High-heat crystal polystyrene (HHCPS) and high-impact polystyrene (HIPS) macroscale parts with microscale features were injection molded using a commercially available mold base with matching steel insert and microfeatured silicon wafer in conjunction with a temperature control system.
Ultraviolet (UV) photolithography was utilized to create orthogonally arrayed microchannels on the surface of the silicon wafers, which served as the back half mold surface. The effect of mold temperature on final part microgeometry was analyzed. It was determined that high mold temperatures facilitated greater filling of the microfeatures. In addition, solid-state processing created an undercut feature in the microchannel, which promoted both pillar elongation and (unfortunately) tensile pillar rupture. The novel addition of tensile elongation of pillars during demolding could lead to pillar aspect ratios not achievable through conventional replication. Material compliance promoted greater pillar heights, as HIPS exhibited greater values at identical mold temperature conditions.
Background
Microinjection molding can be used to create microscale topography on the surface of polymer parts. A microinjection molding machine differs from traditional molding machines in that it utilizes both an electrically controlled system and it contains a plunger as a means of polymer injection in order to increase the precision of filling. Many have attempted to investigate the effect of different processing parameters on part quality. It was previously determined that the filling of micro-cavities is enhanced through higher melt temperatures, higher mold temperatures, and greater injection velocities [1, 2]. Of the three parameters, injection velocity was found to have the smallest effect [2].
