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注塑模具的設(shè)計(jì)和熱分析
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
摘 要
本文介紹塑料注塑模具在生產(chǎn)翹曲測試樣品和進(jìn)行模具內(nèi)模具熱殘余應(yīng)力的影響的熱分析方面的設(shè)計(jì)。對在注塑模具設(shè)計(jì)中所需的技術(shù)，理論，方法以及應(yīng)該考慮的因素進(jìn)行描述。使用商業(yè)計(jì)算機(jī)輔助版本為13.0 Unigraphics設(shè)計(jì)軟件進(jìn)行模具設(shè)計(jì)。由于試樣的不均勻冷卻，用于進(jìn)行殘余熱應(yīng)力分析模型使用被稱為LUSAS分析師，版本為13.5的商用有限元分析軟件進(jìn)行建立和解決。該軟件通過塑料注塑成型周期為模型提供溫度分布的曲線圖和溫度變化程度，這個(gè)注塑成型周期通過繪制時(shí)間響應(yīng)曲線得到。結(jié)果表明，相比其他區(qū)域來說，收縮可能發(fā)生在冷卻通道的附近。這種模具在不同區(qū)域的不平衡冷卻效果歸咎于翹曲.?2005愛思唯爾B.V.版權(quán)所有。
關(guān)鍵詞：塑料注塑模具設(shè)計(jì);熱分析
1 前言
塑料工業(yè)是世界上增長速度最快的行業(yè)之一，位居少量的十億美元的產(chǎn)業(yè)之中。在日常生活中幾乎每一個(gè)產(chǎn)品都涉及到塑料的使用，并且大多數(shù)這些產(chǎn)品可以通過塑料注塑成型方法[1]進(jìn)行生產(chǎn)。注塑
成型工藝以利用較低的成本[2]創(chuàng)造具有各種外觀和復(fù)雜的幾何形狀的產(chǎn)品而著稱。注塑成型過程是一個(gè)循環(huán)的過程。在這個(gè)過程中有四個(gè)重要的階段。這些階段是：加料，注射成型，冷卻和推出。塑料注塑成型過程開始于將樹脂和合適的添加劑從注射機(jī)的料斗送到注塑機(jī)的注射系統(tǒng)[3]中。這是加料階段，在這個(gè)階段熱聚合物熔體在注射溫度下充滿型腔。在聚合物充滿型腔之后，在注射成型階段，額外的聚合物熔體在較高壓力作用下注入型腔，以補(bǔ)償由于熔體凝固而可能引起的收縮。緊接著的是冷卻階段”在這階段，模具被冷卻直到塑件有足夠的剛性被推出。最后一步是“推出階段”在此階段模具被打開，塑料制件被推出，之后模具關(guān)閉重新開始下一個(gè)周期[4]。設(shè)計(jì)和制造具有理想特性的注塑成型塑料制件一個(gè)昂貴的過程，這主要靠經(jīng)驗(yàn)包括反復(fù)修改實(shí)際的工具。在模具設(shè)計(jì)任務(wù)中，設(shè)計(jì)通常在核心部位加一個(gè)特定幾何形狀想凸起和凹坑的模具是非常復(fù)雜，為了設(shè)計(jì)一套模具，許多重要的設(shè)計(jì)因素必須考慮。這些因素是模具大小，模腔的數(shù)量，腔的布局，熱流道系統(tǒng)，澆注系統(tǒng)，補(bǔ)縮和推出系統(tǒng)[6]。在模具的熱分析中，其主要目的是分析熱殘余應(yīng)力或模壓對產(chǎn)品尺寸的影響。熱誘導(dǎo)應(yīng)力主要發(fā)生在注射的冷卻階段，主要是低的熱導(dǎo)率和熔融聚合物與模具之間的溫度差異所導(dǎo)致的。在冷卻過程中在產(chǎn)品內(nèi)存在不均勻的溫度場。
在冷卻過程中，靠近冷卻管道的地方比遠(yuǎn)離冷卻冷卻管道的地方的受的冷卻程大
一些。 這種溫度的不同，使得材料存在不同的收縮而引起熱應(yīng)力。顯著的熱應(yīng)力可以導(dǎo)致翹曲問題。因此，在冷卻階段對注塑成型塑件的殘余熱應(yīng)力進(jìn)行模擬是很重要的[8]。通過了解熱殘余應(yīng)力的分布特點(diǎn)，能對其引起的變形進(jìn)行預(yù)測。在本文中塑料注塑模具在生產(chǎn)翹曲測試樣品和進(jìn)行模具內(nèi)部模具的熱殘余應(yīng)力 影響的分析的設(shè)計(jì)得以描述。
2 方法論
2.1 翹曲測試樣本的設(shè)計(jì)
本節(jié)說明翹曲測試樣的設(shè)計(jì)將用于注塑模具。很明顯翹曲是一些薄的殼形件中存在的主要問題。因此，產(chǎn)品發(fā)展的主要目的是設(shè)計(jì)出一種塑料制件以確定出薄的殼形的注塑成型塑件翹曲問題的有效因素。翹曲測試樣品來源于薄的殼形的塑料制品。樣本的外形尺寸長120毫米，寬50毫米和厚度為1毫米。
用于生產(chǎn)翹曲測試試樣材料是丙烯腈—丁二烯—苯乙烯（ABS），注射溫度，時(shí)間和壓力分別為210?C，3秒和60兆帕。圖1顯示了翹曲測試試樣的生產(chǎn)。
圖1 生產(chǎn)的翹曲測試樣品
2.2 翹曲測試樣本塑料注塑模具設(shè)計(jì)
本節(jié)介紹的設(shè)計(jì)方面和其他與設(shè)計(jì)生產(chǎn)翹曲測試樣品模具相關(guān)的因素。用于生產(chǎn)用于產(chǎn)生翹曲測試樣品的塑料注塑模具的材料是AISI 1050碳鋼。在設(shè)計(jì)模具 過程中四中設(shè)計(jì)方案被考慮包括：
一，三板式（方案1）雙分型面一模一腔模具。由于成本高，不適用。
二，雙板式（方案2）單分型面一模一腔無澆注系統(tǒng)模具，生產(chǎn)效率低，不適用。
三，雙板式（方案3）單分型面一模二腔有澆注系統(tǒng)和推出機(jī)構(gòu)，由于塑件過于薄，推桿可能會(huì)損壞產(chǎn)品。
四，雙板式（方案4）單分型面一模二腔有澆注系統(tǒng)，利用澆口拉出器扮演推桿的角色避免在推出過程中產(chǎn)品受損。
在設(shè)計(jì)得到翹曲測試樣品的模具中，第四種方案被應(yīng)用，不用的設(shè)計(jì)因素在設(shè)計(jì)中已經(jīng)被采用。首先，模具的設(shè)計(jì)是基于注射機(jī)活動(dòng)模板尺寸，所用的注射機(jī)是桿（BOY22D）。這種注射機(jī)有一個(gè)限制，那就是活動(dòng)模板的最大面積通過兩根拉桿之間的距離決定，注射機(jī)的拉桿之間的距離是254毫米。因此，模具板最大寬度不能超過這個(gè)距離。此外，為保證模具的開啟和運(yùn)轉(zhuǎn)，兩根拉桿與模具之間保留4毫米的距離，這使最終模具的最大寬度為250毫米。模具標(biāo)準(zhǔn)模架采用250毫米×250毫米。模架通過壓板從模架或模板的右上角和左下角與注射機(jī)進(jìn)行配合，其他與模板相關(guān)的尺寸如表1中所示。模具的設(shè)計(jì)要使得鎖模壓力產(chǎn)生的鎖模力高于內(nèi)部型腔的壓力（反壓力）以避免發(fā)生溢料?；谀＞邩?biāo)準(zhǔn)件庫提供的尺寸，型芯的寬度和高度分別為200和250毫米，這些尺寸使在型芯上設(shè)計(jì)兩腔時(shí)兩腔的位置能夠均衡的放置，這樣在凹模是空的時(shí)候模具有足夠的空間去容納塑料熔體注入腔內(nèi)用到的澆道襯套，因此，唯一的一個(gè)標(biāo)準(zhǔn)分型面設(shè)計(jì)在產(chǎn)品表面。
表1 模具板尺寸(寬度×高×厚)
頂部夾緊板
250×250×25
凹模
200×250×40
型芯
200×250×40
側(cè)板/支撐板
37×250×70
推桿固定板
