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球磨機(jī)裝補(bǔ)球理論的現(xiàn)狀及發(fā)展趨勢
摘 要:論述了國內(nèi)外磨礦機(jī)裝補(bǔ)球理論及實(shí)踐的現(xiàn)狀,分析了其存在的主要缺點(diǎn),并指出了今后發(fā)展的趨勢。
關(guān)鍵詞:裝補(bǔ)球理論;現(xiàn)狀評述;發(fā)展趨勢;
引言
磨礦往往是金屬礦石粉碎過程的最后一道工序,在此階段,顆粒在沖擊和磨剝的聯(lián)合作用下逐漸減小。其目的是使礦石中的有用礦物與脈石礦物相互解離并在粒度上適合選礦要求.最終得到有用礦物解離充分且粉碎盡量少的入選物料,因而恰當(dāng)?shù)哪サV往往是有效選礦的前提及關(guān)鍵。由于球磨機(jī)中鋼球是產(chǎn)生沖擊及磨剝作用的破碎力實(shí)施體及能量傳遞體,所以合理的鋼球尺寸、配比及裝補(bǔ)球制度將直接決定磨礦指標(biāo)的好壞.并最終影響到選礦廠的選別指標(biāo)和經(jīng)濟(jì)效益。
1磨機(jī)裝補(bǔ)球理論的現(xiàn)狀
1.1球磨機(jī)鋼球尺寸的確定方法
球磨機(jī)中的鋼球尺寸對磨礦影響極大,然而影響鋼球尺寸的因素又很多,致使準(zhǔn)確確定鋼球尺寸成為一個很大的難題?,F(xiàn)代固體力學(xué)還不能從理論上計(jì)算巖礦的破壞強(qiáng)度.只能借助工程測定來解決.現(xiàn)代數(shù)學(xué)也不能求解眾多未知數(shù)的方程[l],因此球徑的純理論計(jì)算公式還不能產(chǎn)生,至多能找到包含主要因素的半理論公式。早先,人們只抓住一兩個主要因素而總結(jié)出一些經(jīng)驗(yàn)球徑公式。在前些年的選礦工程實(shí)踐中,常用的確定鋼球尺寸的經(jīng)驗(yàn)公式主要有以下幾個。
1. V.A.奧列夫斯基公式
(1)
式中 ——所需球徑,mm;
——溢流、給礦粒度,mm.
根據(jù)此公式計(jì)算確定的球徑比實(shí)際偏小過多,所以在實(shí)踐中較少采用,常用的是拉蘇莫夫經(jīng)驗(yàn)公式。
2. 拉蘇莫夫公式
(2)
式中 D——與給礦粒度相應(yīng)的球徑.mm;
d——給礦最大粒度,mm;
i,n——礦料性質(zhì)參數(shù)。
此公式需通過實(shí)驗(yàn)室試驗(yàn)確定i及n值,基本上能反映磨礦過程的實(shí)際特性.使用的較為廣泛。對中硬礦石,拉蘇莫夫公式可轉(zhuǎn)變?yōu)楹喕问健?
(3)
1.2磨機(jī)裝補(bǔ)球制度
1.2.1磨機(jī)初裝球
磨礦實(shí)踐表明.磨機(jī)中存在不同尺寸的混合介質(zhì)時磨礦效果較單一尺寸介質(zhì)的磨礦效果好。前蘇聯(lián)對磨機(jī)合理裝球作了大量的研究并認(rèn)為.要使球磨機(jī)以高效率工作,球荷中不但應(yīng)有足夠數(shù)量的磨碎粗粒物料的大球.同時也應(yīng)有研磨細(xì)粒物料的中球和小球。合理裝球的目的,就是在球荷中保持大球、中球、小球有適當(dāng)?shù)谋壤G懊娴那驈焦街唤鉀Q了計(jì)算單一窄級別球徑的問題.對于給礦為多級別的混合礦料,磨機(jī)內(nèi)的鋼球就必須確定裝球的級別尺寸及各尺寸之間的比例,這就靠初裝球來解決。常用的初裝球方法有兩種:一種是根據(jù)經(jīng)驗(yàn)確定3~5種球,各種球的比例也由經(jīng)驗(yàn)確定,此種方法較簡單,易實(shí)行,但理論性差而經(jīng)驗(yàn)性很強(qiáng),在歐美用得較多;另一種則通過對磨機(jī)全給礦的粒度分析,扣除不需磨細(xì)的部分后,將全給礦按粒度分組,求出各組礦粒所需的球徑,再按各組球的比例與該組礦粒產(chǎn)率相一致的原則確定初裝球比例。此方法是較系統(tǒng)及科學(xué)的方法,但方法本身太繁瑣,其中也有不少經(jīng)驗(yàn)成分,且在生產(chǎn)中難以長期堅(jiān)持及推廣。
