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當前位置: 潤大世紀 > 資訊中心 > 技術文章 > 脫離熱壓罐的預浸料:革命性的預浸料技術

脫離熱壓罐的預浸料:革命性的預浸料技術

文章出處:網絡責任編輯:管理員人氣:發表時間:2015-06-17【

在航空航天用復合材料結構件,特別是主結構件制造領域,采用熱爐固化、真空袋壓預浸料即脫離熱壓罐的預浸料工藝,以其相對于傳統的熱壓罐預浸料工藝更加靈活、成型更快以及更加經濟等優勢顯示出了極好的應用前景。


圖1 這種OOA固化、11.6m長的機翼結構類似于幻影眼示范機的主梁,該機因引領了行業趨勢,贏得了《航空周刊》的供應商創新獎這一最高榮譽。兩者均由Aurora公司建造。圖片來源:Aurora公司

不使用熱壓罐、只使用真空袋(VBO)大氣壓力生產航空航天用復合材料,并不是什么新鮮事。用于次級結構件(襟翼、整流罩等)的真空袋壓、熱爐固化材料系統已行之有效。新的發展是這種材料能夠提供不到1%的孔隙率和具備熱壓罐法產品質量的力學性能,這些特征都是航空航天主結構件,如帶集成補強件的機翼、機身和尾翼零部件所要求的。 內容來自123456

對OOA工藝的興趣也促進了樹脂傳遞模塑(RTM)、真空輔助樹脂傳遞模塑(VARTM)和其他液體成型工藝以及最新一代的模壓成型熱塑性塑料的使用。雖然這些工藝在那些制造高承載結構件的廠商中獲得了越來越多認可,但是VBO預浸料,其中涉及傳統的手工鋪層以及自動化的材料鋪放方法,由于形成了獨特的優勢結合,特別令人感興趣。

美國空軍已確認OOA預浸料對實現快速且具經濟成本的制造是至關重要的,這是美國國防部(DOD)打造未來軍事平臺所需要的,并且空軍認為當一個OOA預浸料系統能用于原型設計、生產和備件時,就會帶來額外的成本節省。商用飛機原始設備制造商的供應商都將采用OOA材料作為一種途徑,來實現生產的靈活性、擺脫尺寸的限制以及模塊化/蜂窩工作流程。

然而,許多問題仍然存在。OOA材料的循環時間實際上可能會更長,這是由于低孔隙率所需要的邊緣-吸氣是一個依賴于時間的過程。其他的問題還包括膠粘劑的相容性、夾層結構的表面質量,以及在鋪層中采用自動化。此外,因為這些材料是新的,必須建立一個B-基礎設計許用值的完整數據庫,這需要時間和金錢。

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“拋開炒作的因素,OOA材料是一個很好的工具嗎?”波音研究與技術公司非熱壓罐(預浸料)制造技術項目經理Gail Hahn說道,“是的,但它對行業中所有的應用都適合嗎?不一定。”


圖2 先進復合材料貨運飛機Dornier 328,證明了用$5000萬和18個月的時間能夠建造一架新的軍用運輸飛機。該機駕駛員座艙后面被隔開以避免開發新的飛行控制系統的費用,它還使用了一個全復合材料、長達18m的機身。圖片來源:洛克希德馬丁公司

為什么采用OOA預浸料?

OOA預浸料可以確保均勻的樹脂分布,并避免灌注過程中常見的干點和富樹脂區。此外,OOA預浸料可以在較低的壓力和溫度下固化(真空壓力相對0.59MPa的典型的熱壓罐固化壓力,在93℃或121℃固化相對傳統的177℃熱壓罐固化)。因此,采用OOA材料生產帶集成補強件的大型復合材料結構件,可以在一個周期內固化完成,但所用工具通常是非常復雜和昂貴的,而現在可以實現更加簡單并具成本效益的制造。再者,工具和部件的熱膨脹系數(CTE)之間的不匹配性在較低的溫度下也更小一些,因而更容易掌控產品的質量。此外,OOA預浸料也可以成為防止因固化溫差造成部件開裂的一個可能的解決方案。 123456

一個被最廣泛引用的OOA材料的好處是其具有降低較高資本和運營成本的潛力。位于俄亥俄州賴特-帕特森空軍基地的美國空軍研究實驗室(AFRL)的防務制造科學與技術項目經理John Russell談到,美國航空航天局(NASA)分析了為固化直徑10m的運載火箭筒身而建造的一個12m×24m熱壓罐固化的成本,發現它的建造成本約為$4000萬,另一項安裝加上氮氣和電力運行的成本為$6000萬。國防部的報告表明,未來的軍用飛機項目將要求小批量生產,預算非常少。未來的NASA項目也將在成本上受到更大程度的制約。Russell指出,對于NASA和未來國防部的項目,如遠程轟炸機和聯合未來戰區運輸機(后者是C-130的繼承者,預期具有55m翼展和43m機身),它們所面臨的挑戰是如何應對低產量——100架飛機,同時盡可能降低對資金的要求??哲姺矫骖A測取消熱壓罐后,將會帶來資金和操作成本上的巨大節省。
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商用飛機原始設備制造商的1級供應商對此表示同意,他們看到了OOA預浸料是一種可實現更快速、更敏捷制造的途徑。 “我們將進行非熱壓罐制造,因為在未來這對許多部件而言是可行的。”英國GKN航空航天公司(以下簡稱GKN公司)技術總監Rich Oldfield解釋說,“我們看到它通過一個完全不同的工作流程可為工廠帶來效率,相對于大型部件的熱壓罐工藝,該流程實現了單元式制造,省去了排列等候的時間。” GKN公司也看到這種靈活的制造方式減少了對傳統制造的依賴,是實現下一代737和A320單通道噴氣客機中復合材料預測含量達到60%~70%的關鍵。


圖3 先進復合材料貨運飛機的機身由8個部件組成,它們采用英國先進復合材料集團(ACG)提供的MTM-45預浸料經過OOA固化制成。圖片來源:洛克希德馬丁公司 123456

20年的革命性開發

據Russell介紹,美國空軍研究實驗室(AFRL)和國防部高級研究計劃局(DARPA)從20世紀90年代中期開始關注預浸料加工,并考慮其如何被用來制作低成本復合材料項目的原型。第一代的OOA VBO材料,包括先進復合材料集團(ACG)的LTM系列,使得大型組合部件的原型能在低溫下真空固化,因而制造起來更加便宜。這方面的知名例子包括:Scaled復合材料公司的太空船一號、白騎士一號和全球飛行器,波音公司的X45A無人作戰飛機(UCAV),DARPA/洛克希德馬丁公司的黑星和麥道公司的猛禽戰機。

這些早期的預浸料比較便宜,因為它們在較低溫度和真空壓力下固化,但它們達不到部件所需的力學性能,而且生產循環時間也較長。

2005~2006年間,兩種OOA預浸料系統——ACG的MTM45和氰特工程材料公司(以下簡稱氰特公司)的CYCOM 5215,已接近熱壓罐固化預浸料的性能。這兩種預浸料具有靈活的固化周期,在66~79℃需要較長的固化時間,但在121℃能提供較短的2h固化。此外,它們在177℃獨立固化后,還獲得了150℃以上的濕法Tg。
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這些系統實現了DARPA所要求的<1%的孔隙率,但其他地方不符合要求。對于其10~12天的粘性期和最高21天的敝模壽命,美國空軍研究實驗室(AFRL)和NASA都感到不滿意,因為大型復雜結構件的鋪層和裝袋,至少需要3~4個星期。然而,最近ACG和氰特公司已經開發出XMTM-47和CYCOM 5320-1,分別具有21天的粘性期和最低30天的敝模壽命。

2007年,一系列革命性制造技術的開發開始啟動,非熱壓罐(預浸料)制造技術是其中主要的5種技術之一。非熱壓罐技術由DARPA和波音公司共同資助,并由美國空軍研究實驗室(AFRL)執行,其既定目標是開發可以提供性能與目前普遍應用的熱壓罐固化環氧材料相同的OOA系統。ACG公司為該項目提供了MTM45-1、MTM-44和MTM-46材料,氰特公司則提供了CYCOM X5320材料。波音公司評估了3個實驗樹脂配方,并選定X5320,隨后氰特公司把它商業化,成為CYCOM 5320。