120×250×15
推板板
120×250×20
底部夾緊板
250×250×25
在模具打開時(shí)，該產(chǎn)品和凝料在分型面被推出來。模具設(shè)有主澆口和側(cè)澆口。為了順利澆注，澆口的底部設(shè)計(jì)成傾斜角為20度，厚度為0.5毫米。并且為了塑料熔體能夠進(jìn)入型腔，澆口設(shè)計(jì)成寬4mm,厚0.5mm。在模具設(shè)計(jì)中主流道,選用拋物線截面類型，這種截面優(yōu)點(diǎn)是在一套模具中可以使注射機(jī)簡化一半，在這種情況下，它就是型芯。然而這種類型的流道與圓截面的類型相比存在有更多的熱量損失和廢料的缺點(diǎn)。這可能會(huì)是塑料熔體凝固的更快。這個(gè)問題可以通過設(shè)計(jì)一個(gè)短和大的直徑的流道來緩解，流道的直徑是6mm。設(shè)計(jì)的流道要在同一時(shí)間，同一溫度和同等的壓力下將材料或者塑料熔體分流到凹模內(nèi)這一點(diǎn)是很重要的。由于這個(gè)原因，型腔布局設(shè)計(jì)成對稱形式。另一個(gè)應(yīng)該被考慮的設(shè)計(jì)因素就是通風(fēng)孔的設(shè)計(jì)，型芯和凹模的配合面是經(jīng)過非常的精細(xì)的加工以防止溢料的發(fā)生。然而這會(huì)導(dǎo)致當(dāng)模具關(guān)閉時(shí)候空氣的進(jìn)入并導(dǎo)致短射或有缺陷。為保證進(jìn)入的空氣被釋放從而避免成型不完全，需要設(shè)計(jì)足夠的通風(fēng)孔。冷卻系統(tǒng)沿著模腔開通，并且均衡的布置在模具中以確保均勻的冷卻。這些冷卻管道在凹模和型芯的兩側(cè)可設(shè)。它在湍流的情況下對模具進(jìn)行充分的冷卻。圖2所示為通風(fēng)孔
和冷卻管道在型腔上的布局。
圖2 通風(fēng)孔和冷卻通道在型芯上的布局
在模具設(shè)計(jì)中，推出機(jī)構(gòu)僅僅有推桿固定板，推桿和推板組成。推桿位于型芯的中間位置，它不僅僅是作為一根推桿準(zhǔn)確的固定塑料制品。同時(shí)在脫模階段，當(dāng)模具打開時(shí)它也充當(dāng)推出器將塑料制品推出模具。在型腔內(nèi)沒有使用和放置額外的推出裝置因?yàn)樯a(chǎn)的產(chǎn)品非常的薄，即1mm。在型腔中加額外的推出裝置可能會(huì)使產(chǎn)品出現(xiàn)小孔并且在推出過程中損壞產(chǎn)品。最后，足夠的尺寸公差給予考慮以補(bǔ)償材料的收縮。圖3顯示了應(yīng)用Unigraphics開發(fā)的模具的三維實(shí)體造型以及線框造型。
圖3 模具的三維造型和線框模型
3 結(jié)果與討論
3.1 產(chǎn)品的生產(chǎn)和修改結(jié)果
在審核過程中，設(shè)計(jì)和制造的模具生產(chǎn)出來的翹曲測試樣品存在一些缺陷。這些缺陷只要是短射，溢料和翹曲。短射可以通過隨后在凹模的拐角處加工額外的通風(fēng)孔以讓更多的進(jìn)入里面的空氣溢出而得以消除。同時(shí)，溢料可以通過減小注射機(jī)的成型壓力而減少。翹曲可以通過控制各種參數(shù)比如注射時(shí)間，注射溫度和熔化溫度來控制。對這些進(jìn)行修正之后，模具可以通過澆口以較低的價(jià)格， 較少的工序生產(chǎn)出高質(zhì)量的翹曲測試樣本。圖4為經(jīng)修改夠加工了額外的通風(fēng)孔以消除短射的模具。
圖4 額外的通風(fēng)孔消除短射
3.2 模具和產(chǎn)品的詳細(xì)分析
模具和產(chǎn)品開發(fā)后，分析模具和產(chǎn)品開展了。在塑料注射成型過程中，熔融的ABS在210?C通過凹模上澆道襯套被注入，直接進(jìn)入型腔中，冷卻開始后，該產(chǎn)品就形成了。一個(gè)產(chǎn)品的周期大約需要35s包括20s的冷卻時(shí)間。用于生產(chǎn)生產(chǎn)翹曲測試試樣的材料是ABS，注射溫度，時(shí)間和壓力分別為210?C，3秒和60兆帕。模具采用AISI 1050碳鋼作為材料。這些材料的性質(zhì)是非常重要的，它決定了模具中的溫度分布，可以利用溫度分布進(jìn)行有限元分析。表2為ABS和AISI 1050碳鋼材料的性質(zhì)。
表2 模具和產(chǎn)品的材料屬性
屬性
模具(碳鋼AISI 1050)
產(chǎn)品（ABS聚合物）
密度(ρ)
7860 kg/m3
1050 kg/m3
楊氏模量(E)
208 Gpa
2.519 Gpa
泊松比(ν)
0.297
0.4
屈服強(qiáng)度(SY)
365.4Mpa
65 Mpa
拉伸強(qiáng)度(SUTS)
636Mpa
636Mpa
熱膨脹系數(shù)(α)
65×10-6K-1
11.65×10-6K-1
電導(dǎo)率(K)
0.135 W/(m K)
49.4 W/(m K)
比熱(C)
1250 J/(kg K)
477 J/(kg K)
對模具最關(guān)鍵的分析部分是在凹模和型芯上，因?yàn)樗鼈兪钱a(chǎn)品成型的部位。因此熱分析來研究溫度的分布和在不同時(shí)間的溫度是使用商業(yè)的有限元分析軟件來完成，這款軟件叫做LUSAS Analyst版本為13.5.為了研究熱殘余應(yīng)力對模具不同部位的影響，進(jìn)行一次二維的熱分析。由于對稱性，熱分析通過凹模和型芯的垂直截面和側(cè)面的上半部分的這個(gè)模型來實(shí)現(xiàn)，凹模和型芯在注射階段就固定在一起了。圖5為無規(guī)律的配合分析的熱分析模型。
圖5 熱分析模型
該模型的建模還涉及分配屬性 ，工序或模型的周期。這需要有限元分析求解器來分析模具建模和繪出時(shí)間響應(yīng)圖以便顯示在某一段時(shí)間不同區(qū)域的溫度變化。
針對產(chǎn)品分析利用版本為13.5LUSAS分析師進(jìn)行一個(gè)二維的拉伸強(qiáng)度的分析。基本上述樣品處于一端受拉另一端固定的狀態(tài)。增加載荷使模型達(dá)到塑性變形。圖6為受載荷作用的模型的分析。
圖6 為分析產(chǎn)品所用的受載荷模型
3.3 模具和產(chǎn)品分析的結(jié)果和討論
針對模具分析，對在不同時(shí)間間隔內(nèi)的熱分布進(jìn)行了觀察。圖7為塑料注塑成型在一個(gè)完整的周期不同的時(shí)間間隔內(nèi)熱或熱量分布的等高線圖二維分析。
圖7 不同時(shí)間間隔內(nèi)的熱分布等高曲線
對模具的二維分析，繪制了時(shí)間響應(yīng)圖表以分析熱殘余應(yīng)力對產(chǎn)品的影響。圖8為為繪制時(shí)間響應(yīng)圖標(biāo)而選取的點(diǎn)。圖9-17為在圖8中所示不同點(diǎn)溫度分布曲線。從圖9-17中繪制的溫度分布中可以看出，圖中的每一個(gè)點(diǎn)的溫度都在上升，即從環(huán)境溫度到某一高于環(huán)境溫度的溫度然后在某一段時(shí)間內(nèi)保持這個(gè)溫度不變。這種溫度的升高是由于產(chǎn)品熔體塑料進(jìn)入型腔所致。在一段時(shí)間之后溫度繼續(xù)上升至最高溫度并且保持在最高溫度，這個(gè)溫度的升高與成型階段高的壓力有關(guān)。這個(gè)溫度一直持續(xù)到冷卻階段的開始，冷卻使得模具的溫度到一個(gè)較低的值且保持在這個(gè)水平。繪制的曲線并不光滑，這是因?yàn)槿鄙佥斎胨芰先垠w填充和冷卻水冷卻速度的功能。圖表只顯示在這個(gè)周期內(nèi)溫度能夠達(dá)到的最大值。在熱殘余應(yīng)力時(shí)至關(guān)重要的階段是冷卻階段。這是因?yàn)槔鋮s階段使材料從高于玻璃化轉(zhuǎn)變溫度到低于這個(gè)溫度。材料應(yīng)成分收縮而產(chǎn)生熱應(yīng)力。熱應(yīng)力有可能造成翹曲。