國內(nèi)的研究始于50年代,基本上是照蘇聯(lián)的方法。在發(fā)現(xiàn)最初裝球三、五天甚至半個月內(nèi)磨礦效率下降,粒度變粗的情況下,選礦廠開始重視最初裝球問題。崔巍等通過試驗(yàn)逐漸擴(kuò)大裝球種類及比例或在清球時僅把其中碎球部分拿掉,再補(bǔ)上相當(dāng)于碎球大小的好球,均取得了一定的效果[4]。東北大學(xué)則提出了線性疊加原理算法,即認(rèn)為單一尺寸的球介質(zhì)對混合粒級磨機(jī)給礦的磨碎作用等于其對組成混合物料的各窄級別物料磨碎作用的線性疊加;混合加球?qū)旌辖o料的磨碎作用等于混合球中各單一尺寸球?qū)Ω髡墑e物料磨碎作用的線性疊加[5]。據(jù)此可通過試驗(yàn)求出某一級別球介質(zhì)磨碎各窄級別或混合物料的磨礦動力學(xué)方程式中參數(shù),并可模擬算出混合加球磨碎混合物料的磨礦產(chǎn)品粒度分布。
1.2.2磨機(jī)補(bǔ)加球制度
初裝球只解決磨機(jī)工作前的裝球問題,磨機(jī)一經(jīng)工作,磨損隨即產(chǎn)生,接著就是補(bǔ)球的問題。球磨機(jī)連續(xù)工作過程中球逐漸磨損,機(jī)體內(nèi)球荷重量不斷減少,磨機(jī)內(nèi)破碎力減弱,因而磨機(jī)生產(chǎn)率降低,需定期向磨機(jī)內(nèi)添加一定數(shù)量的鋼球,以保證磨機(jī)內(nèi)球荷穩(wěn)定及正常工作。要確定合理的補(bǔ)加球種類及比例,就必須計(jì)算球的磨損及消耗,這就要弄清球的磨損規(guī)律。
球磨機(jī)中球的磨損有許多種,有化學(xué)腐蝕磨損、機(jī)械磨損及非正常磨損等。其中化學(xué)腐蝕磨損的情況復(fù)雜,目前還難以作為工程計(jì)算,非正常磨損則主要由球的質(zhì)量不好所致,情況也復(fù)雜,難以定量計(jì)算。目前僅能對機(jī)械磨損作為工程計(jì)算。盡管磨礦介質(zhì)種類繁多,有鑄鐵、碳素鋼、合金鋼等多種其耐磨性也各不相同,但其磨損卻基本遵循相同的磨損規(guī)律。關(guān)于球的磨損規(guī)律有三個假說,較公認(rèn)的為榜德假說,即認(rèn)為球的磨損速度與鋼球直徑的2.3次方成正比,較符合實(shí)際。但以此規(guī)律為基礎(chǔ)發(fā)展起來的補(bǔ)加球制度卻各不相同,歐美國家多補(bǔ)加3~4種球,前蘇聯(lián)及我國多采用合理平衡補(bǔ)加球法,即在認(rèn)為磨機(jī)工作效果好時,停下磨礦機(jī),清理出磨機(jī)內(nèi)鋼球,找出其球徑分布,并認(rèn)定它是最好的球徑分布,以此作為磨損后應(yīng)達(dá)到的標(biāo)準(zhǔn),通過計(jì)算磨損率來確定補(bǔ)加鋼球制度,再根據(jù)現(xiàn)場人員經(jīng)驗(yàn)校核計(jì)算結(jié)果。但是,由于礦石性質(zhì)的變化,這種較粗放的補(bǔ)加球方法常常使磨機(jī)內(nèi)盈球或虧球,很難達(dá)到磨內(nèi)球荷總量及各種球配比的穩(wěn)定狀態(tài)。
2現(xiàn)行磨機(jī)裝補(bǔ)球理論的缺陷
2.1鋼球尺寸確定公式的缺陷
2.1.1與磨礦目的不太相符
