Influence of the Mold Temperature and Part Thickness on the Replication Quality and Molecular Orientation in Compression Injection Molding of Polystyrene

Abstract

It is well known that the process of injection molding with dynamic mold temperature control leads to a good replication quality of high aspect ratio microstructures. However, the inhomogeneous pressure distribution during the holding pressure phase results in an anisotropy of the component properties, low dimensional accuracy and, especially with optical polymers, in undesired stress birefringence. 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. Within the scope of this investigation, the new process was reproduced by an ICM process at elevated mold temperature (ICM_EMT) and compared with injection molding (IM) with regard to molding accuracy and optical properties in dependence of component thickness and mold temperature. 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 was 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.

Birefringence in Injection-Molded Cyclic Olefin Copolymer Substrates and Its Impact on Integrated Photonic Structures

“…Based on the observations discussed in this work, it is possible to outline some general guidelines for the fabrication of photonic structures in injection-molded COCs. While it is theoretically possible to influence the degree of molecular orientation in the workpieces by means of adapting the injection-molding parameters, e.g., process and mold temperature, injection and back pressure as well as mold geometry [ 40 ], it is challenging to avoid these effects completely. Nevertheless, injection molding is the preferred choice to manufacture low-cost and large-quantity polymer-based products, a prerequisite for commercial success in attractive markets, such as medical platforms and sensing applications [ 21 , 22 ].…”

Pressure Equilibrium Time of a Cyclic-Olefin Copolymer

“…Thus, the relaxation of the polymer chains, due to flow and high-pressure levels during the holding pressure phase, is prevented. The orientations introduced in the manufacturing process are primarily determined by the pressure, temperature, and shear conditions during the holding pressure and cooling phase [ 22 ]. During the filling phase, the orientations introduced can relax to a large extent, due to the shear-induced heating and low-pressure gradients at the melt front.…”