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一系列的樣件被制造出來,并經受了有限的非破壞和解剖評價。HITCO碳復合材料公司建造了一個3.65m×4.57m的增硬表皮主結構樣件。Aurora飛行科學公司(以下簡稱Aurora公司)建成了一個11.6m長的無人駕駛飛行器(UAV)翼梁(在一個單獨的開發中,Aurora公司為幻影眼無人機樣機建造了一個3部件、跨度為45.7m的OOA固化機翼結構,并因此贏得《航空周刊》的供應商創新金獎)。波音公司為可長途續航的60%尺度的幻影眼無人機樣機建造了懸臂和尾翼,按計劃該機將在明年初開始飛行試驗。波音公司還提交和公開了碳預浸料的材料規范草案和工藝規范草案。

作為該計劃第二階段的一部分,波音下屬的圣路易斯公司制造了長達21m的部件,包括一個主梁和3種不同的機翼表皮結構:一種厚板增強的表皮,使用了Grafil公司的HR 40 12K高模量碳纖維;一種增硬的表皮,使用了氰特T40/800B中模量碳單向帶以及由氰特Thornel T-650/35 3K碳纖維制成的高強度織物;一種夾層結構的表皮,使用了非金屬的蜂窩芯。

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這4種部件都使用相同的模具制造,使波音公司能對成品板材按照質量、生產的困難程度、可檢測性以及技術成熟水平進行比較。

也許迄今為止,最成功的大型OOA固化結構件的驗證來自先進復合材料貨運飛機(ACCA)。當空軍部長下達任務開發一架新型軍用運輸機,并且只用$5000萬和18個月的時間來完成(國防部下達的指標)時,洛克希德馬丁公司用一架Dornier 328飛機參加了美國空軍研究實驗室(AFRL)的競爭招標。該飛機的駕駛員座艙后面被隔開,以避免開發新的飛行控制系統的費用,并且增加了一個全復合材料、18m長的機身,它分8塊制造,整個結構使用了ACG公司的VBO固化MTM-45材料。雖然該項目持續了7個月(已超過最后期限),但是該項目的制造工程師、美國空軍研究實驗室(AFRL)的Russell表示:“ACCA的花費在預算之內,并且取得了成功。” 123,123


圖4 組裝后的機身內視圖。圖片來源:洛克希德馬丁公司

準備好了,但要等待資格認證

OOA預浸料已達到用于飛機主結構件的熱壓罐固化系統同等的物理性能,并且幾個產品正在進行資格認證當中。先進材料性能國家中心(NCAMP)已完成針對CYCOM 5215和9個MTM45-1產品的許可試驗,包括單向和編織石英纖維、平紋編織碳纖維G30-500、高抗拉強度(HTS)12K碳單向帶,E-玻纖和6781型S-2玻纖預浸料。纖維由AGY公司提供,織造由JPS玻璃纖維織物公司完成。NCAMP對于與MTM46以及CYCOM 5320-1相似的9個系統的測試也將很快完成。ACG宣布,它將在明年發放其XMTM47系統(“X”很快會被刪除)的使用許可,該系統是ACG與NAVAIR合作設立的輕型復合材料結構(LCS)項目的一項研發成果。
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然而,在行業內對于資格認證和獲得一個完整的許用值數據庫也存在著爭論。 “我們的OOA技術處于一個準備好的水平,可以相對較快地把它用于產品中。”GKN公司的Rich Oldfield說,“目前只是缺乏材料許可的可用性,因為現在還沒有任何針對OOA預浸料系統的B基礎許用值數據庫。”這一聯邦航空局(FAA)認可的統計設計數據,來自復合材料片層和層壓板批量試驗,是復合材料獲得在軍用或商用飛機主結構上應用許可的先決條件。ACG公司的研究和技術副總裁Chris Ridgard解釋說:“NCAMP不會對機身制造商提供的每項性能進行測試。”它也不會像防衛項目要求的那樣測試很多的樣品——大約1400~1500件,國防項目則要求3000件。“NCAMP對材料進行測試,并將測試值存儲在數據庫中,但只有一個特定的防衛平臺可以判定一種材料有資格在軍機中使用。”美國空軍研究實驗室(AFRL)的Russell解釋道,“通常要花費$300萬~500萬來開發一個完整的B基礎許用值數據庫,但還沒有一家供應商出面迎接這種挑戰。”Russell指出,NCAMP測試提供的數據,不被任何一家公司或防衛平臺所獨家擁有,比如正在龐巴迪公司和空中客車公司進行的資格認證所提供的數據。 copyright 123456

氰特公司的產品經理Mark Ostermeier稱,CYCOM 5320和5320-1是在單獨的專有項目中進行資格認定的。氰特公司已經宣布它將供應碳纖維復合材料,用于龐巴迪公司Learjet 85飛機的主結構件和次結構件。龐巴迪公司報道該飛機的受壓機身將采用氰特公司提供的一種低壓、熱爐固化及非熱壓罐的碳纖維。

ACG公司的OOA材料正在空中客車公司進行資格認證??罩锌蛙嚬镜姆菬釅汗藜夹g項目負責人David Inston解釋說:“MTM44-1符合空中客車公司為一個特定的2x2斜紋布所制定的規范。然而,它對一個特定的應用還屬于不完全合格。”雖然這是一個機織織物的規格,但它只能用于次級結構而不能用于主結構。該材料的資格認證已接近完成,這標志著MTM44-1將第一次用于實際飛機的部件??罩锌蛙嚬九cGE航空公司達成一項協議,由后者為A350生產輔助機翼結構,包括機翼檢視板和機翼后緣。

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Inston指出,空中客車公司雖公布過一個針對MTM44-1單向產品的規范,并已建造了一個主結構的樣件,但是還沒有對特定應用進行進一步的資格認證測試。“空中客車公司現在正在確定進一步開發OOA材料應用的路線,包括潛在的主結構件用途。”他補充道。


圖5 GKN公司使用微波固化,使OOA固化周期縮短了80%(縮至90min)。微波爐比熱壓罐更具效率,由于它只加熱復合材料,因此縮短了加熱和冷卻的時間,并降低了能耗。圖片來源:GKN公司

為了低孔隙度需要較長的時間

然而,制造商警告說,不要對生產中的OOA預浸料產生不合理的期望。例如,它們“快速”制備原型的優勢,可能無法轉化為“快速”生產,因為孔隙含量目標不能降低。ACG公司的Ridgard解釋道:“在OOA工藝中,揮發性物質不僅包括在鋪層過程中所包埋的空氣,還有環氧樹脂暴露在周圍空氣中時吸收的水分(1%~2%),因此揮發物的去除涉及到一個“邊緣呼吸”的策略。為此,層壓板的邊緣必須與透氣材料接觸,并且必須采用一種保持空氣逃逸路徑的方式來布置材料。真空袋材料和工藝過程與采用熱壓罐工藝是一樣的,但真空的質量至關重要,因為夾帶空氣的抽取是一個隨時間變化的過程,而OOA的固化周期通常較長。”氰特公司推薦在OOA預浸料部件初始固化前應保持真空。這是鋪層過程中除了壓實外所必需的條件,并且在不移除真空袋下進行。保持長度取決于部件的大小和復雜性,范圍從4h成型0.6m×0.6m的部件,到16h成型6m×12m、采用共固化增硬的結構。

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然而,GKN公司的技術負責人John Cornforth警告說:“當你談論OOA在真空中要花費更多的時間時,你必須小心,因為一些熱壓罐材料也需要較長的真空時間。”他指出,對比替代系統是非常困難的,因為真空壓力、固化溫度和循環時間等特定過程的細節,非常依賴于每個部件的具體情況。

