車窗扣座注塑模具設(shè)計【塑料螺釘】

車窗扣座注塑模具設(shè)計【塑料螺釘】,塑料螺釘,車窗,注塑,模具設(shè)計,塑料,螺釘 模具畢業(yè)設(shè)計說明書 設(shè) 計 題 目 ：車窗扣座零件模具畢業(yè)設(shè)計說明書 設(shè) 計 者： 班 級： 指 導(dǎo) 教 師： 摘 要 論文根據(jù)工程實際的需要完成車窗扣座的注射模設(shè)計。在設(shè)計中采用塑料注射成型論文中具體分析了產(chǎn)品的工藝性,確定了所采用塑料的工藝參數(shù)和所采用的成型設(shè)備,確定了模具制作的總體方案,分析并解決了模具的總體結(jié)構(gòu)和各工作部分的具體結(jié)構(gòu),并進行了一些必要的尺寸計算和強度的校核。論文中還對分型面、澆注系統(tǒng)、脫模機構(gòu)和溫度調(diào)節(jié)系統(tǒng)進行了分析設(shè)計，完成了工件工程圖設(shè)計，圓滿完成了模具設(shè)計所要求的各項工作。 本文中針對車窗扣座注射模具制定出合理的設(shè)計結(jié)構(gòu)，其中包括成型部分及其零部件設(shè)計，澆注系統(tǒng)設(shè)計，脫模機構(gòu)設(shè)計，冷卻系統(tǒng)設(shè)計等。根據(jù)分析，設(shè)計了一套塑料注射模具，并對模具以及主要零件進行了CAD繪圖。 關(guān)鍵字：注射模具，澆注系統(tǒng)，脫模機構(gòu)，冷卻系統(tǒng) Abstract This paper according to the actual need to complete the design of the injection mould for the window fastener seat. Used in the design of plastic injection molding in the specific analysis of the process of product, the process parameters of plastic and forming equipment used to determine the overall scheme was determined, mold making, analysis and solve the specific structure of the mold overall structure and each part of the work, and check the necessary size calculation and strength. Also, the paper surface, gating system, demoulding mechanism and temperature control system analysis and design, completed the engineering design, the successful completion of the work required by the die design. In this paper, in view of the window button seat injection mold the development of a reasonable design structure, including molding parts and components design, gating system design, demould mechanism design, the design of the cooling system. According to the analysis, a set of plastic injection mold design and mold, and the main parts of the CAD drawing. Keywords: ejection mechanism of injection mould, gating system, cooling system 目 錄 摘 要 II Abstract III 目 錄 IV 第1章 前言 1 第2章 塑件的工藝分析 2 2.1塑件的工藝性分析 2 2.2塑件的結(jié)構(gòu)和尺寸精度及表面質(zhì)量分析 3 2.2.1結(jié)構(gòu)分析 3 2.2.2尺寸精度分析 3 2.2.3表面質(zhì)量分析 4 2.3 注射機的初選 4 第3章 分型面選擇和澆注系統(tǒng)設(shè)計 6 3.1 注射模具分型面的選擇 6 3.1.1 分型面的基本形式 6 3.1.2 分型面選擇的基本原則 6 3.1.3 分型面的選擇 6 3.2 澆注系統(tǒng)的設(shè)計 7 3.2.1 澆注系統(tǒng)的組成 7 3.2.2 注射模具主流道的設(shè)計 7 3.2.3 分流道的設(shè)計 9 第4章 成型零件的設(shè)計 12 4.1 模具型腔的結(jié)構(gòu)設(shè)計 12 4.2 型芯的結(jié)構(gòu)設(shè)計 13 4.3 成型零件的尺寸確定 13 第5章 頂出機構(gòu)的設(shè)計 18 第6章 冷卻系統(tǒng)的設(shè)計 18 第7章 排氣系統(tǒng) 20 第8章 成型設(shè)備有關(guān)參數(shù)校核 20 第9章 模具特點和工作原理 21 總 結(jié) 22 參考文獻(xiàn) 23 第1章 前言 先進制造技術(shù)的發(fā)展使人們不再單純地依賴產(chǎn)品圖或產(chǎn)品樣件來設(shè)計制作模具,逆向工程技術(shù)的應(yīng)用使產(chǎn)品的圖片、照片或影像資料,甚至產(chǎn)品模具本身,都可以作為模具的設(shè)計依據(jù)。