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注塑模具的設(shè)計(jì)與熱分析
摘 要:本文介紹了用于生產(chǎn)熱變形測(cè)試樣品的注塑模具的設(shè)計(jì)，這種模具能為自身實(shí)現(xiàn)熱分析，從而得到模具的熱殘余應(yīng)力的影響。文章對(duì)技術(shù)，理論，方法以及在注塑模型設(shè)計(jì)中需要的考慮的因素也進(jìn)行了介紹。模具設(shè)計(jì)是通過商用計(jì)算機(jī)輔助設(shè)計(jì)軟件Unigraphics系統(tǒng)的13.0版本實(shí)現(xiàn)的。這種用于分析因樣品不均勻冷卻產(chǎn)生的熱殘余應(yīng)力的模具，已經(jīng)通過使用13.5版的被稱作LUSAS分析員的商業(yè)有限元分析軟件得到了開發(fā)，而且存在問題也已經(jīng)解決。該軟件通過繪制相應(yīng)的時(shí)間反應(yīng)曲線為模具提供了溫度分布等高線圖以及注塑周期中的溫度的變化。結(jié)果表明，與其他區(qū)域相比，收縮可能更容易發(fā)生在冷卻渠道附近的區(qū)域。熱變形就是這種在模具的不同區(qū)域的不均衡降溫效果引起的。
關(guān)鍵詞：注塑模具；設(shè)計(jì)；熱分析
1. 引言
塑料業(yè)是世界上發(fā)展最快的工業(yè)之一，被列入產(chǎn)值達(dá)數(shù)十億美元的產(chǎn)業(yè)。幾乎每一個(gè)在日常生活中使用的產(chǎn)品都涉及塑料的使用，這些產(chǎn)品大部分可通過注塑成型方法生產(chǎn)［1］。注塑成型工藝因其制造過程是以較低的成本生產(chǎn)各種形態(tài)和復(fù)雜幾何形狀的產(chǎn)品而眾所周知［2］。
注塑成型工藝是一個(gè)循環(huán)工藝，整個(gè)過程分為四個(gè)重要的階段，即：充模，保壓，冷卻和噴射。在注塑成型過程是從漏斗中把樹脂和適當(dāng)?shù)奶砑觿┳⑷氲阶⑺艹尚蜋C(jī)的加熱/注射系統(tǒng)開始的［3］。這就是“充模階段”，在這個(gè)過程中，模腔填充了達(dá)到注射溫度的熱聚合物熔體。在模腔填充后的 “保壓”階段，更多的是聚合物熔體在更高的壓力下被裝進(jìn)腔體，以補(bǔ)償因聚合物固化引起的預(yù)計(jì)萎縮。接下來便是冷卻階段，在此過程中模具會(huì)冷卻，直到有足夠的剛性部分被彈出。最后一個(gè)階段是“彈射階段”，這個(gè)階段模具被打開，成型部分被彈出，過后，模具會(huì)再次被關(guān)閉開始下一個(gè)循環(huán)［4］。
因?yàn)橹饕强拷?jīng)驗(yàn)，包括了實(shí)際工具的反復(fù)修改，所以設(shè)計(jì)和制造所需性能的注塑成型聚合物部件的過程很昂貴的。在模具設(shè)計(jì)任務(wù)中，由于包含了噴射和氣壓因素，通常來說，在核心區(qū)為模具設(shè)計(jì)特別附加的幾何結(jié)構(gòu)是相當(dāng)復(fù)雜［5］。
為了設(shè)計(jì)出模具，許多重要的設(shè)計(jì)因素必須加以考慮。這些因素是模具的大小，模腔的數(shù)量、布局，熱流道系統(tǒng)，門控系統(tǒng)，收縮和彈射系統(tǒng)［6］。
在模具的熱分析中，其主要目的是分析熱殘余應(yīng)力或模壓對(duì)產(chǎn)品尺寸方面的影響。熱誘導(dǎo)應(yīng)力主要發(fā)生在注塑零件的冷卻階段，主要是由于其較低的熱傳導(dǎo)率和熔融樹脂和模具之間的溫度差異。在場(chǎng)冷卻過程中的產(chǎn)品腔的溫度是不平衡的［7］。
冷卻過程中，離冷卻通道越近的地方能更大程度的冷卻下來。這種溫度的不同引起了材料的異收縮，從而帶來熱應(yīng)力。明顯的熱應(yīng)力可能會(huì)引起變形問題。因此，在注射成型過程的冷卻階段對(duì)熱殘余應(yīng)力場(chǎng)進(jìn)行模擬是非常重要的［8］。通過了解熱殘余應(yīng)力的分布特點(diǎn)，我們就可以預(yù)測(cè)熱殘余應(yīng)力引起的變形。
在這篇論文中，為生產(chǎn)翹曲測(cè)試樣本設(shè)計(jì)的提出了這樣一種注塑模具設(shè)計(jì)：它能對(duì)模具實(shí)現(xiàn)熱殘余應(yīng)力對(duì)其的影響實(shí)現(xiàn)熱分析。
2. 方法
2.1 翹曲測(cè)試樣本的設(shè)計(jì)
這一部分介紹了用于注塑模具設(shè)計(jì)的翹曲測(cè)試樣本的設(shè)計(jì)。對(duì)于外殼很薄的產(chǎn)品來說，很明顯翹曲是存在其中的主要問題。因此，產(chǎn)品開發(fā)的主要目的就是設(shè)計(jì)一個(gè)塑件能為外殼很薄的注塑模具的翹曲問題確定影響因素。翹曲測(cè)試樣本是由薄塑料殼而開發(fā)的。樣本的總體尺寸是長(zhǎng)120毫米，寬50毫米，厚1毫米。生產(chǎn)翹曲測(cè)試樣本所用材料是丙烯腈丁二烯苯乙烯（ABS）注塑的溫度、時(shí)間和壓力分別是210 ?C, 3 s 和 60MPa,圖1表示翹曲測(cè)試樣本。
圖1 翹曲測(cè)試樣本
2.2 翹曲測(cè)試樣本設(shè)計(jì)注塑模具
本節(jié)介紹了用于生產(chǎn)翹曲測(cè)試樣本的模具的設(shè)計(jì)部分和設(shè)計(jì)中其他需要考慮的因素。用于生產(chǎn)翹曲測(cè)試樣本注塑模具的材料是美國(guó)國(guó)家鋼鐵學(xué)會(huì)的1050碳鋼。在模具設(shè)計(jì)中考慮到的四個(gè)設(shè)計(jì)概念包括：
I. 三板模具（概念1）有兩個(gè)分模線單腔，由于成本高所以不適用。
ii. 二板模具（概念2）有無澆注系統(tǒng)的一個(gè)分模線單腔。由于每注的低生產(chǎn)率而不適用。
iii. 二板模具（概念3）有一分模線與門控噴射系統(tǒng)的雙腔。由于產(chǎn)品太薄，頂針可能損害它，因此也不適用。
iv. 二板模具（概念4）有一分模線與有門控系統(tǒng)的雙腔，只用直澆口拉出器作為噴射器，來避免在噴射過程中對(duì)產(chǎn)品的破壞。在翹曲樣本的模具設(shè)計(jì)中第四個(gè)設(shè)計(jì)概念已經(jīng)得到應(yīng)用。
各種設(shè)計(jì)根據(jù)都已經(jīng)在設(shè)計(jì)中得到應(yīng)用。
首先，模具的設(shè)計(jì)是基于所使用注塑機(jī)器的壓盤尺寸(BOY 22D)。這個(gè)機(jī)器是有一定限制的，這就是兩個(gè)拉桿之間的距離定出的壓板機(jī)的最大面積在機(jī)器拉桿之間的距離為254毫米。因此，模具板最大寬度應(yīng)不能超過這個(gè)距離。此外，為了達(dá)到調(diào)定和處理模具的目的，在兩個(gè)拉桿和模具之間保留了4毫米的空間。最終這使模具的最高寬度為250毫米。標(biāo)準(zhǔn)模具基地有250×250毫米的使用面積。模具基底是用美得麗鉗安裝在模具基座或模具壓板右上角和左下角的。其他相關(guān)模具板的尺寸見表1。 設(shè)計(jì)了夾緊壓力的模具具有高于內(nèi)部空腔力（反應(yīng)力）的夾緊力來避免發(fā)生突然的活動(dòng)。
表1 磨具板尺寸
部件
尺寸：寬度×高度×厚度
頂部壓緊板
250×250×25
腔板
200×250×40
核心板
200×250×40
側(cè)板，支撐板
37×250×70
排出維持板
120×250×15
推出器
120×250×20
底部固定板
250×250×25
基于標(biāo)準(zhǔn)模具系列給出的尺寸，寬度和核心板的高度分別是200和250毫米 這些尺寸使核心板兩腔的設(shè)計(jì)是水平放置的，因?yàn)榍话鍨榭諘r(shí)有足夠的空間，為了填充熔融塑料，只是用澆口襯瓦來固定因此，在產(chǎn)品表面只設(shè)計(jì)了一個(gè)標(biāo)準(zhǔn)分型線。在模具打開的過程中，產(chǎn)品和滑行裝置放置在通過分型面的一個(gè)平面上，標(biāo)準(zhǔn)門或側(cè)門就是轉(zhuǎn)為這個(gè)模具設(shè)計(jì)的。門就處在滑道和產(chǎn)品之間。在門底部的土地，設(shè)計(jì)了20?傾斜，而且厚度只有0.5毫米以便于清鏟。
