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Fig. 7. Variations of the resistance of the silicon-based conducting line with time under (a) cyclic variation of temperature, and (b) cyclic variation of pressure.
4. Application of heat-generable mold insert to the injection molding of micro-structures
In this work, as described above, the developed heat-generable mold insert is adopted to control the thermal/mechanical states of the molding plastic during cooling to solve the de-molding problems that arise in the injection molding of micro-structures. Some molding defects in the micro-structures with high aspect ratios that are caused by the shrinkage of the plastic, as shown in Fig. 9, can easily be detected, when a common injection molding process is used in which the novel mold insert developed in this work is not used. Fig. 9(a) displays the tilt of the molded micro-structure. Such a defect is thought to occur during de-molding as the shrinkage of the plastic causes a small shear stress. Greater shrinkage of the plastic produces more shear stress and mold gripping force, leading to the fracture of the micro-structure, as presented in Fig. 9(b). The most serious situation is that the molded micro-structures are sheared off from their base and remain in the micro-cavities of the mold insert, as presented in Fig. 9(c).
Fig. 8. Effects of original resistance of doped conducting line on stable power value that could be reached by breakdown.
Fig. 9. Various molding defects, caused by the shrinkage of the plastic, and of the micro-structures. (a) tilt, (b) fracture and (c) shear off of the micro-structure.
Fig. 10 illustrates an example of the timing of the power on/off of the developed ‘micro-heaters’ embedded in the mold insert during injection molding. When the temperature of the mold (metal mold plate) descends just to the glass transition temperature of the molding plastic during cooling, the silicon-based conducting lines are charged immediately to heat the surface of the mold insert and thus actively control the distribution of the temperature in the molding plastic. Fig. 11 plots the variations of the temperature in the mold insert at various heating powers in the mold-cooling stage. The range of temperatures increases with the heating power. Additionally, the temperature of the mold inserts declines rapidly to the temperature of the mold when the power is off. Fig. 12 shows some of the results of applying the developed heat-generable mold insert. As shown in Fig. 12, in all of the sub-areas of a rectangular molded area of dimensions of 8 mm by 8 mm, micro-structures can be de-molded perfectly, except at the edge of the sub-area marked ‘9E’ which is farthest from the center of the molded area. A larger power-density of the mold insert or a longer heating duration is necessary to release more thermal stress in the molded plastic and thereby further improve the efficacy or expand the effective moldable area of the Fig. 13(b) displays the weld-line on the surface of the base plate near the base of a micro-structure. The weld-line defect is considered to be caused by the re-melting of a micro-structure after solidification. This study investigates the operation window of the injection molding of micro-structures using the developed novel mold insert with the layout of micro-cavities presented in Fig. 12. Fig. 14(a) and (b) show the operation windows in cases in which the micro-structures have a uniform aspect ratio of three and four, respectively. These data reveal that a higher aspect ratio of the micro-structures corresponds to a stricter requirement of the heating conditions of the mold insert. Moreover, mold inserts with higher power-densities must be developed. novel mold insert. However, excessive heating of the mold insert itself will also cause defects. Fig. 13(a) presents the collapse of the molded micro-structures, which is caused by the poor strength of the de-molded plastic when the temperature is too high. Fig. 13(b) displays the weld-line on the surface of the base plate near the base of a micro-structure. The weld-line defect is considered to be caused by the re-melting of a micro-structure after solidification.