ACG公司的Ridgard聲稱整體時間的損失真的不會比采用熱壓罐生產更大。他說:“如果你在176℃固化MTM45-1,例如,你不得不在121℃駐留2h——因為升溫過高,使樹脂粘度下降過快,從而封閉空氣路徑,這樣你只增加了2h。你也可以靈活選擇在121℃或93℃進行較長時間的固化(額外的真空時間是算在內的),然后進行獨立的后固化,以獲得相同的性能。”

然而,GKN公司可能已發現無需額外循環時間的一種方法。該公司開發了微波固化,可將熱壓罐和OOA材料的典型固化循環時間縮短達80%——只需90min。據報道,微波只加熱復合材料結構,而模具和熱爐室保持冷卻,從而大大縮短了加熱和冷卻時間,以及能源消耗。此外,GKN公司還看到了該工藝的另外一個好處,即能夠有選擇性地固化部件的結構,這可使部分固化的結構整合在一起,將來再共固化成總裝配部件。GKN公司利用ACG公司的MTM材料已開始進行技術儲備,目前正在用微波固化與傳統熱壓罐材料及工藝進行比較,以確定最佳參數。

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夾層表面和OOA膠粘劑

在研究具有重要意義的OOA固化蜂窩芯夾層結構的工作中,ACG公司已經認識到了一些技術上的挑戰。Chris Ridgard解釋說:“袋壓和鋪層技術基本上和熱壓罐固化是相同的,但是芯材的排氣變得非常重要。”用于熱壓罐固化的高壓,不允許蜂窩芯內的空氣流入層壓板的表皮,但是在OOA固化中,這種高壓是不存在的。因此,在樹脂軟化之前,空氣必須從夾芯格子中除去,因為較低的OOA壓力可能讓空氣流入表皮中,導致高孔隙率。加拿大McGill大學在2010年SAMPE會議上發表的論文指出,如果在加熱前,蜂窩內的壓力降低到0.05MPa或更低,空氣就不會溢出面板,即空氣在固化循環中仍然留在芯材中。Ridgard稱,使用一種輕質的玻璃纖維網格布(如54g/m2紗羅織物)作為透氣材料,可防止表皮由于芯材內的壓力而脫粘。這項技術是在25年前英國航空公司針對復合材料修補所確立的,但一些人認為它不是最佳的解決方案,因為透氣材料被層壓到夾層中,增加了部件的重量。

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圖6 美國國家航空航天局(NASA)的運載火箭(先前的“戰神V”)可以成為因采用OOA預浸料而節省大筆金錢的一個很好例證。該項目如果采用12m×24m的熱壓罐,估計成本達到$1億。NASA的復合材料與結構高級顧問Mark Shuart相信,一旦直徑達10m的火箭筒身部分被證明可以使用OOA工藝,那么航空航天復合材料的制造將發生極大的變化。圖片來源:NASA

膜狀膠粘劑的發泡,也可以使空氣遷移到表皮,這會影響膠粘帶的形成,進而損害表皮和芯材的粘接質量。許多用于蜂窩夾芯的普通膜狀膠粘劑,在OOA加工中所使用的高真空水平下都會發泡。ACG公司發現,發泡受到膜狀膠粘劑載體的類型、蜂格尺寸和固化溫度的影響。該公司通過對這些變量進行研究,開發了MTA241,它與MTM材料相容,并且在標準的OOA固化循環內不會發泡。“我們知道,VBO工藝需要一整套的OOA材料。”波音公司的Hahn說道。美國空軍研究實驗室(AFRL)的Russell也同意這種觀點,他說:“膜狀膠粘劑和膏狀膠粘劑需要與OOA系統相容。當OOA向前發展時,整套材料都必須是相容的。” copyright 123456

OOA蜂窩夾層的另一個問題是避免單次固化時發生表面點蝕。“一個漂亮的OOA夾層表面需要有一層表面齊整的薄膜。”Hahn說道。在最近為2個1.2m×2.4m的測試面板進行的試驗中,其中一半使用了表面薄膜,另一半沒有使用,結果有薄膜的一半形成了一個更好的表面(沒有表面孔隙)。ACG公司不太擔憂表面點蝕的問題,因為其在熱壓罐固化夾層結構中經常使用表面薄膜作為銅網或其他防雷保護(LSP)材料的載體。ACG公司已開發了MTM246表面改善薄膜,用于它的OOA預浸料,而氰特公司推薦其靈活固化的SurfaceMaster SM 905產品,該產品已被用于許多OOA項目中。

更高的浸漬用于自動化

自動化已成為OOA發展的一個關鍵領域。非熱壓罐復合材料的纖維鋪放是2009~2011年的一個研發項目,由國防部長辦公室(OSD)、美國空軍研究實驗室(AFRL)、國防后勤局(DLA)和海軍研究辦公室(ONR)制造技術計劃聯合資助。該項目的主要目標是開發用于OOA材料的自動纖維鋪放(AFP)工藝,包括關鍵工藝參數的確定,如鋪放速率,并通過制造全尺寸的航空航天部件來驗證該過程。參加者包括波音圣路易斯公司、GKN公司、HITCO碳復合材料公司、洛克希德馬丁公司和Spirit空間系統公司。該項目所使用的自動化設備則來自于ElectroImpact公司、Ingersoll機床公司以及MAG工業自動化系統公司。 123456

據Ridgard稱,大多數用于手工鋪層的OOA預浸料在一定程度上結合了干纖維的工藝路線,它允許在固化過程中抽取空氣,但結果不能形成100%的浸漬。這就是使用編織緊密的E玻纖或石英織物的OOA預浸料得不到充分浸漬的原因。然而,在AFP中使用的材料通常要求具有更高的浸漬水平。氰特公司解釋說,自動鋪帶(ATL)中使用的OOA預浸料被分割成152~305mm的寬度,它們不像一般用于AFP的3.2mm、6.4mm或12.7mm窄帶子那樣敏感。在這些帶子中沒有干纖維,因為帶子鋪放時,干纖維移向邊緣并聚集,而帶子在使用時其邊緣會發生脫落。這可以解釋為什么用于AFP的OOA預浸料可能和VBO系統雖具有相同的化學性質,但它經常使用最高的浸漬水平。

盡管如此,假定AFP應用的邊緣壓緊力以及在壓力和熱量下材料的鋪放能力能夠減輕空氣夾帶的問題,氰特公司仍然建議自動鋪帶采用和手工鋪層OOA層壓板相同的真空保持度。“這個問題和其他問題正被前面提到的OSD纖維鋪放項目所研究探討,”Russell稱,“我們不知道鋪帶邊緣的壓力是否將阻斷空氣路徑,這可能會導致孔隙度出現問題。”MTM-45和CYCOM 5320-1將被評估,用于AFP制備的戰機機翼表皮部件中。該項目將比較各類自動化設備,并以相同方式跟蹤每個樣件的鋪放時間,所用標準是由聯合攻擊戰斗機(JSF)項目制定的標準。

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GKN公司應用其在OOA材料自動化鋪層方面超過10年的經驗來生產10.5m長、全截面碳纖維復合材料的翼梁樣件,該部件和為A400M軍用運輸機提供的前后翼梁相似,后者是2005~2009年空中客車公司領導的先進低成本飛機結構(ALCAS)項目的一部分。在生產過程中,首先一臺西班牙MTorres公司提供的11軸龍門式高速鋪帶機使用HTS 268g/m2的碳織物MTM44-1預浸料進行鋪層;然后這種“扁平帶”被自動地轉移到碳纖維模具中,模具再被放入一臺雙隔膜成型壓力機中,20min后成型為C型截面半成品;該層壓半成品再經過袋壓和固化(采用真空壓力,所需熱量來自集成在模具中的電器元件,初始固化溫度為130℃,后固化溫度為180℃),一個27mm厚、孔隙率<2%的翼梁部件就得以制造完成。該樣件目前正在空中客車英國公司進行測試。