這個(gè)門還設(shè)計(jì)了四毫米寬0.5毫米厚的熔融塑料入口。在模具設(shè)計(jì)中，選擇了滑道的拋物線交叉部分的類型，因?yàn)樗幸粋€(gè)好處，那就是一模只需一半的簡(jiǎn)單加工，即這一例子中的核心板。
盡管如此，與圓截面類型相比這種滑道有諸如更多的熱流失和廢料。這可能會(huì)引起熔融塑料更快的凝固。這個(gè)問題通過使用更短，直徑更大的滑道得以減少，它的直徑是6毫米。非常重要的一點(diǎn)是滑道的設(shè)計(jì)要同時(shí)在相同的壓力和溫度下把材料或熔融塑料分送到腔體中。由于這一點(diǎn)，腔體的布局設(shè)計(jì)成了對(duì)稱形式。設(shè)計(jì)中另一個(gè)需要考慮的方面就是通氣孔的設(shè)計(jì)。
為了防止發(fā)生滑動(dòng)，核心和板腔之間的配合面板加工非常精細(xì)。但是，這可能導(dǎo)致在模具關(guān)閉時(shí)空氣被封在里面，而使產(chǎn)品出現(xiàn)噴丸不足或部分不完整。為了確保鎖住的空氣能被排出來而避免部分不完整的發(fā)生，我們?cè)O(shè)計(jì)了足夠多的通氣孔。冷卻系統(tǒng)是沿著模具的長(zhǎng)度鉆出來的，在與模具水平的位置使其更好的冷卻這些冷卻渠道在兩鉆腔和和核心板上都有。它們使模具在湍流的情況下能夠充分的被冷卻。圖2 給出了通氣孔和核心板上的冷卻渠道在腔里的布局。
圖2 氣孔在腔中的布局及冷卻通道
在這個(gè)模具設(shè)計(jì)中，彈射系統(tǒng)包括彈射版，直澆口拉出器以及噴射器。位于核心板中心位置的直澆口拉出器的作用不只是作為模具打開時(shí)固定產(chǎn)品位置的拉出器，而且在噴射階段作為噴射器把產(chǎn)品從模具中推出來。在產(chǎn)品腔中沒有放置和使用附加的噴射器，因?yàn)楫a(chǎn)品生產(chǎn)的非常薄，即1毫米。
產(chǎn)品腔中額外的注射器可能會(huì)在噴射階段給產(chǎn)品增加洞眼和損害。最后，要對(duì)尺寸容差進(jìn)行足夠的考慮來補(bǔ)償材料的收縮。圖3顯示了利用unigraphic系統(tǒng)建立的三維實(shí)體造型以及線框模型
圖3 模具的三維實(shí)體建模和線框模型
圖4 用于避免欠注的額外的通風(fēng)孔
3. 結(jié)果和討論
3.1 生產(chǎn)和調(diào)整產(chǎn)品的結(jié)果
從設(shè)計(jì)和制作的模具來看，生產(chǎn)的翹曲測(cè)試樣本在實(shí)驗(yàn)跑中有一些缺陷。這些缺陷是短射，溢料和翹曲。短射隨后被彎道腔排氣孔的邊緣消除來使被困住的空氣能出來。與此同時(shí)，溢料也通過減少保壓機(jī)器的壓力得到降低。變形可以通過控制諸如注射時(shí)間，注射溫度和融融溫度等各種參數(shù)來得到控制。經(jīng)過這些修改，模具生產(chǎn)了高品質(zhì)低成本的翹曲測(cè)試樣本同時(shí)需要盡快完成小門控。圖4顯示了模具的修改，正在加工消除短射的額外排氣孔。
3.2 模具和產(chǎn)品的細(xì)節(jié)分析
模具和產(chǎn)品開發(fā)出來以后, 就進(jìn)行對(duì)模具和產(chǎn)品的分析t. 在注塑成型額過程中, 210度的熔融ABS通過腔板上的澆口套注入模具直接即進(jìn)入到產(chǎn)品腔。冷卻發(fā)生后，產(chǎn)品就成型了One 產(chǎn)品的周期大約需要35秒，而其中冷卻的時(shí)間就有20秒。生產(chǎn)翹曲測(cè)試樣本的材料是ABS，其注塑溫度，時(shí)間，和壓力分別是210度，3秒和60MPa。
模具選的材料是1050碳鋼，這些材料的性能在有限元素分析開發(fā)的模具中決定溫度分布的方面是非常重要的。表2顯示了ABS和AISI1050碳鋼的性能。對(duì)模具進(jìn)行分析的關(guān)鍵部分在于腔體和核心板，因?yàn)檫@是產(chǎn)品成型的地方。
所以熱分析來知道溫度分布和不同時(shí)間的不同溫度是通過采用商業(yè)有限元素分析軟件即所謂的LUSAS分析員的13.5版本來實(shí)現(xiàn)的。為了研究在不同地區(qū)熱殘余應(yīng)力對(duì)模具的影響，進(jìn)行了二維的熱分析，由于是對(duì)稱的，熱分析只要通過垂直截面的上半部分或者在注射階段被夾住的兩腔的側(cè)面和核心板塊。圖5顯示了用不規(guī)則網(wǎng)格分析的熱分析模型。
圖5 熱分析模型
建模還涉及到性能分配和模型的程序或周期的時(shí)間。這使得有限求解分析模具模型建立，并且繪制了時(shí)間反應(yīng)圖來顯示在特定階段的不同地區(qū)的溫度變化。對(duì)于產(chǎn)品的分析，是通過利用LUSAS分析員的13.5版本實(shí)現(xiàn)了一個(gè)二維拉伸應(yīng)力分析。基本上在被緊緊地加載在一端，而另一端是被鎖住的。負(fù)荷會(huì)一直增長(zhǎng)知道模型達(dá)到可塑性。圖6顯示了分析的加載模式。
表格2 模具和產(chǎn)品的材料性能
圖6 分析的加載模式
圖7 不同時(shí)間間隔的二維熱分布等高線圖
3.3 對(duì)模具和產(chǎn)品分析的結(jié)果和討論
對(duì)于模具分析來說，可以觀察到不同的時(shí)間間隔中的熱量分布。圖7顯示了一個(gè)完整注塑模型周期中不同時(shí)間間隔的熱分布二維分布等高線圖。
對(duì)于模具的分析來說，在不同熱量或熱能分布等高線繪制圖上 ，熱量分布在一個(gè)完整的注塑周期中。為了實(shí)現(xiàn)對(duì)于模具的二維分析，繪制了時(shí)間反應(yīng)曲線圖來分析熱殘余壓力對(duì)產(chǎn)品的影響。圖8顯示了為了繪制時(shí)間反應(yīng)曲線選出的結(jié)點(diǎn)。
圖8 時(shí)間反應(yīng)曲線圖選出的結(jié)點(diǎn)
圖9.17顯示了圖8 所示不同節(jié)點(diǎn)上的溫度分布曲線。
從繪制出的溫度分布曲線圖來看，圖9.17中，為繪制曲線圖選擇的每一個(gè)結(jié)點(diǎn)在溫度上會(huì)經(jīng)歷上升。即，從其周圍的溫度到比其周圍溫度高的特定溫度，然后在特定的一個(gè)時(shí)間里，在這個(gè)溫度上保持不變。這個(gè)溫度增長(zhǎng)是由于熔融塑料被注入產(chǎn)品腔引起的。From the temp一段特定的時(shí)間.過后，溫度會(huì)進(jìn)一步升高直到最高溫度，然后保持在那個(gè)溫度上。溫度增長(zhǎng)歸因于引起高壓的保壓階段。這個(gè)階段上引起溫度上升。這個(gè)溫度會(huì)一直維持到冷卻階段開始，這個(gè)階段上把模具的溫度降并維持到一個(gè)較低的值。繪制的曲線由于沒有熔融塑料的注入率以及冷卻劑的制冷率，他是彎曲的。繪制的曲線只是顯示了在周期中能夠達(dá)到的最大值。熱殘余應(yīng)力分析的最關(guān)鍵步驟是冷卻階段。這是因?yàn)椋鋮s階段引起了材料從高于玻璃轉(zhuǎn)化溫度到低于它的轉(zhuǎn)化。材料經(jīng)歷了可能導(dǎo)致翹曲的引起熱應(yīng)力的不同的收縮。從圖9.17所示的冷卻以后的溫度中，由于溫度進(jìn)一步降低冷卻管附近的溫度會(huì)很明顯的受到更大的冷卻效果，而遠(yuǎn)離冷卻管的地方則有較小的冷卻效果冷卻率越大，冷卻效果越多意味著在這個(gè)地方發(fā)生跟大的收縮。盡管如此，最遠(yuǎn)的地方，第284結(jié)點(diǎn)，盡管遠(yuǎn)離冷卻管，由于向環(huán)境中損失的熱量，經(jīng)歷更大的冷卻力。結(jié)果是，產(chǎn)品腔中心位置的冷卻管引起中間高于其他地方的部分的溫度不相同.。由于更大程度的收縮，中間部分產(chǎn)生了壓縮，同時(shí)由于收縮不均等引起了翹曲。盡管如此，冷卻后，不同結(jié)點(diǎn)的溫度差異很小，翹曲影響也不明顯。
