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注射成型模具溫度調(diào)節(jié)系統(tǒng)的設(shè)計(jì)和優(yōu)化
D.E. Dimla a, ?, M. Camilotto b, F. Miani b
伯恩茅斯工程和計(jì)算設(shè)計(jì)大學(xué)， 英國，多塞特，伯恩茅斯，基督城
摘要 隨著消費(fèi)壽命越來越短，諸如手機(jī)的電子產(chǎn)品在人群中變的越來越時(shí)尚，注塑成型依然是成型此類相關(guān)塑料零件產(chǎn)品的最熱門方法。成型過程中熔融聚合物被注入模具型腔內(nèi)，經(jīng)過冷卻，最后頂出塑料零件產(chǎn)品。在一個(gè)完整的注塑成型的過程主要有三個(gè)階段，沖模，冷卻和頂出。成型周期決定生產(chǎn)的成本效益。相應(yīng)地，其中三個(gè)階段中，冷卻階段是最重要的一步，它決定零件的生產(chǎn)速率。這項(xiàng)研究的主要目的在于使用的有限元分析和傳熱分析在注塑成型工具中配置一個(gè)最優(yōu)的和最有效的冷卻/加熱的水道。一個(gè)適合注塑成型典型的組件3D CAD模型的最佳的形狀的設(shè)計(jì)完成之后，用來成型塑料零件的型芯和型腔的設(shè)計(jì)才得以實(shí)現(xiàn)。這些也用在有限元分析和熱分析，首先確定注射入口的最佳位置，然后確定冷卻渠道。這兩個(gè)因素對(duì)成型周期影響最大，如果要減少的成型周期時(shí)間，那么，首先必須對(duì)這些因素進(jìn)行優(yōu)化并使其減至最低。分析虛擬模型表明，與傳統(tǒng)冷卻模具相比，這樣設(shè)計(jì)的冷卻水道將大大減少循環(huán)時(shí)間，以及明顯的改善制品質(zhì)量和表面光潔度。
關(guān)鍵字 模具設(shè)計(jì)優(yōu)化，注塑成型
1 引言：
注射成型是塑料部件工業(yè)生產(chǎn)中其一個(gè)利用最多的生產(chǎn)過程。它的成功在于，與其它成型方式相比，如吹塑成型，有高的三維形狀塑造造能力，能帶來更高的效益。注塑成型的基本原則是一種固體聚合物經(jīng)加熱熔融后注入一模具型腔;然后經(jīng)冷卻后從模具頂出，獲得與型腔結(jié)構(gòu)相似的制品。因此一個(gè)注塑成型的過程的主要階段，涉及充模，冷卻和制品頂出。成型過程的成本效益，由成型所花的時(shí)間即成型周期決定。相應(yīng)地，其中三個(gè)階段中，冷卻階段是最重要的一步，它決定零件的生產(chǎn)速率。因?yàn)樵诖蠖鄶?shù)現(xiàn)代工業(yè)，時(shí)間和成本有很強(qiáng)的聯(lián)系。成型周期越長(zhǎng)，生產(chǎn)該制品的成本就越高。減少塑件被頂出前所用冷卻時(shí)間，可以大大提高產(chǎn)品的生產(chǎn)效率，因此也就降低了該產(chǎn)品生產(chǎn)成本。因此，了解此是非常重要的，從而，優(yōu)化傳熱過程，使得典型的成型工藝效率更高。從歷史上看，這已得到了應(yīng)用，通過在模具（核型和型腔）內(nèi)打幾個(gè)直孔 ，注入冷卻液體進(jìn)行循環(huán)，帶走模具中過多的熱量實(shí)現(xiàn)冷卻，使零件可以很容易地頂出。利用傳統(tǒng)的加工工藝，如鏜孔，加工直孔。然而，這個(gè)簡(jiǎn)單的技術(shù)只能加工直孔，因此，主要問題是目前還沒有生產(chǎn)如等高線一樣的復(fù)雜的冷卻水道和其他三維結(jié)構(gòu)。另一種設(shè)計(jì)冷卻系統(tǒng)的方法'已經(jīng)提出，該系統(tǒng)符合設(shè)在型芯，型腔，甚至兩者中都可以。這種方法采用的等高樣的水道，構(gòu)造是水道盡可能接近成型零件表面 ，以從熔融塑料吸收帶走更多的熱量。這將確保該塑件均勻冷卻，以及使生產(chǎn)效率更高。
該研究的第一部分主要集中在審查和評(píng)價(jià)注塑成型過程，以介紹此研究的理論及背景。然后對(duì)開發(fā)和應(yīng)用冷卻水道的方法進(jìn)行研究，并找出最可行的方法。