逆向工程技術(shù)特別在消化、吸收國外先進模具技術(shù)方面具有突出的優(yōu)勢, 由此還帶來設(shè)計思路上的變化,有時可以先設(shè)計模具型腔,然后據(jù)此再完善產(chǎn)品設(shè)計圖樣[1]。 塑料制品的成型是塑料成為具有實用價值制品的重要環(huán)節(jié)。塑料成型方法已達(dá)40多種。其中最重要的是注射，擠出，吹塑和壓制等。它們幾乎占了整個塑料成型的85%；其中注射尤為突出，占塑料成型的30%以上。注射模具成形是熱塑性塑料成型的一種方法，幾乎所有的熱塑性塑料都可以用此方法成型，有些熱固性塑料也可以用注射模塑成型。 23 第2章 塑件的工藝分析 該塑件是車窗扣座產(chǎn)品，其零件圖如圖所示。本塑件的材料采用ABS，生產(chǎn)類型為大批量生產(chǎn)。 圖2.1 車窗扣座圖 2.1塑件的工藝性分析 該材料為ABS，ABS樹脂是目前產(chǎn)量最大，應(yīng)用最廣泛的聚合物，它將PS，SAN，BS的各種性能有機地統(tǒng)一起來，兼具韌，硬，剛相均衡的優(yōu)良力學(xué)性能。ABS是丙烯腈、丁二烯和苯乙烯的三元共聚物，A代表丙烯腈，B代表丁二烯，S代表苯乙烯。 ABS塑料-名稱 化學(xué)名稱 丙烯腈-丁二烯-苯乙烯塑料 英文名稱 Acrylonitrile Butadiene Styrene plastic 一般性能 ABS外觀為不透明呈象牙色粒料，其制品可著成五顏六色，并具有高光澤度。ABS相對密度為1.05左右，吸水率低。ABS同其他材料的結(jié)合性好，易于表面印刷、涂層和鍍層處理。ABS的氧指數(shù)為18～20，屬易燃聚合物，火焰呈黃色，有黑煙，并發(fā)出特殊的臭味。 力學(xué)性能 ABS有優(yōu)良的力學(xué)性能，其沖擊強度極好，可以在極低的溫度下使用；ABS的耐磨性優(yōu)良，尺寸穩(wěn)定性好，又具有耐油性，可用于中等載荷和轉(zhuǎn)速下的軸承。ABS的耐蠕變性比PSF及PC大，但比PA及POM小。ABS的彎曲強度和壓縮強度屬塑料中較差的。ABS的力學(xué)性能受溫度的影響較大。 熱學(xué)性能 ABS的熱變形溫度為93~118℃，制品經(jīng)退火處理后還可提高10℃左右。ABS在-40℃時仍能表現(xiàn)出一定的韌性，可在-40~100℃的溫度范圍內(nèi)使用。 電學(xué)性能 ABS的電絕緣性較好，并且?guī)缀醪皇軠囟取穸群皖l率的影響，可在大多數(shù)環(huán)境下使用。 分析塑件的結(jié)構(gòu)工藝性塑件尺寸較小，內(nèi)部結(jié)構(gòu)簡單，對塑件的測量和計算沒較大影響，符合塑件的設(shè)計要求。 塑件精度要求,塑件工作要求不高，故選普通精度：４級 塑件表面質(zhì)量分析 該塑件要求外形美觀，外表面沒有斑點及熔接痕，而塑件內(nèi)部沒有較高的表面粗糙度要求。 塑件的結(jié)構(gòu)工藝性分析 ①該塑件的外形為長方體。壁厚均勻，且符合最小壁厚要求。 ② 塑件型腔很大，有尺寸不等的孔，它們均符合最小孔徑要求。 綜上所述，該塑件可采用注射成型加工。 2.2塑件的結(jié)構(gòu)和尺寸精度及表面質(zhì)量分析 2.2.1結(jié)構(gòu)分析 從零件圖上分析，該零件總體形狀為長方形。因此,模具設(shè)計,該零件屬于中等復(fù)雜程度. 2.2.2尺寸精度分析 從塑件的壁厚上來看,壁厚最大處為3.5mm,壁厚均勻,，在制件的轉(zhuǎn)角處設(shè)計圓角，防止在此處出現(xiàn)缺陷，由于制件的尺尺寸中等。 2.2.3表面質(zhì)量分析 該零件的表面除要求沒有缺陷﹑毛刺，內(nèi)部不得有雜質(zhì)外，沒有什么特別的表面質(zhì)量要求，故比較容易實現(xiàn)。 綜上分析可以看出,注塑時在工藝控制得較好的情況下,零件的成型要求可以得到保證. 2.3 注射機的初選 （1）注射容量 國產(chǎn)標(biāo)準(zhǔn)注射機的標(biāo)準(zhǔn)規(guī)定，以注射機注射ABS時在對空注射條件下，注射機螺桿或柱塞做一次最大行程所能達(dá)到的最大容量。由于ABS的密度為1.02-1.05g/cm3，即它的單位容量與單位質(zhì)量向近，所以在目前實際中為便于計算，有時還沿用過去的習(xí)慣，通常也用其質(zhì)量可作粗略計量。 注射容量是選擇注射機的重要參數(shù)，它在一定程度上反映了注射機的注射能力，標(biāo)志著注射機能成型最大體積的塑料制品。 確定了單個塑件的體積（質(zhì)量）和模孔數(shù)量就可以大體上計算出多模塑件的總體積，再加上澆注系統(tǒng)中主流道、分流道、澆口、冷井的體積，即是一模塑料的總體積Vm。 Vm≤0.8Vz 式中 Vm—成型零件與澆注系統(tǒng)體積總和，cm3 ； Vz—注射機最大注射容量，cm3 ； 估算： （2）最大成型面積 最大注射面積是指塑料在模具在分型面上所允許成型的最大投影面積，也就是說在模具設(shè)計時，布局在模具分型面上的塑件及澆注系統(tǒng)的投影面積S，只能小于這個數(shù)據(jù)時才能正?？煽康淖⑸洹? 式中 S—塑料在模具分型面上允許成型的投影面積； （3）模具的閉合高度 注射機動壓板的最大的行程和壓板間最大和最小間距是一個固定的參數(shù)。它決定著所能安裝的模具的閉合高度。對于所用的注射機來說，注射模的閉合高度必須符合下列的要求： H小≤H≤H大 式中 H小—注射機允許的最小厚度，mm，H小=100mm； H —注射機的實際閉合高度，mm； H大—注射機允許的最大厚度，mm，H大=300mm； H=16+32+20+77+10+129+16=300 mm； （4）模具的頂出 注射機的頂出裝置通常有中心頂桿頂出、兩側(cè)頂桿頂出以及液壓頂出幾種形式。