The optical anisotropy which is frozen into transparent plastic parts has often been utilized to assess the level of molecular orientation produced during the injection molding process [1]. The advent of optics based digital information encoding and retrieval technologies (optical discs) have required the development of transparent polymeric materials which can be injection molded into optical components with controlled (low) levels of optical anisotropy. The light beam typically passes through the transparent substrate, and excessive levels of optical anisotropy degrade the performance of the data retrieval system. This optical anisotropy has been shown to be caused by recoverable molecular orientations, which are produced in the filling and holding portions of the injection molding cycle, and are frozen into the part during the molding process. Processing effects on molded in birefringence have been approached from empirical [2], experimental [3], and theoretical [4,5,6] viewpoints. The latter efforts have focused on predicting the optical anisotropy in transparent moldings. This paper will present experimental results obtained for several polycarbonates of bisphenol-A. Concepts from flow birefringence in molten polymers [7] will be used to identify the significant process and melt rheological variables which influence molded in birefringence. Control strategies to minimize orientations produced during both mold filling and the pack-and-hold portions of the molding cycle will be developed. Results will be presented for two mold geometries, an end gated plaque and a center gated disc, to demonstrate the interrelationships between processing and melt theology which govern the optical properties of the moldings. In addition, some preliminary results on the utility of variable-cavity-volume mold designs to further reduce optical anisotropy will be discussed as well as development of a polariscope to measure in greater detail optical properties of compact disc substrates.

Date Published: 13 July 1988
PDF: 20 pages
Proc. SPIE 0896, Replication and Molding of Optical Components, (13 July 1988); doi: 10.1117/12.944466

Published in SPIE Proceedings Vol. 0896:
Replication and Molding of Optical Components
Max J. Riedl, Editor(s)