人們通過應(yīng)用有限元和熱流量的分析對(duì)設(shè)計(jì)工具進(jìn)行精煉，現(xiàn)在特定的軟件已被用來優(yōu)化設(shè)計(jì)與構(gòu)造模具。先后，，基于虛擬模型的冷卻通道的效率的研究是利用軟件I-DEASTM的原型和仿真。這項(xiàng)研究正在進(jìn)行，在此希望其最終發(fā)展到一定的水平，能在決定生產(chǎn)零件成型的規(guī)格上熟練使用所需的虛擬模型。
2 注射成型工藝的簡(jiǎn)要概述
與其它所有的生產(chǎn)方式一樣，注射模具生產(chǎn)目前也需要降低其成本以保持市場(chǎng)競(jìng)爭(zhēng)力。這方面的需求通過利用包括設(shè)計(jì)軟件，計(jì)算機(jī)數(shù)值控制機(jī)械等不同的技術(shù)已得到解決。有了這些技術(shù)后，注射成型生產(chǎn)才得以實(shí)現(xiàn)，其生產(chǎn)成本取決于成型周期時(shí)間的長(zhǎng)短?？梢酝ㄟ^調(diào)整模具結(jié)構(gòu)來縮短成型周期，但最后的分析表明，成型時(shí)間主要取決于模具能使熔融聚合物快速冷卻的能力。冷卻液體會(huì)在設(shè)定的溫度下通過模具上冷卻水孔進(jìn)行冷卻定型。同時(shí)必須保證熔融聚合物均勻流到模腔的各個(gè)部位，且在最短的時(shí)間內(nèi)使其散失熱量進(jìn)行冷卻。直至目前為止，這些水孔只能通過鉆孔方式進(jìn)行加工，而這種方式只能加工直孔。如果將冷卻水道整體設(shè)計(jì)成與塑件形狀一致，同時(shí)改變它們的截面以增加導(dǎo)熱面積，這樣就可以大大提高塑件冷卻的效率。塑料也將更加均勻冷卻，所以也可能有助于減少塑件頂出時(shí)的翹曲變形。
2.1 溫度控制
成型溫度，例如熔融聚合物溫度，模具溫度，環(huán)境溫度和夾緊系統(tǒng)的溫度都必須控制在一定范圍之內(nèi)（圖1 ）。塑料熔體注入模腔后，必須通過冷卻固化后才能獲得所需制品形狀。模具的溫度通過冷卻液在模具內(nèi)冷卻水道中的的循環(huán)進(jìn)行調(diào)節(jié)，一般用水或油作冷卻液。當(dāng)塑件充分冷卻后才能從模腔頂出，冷卻過程中熔體 (95%)會(huì)產(chǎn)生收縮現(xiàn)象，這樣有來料對(duì)其進(jìn)行補(bǔ)償，同時(shí)也會(huì)有一些收縮會(huì)在塑件上保留下來[1]。
圖1. 注射過程中的溫度記錄圖
2.2 壓力控制
注射裝置和夾緊系統(tǒng)都要求要求有一定的壓力，且兩者方向相反（圖2）。注射系統(tǒng)中的壓力可以分為三種：注射壓力，保壓壓力和塑化壓力。所有這些都是通過螺桿的運(yùn)動(dòng)實(shí)現(xiàn)的。在夾緊裝置中，由油泵液壓系統(tǒng)控制移動(dòng)模具所需要的壓力。保壓壓力是在注射完成后對(duì)冷卻凝固過程中產(chǎn)生的收縮空間進(jìn)行補(bǔ)料，以保證塑件形狀而設(shè)定的。
圖2. 注射過程中的壓力記錄圖
2.3 時(shí)間控制
時(shí)間是在整個(gè)成型過程中最重要的參數(shù)。從成型周期時(shí)間可估計(jì)生產(chǎn)成本和機(jī)械效率。原則上在時(shí)間方面應(yīng)加以控制，其中包括：合模時(shí)間，注射時(shí)間和冷卻時(shí)間。圖3 以一個(gè)簡(jiǎn)單的示意圖顯示了一個(gè)典型的成型周期。
圖3 成型周期
2.4 熱參數(shù)
盡管塑料材料有很高的熔融溫度，他們特定的性能與溫度有很大的關(guān)系。過高或過低的溫度都會(huì)對(duì)塑件結(jié)構(gòu)帶來損壞。研究其熱性能有利于了解和預(yù)測(cè)其其它各性能。因此，成型冷卻時(shí)間必須合理設(shè)置，第一，塑化的厚度，第二，熔融熱的耗散。與金屬不同，塑料材料具有特殊的熱容量，且高結(jié)晶塑料比非結(jié)晶擁有更高的熱容量。與其它材料，例如與金屬，比較之下，塑料有一個(gè)大的熱膨脹系數(shù)?？梢酝ㄟ^加入礦物填料，如玻璃纖維，來改善塑料的熱性能。
2.5 冷卻水道