應(yīng)在動模座板與注射機頂出位置相對的位置上，設(shè)置稍大于注射機頂桿的通孔，以便于注射機頂桿通過。 （5）定位環(huán)和澆口套 定位環(huán)是將定模部分裝入注射機定壓板的定位對中位置，應(yīng)與注射機的定位孔采取動配合的連接形式，以保證模具體對中。 （6）模具的截面尺寸 可安裝的注射模具外形最大尺寸取決于注射機的壓板尺寸和拉桿的間距，因為此注射模的最長的邊不應(yīng)超過壓板尺寸，而模具的最短邊應(yīng)小于拉桿間距，才能將注射模裝入注射機，并應(yīng)留有固定模體的壓緊空間。同時，注射模動、定模上的緊固螺栓孔，也應(yīng)與注射機壓板上的標(biāo)準(zhǔn)螺孔一致。 綜合考慮上述條件，現(xiàn)決定采用一模兩腔，注射機選擇SZ-60/450臥式注塑機。見表3.1。 表3.1 SZ-60/450注塑機參數(shù)表 理論注射量/㎝3 105 螺桿（柱塞）直徑/㎜ 35 注射壓力/MPa 125 注射速率/(g/s) 75 塑化能力/(g/s) 10 螺桿轉(zhuǎn)速/(r/min) 14―200 鎖模力/kN 450 拉桿內(nèi)間距/㎜ 400*450 移模行程/㎜ 220 最大模具厚度/㎜ 300 最小模具厚度/㎜ 100 鎖模形式 雙曲肘 定位孔直徑/㎜ 55 噴嘴球半徑/㎜ 20 第3章 分型面選擇和澆注系統(tǒng)設(shè)計 3.1 注射模具分型面的選擇 3.1.1 分型面的基本形式 分型面的形式由塑料的具體情況而定，但大體上有平面式分型面、階梯式分型面、斜面式分型面、曲面式分型面、綜合式分型面。 3.1.2 分型面選擇的基本原則 選擇分型面的基本原則：（1）保持塑料外觀整潔；（2）分型面應(yīng)有利于排氣；（3）應(yīng)考慮開模是塑料留在動模一側(cè)；（4）應(yīng)容易保證塑件的精度要求；（5）分型面應(yīng)力求簡單適用并易于加工；（6）考慮側(cè)向分型面與主分型面的協(xié)調(diào)；（7）分型面應(yīng)與成型設(shè)備的參數(shù)相適應(yīng)；（8）考慮脫模斜度的影響[11]。 3.1.3 分型面的選擇 1、確定成型位置 由于塑件結(jié)構(gòu)簡單,所以不用設(shè)計小型心,型腔直接開設(shè)在定模板和中間板上.采用兩排各8個型腔分布. 2、確定分型面 采用單分型面注射模,從AA分型面一次分型,如下圖所示: 圖3.1 分型面 3.2 澆注系統(tǒng)的設(shè)計 3.2.1 澆注系統(tǒng)的組成 澆注系統(tǒng)是將熔融的塑料從成型設(shè)備噴嘴進入模具型腔所經(jīng)的通道，它包括主流道、分流道、澆口及冷料。在設(shè)計注射模具的澆注系統(tǒng)應(yīng)注意以下幾項原則[12]。 （1）根據(jù)所確定的塑件型腔數(shù)設(shè)計合理的澆注系統(tǒng)布局。 （2）根據(jù)塑件的形狀和大小以及壁厚等諸多因素，并結(jié)合選擇分型面的形式選擇澆注系統(tǒng)的形式及位置。 （3）應(yīng)盡量的縮短物料的流程和便于清除料把，以節(jié)省原料，提升注射效率。 （4）應(yīng)根據(jù)所選用塑件的成型性能，特別是它的流動性能，選擇澆注系統(tǒng)的截面積和長度，并使其圓滑過渡以利于物流的流動。 3.2.2 注射模具主流道的設(shè)計 主流道是熔融塑料由成型設(shè)備噴嘴先經(jīng)過的部位，它與成型設(shè)備噴嘴在同一軸心線上。由于主流道與熔融成型設(shè)備噴嘴反復(fù)接觸、碰撞，一般澆口不直接開設(shè)在定模上，為了制造方便，都制成可拆卸的澆口套，用螺釘或迫合形式在定模板上[13]。 （1）主流道的設(shè)計 主流道是指澆注系統(tǒng)中從成型設(shè)備噴嘴與模具接觸處開始到分流道為止的塑料熔體的流動通道。主流道的形狀與尺寸對塑料熔體的流動速度和充模時間有較大的影響，因此，必須使熔體的溫度降和壓力損失最小。 （2）主流道尺寸 在臥式或立式成型設(shè)備上使用的模具中，主流道垂直于分型面。為了讓主流道凝料能從澆口套中順利拔出，主流道設(shè)計成圓錐形，其錐角 為2o～6o。小端直徑d比成型設(shè)備噴嘴直徑大0.5mm～1 mm。由于小端的前面是球面，其深度為3mm～5 mm，成型設(shè)備噴嘴的球面在該位置與模具接觸并且貼合，因此要求主流道球面半徑比噴嘴球面半徑大1mm～2mm。流道的表面粗糙度值Ra為0.08 。 （3）主流道澆口套 主流道澆口套一般采用碳素工具鋼如T8A、T10A等材料制造，熱處理淬火硬度53HRC—57HRC。 澆口套的材料應(yīng)選用優(yōu)質(zhì)鋼T8A，并應(yīng)進行淬火處理，為了防止成型設(shè)備噴嘴不被碰撞而損壞，澆口套的硬度應(yīng)低于成型設(shè)備噴嘴的硬度。為了便于澆注凝料從主流道中取出，主流道采用α為3o～6o左右的圓錐孔。澆口套于成型設(shè)備的噴嘴頭的接觸球面必須吻合，由于成型設(shè)備噴嘴是球面，半徑是固定的，所以為使熔融塑料從噴嘴完全進入主流道而不溢出，應(yīng)使?jié)部谔锥嗣娴陌记蛎媾c成型設(shè)備噴嘴端的凸面接觸良好，圓錐孔的小端直徑則大于噴嘴的內(nèi)孔直徑，球面與主流道孔應(yīng)以清角連接，不應(yīng)有倒拔痕跡。為了便于澆注凝料從主流道中取出，主流道采用α為3o～6o度左右的圓錐孔，對流動性較差的塑料也可取得稍大一些，但過于大則容易引起注射速度緩慢，并容易形成渦流。 澆口套與塑料注射區(qū)直接接觸時，其出料端端面直徑應(yīng)盡量選得小些。澆口套于成型設(shè)備的噴嘴頭的接觸球面必須吻合，由于成型設(shè)備噴嘴是球面，所以為使熔融塑料從噴嘴完全進入主流道而不溢出，應(yīng)使?jié)部谔锥嗣娴陌记蛎媾c成型設(shè)備噴嘴端的凸面接觸良好，圓錐孔的小端直徑則大于噴嘴的內(nèi)孔直徑，球面與主流道孔應(yīng)以清角連接，不應(yīng)有倒拔痕跡，以保證主流道凝料順利脫模[14]。 