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購買設(shè)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(yù)覽，資料完整，充值下載就能得到?！咀ⅰ浚篸wg后綴為CAD圖，doc,docx為WORD文檔，有不明白之處，可咨詢QQ:1304139763 編號 畢業(yè)設(shè)計 論文 外文翻譯 譯文 學(xué) 院 機電工程學(xué)院 專 業(yè) 機械設(shè)計制造及其自動化 學(xué)生姓名 韋良華 學(xué) 號 1000110129 指導(dǎo)教師單位 機電工程學(xué)院 姓 名 陳虎 職 稱 助教 2014 年 5 月 26 日 1 通過實驗設(shè)計優(yōu)化微注射成型工藝 摘要 本文提出通過試驗設(shè)計 DOE 優(yōu)化微注射成型 MIM 過程 MIM 是一種相對較新的用 于微部件的快速制造的技術(shù) 由于改變工藝參數(shù) 為了滿足質(zhì)量和可靠性的限制 減少操作 過程中變異的是非常重要 在這項研究中 對 MIM 工藝的理解 它是通過 DOE 的六個影響表 面質(zhì)量的參數(shù) 流動長度和長寬比來優(yōu)化的 顯著單一的工藝參數(shù)以及它們之間的相互作用 是通過統(tǒng)計分析確定 為 2 級的試驗中 20 21 20 的縱橫比 分別對應(yīng)聚丙烯 PP 丙 烯腈 丁二烯 苯乙烯 ABS 和聚甲醛 POM 實現(xiàn) 關(guān)鍵詞 微注射成型 MIM 試驗設(shè)計 DOE 全因子 部分因子 優(yōu)化設(shè)計的設(shè)計 第一章 引言 因為它的大批量生產(chǎn)能力和低元件成本 微注射成型 MIM 是一種在微型制造行業(yè)內(nèi) 流行的相對較新的技術(shù) 為了使 MIM 以最小的成本實現(xiàn)最高品質(zhì)的元件 理解的過程并確 定不同的獨立參數(shù)的影響是很重要的 一種可以采用的調(diào)查 MIM 的整體操作的方法是試驗 設(shè)計 DOE 的設(shè)計 在一般情況下 DOE DoE 可用于收集從每個過程 并通過數(shù)據(jù) 分析獲得加工工藝的理解 這個程序可以幫助優(yōu)化過程 并最終使得質(zhì)量的提高 本文的結(jié)構(gòu)如下 在 MIM 工藝在第 2 節(jié)所述 在第 3 節(jié) DOE 的介紹 實驗數(shù)據(jù)的收 集之后第 4 節(jié)解釋 結(jié)果和數(shù)據(jù)分析進行說明在第 5 節(jié)說明 結(jié)果的討論 在第 6 節(jié)提出 最后在第 7 節(jié)給出結(jié)論的文件結(jié)束 2212 8271 2013 的作者 由 Elsevier BV 公司負(fù)責(zé)出版 羅伯托特提教授同行評議 DOI 10 1016 j procir 2013 09 052 第二章 微注射成型 MIM 微注射成型 1 是在制造世界一個相對較新的技術(shù) 因此 它需要被深入研究調(diào)查 據(jù) Liu 等人 2 進行微粉末注射成型 因為它在許多不同的領(lǐng)域 例如醫(yī)學(xué) 光學(xué)和電信 成 2 功的應(yīng)用 使得微系統(tǒng)技術(shù)被廣泛使用在新的 21 世紀(jì) 帶有大批量生產(chǎn)能力和低元件成本 使得 MIM 技術(shù)是進行微制造中的一個關(guān)鍵生產(chǎn)工序 MIM 的組件分為以下兩個類別之一 A 型 外形尺寸小于 1mm B 型 微特征小于 200 由 Sha 等人 3 在美國 DOE 進行初步工作和 MIM 的數(shù)據(jù)分析 主要集中在 5 個不同的受 三個不同的聚合物材料可達(dá)到的高寬比影響的因素 熔體和模具溫度 注射速度 壓力和流 動狀態(tài) 的分析 本實驗縱橫比是一個特殊設(shè)計的微特征 其為較長尺寸與較短尺寸的的比 率 他們的研究結(jié)論是 熔體溫度 TB 和注射速度 六 是受在復(fù)制所有三種聚合物材料 的微觀特性中可達(dá)到的長寬比的影響的關(guān)鍵因素 由 Griffiths 等人 4 進行的 MIM 工具的表面質(zhì)量效果主要集中于影響熔體流動和模具 表面之間的流動行為 并相互作用的因素 這些早期的調(diào)查結(jié)果都考慮到了這項研究 圖 1 示出了 MIM 型機的畫面 DOE 的規(guī)劃和數(shù)據(jù)分析使用的統(tǒng)計軟件包 Minitab 16 進行 圖 1 微型注塑機 5 第三章 設(shè)計實驗 DOE 在實驗中定義和調(diào)查所有可能的條件涉及多重因素的技術(shù)被稱為實驗的設(shè)計 這兩種 DOE 類型被廣泛采用是析因設(shè)計與田口方法 根據(jù)實驗 Minitab 的設(shè)計 6 析 因設(shè)計是一種設(shè)計的實驗 允許同時影響研究 一些因素可能對產(chǎn)生同一個影響結(jié)果 當(dāng)進 行實驗 不同的所有因素的水平同步 而不是一次一個 允許相互作用的因子的研究 3 在全面析因?qū)嶒?響應(yīng)于實驗因子水平的所有組合計算 因子水平的組合代表了在響應(yīng) 將被測量的條件 每個實驗條件稱為運行和響應(yīng)測量觀察 整組運行的是 設(shè)計 為了最大限度地減少時間和成本 因此能夠排除一些因子水平的組合 因子設(shè)計中 一 個或多個電平組合被排除被稱為部分因子設(shè)計 有用的部分因子設(shè)計的因素中篩選出來 因為它們減少運行次數(shù)以達(dá)到可管理的大小 被執(zhí)行的運行是一個選擇的子集或完全析因設(shè)計的一小部分 但 Roy 7 提到 使用全因子 和部分因子能源部可能會導(dǎo)致以下問題 實驗在成本和時間變量的數(shù)目是大的而變得笨拙 兩種設(shè)計為相同的實驗可能會產(chǎn)生不同的結(jié)果 這些設(shè)計通常不允許確定各因素的貢獻 實驗 用的大量因素的解釋可能是相當(dāng)困難的 因此 田口方法 以克服這些問題被開發(fā)了 田口方法是定義和調(diào)查所有可能的條件中 涉及到多個因素的實驗技術(shù) 田口方法首先由田口玄一博士在第二次世界大戰(zhàn) 8 9 后提出 他想出了三個基本概念 7 1 質(zhì)量應(yīng)該設(shè)計到產(chǎn)品中 而不是檢查了進去 2 質(zhì)量最好通過最小化從一個目標(biāo) 的偏差來實現(xiàn) 本產(chǎn)品應(yīng)設(shè)計成使得它是免疫不可控的環(huán)境因素 3 質(zhì)量成本應(yīng)作為衡量 偏離標(biāo)準(zhǔn)的函數(shù)和損失應(yīng)該是衡量整個系統(tǒng)的函數(shù) 田口博士建立了一個三階段的過程 實現(xiàn)產(chǎn)品質(zhì)量的依據(jù)上述概念的增強 DOE 即系 統(tǒng)設(shè)計 參數(shù)設(shè)計和容差設(shè)計 在第一階段 系統(tǒng)設(shè)計是確定的設(shè)計因素的合適的工作水平 它包括設(shè)計 并根據(jù)選定 的材料 零件和標(biāo)稱產(chǎn)品 工藝參數(shù)的系統(tǒng)測試 參數(shù)設(shè)計是一個尋找可以實現(xiàn)產(chǎn)品 過程的最佳性能的因子水平 公差設(shè)計的最后階段是降低其顯著影響產(chǎn)品 工藝因素的耐受性 構(gòu)建一組特殊的陣列稱為正交陣列 OAS 奠定了實驗 在 OA 簡化了實驗設(shè)計過程 它是通過選擇最合適的 OA 完成的 分配的因素 以適當(dāng)?shù)牧胁⒚枋龇Q為試驗條件的個別實 驗的組合 在這項研究中 一個部分因子 DOE 與 Taguch 的設(shè)計理念為提高質(zhì)量相結(jié)合進行 第四章 實驗數(shù)據(jù)收集 該實驗由沙等人 10 所定義的來設(shè)計和設(shè)置 該實驗的目的是分析六個可實現(xiàn)的高寬比 的因素影響 并找到最顯著因素 以達(dá)到給予最高的長寬比的最佳的設(shè)置 圖 2 示出了測試 4 微特征的一部分和腿具的有兩個水平寬度 W 200 或 500 微米 和深度 D 70 D1 或 100 D2 