圖8 在產(chǎn)品內(nèi)部選擇的點(diǎn)作的時(shí)間響應(yīng)圖
圖9 第284點(diǎn)的溫度分布圖
圖10 第213點(diǎn)的溫度分布圖
圖11 第302點(diǎn)的溫度分布圖
圖12 第290個(gè)點(diǎn)的溫度分布圖
圖13 第278點(diǎn)的溫度分布圖
圖14 第1838點(diǎn)的溫度分布圖
圖15 第1904點(diǎn)的溫度分布圖
圖16 第1853點(diǎn)的溫度分布圖
圖17 第1866點(diǎn)的溫度分布圖
從圖9-17所示的冷卻階段之后的溫度可以看出，靠近冷卻管道的區(qū)域由于溫度降低快而冷卻程度大一些，而遠(yuǎn)離冷卻管道的區(qū)域冷卻程度要小一些。冷卻速度快，冷卻程度大意味著在這部分區(qū)域發(fā)生更多的收縮。然而最遠(yuǎn)的點(diǎn)，第284點(diǎn)由于熱量散失到環(huán)境中而冷卻程度更大。
結(jié)果，由于冷卻管道位于型腔的中間位置而使中間部分的溫度差異比其余的位置要大。由于收縮量大使得塑件中間部分產(chǎn)生壓應(yīng)力。不均勻收縮導(dǎo)致翹曲。但是，冷卻之后各個(gè)點(diǎn)的溫度差異小了，翹曲變形就不會(huì)那么顯著了。對于一個(gè)設(shè)計(jì)師，設(shè)計(jì)一套具有高效冷卻系統(tǒng)和較小熱殘余應(yīng)力的模具是很重要的。
對于產(chǎn)品分析，從開展研究的第一步到分析注塑成型產(chǎn)品整個(gè)過程，產(chǎn)品在不同載荷因子作用下的應(yīng)力分布都進(jìn)行了二維觀察分析。圖18-21為在不同載荷作用下對應(yīng)的應(yīng)力等高線圖。一個(gè)至關(guān)重要的點(diǎn)127，這點(diǎn)處塑料制品收到最好的拉應(yīng)力作用，這一點(diǎn)的應(yīng)力-應(yīng)變關(guān)系以及受力情況與應(yīng)力的關(guān)系曲線都繪制在圖22和圖23中。從圖23中受力情況與應(yīng)力關(guān)系的曲線來看，制品所受的拉力一直增加直到達(dá)到圖23中的載荷因子的值，這個(gè)值是1150N。這就意味著制品可以承受1150N的拉力。高于這個(gè)值的載荷會(huì)導(dǎo)致制品破壞。在圖23中，這種破壞更可能發(fā)生在試樣的固定端所在的區(qū)域，這個(gè)最大應(yīng)力為3.27x107Pa.
試樣應(yīng)力分析由于產(chǎn)品是為了進(jìn)行翹曲測試并且與拉力分析無關(guān)而使得其解釋非常有限的信息。然而在將來，研究者指出產(chǎn)品的所處狀態(tài)應(yīng)該定下來以便于在不同的載荷作用下試樣的變形展開更深入的研究。
圖18 載荷增量1作用下的相對應(yīng)力圖
圖19 載荷增量14作用下的相對應(yīng)力圖
圖20 載荷增量16作用下的相對應(yīng)力圖
圖21 載荷增量23作用下的相對應(yīng)力圖
4 結(jié)論
設(shè)計(jì)出來的模具使生產(chǎn)高質(zhì)量的翹曲測試樣品成為可能。這種成為是樣品可以決定出影響翹曲的參數(shù)。這種測試樣品以較低的價(jià)格生產(chǎn)并且只需要比較少的加工。塑料注塑成型模具的熱分析使得熱殘余應(yīng)力對產(chǎn)品的變形形狀的影響被理解，產(chǎn)品的拉伸應(yīng)力分析能夠成功的預(yù)測翹曲測試樣品能夠承受的最大拉伸應(yīng)力。
圖22 ABS材料的應(yīng)力—應(yīng)變關(guān)系曲線
圖23 ABS材料的應(yīng)力—載荷關(guān)系曲線
致謝
作者要感謝馬來西亞博特拉大學(xué)工程學(xué)院推出的本出版物。
參考文獻(xiàn)
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Journal of Materials Processing Technology 171 2006 259 267 Abstract the in Unigraphics using distrib results at K 1 industries Almost usage by molding to at There are tion appropriate system This with ity pack e 0924 0136 doi 10 1016 j jmatprotec 2005 06 075 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 This paper presents the design of a plastic injection mould for producing warpage testing specimen and performing thermal analysis for mould to access on the effect of thermal residual stress in the mould The technique theory methods as well as consideration needed designing of plastic injection mould are presented Design of mould was carried out using commercial computer aided design software Version 13 0 The model for thermal residual stress analysis due to uneven cooling of the specimen was developed and solved a commercial finite element analysis software called LUSAS Analyst Version 13 5 The software provides contour plot of temperature ution for the model and also temperature variation through the plastic injection molding cycle by plotting time response curves The show that shrinkage is likely to occur in the region near the cooling channels as compared to other regions This uneven cooling effect different regions of mould contributed to warpage 2005 Elsevier B V All rights reserved eywords Plastic Injection mould Design Thermal