An experimental study of yellow shift in injection-molded light guide plate

Abstract

Recently, the size of light guide plate (LGP) in LCD-BLU (Backlight Unit) module is getting thinner and larger than ever. The part reached critical thickness to mold it by conventional injection molding methods due to the low flowability, melt solidification, machine limitation, and so on. Therefore, severe conditions have been applied to the part to increase the flowability such as high injection speeds and higher melt temperature. However, these approaches lead to the degradation of material and loss of optical properties. These defects are connected to the invisible part failure, so called, yellowing and color shift. In the present study a series of injection molding experiments were conducted to understand the distribution of yellowness in injection-molded LGP, and how the optical properties change under various injection molding conditions. Optical properties of yellow index (YI), CIE xy, and spectral transmittances of LGP sample were analyzed by the UV-visible spectrophotometer. Also, correlations between optical properties and process conditions were investigated from the Design of experiment (DOE). Interestingly, the value of YI, i.e., yellow shift in CIE diagram showed the maximum near the gate and decreased as the distance from the gate increased. Furthermore, as far as yellow shift concerned the data of direct transmittance are much more useful than total transmittance for evaluating color behavior. Meanwhile, analysis of variance (ANOVA) was conducted to see the effectiveness of chosen processing parameters. Mold temperature was found to be the most influential factor on the color shift and injection speed, melt temperature, packing pressure were followed.