微米的形式 其中具有相同深度的特征 D1 或 D2 分別組成上部分的一側(cè) 上 圖 2 能源部測試部分 三種不同的材料 即 半結(jié)晶聚合物 如聚丙烯 PP 聚甲醛 POM 和無定形聚合 物 如丙烯腈 丁二烯 苯乙烯 ABS 是在本研究中 調(diào)查的參數(shù)為料筒溫度 TB 模具溫度 Tm 注射速度 V 保壓壓力 PH 空氣疏散 VA 的存在和微腿寬度 W 縱橫比 即 微特征和它們的深度的長度之間的比率 D1 或 D2 是在實驗過程中測定 具有相同的 W 和 D 2 每部分 同時施加于表 1 中給出的過程設(shè)置 24 次的測量的響應(yīng)的 平均值被用于本研究 表 1 2 DOE 二級 MIM 工藝參數(shù) MIM 工藝參數(shù)和 DoE 水平 聚合物 級別 鋱 C C m 毫 米 秒 W 微 米 PP 1 200 35 50 No No 250 5 2 225 50 100 Yes Yes 500 1 180 35 50 No No 250POM 2 200 60 100 Yes Yes 500 1 248 60 50 No No 250ABS 2 258 75 100 Yes Yes 500 第五章 實驗結(jié)果與數(shù)據(jù)分析 在這個實驗中應(yīng)用一個 2 級六個因素部分因子設(shè)計 26 2 DOE 被用來確定處于活動 狀態(tài)的顯著因素 并研究微流道的填充因子 這個練習(xí)的目的是看 DOE 響應(yīng)的結(jié)果以了解 該過程 然后選擇顯著因素及其達(dá)最佳性能所必需的相應(yīng)的設(shè)置 5 1 結(jié)果 這是 DOE 測定實驗熔體填充的長度和通道的深度之間的比率的的反應(yīng) 或 被記錄D1 2 在表 2 中 和 上表中所示的值是 24 次測量的平均值的值 D1 2 表 2 為 2 級 MIM 工藝參數(shù)的實驗結(jié)果 MIM 工藝參數(shù) PP POM ABS運 行 試 驗 編 號 T T V P V W D1 D2 D1 D2 D1 D2 1 1 1 1 1 1 1 4 9 2 4 0 5 8 2 2 1 1 1 2 1 6 13 4 5 4 7 3 1 2 1 1 2 2 7 15 4 6 5 17 4 2 1 1 1 2 8 20 6 12 6 19 5 1 1 2 1 2 2 11 20 1 5 6 20 6 2 1 2 1 1 2 17 18 6 12 7 20 6 7 1 2 2 1 1 1 10 18 3 6 6 19 8 2 2 2 1 2 1 15 20 6 14 7 20 9 1 1 1 2 1 2 7 11 3 4 3 5 18 10 2 1 1 2 2 2 7 19 4 5 5 20 11 1 2 1 2 2 1 5 10 3 5 0 8 8 12 2 2 1 2 1 1 7 14 5 8 1 2 9 13 1 1 2 2 2 1 9 16 4 6 6 18 14 2 1 2 2 1 1 12 20 5 11 7 5 20 15 1 2 2 2 1 2 11 20 5 11 7 20 16 2 2 2 2 2 2 17 20 8 16 7 5 19 5 2 數(shù)據(jù)分析 統(tǒng)計軟件包 Minitab16 是用來分析從實驗獲得的結(jié)果 該分析用于在 D1 和 D2 兩種情 況下 PP 的結(jié)果 如表 3 所示 表 3 估計效果和 PP D1 數(shù)據(jù)的 DOE 系數(shù) 術(shù)語 效果 系數(shù) 系數(shù)標(biāo) 準(zhǔn)誤差 T P T 3 125 1 5625 0 3125 5 00 0 038 T 0 8750 0 4375 0 3125 1 40 0 296 V 6 375 3 1875 0 3125 10 20 0 009 P 0 3750 0 1875 0 3125 0 60 0 609 V 0 1250 0 0625 0 3125 0 20 0 86 W 2 1250 1 0625 0 3125 3 40 0 077 單 因 素 T T 0 3750 0 1875 0 3125 0 60 0 609 T V 1 8750 0 9375 0 3125 3 00 0 095 T P 0 3750 0 1875 0 3125 0 60 0 609 T V 0 1250 0 0625 0 3125 0 20 0 860 相 互 7 T W 0 1250 0 0625 0 3125 0 20 0 860 T P 0 3750 0 1875 0 3125 0 60 0 609 T W 0 6250 0 3125 0 3125 1 00 0 423 作 用 第六章 結(jié)果討論 上述結(jié)果分別用于生產(chǎn)更多的證據(jù)來支讓 MIM 工藝因素的技術(shù)支持 使用 0 05 適用于 PP 發(fā)現(xiàn) 值是 0 038 和 為 0 009 表明 這兩個單因素 和 D1 T V T 是顯著主要影響 即它們的 p 值小于 0 05 這兩個單因素 其作用和其它計算值在表 3 中V 顯示 此外 上述結(jié)果表明 沒有一個雙向的交互是顯著的 這顯然是受了 標(biāo)準(zhǔn)化效應(yīng)正 態(tài)圖 圖 3 和 帕累托圖 theStandardized 的影響 圖 4 所示 圖 3 對 PP 的正常影響D1 8 圖 4 用于 PP 帕累托圖D1 6 1 正常效果圖 一個正常的效果圖用于比較相對大小和主 交互效應(yīng)的統(tǒng)計顯著性 如圖 3 Minitab 中繪 制一條直線來指示該點預(yù)計將下降 如果所有的效果都接近于零 不屬于直線附近的 點 通常有顯著信號因素的作用 這樣較大的效果一般去進一步遠(yuǎn)離擬合直線相比不重要的影響 默認(rèn)情況下 Minitab 中使用 0 05 和標(biāo)簽效果顯著 因子 C 和 A 明確標(biāo)示標(biāo)簽的示于圖 3 這是通過在 MIM 工藝對 PP 具有更大的權(quán)重的系數(shù) C 相比 在該圖中可以看到系數(shù)D1 a 6 2 帕累托圖 帕累托圖的作用是用來比較相對大小和主 交互效應(yīng)的統(tǒng)計顯著性 如圖 4 Minitab 繪制以絕對值的因素影響遞減順序的 圖表上的參考線指示哪些因素影響顯著 當(dāng)你的模型 中包含的誤差項 默認(rèn)情況下 Minitab 中使用 0 05 繪制參考線 在圖 3 的結(jié)果確認(rèn)圖 4 中顯示的結(jié)果為因子 C 和 A 是已通過參考線僅有的兩個因素的影響 并且因子 C 比因子 A 具有更大的影響 6 3 主效應(yīng)圖 在分析中的下一個步驟是看的顯著相互作用 表 3 計算的雙向互動效應(yīng) 可以直觀地顯 示在交互作用圖 看看這些影響有多大 交互作用圖顯示了兩個可疑的相互作用的因素 改 變一個因子的設(shè)置對另一個因子的影響 因為交互可以放大或減小主效應(yīng) 即取決于相互作 用是否是正或負(fù) 評估相互作用是極其重要的 而接近平行線表示因子之間很少或沒有相互 9 作用 相交線信號的交互 交互量是成正比的交角 即接近 90 表達(dá)了強烈的相互作用 在圖 6 中的交互作用圖顯示 即 在 100 的高寬比 在 50 更在兩個 同 級別 的 T 響 應(yīng) V V 高 但是 可以看出 使用 在 100 運行和使用 在 50 運行其響應(yīng)差的差比T 設(shè) 置 為 225 V V 使用 在 100 運行和使用 在 50 運行的縱橫比差別更大 這表明 以獲得最高T 設(shè) 置 為 200 V V 的長寬比應(yīng)定為 225 而 保持在 100 V 圖 6 PP 交互作用圖D1 這項研究表明 除了在聚甲醛 ABS 和 ABS 用的雙向互動 在大多數(shù)情況下 2 1 2 縱橫比是通過單因素的影響 對于 PP 只在 PP 和 Vi 的情況下 對于 POM 2 i 1 和 W 和對于 POM W 和 X 當(dāng) ABS 用于 中的影響 1 T m i 2 m i b i 1 因素分別為 W 和 X 對于 的顯著因素 W 和 X 在表 4 中以粗體顯示的T i m 2 i m 條目指示所選設(shè)置的顯著因素 陰影部分在表 4 中示出的因素之間的雙向交互 使用消除過程中的關(guān)鍵因素的 PP 被確定為機筒溫度 和噴射速度 對于聚 i 甲醛為機筒溫度 模具溫度 噴射速度 和寬度 W 以及 ABS 為機筒溫 m i 度 的 噴射速度 和寬度 W 與模具溫度 固定在 75 因此該因素保持 i m 壓力 PH 和空氣排出的存在 Va 能在 MIM 工藝被忽略 這給出了 4 項試驗適用于 PP 16 項試驗的聚甲醛和 8 個試驗的 ABS 全階乘 另外 作為本研究的結(jié)果是 最優(yōu)設(shè) 置 為使用不同的材料實現(xiàn)最高的比率方面可以被概括如下 PP 在 225 和六 100 1 PP 為 100 2 i 10 POM 200 為 60 在 100 和 W 為 500 1 m i POM 除了 W 同為 2 1 ABS TB 為 258 六 100 W500 而 1 是固定在 75 m ABS 100 W500 而 為固定在 75 2 i m 驗證試驗中進行驗證為已選定的理論上和重復(fù) 24 次平均測得的反應(yīng) 得到最好的縱橫 比迄今發(fā)現(xiàn)上述設(shè)定的最佳性能 它們?nèi)缦?