與其它多數(shù)制造業(yè)相同，產(chǎn)品的制造時(shí)間與其成本有密切的關(guān)系。生產(chǎn)零件的時(shí)間越長(zhǎng)，其成本就越高，注射模具成型制造的生產(chǎn)周期主要在于其冷卻時(shí)間。所以，減少冷卻時(shí)間將能降低零件的生產(chǎn)成本?？梢栽谀＞咧屑庸こ隼鋮s孔，然后以一定壓力壓入冷卻液體使其在水孔內(nèi)循環(huán)，這樣通過熱交換來控制模具溫度。常規(guī)的鉆孔機(jī)械如CNC客以用來鉆直孔。其中最主要的問題在于，不能制造加工在三維方向上的比較復(fù)雜的冷卻孔，尤其是在靠近模具壁的地方。 這樣模具冷卻不均勻，引起制件收縮翹曲，冷卻時(shí)間增長(zhǎng)（圖4），導(dǎo)致冷卻系統(tǒng)的效率大大降低。另一方面來說，如果盡量依照零件的形狀(圖5)加工成型冷卻的水道，塑件的冷卻也會(huì)變的比較均勻，這樣冷卻時(shí)間可以顯著減少，冷卻效率提高。此外，如果塑件能在非常均勻的溫度下被頂出，出模是的收縮也將是均勻的，這樣也可避免塑件在頂出時(shí)產(chǎn)生翹曲現(xiàn)象。最后，設(shè)有這種冷卻系統(tǒng)的模具比設(shè)有常規(guī)標(biāo)準(zhǔn)冷卻系統(tǒng)的模具更易達(dá)到設(shè)定的操作溫度[3,4]。
這樣就可以減少啟動(dòng)模具到所生產(chǎn)操作條件所需要的時(shí)間。當(dāng)聚合物熔體被注入模具后，遇到模具壁就立即冷卻固化。如果塑件的體積較大，且其厚度不太小時(shí)，冷卻固化的聚合物可能增大注射的阻力，導(dǎo)致模腔不能被完全填充。在這種情況下，需要對(duì)物料加熱到一定的溫度，以增加其流動(dòng)性。盡管有這些好處，但我們也要注意到，模具中一定形狀的復(fù)雜的冷卻孔的加工制造將大大增加其最初成本，盡管這門技術(shù)能給生產(chǎn)帶來很多好處。
圖4 一副模具型芯型腔的冷卻水道
圖5 同一模具的隨形冷卻水道
3 隨形冷卻孔—概述
關(guān)于隨形冷卻孔的效率問題，Ring等人對(duì)三個(gè)具有不同結(jié)構(gòu)的模具進(jìn)行了研究，其中有的加工有隨形冷卻水孔，有的沒有，結(jié)果表明，由于改善了熱傳遞效率，配置有隨形冷卻孔的模具，其生產(chǎn)周期大大縮短，生產(chǎn)效率大大提高。Jacobs發(fā)表了一篇文獻(xiàn)，文獻(xiàn)主要論述了隨形冷卻孔的重要性，同時(shí)介紹了一種高導(dǎo)熱性能的新材料[6]。這項(xiàng)研究表示，與配有常規(guī)冷卻水孔的模具相比較，隨形冷卻孔的設(shè)置(層狀的銅的鎳或銅模子)使模具成型生產(chǎn)效率提高大約70%。Sachs等也對(duì)隨形冷卻孔和鉆孔法加工的冷卻孔進(jìn)行了比較研究[3]。根據(jù)他們的調(diào)查,他們對(duì)模具核心腔加上軟件的使用技巧,進(jìn)而建構(gòu)模具理論和實(shí)驗(yàn)數(shù)據(jù)作比較.分析顯示,隨后模形通道溫度高于常規(guī)，實(shí)現(xiàn)一個(gè)更加有效的溫度分布均勻傳熱能力。
拜恩[7]提出了一種控制模具溫度的方法，即通給隨形冷卻孔以一個(gè)準(zhǔn)確的定位來實(shí)現(xiàn)控溫的目的。同時(shí)他也分析了系統(tǒng)與環(huán)境的熱交換的影響，以通過精確計(jì)算，設(shè)計(jì)更為有效的冷卻系統(tǒng)。
Park and Know [8] 進(jìn)行了根據(jù)修改過的邊界元素法的對(duì)鑄造的過程進(jìn)行了熱分析，這是一種靈敏度分析，有利于模具的優(yōu)化設(shè)計(jì)過程有效進(jìn)行。通過熱分析優(yōu)化設(shè)計(jì)可對(duì)冷卻時(shí)間和表面溫差進(jìn)行準(zhǔn)確的預(yù)測(cè)，這表明對(duì)于較理想模具設(shè)計(jì)，特殊在模具型號(hào)和冷卻孔的設(shè)計(jì)方面，靈敏度分析法是一個(gè)很好的設(shè)計(jì)方法。這樣在同一臺(tái)機(jī)器中設(shè)置了有利于塑件冷卻的加工條件，使影響產(chǎn)品質(zhì)量和生產(chǎn)力因素降到最少。