analysis Introduction by cooling stage where the mould is cooled until the part is sufficiently rigid to be ejected The last step is the ejection Plastic industry is one of the world s fastest growing ranked as one of the few billion dollar industries every product that is used in daily life involves the of plastic and most of these products can be produced plastic injection molding method 1 Plastic injection process is well known as the manufacturing process create products with various shapes and complex geometry low cost 2 The plastic injection molding process is a cyclic process are four significant stages in the process These stages filling packing cooling and ejection The plastic injec molding process begins with feeding the resin and the additives from the hopper to the heating injection of the injection plastic injection molding machine 3 is the filling stage in which the mould cavity is filled hot polymer melt at injection temperature After the cav is filled in the packing stage additional polymer melt is ed into the cavity at a higher pressure to compensate the xpected shrinkage as the polymer solidifies This is followed Corresponding author E mail address saihong eng upm edu my S H Tang stage after 4 meric nated actual the core and f mould gating to stresses de molded conducti molten e see front matter 2005 Elsevier B V All rights reserved in which the mould is opened and the part is ejected which the mould is closed again to begin the next cycle The design and manufacture of injection molded poly parts with desired properties is a costly process domi by empiricism including the repeated modification of tooling Among the task of mould design designing mould specific supplementary geometry usually on the side is quite complicated by the inclusion of projection depression 5 In order to design a mould many important designing actors must be taken into consideration These factors are size number of cavity cavity layouts runner systems systems shrinkage and ejection system 6 In thermal analysis of the mould the main objective is analyze the effect of thermal residual stress or molded in on product dimension Thermally induced stresses velop principally during the cooling stage of an injection part mainly as a consequence of its low thermal vity and the difference in temperature between the resin and the mould An uneven temperature field xists around product cavity during cooling 7 260 Processing ences channel e Significant fore of understanding tion be producing mal residual 2 2 1 specimen that thin uct the moulded shell 120 material w temperature respecti duced 2 2 testing erations testing injection carbon the iii i imen design sion is machine The Therefore not reserv setting up S H Tang et al Journal of Materials During cooling location near the cooling channel experi more cooling than location far away from the cooling This different temperature causes the material to xperience differential shrinkage causing thermal stresses thermal stress can cause warpage problem There it is important to simulate the thermal residual stress field the injection molded part during the cooling stage 8 By the characteristics of thermal stress distribu deformation caused by the thermal residual stress can predicted