礦料的磨碎有各種目的。一種情況只要求將原始固體的尺寸減小到某一要求的尺寸,例如獲得粉體。此情況下一般礦料磨得越細(xì)越好,水泥磨礦即如此,這種以粉碎礦料為目的的磨碎稱為粉碎性磨礦;還有一種情況是磨礦只是為了擦洗或暴露新鮮表面,并非為了磨細(xì)礦料,此種以擦洗為目的的磨礦稱擦洗性磨礦;而有用礦物選礦工藝中對磨礦的要求又不同于以上兩種情況,在這種情況下要求礦物在較粗粒度下即可解離,以減少不必要的磨碎及減輕過粉碎,為此要解離脈石與有用礦物之間的連生體,即磨礦的目的是獲得解離充分且過粉碎輕的礦料,為后續(xù)的選別作業(yè)創(chuàng)造條件,此種以解離礦物為目的的磨礦稱為解離性磨礦。金屬礦選礦廠中的磨礦均屬于解離性磨礦。而以上列舉的球徑經(jīng)驗(yàn)公式由于誤差較大。通常算得的球徑偏大,過大的球徑產(chǎn)生過大的破碎力,使礦粒產(chǎn)生貫穿破碎。只能保證一定的磨礦細(xì)度,適應(yīng)不了解離性磨礦的要求。解離性磨礦要求破碎力精確,故目前的球徑計(jì)算方法與磨礦目的不太相符。
一般而言,礦物的單體解離度與礦石的磨礦細(xì)度密切相關(guān),但由于解離性磨礦的存在而使二者不存在正比例的關(guān)系。當(dāng)球徑過大時,雖然磨礦細(xì)度高,但礦物的單體解離度不一定就高,而且過分追求高的磨礦細(xì)度,就必然造成過粉碎嚴(yán)重,使選礦指標(biāo)降低,礦物資源浪費(fèi)。當(dāng)球徑精確時,雖然磨礦細(xì)度不很高,也能得到高的礦物單體解離度,這不僅有利于選礦,也能節(jié)能降耗。
2.1.2考慮因素過分簡單
磨礦過程是一個復(fù)雜的過程,其影響因素很多。給礦粒度、礦石的力學(xué)強(qiáng)度、磨機(jī),工作參數(shù)及礦漿的物理和化學(xué)性質(zhì)都直接影響著球徑的選擇。如此多的復(fù)雜因素要一一考慮是不可能的。只有抓住主要因素,簡化甚至省略次要因素,才是解決復(fù)雜問題的關(guān)鍵。但過于簡單化,只考慮一兩個因素,必然導(dǎo)致過大的誤差。以上列舉的球徑尺寸經(jīng)驗(yàn)公式大多從實(shí)踐經(jīng)驗(yàn)出發(fā),對鋼球尺寸的計(jì)算進(jìn)行了探討,有其簡單實(shí)用的方面,但由于它們考慮的因素太少,過分簡單及經(jīng)驗(yàn)性強(qiáng),局限性大,計(jì)算結(jié)果必然誤差較大。
2.1.3適用范圍窄
由于以上經(jīng)驗(yàn)公式大多根據(jù)實(shí)際經(jīng)驗(yàn)獲得,經(jīng)驗(yàn)性強(qiáng),局限性大,則適用性就差。對于礦石性質(zhì)復(fù)雜多變,每個選礦廠都有自己特殊的情況,應(yīng)用以上公式準(zhǔn)確性就差。如對于粒度大小相同的不同礦料,由于它們的力學(xué)強(qiáng)度不同,所需的球徑也不相同,而運(yùn)用經(jīng)驗(yàn)公式計(jì)算出的結(jié)果卻無法滿足這一點(diǎn),且不同的經(jīng)驗(yàn)公式有不同的結(jié)果,大的誤差對指導(dǎo)實(shí)際生產(chǎn)必然有較大偏差,仍需要不斷調(diào)整。半理論公式考慮了較多因素,在計(jì)算細(xì)磨時誤差極小,在粗磨情況下仍需校正使用。最近,公式推導(dǎo)者已找到了具體的修正辦法[6],在粗磨下精確選擇球徑。
2.2磨機(jī)裝補(bǔ)球制度的缺陷
現(xiàn)行磨機(jī)初裝及補(bǔ)加球方法仍然存在缺陷。補(bǔ)裝球的種類及比例按“與被磨物料性質(zhì)相適應(yīng)”的原因確定,但很大程度上則依賴于操作人員的經(jīng)驗(yàn)。補(bǔ)加球則是通過新裝球運(yùn)行一段時間后對磨內(nèi)鋼球的分析獲得,這段時間補(bǔ)加球的種類及比例只有根據(jù)經(jīng)驗(yàn)邊摸索邊調(diào)整,直至達(dá)到操作人員認(rèn)為合適為止。因此,這種意義上的合理平衡也是受操作人員經(jīng)驗(yàn)的限制,僅能視為一定程度上的合理,即經(jīng)驗(yàn)性強(qiáng)而科學(xué)性明顯較差。這是現(xiàn)行裝補(bǔ)球方法的主要問題之一。