對(duì)設(shè)計(jì)師而言，設(shè)計(jì)一個(gè)有較少熱殘余應(yīng)力及及有效的冷卻系統(tǒng)的模具是非常重要的。對(duì)產(chǎn)品的分析，從開始進(jìn)行分析注塑產(chǎn)品的階段，不同的負(fù)載因子在產(chǎn)品上的壓力分布在這兩個(gè)尺寸分析中得到觀察。圖18—21顯示了不同負(fù)荷應(yīng)力增量的等高線圖。一個(gè)臨界點(diǎn)，即127節(jié)點(diǎn)上，產(chǎn)品的最大拉應(yīng)力被選定出來進(jìn)行分析。在圖22和23 中繪制了這一點(diǎn)上的壓力應(yīng)變曲線和負(fù)載應(yīng)力曲線從圖23繪制的這一點(diǎn)上的負(fù)載應(yīng)力曲線，很清楚的看到產(chǎn)品經(jīng)受很大的負(fù)荷，直到23的負(fù)載力，即1150牛。這意味著產(chǎn)品能夠承受最大在1150牛的拉伸力。高于這個(gè)值的負(fù)荷會(huì)引起產(chǎn)品的失靈?；趫D23，失靈很可能會(huì)發(fā)生在產(chǎn)品的固定端附近的區(qū)域，這里最大應(yīng)力是3.27×107 Pa 。由于該產(chǎn)品的生產(chǎn)是以為了翹曲測(cè)試，也沒有分析與拉伸荷載的關(guān)系，所以其應(yīng)力分析揭示的是很有限的信息。未來，盡管如此，建議產(chǎn)品服務(wù)條件應(yīng)該確定，以便能夠進(jìn)行各種其他的負(fù)載下的進(jìn)一步分析。
圖9 284結(jié)點(diǎn)溫度分布圖
圖10 213結(jié)點(diǎn)的溫度分布圖
圖11 302結(jié)點(diǎn)的溫度分布圖
圖12 290結(jié)點(diǎn)的溫度分布圖
圖13 278結(jié)點(diǎn)的溫度分布圖
圖14 1830結(jié)點(diǎn)的溫度分布圖
圖15 1904結(jié)點(diǎn)的溫度分布圖
圖16 1853結(jié)點(diǎn)的溫度分布圖
圖17 1866結(jié)點(diǎn)的溫度分布圖
圖18 在荷載增量為1時(shí)的等效應(yīng)力圖
圖19 在荷載增量為14時(shí)的等效應(yīng)力圖
圖20 在荷載增量為16時(shí)的等效應(yīng)力圖
圖21 在荷載增量為23時(shí)的等效應(yīng)力圖
圖22 ABS的應(yīng)力與應(yīng)變曲線
圖23 ABS樹脂的應(yīng)力與荷載增量曲線
4. 結(jié)論
設(shè)計(jì)的模具可以生產(chǎn)用于確定影響翹曲參數(shù)的高質(zhì)量的翹曲測(cè)試樣本。該測(cè)試樣本生產(chǎn)成本低，只涉及很少德控門的修整。注塑模具的熱分析提供了熱殘余應(yīng)力對(duì)標(biāo)本的變形后形狀的影響的解釋另外對(duì)產(chǎn)品拉伸應(yīng)力的分析成功的預(yù)測(cè)翹曲測(cè)試樣本在出現(xiàn)失靈之前的拉伸載荷。
鳴謝：作者向發(fā)起本文出版的馬來西亞博特拉大學(xué)工程學(xué)院致謝。
參考文獻(xiàn)
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Journal of Materials Processing Technology 171 2006 259 267 Design and thermal analysis of plastic injection mould S H Tang Y M Kong S M Sapuan R Samin S Sulaiman Department of Mechanical and Manufacturing Engineering Universiti Putra Malaysia 43400 Serdang Selangor Malaysia Received 3 September 2004 accepted 21 June 2005 Abstract This paper presents the design of a plastic injection mould for producing warpage testing specimen and performing thermal analysis for the mould to access on the effect of thermal residual stress in the mould The technique theory methods as well as consideration needed in designing of plastic injection mould are presented Design of mould was carried out using commercial computer aided design software Unigraphics Version 13 0 The model for thermal residual stress analysis due to uneven cooling of the specimen was developed and solved using a commercial nite element analysis software called LUSAS Analyst Version 13 5 The software provides contour plot of temperature distribution for the model and also temperature variation through the plastic injection molding cycle by plotting time response curves The results show that shrinkage is likely to occur in the region near the cooling channels as compared to other regions This uneven cooling effect at different regions of mould contributed to warpage 2005 Elsevier B V All rights reserved Keywords Plastic Injection mould Design Thermal analysis 1 Introduction Plastic industry is one of the world s fastest growing industries ranked as one of the few billion dollar industries Almost every product that is used in daily life involves the usage of plastic and most of these products can be produced by plastic injection molding method 1 Plastic injection molding process is well known as the manufacturing process to create products with various shapes and complex geometry at low cost 2 The plastic injection molding process is a cyclic process There are four signi cant stages in the process These stages are lling packing cooling and ejection The plastic injec tion