模具冷卻水孔的方面存在的問題不僅僅是去設(shè)計(jì)加工冷卻孔結(jié)構(gòu)，更在于怎么將這種技術(shù)很好地應(yīng)用到多種多樣結(jié)構(gòu)的型腔冷卻孔的設(shè)計(jì)加工。徐等[9]對(duì)如何克服這個(gè)問題作出了解答和提議。即將原型劃分為多個(gè)小的區(qū)域，這樣有利于對(duì)整體結(jié)構(gòu)的研究分析，再對(duì)每個(gè)區(qū)域進(jìn)行設(shè)置冷卻水孔。然后綜合前面的分析結(jié)果，對(duì)整個(gè)結(jié)構(gòu)進(jìn)行構(gòu)造。為解決冷卻問題，李[10]也提出了類似的方法，建議通過公認(rèn)算法將模具按其區(qū)域結(jié)構(gòu)特征不同劃分成不同的小塊。然后，根據(jù)每小塊的結(jié)構(gòu)特點(diǎn)，分別設(shè)定不同結(jié)構(gòu)的冷卻孔，并對(duì)其進(jìn)行最后的組合形成完整的冷卻系統(tǒng)。算法是基于“超二次”,即模具形狀特征參數(shù),諸如在采用計(jì)算機(jī)繪圖時(shí)需要參數(shù).這種方法是解決問題的最好的選擇,可以近似估計(jì)整個(gè)塑件的結(jié)構(gòu).這樣，冷卻系統(tǒng)變得容易被仿效. 當(dāng)有著復(fù)雜的零件制造時(shí)這種做法是十分有效的.
4 模具零件（MPA）分析
基本思路是，首先應(yīng)用I-DEASTM構(gòu)造一個(gè)虛擬模型，再進(jìn)行模擬分析確定最佳的流道位置。 然后進(jìn)行冷卻系統(tǒng)的設(shè)計(jì)。先后對(duì)示范作進(jìn)一步的分析,如有限元分析改進(jìn)設(shè)計(jì)，為了幫助設(shè)計(jì)者確定模具制造的零部件，應(yīng)用MPA對(duì)虛擬模型進(jìn)行最后的分析。對(duì)軟件唯一的要求就是能對(duì)零件材料作出正確的選擇。
4.1 模型
分析試驗(yàn)中進(jìn)行模型結(jié)構(gòu)的選擇，其依據(jù)是零件基本規(guī)格和成型塑件的特點(diǎn)，如模具表面斜度的設(shè)置要保證塑件冷卻成型后能順利被頂出。這個(gè)模具 (圖6)表面形成一個(gè)長(zhǎng)方形并設(shè)有一個(gè)大小有 8?角度。塑件型腔內(nèi)設(shè)有加強(qiáng)筋，這樣可以增強(qiáng)其機(jī)械性能，也可避免成型中可能產(chǎn)生的變形。
圖6 模具的3D視圖
4.2 流道澆口位置
最佳澆口的設(shè)置，要通過反復(fù)的試驗(yàn)才能實(shí)現(xiàn)，可以同多塑件質(zhì)量來衡量設(shè)置的好壞，諸如表面熔接痕，氣泡數(shù)量等(圖7為一個(gè)典型的熔接痕情況)。澆口可以設(shè)計(jì)在底部中心位置和內(nèi)外表面。流動(dòng)分析表面，兩種設(shè)置的整個(gè)成型周期相同，但澆口設(shè)置在外表面時(shí)塑件表面的成型熔接痕要小一些。選擇澆口位置的標(biāo)準(zhǔn)在于是否能保證塑件的表面質(zhì)量以及短的成型周期。如果在塑件外表面設(shè)澆口，則會(huì)有聚合物凝料在澆口處形成，塑件被頂出后需要對(duì)其進(jìn)行切割處理，由此比較浪費(fèi)時(shí)間，不適合進(jìn)行制件生產(chǎn)。另外，如果把流倒設(shè)在型腔內(nèi)，將導(dǎo)致有新的問題出現(xiàn)，形狀復(fù)雜的冷卻水孔的布置將受到限制，澆注系統(tǒng)難以配置。
設(shè)置較多的澆口并不能改善塑件質(zhì)量，也不利于降低冷卻時(shí)間，只會(huì)使冷卻孔的配置更為復(fù)雜。所以，一個(gè)澆口還是最為理想的。
以下是通過計(jì)算機(jī)模擬預(yù)測(cè)而設(shè)置局部?jī)?yōu)化解決方案：
（1） 著力減少內(nèi)外表面的熔接痕，如圖8與圖7相比較。
（2） 加強(qiáng)加工質(zhì)量的預(yù)測(cè)（圖9）
（3） 進(jìn)一步的質(zhì)量檢查，設(shè)置冷卻孔。結(jié)果表明，塑件上沒有明顯的痕跡出現(xiàn)，定出時(shí)間也在1s左右，非常合理，即澆口位置合適（圖10）。
