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外文翻譯
快速成型技術(shù)和系統(tǒng)的回顧
快速成型制造技術(shù)已經(jīng)為很快地直接地從計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)創(chuàng)造 3D立體產(chǎn)品出現(xiàn)。這些技術(shù)在工業(yè)成型制造中起著重要的作用。這一篇文章回顧了 RP&M 的主要技術(shù)和應(yīng)用程序。RP&M的原則和技術(shù)在這里被展現(xiàn)。在這些新的技術(shù)方面的一些現(xiàn)有的問(wèn)題和研究議題被介紹。我們也在使用快速設(shè)計(jì)原型作為較進(jìn)一步的例證。
關(guān)鍵字：快速成型，分層堆積制造
簡(jiǎn)介
產(chǎn)品制造工業(yè)正在面對(duì)二個(gè)重要的挑戰(zhàn)性的任務(wù):(1) 縮減產(chǎn)品發(fā)展時(shí)間的; (2) 制造產(chǎn)品的小整批尺寸產(chǎn)品和多種類(lèi)型產(chǎn)品在柔性方面的改進(jìn)，CAD和CAM已經(jīng)著重改良傳統(tǒng)的生產(chǎn)設(shè)計(jì)和制造形式。然而, 對(duì)于新產(chǎn)品的迅速發(fā)展在和計(jì)算機(jī)輔助制造的計(jì)算機(jī)輔助設(shè)計(jì)的真實(shí)整合中有一些障礙。 雖然可觀的研究已經(jīng)在過(guò)去被為計(jì)算機(jī)輔助設(shè)計(jì)和制造業(yè)整合, 像功能辨識(shí), CNC 程序規(guī)劃和程序計(jì)劃，但是 CAD 和 CAM 之間還是存在著一些空白區(qū)域:
（1） 3D立體模型和原尺的迅速創(chuàng)造。
（2） 創(chuàng)建復(fù)雜模型表面產(chǎn)品的成本問(wèn)題
這是一篇和快速成型有關(guān)的指導(dǎo)性文章
RP& M 能做什么
為了從實(shí)質(zhì)上短縮發(fā)展中的圖案，鑄模和原形的時(shí)間,一些制造業(yè)的企業(yè)已經(jīng)啟動(dòng)使用迅速設(shè)計(jì)成型方法作為制造復(fù)雜圖案和元件設(shè)計(jì)方法。在過(guò)去幾年內(nèi), 多種新的快速制造業(yè)的技術(shù), 通常叫做快速成型制造 (RP&M)的技術(shù) 已經(jīng)出現(xiàn); 這項(xiàng)技術(shù)發(fā)展包含 Stereolithography;SLS;FDM;LOM;BPM和3D印刷。這些技術(shù)能夠直接地產(chǎn)生來(lái)自 CAD 數(shù)據(jù)庫(kù)的實(shí)際的物件。他們通常都有一個(gè)的重要功能: 加工零件模型時(shí)用堆積成型方法要多于材料切除法。這就把3D模型的加工簡(jiǎn)化到2D書(shū)屋堆積成型，因此便能直接的調(diào)用計(jì)算機(jī)模型數(shù)據(jù)庫(kù)中的模型直接加工。
RP&M 的基本程序
如圖 1 所示，一個(gè)零件首先被模型制造者做出一個(gè)幾何模型, 例如一個(gè)實(shí)體模型。零件進(jìn)入一系列的平行截面塊之內(nèi)然后算術(shù)地被區(qū)分之。 對(duì)于每個(gè)模塊, 硬化或綴合，路徑已經(jīng)產(chǎn)生,在 圖2 中顯示。這些硬化或粘合路徑直接地被用倆作為加工指令加工工件材料。在一個(gè)層被建造之后, 一個(gè)新的層被用同樣的方法建在先前的那個(gè)層上。因此，模型是從底層一步步建到高層的。在摘要中, 迅速成型設(shè)計(jì)的犯法包含兩個(gè)方面：: 數(shù)據(jù)準(zhǔn)備和模型生產(chǎn)。