對聚丙烯和聚甲醛 20 的最佳縱橫比和 21A 的 BS 第七章 結(jié)論 在本文中已被提出對于理解 MIM 工藝和利用 DOE 的工藝參數(shù)的分析方法優(yōu)化 已經(jīng)進 行一個部分因子實驗 Taguch 的質(zhì)量概念以節(jié)省時間和精力進行判斷 在測量的響應(yīng)的形式 收集的數(shù)據(jù)已被成功地分析 以確定顯著單因素以及雙向的相互作用 進一步 在研究中通 過 DOE DoE 方法使用不同的材料所確定最佳工藝參數(shù)設(shè)置已經(jīng)由運行試驗驗證和測量 以符合 MIM 工藝參數(shù)的最佳設(shè)定值實現(xiàn)的高寬比的響應(yīng)驗證了理論結(jié)果 通過這項研究的 MIM 獲得的知識將有助于理解和優(yōu)化納米注射成型 NIM 的過程 11 致謝 感謝歐盟 FP7 FlexiTool 項目支持這項工作 文獻 1 Trotta G Surace R Modica F Spina R Fassi I 2011 聚合物組分 AIP 機密的微注 射成型 PROC 2011 1315 1273 8 2 Liu ZY Loh NH Tor SB Khor KA Murakoshi Y Maeda R Shimizu T 2002年 微 粉末注射成型 材料加工技術(shù)2002 127 2 中 p 165 3 Sha B Dimov S Griffiths C Packianather MS 2007年 微型注塑調(diào)查 影響因素 復(fù)制質(zhì)量 材料加工技術(shù)2007 183 頁 284 4 Griffiths CA Dimov SS Brousseau EB Hoyle RT 2007 工具表面質(zhì)量的微注射 11 成型的效果 材料加工技術(shù)2007 189 1 號碼 418 5 Griffiths CA Dimov SS Brousseau EB Chouquet C Gavillet J Bigot S 2010年 調(diào)查微注射成型 詮釋J先進制造業(yè)技術(shù)2010 47 1 號碼 99 6 Minitab的手冊 第5版 加拿大 柯特Hinrichs先生 2005在田口方法 7 Roy R 1990 入門 美國 范NOSTRAND 萊因霍爾德 1990 8 Sudhakar PR 1995 簡介質(zhì)量改進 通過田口方法 質(zhì)量1995年 第 54 9 Taguchi G 1996 D O E 的角色對于強大的工程 一科芒特里 詮釋J質(zhì)量和1996年可靠性工程 12 號碼 73 10 Sha B Dimov S Griffiths C Packianather MS 2007 顯微注射成型 影響所 能達(dá)到的高寬比的因素 詮釋J高級制造業(yè) 通力2007 33 第147 11 Zhang N Cormac J Byrne CJ Browne DJ Gilchrist MD 2012 邁向納米注塑成型材料今天2012 15 5 第216 M Packianathera F Cha C Griffith S Dimo D T Phan aIMME 英國卡迪夫 CF243AA 游行皇后大廈卡迪夫大工程學(xué)院 英國伯明翰 B152TT 伯明翰大學(xué)機械工程學(xué)院 通訊作者聯(lián)系電話 44 29 20875911 傳真 44 29 20874695 電子郵件 地址 packianatherms cf ac uk 編號 畢業(yè)設(shè)計 論文 外文翻譯 原文 學(xué) 院 機電工程學(xué)院 專 業(yè) 機械設(shè)計制造及其自動化 學(xué)生姓名 韋良華 學(xué) 號 1000110129 指導(dǎo)教師單位 機電工程學(xué)院 姓 名 陳虎城 職 稱 助教 2014 年 5 月 26 日 a r t i c l e i n f o Article history Received 25 October 2010 Received in revised form 12 January 2011 Accepted 14 January 2011 Available online 21 January 2011 Keywords Microcellular injection molding Plastic foaming Swirl free surface a b s t r a c t Microcellular injection molding is the manufacturing method used for producing foamed plastic parts Microcellular injection molding has many advantages including material energy and cost savings as well as enhanced dimensional stability In spite of these advantages this technique has been limited by its propensity to create parts with surface defects such as a rough surface or gas flow marks Methods for improving the surface quality of microcellular plastic parts have been investigated by several researchers This paper describes a novel method for achieving swirl free foamed plastic parts using the microcellular injection molding process By controlling the cell nucleation rate of the polymer gas solution through material formulation and gas concentration microcellular injection molded parts free of surface defects were achieved This paper presents the theoretical background of this approach as well as the experimental results in terms of surface roughness and profile microstructures mechanical properties and dimensional stability Introduction The commercially available microcellular injection molding process a k a the MuCell Process consists of four distinctive steps namely gas dissolution nucleation cell growth and