DNA molecules in a solution can be immobilized and stretched into a highly ordered array on a solid surface containing micropillars by molecular combing technique. However, the mechanism of this process is not well understood. In this study, we demonstrated the generation of DNA nanostrand array with linear, zigzag, and fork-zigzag patterns and the microfluidic processes are modeled based on a deforming body-fitted grid approach. The simulation results provide insights for explaining the stretching, immobilizing, and patterning of DNA molecules observed in the experiments.

A numerical algorithm is developed to simulate the injection-compression molding (ICM) process. A Hele-Shaw fluid-flow model combined with a modified control-volume/finite-element method is implemented to predict the melt-front advancement and the distributions of pressure, temperature, and flow velocity dynamically during the injection melt filling, compression melt filling, and postfilling stages of the entire process. Part volumetric shrinkage was then investigated by tracing the thermal-mechanical history of the polymer melt via a path display in the pressure-volumetemperature (PVT) diagram during the entire process. Influence of the process parameters including compression speed, switch time from injection to compression, compression stroke, and part thickness on part shrinkage were understood through simulations of a disk part. The simulated results were also compared with those required by conventional injection molding (CIM). It was found that ICM not only shows a significant effect on reducing part shrinkage but also provides much more uniform shrinkage within the whole part as compared with CIM. Although using a higher switch time, lower compression speed, and higher compression stroke may result in a lower molding pressure, however, they do not show an apparent effect on part shrinkage once the compression pressure is the same in the compression-holding stage. However, using a lower switch time, higher compression speed, and lower compression stroke under the same compression pressure in the postfilling stage will result in an improvement in shrinkage reduction due to the melt-temperature effect introduced in the end of the filling stage.

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SYNOPSISAnalysis of the injection-molding process based on Leonov viscoelastic fluid model has been employed to study the effects of process conditions on the residual stress and birefringence development in injection-molded parts during the entire molding process. An integrated formulation was derived and numerically implemented to solve the nonisothermal, compressible, and viscoelastic nature of polymer melt flow. Simulations under process conditions of different melt temperatures, mold temperatures, filling speeds, and packing pressures are performed to predict the birefringence variation in both gapwise and planar direction. It has been found that melt temperature and the associated frozen layer thickness are the dominant factors that determine the birefringence development within the molded part. For a higher mold temperature, melt temperature, and injection speed, the averaged birefringence along gapwise direction is lower. The birefringence also increases significantly with the increased packing pressure especially along gate area. The simulated results show good consistency with those measured experimentally.

Numerical simulations and experimental studies concerning melt flow and primary as well as secondary gas penetration during the filling and the postfilling stages in gas-assisted injection molding of a thin plate with a semicircular gas channel design were conducted. Distribution of the skin melt thickness along the gas-penetration direction was measured to identify primary and secondary gas penetration. Melt and gas flow within the gas channel of a semicircular cross section is approximated by a model which uses a circular pipe of an equivalent hydraulic diameter superimposed on the thin part. An algorithm based on the control-volume/finite-element method combined with a dual-filling parameter technique suitable for the tracing of two-component flow-front advancements is utilized and numerically implemented to predict both melt-and gas-front advancements during the melt-filling and the gas-assisted filling processes. A flow model of the isotropic melt-shrinkage origin combined with a gapwise layer tracing algorithm was implemented to assist the prediction of secondary gas penetration and melt flow in the post-filling stage. Simulated results on the gas front locations at the end of both primary and secondary penetration phases show reasonably good coincidence with experimental observations.

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