購買設計請充值后下載，，資源目錄下的文件所見即所得，都可以點開預覽，，資料完整，充值下載可得到資源目錄里的所有文件。。?！咀ⅰ浚篸wg后綴為CAD圖紙，doc,docx為WORD文檔，原稿無水印，可編輯。。。具體請見文件預覽，有不明白之處，可咨詢QQ:12401814
南京理工大學泰州科技學院
畢業(yè)設計(論文)外文資料翻譯
系 部： 機械工程系
專 業(yè)： 機械設計及其自動化
姓 名： 周 志 宇
學 號： 05010158
外文出處：Composites scienc and techology
2007
附 件：1.外文資料翻譯譯文；2.外文原文。
指導教師評語：
譯文基本能表達原文思想，語句較流暢，條理較清晰，專業(yè)用語翻譯基本準確，基本符合中文習慣，整體翻譯質量一般。
簽名：
年 月 日
PBT玻璃纖維增強復合材料水輔注塑成型的實驗研究
摘要：本報告的目的是通過實驗研究聚對苯二甲酸丁二醇復合材料水輔注塑的成型工藝。實驗在一個配備了水輔注塑統(tǒng)的80噸注塑機上進行，包括一個水泵，一個壓力檢測器，一個注水裝置。實驗材料包括PBT和15%玻璃纖維填充PBT的混合物以及一個中間有一個肋板的空心盤。實驗根據(jù)水注入制品的長度的影響測得了各種工藝參數(shù)以及它們的機械性能。XRD也被用來分別材料和結構參數(shù)。最后,作了水輔助和氣體輔助注塑件的比較。實驗發(fā)現(xiàn)熔體壓力,熔融溫度，及短射類型是影響水注塑行為的決定性參數(shù)。材料在模具一面比在水一面展示了較高的結晶度。氣輔成型制品也要比水輔成型制品結晶度高。另外，制品表面的玻璃纖維大部分取向與流動方向一致，而隨著離制品表面距離的增加，越來越多的垂直與流動方向。
關鍵詞：水輔注塑成型，玻璃纖維增強PBT，工藝參數(shù)，機械性能，結晶
1．前言
依靠重量輕，成型周期短，消耗低，水輔注塑成型技術在塑料制品制造方面已經(jīng)取得了突破。在水輔注塑成型中，模具行腔被部分注入聚合物熔體，而后向這些聚合物中心注入水。水輔注塑成型的原理如圖1
圖1 水輔注塑成型的原理如圖
水輔注塑成型能夠在更短的循環(huán)時間內生產(chǎn)出收縮小，翹曲小，表面質量好的各種薄厚的制品。水輔注塑成型工藝也可根據(jù)工具及設備的承受壓力在設計，節(jié)省材料，減輕重量，減少成本方面取得更大的自由。典型的應用有棒，管材，水路管網(wǎng)建設用的大型復合結構管。另一方面，盡管有很多優(yōu)勢，由于加入了額外的工藝參數(shù)，模具和工藝控制變的更加嚴峻和困難。水也可能腐蝕模具鋼，同時一些材料包括熱塑性塑料難以成型。成型后水的清除也是對這個新技術的一個挑戰(zhàn)。表1列出了水輔注塑成型技術的優(yōu)勢和局限性。
優(yōu)勢
局限性
1，成型周期短
2，成本低（水更便宜而且可方便地循環(huán)利用）
3，制品內部不產(chǎn)生泡沫現(xiàn)象。
1，水腐蝕模具
2，需要較大的注塑元件。（容易陷入聚合物熔體）
3，一些材料難以成型（尤其是非晶態(tài)熱塑性材料）
4，成型后需要清除水
表1
水輔注塑成型有優(yōu)勢超過它更有名的競爭對手，氣輔注塑成型，因為依靠水在成型過程中更好的冷卻能力，水輔注塑成型獲得了更短的成型周期。它的不可壓縮性，低成本以及易循環(huán)利用，水成為這一過程的理想媒介。既然水不會溶解和擴散到聚合物熔體中，那么經(jīng)常在氣輔成型工藝出現(xiàn)的氣泡現(xiàn)象也便消除了。另外，水輔注塑成型能更好的用小剩余壁厚成型大型制件。表2是對水輔和氣輔成型工藝的一個比較。
表2水輔和氣輔成型工藝比較。
水輔
氣輔
1成型周期
2介質成本
3氣泡現(xiàn)象
5殘余壁厚
6表面粗糙度
7表面光澤
8指形效應
9非均勻穿透
10制品透明度
11內表面（熱塑性半晶）
12內表面（熱固性）
短
低
無
小
小
高
大
穩(wěn)定