shaping 1 In the gas dissolution stage polymer in the injection barrel is mixed with supercritical fluid SCF nitrogen carbon dioxide or another type of gas using a special screw which is designed to maximize the mixing and dissolving of the gas in the polymer melt During injection a large number of nucleation sites orders of magnitude higher than conventional foaming processes are formed by a rapid and substantial pressure drop as the polymer gas solution is injected into the mold cavity thus causing the formation of cells bubbles During the rest of the injection molding cycle cells continue to grow to fill and pack out the mold and subsequently compensate for the polymer shrinkage as the material cools inside the mold The cell growth is driven by the amount and spatial distribution of the dissolved gas The cell growth is also controlled by processing conditions such as melt pressure and temperature as well as material properties such as melt strength and gas solubility Finally the shaping of the part takes place inside the mold until the mold opens allowing the part to be ejected Since the microcellular injection molding process was invented there have been numerous studies on process material and technical developments aimed at materializing the full process potential According to previous studies 1 5 microcellular injection molding offers a number of advantages such as cost savings weight reduction ease in processing due to low viscosity and outstanding dimensional accuracy Due to these advantages the microcellular injection molding process has been used in many industries such as automotive electrical goods and home appliances using a broad range of thermoplastics Despite these advantages however the surface imperfections associated with microcellular injection molded partsdsuch as unique gas flow marks referred to as swirl marks throughout this paper and a lack of smoothnessdstill remain one of the main drawbacks surrounding microcellular injection molding In order to eliminate or reduce these surface imperfections there have been several studies attempted as reported in Refs 6 14 Some researchers have focused on temperature modification of the mold surface to improve the surface quality of microcellular injection molded parts 6 8 With polymeric foam it was found that bubbles forming at the advancing melt front are first stretched by the fountain flow behavior toward the mold surface and subsequently dragged against the mold wall causing swirl marks 9 During the filling stage of polymer melts keeping the mold wall temperature high enough for bubbles at the mold surface to beeliminated improves the surface quality of microcellular injection molded parts By controlling the mold temperature rapidly and precisely using mold temperature control units or other kinds of thermal or surface heating devices microcellular foamed plastics with glossy and swirl free surfaces can be produced There have also been efforts to eliminate the swirl marks on microcellular injection molded parts without any mold temperature controller In particular it was proposed that inserting an insulator onto the mold wall might help keeping the interface temperature between the mold and the polymer melt high This technique basically yields the same result as temperature modification of the mold 10 Thermal analysis and experimental results prove that the addition of an insulator layer on the mold can improve the surface quality of microcellular injection parts 11 Another method of producing parts with an improved surface quality leads to a microcellular co injection molding process 12 In this technique a proper amount of solid skin material