圖-1 固體模型實(shí)例
圖-2 切片掃描
RP&M 的目前應(yīng)用領(lǐng)域
雖然 RP&M 技術(shù)仍然在他的早期階段,但是一些工業(yè)公司 , 像德克薩斯儀器公司，克萊斯勒公司, 安培公司和福特電動(dòng)機(jī)公司已經(jīng)從技術(shù)改良下列三個(gè)方面的他們的產(chǎn)品發(fā)展獲益。
工程學(xué)設(shè)計(jì)
視覺(jué)化。概念上的模型在產(chǎn)品設(shè)計(jì)非常重要。設(shè)計(jì)者使用 計(jì)算機(jī)中的CAD程序產(chǎn)生他們的設(shè)計(jì)觀念。然而，無(wú)論工程師用CAD能夠多么好的解釋藍(lán)圖和復(fù)雜工件的圖像，它仍然非常困難的完全地展示實(shí)際的復(fù)雜產(chǎn)品將會(huì)看起來(lái)像什么。一些錯(cuò)誤可能仍然從工程師和設(shè)計(jì)者的檢查中逃脫。實(shí)際物件的觸覺(jué)有時(shí)能揭露未預(yù)料到的問(wèn)題。閃爍一個(gè)較好的設(shè)計(jì)。由RP&M產(chǎn)生一個(gè)較好的設(shè)計(jì)思路，復(fù)雜工件的成型加工能夠在短時(shí)間內(nèi)建造, 因此工程師能非?？斓卦u(píng)估一個(gè)設(shè)計(jì)。
證明和最佳化。改良產(chǎn)品質(zhì)量總是制造加工中的一個(gè)重要議題。由傳統(tǒng)的方法, 模型的研發(fā)或者一個(gè)最佳化的有效設(shè)計(jì)是要耗費(fèi)大量時(shí)間和資金的。在此對(duì)比, RP&M 成型能被很快地設(shè)計(jì)加工再?zèng)]有現(xiàn)成的用工具工作和勞動(dòng)力的情況下。結(jié)果證明設(shè)計(jì)觀念變的很簡(jiǎn)單; 產(chǎn)品質(zhì)量能被在有限制的時(shí)間里和可負(fù)擔(dān)成本的前提下得到改善。反復(fù)。就像汽車(chē)工業(yè)，廠家經(jīng)常把性產(chǎn)品投放到市場(chǎng)中。產(chǎn)品投放市場(chǎng)的時(shí)間快慢是在今天的競(jìng)爭(zhēng)市場(chǎng)中區(qū)分勝利者和失敗者的主要因素之一。由于 RP&M 技術(shù)存在，在一個(gè)短時(shí)間內(nèi)完成多樣化設(shè)計(jì)是可能的。 總體上減少模型的研發(fā)時(shí)間。
制造業(yè)
我們能使用 RP&M 成型技術(shù)作為可生產(chǎn)研究。 在產(chǎn)品的設(shè)計(jì)初期提供一個(gè)實(shí)物模型，我們能加速程序計(jì)劃和工具工作設(shè)計(jì)的程序。 除此之外，由于正確地描述復(fù)雜的幾何形狀，這種方法能幫助在工場(chǎng)上解釋藍(lán)圖方面減少問(wèn)題的出現(xiàn)。 另外的一個(gè)應(yīng)用程序正在為鑄模用工具工作研發(fā)。 這種方法也能為鑄件被用作主圖案。
市場(chǎng)
