明膠玻璃之溫間材料性質與成形特性探討

Abstract

在製造飛機之透明擋風玻璃或座艙罩時，會採用明膠玻璃材料進行溫間拉伸成形製程，但在實際加工中當材料升溫至玻璃轉換溫度附近時會遇到因熱行為所產生的材料變形狀況，此行為不僅會對明膠玻璃的成形造成影響，也會導致成品缺陷的產生，其中又以光學缺陷和幾何缺陷為主。本論文旨在透過對明膠玻璃進行玻璃轉換溫度附近的材料實驗，來對材料的溫間性質進行討論，並透過實際數據建立其材料模型，以提高CAE所使用的分析模型準確性，最後透過成形模擬的結果對成品的回彈缺陷進行探討。 透過文獻蒐集和整理建立出CAE中明膠玻璃在溫間成形模擬時所需要的材料模型參數，並進行與其對應的材料實驗，包含單軸拉伸、熱膨脹、比熱和潛變實驗，發現材料對於溫度和應變率的敏感性、平面和垂直方向的膨脹收縮差異性、隨溫度增加的比熱以及在加工溫度下發生的潛變行為。 觀察材料在加工溫度下所展現的變形行為以對明膠玻璃之材料模型進行建立，分別探討可用於描述此材料的彈塑性模型、彈塑黏模型之特性。使用曲線擬合的方式取得材料的黏性參數並透過實驗數據完成彈塑黏模型的模擬驗證，以確定參數的正確性。在詳細說明彈塑性模型與彈塑黏模型在成形模擬的使用上之限制後，選擇彈塑性模型作為後續之成形模擬分析使用。 在成品性質的探討方面，主要針對光學性質和幾何缺陷進行討論，說明與CAE模擬結果相對應的缺陷關係，包含光學方面之偏折、扭曲和霧度，以及幾何方面之回彈現象。分析影響回彈模擬結果的因子，分別對接觸面摩擦係數的改變、材料模型中黏性參數的有無和經過6個小時的退火製程之回彈模擬結果比較差異，得到加入黏性參數對成品回彈量的下降有明顯的趨勢，因此在未來對於成品性質方面的進一步探討中，正確的、多應變率的材料黏性參數取得會是一個主要需要探討的方向。

In the manufacturing of transparent windshields or canopies for aircraft, PMMA materials are utilized in a warm stretching forming process. However, during processing, when the material is heated to temperatures near its glass transition point, thermal behavior induces material deformation. The sensitivity of polymeric materials to temperature and processing speed also impacts manufacturers’ inspection requirements for finished products, with optical and geometric defects being the primary concerns. This study aims to characterize the properties of PMMA through material experiments conducted near its glass transition temperature, develop a material model based on empirical data to enhance the accuracy of finite element analysis (FEA) models used in computer-aided engineering (CAE), and investigate springback defects in the finished product through forming simulation results. Through a systematic literature review, the material model parameters required for CAE simulations of PMMA during warm forming were established. Corresponding material experiments, including uniaxial tensile tests, thermal expansion measurements, specific heat capacity evaluations, and creep tests, were conducted. These experiments revealed the material’s sensitivity to temperature and strain rate, differential expansion and contraction in planar and vertical directions, increasing specific heat with rising temperature, and creep behavior under processing conditions. The deformation behavior observed under processing temperatures was used to construct a material model for PMMA. The characteristics of elastoplastic and viscoelastic-plastic models suitable for describing this material were analyzed. Viscoelastic parameters were obtained through curve-fitting techniques, and the viscoelastic-plastic model was validated using experimental data to ensure parameter accuracy. After a detailed discussion of the limitations of elastoplastic and viscoelastic-plastic models in forming simulations, the elastoplastic model was selected for subsequent forming simulation analyses. Regarding the properties of the finished product, the discussion focuses on optical and geometric defects, elucidating their correlation with CAE simulation outcomes. Optical defects, such as deflection, distortion, and haze, and geometric defects, notably springback, were examined. Factors influencing springback simulation results were analyzed, including variations in contact surface friction coefficients, the presence or absence of viscoelastic parameters in the material model, and the effects of a six-hour annealing process. The results indicate a significant trend of reduced springback when viscoelastic parameters are included. Consequently, the accurate determination of viscoelastic parameters across multiple strain rates emerges as a critical direction for future investigations into the properties of finished products.