高
平滑
粗糙
長
高
有
大
高
低
小
不穩(wěn)定
低
粗糙
平滑
隨著對密度小，強度高，價格便宜，成型周期短的優(yōu)良性能材料需求的增加，塑料工程是一個不可忽視的工藝。這些塑料包括熱塑性和熱固性塑料。一般來說，熱塑性塑料以其更高的沖擊強度，斷裂阻力，疲勞強度而更有優(yōu)勢。這使得熱塑性塑料在工程建設中廣泛使用。
PBT是廣泛使用的熱塑性工程塑料之一，它有1，4—丁烯乙2醇和DMT聚合而成。玻纖增強混合材料適用于提高原材料的機械性能。今天，短玻璃纖維增強PBT已被廣泛應用與電子，通信，汽車領域。所以，對玻璃纖維增強PBT的研究更加重要了。本文是通過實驗研究聚對苯二甲酸丁二醇水輔注塑的成型工藝，實驗在一個配備了水輔注塑統(tǒng)的80噸注塑機上進行，包括一個水泵一個壓力檢測器，一個注水裝置。實驗材料包括PBT和15%玻璃纖維填充PBT的混合物以及一個中間有一個肋板的空心盤。實驗根據(jù)水注入制品的長度的影響測得了各種工藝參數(shù)以及它們的機械性能。XRD也被用來分別材料和結構參數(shù)。最后,作了水輔助和氣體輔助注塑件的比較。
2．實驗步驟
2.1 材料
實驗材料包括PBT（牌號1111FB，南亞塑料，臺灣）和15%玻璃纖維填充PBT的混合物（牌號1210G3，南亞塑料，臺灣）。表3列出了此混合材料的特征。
表3 纖維增強PBT復合材料特征
性質
ASTM
PBT
15%G.F.PBT
屈服應力(kg/cm2)
彎曲應力(kg/cm2)
硬度
熱變形溫度（℃）
MFI
沖擊強度
熔點（℃）
D-638
D-570
D-785
D-648
D-1238
D-256
DSC
600
900
119
60
40
5
224
1000
1500
120
200
25
5
224
2.2 水輔注塑元件
一個實驗室注水元件，包括一個水泵，一個壓力檢測器，一個注水閥，一個配備了溫度調節(jié)裝置的水箱，以及一個控制電路。這個孔板型注水閥每邊有兩個孔，用來成型制件。實驗過程中，注水閥的控制電路收到由注塑機產(chǎn)生的信號實現(xiàn)對時間和注水壓力的控制。在注入模具行腔之前，水在有溫控裝置的水箱里加熱30分鐘。
2.3注塑機和模具
水輔注塑成型實驗在一個最高注塑速率109cm3/s的80噸注塑機上進行。研究使用了一個中間有一個肋板的空心盤。圖2顯示了這個行腔的尺寸。模具溫度由一個水循環(huán)模溫控制元件調節(jié)。實驗根據(jù)水注入制品的長度的影響測得了各種工藝參數(shù)，包括熔體溫度，模具溫度，熔體充模壓力，水溫和水壓，注水延遲時間和保持時間，以及熔體短射類型。表4列出這些工藝參數(shù)及在實驗中的數(shù)值。
A
B
C
D
E
F
熔體壓力
熔體溫度
短射類型
水 壓
水 溫
模具溫度
140
126
114
98
84
280
275
270
265
260
76
77
78
80
81
8
9
10
11
12
80
75
70
65
60
80
75
70
65
60
表 4
2.4氣輔注塑元件
為了對水輔和氣輔注塑成型制件進行比較，氣輔注塑成型實驗使用了一個商用氣輔注塑成型元件，其具體配置可參考RCFS。氣輔注塑成型工藝控制和水輔注塑成型一樣，除了氣體溫度設置為25外。
圖2 模具行腔的尺寸和外形
2.5 XRD
為了分析水輔注塑成型制品的晶體結構，實驗使用了具有二維探測分析傳輸模式的廣角X射線衍射儀。更特別的是實驗對水輔注塑成型制品模具一邊和水一邊的樣品在7到40的范圍內進行測量。分析所用的樣品來自制品中心。為了獲得XRD樣品要求的厚度，多余的部分在一個旋轉輪上打磨掉。首先用濕的碳硅紗布，而后用粒度300的，再用粒度600和1200的，以獲得更好的表面質量。
2.6機械性能
拉伸強度和彎曲強度測試在一個拉力測試機上進行。實驗對水輔注塑成型制件樣本進行拉力測試以評估水溫對拉伸性能的影響。樣本的尺寸為30mm*10mm*1mm.
