We have demonstrated a robust platform that can not only sustainably fabricate a lab-on-a-chip (LOC) device using microinjection molding but also elucidate the filling process of microstructures based on the multiscale analysis. In addition, a novel dimensionless number, i.e., the filling number μ(f) which can provide an insight into the underlying filling mechanism for micropillars, has been proposed based on the understanding of the characteristics of polymeric flow and cavity dimension. This study suggests a solid experimental and numerical tool for the production of microfluidic devices with the use of a micromolding technique, which is expected to help materialize the commercialization of the LOC devices in a more sustainable manner.