（4） 同樣地，表面溫度和凍結(jié)時(shí)間的預(yù)測(cè)率也達(dá)到了預(yù)期目標(biāo)（圖11，圖12）。
圖10從根本上表明，優(yōu)化條件的選擇使得成型工藝過程可順利成型，這樣更加保證了成型塑件的質(zhì)量。這種條件下，注射時(shí)間預(yù)計(jì)為1.18 s。
圖7 模具表面的熔接狠 圖8 減少模具外表面的熔接痕
圖9 獨(dú)立澆口位置：質(zhì)量預(yù)測(cè) 圖10 計(jì)算機(jī)分析結(jié)果
圖11 表面溫度 圖12 冷卻時(shí)間
5 冷卻水道定位
5.1 一般考慮因素
制件的最終輪廓和形狀精確性不僅僅是由其成型工藝條件決定，而且也取決于成型時(shí)型腔壁的溫度高低[11]。物料的冷卻過程即為結(jié)晶過程，為了是物料順利冷卻結(jié)晶，對(duì)模具溫度必須進(jìn)行嚴(yán)格的控制，因此，冷卻系統(tǒng)的準(zhǔn)確定位對(duì)于生產(chǎn)合格的制品來說是非常重要的。確定流道位置關(guān)鍵問題在于如何保證型芯和型腔整體有均一的溫度。如果型腔內(nèi)兩個(gè)部位有很大的溫度梯度，這將導(dǎo)致塑件翹曲或結(jié)構(gòu)的破壞（圖13）。
圖13. 溫度梯度影響[11]
既要盡可能縮短成型周期，又要保證熔體的順利結(jié)晶，一個(gè)折中的辦法是，冷卻系統(tǒng)的設(shè)置要保證模具壁有一個(gè)均一的溫度，另外也要保證聚合物物料均勻冷卻，這樣就比較符合生產(chǎn)的要求。
5.2 溫度特性
由于諸如模具材料性能和聚合物原料性能等多種因素的影響，在成型周期中，模具的溫度有周期性的變化（圖14）。冷卻系統(tǒng)不能控制這種波動(dòng)的幅度，但是，最重要的是當(dāng)具有高溫的聚合物熔體流動(dòng)到型腔并與型腔壁接觸時(shí)，會(huì)產(chǎn)生最大的溫度回升現(xiàn)象[11]。所以，必須對(duì)相關(guān)的物理效應(yīng)進(jìn)行監(jiān)測(cè)，來保持模內(nèi)溫度的均一性。
模具內(nèi)凸高的區(qū)域容易產(chǎn)生熱量的集中，因此需要比較集中的冷卻水孔。相反，在凹低的區(qū)域，由于有較多的物料存在有利于散熱，因此其不需要太集中的冷卻孔。由此，必須注意這些區(qū)域彎道和冷卻系統(tǒng)的設(shè)計(jì)及配置。在三維冷卻系統(tǒng)的設(shè)計(jì)中，需要考慮三面的因素：截面的直徑（或當(dāng)不是圓形時(shí)截面面積）;孔與孔之間的距離；孔與模具壁制件的距離；冷卻孔直徑的選擇和流道的設(shè)計(jì)時(shí)需要考慮的問題是其使系統(tǒng)產(chǎn)生壓力損失。Zollner [11]中對(duì)加熱與冷卻中壓力損失的關(guān)系進(jìn)行了研究論述，并指出了確定冷卻孔位置的方法。對(duì)此關(guān)系的研究結(jié)果表明，在半結(jié)晶熱塑性塑料中為2.5到5 ％之間，在無定形熱塑性塑料中為5到 10 ％。
5.3 型腔冷卻水道定位
在此研究中，對(duì)于型芯及型腔中冷卻孔的設(shè)置提出了多種不同的方案，并將隨形冷卻系統(tǒng)（圖15）和直孔冷卻系統(tǒng)（圖16）相對(duì)比。由于有限元分析軟件包只對(duì)其四分之一進(jìn)行了插入分析，因此還必須對(duì)這些零件進(jìn)行全面的分析。順著塑件的表面，要設(shè)置四條冷卻水道，其中三條用于其側(cè)表面降溫，另外一條對(duì)其底部進(jìn)行冷卻降溫。
圖15 型腔隨形冷卻水道
圖16 型腔直線冷卻水道
5.4 型芯冷卻水道定位
型芯的冷卻系統(tǒng)由兩條順著型芯幾何表面形狀的冷卻孔組成，其中一條流道對(duì)其頂部和短尺寸表面進(jìn)行冷卻，另外一條用來冷卻其相對(duì)較大的表面?？梢酝ㄟ^設(shè)置彎道來減少.冷卻系統(tǒng)中流體動(dòng)力的損失。 在直流道的方案中，一條水道（圖18）的加工需要三種鉆孔操作。在這種情況下不可能有圓角，這要對(duì)冷卻液體的損失較前一種方案要大。下一步是有限元分析，以檢查該零部是否能夠抵抗注射是的巨大壓力。