In this paper the design of a plastic injection mould for warpage testing specimen and for performing ther analysis for the mould to access on the effect of thermal stress in the mould is presented Methodology Design of warpage testing specimen This section illustrates the design of the warpage testing to be used in plastic injection mould It is clear warpage is the main problem that exists in product with shell feature Therefore the main purpose of the prod development is to design a plastic part for determining effective factors in the warpage problem of an injection part with a thin shell The warpage testing specimen is developed from thin plastics The overall dimensions of the specimen were mm in length 50 mm in width and 1 mm in thickness The used for producing the warpage testing specimen as acrylonitrile butadiene stylene ABS and the injection time and pressure were 210 C 3 s and 60 MPa vely Fig 1 shows the warpage testing specimen pro Design of plastic injection mould for warpage specimen This section describes the design aspects and other consid involved in designing the mould to produce warpage specimen The material used for producing the plastic Fig 1 Warpage testing specimen produced imum base fitted lo sions ha reaction the respecti on space with Therefore T Mould Components T Ca Core Side Ejector Ejector Bottom Technology 171 2006 259 267 mould for warpage testing specimen was AISI 1050 steel Four design concepts had been considered in designing of 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 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 v 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 the fourth design concept had been applied Various considerations had been applied in the design Firstly the mould was designed based on the platen dimen of the plastic injection machine used BOY 22D There a limitation of the machine which is the maximum area of platen is given by the distance between two tie bars distance between tie bars of the machine is 254 mm the maximum width of the mould plate should exceed this distance Furthermore 4 mm space had been ed between the two tie bars and the mould for mould and handling purposes This gives the final max width of the mould as 250 mm The standard mould with 250 mm 250 mm is employed The mould base is to the machine using Matex clamp at the upper right and wer left corner of the mould base or mould platen Dimen of other related mould plates are shown in Table 1 The mould had been designed with clamping pressure ving clamping force higher than the internal cavity force force to avoid flashing from happening Based on the dimensions provided by standard mould set width and the height of the core plate are 200 and 250 mm vely These dimensions enabled design of two cavities core plate to be placed horizontally as there is enough while the cavity plate is left empty and it is only fixed sprue bushing for the purpose of feeding molten plastics it is only one standard parting line was designed at able 1 plates dimensions Size mm width height thickness op clamping plate 250 250 25 vity plate 200 250 40 plate 200 250 40 plate support plate 37 250 70 retainer plate 120 250 15 plate 120 250 20 clamping plate 250 250 25 Processing the released