Effects of Injection Molding Holding Pressure on the Replication of Surface Microfeatures

Abstract

The injection molding of an optical grating was studied using two different polycarbonates. The grating had period 10 μm and peak-to-valley distance ∼1 μm. Parts were molded using different holding pressures and mold temperatures. After production, the parts were annealed at 100°C. The replication was investigated using white light interferometry (WLI) before and after annealing. WLI was performed using high definition vertical-scanning interferometry (HDVSI) to resolve the details of the molded gratings with a noise level below 2 nm.It was observed that increasing the holding pressure could have either a positive or a negative effect on the replication. When the microfeatures were not fully filled, an increased holding pressure improved the definition of the features. However, for both polymers, it was observed that the replication as a function of holding pressure started to drop when the holding pressure was increased above an optimal value. This could be due to an elastic recoil occurring after releasing the holding pressure.The peak-to-valley distance of the grating was reduced after annealing. This effect was larger for parts molded using a low mold temperature. This is probably due to a higher cooling rate giving higher internal stresses, which will relax during annealing.

A statistical analysis of nanocavities replication applied to injection moulding

“…We see clearly that with larger filling times, the pressure that reaches the cavitiy during the time before freezing (which is for all the polymers fewer than the 0.692s) decreases. The positive effect of the pressure in the replication has already been addressed in the literature by Chen [18] and Pranov [23]. From an industrial point of view it is concluded therefore, that for improving the replication of cavities larger injection pressures are needed, which are obtained only with injection machines with large hydraulic pressures available.…”

Surface characterization and analysis of textured injection moulded PC-ABS automotive interior components

“…Based on the results, it is observed on fine and coarse grain surfaces that the difference in surface replication is mostly in the peak and core regions. This is primarily due to the injection moulding mechanism in which the plastic melt initially fills the peak-core regions of the mould surface and later the core-valley regions [29].…”

Influences of micro-ridges orientation and position on the replication of micro-structured surfaces by injection molding

“…It was also observed that increasing the holding pressure had a positive effect on replication. Anyhow, the same author [7] noted that increasing the holding pressure could have either a positive or a negative effect on the replication of an optical surface. In fact, the definition of the feature was improved increasing the pressure when the micro-features were not fully filled.…”