Rapid thermal response injection molding for microfeature fabrication
The freezing problem in the injection molding filling stage not only deteriorates the quality of molded parts, but also results in difficulty in molding new-generation plastic parts that are rich in extremely thin-wall sections and microfeatures. A rapid thermal response (RTR) molding process was developed to overcome the freezing problem. In RTR molding, the mold surface can be heated from room temperature to over 200°C in several seconds and cooled to the ejection temperature within the normal cycle time. Although the RTR technology can be used in numerous applications, the ideas and findings reported in this thesis were collected and organized towards increasing moldability of high-aspect-ratio microfeatures. Changes of molding characteristics as parts become thinner and smaller were studied in both simulations and experiments. In conventional molding, the molding difficulty drastically increases as the thickness reduces. A polymer melt thinner than 0.25 mm tends to copy the mold temperature in a tenth of a second, thus resulting in premature freezing when molding high-aspect-ratio ultra-thin sections and microfeatures. In contrast, the melt in RTR molding does not freeze, and therefore can fill an extremely long and thin path. A concept of scalable filling under isothermal molding condition was presented to address the benefit of the RTR molding process. Experimental results of high-aspect-ratio micro wells, injection molded with variable RTR heating temperature, were discussed. With the conventional mold temperature, the molded depth is less that 5% of the to-be-copied one. A sharp increase in the replicated depth was observed near the polymer melting temperature. For high-density polyethylene, complete replication of the micro wells was achieved with a 2-s heating duration and a total cycle time of 15 seconds. Another area explored in this research is micro filling simulation. The standard simulation scheme was modified to include microscale effects. The simulation results indicate the importance in employing size-dependent viscosity and wall slip to predicate micro filling behaviors. Development of integrated technology for replicating microfeatures with simulation capability will push the current technical envelope of micro injection molding further into those areas that have so far been predominated by etching- and lithography-based fabrications.