現(xiàn)行裝補(bǔ)球方法的另一主要問題是方法繁瑣,而效果又不顯著,因此很少有廠礦堅(jiān)持使用,多數(shù)廠礦完全憑經(jīng)驗(yàn)操作。而由于球徑公式誤差大,導(dǎo)致球徑在大的方面很不精確,而在小的方面則過分追求精確,要求補(bǔ)加球種類達(dá)四至五種,粗磨甚至達(dá)到七種,給生產(chǎn)和管理帶來不便,難以堅(jiān)持。只有在大的方面精確,小的方面粗放的裝補(bǔ)球方法才能在生產(chǎn)中堅(jiān)持應(yīng)用,才有生命力。
東北大學(xué)以線性疊加算法確定裝補(bǔ)球的方法,理論性很強(qiáng),缺陷在于該法需在實(shí)驗(yàn)室條件下計(jì)算參數(shù)K、n值,但現(xiàn)場生產(chǎn)條件與實(shí)驗(yàn)室條件相差太大,所以必然會產(chǎn)生誤差,即使能用于工業(yè)生產(chǎn),方法也太繁瑣,加之該辦法也未解決球徑精確計(jì)算的問題,所以,仍需作進(jìn)一步改進(jìn)。
3磨機(jī)裝補(bǔ)球理論的發(fā)展趨勢
基于以上裝補(bǔ)球理論的缺陷,要優(yōu)化磨礦操作,提高磨礦效率,裝補(bǔ)球應(yīng)從以下幾方面展開研究。
3.1增強(qiáng)以提高單體解離度為目的的意識
著名選礦學(xué)者A.F,塔加爾特把解離礦物視為磨礦的第一目的,而減小粒度作為第二目的[7]。那么確定磨機(jī)的工作參數(shù)就必須以保證獲得解離充分且過粉碎輕的物料為基本原則,特別是確定鋼球尺寸時,應(yīng)以提高礦物的單體解離度及減輕過粉碎為主要準(zhǔn)則,這就必須使球徑精確化,由此使礦石所需的破碎力精確化,使破碎沿礦石中聚合力弱的礦物界面發(fā)生,提高解離度。避免因球徑過大,破碎力過大而導(dǎo)致貫穿破碎及過度粉碎,解離度降低和金屬損失升高的現(xiàn)象及因球徑太小,破碎力不足所導(dǎo)致的需多打擊礦石才能破碎,磨礦能力降低及消耗升高的現(xiàn)象。在攀枝花密地選礦廠的試驗(yàn)也證明球徑精確化后不但可以在較粗的粒度下獲得好的解離度,而且可以使電耗球耗等降低[8]。因此,球徑精確化應(yīng)該是今后新的裝補(bǔ)球方法研究的主要內(nèi)容之一。
3.2采用科學(xué)性強(qiáng)及較簡單的方法
磨機(jī)中的鋼球破碎作用是一種隨機(jī)性行為,只有用統(tǒng)計(jì)力學(xué)的辦法來解決,用精確的球徑加上統(tǒng)計(jì)力學(xué)的辦法,使磨機(jī)中有最高的破碎概率及單體解離度,這才是科學(xué)的裝球方法。同時,又要考慮生產(chǎn)管理上的方便及操作性強(qiáng),以入磨物料的性質(zhì)及新球徑確定方法來確定合理的初裝球及在基本磨損規(guī)律的基礎(chǔ)上確定補(bǔ)加球制度。而既然初裝球是根據(jù)給礦粒度組成科學(xué)地確定的,就可以以初裝球比例作為補(bǔ)球計(jì)算的依據(jù),不必再去搞經(jīng)驗(yàn)性強(qiáng)的鋼球平衡資料。裝補(bǔ)球的方法應(yīng)堅(jiān)持大的方面精確,小的方面粗放,粗磨只需2~3種球,細(xì)磨僅1~2種。有較強(qiáng)的理論性,且不增加操作人員負(fù)擔(dān)的方法才能在生產(chǎn)中堅(jiān)持應(yīng)用,才是有生命力的方法。
3.3穩(wěn)定磨機(jī)內(nèi)球荷總量及配比
當(dāng)原礦給入的性質(zhì)是穩(wěn)定的,即主要指粒度組成及強(qiáng)度穩(wěn)定,磨機(jī)內(nèi)的球荷及配比也應(yīng)是穩(wěn)定的。所以應(yīng)該在裝補(bǔ)球時應(yīng)用自動控制的方法,使磨機(jī)內(nèi)的裝球符合生產(chǎn)要求,這也應(yīng)是今后研究的方向。
參考文獻(xiàn)
[1]段希祥.球磨機(jī)鋼球尺寸的理論計(jì)算研究[J],中國科學(xué)(A輯),1989,(8).