為了協(xié)助產(chǎn)品售賣(mài)，一個(gè)原型能用來(lái)示范觀念，設(shè)計(jì)主意,同時(shí)也代表了一個(gè)公司的生產(chǎn)能力。實(shí)際模型的真實(shí)舉例說(shuō)明設(shè)計(jì)的可行性。同時(shí), 原型能用來(lái)修正設(shè)計(jì)得到客戶的回應(yīng)，以便最后的產(chǎn)品將會(huì)符合客戶的需求。及時(shí)的領(lǐng)會(huì)客戶的想法, 方法在90年代是市場(chǎng)競(jìng)爭(zhēng)的關(guān)鍵。
及時(shí)的領(lǐng)會(huì)客戶的想法在90年代是市場(chǎng)競(jìng)爭(zhēng)的關(guān)鍵。RP& M 技術(shù)有潛力能確定提高產(chǎn)品質(zhì)量。主要有兩方面: 幾乎沒(méi)有幾何形狀方面的限制; 分層堆積制造方式允許從CAD到CAM的直接的簡(jiǎn)單的接口，而完全忽略了計(jì)算機(jī)輔助設(shè)計(jì)程序。
快速原型制造和制造技術(shù)
正如以前被提到的。有幾種遵循“生長(zhǎng)”和“添加制造”的原理的現(xiàn)代生產(chǎn)方式。在這些技術(shù)的主要不同點(diǎn)有二個(gè)方面: (1) 選用的材料(2) 零件制造技術(shù)。 以下部分將會(huì)在這兩方面詳細(xì)地說(shuō)明快速原型制造。
光敏液相固化法 (SLA)
SLA 被Charle Hull發(fā)明的3D立體系統(tǒng)Inc.2。它是首個(gè)商業(yè)化的快速原型制造技術(shù)并且應(yīng)用的最廣泛。 用的材料是液體樹(shù)脂。在光的照射之下, 小的分子 (單體)聚合成較大的分子?；谶@個(gè)原則,零件在如圖 3 所顯示的一個(gè)盛放液體樹(shù)脂的桶中制造出來(lái)
SLA 機(jī)器制造原型是借激光探頭在液體表面上追尋分層路線制造的。與數(shù)控機(jī)床得之字形切削路徑不同,探頭沿著平行線移動(dòng)，然后再沿著垂直的方向。在液體下面有一個(gè)提升的支撐面。表面就在廣可以照射的深度。激光光線在檢流計(jì)驅(qū)動(dòng)鏡子旁邊的 X 和 Y 軸中被水平地偏斜，以便它移動(dòng)。樹(shù)脂的表面生產(chǎn)一個(gè)固體形狀。在一層被建造之后,支撐面下降一個(gè)用戶指定的距離，一層液體樹(shù)脂的新涂料覆蓋了原始層。一個(gè)掃帚幫助填充用來(lái)制造下一層的新的樹(shù)脂。激光在已經(jīng)做好的一層上畫(huà)新的層。這樣，模型從底部到頂端的逐層建造出來(lái)。當(dāng)所有的層被完成的時(shí)候，原型被制造了大約95% 。后續(xù)制造要等到原型完全凝固之后。這在一個(gè)充滿紫外線的烤箱中制做。SLA有一些值得提及的特點(diǎn)。
材料：有五種商業(yè)化得的原型材料。它們?nèi)菢?shù)脂。
支持：因?yàn)橐粋€(gè)模型在液體被產(chǎn)生,在制造過(guò)程中，懸于零件(在下面不支持)之上的下垂或者飄浮離開(kāi)。原型如此需要一些初步設(shè)計(jì)支持直到它被制造或固化。支持可能是柱子，橋和構(gòu)架。有時(shí)需要輸入或者把硬加入填充在的薄壁件。這些附加的物體在模型原型的制造的過(guò)程中產(chǎn)生和必須在模型完成之后清除。
模型準(zhǔn)確性和表現(xiàn)性。 被達(dá)成的準(zhǔn)確性是大約 0.1% 的全部尺寸而且除了不超過(guò) 0.5% 之外以較大的大小。 層厚度是在 0.004 和 0.03之間。目前，被 3D立體系統(tǒng)公司做的 SLA 機(jī)器是在 RP& M 系統(tǒng)之中的最精確的機(jī)器。。
photopolymer 制作的原型是易碎的而且可能不夠結(jié)實(shí)來(lái)抵抗高的壓迫力測(cè)試。 同時(shí), ，材料的收縮可能會(huì)使原型變形。