這項(xiàng)研究工作一直在進(jìn)行，而且由于在注射過程中，其底部承受的壓力為最大，研究也將集中在這樣的部位。
圖17 型芯隨形冷卻水道
圖18 型芯直線水道
6 結(jié)論
作為注射模具結(jié)構(gòu)組成的一部分，隨形冷卻系統(tǒng)的設(shè)計(jì)和優(yōu)化已可以通過虛擬樣機(jī)模擬進(jìn)行。這種方法包括塑件三維CAD模型的建立，其中型芯和型腔的構(gòu)造都是通過這種方式實(shí)現(xiàn)的。模擬研究表明，與直流道的設(shè)置相比，可以對(duì)隨形冷卻水道進(jìn)行優(yōu)化設(shè)計(jì)并預(yù)測(cè)流道的最佳位置，更有利于縮短制件的冷卻時(shí)間，提高生產(chǎn)效率。該研究正在進(jìn)行，在此希望通過虛擬模型技術(shù)對(duì)零件生產(chǎn)成型規(guī)格進(jìn)行確定，最終將此項(xiàng)研究提高到一個(gè)較高的能力水準(zhǔn)。引用有限元分析法對(duì)型腔型芯樣本進(jìn)行測(cè)試，這方面需要做更多的工作，一檢查模具對(duì)注射壓力了承受能力，并最終設(shè)定模板的厚度。對(duì)于一些嚙合結(jié)構(gòu)的塑件，需要對(duì)其進(jìn)行結(jié)構(gòu)平面化，并利用平面要素處理，這樣也有利于在隨形冷卻系統(tǒng)和傳統(tǒng)冷卻系統(tǒng)中理解冷卻時(shí)間。
參考文獻(xiàn)：
[1] D.M. Bryce, Plastic Injection Moulding, Society of Manufacturing Engineers, Dearborn, MI, 1996.
[2] Anon., Intelligent Systems Laboratory, Michigan State University, 1999 [accessed October 30, 2003]. http://islnotes.cps.msu.edu/ trp/inj/inj time.html.
[3] E. Sachs, et al., Production of injection molding with conformal cooling channels using the three dimensional printing process, Polym. Eng. Sci. 40 (5) (2000) 1232–1247.
[4] K.W. Delgarno, Layer manufactured production tooling incorporating conformal heating channels for transfer moulding of elastomer compounds, Plastic Rubber Compos. 30 (8) (2001) 384–388.
[5] M. Ring, et al., An investigation of effectiveness of conformal cooling channels and selective laser sintering material in injection moulding tools, RPD (2002) 1–5.
[6] F. Jacobs, High-conductivity Materials and Conformal Cooling Channels, Warwick Manufacturing Group, Warwick University, UK [accessed September 29, 2003]. http://www.nasatech.com/NEWS/ rpd399.xpress.html.
[7] Anon., Rapid tooling/rapid prototyping, EGS Associates Corporation [accessed September 29, 2003]. http://www.esgn.com/services/ rapid tooling.htm.