水輔注塑成型制件的彎曲實驗也在室溫下進行。彎曲樣本的尺寸為20mm*10mm*1mm
2.7顯微鏡觀察
用電子掃描顯微鏡（型號5410）觀察制品中纖維的分子取向。樣品為取自注塑成型制件厚度方向上（圖3）。在垂直于流動方向了對截面進行觀察。觀察前，所有樣品表面鍍金。
圖3拉伸和彎曲測試切取樣品的位置圖示
3結果和討論
所有實驗在一個最高注塑速率109cm3/s的80噸注塑機上進行。所有研究中使用了一個中間有一個肋板水道的空心盤。
3.1制品的指形效應
所有制品都在水道的過度區(qū)域出現(xiàn)了指形效應。并且，玻璃纖維增強的復合材料指形效應比不增強的更嚴重，如圖4所示。指形效應一般在一種密度小，粘性低的液體穿過另一種密度大，粘性高的不相溶液體時產(chǎn)生?？紤]一個密度和黏度變化都很快的兩相界面或區(qū)域。流體移動的壓力P2-P1導致有效的置換量用下式描述
這里U是特性速率，K是穿透性。當壓力為正時，任何很小的置換量都會被放大，導致不穩(wěn)定并出現(xiàn)指形效應。當一種液體被比它密度低，黏度小的液體置換時，我們知道ｕ=ｕ1-ｕ2》0，而且U》O。這時，當一個黏度較高的液體被一種黏度較低的液體置換時，這種液體流動性較高，會出現(xiàn)不穩(wěn)定和指形效應。這次研究的結果顯示玻璃纖維增強的復合材料更傾向于指形效應。這也許是因為玻璃纖維增強的復合材料和水的黏度差比較大。因此水輔注塑成型復合材料顯示了更嚴重的指形效應。
3.2水穿透對工藝參數(shù)的影響
圖4 PBT復合材料水輔注塑成型照片
3.2工藝參數(shù)對水穿透的影響
實驗根據(jù)水穿透行為的影響測得了各種工藝參數(shù)。表4列出這些工藝參數(shù)以及實驗中使用的數(shù)值。為了成型制件，引用了一個重要工藝條件。通過在每一個實驗中改變一個參數(shù)，我們可以更好的理解在復合材料水輔注塑成型中每個參數(shù)對水穿透行為的影響。成型后，實驗測量了水注塑的長度。圖5-10顯示了工藝參數(shù)對水注塑長度的影響，包括熔體充模壓力，熔體溫度，模具溫度，短射類型，水溫以及水壓。
實驗結果顯示，水在純凈PBT中比在玻璃纖維增強PBT復合材料中穿透更深。這是由于玻璃纖維增強復合材料冷卻過程中體積收縮更小，因此，制品被水穿過的長度要短些。
熔體充模壓力
圖5，熔體充模壓力對水穿過長度的影響
熔體溫度
圖6 熔體溫度對水穿過長度的影響
模具溫度
圖7模具溫度對水穿過長度的影響
短射類型
圖8短射類型對水穿過長度的影響
水溫
圖9水溫對水穿過長度的影響
水壓
圖10水壓對水穿過長度的影響
由圖5可以看出，水穿過長度隨著熔體充模壓力的增大而減小。這可以解釋為由于熔體充模壓力增大，模具行腔對流動的阻力增加，因此水更難以進入材料的內部。水穿過長度因而變短。
圖6可以看出成型PBT復合材料制品時，隨著熔體溫度是增加水穿過長度也會變短。這也許是因為隨著溫度增加聚合物熔體的黏度降低。較低的熔體黏度有利于水包裹住水道，減少空閑區(qū)域，而不是更深的穿透。水道開頭孔的變小導致了水穿過長度的變短。
如圖7，增加模具溫度稍微降低了水在成型制品中的穿過長度。這也許是因為增加模具溫度降低了冷卻速率以及材料的黏度。于是水就包裹了水道，減少了水道口附近的空閑空間，而不是更深的穿透制品。
如圖8，增加短射率降低了水穿過長度。在水輔注塑成型中，模具行腔被部分注入聚合物熔體，而后向這些聚合物中心注入水。聚合物熔體短射率的增加降低了水在成型制品中的穿過長度。
作為實驗中的工藝參數(shù)，增加水溫或者水壓都增加了水在成型制品中的穿過長度。增加水溫降低了冷卻速率，是聚合物熔體更長時間內保溫，它的黏度也因此降低。這有利于水更深的進入進品中心。增加水壓也有利于水穿過物體，因此而獲得更深的穿透長度。
最后，制品的偏差，各種工藝參數(shù)測量的主觀性，
最大的制品偏差是翹曲。表11的結果顯示制品翹曲隨著水在成型制品中的穿過長度的降低而減少。這是因為水穿過制品的長度越長，包裹聚合物材料的水就越多。制品的翹曲和收縮也因而降低。
水穿過制品的長度
3.3成型制品的結晶