采用加熱的模具已經成為GKN公司固化OOA的傳統方法。“電或液體加熱的模具使你能夠不使用熱壓罐和熱爐,”Rich Oldfield說道,“它還允許更精確地控制固化過程,包括局部加熱,以適應不同厚度的部件,并消除熱點和冷點。”GKN公司認為即使整體加熱的模具,其成本也只比標準模具略高一些,但它大大縮短循環時間以及擺脫傳統約束的好處提供了一個整體上的經濟收益。此外,這項技術也非常有利于更高速的制造。 123456

批評者爭辯說,自動化鋪層依然依賴大量的資金成本,因此這只是“換湯不換藥”。而GKN公司卻進行了機器投資,它認為與OOA固化相結合的自動化鋪層會帶來非常顯著的成本節省。自動鋪層也不必像典型的手工鋪層那樣,因使用了預浸料支撐薄膜而需要對產品進行100%的檢測。

BMI和超越

OOA雙馬來酰亞胺(BMI)系統的開發,是OOA工藝發展中一個“安靜的革命”。直到Russell在鹽湖城召開的秋季SAMPE會議上稱:這應該成為每一家公司的一個目標,BMI 才為業界所關注。ACG公司在今年5月份通過演示宣布,它正在開發一種OOA BMI,并將在2011年年底之前完成篩選,2012年產生數據。氰特公司也表示,它將開發OOA BMI。

Maverick公司已生產聚酰亞胺樹脂達17年之久,并向預浸料制造商銷售這類樹脂,如Renegade材料公司。事實上,Maverick公司和Renegade材料公司自2008年以來,就保持密切合作。最近,Akron聚合物系統公司和NASA Glenn公司作為合作伙伴也加入進去,共同開展OOA材料用于高溫復合材料應用項目的研究。這項工作受益于最近從美國俄亥俄州獲得的Third Frontier Grant基金。該團隊自籌和俄亥俄州的配套資金將在兩年內達$200萬,在此期間,他們將開發使用溫度達204~371℃的BMI和聚酰亞胺系統。 本文來自123

Maverick公司總裁兼產品開發總監Robert Gray博士稱,聚酰亞胺的使用溫度為204~371℃,成本在$220~551/kg之間,而BMI樹脂可提供高達177℃的熱/濕性能,成本僅為$154~220/kg。BMI樹脂更容易加工,因為其不像聚酰亞胺在固化過程中會產生反應副產物,例如水和乙醇。Gray相信大多數加工廠受限于加工只針對高溫應用的BMI材料。因此,Maverick公司計劃在配制VBO固化預浸料時采用分子量更高的系統以代替粘度非常低的樹脂,來減少加工聚酰亞胺的復雜性。

聚酰亞胺復合材料的軍事用途包括:噴氣發動機中的導向壓縮機的定子葉片、發動機背后的排氣瓣及外部旁路管道。聚酰亞胺結構也可以應對商業飛機發動機里面和周圍的極端環境。Maverick公司和Renegade公司已經確定了聚酰亞胺的許多結構性應用,包括無人機(UAV)和隱形應用。通過提供加工更經濟的OOA聚酰亞胺,Maverick公司的項目可以幫助飛機設計師通過減少絕緣和屏蔽的需要來減輕部件的重量。這對于對重量異常敏感的運載火箭和其他空間平臺尤為重要。

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目前,PMR-15(氰特公司提供的牌號為2237 CYCOM)是行業標準的聚酰亞胺,需要在1.38MPa進行熱壓罐固化。Gray解釋說:“復合材料制造商很少有大型的高溫熱壓罐。如果我們能脫離熱壓罐加工聚酰亞胺,就可以極大地增強高溫復合材料部件供應商的能力,并擴展這些輕質材料的應用。”

然而,推動OOA BMI的因素是什么呢?行業顧問Jeff Hendrix認為,大多數要求耐熱溫度在135℃以上的BMI和聚酰亞胺部件都被用于發動機中,并且通常要求它們的尺寸不是很大。這些部件適合于目前的熱壓罐工藝,實際上大多數是模壓成型的。“不是使用溫度高促使人們選擇BMI,而是其在較低溫度下的卓越缺口沖擊性能使它們對許多方案都有吸引力。”Hendrix解釋道,“真正令人滿意的OOA系統是其可以在82~121℃范圍內匹配BMI的缺口沖擊性能,這對環氧樹脂基系統頗為理想。” 123,123

美國空軍研究實驗室(AFRL)的Russell表示同意,并指出:“5250-4 BMI(氰特公司生產)被大量地用于JSF項目中,該應用不需要較高的工作溫度,選擇5250-4 BMI僅僅是因為其熱/濕性能提供了更高的比剛度和比強度,進而能夠實現更輕的結構。”

Russell還看到了將來的項目對于更高耐溫性能的需求,他說:“一些涉及聯合未來戰區運輸機的概念,將催生一種類似于巨大的F-22的運輸機,其發動機緊挨著機身,排放的廢氣穿過水平尾翼表面。這些主結構的溫度可能會超過環氧樹脂的耐熱能力。因此,我們對OOA BMI感興趣。”Russell稱,NASA對BMI產生興趣的原因是,在運載火箭的頭錐使用BMI可以避免使用昂貴、笨重的絕緣材料。單單減輕重量就足以證明使用更貴的BMI是合理的,更不用說它還有利于原材料和勞動力的節省。“我們與Stratton復合材料公司有$60萬的合作項目涉及非熱壓罐BMI的開發。”Russell指出。
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波音公司的Hahn說:“有些人看不到OOA BMI所帶來的許多好處。作為一家飛機制造商,我會問材料供應商,他們是否有一種OOA BMI可在相當低的固化溫度下可以達到足夠的初始強度。因為開發新的項目時,所要求的使用溫度通常升高而不是下降。”

不再使用熱壓罐嗎?

NASA的復合材料結構高級顧問Mark Shuart談到,當截面直徑為10m的部件被證明可以采用OOA成功加工時,航空航天復合材料的制造將會呈現很大的相同。但有些人會說沒有那么簡單。“對于要采用的技術來說,它必須具有經濟意義。”Hahn說道。Hendrix表示同意。“如果你的生產計劃已經有了一個熱壓罐和材料數據庫,那么你不必刻意采用OOA,”他說道,“只有從原型到生產更經濟,或者為大型共固化結構件減少模具的復雜性,才會有意義。”

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氰特公司的Mark Ostermeier預測,OOA中會更加占據主導地位。但他警告說,航空航天業將仍然是保守的。“龐巴迪公司的Learjet機型在謀求一個全OOA復合材料機身方面已經邁開了一大步,”他說道,“但大多數廠家的大型商業結構件可能還在等等再看。”

Ostermeier看到軍用航天航空領域走向OOA的速度比商用飛機更快,并且如果NASA可靠地生產出非熱壓罐的運載火箭,他預測這可以相當大地改變復合材料的制造方式。Hendrix對此保持懷疑,他說:“我相信OOA的前途是光明的,但人們應該小心,因為他們認為它十分便宜。”

Hahn認為,OOA用于實際生產和研發項目的功效比較是一個大問題,決策的推動力往往是一個項目的截止日期。“我們正在考慮的OOA材料的成本與使用的熱壓罐主結構材料相同,”她說道,“如果這些OOA材料允許我們在6周內完成模具,然后更快速地進行原型開發和生產,那么它們可以在針對某一應用的商業開發中獲勝。”
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性能要求:CAI與OHC

美國空軍研究實驗室(AFRL)非熱壓罐研究項目的負責人John Russell最近在猶他州鹽湖城召開的秋季SAMPE會議上宣稱:“使用沒有微裂紋的高模量纖維,可使我們提高25%的缺口沖擊性能。”盡管OOA預浸料供應商在減少纖維微裂紋方面做得不是很多,但ACG公司已經宣布其XMTM47材料將于明年商業化,該材料的設計基于120℃的使用溫度及所要求的缺口性能的改善。缺口壓縮強度通常采用開孔壓縮(OHC)實驗來測量。ACG的XMTM47材料的目標是104℃濕法OHC 達到43ksi?,F有的一個更韌性的系統如MTM45-1的OHC約為35ksi。

