Effects of fast mold temperature evolution on micro features replication quality during injection molding

The growing demand to manufacture, with high accuracy, functional structures in the micro and sub-micrometer range polymer based microsystem products calls for reliable mass production processes. Being injection molding (IM) the preferential technology employed for polymer mass fabrication and mold temperature one of the most relevant process parameter to enhance polymer replication at the micro meter scale, the present study investigates effects of fast mold temperature evolution on final replication quality of produced injection molded parts. Micro features master geometries were produced by UV lithography and subsequent nickel electroplating. The mold temperature was controlled by a thin heating device (composed by polyimide as insulating layer and polyimide carbon black loaded as electrical conductive layer) able to increase the temperature on mold surface in a few seconds (40°C/s) by Joule effect and let the surface to cool down soon after. This heating device allowed to maintain mold temperature at a constant value for a time that could be equal to the filling time or longer. A fully characterized isotactic polypropylene was used as the polymer material during the injection molding experiments. The experiments revealed that the replication was mostly sensitive to cavity pressure and mold temperature. In particular, an increase of holding pressure and mold temperature enhanced the replication. Also, the heating time increased the replication quality.