定位環(huán)是模體與成型設(shè)備的定位裝置，它保證澆口套與成型設(shè)備的噴嘴對中定位，定位環(huán)的外徑應(yīng)與成型設(shè)備的定位孔間隙配合。澆口套端面應(yīng)與定模相配合部分的平面高度一致。成型設(shè)備SZ-63／400的噴嘴球半徑為18 mm，噴嘴孔徑為2 mm。所以要使?jié)部谔锥嗣娴陌记蛎媾c成型設(shè)備噴嘴的端凸球面接觸良好，凹球面半徑取19 mm，圓錐孔的小端直徑則應(yīng)大于噴嘴口內(nèi)徑，取3 .2mm，如圖3.2。 圖3.2 澆口套 主流道垂直于分型面。為了讓主流道凝料能順利從澆口中拔出，主流道設(shè)計成圓錐形，其錐角為 3o。小端直徑d比成型設(shè)備噴嘴直徑大0.5－1mm。由于小端的前面是球面，其深度為3－5mm，取值為5mm，成型設(shè)備噴嘴的球面在該位置與模具接觸并且貼合，因此要求主流道球面半徑比噴嘴球面大1－2mm。 3.2.3 分流道的設(shè)計 分流道是將熔融塑料從主流道截面及其方向的變化，平穩(wěn)進入單腔中的進料澆口或主流道進入多腔的澆口的通道，它是主流道與澆口的中間連接部分，起分流和轉(zhuǎn)換方向的作用，通常分流道設(shè)置在分型面的成型區(qū)域內(nèi)。 在注射過程中，熔融的塑料在流經(jīng)分流道時，應(yīng)是它的壓力損失以及熱量損失最小，而以分流道中產(chǎn)生的凝料最少為原則，分流道的設(shè)計要點總體歸納如下： 分流道的形狀要考慮分流道的截面積與其周邊長度的比最大為好，這樣可以減少熔料的散熱面積和摩擦阻力，減少壓力損失。 在可能情況下，分流道的長度應(yīng)盡量的短，以減少壓力損失，避免模體過大影響成本，在多型腔模具中和型腔的分流道長度盡量相等，以達(dá)到注射大時壓力傳遞的平衡，保證塑料盡可能同時均勻的充滿各個型腔。在有些情況下分流道長度不能相等時，則應(yīng)在澆口處作必要的補救措施，如果分流道較長時，應(yīng)在其末端設(shè)置冷料穴，放置冷料和空氣進入模腔[15]。 在滿足注射成型工藝的前提下，分流道的截面積應(yīng)盡量的小，但分流道的截面積過小會降低注射速度，使填充時間延長，同時可能出現(xiàn)缺料、焦燒、皺紋、縮孔等塑件缺陷，而分流道過大則增大冷卻時間應(yīng)比型腔中塑件的冷卻時間要短，才不影響注射時的效率。因此在設(shè)計時應(yīng)采用較小的截面積，以便于在試模是為不要的修正留有余地。 分流道和型腔的分布是排列緊湊，距離合理，應(yīng)采用軸對稱或中心對稱，使其平衡，盡量縮小成型區(qū)域的總面積。最好使型腔和分流道在分型面上的總投影面積的幾何中心和鎖緊力的中心相重合。 在分流道上的轉(zhuǎn)向次數(shù)盡量少，在轉(zhuǎn)向處應(yīng)圓滑過渡，不能有尖角，這些都是為了減小壓力損失，有利于物料的流動。 當(dāng)分流道設(shè)在定模一側(cè)或分流道延伸較長時，應(yīng)在澆口附近或分流道的交叉處設(shè)置鉤料桿，以便于在開模時在鉤料桿的作用下首先從定模中拉出分流道的凝料，并與塑料一起頂出。 分流道的內(nèi)表面不必要求很光，一般表面粗糙度取1.6μm即可，這樣可以在分流道的摩擦阻力下使料流外層的流動小些，使其分流道的冷卻皮層固定，有利于熔融塑料的保溫。 在總體分布中，應(yīng)綜合考慮冷卻系統(tǒng)的方式和布局，并留出冷卻水路的空間。 a．分流道的形狀和尺寸 分流道開設(shè)在定模板上，其截面形狀為半圓形，底部以圓角相連。分流道為二次分流道 3、 澆注系統(tǒng)的設(shè)計 ①主流道設(shè)計 根據(jù)手冊查得SZ-60/450型注塑機噴嘴的有關(guān)尺寸。 噴嘴球半徑：R=12mm 噴嘴孔直徑：d=Φ4mm 根據(jù)模具主流道與噴嘴的關(guān)系：R=Ro+（1～2）mm，d=do+0.5mm 取主流道球面半徑：R=14mm 取主流道的小端直徑：d=Φ5 為了便于將凝料從主流道中拔出，其斜度為1～3°。經(jīng)換算得主流道大端面直徑D=5.5mm。同時為了使熔料順利進入分流道，在主流道出料端設(shè)計r=3mm的圓弧過渡。對小型模具可將主流道襯套與定位圈設(shè)計成整體式。但在大多數(shù)情況下是將主流道襯套與定位圈設(shè)計成兩個零件，然后配合固定在模板上。主流道襯套與定模座板采用H7/m6過渡配合，與定位圈的配合采用間隙配合。主流道襯套一般選用T8、T10制造，熱處理強度為52～56HRC。 ②分流道設(shè)計 分流道的形狀及尺寸與塑件的體積、壁厚、形狀的復(fù)雜程度、注射速率等因素有關(guān)。該塑件形狀不算太復(fù)雜，且壁厚均勻，從便于加工的方面考慮，采用截面形狀為半圓形的分流道，查有關(guān)手冊得R=3mm ③澆口設(shè)計 澆口的結(jié)構(gòu)形式很多，按照澆口的形狀可以分為點澆口、扇形澆口、盤形澆口、環(huán)形澆口、及薄片式澆口。綜合對塑料成型性能、澆口和模具結(jié)構(gòu)的分析比較，確定成型該塑件的模具采用點澆口形式。如圖所示： ④型芯、型腔結(jié)構(gòu)的確定 型芯、型腔可采用整體式或組合式結(jié)構(gòu)。 該塑件型芯形狀比較復(fù)雜，因此應(yīng)采用組合式形式，而型腔形狀比較簡單，可采用整體式結(jié)構(gòu)。型腔尺寸如下： ⑤ 推件方式的選擇 根據(jù)塑件的形狀特點，模具型腔在定模部分。開模后，塑件和型芯一塊向后運動。其推出機構(gòu)可采用推塊或推桿推出。綜合對塑件形狀結(jié)構(gòu)分析，該塑件可采用推桿推出結(jié)構(gòu)。 