molding process begins with feeding the resin and the appropriate additives from the hopper to the heating injection system of the injection plastic injection molding machine 3 This is the lling stage in which the mould cavity is lled with hot polymer melt at injection temperature After the cav ity is lled in the packing stage additional polymer melt is packed into the cavity at a higher pressure to compensate the expected shrinkage as the polymer solidi es This is followed Corresponding author E mail address saihong eng upm edu my S H Tang by cooling stage where the mould is cooled until the part is suf ciently rigid to be ejected The last step is the ejection stage in which the mould is opened and the part is ejected after which the mould is closed again to begin the next cycle 4 The design and manufacture of injection molded poly meric parts with desired properties is a costly process domi nated by empiricism including the repeated modi cation of actual tooling Among the task of mould design designing the mould speci c supplementary geometry usually on the core side is quite complicated by the inclusion of projection and depression 5 In order to design a mould many important designing factors must be taken into consideration These factors are mould size number of cavity cavity layouts runner systems gating systems shrinkage and ejection system 6 In thermal analysis of the mould the main objective is to analyze the effect of thermal residual stress or molded in stresses on product dimension Thermally induced stresses develop principally during the cooling stage of an injection molded part mainly as a consequence of its low thermal conductivity and the difference in temperature between the molten resin and the mould An uneven temperature eld exists around product cavity during cooling 7 0924 0136 see front matter 2005 Elsevier B V All rights reserved doi 10 1016 j jmatprotec 2005 06 075260 S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 During cooling location near the cooling channel experi ences more cooling than location far away from the cooling channel This different temperature causes the material to experience differential shrinkage causing thermal stresses Signi cant thermal stress can cause warpage problem There fore it is important to simulate the thermal residual stress eld of the injection molded part during the cooling stage 8 By understanding the characteristics of thermal stress distribu tion deformation caused by the thermal residual stress can be predicted In this paper the design of a plastic injection mould for producing warpage testing specimen and for performing ther mal analysis for the mould to access on the effect of thermal residual stress in the mould is presented 2 Methodology 2 1 Design of warpage testing specimen This section illustrates the design of the warpage testing specimen to be used in plastic injection mould It is clear that warpage is the main problem that exists in product with thin shell feature Therefore the main purpose of the prod uct development is to design a plastic part for determining the effective factors in the warpage problem of an injection moulded part with a thin shell The warpage testing specimen is developed from thin shell plastics The overall dimensions of the specimen were 120 mm in length 50 mm in width and 1 mm in thickness The material used for producing the warpage testing specimen was acrylonitrile butadiene stylene ABS and the injection temperature time and pressure were 210 C 3 s and 60 MPa respectively Fig 1 shows the warpage