opening is land only w for runner ing case more type This the diameter or same the air and flashing in or that occurring ca e ca cient sho core the 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 S H Tang et al Journal of Materials surface of the product The product and the runner were in a plane through the parting line during mould Standard or side gate was designed for this mould The gate located between the runner and the product The bottom of the gate was designed to have 20 slanting and has 0 5 mm thickness for easy de gating purpose The gate as also designed to have 4 mm width and 0 5 mm thickness the entrance of molten plastic In the mould design the parabolic cross section type of was selected as it has the advantage of simpler machin in one mould half only which is the core plate in this However this type of runner has disadvantages such as heat loss and scrap compared with circular cross section This might cause the molten plastic to solidify faster problem was reduced by designing in such a way that runner is short and has larger diameter which is 6 mm in It is important that the runner designed distributes material molten plastic into cavities at the same time under the pressure and with the same temperature Due to this cavity layout had been designed in symmetrical form Another design aspect that is taken into consideration was vent design The mating surface between the core plate the cavity plate has very fine finishing in order to prevent from taking place However this can cause air to trap the cavity when the mould is closed and cause short shot incomplete part Sufficient air vent was designed to ensure air trap can be released to avoid incomplete part from The cooling system was drilled along the length of the vities and was located horizontally to the mould to allow ven cooling These cooling channels were drilled on both vity and core plates The cooling channels provided suffi cooling of the mould in the case of turbulent flow Fig 2 ws cavity layout with air vents and cooling channels on plate In this mould design the ejection system only consists of ejector retainer plate sprue puller and also the ejector 3 3 1 testing run short air Fig 3 3D solid modeling and wireframe Technology 171 2006 259 267 261 Results and discussion Results of product production and modification From the mould designed and fabricated the warpage specimens produced have some defects during trial The defects are short shot flashing and warpage The shot is subsequently eliminated by milling of additional vents at corners of the cavities to allow air trapped to modeling of the mould 262 Processing escape packing by injection ity little the eliminate 3 2 of tion the directed the 35 men sure selected ing finite and core formed Fig 5 Model for thermal analysis T Material Carbon Density Y Poisson Y T Thermal Conducti Specific S H Tang et al Journal of Materials Fig 4 Extra air vents to avoid short shot Meanwhile flashing was reduced by reducing the pressure of the machine Warpage can be controlled controlling various parameters such as the injection time temperature and melting temperature After these modifications the mould produced high qual warpage testing specimen with low cost and required finishing by de