is injected prior to the injection of a foaming core material This can yield a sandwiched solid skinefoamed coreesolid skin structure with a surface finish similar to a conventionally molded component while partially maintaining the advantages of microcellular injection molding Another approach for improving the surface quality of microcellular injection molded parts is the gas counter pressure process 13 14 In this process a high pressure gas is injected into the mold prior to the polymer gas solution to suppress cell nucleation and bubble growth while the polymer gas solution is being injected into the mold cavity Toward the end of injection counter gas pressure is released and bubbles begin to form within the cavity Since a majority of the part surface is already solidified gas flow marks are eliminated In spite of these efforts to improve the surface quality there have been difficulties in applying the microcellular injection molding process in industries requiring parts with high surface qualities because these techniques entail additional equipment which results in high costs or maintenance There have been no reported studies on improving the surface quality of microcellular injection molded parts without any additional equipment or modification to existing equipment This paper proposes a novel approach to improve the surface quality of microcellular injection molded parts by controlling the cell nucleation rate In this study the cell nucleation rate was dramatically lowered or delayed by controlling the degree of supersaturation so that cell nucleation was delayed during the filling stage After the polymer gas solution volumetrically filled the mold cavity intentionally delayed nucleation occurred and bubbles formed in the polymer matrix except on the surface where the material had already solidified upon touching the mold surface Theoretical background and experimental results are described in this paper Microstructure surface profile surface roughness mechanical properties and dimensional stability are also investigated in this study 2 Theoretical 2 1 Nucleation theory for polymeric foams In polymeric foams nucleation refers to the initial stage of the formation of gas bubbles in the polymeregas solution For nucleation gas bubbles must overcome the free energy barrier before they can survive and grow to macroscopic size 15 According to classical nucleation theories 16 18 the nucleation rate is controlled by the macroscopic properties and states of the polymer and gas such as solubility diffusivity surface tension gas concentration temperature and the degree of super saturation One representative equation for the nucleation rate of polymeric foams was reported by Colton and Suh 19 20 In addition to the mathematical representation they also verified their nucleation theory experimentally for a batch foaming process using a high pressure vessel The nucleation equation for microcellular foams dominated by the classical nucleation theory 16e18 can be expressed as where N is the nucleation rate f is the frequency of atomic molecular lattice vibration C is the concentration of gas molecules k is the Boltzmann s constant T is the absolute temperature and is the activation energy barrier for nucleation According to previous studies 19 20 the nucleation rate of polymeric foams is composed of two components a homogeneous term and a heterogeneous term The activation energy for homogeneous nucleation is given by 16 33 2 where g is the surface energy of the bubble interface and is assumed to be the gas saturation pressure More precisely where Pr is the pressure that is exerted in a high P pressure vessel and Pr is