testing specimen pro duced 2 2 Design of plastic injection mould for warpage testing specimen This section describes the design aspects and other consid erations involved in designing the mould to produce warpage testing specimen The material used for producing the plastic Fig 1 Warpage testing specimen produced injection mould for warpage testing specimen was AISI 1050 carbon steel Four design concepts had been considered in designing of the mould including i Three plate mould Concept 1 having two parting line with single cavity Not applicable due to high cost ii Two plate mould Concept 2 having one parting line with single cavity without gating system Not applicable due to low production quantity per injection iii Two plate mould Concept 3 having one parting line with double cavities with gating and ejection system Not applicable as ejector pins might damage the product as the product is too thin iv Two plate mould Concept 4 having one parting line with double cavities with gating system only used sprue puller act as ejector to avoid product damage during ejection In designing of the mould for the warpage testing spec imen the fourth design concept had been applied Various design considerations had been applied in the design Firstly the mould was designed based on the platen dimen sion of the plastic injection machine used BOY 22D There is a limitation of the machine which is the maximum area of machine platen is given by the distance between two tie bars The distance between tie bars of the machine is 254 mm Therefore the maximum width of the mould plate should not exceed this distance Furthermore 4 mm space had been reserved between the two tie bars and the mould for mould setting up and handling purposes This gives the nal max imum width of the mould as 250 mm The standard mould base with 250 mm 250 mm is employed The mould base is tted to the machine using Matex clamp at the upper right and lower left corner of the mould base or mould platen Dimen sions of other related mould plates are shown in Table 1 The mould had been designed with clamping pressure having clamping force higher than the internal cavity force reaction force to avoid ashing from happening Based on the dimensions provided by standard mould set the width and the height of the core plate are 200 and 250 mm respectively These dimensions enabled design of two cavities on core plate to be placed horizontally as there is enough space while the cavity plate is left empty and it is only xed with sprue bushing for the purpose of feeding molten plastics Therefore it is only one standard parting line was designed at Table 1 Mould plates dimensions Components Size mm width height thickness Top clamping plate 250 250 25 Cavity plate 200 250 40 Core plate 200 250 40 Side plate support plate 37 250 70 Ejector retainer plate 120 250 15 Ejector plate 120 250 20 Bottom clamping plate 250 250 25S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 261 the surface of the product The product and the runner were released in a plane through the parting line during mould opening Standard or side gate was designed for this mould The gate is located between the runner and the product The bottom land of the gate was designed to have 20 slanting and has only 0 5 mm thickness for easy de gating purpose The gate was also designed to have 4 mm width and 0 5 mm thickness for the entrance of molten plastic In the mould design the parabolic cross section type of runner was selected as it has the advantage of simpler machin ing in one mould half only which