Читать книгу Plastics Process Analysis, Instrumentation, and Control - Группа авторов - Страница 39

The process of injection molding with dynamic mold temperature control leads to a good replication quality of high-aspect-ratio microstructures. However, an inhomogeneous pressure distribution during the holding pressure phase results in an anisotropy of the component properties, low dimensional accuracy and, especially with optical polymers, an undesired stress birefringence (55).

The anisotropy is based on the orientation of the molecular chains in the flow direction, which can be reduced by an injection-compression molding (ICM) process. In order to use the synergy from both processes, an injection-compression molding process with dynamic mold temperature control can be utilized.

This process was reproduced by an ICM process at elevated mold temperature and compared with injection molding with regard to molding accuracy and optical properties in dependence of component thickness and mold temperature (55). In order to evaluate the molding accuracy, the roughness of a wire-eroded microstructure on the cavity surface was measured.

To determine the degree of orientation, the optical properties considered were the transmission and the path difference. It could be shown that the adapted ICM process was able to achieve a high degree of replication accuracy with a low degree of orientation, especially for thin-walled components. ICM at elevated mold temperature reduced the path difference in the components with the lowest wall thickness by a factor of two while at the same time optimizing the replication of the microstructure. This could also be confirmed by transmission measurements (55).