the pressure of the supersaturated vapor in the sample 16 That is DP is the pressure difference between the pressure that is applied to the sample and the pressure of the supersaturated vapor in the sample When the pressure that saturates the gas in a high pressure vessel is suddenly released to trigger the so called thermodynamic instability by rendering the sample into the supersaturated state Pr becomes 1 bardso low compared to Pr that DP can be approximated as Pr On the other hand the activation energy for heterogeneous nucleation is affected by a geometric factor that depends on the contact wetting angle between the polymer and the particle and can be expressed as 3a 3b 12 34cos 14cos 3 where f q is a geometric factor that is dependent upon the contact angle of the interface between the polymer and a second phase and has values of less than or equal to 1 For a typical wetting angle of around on the interface between a solid particle and the polymer melt the 200 geometric factor is 2 7X suggesting that the energy barrier for heterogeneous 10 3 nucleation can be reduced by three orders of magnitude with the presence of an interface 20 21 2 2 Nucleation theory for microcellular injection molding In the batch foaming process the theory of Colton and Suh was verified by their experiments Due to the large difference between the pressure exerted in a high pressure vessel and the pressure of the supersaturated vapor in the sample the gas pressure dissolved in the polymer the in the Gibbs free energy equation can be P approximately assumed to be the saturation gas pressure The assumption that is P the gas saturation pressure is fairly reasonable in a batch foaming process although the can still have an error of about 30 40 due to overestimation as reported in a P previous study 15 The nucleation theory by Colton and Suh is a simplified form derived and modified from classic nucleation theories 16 18 and is generally adequate for the batch foaming process However there is a need for this theory to be modified in cases of microcellular injection molding and extrusion systems because cannot be directly P controlled and measured To predict nucleation in microcellular injection molding and extrusion processes more precisely this paper proposes a cell nucleation theory of a different form which includes a term for the degree of supersaturation because it is a directly controllable factor To avoid misestimating and to consider the degree of supersaturation a more P proper activation energy equation for nucleation can be derived from the following equation 16 17 4 P 2 where is the radius of a characteristic droplet and the W Thomson equation 5 ln 2 where is the pressure of the saturated vapor i e the equilibrium pressure R is the universal gas constant M is the molar mass and is the density These equations yield 6 ln which can be alternatively expressed as 7 1ln where is the molecular density of the bulk liquid and S 1 is defined as the degree of supersaturation Thus the activation energy equation cf Equation 2 for nucleation in the microcellular injection molding process can be given by 8 16 33 1ln 2 Hence it can be stated that the activation energy for nucleation is inversely proportional to the square of the natural logarithm of the supersaturation degree In the microcellular injection molding process the polymer gas solution becomes a metastable supersaturation solution when it is injected into the mold cavity This is because the amount of gas able to be dissolved in the polymer in the presence of a rapid pressure drop is less than the gas amount originally dissolved in polymer melts In particular assuming the air in the