PBT是一個結晶速率很高的半結晶熱塑性聚脂。在水輔注塑成型過程中，結晶在非等溫條件下發(fā)生，冷卻速率隨著冷卻時間而變化。這里研究了各種工藝參數(shù)包括充熔體溫度，模具溫度，以及水溫對成型制品結晶的影響。測量使用了2維廣角X射線衍射儀。表12的結果顯示所有材料在模具層的結晶比在水層的結晶度要高。這個結果標志著在冷卻過程中水有著更好的冷卻能力。這與我們早先通過測量模內溫度分布得到的結果一致。另外，表12C的實驗結果顯示成型材料的結晶隨著水溫的增加而增加。這是因為增加水溫降低了冷卻過程中的材料冷卻速率。成型制品因而有更高的結晶度。
熔體溫度，模具溫度，以及水溫對水輔成型制品結晶的影響
另一方面，為了對水輔和氣輔注塑成型制品的結晶作一個比較，我們在同一臺注塑機上做了實驗，不同的是注塑機裝備了一個高壓氮氣注塑裝置。圖13的結果顯示氣輔注塑成型制品比水輔注塑成型制品有著更高的結晶度。這是因為水比空氣的冷卻能力高，冷卻快。因而水輔注塑成型制品比氣輔注塑成型制品的結晶度要低些。
圖13，水輔和氣輔成型制品的結晶度
3.4機械性能
對水輔注塑成型制品樣本進行拉伸測試以觀察水溫對拉伸性能的影響，表14的測量結果顯示其隨水溫增高而降低。正如我們看到的，PBT材料的屈服應力和拉伸應力都隨著溫度增高而降低。另一方面，PBT水輔注塑成型制品彎曲強度測試也在室溫下進行。圖15的測試結果顯示，制品的彎曲強度也隨溫度升高而降低。
圖15 水溫對PBT制品彎曲強度的影響
圖14水溫對PBT制品拉伸性能的影響
一般來說，增高水溫降低了冷卻速率，使制品的結晶度增高。正如我們所知，對于半結晶熱塑性塑料，較高的結晶度意味著較低的自由體積因而增加了制品的剛度。但是，實驗結果顯示，結晶度對PBT力學性能的影響是微不足道的，有更重要的增加了PBT材料的拉伸和彎曲應力。成型材料的機械性能取決于成型過程中結晶的數(shù)量和晶體類型。PBT的延展性隨著結晶降低的事實說明PBT在冷卻速率較低的成型過程中結晶度和剛性增加，因為缺乏延展性，成型制品在拉伸測試中的數(shù)值較高，而剛度沒有預期的高。無論如何，需要更詳細 的實驗研究水輔注塑成型制品的形態(tài)參數(shù)以及相關的機械性能。
3.5成型制品中纖維取向
從制品的中間切取小的樣品用來觀察纖維的取向。觀察的位置如圖3所示。觀察前，所有樣品的表面被磨光并鍍金。圖16顯示了水輔注塑成型制品的微型結構。
圖16 PBT復合材料水輔注塑成型制品的纖維取向
測量結果顯示水輔注塑成型制品中的纖維取向與常規(guī)注塑制品有明顯區(qū)別。
在常規(guī)注塑制品中一般觀察兩個區(qū)域：薄壁處與中心。在薄壁區(qū)域，所有纖維取向與流動方向平行，而在中心，纖維在流動平面內取向隨意。與常規(guī)注塑成型相比，水輔注塑成型技術的充模方式不同。對于常規(guī)注塑機，一個循環(huán)周期被定義為充模，保壓，冷卻3個階段。而在水輔注塑成型過程中，模具行腔被部分注入聚合物熔體，而后向這些聚合物中心注入水。這個新穎的充模方式明顯影響了纖維的取向。
由圖16可以看出，水輔注塑成型制品的纖維取向大致可分為3個區(qū)域，在模具一邊的表面，這里充模時剪切很嚴重，纖維很規(guī)則的平行。在水一側的表面，剪切作用不明顯，速率快，在這種情況下，纖維更傾向與垂直與注射方向。在制品中心，纖維取向很隨意。 總的來說，模具一邊的制品表面的玻璃纖維取向大部分與流動方向一致，而隨著離這一表面距離的增加，纖維取向逐漸的垂直與流動方向。最后，應該注意的是，我們實驗室應該在今后的研究中對水輔注塑成型和常規(guī)注塑成型的纖維取向和形態(tài)做一個定量的比較。
4結論
本報告的目的是通過實驗研究聚對苯二甲酸丁二醇復合材料水輔注塑的成型工藝?；诋斍皩嶒灴傻贸鲆韵陆Y論
1. 水輔注塑成型制品在水道的過度區(qū)域出現(xiàn)了指形效應。并且，玻璃纖維增強復合材料的指形效應比不增強的更嚴重
2. 研究的實驗結果顯示PBT復合材料的水穿透長度隨著水溫和水壓的增加而增加。隨著熔體充模壓力，熔體溫度，模具溫度，短射量的增加而降低。，
3. 制品的翹曲隨著水穿透的程度而降低了。
4. 注塑制品的結晶度隨著水溫的升高而提高。水輔成型制品的結晶度比氣輔的要低。
5. 模具一邊的制品表面的玻璃纖維取向大部分與流動方向一致，而隨著離這一表面距離的增加，纖維取向逐漸的垂直與流動方向
感謝
感謝臺灣科學委員會對研究工作的資金支持！
參考文獻
[1] Knights M. Plast Technol 2002(April):42.