第4章 成型零件的設(shè)計 4.1 模具型腔的結(jié)構(gòu)設(shè)計 型腔大體有以下幾種結(jié)構(gòu)形式：整體式、整體組合式、局部組合式和完全組合式。 型腔由整塊材料制成，用臺肩或螺栓固定在模板上。它的主要優(yōu)點是便于加工，特別是在多型腔模具中，型腔單個加工后，在分別裝入模板，這樣容易保證各型腔的同心度以及尺寸精度要求，并且便于部分成型件進行處理等。 型腔由整塊材料制成，但局部鑲有成型嵌件的局部組合式型腔。局部組合式型腔多于型腔較深或形狀較為復(fù)雜，整體加工比較困難或局部需要淬硬的模具。 完全組合式是由多個螺栓拼塊組合而成的型腔。它的特點是，便于機加工，便于拋光研磨和局部熱處理。節(jié)約優(yōu)質(zhì)鋼材。這種形式多用于不容易加工的型腔或成型大面積塑件的大型型腔上。這里選擇整體式型腔。 在塑料注射模具的注射過程中，型腔從合模到注射保證過程中受到高壓的沖擊力，因此模具型腔應(yīng)該有足夠的硬度和剛度，總的來說，型腔所承受的力大體有合模時的壓應(yīng)力、注射過程中塑料流動的注射壓力、澆口封閉前一瞬間的壓力保證和開模時的壓應(yīng)力，但型腔所承受的力主要是注射壓力和保證壓力，并在注射過程中總是在變化。在這些壓力作用下，當(dāng)型腔的剛度不足時，往往會產(chǎn)生彈性變形，導(dǎo)致型腔向外膨脹，它將直接影響塑件的質(zhì)量和尺寸精度。所以在模具設(shè)計時要首先考慮使型腔的壁厚和底板厚度都有足夠的強度和剛度，以保證型腔在注射過程中產(chǎn)生超過規(guī)定限度的彈性變形。因此型腔壁厚和底板的計算和選擇是十分重要的。 （1）型腔側(cè)壁厚度的計算 按強度計算 其壁厚S按下列公式計算 式中 [σ]— 型腔材料的許用應(yīng)力，[σ]=156.8MPa p—型腔內(nèi)單位平均壓力，P=38.4MPa r—型腔內(nèi)半徑，r=10mm 代入公式得：S=4mm （2）底板厚度的計算 按強度計算 其壁厚H按下面公式計算 式中 [σ]— 型腔材料的許用應(yīng)力，[σ]=156.8MPa p—型腔內(nèi)單位平均壓力，P=38.4MPa r—型腔內(nèi)半徑，r=10mm 代入公式得：H=5.5mm 4.2 型芯的結(jié)構(gòu)設(shè)計 型芯的結(jié)構(gòu)形式大體有：整體式、整體復(fù)合式、局部組合式、完全組合式。 4.3 成型零件的尺寸確定 （1）平均收縮率計算型腔尺寸 ABS的收縮率一般為0.3%～0.8%,從而得出ABS的平均收縮率為0.6%。 徑向尺寸 ABS的一般精度等級為6級。同時得出塑料制件的尺寸公差。 又由于塑件的外徑D=25.00㎜，所以查表得Δ=0.45 按照平均收縮率計算凹模徑向尺寸公式 式中 LM——凹模的徑向尺寸，mm Scp——塑料的平均收縮率，% Ls——塑件徑向公稱尺寸，㎜ Δ——塑件公差值，㎜ δz——凹模制造公差，㎜ 已知　Ls =25.00㎜　　Scp =0.006　　Δ=0.45㎜ 所以 δz=Δ/3=0.15㎜ 深度尺寸 ABS的一般精度等級為6級。同時得出塑料制件的尺寸公差。 又由于塑件的深度尺寸Hs =15.00㎜，所以查表得Δ=0.40㎜ 按照平均收縮率計算凹模深度尺寸公式 式中　HM——凹模的深度尺寸，㎜ Scp——塑料的平均收縮率，% Hs——塑件高度公稱尺寸，㎜ Δ——塑件公差值，㎜ δz——凹模深度制造公差，㎜ 已知　Hs =15.00㎜　　Scp =0.006　　Δ=0.40㎜ 所以　δz=Δ/3=0.13㎜ （2）按平均收縮率計算組合型芯尺寸 徑向尺寸 ABS的一般精度等級為6級。同時得出塑料制件的尺寸公差。 所以　d=9㎜，所以查表得Δ=0.37 按照平均收縮率計算型芯徑向尺寸公式 式中　LM——組合型芯的徑向尺寸，㎜ Scp——塑料的平均收縮率，% Ls——塑件徑向公稱尺寸，㎜ Δ——塑件公差值，㎜ δz——組合型芯制造公差，㎜ 已知　Ls =9.00㎜　　Scp =0.006　　Δ=0.37㎜ 所以　δz =Δ/3=0.12㎜ 高度尺寸 ABS的一般精度等級為6級。同時得出塑料制件的尺寸公差。 又由于塑件的深度尺寸Hs=15.00-3.00=12.00㎜，所以查表得Δ=0.36㎜ 按照平均收縮率計算組合型芯高度尺寸公式 式中　HM——型芯高度尺寸，㎜ Scp——塑料的平均收縮率，% Hs——塑件孔深度公稱尺寸，㎜ Δ——塑件公差值，㎜ δz——組合型芯高度制造公差，㎜ 已知　Hs =13.00㎜　　Scp =0.006　　Δ=0.36㎜ 所以　δz =Δ/3=0.12㎜ （3）分流道的設(shè)計 采用半圓形截面流道。因為塑料熔體在流道中流動時，表面冷凝凍結(jié)，起絕熱的作用，熔體僅在流道中心流動，因此分流道的理想狀態(tài)應(yīng)是其中心線與澆口的中心線位于同一直線上，而半圓形截面可以滿足。 分流道的長度取決于模具型腔的總體布置方案和澆口的位置，從輸送熔體時的減少壓力損失和熱量損失及減少澆道凝料的要求出發(fā)，應(yīng)力求縮短。 對于壁厚小于3㎜，質(zhì)量在200g以下的塑件可用公式 式中 W——流經(jīng)分流道的塑料量，g 　L——分流道長度，㎜ 　D——分流道直徑，為6㎜ 其中　 n——型腔數(shù)目 m——塑件質(zhì)量，g 得出 取分流道的長度為112㎜ 分流道的布置取決于型腔的布局，兩者相互影響。分流道的布置形式有平衡式和非平衡式兩種。此設(shè)計中我采用的是平衡式布置。平衡式布置可以使各型腔同時均衡的進料，從而保證了各型腔成型出來的塑件在強度.性能.重量上的一致性。 4.6確定主要零件結(jié)構(gòu)及尺寸 經(jīng)過初步設(shè)計,預(yù)選中小型315×400×194標(biāo)準(zhǔn)A1模架，各板厚數(shù)值皆已有國際規(guī)定，其強度足夠。 