Injection molded plastics have been used as replacements for metals in optical pickup cases. These plastics are short fiber reinforced composite materials, so the mechanical properties of the pickup case are heterogeneous and anisotropic. We have developed a method for predicting the position deviation of the beam spot on the detector for optical pickups. This method can be used to quantitatively assess to what extent the orientation distribution of short fibers in the pickup case affect the displacement and tilt of optical parts with change in temperature. The sequence for considering the orientation distribution of short fibers in the pickup case is: first divide the pickup case into multiple regions, then define the three-dimensional fiber orientation state as the degree of orientation in each region, and finally set the mechanical properties as a function of the degree of orientation in every region. The degrees of orientation were evaluated by second-order tensor based on cross-sectional observation of the pickup case using a scanning electron microscope. The mechanical properties were obtained by homogenization in three steps. The difference between the calculated result and the measured result for the beam spot deviation at an ambient temperature of 60 °C decreased to one fourth of its previous value by considering the orientation distribution of short fibers in the pickup case.