is the core plate in this case However this type of runner has disadvantages such as more heat loss and scrap compared with circular cross section type This might cause the molten plastic to solidify faster This problem was reduced by designing in such a way that the runner is short and has larger diameter which is 6 mm in diameter It is important that the runner designed distributes material or molten plastic into cavities at the same time under the same pressure and with the same temperature Due to this the cavity layout had been designed in symmetrical form Another design aspect that is taken into consideration was air vent design The mating surface between the core plate and the cavity plate has very ne nishing in order to prevent ashing from taking place However this can cause air to trap in the cavity when the mould is closed and cause short shot or incomplete part Suf cient air vent was designed to ensure that air trap can be released to avoid incomplete part from occurring The cooling system was drilled along the length of the cavities and was located horizontally to the mould to allow even cooling These cooling channels were drilled on both cavity and core plates The cooling channels provided suf cient cooling of the mould in the case of turbulent ow Fig 2 shows cavity layout with air vents and cooling channels on core plate In this mould design the ejection system only consists of the ejector retainer plate sprue puller and also the ejector Fig 2 Cavity layout with air vents and cooling channels plate The sprue puller located at the center of core plate not only functions as the puller to hold the product in position when the mould is opened but it also acts as ejector to push the product out of the mould during ejection stage No addi tional ejector is used or located at product cavities because the product produced is very thin i e 1 mm Additional ejec tor in the product cavity area might create hole and damage to the product during ejection Finally enough tolerance of dimensions is given consid eration to compensate for shrinkage of materials Fig 3 shows 3D solid modeling as well as the wireframe modeling of the mould developed using Unigraphics 3 Results and discussion 3 1 Results of product production and modi cation From the mould designed and fabricated the warpage testing specimens produced have some defects during trial run The defects are short shot ashing and warpage The short shot is subsequently eliminated by milling of additional air vents at corners of the cavities to allow air trapped to Fig 3 3D solid modeling and wireframe modeling of the mould 262 S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 Fig 4 Extra air vents to avoid short shot escape Meanwhile ashing was reduced by reducing the packing pressure of the machine Warpage can be controlled by controlling various parameters such as the injection time injection temperature and melting temperature After these modi cations the mould produced high qual ity warpage testing specimen with low cost and required little nishing by de gating Fig 4 shows modi cations of the mould which is machining of extra air vents that can eliminate short shot 3 2 Detail analysis of mould and product After the mould and products were developed the analysis of mould and the product was carried out In the plastic injec tion moulding process molten ABS at 210 C is injected into the mould through the sprue bushing on the cavity plate and directed into the product cavity After cooling takes place the product is formed One cycle of the product