화학공학소재연구정보센터
Polymer, Vol.51, No.23, 5623-5639, 2010
Frozen-in birefringence and anisotropic shrinkage in optical moldings: II. Comparison of simulations with experiments on light-guide plates
The thermally-, flow-induced and total birefringence components and anisotropic shrinkages in LOP moldings were simulated by using a combination of a CV/FEM/FDM technique nonlinear viscoelastic and photoviscoelastic constitutive equations and orientation functions, as described in Part I of this study. The simulated results were compared with measurements on LOP moldings of a polystyrene (PS) and two optical grade polycarbonates (PCs) OQ1030 and OQ3820 having low and high molecular weights. The thermally-induced birefringence was simulated by a combination of constrained and free cooling during molding. In LGP moldings of PS, the simulated thermally-induced birefringence indicated a minor variation with location in the mold plane, a parabolic shape in the core region and an increase towards the wall. Compared to the flow-induced birefringence, the thermal birefringence provided a minor contribution to the total transverse birefringence Delta n(12). In LOP moldings of PCs, the simulated thermally-induced birefringence showed a significant variation with location in the mold plane, nearly constant value in the core region and high value in the wall region. In LOP moldings of both PCs, the contributions of the thermally- and flow-induced birefringence to the total transverse birefringence Delta n(12) were significant. The effect of processing conditions on the development of the normal birefringence in LOP moldings of PCs was ranked from most to least: the packing pressure, mold temperature, melt temperature, injection speed and packing time. However, in LGP moldings of PS the packing time effect was significant due to a longer gate freezing time. Simulated and measured normal birefringence along the flow direction was in fair agreement, but simulations were unable to describe the observed birefringence maximum arising near the gate. The averaged luminance of LOP moldings exhibited some correlation with the averaged normal birefringence. LOP moldings of PC OQ1030 indicated a pronounced maximum in the simulated transverse flow birefringence in the core but a low value near the wall. In contrast, the LGP molding of PC OQ3820 showed a high simulated birefringence near the wall and a low value of maximum in the core. The simulated and measured total transverse birefringence in LOP moldings was in fair agreement. LOP molding of both PCs showed similar tendency in shrinkage variation with processing conditions. However, the thickness shrinkage was higher in LGP moldings of PC OQ3820. The effect of processing conditions on the development of shrinkage in LGP moldings of both PCs was ranked from most to least: the packing pressure, melt temperature, mold temperature, injection speed and packing time. In LGP moldings of PS, the thickness shrinkage slightly increased with increasing melt temperature and significantly increased with reducing packing time. A good agreement between the simulated and measured anisotropic shrinkages in LOP moldings at various processing conditions was observed. (C) 2010 Elsevier Ltd. All rights reserved.