gating Fig 4 shows modifications of mould which is machining of extra air vents that can short shot Detail analysis of mould and product After the mould and products were developed the analysis mould and the product was carried out In the plastic injec moulding process molten ABS at 210 C is injected into mould through the sprue bushing on the cavity plate and into the product cavity After cooling takes place product is formed One cycle of the product takes about s including 20 s of cooling time The material used for producing warpage testing speci was ABS and the injection temperature time and pres were 210 C 3 s and 60 MPa respectively The material for the mould was AISI 1050 carbon steel Properties of these materials were important in determin temperature distribution in the mould carried out using element analysis Table 2 shows the properties for ABS AISI 1050 carbon steel The critical part of analysis for mould is on the cavity and plate because these are the place where the product is Therefore thermal analysis to study the temperature distrib performed called 2D of modeling side together analysis and element response duration analysis Basically the the the 3 3 analysis time able 2 properties for mould and product Steel AISI 1050 mould 7860 kg m 3 oung s modulus E 208 GPa s ratio 0 297 ield strength S Y 365 4 MPa ensile strength S UTS 636 MPa expansion 11 65 10 6 K 1 vity k 49 4 W m K heat c 477 J kg K Technology 171 2006 259 267 ution and temperature at through different times are using commercial finite element analysis software LUSAS Analyst Version 13 5 A two dimensional thermal analysis is carried out for to study the effect thermal residual stress on the mould at different regions Due to symmetry the thermal analysis was performed by only the top half of the vertical cross section or view of both the cavity and core plate that were clamped during injection Fig 5 shows the model of thermal analyzed with irregular meshing Modeling for the model also involves assigning properties process or cycle time to the model This allowed the finite solver to analyze the mould modeled and plot time graphs to show temperature variation over a certain and at different regions For the product analysis a two dimensional tensile stress was carried using LUSAS Analyst Version 13 5 the product was loaded in tension on one end while other end is clamped Load increments were applied until model reaches plasticity Fig 6 shows loaded model of analysis Result and discussion for mould and product For mould analysis the thermal distribution at different intervals was observed Fig 7 shows the 2D analysis ABS Polymer product Density 1050 kg m 3 Young s modulus E 2 519 GPa Poisson s ratio 0 4 Yield strength S Y 65 MPa Thermal expansion 65 10 6 K 1 Conductivity k 0 135 W m K Specific heat c 1250 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 interv are 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 ature als in one complete cycle of plastic injection molding For the 2D analysis of the mould time response graphs plotted to analyze the effect of thermal residual stress on Fig 7 Contour plots of heat distribution constant at that temperature Increase in temper was due to packing stages that involved high pressure at