微/次微米結構的射出成形特性暨可自發熱模仁的發展與應用

Abstract

摘 要 本論文致力於高深寬比微米/次微米塑膠結構的射出成形特性及脫模行為研究。首先使用傳統射出成形機進行微/次微米結構充填行為探討，其次針對傳統微射出成形的缺失，設計一新型模仁並提出新的射出成形方法。在微結構模穴充填方面，實驗使用PMMA、PP及HDPE等材料觀察微小模穴的特殊充填行為及成形缺陷。同時，為了避免直接脫模法對微結構所造成的破壞，嘗試以消失模法來克服成形後的脫模問題。研究發現微米/次微米結構的成形遠比一般尺寸者的成形困難。如果模溫在材料的玻璃轉移溫度Tg點以下，射出壓力傳遞只能到達模具的微模穴基部，塑料無法充填進入微結構模穴。若要完整的充填，其模具溫度需在Tg點以上，且主射出壓力和持壓時間對微結構的可成形深寬比有很大的影響。在製品的收縮控制上，高於Tg且靠近此點的模溫與高的保壓力皆能減少收縮量，其中，保壓力提升可使收縮量曲線的變化趨於線性，然而製品與模仁彼此的夾持力卻無法消除，此種不規則收縮的複雜問題若沒有特殊的應力釋除處理將無法獲得良好的成形品質。 因此本研究針對微結構射出成形的收縮行為，發展一種在矽基模仁上創建表面具有可自發熱功能的微結構模仁，以解決微/次微米元件在成形冷卻過程的脫模問題。此種設計不僅在矽晶片製作微溝槽，並在微溝槽表面埋植電熱線，而該電熱線係根據摻雜改質的原理，利用離子植佈法對矽基模仁表面進行摻雜磷離子，使其具有適當的導電性，此新型模仁被設計應用在協助解決高深寬比微結構的脫模問題。實驗結果顯示，矽基電熱線可提供穩定的加熱功率，在射出成形的冷卻階段，以足夠的功率和適當的時機加熱模仁表面的局部區域和近旁的成形塑料，可有效地降低微溝槽模仁和製品彼此收縮的夾持力，獲得自然脫模的高成形精度的製品。此種模仁實際應用於微射出成形時具有極佳的加溫與溫控效率，對節省能源、縮短成形週期及工業化極具潛力。

ABSTRACT This dissertation is concerned with the injection molding characteristics and demolding behavior of polymer micro- and sub-micron-structures with high-aspect-ratios. The filling behavior of the polymer injected by a traditional injection molding machine into the microcavities was investigated first. Then a new injection molding strategy was proposed along with the development of a novel mold insert learned from the drawbacks of the traditional injection molding process in fabricating microstructures. In the filling of the microcavities, distinctive mold-filling behaviors and resulting defects were observed for various types of polymers such as PMMA, PC, PP and HDPE. Here, sacrificial molds were used to overcome the demolding problems in order to prevent the microstructures from direct damage as demolding. Experimental results revealed that injection molding of micro- and sub-micro-structures are more difficult than that of products with common dimensions. If the mold temperature is lower than the glass transition temperature (Tg) of the molding polymer, the filling resistance of the polymer in micro injection molding will be markedly high, and therefore, the polymer melt can only fill up the mold cavity of the base part of the micro-structure products and can not fill into the microcavities. To fill up the micro- and sub-micron-cavities successfully, the mold temperature must normally be above the Tg of the polymer used. Moreover, the main injection pressure and the main injection time substantially affect the achievable aspect ratio of the micro- and sub-micron-structures. As for the shrinkage control, both a mold temperature, which is above the Tg and a higher holding pressure can decrease the shrinkage. In addition, the shrinkage curve becomes linear with the increase of the holding pressure. However, the griping force due to the difference of shrinking ratio between the polymer and the mold insert can not be released, which results in the demolding defects. Therefore, a special in-mold process of stress relief is strongly necessary for solving the demolding problem to guarantee high micro-molding quality. This thesis, in accordance with the shrinking behavior of the injection molding of microstructure, presents a silicon-based heat-generable mold insert for micro injection molding to solve the problem of demolding destruction, as mentioned earlier. Design of this mold insert not only has micro-channels constructed on the silicon wafer as the microcavities but also includes micro electrical heating lines embedded in the wall of the microcavities. These electrical heating lines with specified resistance were fabricated by doping phosphorus ions precisely into the surface of the silicon cavity wall using the ion implanting process. The performance of the novel mold insert was studied and then the mold insert was applied to the injection molding of micro-structures with high aspect ratios for possibly resolving the demolding problem. Experimental results indicated that silicon-based electrical heating lines embedded in the novel mold insert can provide stable heating power. By heating the cavity wall and the nearby plastic with appropriate timing and sufficient power in the cooling stage, we could effectively reduce the griping force between the patterned plastic microstructures and the micro-channels of the mold insert. Consequently, defect-free demolded products with high-precision were obtained. This novel mold insert provided extremely high thermal efficiency in heating and temperature control when it was applied to the micro injection molding in practice. The above mentioned advantages can in turn result in power saving and cycle time shortening, and thus, enhancing industrial applications.