定模座板 外形尺寸：400×315×25mm；材料：Q235A；調(diào)質(zhì)HB216-260；澆口套與板之間采用φ20H7/k6過渡配合，四個孔距為260×160mm，四個小孔為160×100的銷釘孔。如圖5所示。 圖5 定模座板 4.6.3、型腔 外形尺寸：315×315×32mm；材料：45鋼；調(diào)質(zhì)HB230-270；板上開16腔孔；采用四個φ30,孔距為230*6mm的導(dǎo)套孔采用過渡配合（H7/k6）。 4.6.3、型芯 外形尺寸：315×315×32mm；材料：45鋼；調(diào)質(zhì)HB230-270；板上開24腔孔；采用四個φ20mm、孔距為258×260mm的導(dǎo)柱與孔采用過渡配合（H7/k6）；260×160mm。 4.6.7、推桿固定板 外形尺寸：199×315×20mm；材料：Q235A；四個與φ2.6推桿過渡配合、孔距為150×240mm的孔；四個用于連接推板的M12螺釘孔，孔距為285×160mm，如圖8所示。 4.6.8、推板 外形尺寸：315×199×20mm；材料：45鋼；淬火HRC43-48；四個用于連接推桿固定板的φ12孔，孔距為285×160mm。如圖9所示。 圖9 4.6.9、動模座板 外形尺寸：400×315×25mm；材料：Q235A；調(diào)質(zhì)HB216-260；四個孔距為260×160mm的M16螺釘孔。如圖10所示。 圖10 動模座板 第5章 頂出機構(gòu)的設(shè)計 自動脫螺紋機構(gòu) 對于某些帶有螺紋的塑件，采用自動脫螺紋機構(gòu)方便塑件的取出，而且運動平穩(wěn)，塑件不易變形。 在該模具設(shè)計中考慮到塑件體積不是很大且有內(nèi)螺紋，所以選擇自動脫螺紋機構(gòu)。 第6章 冷卻系統(tǒng)的設(shè)計 在注射成型過程中，模具溫度直接影響到塑件的質(zhì)量如收縮率、翹曲變形、耐應(yīng)力開裂性和表面質(zhì)量等，并且對生產(chǎn)效率起到?jīng)Q定性的作用，在注射過程中，冷卻時間占注射成型周期的約80%，然而，由于各種塑料的性能和成型工藝要求不同，模具溫度的要求不盡相同，因此，對模具冷卻系統(tǒng)的設(shè)計及優(yōu)化分析在一定程度上決定了塑件的質(zhì)量和成本，模具溫度直接影響到塑料的充模、塑件的定型、模塑的周期和塑件質(zhì)量，而模具溫度的高低取決于塑料結(jié)晶性，塑件尺寸與結(jié)構(gòu)、性能要求以及其它工藝條件如熔料溫度、注射速度、注射壓力、模塑周期等。影響注射模冷卻的因素很多，如塑件的形狀和分型面的設(shè)計，冷卻介質(zhì)的種類、溫度、流速、冷卻管道的幾何參數(shù)及空間布置，模具材料、熔體溫度、塑件要求的頂出溫度和模具溫度，塑件和模具間的熱循環(huán)交互作用等。 （1）低的模具溫度可降低塑件的收縮率。 （2）模具溫度均勻、冷卻時間短、注射速度快，可降低塑件的翹曲變形。 （3）對結(jié)晶性聚合物，提高模具溫度可使塑件尺寸穩(wěn)定，避免后結(jié)晶現(xiàn)象，但是將導(dǎo)致成型周期延長和塑件發(fā)脆的缺陷。 （4）隨著結(jié)晶型聚合物的結(jié)晶度的提高，塑件的耐應(yīng)力開裂性降低，因此降低模具溫度是有利的，但對于高粘度的無定型聚合物，由于其耐應(yīng)力開裂性與塑料的內(nèi)應(yīng)力直接相關(guān)，因此提高模具溫度和充模，減少補料時間是有利的。 （5）提高模具溫度可以改善塑件的表面質(zhì)量。 在注射成形過程中，模具的溫度直接影響塑件的成型質(zhì)量和生產(chǎn)效率，根據(jù)塑料的要求，注射到模具內(nèi)的塑料溫度為2000C左右，而從模具中取出塑件的溫度約為600C，溫度降低是由于模具通入冷卻水，將溫度帶走了，普通的模具通入常溫的水進行冷卻，通過調(diào)節(jié)水的流量就可以調(diào)節(jié)模具的溫度。 因電風(fēng)扇葉片鎖緊螺母使用的塑料是PE，要求模溫高，若模具溫度過低則會影響塑料的流動性，增加剪切阻力，使塑件的內(nèi)應(yīng)力較大，甚至還出現(xiàn)冷流痕、銀絲、注不滿等缺陷。因此在注射開始時，為防止填充不足，充入溫水或者模具加熱。 總之，要做到優(yōu)質(zhì)、高效率生產(chǎn)，模具必須進行溫度調(diào)節(jié)。 對溫度調(diào)節(jié)系統(tǒng)的要求： （1）確定加熱或是冷卻； （2）模溫均一，塑件各部分同時冷卻； （3）采用低的模溫，快速且大量通冷卻水； 溫度調(diào)節(jié)系統(tǒng)應(yīng)盡量結(jié)構(gòu)簡單，加工容易，成本低謙。 根據(jù)模具冷卻系統(tǒng)設(shè)計原則：冷卻水孔數(shù)量盡量多，尺寸盡量大的原則可知，冷卻水孔數(shù)量大于或等于3根都是可行的。這樣做同時可實現(xiàn)盡量降低入水與出水的溫度差的原則。根據(jù)書上的經(jīng)驗值取4根，冷卻水口口徑為6mm. 另外，具冷卻系統(tǒng)的過程中，還應(yīng)同時遵循： （1）澆口處加強冷卻； （2）冷卻水孔到型腔表面的距離相等； （3）冷卻水孔數(shù)量應(yīng)盡可能的多，孔徑應(yīng)盡可能的大； （4）冷卻水孔道不應(yīng)穿過鑲快或其接縫部位，以防漏水。 （5）進水口水管接頭的位置應(yīng)盡可能設(shè)在模具的同一側(cè)，通常應(yīng)設(shè)在注塑機的面。 （6）冷卻水孔應(yīng)避免設(shè)在塑件的熔接痕處。 而且在冷卻系統(tǒng)內(nèi)，各相連接處應(yīng)保持密封，防止冷卻水外泄。 