takes about 35 s including 20 s of cooling time The material used for producing warpage testing speci men was ABS and the injection temperature time and pres sure were 210 C 3 s and 60 MPa respectively The material selected for the mould was AISI 1050 carbon steel Properties of these materials were important in determin ing temperature distribution in the mould carried out using nite element analysis Table 2 shows the properties for ABS and AISI 1050 carbon steel The critical part of analysis for mould is on the cavity and core plate because these are the place where the product is formed Therefore thermal analysis to study the temperature Fig 5 Model for thermal analysis distribution and temperature at through different times are performed using commercial nite element analysis software called LUSAS Analyst Version 13 5 A two dimensional 2D thermal analysis is carried out for to study the effect of thermal residual stress on the mould at different regions Due to symmetry the thermal analysis was performed by modeling only the top half of the vertical cross section or side view of both the cavity and core plate that were clamped together during injection Fig 5 shows the model of thermal analysis analyzed with irregular meshing Modeling for the model also involves assigning properties and process or cycle time to the model This allowed the nite element solver to analyze the mould modeled and plot time response graphs to show temperature variation over a certain duration and at different regions For the product analysis a two dimensional tensile stress analysis was carried using LUSAS Analyst Version 13 5 Basically the product was loaded in tension on one end while the other end is clamped Load increments were applied until the model reaches plasticity Fig 6 shows loaded model of the analysis 3 3 Result and discussion for mould and product analysis For mould analysis the thermal distribution at different time intervals was observed Fig 7 shows the 2D analysis Table 2 Material properties for mould and product Carbon Steel AISI 1050 mould ABS Polymer product Density 7860 kg m 3 Density 1050 kg m 3 Young s modulus E 208 GPa Young s modulus E 2 519 GPa Poisson s ratio 0 297 Poisson s ratio 0 4 Yield strength S Y 365 4 MPa Yield strength S Y 65 MPa Tensile strength S UTS 636 MPa Thermal expansion 65 10 6 K 1 Thermal expansion 11 65 10 6 K 1 Conductivity k 0 135 W m K Conductivity k 49 4 W m K Speci c heat c 1250 J kg K Speci c heat c 477 J kg K S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 263 Fig 6 Loaded model for analysis of product contour plots of thermal or heat distribution at different time intervals in one complete cycle of plastic injection molding For the 2D analysis of the mould time response graphs are plotted to analyze the effect of thermal residual stress on the products Fig 8 shows nodes selected for plotting time response graphs Figs 9 17 show temperature distribution curves for dif ferent nodes as indicated in Fig 8 From the temperature distribution graphs plotted in Figs 9 17 it is clear that every node selected for the graph plotted experiencing increased in temperature i e from the ambient temperature to a certain temperature higher than the ambient temperature and then remained constant at this temperature for a certain period of time This increase in tem perature was caused by the injection of molten plastic into the cavity of the product After a certain period of time the temperature is then further increased to achieve the highest temperature and remained constant at that