Study On Replication Capability And Morphology Of Injection Molded Micro-structures

The micro-injection molding technology is capable of producing plastic micro-parts or parts with micro-features. The advantages of this technology include low cost, short cycle time and easiness for volume production and so forth. Nowadays, the micro-injection molded parts have been widely used in many applications such as the micro-fluidic devices, biomedicine, photoelectric communication, and micro-mechanical-electronic-systems and so on, with a huge application potential in the future. However, the current study on the micro-injection molding is mainly focused on the effects of the processing condition and geometric factors on the replication capability of the molded parts. In this regard, in order to resolve certain issues encountered in micro-injection molding process, the present work aims to gradually investigate the rheological properties of the polymer, replication capability of the micro-features with various aspect ratios, surface morphology of the molded micro-features, and the crystal morphology of the micro-injection molded parts.First of all, the micro-features with small aspect ratio were designed in order to investigate the replication capability with different molding materials, processing conditions, and geometric factors. The results showed that various materials had different effects on the replicated height of the micro-features. The replicated height of the molded PMMA micro-features fell into a large range of 4.4~99.5%, while that for molded PP micro-features was higher than 92.3%. Moreover, for PMMA micro-features, the melt temperature, mold temperature and injection velocity were found to be important factors influencing the replication height, while the packing pressure and packing time had no obvious effect; for PP micro-features, the melt temperature and injection veloctiy were the most important factors, followed by the mold temperature, packing pressure and packing time. As for the geometric factors, the aspect ratio and the distance from the micro-feature to the injection gate had different effect on the replication capability of the molded micro-features.Based on the aforementioned study, effects of the layout, cross sectional shape, surface roughness, and the processing condition on replication capability of the micro-features with large aspect ratio were futher investigated. It was found that with the same processing conditon, the micro-feature molded with transverse layout had larger filled length than that with parallel layout. The filled length of the micro-features increased with the cross section in the order of triangle, rectangle, and semicircle, and this is especially the case in the parallel layout. In addition, the filled length decreased with increasing surface roughness of the micro-channels. Furthermore, the effect of the processing condition on the filled length of the micro-features in parallel and transverse layouts was different due to the various resistance of the trapped air inside micro-channels during the injection process.In additon, the surface morphology of the micro-features molded under different conditions was investigated using scanning electron microscope (SEM). It was found that the surfaces of micro-features molded in parallel layout were very smooth and clear. As comparison, there were defects on the surface of the micro-features molded in transverse layout, and the appearance of defects strongly depended on the processing condition and flow direction in the main cavity. Increase in melt temperature and/or injection velocity could improve or even eliminate the surface defects. The short shot experiment indicated that the formation of the surface defect was mainly influenced by the melt temperature and melt pressure at the entrance, and the cooling rate of the melt inside the micro-channel, however, independent of the cross sectional shape of the micro-features.Using the SEM and differential scanning calorimetry, the crystal morphology of the injection molded parts with different dimensions was investigated. For conventional injection molded parts, there were only lamellae disorderly arranged throughout the entire thickness. With the dimension decreasing, the oriented lamellae started to appear from the skin layer of the parts, and the density of the lamellae increased. For the micro-injection molded part with a thickness of only 175?m, there were many shish-kebab-like structures in the tickness direction.At last, from a practical viewpoint, the micro-injection molding of the micro-mixer and precision optical lens was investigated. It was found that it is fairly important to master the influencing factors and the corresponding rule of the replication capability and morphology of the micro-features for practical production.