[2] Michaeli W, Juntgen T, Brunswick A. Kunststo. Plast Europe
[3] Liu SJ, Chen YS. Polym Eng Sci 2003;43:1806.
[4] Liu SJ, Chen YS. Compos Part A: Appl Sci Manuf 2004;35:171.
[5] Johnson L, Olley P, Coates PD. Plast Rubber Compos 2000;29:31.
[6] Potente H, Moritzer E, Oberman CH. Polym Eng Sci 1996;36:2163.
[7] Fung CP. Plast Rubber Compos 2004;33:170.
[8] Ludwig H-J, Eyerer P. Polym Eng Sci 1988;28:143.
[9] Chisholm BJ, Fong PM, Zimmer JG, Hendrix R. J Appl Polym Sci
1999;74:889.
[10] Yoshioka T, Tsuji M, Kawahara Y, Kohjiya S, Manabe N, Yokota
Y. Polymer 2005;46:4987.
[11] Liu SJ, Wu YC. Int Polym Process 2000;15:297.
12] Liu SJ, Chang JH, Ho CY, Hung SW. Int Polym Process 1999;14:
191.
[13] Liu SJ, Chang JH. Polym Compos 2000;21:322.
[14] Liu SJ, Yang CY. Plast Rubber Compos 2002;31:36.
[15] Liu SJ, Lin IH. Plast Rubber Compos 2002;31:28.
[16] Homsy GM. Ann Rev Fluid Mech 1987;19:271.
[17] Liu SJ, Chen WK. Plast Rubber Compos 2004;33:260.
[18] Sarasua JR, Lopez Arraiza A, Balerdi P, Maiza I. Polym Eng Sci
[19] Ludwig H-C, Fischer G, Becker H. Compos Sci Technol 1995;53:235.
[20] Mlekusch B, Lehner EA, Geymayer W. Compos Sci Techol
[21] Tucker III CL. In: Isayev AI, editor. Injection and compression
molding fundamentals. Marcel Dekker; 1987 [Chapter 7
form 7 November Fig. 1. Water-assisted injection molding can produce parts incorporating both thick and thin sections with less shrink- involved. Water may also corrode the steel mold, and some materials including thermoplastic composites are di?cult to mold successfully. The removal of water after molding is also a challenge for this novel technology. Table 1 lists the advantages and limitations of water-assisted injection molding technology. * Corresponding author. Address: 259, Wen-Hwa 1st Road, Kwei-San, Tao-Yuan 333, Taiwan. E-mail address: shihjung@mail.cgu.edu.tw (S.-J. Liu). Composites Science and Technology COMPOSITES 1. Introduction Water-assisted injection molding technology [1] has proved itself a breakthrough in the manufacture of plastic parts due to its light weight, faster cycle time, and relatively lower resin cost per part. In the water-assisted injection molding process, the mold cavity is partially filled with the polymer melt followed by the injection of water into the core of the polymer melt. A schematic diagram of the water-assisted injection molding process is illustrated in age and warpage and with a better surface finish, but with a shorter cycle time. The water-assisted injection molding process can also enable greater freedom of design, material savings, weight reduction, and cost savings in terms of tool- ing and press capacity requirements [2–4]. Typical applica- tions include rods and tubes, and large sheet-like structural parts with a built-in water channel network. On the other hand, despite the advantages associated with the process, the molding window and process control are more critical and di?cult since additional processing parameters are Abstract The purpose of this report was to experimentally study the water-assisted injection molding process of poly-butylene-terephthalate (PBT) composites. Experiments were carried out on an 80-ton injection-molding machine equipped with a lab scale water injection sys- tem, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The materials included virgin PBT and a 15% glass fiber filled PBT composite, and a plate cavity with a rib across center was used. Various processing variables were examined in terms of their influence on the length of water penetration in molded parts, and mechanical property tests were performed on these parts. X-ray di?raction (XRD) was also used to identify the material and structural parameters. Finally, a comparison was made between water-assisted and gas-assisted injection molded parts. It was found that the melt fill pressure, melt temperature, and short shot size were the dominant parameters a?ecting water penetration behavior. Material at the mold-side exhibited a higher degree of crystallinity than that at the water-side. Parts molded by gas also showed a higher degree of crystallinity than those molded by water. Furthermore, the glass fibers near the surface of molded parts were found to be ori- ented mostly in the flow direction, but oriented substantially more perpendicular to the flow direction with increasing distance from the skin surface. C211 2006 Elsevier Ltd. All rights reserved. Keywords: Water assisted injection molding; Glass fiber reinforced poly-butylene-terephthalate (PBT) composites; Processing parameters; B. Mechanical properties; Crystallinity; A. Polymer matrix composites; Processing An experimental study of the water-assisted glass fiber filled poly-butylene-terephthalate Shih-Jung Liu * , Ming-Jen Polymer Rheology and Processing Lab, Department of Mechanical Received 12 September 2005; received in revised Available online 0266-3538/$ - see front matter C211 2006 Elsevier Ltd. All rights reserved. doi:10.1016/pscitech.2006.09.016 injection molding of (PBT) composites Lin, Yi-Chuan Wu Engineering, Chang Gung University, Tao-Yuan 333, Taiwan 29 June 2006; accepted 11 September 2006 2006 67 (2007) 1415–1424 SCIENCE AND TECHNOLOGY Table 2 A comparison of water and gas-assisted injection molding Water Gas 1. Cycle time Short Long 2. Medium cost Low High 3. Internal foaming No Yes 4. Residual wall thickness Small Large 5. Outside surface roughness Low High 6. Outside surface gloss High Low 1416 S.