第7章 排氣系統(tǒng) 在注塑模具的設(shè)計過程中，必須考慮排氣結(jié)構(gòu)的設(shè)計，否則，熔融的塑料流體進入模具型腔內(nèi)，在填充模具的型腔過程中同時要排出型強及流道原有的空氣，氣體如不能及時排出會使制件的內(nèi)部有氣泡， 除此以外，塑料熔體會產(chǎn)生微量的分解氣體。這些氣體必須及時排出。否則，被壓縮的空氣產(chǎn)生高溫，會引起塑件局部碳化燒焦，或塑件產(chǎn)生氣泡，或使塑件熔接不良引起強度下降，甚至充模不滿甚至?xí)a(chǎn)生很高的溫度使塑料燒焦，從而出現(xiàn)廢品。 排氣方式有兩種：開排氣槽排氣和利用合模間隙排氣。 由于車窗扣座注塑模是小型鑲拼式模具，可直接利用分型面和鑲拼間隙進行排氣，而不需在模具上開設(shè)排氣槽。 第8章 成型設(shè)備有關(guān)參數(shù)校核 1、模具閉合高度的確定 根據(jù)支承與固定零件中提供的數(shù)據(jù)測量確定： H=206mm 2、 注射機有關(guān)參數(shù)的校核 1、模具閉合高度的確定和校核 ⑴模具閉合高度的確定。根據(jù)標(biāo)準(zhǔn)模架各模板尺寸及模具設(shè)計的其他零件尺寸：定模座板H定=20mm。 ⑵定模板H=30mm，型芯固定板H固=20mm，模腳H模=60mm，動模固定板H動=20mm模具閉合高度H閉=20+30+20+60+20mm=150mm 2模具安裝部分的校核 該模具外形尺寸為315mmX315mm，注射機模板最大安裝尺寸為400mmX450mm，固滿足模具安裝要求。 注射機允許模具最小厚度為Hmin=70mm，最大厚度為Hmax=200mm，所以模具閉合高度滿足Hmin＜H閉＜Hmax的安裝條件 第9章 模具特點和工作原理 1、模具的特點： 該模具是兩板模，設(shè)計了1 個水平分型面。設(shè)計了定距拉桿， A 分 型面是為了取出制件。該模具一模2件，節(jié)省了成本，降低了制造周期，提高了生產(chǎn)效率。 2、模具的工作過程 模具裝配試模完畢后，模具進入正式工作狀態(tài)，其基本工作過程如 下。 （1）對塑料進行烘干，并裝入料斗。 （2）清理模具型芯、型腔，并噴上脫模劑，進行適當(dāng)?shù)念A(yù)熱。 （3）合模、鎖緊模具。 （4）對塑料進行預(yù)塑化，注射裝置準(zhǔn)備注射。 （5）注射過程包括充模、保壓、倒流、澆口凍結(jié)后的冷卻和脫模。 （6）脫模過程。制件的推出同一般注塑模具推出方式相同，即由注 塑機推桿推動模具推板，從而推動推件桿將之間頂出。 總結(jié) 總 結(jié) 這次畢業(yè)設(shè)計針對設(shè)計內(nèi)容進行了大量的工作，順利完成了畢業(yè)設(shè)計中所提出的各項任務(wù)，達(dá)到了畢業(yè)設(shè)計的目的。 通過此畢業(yè)設(shè)計，掌握了模具設(shè)計的方法和步驟，并結(jié)合具體的零件進行了具體的設(shè)計工作，包括確定型腔的數(shù)目、選擇分型面、確定澆注系統(tǒng)、脫模方式、溫度調(diào)節(jié)系統(tǒng)的設(shè)計、注射模成型零件尺寸的計算等。 畢業(yè)設(shè)計進行三維造型繪制；完成塑件注射模具方案設(shè)計和相關(guān)設(shè)計計算；最后完成模具加工，掌握了完整的工程設(shè)計過程，工程設(shè)計應(yīng)用能力得到了鍛煉和提高。 參考文獻(xiàn) 1. 屈華昌主編.《塑料成型工藝與模具設(shè)計》.北京:高等教育出版社,2001 2. 李澄,吳天生,聞百橋主編.《機械制圖》.北京:高等教育出版社,1997 3.許發(fā)樾主編.《實用模具設(shè)計與制造手冊》.北京：機械工業(yè)出版社，2002 4. 李軍主編 , 《精通PRO/E中文野火版模具設(shè)計》.北京:中國青年出版社,2004 5．《塑料模設(shè)計及制造》．李學(xué)鋒主編．北京：機械工業(yè)出版社，2001年 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. l 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 N=fCex(-?G**/kT) 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 ?G**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 ?Ghom**?16πr33?P2 where g is the surface energy of the bubble interface and ?P.is assumed to be the gas saturation pressure. More precisely, ?P=|Pr'-Pr| where Pr` is the pressure that is exerted in a high 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 ?Ghet**=?Ghom**×f(θ) (3a) fθ=12-34cosθ+14cosθ3 (3b) where f(q) is a geometric factor that is dependent upon the contact angle, θ, 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 200 on the interface between a solid particle and the polymer melt, the geometric factor is 2.7X10-3, suggesting that the energy barrier for heterogeneous nucleation can