[2]劉如盤.確定球磨機(jī)適宜鋼球尺寸的公式[M],礦冶工程,1991,(4).
[3]段希祥.選擇性磨礦及其應(yīng)用[M],北京:冶金工業(yè)出版社,1991,8.
[4]崔巍球.磨機(jī)的臺理平衡裝球[M],北京:冶金工業(yè)出版社,1959,10.
[5]陳丙辰主編.選礦手冊[M](第二卷第二分冊,磨礦部分),北京:冶金工業(yè)出版社,1993,10.
[6]段希祥.球徑半理論計(jì)算公式的修正研究[J],中國科學(xué)(E輯),1997(12).
[7]A.F塔加爾特主編.選礦手冊[M](第二卷第二分冊,濕式磨碎),北京:冶金工業(yè)出版社,1959,9.
[8]曹亦俊等.提高磨礦產(chǎn)品單體解離度的研究[J],礦物巖石地球化學(xué)通報,1997,16(增刊).
畢業(yè)設(shè)計(jì)
文獻(xiàn)綜述
院(系)名稱
工學(xué)院機(jī)械系
專業(yè)名稱
機(jī)械設(shè)計(jì)制造及其自動化
學(xué)生姓名
牛念念
指導(dǎo)教師
閆存富
2012年 03 月 10 日
CHrNESE JOURNAL OF MECHANICAL ENGINEElUNG 、,ol-23,No.5,2010 。547’ DOI:10.3901/CJME.2010.05.547,available online at www.cjmenet.corn;www.cjmenet.com.cn Design and Verification of an Auxiliary System for High Vacuum Die Casting GE Xiaohon91'2一,HUANG Hongwul,LI Hui2,and LIU Yadanl 1 School ofPhysics and Mechanwal&Electrical Engineering,Xiamen University,Xiamen 361005.China 2 Institute ofMaterial Processing and Die&Mould Integrated Technology,Xiamen University ofTechnology, Xiamen 36102幺China Received January26,2010;revisedAugustl8,2010;acceptedAugust25,2010;published electronicallyAugust26,2010 Abstract:Vacuum die casting is the optimal method to produce high quality aluminum alloy components.At present,there arc still very few systematic studies on Vacllum die casting theory and equipment design.On the basis of the existing theories of the vacuum die casting pumping and venting systems.a(chǎn) simplified model is established in this research.The model has an aggregate unit consisted of ‘'vacuum pump+buffer tank”and a cylindrical container(including the shot sleeve,cavity and exhaust channel).The theoretical analysis is carried out between the cavity pressure and the pumping time under different volume models.An auxiliary system for high vacuunl die casting is designed based on the above analysis.This system is composed of a vacuum control machine and a new vacuum stop valve.The machine has a human-computer control mode with“programmable logic controller(PLC)+touch SCl'eal'’and a real-time monitoring function of vacuunl degree for buffer tank and die cavity.The vacuum stop valve with the“compressed gas+ piston rod+labyrinth groove”structure can realize the function of whole-process vacuum venting.The henri,,system shows great advantages on vacuuming the cavity with a much faster speed by making tests oil all existing die casting mold and a 250 t die casting machine.A die cavity pressure less than 10 kPa can be reached within 0.8 s in the experiment and the porosity ofcastings can be greatly decreased.The systematic studies on vacuum die casting theory and equipment have a great guiding significance for high vacuuin die casting,and call also be applied to other high vacuum forming in related theoretical and practical research. Key words:vacuum die casting,pumping model,high vacuum,vacIllmll stop valve,auxiliary system 1 Introduction Higll vacuum die casting technology is a special die casting process during which a cavity pressure less than 10 kPa call be obtained.This technology can reduce or eliminate the porosity in die castings,with the air entrainment content being merely 1-3 mL per 1 00 g.The obtained castings can be used for further processing such as heat treatment and welding。etctl-21.The key technology of lligh vacuun'l die casting is to design a high—sensitive vacuum pumping system.The difficulty lies in establishing a modeling of pumping process.Some scholars have studied on the theoretical model or practical application in this aspect. BAR-M匝IlL et alt3-41.studied the air venting of die casting and the ph)rsical modeling of vacuunl pumping process.They believed there was a critical vent area during the die casting process.Whca the effective vent area of a vent valve was smaller than the critical area,the venting process was insufficient.hence a lot of gases were involved in the liquid metal.When the effective vent area was larger, the liquid metal was overflowed easily.NOURI—BORUJE一 ’Corresponding author.E—mail:xmlggxh@163.corn This project is supported by Fujian Provincial Natural Science Foundation of China(Grant No.2007J0 1 70),and Xiamen Municipal Natural Science Foundation ofChina(Grant No.3502220093034) RDI.et alt51.built a transient model for the residual air mass in the die cavity during the injection process.Thev considered the influence of the molten metal viscosity. temperature,injection velocity and friction factor,and thus improved the numerical simulation calculation method in this