然而,根據NCAMP(國家先進材料性能中心)副主任Yeow Ng所言,如果OOA預浸料被用于商用飛機,則其壓縮后抗沖擊性能(CAI,表明損傷容許度)將增加。在Stephen Trimble為《Flight Daily News》最新撰寫的文章中,Ng指出,MTM45-1達不到商業飛機監管機構所要求的強度公差。 波音787結構的沖擊后壓縮(CAI)強度的測量值是40s(ksi),但MTM45-1的CAI 值在30ksi以下。 本文來自123

ACG公司的研究和技術副總裁Chris Ridgard回應道:“許多用在Hawker Beech、Embraer、Gulfstream以及龐巴迪飛機中的結構件都是用熱壓罐固化Hexcel 8552(CAI≈30 ksi)和氰特977-2(CAI≈37ksi)預浸料制成的。”他還指出,用于Learjet 85、正在進行資格認證的CYCOM 5320,其公布的CAI值為26.5ksi。他說:“在OOA成為一種趨勢之前,這種辯論已經存在很久了,它起始于20世紀70年代的熱壓罐材料和建立在特定應用需求基礎上的不同設計理念。”曾是英國航空航天公司結構工程師的Ridgard解釋道:“軍事上的應用是被缺口應變推動的。軍事應用要求結構上的任何地方都可有一個孔存在,并提供一定程度的余量用于戰斗損傷和螺栓的維修。因此,軍用飛機都傾向于使用具有較高OHC值而韌性較低的樹脂系統。” 他繼續說道,“對于具有相對較少扣件的大型結構件的大型商用飛機來說,一些飛機制造商偏好使用具有極高CAI值和高破壞應變的高增韌樹脂系統。” 123,123

ACG公司稱將開發一種CAI高于40s的OOA系統,這是一種高度增韌的系統,用于商用飛機領域。氰特公司稱,這也將是其下一個工作的重點。當被問及是OHC還是CAI推動Aurora飛行科學公司的無人機設計時,該公司的結構工程師Ed Wen回應道:“兩者都有推動作用。”這與波音公司Gail Hahn的觀點一致:“我們知道采用不同材料系統的重要性,因此我們將采用一系列的OOA預浸料來滿足不同的設計要求。”

真空粘接蜂窩夾層

在《先進復合材料的保養和維修》一書(由汽車工程師協會所屬的Warrendale公司在1997年出版)中,英國航空公司擁有復合材料制造和修復24年經驗的Keith Armstrong博士講述了有關“在蜂窩中,單獨用0.1MPa的真空獲得足夠的膜狀膠粘劑的粘合壓力”的問題,他寫道:當真空泵開始從真空袋中抽取空氣時,一些空氣從蜂格中去除,尤其是靠近板材的邊緣。然而,當真空壓力增加時,膜狀膠粘劑在蜂格的兩端形成一個密封,并且空氣不再被抽取。 123456

在英國航空公司的試驗中,一塊邊長為0.3m的正方形測試板因真空條件下加熱造成的內部壓力而不能在中心區域粘接。Armstrong稱,在120℃固化過程中,甚至更高的180℃固化中,蜂格內的壓力將增加1.5倍,因此他建議相應地減小蜂窩夾層內的壓力。對于120℃的固化,如果一個0.1MPa的真空施加于蜂窩夾層,則夾層內的壓力在加熱之前需要減小到4MPa。Armstrong解釋道:“這種非穿孔蜂窩板的制造技術,是在膠粘劑和蜂窩之間夾層的一個面上使用了一種合適的織物。英國航空公司選用了一種無紡聚酯單絲織物,有時也被稱為定位布,它通常用作膜狀膠粘劑的載體,具有較低的吸水性能。

他強調說,無論選擇什么織物,其厚度不應超過0.076mm(英國航空公司選用的兩層織物的厚度為0.038mm),膜狀膠粘劑應包括1層1.36kg/m3層或2層0.096kg/m3層,以確保能有足夠的膠粘劑量來吸附織物,并且在表皮和蜂格末端之間形成充分的膠粘帶。 123456


原文資料:

Out-of-autoclave prepregs: Hype or revolution?

Producing aerospace composites without an autoclave, using vacuum-bag-only (VBO) atmospheric pressure, is nothing new. Vacuum-bagged, oven-cured material systems for secondary structures (flaps, fairings and the like) are well established. What is new is the ability for such materials to deliver the less than 1 percent void content and autoclave-quality mechanical properties required for aerospace primary structures, such as wings, fuselages and empennage components with integrated stiffeners.

Interest in OOA processing also has spurred the use of resin transfer molding (RTM), vacuum-assisted RTM (VARTM) and other liquid molding processes as well as the latest generation of compression molded thermoplastics (see “Aerospace-grade compression molding,” under "Editor's Picks," at top right). Although these processes are gaining acceptance among those who fabricate highly loaded structures, VBO prepregs, which encompass traditional hand layup as well as automated material placement methods, are of particular interest due to a combination of unique advantages. copyright 123456

The U.S. Air Force has identified OOA prepregs as vital to achieving the fast and affordable manufacturing that the U.S. Department of Defense (DoD) will require for future military platforms, and the Air Force sees additional cost savings when one OOA prepreg system can be used for prototyping, production and spares. Suppliers to commercial aircraft OEMs see OOA materials as a way to achieve manufacturing flexibility, freeing them from size constraints and enabling modular/cellular workflows.

Yet many issues remain. OOA cycle times might actually be longer, due to the time-dependent process of edge-breathing required for low void content. Other issues include compatibility of adhesives, surface quality of sandwich structures, and using automation in lay-up. Also, because these materials are new, a complete database of B-basis design allowables will have to be established, requiring time and money.

“Aside from the hype, is OOA a good tool?” asks Gail Hahn, Non-autoclave (Prepreg) Manufacturing Technology program manager at Boeing Research & Technology (St. Louis, Mo.), “Yes, but is it a game changer for all applications in the industry? Not necessarily.”

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Why OOA prepreg?

OOA prepregs ensure even resin distribution, avoiding the dry spots and resin-rich pockets common with infusion processes. Also, OOA prepregs can be cured at lower pressures and temperatures (vacuum pressure vs. a typical autoclave pressure of 85 psi and cure at 200°F/93°C or 250°F/121°C vs. a traditional 350°F/ 177°C autoclave cure). Therefore, tooling for large composite structures with integrated stiffeners that can be cocured in a single cycle, which is typically very complex and expensive, now might be fabricated much more simply and cost-effectively. Further, mismatches between tool and part coefficients of thermal expansion (CTEs) are smaller at lower temperatures and, therefore, more easily managed, positioning OOA prepregs as a potential solution for part cracking caused by cure-temperature differentials.

One of the most widely cited OOA benefits is the potential to reduce high capital and operating costs. John Russell, Defense-Wide Manufacturing Science and Technology program manager for the Air Force Research Laboratory (AFRL, Wright-Patterson Air Force Base, Ohio), relates that NASA explored the costs of building a 40-ft by 80-ft (12m by 24m) autoclave to cure 10m/33-ft diameter launch vehicle barrels and found that it would cost roughly $40 million to build and another $60 million to install, plus the cost of nitrogen and power to operate it. DoD advisories indicate that future military aircraft programs will be produced in small volumes with very small budgets. Future NASA programs will be cost-constrained to an even greater degree. Russell points out that for NASA and for coming DoD programs, such as the Long Range Strike and Joint Future Theatre Lift (the latter a C-130 successor with planned 180-ft/55m wingspan and 140-ft/43m fuselage), “the challenge is how to deal with low production volumes —100 aircraft — while reducing capital requirements as much as possible.” The Air Force foresees a huge savings from the elimination of autoclave capital and operating costs. 內容來自123456

Tier 1 suppliers to commercial aircraft OEMs agree, seeing OOA prepregs as a way to attain faster, more agile manufacturing. “We will pursue nonautoclave manufacturing for as many parts as is feasible in the future,” comments GKN Aerospace (Redditch, Worcestershire, U.K.) director of technology, Rich Oldfield. “We see it as a way to bring efficiency to the factory, including an entirely different workflow due to cellular manufacturing vs. large parts queued up for an autoclave.” GKN also sees this flexible manufacturing, which is less reliant on traditional manufacturing monuments, as critical to the task of achieving the forecast 60 to 70 percent composites content on the next generation 737 and A320 single-aisle jetliner programs.