different time intervals 264 Fig S H Tang et al Journal of Materials Processing 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 which remains reduction at absence plastic plotted be ysis Technology 171 2006 259 267 Fig 11 Temperature distribution graph for Node 302 Fig 12 Temperature distribution graph for Node 290 caused the temperature to increase This temperature constant until the cooling stage starts which causes in mould temperature to a lower value and remains this value The graphs plotted were not smooth due to the of function of inputting filling rate of the molten as well as the cooling rate of the coolant The graphs only show maximum value of temperature that can achieved in the cycle The most critical stage in the thermal residual stress anal is during the cooling stage This is because the cooling Fig 13 Temperature distribution graph for Node 278 stage glass ential in Figs cooling S H Tang et al Journal of Materials Processing Fig 14 Temperature distribution graph for Node 1838 Fig 15 Temperature distribution graph for Node 1904 causes the material to cool from above to below the transition temperature The material experiences differ shrinkage that causes thermal stress that might result warpage From the temperature after the cooling stage as shown in 9 17 it is clear that the area node located near the channel experienced more cooling effect due to fur Fig 16 Temperature distribution graph for Node 1853 ther cooling ing is 284 channel product middle stress more that cooling is mould cooling analyze on dimensional equi maximum v this in in is load to the stress tion purposes sis vice may loading Technology 171 2006 259 267 265 Fig 17 Temperature distribution graph for Node 1866 decreasing in temperature and the region away from the channel experienced less cooling effect More cool effect with quite fast cooling rate means more shrinkage occurring at the region However the farthest region Node experience more cooling although far away from cooling due to heat loss to environment As a result the cooling channel located at the center of the cavity caused the temperature difference around the of the part higher than other locations Compressive was developed at the middle area of the part due to shrinkage and caused warpage due to uneven shrinkage happened However the temperature differences after for different nodes are small and the warpage effect not very significant It is important for a designer to design a that has less thermal residual stress effect with efficient system For the product analysis from the steps being carried out to the plastic injection product the stress distribution product at different load factor is observed in the two analysis Figs 18 21 show the contour plots of valent stress at different load increments A critical point Node 127 where the product experiences tensile stress was selected for analysis The stress ersus strain curve and the load case versus stress curves at point were plotted in Figs 22 and 23 From the load case versus stress curves at this point plotted Fig 23 it is clear that the product experiencing increased tensile load until it reached the load factor of 23 which 1150 N This means that the product can withstand tensile until 1150 N Load higher