-J. Liu et al. / Composites Science and Technology 67 (2007) 1415–1424 Water assisted injection molding has advantages over its better known competitor process, gas assisted injection molding [5], because it incorporates a shorter cycle time to successfully mold a part due to the higher cooling capac- Fig. 1. Schematic diagram of water-assisted injection molding process. ity of water during the molding process. The incompress- ibility, low cost, and ease of recycling the water makes it an ideal medium for the process. Since water does not dis- solve and di?use into the polymer melts during the molding process, the internal foaming phenomenon [6] that usually occurs in gas-assisted injection molded parts can be elimi- nated. In addition, water assisted injection molding pro- vides a better capability of molding larger parts with a small residual wall thickness. Table 2 lists a comparison of water and gas assisted injection molding. With increasing demands for materials with improved performance, which may be characterized by the criteria of lower weight, higher strength, and a faster and cheaper production cycle time, the engineering of plastics is a pro- cess that cannot be ignored. These plastics include thermo- plastic and thermoset polymers. In general, thermoplastic polymers have an advantage over thermoset polymers in Table 1 Advantages and disadvantages of water-assisted injection molding Advantages Disadvanta 1. Short cycle time 2. Low assisting medium cost (water is much cheaper and can be easily recycled) 3. No internal foaming phenomenon in molded parts 1. 2. 3. 4. terms of higher impact strength, fracture resistance and strains-to-failure. This makes thermoplastic polymers very popular materials in structural applications. Poly-butylene-terephthalate (PBT) is one of the most frequently used engineering thermoplastic materials, which is formed by polymerizing 1.4 butylene glycol and DMT together. Fiber-reinforced composite materials have been adapted to improve the mechanical properties of neat plas- tic materials. Today, short glass fiber reinforced PBT is widely used in electronic, communication and automobile applications. Therefore, the investigation of the processing of fiber-reinforced PBT is becoming increasingly important [7–10]. This report was made to experimentally study the water- assisted injection molding process of poly-butylene-tere- phthalate (PBT) materials. Experiments were carried out on an 80-ton injection-molding machine equipped with a lab scale water injection system, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The materials included a virgin PBT and a 15% glass fiber filled PBT composite, and a plate cavity 7. Fingering Greater Less 8. Asymmetrical penetration More stable Unstable 9. Material crystallinity Low High 10. Part transparency High Low 11. Internal surface (semi-crystalline materials) Smooth Less smooth 12. Internal surface (amorphous materials) Rough Smooth with a rib across center was used. Various processing vari- ables were examined in terms of their influence on the length of water penetration in molded parts, which included melt temperature, mold temperature, melt filling speed, short-shot size, water pressure, water temperature, water hold and water injection delay time. Mechanical property tests were also performed on these molded parts, and XRD was used to identify the material and structural ges Corrosion of the steel mold due to water Larger orifices for the injection pin required (easier to get stuck by the polymer melt) Some materials are more di?cult to mold (especially amorphous thermoplastics) Removal of water after molding is required parameters. Finally, a comparison was made between water-assisted and gas-assisted injection molded parts. 2. Experimental procedure 2.1. Materials The materials used included a virgin PBT (Grade 1111FB, Nan-Ya Plastic, Taiwan) and a 15% glass fiber filled PBT composite (Grade 1210G3, Nan-Ya Plastic, Tai- wan). Table 3 lists the characteristics of the composite materials. polymer melt. Table 4 lists these processing variables as well as the values used in the experiments. 2.4. Gas injection unit In order to make a comparison of water and gas-assisted injection molded parts, a commercially available gas injec- tion unit (Gas Injection PPC-1000) was used for the gas- assisted injection molding experiments. Details of the gas injection unit setup can be found in the Refs. [11–15]. The processing conditions used for gas-assisted injection molding were the same as that of water-assisted injection molding (terms in bold in Table 4), with the exception of gas temperature which was set at 25 C176C. 2.5. XRD In order to analyze the crystal structure within the water-assisted injection-molded parts, wide-angle X-ray di?raction (XRD) with 2D detector analyses in transmis- sion mode were performed with Cu Ka radiation at 40 kV and 40 mA. More specifically, the measurements were performed on the mold-side and water-side layers of the water-assisted injection-molded parts, with the 2h angle ranging from 7C176 to 40C176. The samples required for these analyses were taken from the center portion of these Fig. 2. Layout and dimensions of mold cavity (unit: mm). S.