Study of Polymer Flow Behavior in Cavity Filling of Alignment Structures in Micro Hot Embossing
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The behavior of polymer flow is a critical aspect in micro hot embossing. The flow behavior significantly depends on the process parameters including molding temperatures and displacement of a mold with respect to a polymer substrate. To achieve accurate replication of microstructures, it is necessary to predict the flow behavior depending on the process parameters.
Two alignment structures were chosen as embossing features to demonstrate the flow behavior of polymer in hot embossing. The post has a height of 925 µm and a radius of 500 µm. Polymethyl-Methacrylate (PMMA) was selected as a substrate material. Computer-aided engineering models (CAE), using DEFORM 2D (SFTC, Columbus, OH), were used to simulate the flow behavior in the filling of a hemisphere-tipped cavity in molding. Simulations were performed to help understand the flow behavior of the polymer while varying the molding temperatures and the displacement of the mold. The simulation conditions were mold temperatures of 75, 100, 125, and 150˚C, displacement of mold of 0.5, 1.0 2.0, 2.5 mm, and speed of embossing 0.008 mm/sec.
Replication fidelity of the post was evaluated using the height of molded hemisphere-tipped post. The height of the molded post increases while the molding temperature and displacement increase. Incomplete filling of polymer at the cavity was observed at molding temperature of 75°C when the displacement increased from 0.5 mm to 2.5 mm. As molding temperature increased to 125°C, the cavity was completely filled at the displacement of 1.5, 2.0, and 2.5 mm. The increase of mold temperature yielded better cavity filling because the higher molding temperature decreased the fluidic resistance of the polymer. It can be explained by a glass transition temperature of PMMA, about 105°C, so PMMA significantly softens above this temperature.
Numerical simulation predicted the flow behavior of polymer in micro hot embossing of the alignment structure. These results can be used to determine proper process parameters for the microfabrication of polymer microstructures.