cavity is properly vented the pressure at the advancing melt front is at the atmospheric pressure The solubility of a gas in a polymer at atmospheric pressure and processing temperature can be obtained by an Arrhenius type expression with regard to temperature 22 9 1 1 1298 where is the solubility of the gas in the polymer at standard temperature and pressure conditions 298 K and 1 atm The parameter DHs is the molar heat of sorption and Tmelt is the polymer melt temperature Thus the degree of supersaturation is given by 10 1 1298 where is the gas dosage which can be controlled by the supercritical fluid SCF supply system The heat of sorption of various polymer gas systems at standard temperature has been studied and summarized in many previously published studies In order to obtain the degree of supersaturation for a polymer gas solution in the microcellular injection molding process one has to either measure the solubility of the gas in the polymer at standard temperature and pressure or consult published data on the solubility of the gas in the polymer Then the activation energy barrier for nucleation in Equation 8 G can be obtained based on the calculated degree of supersaturation and the surface energy of the bubble interface Given the activation energy barrier and the frequency factor f the nucleation rate expressed in Equation 1 can then be calculated The estimate of the surface energy of the bubble interface and the frequency factor is discussed below In microcellular injection molding the polymer gas solution can be treated as a liquid mixture Thus the surface energy of the bubble interface g can be expressed as 23 24 11 4 1 where is the surface energy of the polymer are the densities and is the weight fraction of gas In addition a frequency factor for a gas molecule f in Eq 1 can be expressed as 24 26 12 4 2 where z is the Zeldovich factor which accounts for the many clusters that have reached the critical size but are still unable to grow to sustainable bubbles The parameter b is the impingement rate at which gas molecules collide with the wall of a cluster The parameter can be used as a correction factor and is determined experimentally Once the nucleation rate as a function of the degree of supersaturation is obtained one can control the gas SCF content in the polymer melt to control or delay the onset of cell nucleation so that no bubble will form at the advancing melt front during the injection filling stage thus allowing microcellular parts with solid swirl free surface to be injection molded 3 Experimental 3 1 Materials The material used in this study was an injection molding grade low density polyethylene LDPE Chevron Phillips Chemical Company Texas USA It has a melt index of 25 g 10 min and a density of 0 925 g 3 To confirm the theory for improving surface quality by controlling the degree of supersaturation a random copolymer polypropylene PP was also used in this study The PP used in this study was Titanpro SM668 Titan Chemicals Corp Malaysia with a melt flow index of 20 g 10 min and a density of 0 9 g Both 3 materials were used as received without any colorant fillers or additives Commercial grade nitrogen was used as a physical blowing agent for the microcellular injection molding trials 3 2 Microcellular injection molding In this study an Arburg 320S injection molding machine Arburg Germany was used for both the solid conventional and microcellular injection molding experiments The supercritical fluid SCF supply system used in this study was the S11 TR3 model Trexel Woburn MA USA The total gas dosagewas controlled by adjusting the gas injection time t and the gas injection flowrate m g A