A revolution 20 years in the making

According to Russell, AFRL and the Defense Advanced Research Projects Agency (DARPA) began to look at tooling prepregs and how they might be used to make prototypes in the Affordable Composites program during the mid-1990s. The first generation of OOA VBO materials, including Advanced Composites Group’s (ACG, Tulsa, Okla. and Heanor, Derbyshire, U.K.) LTM series, enabled cheaper, low-temperature vacuum-cure prototypes of large, unitized parts. Notable examples: Scaled Composites’ (Mojave, Calif.) SpaceShipOne, WhiteKnightOne and Global Flyer, Boeing’s X45A unmanned combat aerial vehicle (UCAV), DARPA/Lockheed Martin’s Darkstar, and the McDonnell Douglas Bird of Prey. 123456

These early prepregs were cheaper because they cure at lower temperatures under vacuum pressure, but they had neither the mechanical properties nor the sufficiently short cycle times necessary for production parts.

By 2005-2006, two OOA prepreg systems — ACG’s MTM45 and Cytec Engineered Materials’ (Tempe, Ariz.) CYCOM 5215 — were approaching the properties of autoclave-cured prepregs. Both prepregs feature flexible cure cycles, requiring longer cure times at 150°F to 175°F (66°C to 79°C) but providing shorter two-hour cures at 250°F/121°C. Additionally, they achieve a wet Tg of more than 300°F/150°C after a 350°F/177°C freestanding postcure.

These systems achieved DARPA’s required <1 percent void content, but fell short elsewhere. The 10- to 12-day tack life and maximum 21-day open mold life were deemed insufficient by both AFRL and NASA because layup and bagging of large, complex structures require, at minimum, three to four weeks. Recently, however, ACG and Cytec have developed XMTM-47 and CYCOM 5320-1, respectively, targeted to a longer tack life of 21 days and minimum 30-day open mold life.

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In 2007, the Disruptive Manufacturing Technologies Initiative began, with Non-Autoclave (Prepreg) Manufacturing Technology as one of five concentration areas. Cofunded by DARPA and Boeing, and executed with AFRL, the Non-Autoclave initiative’s established goal was to develop OOA systems that could provide the same performance as currently qualified autoclave-cure epoxy materials. ACG responded with MTM45-1, MTM-44 and MTM-46 and Cytec responded with CYCOM X5320. Boeing evaluated three experimental resin formulations, and selected X5320, which was later commercialized by Cytec as CYCOM 5320.

A range of demonstrator parts were built and subjected to limited nondestructive and dissection evaluations. HITCO Carbon Composites (Gardena, Calif.) built a 3.65m by 4.57m (12ft by 15 ft) hat-stiffened skin primary structure demonstrator. Aurora Flight Sciences (Columbus, Miss.) built a 38-ft/11.6m unmanned aerial vehicle (UAV) wing spar. (In a separate development, Aurora built a three-part, OOA-cured wing structure with a 150-ft/45.7m span for the Phantom Eye UAV demonstrator, which won top honors in Aviation Week’s Supplier Innovation awards competition, as a potential game-changer in the industry.). Boeing (Irvine, Calif.) built the boom and empennage for a 60 percent-scale demonstrator of the long-endurance Phantom Eye UAV, which is scheduled to begin flight trials in early 2011. Boeing has also submitted draft material specifications for carbon prepreg and a draft process specification, which are available to the public.

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As part of the program’s Phase II, Boeing’s St. Louis operation manufactured 68-ft/21m parts, including a spar and three different wing skin configurations:


A plank-stiffened skin, using Grafil (Sacramento, Calif.) HR 40 12K high-modulus carbon fiber;
A hat-stiffened skin. using Cytec T40/800B intermediate modulus carbon unitape as well as high-strength fabric made from Cytec Thornel T-650/35 3K carbon fiber;
A sandwich construction skin, using nonmetallic honeycomb core.

All were made with the same tooling, enabling Boeing to compare the finished panels in terms of quality, degree of production difficulty, inspectability and technology maturity level.

Perhaps the most successful demonstration of large OOA-cured structure to date is the Advanced Composite Cargo Aircraft (ACCA). When tasked by the Secretary of the Air Force to develop a new military transport aircraft with only $50 million and (indicative of DoD’s future intentions) 18 months to completion, Lockheed Martin (Ft. Worth, Texas) responded to an AFRL Broad Agency Announcement (BAA) competitive solicitation with a Dornier 328, cut off behind the cockpit to avoid the expense of developing new flight controls, and added an all-composite, 60-ft/18m long fuselage, built in eight pieces using ACG’s VBO-cured MTM-45 throughout the structure. Although the program ran seven months past its deadline, AFRL’s Russell, the program’s manufacturing engineer, says ACCA stayed on-budget and was a success (see "Advanced Composite Cargo Aircraft ....” under "Editor's Picks").
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Ready but awaiting qualification

OOA prepregs have now reached physical property parity with autoclave-cure systems for primary aircraft structure, and several are already in qualification processes. The National Center for Advanced Materials Performance (NCAMP, Wichita, Kan.) has completed allowables testing for CYCOM 5215 and nine MTM45-1 product forms, including uni and woven quartz fiber, plain-weave G30-500 carbon fiber, high tensile strength (HTS) 12K carbon uni, E-glass, and 6781-style S-2 glass prepreg, with fiber supplied by AGY (Aiken, S.C.) and woven by JPS Glass Fabrics (Anderson, S.C.). NCAMP testing of the same nine systems for MTM46 and also for CYCOM 5320-1 will soon be completed. ACG has announced that it will be releasing allowables for its XMTM47 system (“X” soon to be removed) in 2011 via its Lightweight Composite Structures (LCS) program with NAVAIR.

However, there is debate in the industry with regard to qualification and the availability of a complete allowables database. “We are at quite a high readiness level with our OOA technology and could deploy it into products relatively quickly,” says GKN’s Rich Oldfield, “except for the lack of availability of qualified materials, because there is no B-basis allowables database currently for any OOA prepreg system.” This FAA-approved statistical design data from composite lamina and laminate batch testing is a prerequisite to qualification for use in military or commercial aircraft primary structure. ACG’s VP of research and technology, Chris Ridgard, explains, “NCAMP does not run every test that defense airframers list as necessary.” It also does not run as many samples as defense programs require — approximately 1,400 to 1,500 vs. the 3,000 for defense qualification. “NCAMP tests materials and stores the values in a database, but only a specific defense platform can qualify a material for use on that vehicle” explains AFRL’s Russell. “It typically costs $3 million to $5 million to develop a complete B-basis allowables database, and none of the suppliers has stepped up to the plate just yet.” Russell notes that NCAMP testing offers data that is not held as proprietary by any one company or defense platform, such as that developed in the ongoing qualifications at Bombardier and Airbus.

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Cytec product manager Mark Ostermeier says CYCOM 5320 and 5320-1 are being qualified in separate, proprietary programs. Cytec has announced it will supply carbon fiber composite materials for both primary and secondary structures on Bombardier’s (Montreal, Quebec, Canada) Learjet 85, and Bombardier reported that the pressurized fuselage will feature “a low-pressure, oven-cured, out-of-autoclave carbon fiber supplied by Cytec.”