be reduced by three orders of magnitude with the presence of an interface [20,21]. l 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?P in the Gibbs free energy equation, can be approximately assumed to be the saturation gas pressure. The assumption that ?P is the gas saturation pressure is fairly reasonable in a batch foaming process although the ?Pcan still have an error of about 30-40% due to overestimation as reported in a 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 ?P cannot be directly 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 ?P, and to consider the degree of supersaturation, a more proper activation energy equation for nucleation can be derived from the following equation [16,17] ?P=|Pr'-Pr|=2rrc （4） where rc is the radius of a characteristic droplet, and the W. Thomson equation RTlnPrP∞=2r?Mr?p （5） where P∞ is the pressure of the saturated vapor (i.e., the equilibrium pressure), R is the universal gas constant, M is the molar mass, and p is the density. These equations yield ?P=RTρlnPrP∞M （6） which can be alternatively expressed as ?P=ktρ1lnS (7) whereρ1is the molecular density of the bulk liquid, and S(=PrP∞) 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 ?G**=16πr33(kTρ1lnS)2 （8） 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] S@1 atm; melt temperature=S@STPexp?(-?HsR(1Tmelt-1298)) (9) where S@STP 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 S=mgS@STPexp?(-?HsR(1Tmelt-1298)) (10) where mg is the gas dosage which can be controlled by the supercritical fluid (SCF) supply system. The heat of sorption, ?HsRg, 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, γ. 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] γmix=γpolymerρmixρpolymer4(1-wgas) （11） where γpolymer is the surface energy of the polymer, P′S are the densities, and wgas 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] f=Zβ(4πrc2) (12) where z is the Zeldovich factor, which accounts for the many clusters that have reached the critical size, rc., 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 Zβcan 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/cm3. 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/cm3. Both 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 。C and 25 。C, 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 cm3 and a packing pressure of 800 bars were employed for 6 s. For the microcellular injection molding experiments, the shot size of the polymer melt was 19.2 cm3 and 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 ima