tensile test mold which produces tensile test specimens that meet the ASTM D638 Type I standards was used for this experiment For injectionmolding of both LDPE and PP tensile test specimens nozzle and mold temperatures were set at 221 and 25 respectively The cycle time was 40 s An injection speed of 80 cm3 s was employed In this study six different gas dosages concentrations were used for injection molding of LDPE as shown in Table 1 Also four different gas dosages were employed for microcellular injection molding of PP The supercritical fluid was injected into the injection barrel at 140 bar pressure to be mixed with the polymer melts in this experiment The weight reduction of foamed versus solid plastic partswas targeted at 8 0 5 for each specimen For the conventional injectionmolding experiment the shot size of 20 2 and a packing pressure of 800 bars were employed for 6 s For the microcellular 3 injection molding experiments the shot size of the polymer melt was 19 2 and 3 the packing stage was eliminated 3 3 Analysis methods To analyze the surface roughness of the molded tensile bar specimens a Federal Surfanalyzer 4000 Federal Product Corporation RI USA was used The surface roughnesses of conventional and microcellular injection molded parts were evaluated at three locations shown in Fig 1 and the averaged surface roughness based on measurementsdone at all three locationswas recordedandreported The cutoff drive speed and drive length for the test were 0 75 mm 2 5 mm s and 25 mm respectively For each process condition ten specimens and three points on each specimen were tested In addition to the surface roughness swirl marks commonly observed in microcellular injection molded samples can also be clearly revealed by a 3 D surface profiler Zygo NewView Zygo Corporation CT USA a non contact 3 D surface profiler was employed to examine the surface profile of injection molded parts in this study using a scan distance of 10 mm A JEOL JSM 6100 scanning electron microscope with an accelerating voltage of 15 kV was employed for observing the microstructures of the foamed parts To observe the cross section of the microcellular injection molded parts test specimens were frozen by liquid nitrogen and subsequently fractured Representative images of each process condition were selected and cell sizes and densities were analyzed A UTHSCSA Image Tool was employed as the image analysis software to evaluate cell densities and sizes A MTS Sintech 10GL screw driven machine was used to test the mechanical properties of the molded specimens including the yield stress strain at break modulus and ultimate stress Ten specimens for each condition were tested The tensile test speed was 50 8 mm min The schematic of the ASTM tensile bar and locations of the various analyses are shown in Fig 1 To test the dimensional stability of the injection molded specimens a dial caliper made by Mitutoyo was used Dimensions of the mold cavity were first measured and then the injection molded parts were measured and compared with the actual dimensions of the mold cavity 4 Results and discussion 4 1 Surface profile and roughness measurement As a visual illustration Fig 2 shows the representative injection molded low density polyethylene LDPE parts To better reveal the surface quality in the photo 5wt colorant was added to the material although the same surface quality was obtained without colorant As anticipated the conventional solid injection molded part Fig 2 a has a glossy and flawless surface On the other hand the typical microcellular injection molded part Fig 2 b produced with a moderate or high gas concentration has a lusterless surface due to s