[8] S.J. Park, T.H. Know, Thermal and design sensitivity analyse for cooling system of injection mold, Part 1 and Part 2, J. Manuf. Sci. Eng. 120 (1998) 287–305.
[9] X. Xu, et al., The design of conformal cooling channels in injection molding tooling, Polym. Eng. Sci. 41 (7) (2001) 1265–1279.
[10] C.L. Li, A feature-based approach to injection mould cooling system design, Comput.-Aided Des. 33 (2000) 1073–1090.
[11] O. Zollner, Optimised mould temperature control, Appl. Technol. Inform. (1997) 1104.
Design and optimisation of conformal cooling channels in
injection moulding tools
D.E. Dimla a, ?, M. Camilotto b, F. Miani b
a School of Design, Engineering and Computing, Bournemouth University, 12 Christchurch Road, Bournemouth, Dorset BH13NA, UK
DIEGM, Universit`a Degli Studi di Udine, via delle Scienze 208, 33100 Udine, Italy
Abstract
With increasingly short life span on consumer electronic products such as mobile phones becoming more fashionable, injection moulding remains the most popular method for producing the associated plastic parts. The process requires a molten polymer being injected into cavity inside a mould, which is cooed and the part ejected. The main phases in an injection moulding process therefore involve filling, cooling and ejection. The cost-efficiency of the process is dependent on the time spent in the moulding cycle. Correspondingly, the cooling phase is the most significant step amongst the three, it determines the rate at which the parts are produced. The main objective of this study was to determine an optimum and efficient design for conformal cooling/heating channels in the configuration of an injection moulding tool using FEA and thermal heat transfer analysis. An optimum shape of a 3D CAD model of a typical component suitable for injection moulding was designed and the core and cavity tooling required to mould the part then generated. These halves were used in the FEA and thermal analyses, first determining the best location for the gate and later the cooling channels. These two factors contribute the most in the cycle time and if there is to be a significant reduction in the cycle time, then these factors have to be optimised and minimised. Analysis of virtual models showed that those with conformal cooling channels predicted a significantly reduced cycle time as well as marked improvement in the general quality of the surface finish when compared to a conventionally cooled mould.
Keywords: Tool design optimisation; Injection moulding
1. Introduction
Injection moulding is one of the most exploited industrial processes in the production of plastic parts. Its success relies on the high capability to produce 3D shapes at higher rates than, for example, blow moulding. The basic principle of injection moulding is that a solid polymer is molten and injected into a cavity inside a mould; which is then cooled and the part ejected from the machine. The main phases in an injection moulding process therefore involve filling, cooling and ejection. The cost-efficiency of the process is dependent on the time spent in the moulding cycle. Correspondingly, the cooling phase is the most significant step amongst the three, it determines the rate at which the parts are produced. As in most modern industries, time and costs are strongly linked. The longer is the time to produce parts the more are the costs. A reduction in the time spent on cooling the part before its is ejected would drastically increase the production rate, hence reduce costs. It is therefore important to understand and thereby optimise the heat transfer processes within a typical moulding process efficiently. Historically, this has been achieved by creating several straight holes inside the mould (core and cavity) and forcing a cooler liquid to circulate and conduct the excess heat away so the part can be easily ejected. The methods used for producing these holes rely on the conventional machining process such as drilling.However this simple technology can only create straight holes and so the main problem is the incapability of producing complicated contour-like channels or anything vaguely in 3D space. An alternative method that provides a cooling system that ‘conforms’ to the shape of the part in the core, cavity or both has been proposed. This method utilises a contour-like channel, constructed as close as possible to the surface of themould to increase the heat absorption away from the molten plastic. This ensures that the part is cooled uniformly as well as more efficiently.
The first part of this investigation concentrates on reviewing and evaluating the injection moulding process, to set the knowledge and background on the subject. Then a study of proposed methods for developing and applying conformal channels is conducted, identify the most viable method.
Specific softwarewas used to optimise the design and construction of the mould, with attention on refining the tool design through application of finite element and thermal flow analyses.Successively, a study on the effectiveness of the conformal cooling channel based on virtual models was performed using I-DEASTM software for prototyping and simulation. The study is on going and hopefully would culminate in the suggestion of the level of proficiency required using virtual models in deciding moulding specifications for production parts.
