The quality problem of plastics part caused by difficulty in melt filling mold flow in molding parts with micro size or with local tiny structure was investigated. Sheet parts with micro cylindrical array structure were taken as the research object. For two kinds of polymer materials

high-density polyethylene (HDPE) and polypropylene (PP)

the micro injection mold which integrated the technologies of vacuum exhaust and ultrasonic vibration was used to study the change of micro cylindrical fillet curvature radius with and without ultrasonic field and under the condition of varying process parameters and ultrasonic power. Without ultrasonic field

raising temperatures of melt and mold and increasing injection rate could decrease the fillet radius of curvature for the micro-column of the two polymer materials. While the fillet radius of curvature for the micro-column could further decrease with ultrasonic field

thereby improving the filling quality of parts. Meanwhile

no matter whether applying ultrasonic field or not

the fillet radius of curvature for the micro-column filled by HDPE polymer material was significantly smaller than that by PP polymer material.