-J. Liu et al. / Composites Science and 2.2. Water injection unit A lab scale water injection unit, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit, was used for all experiments [3]. An ori- fice-type water injection pin with two orifices (0.3 mm in diameter) on the sides was used to mold the parts. During the experiments, the control circuit of the water injection unit received a signal from the molding machine and con- trolled the time and pressure of the injected water. Before injection into the mold cavity, the water was stored in a tank with a temperature regulator for 30 min to sustain an isothermal water temperature. 2.3. Molding machine and molds Water-assisted injection molding experiments were con- ducted on an 80-ton conventional injection-molding machine with a highest injection rate of 109 cm 3 /s. A plate cavity with a trapezoidal water channel across the center was used in this study. Fig. 2 shows the dimensions of the cavity. The temperature of the mold was regulated by a water-circulating mold temperature control unit. Various processing variables were examined in terms of their influ- ence on the length of water penetration in water channels of molded parts: melt temperature, mold temperature, melt fill pressure, water temperature and pressure, water injec- tion delay time and hold time, and short shot size of the Table 3 Characteristics of the glass–fiber reinforced PBT composite Property ASTM PBT 15% G.F. PBT Yield strength (kg/cm 2 ) D-638 600 1000 Bending stress (kg/cm 2 ) D-570 900 1500 Hardness (R-scale) D-785 119 120 Heat distortion temperature (C176C) (18.6 kg/cm 2 ) D-648 60 200 Melt flow index (MFI) D- 1238 40 25 Impact strength (Kg-cm/cm) D-256 5 5 Melting temperature (C176C) DSC 224 224 Technology 67 (2007) 1415–1424 1417 molded parts. To obtain the desired thickness for the XRD samples, the excess was removed by polishing the samples on a rotating wheel on a rotating wheel, first with wet silicon carbide papers, then with 300-grade silicon car- bide paper, followed by 600- and 1200-grade paper for a better surface smoothness. 2.6. Mechanical properties Tensile strength and bending strength were measured on a tensile tester. Tensile tests were performed on specimens obtained from the water-assisted injection molded parts (see Fig. 3) to evaluate the e?ect of water temperature on 20 mm · 10 mm · 1 mm. Bending tests were performed in a micro tensile tester according to the ASTM D256 test. A 200 N load cell was used and the crosshead speed was 50 mm/min. 2.7. Microscopic observation The fiber orientation in molded specimens was observed under a scanning electron microscope (Jeol Model 5410). Specimens for observation were cut from parts molded by water-assisted injection molding across the thickness Table 4 The processing variables as well as the values used in the experiments AB CD E F Melt pressure (Mpa) Melt temperature (C176C) Short shot size (%) Water pressure (Mpa) Water temperature (C176C) Mold temperature (C176C) 140 280 (270) 76 8 80 80 126 275 (265) 77 9 75 75 114 270 (260) 78 10 70 70 98 265 (255) 80 11 65 65 84 260 (250) 81 12 60 60 * The values in the parentheses are the melt temperatures used for virgin PBT materials. 1418 S.-J. Liu et al. / Composites Science and Technology 67 (2007) 1415–1424 the tensile properties. The dimensions of specimens for the experiments were 30 mm · 10 mm · 1 mm. Tensile tests were performed in a LLOYD tensiometer according to the ASTM D638M test. A 2.5 kN load cell was used and the crosshead speed was 50 mm/min. Bending tests were also performed at room tempera- ture on water-assisted injection molded parts. The bend- ing specimens were obtained with a die cutter from parts (Fig. 3) subjected to various water temperatures. The dimensions of the specimens were Fig. 3. Schematically, the positioning of the samples cut from the molded (Fig. 3). They were observed on the cross-section perpen- dicular to the flow direction. All specimen surfaces were gold sputtered before observation. 3. Results and discussion All experiments were conducted on an 80-ton conven- tional injection-molding machine, with a highest injection rate of 109 cm 3 /s. A plate cavity with a trapezoidal water channel across the center was used for all experiments. parts for tensile and bending tests and microscopic observations. 