temperature Increase in temper ature was due to packing stages that involved high pressure Fig 7 Contour plots of heat distribution at different time intervals 264 S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 Fig 8 Selected nodals near product region for time response graph plots Fig 9 Temperature distribution graph for Node 284 Fig 10 Temperature distribution graph for Node 213 Fig 11 Temperature distribution graph for Node 302 Fig 12 Temperature distribution graph for Node 290 which caused the temperature to increase This temperature remains constant until the cooling stage starts which causes reduction in mould temperature to a lower value and remains at this value The graphs plotted were not smooth due to the absence of function of inputting lling rate of the molten plastic as well as the cooling rate of the coolant The graphs plotted only show maximum value of temperature that can be achieved in the cycle The most critical stage in the thermal residual stress anal ysis is during the cooling stage This is because the cooling Fig 13 Temperature distribution graph for Node 278 S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 265 Fig 14 Temperature distribution graph for Node 1838 Fig 15 Temperature distribution graph for Node 1904 stage causes the material to cool from above to below the glass transition temperature The material experiences differ ential shrinkage that causes thermal stress that might result in warpage From the temperature after the cooling stage as shown in Figs 9 17 it is clear that the area node located near the cooling channel experienced more cooling effect due to fur Fig 16 Temperature distribution graph for Node 1853 Fig 17 Temperature distribution graph for Node 1866 ther decreasing in temperature and the region away from the cooling channel experienced less cooling effect More cool ing effect with quite fast cooling rate means more shrinkage is occurring at the region However the farthest region Node 284 experience more cooling although far away from cooling channel due to heat loss to environment As a result the cooling channel located at the center of the product cavity caused the temperature difference around the middle of the part higher than other locations Compressive stress was developed at the middle area of the part due to more shrinkage and caused warpage due to uneven shrinkage that happened However the temperature differences after cooling for different nodes are small and the warpage effect is not very signi cant It is important for a designer to design a mould that has less thermal residual stress effect with ef cient cooling system For the product analysis from the steps being carried out to analyze the plastic injection product the stress distribution on product at different load factor is observed in the two dimensional analysis Figs 18 21 show the contour plots of equivalent stress at different load increments A critical point Node 127 where the product experiences maximum tensile stress was selected for analysis The stress versus strain curve and the load case versus stress curves at this point were plotted in Figs 22 and 23 From the load case versus stress curves at this point plotted in Fig 23 it is clear that the product experiencing increased in tensile load until it reached the load factor of 23 which is 1150 N This means that the product can withstand tensile load until 1150 N Load higher than this value causes failure to the product Based on Fig 23 the failure is likely to occur at the region near to the xed end of the product wi