2. Brief overview of the injection moulding process
The injection moulding industry, like all industries, at present needs to reduce costs to remain competitive. This need has been addressed using various technologies ranging from design software to computer numerical control machinery. After these technologies are in place and moulding begins the cost is usually based on cycle time. Adjustments can be made to the moulding machine to help reduce the time to mould but in the final analysis the time is dictated by the ability of the mould to carry the heat away from the molten polymer. Liquid is passed through cooling channels in the mould at the required temperature. This must allow the molten polymer to flow into all sections of the cavity while at the same time remove the heat as quickly as possible. Up to nowthese channels have been produced by drilling which can only produce straight lines. If the channels carrying the water could be conformed to the shape of the part and their crosssection changed to increase the heat conducting area then a more efficient means of heat removal could be realised. This may also help to reduce warpage when the part is ejected, as the plastic would be cooled more uniformly.
2.1. Temperature control
Temperatures such as those for the molten polymer, the mould, the surround temperature and the clamping system temperature need to be controlled (Fig. 1). When molten plastic is injected in the mould it must be solidified to form the object. The mould temperature is regulated by circulation of a liquid cooler, usually water or oil that flows inside channels inside the mould parts. When the part is sufficiently cooled it can be ejected. Most (95%) of the shrinkage happens in the mould and it is compensated by the incoming material; the remainder of the shrinkage takes place sometime following the production of the part [1].
2.2. Pressure control
Both the injection unit and the clamping system require npressure with the latter developed to resist the former (Fig. 2). Three different pressures can be distinguished in the injection unit: initial, hold and back. All these are obtained by the action of a screw. In the clamping unit the oil pump of the hydraulic system controls the pressure needed to move the mould. Holding pressure is required to finish the filling operation and maintained during solidification to supply the shrinkage.
2.3. Time control
Time is the most significant parameter in the entire operation. Cost and machine efficiency can be estimated from the cycle time. The principle temporal aspects to be controlled include: gate-to-gate time, injection time and cooling time. A simple schematic illustration of a typical cycle time is shown in Fig. 3.
2.4. Thermal proprieties
Despite their large diffusion, for all plastic materials temperature range is a limit to their purpose. Both high and low temperature can create damage to plastic components. It is important to study thermal proprieties to understand and predict this behaviour. Therefore cooling times in
moulding machines must be set carefully to permit, first, plasticization of the thickness and secondly dissipation of melting heat. Unlike metals, the thermal capacity of plastics is high with crystalline plastics having a higher capacity than non-crystalline. Plastics have a large coefficient of thermal expansion if compared, for example, with metals. A way to modify these values is to use mineral fillers such as fibre glass.
2.5. Cooling channels
As with most manufacturing fields, production time and costs (lead and lag) are strongly correlated. The longer it takes to produce parts the more are the costs, and with injection moulding production industries cooling time is often taken as the indicator of cycle time. Improving cooling systems will reduce production costs. A simple way to control temperature and heat interchange is to create several channels inside the mould where a cooler liquid is forced to circulate. Conventional machining like CNC drilling can be used to make straight channels. Herein, the main problem is the impossibility of producing complicated channels in three-dimension, especially close to the wall of the mould. This produces an inefficient cooling system because the heat cannot
be taken away uniformly from the mould and the different shrinkage causes warpage and cooling time increase (Fig. 4). On the other hand, if the cooling channels can be made to conform to the shape of the part as much as possible (Fig. 5), then the cooling system the cycle time can be significantly reduced with cooling taking place uniformly in all zones. Furthermore, if the part is ejected with the same temperature in every point the subsequent shrinkage outside the mould is also
uniform and this avoids post-injectionwarpage of parts. Another advantage is that a mould equipped with conformal channels reaches the operation temperature quicker than a normal one equipped with standard (or drilled) cooling channels [3,4].
In this way one can reduce the time required when the moulding machine is started. When the polymer is injected, it solidifies immediately touching the wall of the mould. If the volume of the part is sufficiently big and its thickness is too small, polymer solidified can obstruct the flow and hinder a complete filling of the cavity. In this case the mould must be heated to a particular temperature in order to permit the polymer to flow. Despite all these advantages it may be noticed that newtechnologies involved in the production of moulding tools with conformal channels can increase initial costs for the additional complexity of the construction process.
3. Conformal channels—an overview
Results from an investigation of the effectiveness of conformal channels by Ring et al. [5] through the construction of three different moulds with and without conformal cooling, showed that the latter technique led to significant improvements and a general reduction of the cycle time while ameliorating heat transfer. A contribution to understanding the importance of conformal
channels and the employment of new high-conductivity materials is given by Jacobs [6]. This research showed that using nickel/copper moulds with conformal channels (copper layered) led to productivity improvements of about 70% when compared to a similar mould made with conventional steel with drilled cooling channels. A comparison between conformal channels and drilled cooling channels has also been conducted by Sachs et al. [3]. They based their inve