Insight into replication effectiveness of laser-textured micro and nanoscale morphology by injection molding

Detailed analysis and characterization of injection-molded polypropylene (PP) specimens produced with laser-textured molds has been performed. Ultrashort pulsed laser surface structuring was exploited to produce sub-micrometric surface features on injection molds over areas of 2400 mm2, including two-dimensional hole and cone arrays, and ridges parallel and perpendicular to the injection direction. Replication effectiveness was evaluated in terms of surface roughness parameters, Filling Volume Fraction (FVF) and Power Spectral Density (PSD), after which detailed comparison of the topography in the same region on plastic samples and molds was performed. A scatterometry setup was employed based on optical diffraction of a probe laser beam to assess replication fidelity over the entire surface of each PP sample with a measurement technique suitable for optimization and quality control in an industrial production environment. Of the tested surface morphologies, greatest replication effectiveness was achieved with LIPSS oriented parallel to the injection direction, where the arithmetic mean height of plastic samples (Sa = 37 ± 4 nm) was 97 % of the corresponding value of the mold (Sa = 38 ± 6 nm), while the average FVF was 86 ± 9 % and the PSD indicated ripple-like features with a spacing of λ≃0.9 μm. Direct comparison of the topography in selected regions highlighted local variability in the transfer effectiveness of individual surface features. The investigation demonstrated that micro and nanoscale morphology can effectively be transferred via injection molding over large areas relevant to industrial applications, while careful attention must be paid to the size and nature of defects in relation to the specific functional surface under consideration.