聚合物熱壓成形之有限元素分析研究

Abstract

本研究針對以熱壓成形法製作聚合物微光學結構的製程，運用有限元素法來分析聚合物材料在熱壓過程中的成形行為。 研究中使用之PMMA材料，經過文獻探討及橡膠態下之機械及流變特性實驗，歸納出其為一種熱塑性、不定形體的塑膠材料；具有非線性黏彈性的特性，而且其材料特性會隨著溫度、壓力、時間而改變。 研究過程中，針對PMMA進行了一系列的材料實驗以取得其力學特性，然後將這些材料參數代入有限元素軟體中以建立有限元素材料模組；隨後以建立之熱壓模擬模式進行PMMA之熱壓模擬並與實驗做驗證，最後以此有限元素模組模擬熱壓製程參數對產品的影響，從中獲得較佳的製程參數。 模擬時採用了可同時考慮時間及溫度效應的非線性黏彈性有限元素軟體Marc，有效地模擬出聚合物在熱壓過程中材料的壓力分布情況，並配合其壓力-比容-溫度(PVT)的特性進一步預測PMMA材料的收縮變形情況。在與實驗結果及理論公式之預測做比較後，證實此有限元素模組能準確的模擬PMMA以熱壓法成形光學微結構時的行為機制。 在熱壓過程中，聚合物材料在溫度高於其玻璃轉移溫度時受壓成形，而後降溫脫模；因為壓印過程造成的材料不均勻壓力分佈以及降溫過程的溫度變化，材料的不均勻收縮變形是不可避免的，因此良好的轉印性以及可容許的均勻收縮變形是熱壓製程的控制重點。研究中經由模擬製程壓力對收縮率的影響，並與理論公式的計算結果做比較後，歸納出一個良好的熱壓製程控制應包含下列三點： 1.在壓印的過程中，施加較大的壓印力可獲得良好的轉印效果並縮短壓印時間。 2.在冷卻的初期，也就是溫度仍然高於材料的玻璃轉移溫度時，改用較小的壓印力，可獲得較均勻的壓力分布，進而縮小材料的體積變化變異量。 3.當溫度低於材料的玻璃轉移溫度後，施加較大的保壓力，將能進一步減小材料表面上沿工作平面之應變差與壓力差，而達到均勻收縮的目的。 本研究除了探討聚合物熱壓製程外，也建立了橡膠態聚合物的材料特性蒐集實驗方式及有限元素材料行為的表示模式，此方法可運用於其他工作溫度需高於材料的玻璃轉移溫度的製程。

This dissertation adopts the finite element method to analyze the deformation behaviors of polymer optical microstructures in the hot embossing process. The polymer PMMA used in this study is a thermoplastic and amorphous polymer, and exhibits nonlinear viscoelastic properties that are dependent on temperature, working pressure, geometry and load history when the polymer PMMA is in the rubber-like state. During the research procedures, first, some experiments were performed to determine the characteristics of PMMA. Second, the finite element material model was built using parameters from material property experiments. Subsequently, a simulation model of the hot embossing of PMMA was constructed and verified experimentally. Finally, the proposed simulation model was used to analyze the influence of the process parameters on the products. The nonlinear viscoelastic material model which depends on time and temperature was incorporated into FEM code Marc, and successfully simulated the surface pressure distribution of the workpiece during hot embossing. Meanwhile, final product shrinkage after cooling was also predicted by further including the PVT relationship of PMMA with the results of pressure distribution. Simulation results confirmed by experimental and theoretical analyses indicated that this approach can accurately simulate polymer deformation during the hot embossing process. During hot embossing, polymers was compressed with temperatures exceeding the glass transition temperature (Tg) and taken off after cooling. Because the non-uniform pressure distributions and the temperature change during hot embossing and cooling, respectively, the final products will display non-uniform shrinkage. Therefore, uniform shrinkage was required for hot embossing procedures. This study thus designed a better embossing process control consisting of the following three stages. 1.Apply larger load during the embossing stage to improve replication. 2.Apply smaller load during the initial cooling stage, while the temperature still exceeds Tg, to minimize pressure variation and thus volume change variation. 3.Increase the holding load in the final cooling stage, when the temperature drops below Tg, to reduce the pressure variation and increase overall shrinkage uniformity. Besides the applications on hot embossing, the experimental methods for collecting the material characteristics of semi-molten polymers and the finite element material model established in this study also can be applied in any processes involving working temperatures exceeding Tg.