ACG’s OOA materials are in qualification at Airbus. David Inston, project leader, out-of-autoclave technologies at Airbus’ Bristol, U.K. location, explains, “MTM44-1 is qualified with an Airbus specification for a particular 2x2 twill fabric. However, it is not fully qualified yet for a specific application.” Although this is a woven fabric specification and, therefore, will be used for secondary rather than primary structure, qualification is nearly complete and marks the first use of MTM44-1 for an actual aircraft part. Airbus has an agreement with GE Aviation (Hamble, U.K.) for the production of secondary wing structures for the A350, including wing access panels and trailing edge details. Inston notes that Airbus does have a specification published for an MTM44-1 unidirectional product, but no further qualification testing has been launched yet for a particular application, although a primary structure demonstrator has been built. “Airbus is in the process now of defining its road map to further develop OOA materials, including potential use in primary structures,” he adds.
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Long time for low porosity

Manufacturers, however, warn against unreasonable expectations for OOA prepregs in production. Their reputation for “fast” prototyping, for example, might not translate to “fast” production, where void content goals can’t be compromised. ACG’s Ridgard explains that in OOA processing, removal of volatiles, which include not only air entrapped during layup but also the moisture (1 to 2 percent) that epoxies absorb when they are exposed to ambient air, involves an “edge-breathing” strategy. The laminate edges must be in contact with the breather and materials must be arranged in a way that maintains air escape paths. The vacuum bag materials and sequence are the same as with autoclave processing, but vacuum quality is vital, and because entrapped air extraction is a time-dependent process, OOA cure cycles are typically longer. Cytec recommends a vacuum hold before initiating cure for parts made using its OOA prepregs. This is in addition to any debulks required during layup and is performed without removing the bag prior to cure. Hold length depends on the part size and complexity, ranging from as low as four hours for a 2-ft by 2-ft (0.6m by 0.6m) part to 16 hours for a 20-ft by 40-ft (6m by 12m) structure with cocured hat stiffeners.
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But John Cornforth, GKN Aerospace’s head of technology, cautions, “You have to be careful when you talk about OOA taking more time in vacuum, because some autoclave materials also require long vacuum times.” He notes that it is very hard to compare alternative systems because the specific process details of vacuum presssure, cure temperatures and cycle times become very dependent upon the specifics of each part.

ACG’s Ridgard asserts that the overall time penalty is really not any greater than with autoclave production: “If you cure MTM45-1 at 350°F [176°C], for example, you have to dwell at 250°F [121°C] for two hours because ramping too high too quickly drops the resin viscosity to where it will block off air paths. You are only adding two hours, plus you have the flexibility of a longer cure at 250°F [121°C] or 200°F [93°C], where the extra vacuum time is built-in, and then do a freestanding postcure to get the same properties.” 123,123

GKN, however, may have found a way around the extra cycle time. The company has developed microwave curing, which reduces typical cure cycles for both autoclave and OOA materials by up to 80 percent, to only 90 minutes. Microwaves reportedly heat only the composite structure, while the tooling and oven chamber remain cool, which drastically cuts heating and cooling time, as well as energy consumption. GKN sees an additional benefit in the ability to selectively cure parts of the structure, which could make integrating partially cured structures and then cocuring the final assembly possible in the future. GKN has begun technology readiness development, using ACG’s MTM materials, and is currently comparing traditional autoclave materials and processes with microwave curing, to define optimum parameters.


Sandwich surface and OOA adhesives

In its significant work with OOA-cured honeycomb-cored sandwich structures, ACG has identified some technical challenges. Chris Ridgard explains, “the bagging and layup techniques are basically the same as in autoclave curing, but venting of the core becomes important.” The high pressures used in autoclave curing, which do not permit air inside a honeycomb core to flow into skin laminates, are not present in OOA curing. Therefore, air must be removed from the core cells before the resin softens, because the lower OOA pressure might otherwise allow air to flow into the skin, resulting in high void content. Research performed at McGill University (Montreal, Quebec, Canada) and presented in a 2010 SAMPE paper shows that if pressure inside the honeycomb is reduced to 500 mbar (7.25 psi) or less prior to heating, the air does not escape into the facesheets. Instead, it remains in the core during the cure cycle. Ridgard says a lightweight glass fiber mesh (such as a 54 g/m2 or 1.6 oz/yd2 leno weave) is used as a breather to prevent skin disbonding due to pressure within the core. This technique was established at British Airways for composite repair more than 25 years ago (see “Old is new ...." sidebar, at the end of this article, or click on it under "Editor's Picks"). However, some believe it is not an optimal solution because the breather is laminated into the sandwich, increasing parasitic weight.
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Air also can migrate into the faceskins due to foaming of the film adhesive, which compromises the adhesive fillet formation and resulting faceskin-to-core bond quality. Many common film adhesives for honeycomb sandwich will foam under the high vacuum levels used in OOA processing. ACG found that foaming was affected by film adhesive carrier type, honeycomb cell size and cure temperature. The company worked through these variables to develop MTA241, which is compatible with its MTM materials and will not foam during standard OOA cure cycles. “We see that a whole suite of OOA materials is needed for VBO processing,” says Boeing’s Hahn. AFRL’s Russell agrees, “Both film and paste adhesives compatible with OOA systems are needed. As OOA moves forward, the complete materials suite must all be compatible.”

Another issue for OOA honeycomb sandwich is to avoid surface pitting with single shot cures. “Surfacing film is required for a beautiful OOA sandwich surface,” says Hahn. On two recent 4-ft by 8-ft (1.2m by 2.4m) test panels where half used a surface film and half did not, the half with the film produced a better surface without surface porosity. ACG is not too concerned, because autoclave-cured sandwich structures often use a surfacing film as a carrier for copper mesh or other lightning strike protection (LSP) materials. ACG has developed MTM246 Surface Improvement Film for use with its OOA prepregs, while Cytec recommends its flexible-cure SurfaceMaster SM 905 product, which has been used in many OOA projects. 內容來自123456


Higher impregnation for automation

Automation has become a critical area for OOA development. Fiber Placement of Out of Autoclave Composites is a 2009-2011 R&D program jointly funded by the Office of the Secretary of Defense (OSD), AFRL, Defense Logistics Agency (DLA), and the Office of Naval Research (ONR) Manufacturing Technology (ManTech) program. The Fiber Placement program’s chief goals are to develop the automated fiber placement (AFP) process for OOA materials, including specification of key process parameters, such as lay-down rate, and to demonstrate the process through fabrication of full-scale aerospace components. Participants include Boeing St. Louis, GKN Aerospace, HITCO Carbon Composites, Lockheed Martin, and Spirit Aerosystems (Wichita, Kan.). The program will use automated equipment from ElectroImpact (Mukilteo, Wash.), Ingersoll Machine Tools (Rockford, Ill.) and MAG Industrial Automation Systems (Hebron, Ky.).
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According to Ridgard, most OOA prepregs for hand layup incorporate dry fiber paths to some degree, which permit air extraction during cure but result in less than 100 percent impregnation. This is why OOA prepregs that use tightly woven E-glass or quartz fabrics are deliberately under-impregnated. However, materials used in AFP typically have higher degrees of impregnation. Cytec explains that OOA prepreg used in automated tape laying (ATL) is slit into 6-inch to 12-inch (152-mm to 305-mm) widths, which are not as sensitive as the typical 0.125-, 0.25-. or 0.5-inch (3.2-, 6.4- or 12.7-mm) narrow-width tapes used in AFP. There can be no dry fibers in the middle of these tapes because they will hang off the edges as the tape is layed, building up and then stripping off the tape edges during application. This is why OOA prepreg for AFP may have the same chemistry as VBO systems, but it often uses the highest level of impregnation possible.