field.HERNANDEZ.et al【….improved the model of BAR.M【EIR et a1.a(chǎn)nd took into account such unsteady factors as friction and air flow variation during vacuum pumping.1nhis improved physical model was suitable for both conventional die casting and vacuum die casting. There were many other studies on the die casting venting and vacuum die casting pumping internationally,but most of them stayed at the theoretical level and were mostly numerical simulations for the venting process. HU.et al【卜Ⅵof Tsinghua University,China,carried out the theoretical calculation and experimental study of the decline of vacuum pressure in the cavity pumped directly by a vacuunl pump.They compared a theoretical curve with a measured curve of the pressure in the cavity.Only when the time of vacuum pumping exceeded 2.5 s.a(chǎn)n ideal cavity vacuum pressure less than l 0 kPa could be obtailled. 、)liA N.et alL。厶刈from Huazhong University of Science and Technology,China,independently developed a key technology related to high vacuum die casting based on minimum filling time(MFT)method.They designed an auxiliary system with a bufrer tank that the mechanical 萬方數(shù)據(jù) ·548· GE Xiaohong,et al:Design and Verification ofan Auxiliary System for High Vacuum Die Casting valve could be closed by the impact of liquid metal flow. This system could decrease the pressllre of cavity to 10 kPa within 1 s. Based on the current Iiterature,few studies provide theoretically sound analysis and practical sound design for a vacuum die casting system with a buffer tank.This paper is to carry out theoretical research and calculation analysis of the high vacuum pumping system model with a buffer tank,to study the relationship between pressure in die cavity and the pumping time,and to develop a high vacuum die casting auxiliary system which Call satisfy"the high vacuunl experiment requirements. 2 Theoretical Model of Pumping System with a Buffer Tank The vacuum pumping scheme with a buffer tank is shown in Fig.1【4】.A large buffer tank,as a vacuum source, is added to extract the gas out of the cavity between the vacuum pump and the mould.The gas in the buffer tank is drawn out unceasingly by the vacuum pump. Fig.1.Scheme ofvacuum pumpkng system with a buffer tank A simplified model is given in Fig.2 for the theoretical calculation of hi【曲vacuum venting based on the above scheme.必n is the Math number at duct entrance,Jjl‰t is the Mach number at duct exit,and re(t)is the number of moles of gas in die cavity. vacutlrtl entrance Fig.2.Theoretical calculation model for air venting In this model。the unfilled shot sleeve。the runner and the die cavity are combined and regarded as a cylindrical container fcalled the cylinder)【jJ.Gases are vented through the duct to the buffer tank.功e main resistance to gas flow is in the duct.The following assumptions about this model are made for the theoretical calculation: (1)AU gases are ideal gases. (2)1nhe venting period is very short,the gases temperatu- re is at 300 k and the heat transfer of gases is not considered. (3)111e effect of gases leakage on the venting process is neglected. (4)The volume of vacuum valve duct is sinail compared to the gas duct,thus can be negligible. According to the model established for the venting process in high vacuum die casting,the following formulas can be established to relate the gas pressure in the cavity to the pumping time.In performing the calculations,‰and 朋rout at the duct entrance and exit can be obtained by solving Eqs.(1)一(3)‘3一'10l as follows: 警=愕L一陪1‰, m 篇P=(·+盟2 M二廠老。(f) I ”J M。 ,+等M二 ·+竿帆2 t+l I—I (2) (3) Within the time interval At,the gases mass Am flowing from the cylinder through the vent to the outside can be obtained by Eq.(4)t5-6]: Am=pgAAt=PoM,.(-b]i[·+等M:產(chǎn)觸∽ The cavity pressures po(t)and p“力can be obtained by Eqs.(5),(6),respectively: (5) (6) where the dimensionless quantity 4fL/D is the resistance coefficient in the duct,usually taking 3 to 7.p“f)is the pressure at duct exit,po(t)is the pressure in die cavity,k is the specific heat ratio of air,Am is the number of moles of gas entering the duct in unit time.P is the air density,R is the molar gas constant.