Modelling and simulation of residual stresses and birefringence in the precision injection moulding of microlens arrays

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Modelling and simulation of residual stresses and birefringence in the precision injection moulding of microlens arrays


Author: Weng, Can
Title: Modelling and simulation of residual stresses and birefringence in the precision injection moulding of microlens arrays
Degree: Ph.D.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Lenses -- Design and construction
Injection molding of metals
Residual stresses
Department: Dept. of Industrial and Systems Engineering
Pages: xxxi, 207 leaves : ill. (some col.) ; 30 cm.
InnoPac Record:
Abstract: Microlens arrays are being increasingly used in the backlight modules of LCD displays. The injection moulding process will induce residual stresses in the moulded components which may affect the performance of the finished products, especially in precision optics. In this thesis, mathematical models are established to investigate the residual stresses and birefringence in the precision injection moulding of microlens arrays. Two constitutive models have been employed to describe material properties under two different states of the optical plastics. In the liquid state, the compressible Leonov model is adopted as a nonlinear viscoelastic constitutive equation and the stress-optical law is used to calculate the birefringence. In contrast, in the solid state, the linear viscoelastic model is employed as the constitutive equation and photoviscoelasticity theory is recommended for calculating the birefringence. The two important boundary conditions, namely non-zero slip and pressure at the flow front, are highlighted. ANSYS software is selected and complemented with programmed material models to numerically solve the established mathematical models. Both 2-dimensional and 3-dimensional finite element models are employed to simulate the injection moulding process and to predict the residual stresses designed with different geometrical configurations. The residual stresses are found to be concentrated in the regions near the gate, and the value of the maximum residual stresses can be obtained from the simulation results.
In order to validate the results of the simulation, a series of fabrication procedures and a birefringence measurement method are proposed. A custom-built polarimeter system is designed and implemented in the evaluation of the residual stresses. The effects of the six main processing parameters on the value of maximum residual stresses are investigated using the simulation based on ANSYS, the birefringence measurements and the simulation using the Moldflow Plastics Insight (MPI) software. The data from the experiments and the simulations of ANSYS are found to be in good agreement and reflect almost the same trends with change of the processing parameters. However, the values from the MPI are approximately eight times larger than the values from the experiments and the simulation of ANSYS, and the results are in general different. Four convex and four concave aperture structures were designed for the purpose of studying the effects of aperture shape on the maximum residual stresses. The aperture shape of microlens arrays is found to exert a significant influence on the value of the maximum residual stresses induced during the moulding process. It is concluded that the value of the maximum residual stress decreases with increase of the mould temperature, increases with increase of the flow rate, and increases with increase of the packing pressure. It is also observed that the cooling time has the least effect on the value of the maximum residual stress. In the macro-rheology studies, a parameter called the SSA (Specific Surface Area) factor is introduced to describe the aperture characteristics of the microlenses. A complex relationship is found between the aperture structure and the maximum residual stress. This study will provide a helpful guidance for the selection of the SSA factor and the processing parameters for different aperture types to minimize the residual stresses in moulded microlens arrays.

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