Even so, it is assumed that the compaction forces of the AFP application head and its ability to lay materials down with pressure and heat will mitigate issues with entrapped air. Cytec still recommends the same vacuum hold as for hand layup OOA laminates. This and other issues are being explored in the aforementioned OSD Fiber Placement program, says Russell. “We are not sure if the pressure applied by the tape-laying head will block the air paths, which could result in porosity issues.” Both MTM-45 and CYCOM 5320-1 will be evaluated in fighter wingskin-sized parts made using AFP. The program will compare various types of automated equipment and will track the lay-down time in the same manner for each demonstrator part, using standards developed by the Joint Strike Fighter (JSF) program. 內容來自123456

GKN Aerospace applied more than 10 years of experience with automated layup of OOA materials to produce a 10.5m/34.5-ft long, full cross-section carbon fiber composite wing spar demonstrator, similar to the front and rear wing spars it supplies for the A400M, as part of the 2005-2009 Airbus-led Advanced Low-Cost Aircraft Structures (ALCAS) program. An MTorres (Torres de Elorz, Spain) gantry-mounted 11-axis high-speed tape-layer-applied HTS 268 g/m2 (8 oz/yd2) carbon fabric MTM44-1 prepreg. This “flat pack” was then robotically transferred to a carbon fiber tool, placed in a double-diaphragm forming press, and shaped into a C-section in 20 minutes. The shaped spar laminate was then bagged and cured using only vacuum pressure with heat supplied by electrical elements integrated into the tool, and an initial cure of 130°C/266°F with a 180°C/356°F postcure. The 27-mm/1.1-inch thick spar showed less than 2 percent porosity. It is being tested at
Airbus UK.

Heated tools have been GKN’s conventional approach to OOA curing. “An electrically or fluid-heated tool enables you to avoid both autoclaves and oven,” says Rich Oldfield. “It also allows more accurate control of the curing process, including local heating to accommodate parts with different thicknesses, eliminating hot and cold spots.” Even though an integrally heated tool costs slightly more than a standard tool, GKN believes the benefits of greatly reduced cycle time and freedom from monument constraints deliver an overall economic gain. This technology also is much more conducive to higher-rate manufacturing.

123456



Critics contend that automated layup is again dependent on large capital costs, and is really just swapping out one monument for another. But GKN has already made the machine investment, and sees very significant cost savings in combining automated layup with OOA curing. Automated layup also might serve to reduce the burden of 100 percent inspection typical for hand layup because of the use of prepreg backing film.


BMIs and beyond

The development of OOA bismaleimide (BMI) systems has been a “quiet revolution” in OOA processing development. Or it was until Russell claimed, “This needs to be a goal for everyone,” at the Fall SAMPE conference in Salt Lake City, Utah. ACG announced via presentations in May that it is developing an OOA BMI and stated that it would finalize candidates by the end of 2010 and generate data in 2011. Cytec also says it will develop an OOA BMI.

For 17 years, Maverick Corp. (Blue Ash, Ohio) has produced polyimide resins, which they sell to prepreggers like Renegade Materials Corp. (Dayton, Ohio). Maverick and Renegade, in fact, have worked closely together since 2008 and were recently joined by Akron Polymer Systems (Akron, Ohio) and NASA Glenn (Cleveland, Ohio) as partners in an OOA Materials for High-Temperature Composite Applications project. The effort benefits from a recently awarded Third Frontier Grant from the U.S. state of Ohio. The team’s contribution and Ohio’s matching funds will total $2 million over two years, during which BMI and polyimide systems will be developed for service temperatures of 400°F to 700°F (204°C to 371°C). 123456

Dr. Robert Gray, Maverick president and director of product development, reports that polyimides enable service temperatures between 400°F/204°C and 700°F/371°C and cost $100/lb to $250/lb, while BMI resins provide up to 350°F/177°C hot/wet performance and cost $70/lb to $100/lb. But processing BMI resins is a little easier because, unlike polyimides, they do not form reaction byproducts, such as water and alcohol, during cure. Gray believes that most fabricators are limited to processing only BMI materials for high-temperature applications. However, Gray plans to reduce the complexity of processing polyimides by moving away from very low viscosity resins toward systems of greater molecular weight, formulated for VBO-curing prepregs.

Military applications for polyimide composites include stator vanes leading into the compressor in jet engines, exhaust flaps in back of the engines, and outer bypass ducts. Polyimide structures also can handle extreme environments in and around commercial aircraft engines. Maverick and Renegade have identified many structural applications for polyimides, including UAV and low-observable applications. By offering more affordably processed OOA polyimides, Maverick’s program could help aircraft designers cut weight by reducing the need for insulation and shielding, especially on launch vehicles and other space platforms where weight is super critical. 123,123

Currently, PMR-15 (supplied by Cytec as CYCOM 2237), the industry-standard polyimide, requires autoclave cure at 200 psi/13.8 bar. Gray explains, “Very few composites manufacturers have large, high-temperature autoclaves. If we can process polyimides out-of-autoclave, we could dramatically expand the high-temp composite parts supplier base and broaden the use of these lightweight materials.”

But what is the driver for an OOA BMI? According to industry consultant Jeff Hendrix, most BMI and polyimide parts that require performance above ~275?F/~135?C are used in engines, and don’t typically need to be large. These parts fit into current autoclaves, and most, in fact, are compression molded. “It’s not the upper use temperature that drives selection of BMIs, but the superior notched properties at even lower temperatures that makes them attractive to many programs,” Hendrix explains, adding, “What’s really desired is an OOA system that can match BMI notched properties in the 180°F to 250°F (82°C to 121°C) range, which would ideally be an epoxy-based system.”

copyright 123456


AFRL’s Russell agrees, noting that “5250-4 BMI [produced by Cytec] was used
heavily in the JSF [program], even where no elevated service temperature was
needed, simply because it was able to deliver higher stiffness-to-weight and
strength-to-weight ratios due to its hot/wet performance, which means lighter
structures.”
Russell also sees the need for higher temperature performance in future
programs: “Some concepts for the Joint Future Theater Lift vehicle show an
aircraft resembling a giant F-22, with engines embedded next to the fuselage,
dumping exhaust across the horizontal tail surfaces. Temperatures for these
primary structures likely will exceed epoxy capability. Thus, OOA BMIs are of
interest to us.” Russell says that NASA has an interest because the use of BMI
in the nose-cone of its launch vehicles could eliminate costly, heavy insulation
materials. Eliminating that weight alone could justify BMI’s extra cost, not to 本文來自123
mention the material and labor reductions. “We have a $600,000 program with
Stratton Composites Solutions (Marietta, Ga.) to look at developing an
out-of-autoclave BMI,” notes Russell.
Boeing’s Hahn says some people don’t see much benefit in an OOA BMI but others
do, and she can see both sides. “But as an airframer,” she says, “I would ask
material suppliers if they have an OOA BMI that achieves sufficient green
strength at reasonably low cure temperatures because in-use temperature needs
usually go up not down as new programs develop.”


No more autoclaves?
Mark Shuart, senior advisor for Composites & Structures at NASA Langley
(Hampton, Va.), remarks that when 10m/33-ft diameter sections are demonstrated
successfully using OOA processing, aerospace composites manufacturing will never
be the same. But others say it’s not that simple. “For the technology to be 內容來自123456
adopted, it will have to make economical sense,” says Hahn. Hendrix agrees. “If
your production program already has an autoclave and a materials database, you
simply cannot make a business case for looking at OOA,” he argues. “It only
makes sense in the case of going from prototyping to production more
economically or reducing tooling complexity for large cocured structures.”
“OOA will become more predominant,” predicts Cytec’s Mark Ostermeier, but he
warns that the aerospace industry will remain conservative. “Bombardier’s
LearJet has taken a major step by pursuing an all-composite OOA fuselage,” he
observes, “but most manufacturers of large commercial structures will probably
wait and see.”
Ostermeier sees military aerospace moving toward OOA faster than commercial
aircraft, and if NASA does produce out-of-autoclave launch-vehicle sections
reliably, he predicts that it could change composites manufacturing quite a bit.
copyright 123456

Hendrix maintains his skepticism, “I believe OOA has a bright future, but people
should be careful because they think it is so much cheaper.”
For Hahn, however, the issue is OOA’s utility in real-world production vs. R&D
programs, where the driver for decision-making often is a program deadline. “The
OOA materials we are looking at right now cost the same as the autoclave primary
structure materials in use,” she notes. “But if they allow us to complete
tooling in six weeks, and then proceed to prototyping and production much more
quickly, it could win in trade studies for an application.” 
 

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