane diacrylate polymers from un-patterned nickel plates. The interface energy between the polymer and the mold was calibrated based on the peel test results via numerical simulation of the peel process. Micro-patterned nickel molds of aspect ratio 3 were used for polymer replication and the peel force recorded was found to be at least 4 times larger than the un-patterned counterpart. Numerical simulation via cohesive zone modeling was used to model both the cure and demolding processes using finite element software, ABAQUS. The numerical peel forces were found to be larger than the experimental peel forces. The major contributing factor for this disagreement is the over estimation of interfacial fracture toughness values. This over estimation could be attributed to the large scatter of of rather low interfacial energy values determined from peel tests results. Based on the experimental and numerical findings, for optimal demolding, it is recommended that the polymer should be UV cured for a longer period of time (at least 45s to 60s) to achieve higher cross-linking density, which will lead to increased polymer stiffness and higher U.T.S. This will reduce polymer premature fracture in the mold during the demolding process.
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Appears in Collections:SMA Theses

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