超音波輔助玻璃熱壓成形之有限元素分析研究

Abstract

超音波輔助成形近年來廣泛應用於高溫成形中，此技術是藉由超音波之高能量提高試片溫度以提高其成形性。實驗研究結果指出超音波輔助成形能減少成形所需之外力及增加材料之成形性，但相對於高分子成形，超音波輔助玻璃熱壓成形之文獻相當的少，大都以實驗觀察為主。雖有部份的研究採用有限元素法對傳統玻璃熱壓成形進行數值分析，但因材料模型的限制尚無法進行超音波輔助玻璃熱壓成形之模擬。因此本研究之目的是建立超音波輔助玻璃熱壓成形之模型，藉由此模型來驗證超音波輔助技術能在熱壓過程中減少成形所需之力及提升玻璃對模穴之填充性。 為了建立超音波輔助玻璃熱壓成形的有限元素分析模型，首先利用動態響應的理論背景來解釋玻璃在超音波輔助熱壓成形過程中的表現，並藉由標準線性固體方程式來推導玻璃的損耗模數，計算出玻璃在超音波振動下的熱吸收量；並且假設玻璃在超音波振動時吸收的能量將完全轉換成熱量，這些熱量將被設為有限元素模型的熱傳計算邊界條件，經由熱固耦合之模擬來探討超音波輔助成形時的外力減少量與玻璃微結構填充性的提升量。 本研究亦使用K-PSK100之光學玻璃進行平板式熱壓及微結構熱壓成形實驗，在成形過程中施加頻率35 kHz、振幅3 m之超音波，藉以比較有無施加超音波對玻璃成形所需之外力及填充性之影響，並與模擬結果相互驗證。實驗結果顯示，施加超音波能明顯地減少成形所需之外力(高達74.9 %)且能提高玻璃材料對微結構模穴的填充性(約17 %)。由成形時的力量響應及成形金字塔結構之高度比較結果，發現模擬與實驗之結果相當吻合，驗證了有限元素模型之有效性。基於此模型之有效性，本研究對其他熱壓參數例如熱壓溫度、熱壓速度、下模具之幾何形狀及玻璃鬆弛之特性對熱壓成形之影響亦進行了探討。本研究之結果可用來探討重要製程參數對最終熱壓成品之影響，例如熱壓速度、熱壓溫度、振動頻率、振幅以及施加超音波之時間等等。

關鍵字：超音波振動、玻璃熱壓成形、有限元素分析、金字塔結構

Ultrasonic vibration technology has recently been applied in hot embossing process. High energy of ultrasonic vibration have been used to increase the temperature of the specimen so that material formability could be improved. Unlike polymer materials, very few research has been studied effect of ultrasonic vibration on hot glass embossing process. Experimental research has delineated the effects of ultrasonic vibration on reducing required forces and improving the formability of materials. Although numerical analysis of conventional glass molding, also glass hot embossing, using finite element method has been carried out deeply, it is inconvenient to utilize these material models for ultrasonic vibration-assisted hot glass embossing process due to the appearance of ultrasonic vibration. Therefore, the purpose of this study was to construct a finite element model for the ultrasonic vibration-assisted hot glass embossing process. The proposed model was used to explain experimental phenomena, such as reduction of embossing force during the embossing stage and the improvement of filling ability of glass material into the microcavities under effect of ultrasonic vibration. In order to construct the finite element model for the ultrasonic vibration-assisted hot glass embossing process, this study firstly utilized theoretical background relating to the dynamic response of glass material to explain the glass behavior during the embossing stage assisted by ultrasonic vibration. By predicting loss modulus of the glass from constitutive equation of Standard Linear Solid model, ultrasonic vibration absorption of the glass was calculated. The amount of energy which the glass absorbed from ultrasonic vibration was assumed to convert into heat completely. This heat generation was then inputted to finite element simulations through a thermal boundary condition. Instead of performing mechanical displacement of ultrasonic vibration, adding such thermal boundary condition could illustrate the reduction of embossing force as well as the improvement of filling ability of the glass into microcavities. The proposed finite element model was verified with experimental data. Using K-PSK100 optical glass specimens, flat hot embossing and microstructure hot embossing experiments were carried out with and without the assistance of ultrasonic vibration (amplitude of 3 m, frequency of 35 kHz) to express effect of ultrasonic vibration on embossing force and filling ability of glass material, respectively. Experimental results showed that ultrasonic vibration could reduce the embossing force significantly (up to 74.9 %) as well as help glass fill into the microcavities better (up to 17 %). Comparisons of embossing force response and the final height of pyramid structure between simulations and experiments were performed. Simulated results fitted quite well with experimental data, which validated the proposed model. Based on this proposed model, effect of some parameters such as embossing temperature, embossing speed, lower mold geometry and relaxation properties of the glass were also investigated. The findings of this research may be used to study effect of some other parameters, such as vibration frequency, vibration amplitude, the ultrasonic vibration applying time, etc., on the quality of the final product.

Keywords: ultrasonic vibration, hot glass embossing, finite element analysis, pyramid microstructures