“is the gas velocity at the vent entrance,A=n(D/2)2 is the vent cross-sectional are41,K is the cylinder volume,瑪is the buffer tank volume,and K is the duct volume,V4=“+%. The gas duct has diameter of 20 mill and length of 3 m. The buffer tank is selected with l 50 L.The vacuum pump with the pumping speed of 8 L/s is to be used,and the bufrer tank is pumped to have an initial pressure of 0.8 kPa. r— r一 豎 堅(jiān)守半 ∽一 ∽一 m一 陋~ II =、●,、J f f,L,L 風(fēng) 風(fēng) 萬方數(shù)據(jù) CHINESE JOURNAL OF MECHANICAL ENGⅢEERING ·549· Table presents the values for the calculation. Table.Values for the initial calculation conditions Initial condition Value Cylinder volume VI/L ‰dllct Di8n柵D/mm Length L/m Pressure at 0 spo(O)/kPa Pressure in final po(t)/kPa Pressure at 0 sp,(O)/kPa Specific heat ratio ofairK Resistance coefficient 4fLIrt o.5,1.0,1.5 20 4 101.3 lO 8 1.41 In the range of3—7.here taking the value of5 Vacuum tank volume圪/L 150 Duct volume n/L 9.4 Pumping speed of the vacuulll pump 。 S/(L·5-1) 6 Molar gas constant R 8.3 I 4 Ambient temperature r/K 300 Cylinder(shot sleeve.runner and die cavity)with the volumes of 0.5 L.1.0 L and 1.5 L are taken for the calculation.ne calculation results are as shown in Fig.3. Whell¨=1.5 L,it only takes 0.73 s to decrease the pressure in the die cavity to 9.885 kPa by pumping and the lowest pressure can reach 8.974 kPa.taking only 0.83 s. £ jd 瓷 善 躥 蘭 營 § .竺 凸 Fig.3.Relation curves between the die cavity pressure and time The calculation equations for the above theoretical model are established without considering the unceasing evacuation of the vacuum tank by the vacuum pump.The time of pumping the buffer tank call be estimated by Eqs. (7),(8)[1l】: 一咕·g營, v=巧+%. (7) (8) where t is the pumping time,昂is the nominal pumping speed,V is the volume of the target container,n is the volume of the buffer tank,坎is the volume of the pumping duct,pi is the initial pressure in the target container,P is the target pressure in the target container, and the dimensionless quantity K is the correction cocfficient, taking l herein】. 111e time to pump the buffer tank to 0.8 kPa in advance is calculated as follows: ,=2·3量,q喜÷·g—P歹l一..?!ぁひ唬?、-··.,,、150i+94。x lg—100—000≈63 s. (9)l口一≈R. ●V-9 8 000 、7 There is a relatively long time not necessarily to be of vacuum before the liquid metal filling the cavity.This time can be used to pre·evacuate the vacuum tank.. 3 Design and Realization of a High Vacuum Auxiliary System On the basis of the above theoretical model.a(chǎn) high vacuum die casting auxiliary system is designed with following core parts such as the control system.vao]unl valve and pneumatic system.The design and realization processes will be introduced briefly in the following sections. 3.1 Design and realization of the control system The control system adopts the industrial control mode of “PLC+touch screen”.It can perform real—time monitoring of the vacuum pump,various solenoid valves,buffer tank, vacuum valve and die cavity.The schematic diagram and photo of the control system are given in Fig.4.The operation interfaces are given in Fig.5. 3.2 Design of vacuum valve and pneumatic control To achieve the best vacuum die casting result,it must be guaranteed that high vacuum level in the die cavity can be maintained until the cavity is completely filled with liquid metal.The vacuum pumping charmel should be closed at the moment that the filling ends.Therefore。the vacuum valve working mode is the key to realize this process.The developed vacuum valve has different structure from the existing onestl2-19].A mode of“compressed gas+piston rod J.1abyrinth groove’’is originally created and the inertial impact of the liquid metal is used to close the vent channel.whose response time is only 1.5 ms.Therefore the ‘'whole·process vacuum venting'’is realized in die casting. Finally,the automatic reset of the vacuum valve is achieved with the assistance of the compressed gas.The structure and photos of the vacuum valve are given in Fig.6.The vacuum valve can also be controlled by the compressed air. The circuit diagram of pneumatic control of vacuum valve isgiven inFig.7. 