3.1. Fingerings in molded parts All molded parts exhibited the water fingering phenom- enon at the channel to plate transition areas. In addition, molded glass fiber filled composites showed more severe water fingerings than those of non-filled materials, as shown photographically in Fig. 4. Fingerings usually form when a less dense, less viscous fluid penetrates a denser, more viscous fluid immiscible with it. Consider a sharp two phase interface or zone where density and viscosity change rapidly. The pressure force (P 2 C0P 1 ) on the dis- placed fluid as a result of a virtual displacement dx of the interface can be described by [16], dP ?eP 2 C0 P 1 T??el 1 C0 l 2 TU=KC138dx e1T where U is the characteristic velocity and K is the perme- ability. If the net pressure force is positive, then any small displacement will be amplified and lead to an instability and part fingerings. For the displacement of a dense, vis- cous fluid (the polymer melt) by a lighter, less viscous one (water), we can have Dl = l 1 C0l 2 > 0, and U >0[16]. In this case, instability and the relevant fingering result when a more viscous fluid is displaced by a less viscous one, since the less viscous fluid has the greater mobility. The results in this study suggest that glass fiber filled com- posites exhibit a higher tendency for part fingerings. This might be due to the fact that the viscosity di?erence Dl be- tween water and the filled composites is larger than the dif- ference between water and the non-filled materials. Water- assisted injection molded composites thus exhibit more se- vere part fingerings. S.-J. Liu et al. / Composites Science and Technology 67 (2007) 1415–1424 1419 Fig. 4. Photograph of water-assisted injection molded PBT composite part. 3.2. E?ects of processing parameters on water penetration Various processing variables were studied in terms of their influence on the water penetration behavior. Table 4 lists these processing variables as well as the values used in the experiments. To mold the parts, one central process- ing condition was chosen as a reference (bold term in Table 4). By changing one of the parameters in each test, we were able to better understand the e?ect of each parameter on the water penetration behavior of water assisted injection molded composites. After molding, the length of water penetration was measured. Figs. 5–10 show the e?ects of these processing parameters on the length of water penetra- tion in molded parts, including melt fill pressure, melt tem- perature, mold temperature, short shot size, water temperature, and water pressure. The experimental results in this study suggest that water penetrates further in virgin PBT than in glass fiber filled PBT composites. This is due to the fact that with the rein- forcing glass fibers the composite materials have less volu- metric shrinkage during the cooling process. Therefore, they mold parts with a shorter water penetration length. 84 98 112 126 140 10 12 14 16 18 20 PBT PBT+15% G.F. Melt fill pressure (MPa) Length of penetration (cm) Fig. 5. E?ects of melt fill pressure on the length of water penetration in molded parts. and 20 PBT PBT+15% G.F. 1420 S.-J. Liu et al. / Composites Science The length of water penetration decreases with the melt fill pressure (Fig. 5). This can be explained by the fact that increasing the melt fill pressure increases the flow resistance inside the mold cavity. It is then more di?cult for the water 250 255 260 265 270 275 280 8 10 12 14 16 18 Length of penetration (cm) Melt temperature (?C) Fig. 6. E?ects of melt temperature on the length of water penetration in molded parts. 60 65 70 75 80 10 12 14 16 18 20 PBT PBT+15% G.F. Mold temperature (?C) Length of penetration (cm) Fig. 7. E?ects of mold temperature on the length of water penetration in molded parts. 76 77 78 79 80 81 10 12 14 16 18 20 PBT PBT+15% G.F. Short shot size (%) Length of penetration (cm) Fig. 8. E?ects of short shot size on the length of water penetration in molded parts. 20 PBT PBT+15% G.F. 60 65 70 75 80 10 12 14 16 18 20 PBT PBT+15% G.F. Water temperature (?C) Length of penetration (cm) Fig. 9. E?ects of water temperature on the length of water penetration in molded parts. Technology 67 (2007) 1415–1424 to penetrate into the core of the materials. The length of water penetration decreases accordingly [3]. The melt temperature was also found to reduce the water penetration in molded PBT composite parts (Fig. 6). This might be due to the fact that increasing the melt temperature decreases viscosity of the polymer melt. A lower viscosity of the materials helps the water to pack the water channel and increase its void area, instead of penetrating further into the parts [4]. The hollow core ratio at the beginning of the water channel increases and the length of water penetration may thus decrease. Increasing the mold temperature decreases somewhat the length of water penetration in molded parts (Fig. 7). This is due to the fact that increasing the mold temperature decreases the cooling rate as well as the viscosity of the materials. The water then packs the channel and increases its void area near the beginning of the water channel, instead of penetrating further into the parts [3]. Molded parts thus have a shorter water penetration length. Increasing the short shot size decreases the length of water penetration (Fig. 8). In water-assisted injection molding, the mold cavity is partially filled with the polymer 8 9 10 11 12 10 12 14 16 18 Water pressure (MPa) Length of penetration (cm) Fig. 10. E?ects of water pressure on the length of water penetration in molded parts. melt followed by the injection of water into the core of the polymer melt [4]. Increasing the short shot size of the poly- mer melt will therefore decrease the length of water pene- tration in molded parts. For the processing parameters used in the experiments, increa