Hot-embossing Simulation Of Amorphous Polymer Micro-Nanostructures Under Combined Capillary And Extrusion Forces

Micro-nano hot embossing technology has become the most popular manufacturing technology in micro-structure forming due to its advantages of fast,high efficiency,low consumption and simple die.Numerical simulation and experimental technique are the two methods we usually take to study a technology.This paper mainly studies the material model and capillary forming mechanism of amorphous polymer from the perspective of numerical simulation.Since micro-nano hot embossing is usually carried out above the glass transition temperature,at which the polymers have both fluid characteristics and elastic characteristics,and the deformation behavior of materials is extremely complicated,a constitutive model that properly describes material behavior becomes a key factor affecting the correctness of numerical simulation prediction.A classic linear viscoelastic theory cannot accurately predict the mechanical response of materials,therefore,it is necessary to study a non-linear thermal-mechanical coupling model to improve the accuracy of numerical simulation.This paper studies the thermal-mechanical coupling model of amorphous polymer,uses second-development interface on nonlinear finite element software ABAQUS and the written user material subroutine UMAT,material constants in a visco-elastic-plastic constitutive model were obtained by fitting the polymer compression experimental data of the amorphous polymer on finite element software ABAQUS.Considering that the upper end surface of the nano-cylinder in the amorphous polymer(PC)nano-embossing is concave and exhibits meniscus,it indicates the existence of capillary force.At the same time,based on the visco-elastic-plastic thermal-mechanical coupling model,the hot embossing simulation that considers or does not consider the capillary force was studied.The simulation results found that the capillary force after demolding is higher than that of the cylinder without capillary force,which is close to the published experiment data.This futher confirms that for nano-precision hot embossing,the effect of capillary force cannot be neglected.Finally,in order to obtain more accurate time-dependent changes in the concave surface of the upper end surface caused by the capillary force,a time-dependent contact angle model was adopted,and the model parameters were obtained by fitting experimental data.The variation law of the curvature of the meniscus can be controlled by the ratio of the end point curvature to the initial point curvature,and the dynamic change of liquid meniscus at different times is obtained by the method of arc segmentation approximation.The algorithm solves the height of the liquid column under the action of Newtonian fluid alcohol capillary and compares it with Hamraoui’s experiment to obtain the fitting physical parameters.The algorithm is further extended to solve the flow height in the micro-nano channel during the hot embossing process of amorphous material PC.Since the ratio of depth to width of the embossing mold is relatively large,the lubrication approximation can be used to establish the flow analysis model in the Micro-Nano hot embossing process under the combine action of capillary force and extrusion force.From this,the influence of non-newton index on liquid column height and Bond number on meniscus is further discussed in this paper.