萬方數(shù)據(jù) ·550‘ GE Xiaohong,et al:Design and Verification ofan Auxiliary System for High Vacuum Die Casting (a)Control system diagram I,2-Programmable controller;3-Touch screen; 4,5-Solenoid valve;6-Pressure transmitier; 7-Pressure regulating valve (b)Control system object Fig.4.Schematic diagram and object ofthe control system Fig.5.Operation interfaces ofthe control system (a)Inlemal structure ofthe vacuLim valre module ∞Outside configuration andphoto ofthe vacuumvalve Fig.6.Structure and photos of the vacuum valve Fig.7.Circuit diagram ofthe pneumatic control ofthe vacuum valve 1,5-2/2 way pilot-operated solenoid valve; 2,6~3/2Way pilot—operated solenoid valve; 3-3/2way direct-operated solenoid valve; 4-2/2way direct.operated solenoid valve; 7,8一Pressure regulating valve; 9一Pressure transmitter;10一Filter 4 Verification and Analysis of the Vacuum Pumping with the High Vacuum Auxiliary System 4.1 Experimental equipment and method ADCl2 alloy is used as the test material and the vacuum 萬方數(shù)據(jù) CHINESE JoURNAL OF MECHANICAL ENGNEERING ·551· die casting mould is designed for an automobile part(a bar), whose mass is 1.04 kg containing casting itself,the gating system and the vent.As shown in Fig.8(a),the developed vacuum valve and mould are installed on the die casting machine,F(xiàn)ig.8(b)shows the bar produced by this system. Fig.8.Experiment ofvacuum die casting A ccrtain amount of ADC 1 2 iS melted to 953 K.refined for 1 5 rain by blowing argon.a(chǎn)nd stood still for 1 0 min for thermal insulation casting.The mould iS heated to a preheating temperature 523 K with all electric heating rod. The vacuum pressure in the buffer tank iS set to 1 0 kPa. The vacuum levels following time in the cavity are shown in Fig.9.One curve is obtained by theoretical calculation and the other by measuring in the experiment process.ne two curves have the salne trend in the pressure drop section. 芒 暮 l 璺 魯 套 § 者 Pumping time t/s Fig.9.Measurement and theoretical calculation resuk 4.2 Observation of porosity castings The fatal defect of aluminum alloy die castings iS the porosity.By sampling,the X-ray inspection of the bars in connecting part without heat treatment indicates that the porosity of castings produced by conventional die casting is 1 00%.And the gas holes are widely distributed,whereas barely any gas hole can be seen in the castings by high vacuum die casting.The typical distribution of the gas holes are as shown in Fig.1 0.The‘'whole-process vacuunl'’ of this system iS synchronized with the iniection.a(chǎn)chieving the whole-process venting,and satisfying the requirement of low porosity to the greatest extent. Fig.10.X—ray pictures ofcastings produced by conventional and vacuum die casting 5 Conclusions (1)The cavity venting process with a buffer tank in high vacuum die casting iS calculated and analyzed.On the basis of the specific parameters selected for the developed high Vacuum die casting auxiliary system.the theoretical curves of cavity venting with different volumes are plotted. ResulB show that,for aluminum castings less than 4 kg,the cavity pressure Can be decreased to l 0 kPa within 0.8 S theoretically using the vacuum pumping with a buffer tank of 1 50 L.To a certain degree.it reflects the vacuum pressure variation仃end in the cavity. (2)A high vacuum die casting auxiliary system is developed based on the above calculation and analysis. This system has a touch control interface with advantages of the simplicity and fast response.a(chǎn)nd a mechanical 萬方數(shù)據(jù) 。552。 一鯉Xiaohong,et al:Design and Verification ofan Auxiliary System for High Vacuum Die Casting vaL:Illlnl valve that call achieve vacuunl venting during the whole process with the response time of only 1.5 ms. (3)The higIl vacuunl die casting auxiliary system has been used for trial production of ADCl2 alloy bars.The porosity decreases significantly compared to the conventional die casting technique. (4)The systematic studies on vacuunl die casting theory and equipment have a great guiding significance for high vacutlnl die casting,and can also be applied to other hiigh vacuum forming such as vacuulTl injection. 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Biographical notes GE Xiaohong。bom in 1964.is currently a Phi)candidate in School of Physics and MechanicalInstitute of Material Processing and Die ECKERT E R G;GOLDSTEIN R J Air venting in pressure die casting[外文期刊] 1997(02) 2.ZHAO Yunyun;WAN Li;PAN Huan Development and application of high vacuum die casting in aluminum alloy[期刊論文]-Special Casting PAN Huan;LUO Jirong Application and development trends of high vacuum die casting process and die casting aluminum alloys with high strength and ductility[期刊論文]-Special Casting XIONG Shoumei;MASAYAKI M Comparison experiments on evacuating capability of a high vacuum valve and a zig-zag valve in vacuum die casting process[期刊論文]-China Foundry 2006(04) 5.HU Bo;XIONG Shoumei;MASAYAKI M Calculation and experiment on cavity vacuum pressure in vacuum die casting process[期刊論文]-China Foundry 2007(03) 6.HERNANDEZ J;LOPEZ J;FAURA F Influence of unsteady effects o