物理發泡管材壓出製程及吹袋壓出製程之模頭最佳化設計

Abstract

本文目的在建立一套供以使用物理發泡劑的一套管狀模頭設計及分析軟體。我們首先以數學模式直接預測含有溶解氣體但不發泡的膠料其剪切黏度η，再以計算流體力學建立其管狀模頭內的流動模式。接著利用Sanchez-Lacombe equation 計算在任一壓力與溫度下，物理發泡劑溶解在膠料中的重量百分比。結果如我們所預期：熔膠的飽和壓力上升或是溫度的下降，都能使發泡劑溶解度上升。而模頭參數包括有起始溝深DEPTH、起始溝寬WCHAN、起始間隙GIBC、taper分段高度HBC、overlap數NOSEC、螺旋角θ以及軸心傾斜角α，第二步便配合直交表引入設計參數，在等溫清況下以流量均勻度及混合度為指標搜尋最佳模頭幾何形狀，並建立其變數分析表。由變數分析表可知吾人所選擇的七個變數的確可以完整涵蓋模頭內流動的變異。同時可看出不論溝道數為何，影響MD的前三名變數分別為DEPTH、WCHAN及GIBC；影響UNI最多的變數是WCHAN及NOSEC；影響△P最多的變數是GIBC及ALFAR，且其貢獻度在4、5及6溝道時的變化都不大。由於在膠料發泡過程中，若模頭內部壓力(local die pressure)無法達到將氣體溶解在膠料內之臨界壓力Pc時，則膠料會產生預發泡(prefoaming)的現象。因此在模頭出口處(亦即模唇，die land)會將流道深度加以壓縮造成一個壓縮的導角(taper)，使得模頭內部各點壓力超過對應的臨界壓力。因此最後檢視其在加入模唇之後及非等溫狀況的表現，建立一套供以使用物理發泡劑的一套管狀模頭設計及分析軟體。

The purpose of the present study is to establish an analytical program for spiral die with physical foaming agent. First, we predict the viscosity of melt polymer with foaming agent mathematically, and establish the flow model by fluid dynamics. Applying the Sanchez-Lacombe equation, we can calculate the weight percentage of foaming agent in melt polymer at any pressure and temperature. As predicted, more foaming agent can be dissolved into melt polymer at a high pressure or low temperature. The parameters of spiral die include DEPTH, WCHAN, GIBC, HBC, NOSEC, θ and α. Second, we use the Tguchi method to search several optimal sets of die geometric parameters by assessing the flow uniformity(UNI) and mixing degree(MD) isothermally and create ANOVA tables. From ANOVA tables, we realize that the seven parameters can totally describe the flow situation in spiral die. At the same time, we find that whatever the number of channels is, the most relevant parameters with respect to MD are DEPTH,WCHAN and GIBC; with respect to UNI, WCHAN and NOSEC are the most relevant parameters; furthermore, with respect to pressure gradient, GIBC及ALFAR are the most relevant. During polymer foaming, if the local die pressure cannot reach the critical pressure, prefoaming occurs. Therefore, at the outlet of spiral die, we narrow down the channel depth to increase the local pressure, which is thus greater than critical pressure. Finally, with die lip being attached to spiral die, we examine the non-isothermal condition, so that an analytical program for spiral die with physical foaming agent can be established.