再循環(huán)。 Photopolymers 是thermoset材料和不能夠再一次為重復(fù)使用被融化。
選區(qū)激光繞結(jié)法(SLS)
動(dòng)態(tài)同步傳輸套式公司 (奧斯汀 TX) 用在德克薩斯大學(xué)的機(jī)械工程部門(mén)被卡爾 Deckard 和約瑟 Beaman 研發(fā)的 SLS 系統(tǒng)提供治療液體的系統(tǒng)替代5代品. SLS 使用對(duì)泉華的一個(gè)二氧化碳激光連續(xù)數(shù)層的粉代替液體。 在 SLS 處理粉的表層在工作地方之上被一個(gè)替換柜臺(tái)滾筒機(jī)制應(yīng)用。 粉狀材料被些微地在它的熔點(diǎn)下面預(yù)先加熱到溫度。 激光光線在粉狀表面上追蹤跨區(qū)段加熱對(duì)泉華溫度的粉。以便粉保證被激光掃描到。 不是被激光掃描的粉將會(huì)適當(dāng)?shù)乇３忠暈閷?duì)粉下層的支持,這在減少扭曲方面是有幫助的。 當(dāng)跨區(qū)段的層被完成的時(shí)候, 滾筒消除粉的另層在那之上為下途徑泉華一。 圖 5 演示SIS 的工作原理。 SIS 有一些特征。
材料。 SIS 使用各類(lèi)型的材料包括 polycarbonate ， pvc ，ABS ，尼龍，樹(shù)脂， polyster ， polypropane ，聚亞安酯作為投資投擲臘的樣板的生產(chǎn)。 能夠使用金屬制的和陶瓷粉的機(jī)器正在研發(fā)過(guò)程中。
支持。 SLS 系統(tǒng)通常不需要預(yù)先設(shè)計(jì)支持結(jié)構(gòu)。 在建筑物期間，如支持在每個(gè)層上的為不融合的粉末處理。 做模型準(zhǔn)確性和表現(xiàn)。 平均的準(zhǔn)確性 " 為一個(gè)部份由從 -0.005 到 +0.015 達(dá)成了范圍直徑和 15"高度。 層厚度是在 0.003 和 0.02之間。
產(chǎn)品可能遭受收縮和變形由于冷卻。 那二個(gè)因素能根據(jù)選擇更小顆粒的粉末所限制, 和在粉末的軟化處理點(diǎn)上面的高方面比和空氣流程溫度的粉狀粒子除去, 但是在泉華點(diǎn)下面。
再循環(huán)。 原型能是 '粉碎' 成粉末以便重復(fù)使用。
熔絲沉積成型法(FDM)
快速原型系統(tǒng)--3D立體制造模型由Stratasys 公司研發(fā)用來(lái)的構(gòu)造以被擠出的熱后可塑性的材料的沉淀為基礎(chǔ)的部份呼叫 FDM. 在一個(gè) FDM 程序中，一個(gè)熱后可塑性細(xì)絲材料進(jìn)入一個(gè)加熱的 FDM中之后被擠出。 FDM 頭的運(yùn)動(dòng)被計(jì)算機(jī)控制。 在飛的擠出頭內(nèi)，細(xì)絲進(jìn)入設(shè)備內(nèi)被一個(gè)反抗加熱器融化。 噴頭追蹤部份的一個(gè)每跨區(qū)段層的精確部分。 當(dāng)噴頭在空間的x軸和y軸移動(dòng)時(shí)拉絲材料就在噴頭內(nèi)進(jìn)行預(yù)熱然后再?gòu)膰婎^擠出。當(dāng)它工作的時(shí)候，材料以 1/10s的速度直接堆積在工作點(diǎn)上。在一層堆積完成后, 噴頭按照程序預(yù)先設(shè)計(jì)好的為堆積下一層在 z 方向移動(dòng)。 每層的堆積都是對(duì)先前的堆積作加固。 圖 6 演示FDM 的工作原理。
FDM 有下列的主要特征:
材料。 FDM 技術(shù)為樣板的堆積提供了多種模型材料和顏色。 可提供的材料有填充了塑料的聚合物質(zhì)的尼龍 , 和臘。 專(zhuān)有的尼龍 , 和投資投擲變大。 所有的材料是無(wú)毒的而且可能有不同的顏色。 而且有消耗材料最小的方法。 沒(méi)有在硬化之后的被需要。
支撐情況。 在許多情況下， FDM 程序不需要支持生產(chǎn)部份。 FDM 噴頭噴出的錫先是材料在空便凝固。 在上層部分，支撐能力可能仍然是減少扭曲部分所必要的。
模型準(zhǔn)確性。 全部的寬度是誤差在 ~0.005。連續(xù)的制成薄板在 0.001-0.05之間, 而壁厚范圍從 0.01 到 0.25 之間。
用FDM制作原型時(shí)有1.2%的收縮率。
性能。3D立體模型加工的邊框是12×12×12。模型加工的速度是60r/s。
選區(qū)片層粘結(jié)法(LOM)
LOM 程序教工零件用的是紙塑料，金屬或者是鋼件。LOM 要加工一個(gè)鋼件工件需要按照預(yù)先設(shè)計(jì)好的程序來(lái)進(jìn)行，然后激光在CAD的引導(dǎo)下加工出需要的形狀。涂層能被黏在一起或焊接。多余的材料要莫被移走要莫就作為下一層噴涂的基礎(chǔ)。圖 7 演示LOM 的工作原理。
LOM 有如下特征:
材料。 事實(shí)上任何的箔 (材料) 都能被應(yīng)用: 紙，金屬，塑料，纖維，合成物質(zhì)材料，玻璃或合成物。 Helisys 公司現(xiàn)在使用纖維素箔。
支撐情況. LOM 程序使用使用電晶體的材料因此通常不需要預(yù)先設(shè)計(jì)。支援結(jié)構(gòu)。
模型準(zhǔn)確表現(xiàn)性. 模型能準(zhǔn)確的表現(xiàn)實(shí)物構(gòu)造在 ±0.005之間。而且因?yàn)椴牧系氖褂貌皇湛s或者扭曲。 層的厚度是在 0.002 和 0.02之間. 用這些材料制造的原型不易碎而且由于用 photopolymers 做成的那些的原型。
能力. LOM 機(jī)器— LOM-1015 使用一個(gè) 40瓦特二氧化碳激光器。LOM-1015 構(gòu)造的原型范圍的大小是 15×10×15".因?yàn)橹恍枰枥L切斷去代替診斷一個(gè)硬化區(qū)域,LOM 相比之下就要快一些。
Ballistic particle manufacturing (BPM)
感知沖擊粒子制造業(yè)技術(shù)是由一個(gè)感知系統(tǒng)使用一個(gè) piezo 驅(qū)動(dòng)的噴墨機(jī)制射擊融化的材料的小滴, 同時(shí)冷卻在先前存放的層之上。一個(gè)層由在移動(dòng) x軸 和 y軸 方向的小滴噴嘴噴射產(chǎn)生。從碟子的底部指定一個(gè)距離，一個(gè)新的層在先完一個(gè)成的層的頂端產(chǎn)生。自動(dòng)化動(dòng)力學(xué)公司也獨(dú)立地研發(fā)了一部相似的機(jī)器。 圖 8 演示 BPM 的工作原理。
圖-8 BPM 工作原理
BPM 的特征是:
材料。 材料應(yīng)該容易地被融化而且凝固,例如熱后可塑性物質(zhì)，鋁和臘。 感知系統(tǒng)現(xiàn)在使用的是蠟,自動(dòng)化動(dòng)力學(xué)公司使用鋁。
支持。 在構(gòu)造模型程序時(shí)，對(duì)于突出部份和空虛結(jié)構(gòu)的支持是必要的,支持的材料是水溶性的蠟的合成物質(zhì)。 當(dāng)模型被完成的時(shí)候, 支持物被用溫水洗去。
模型準(zhǔn)確表現(xiàn)性。 全部的準(zhǔn)確性是 +0.004之間。 層厚度大約在0.0035之間。 沒(méi)有表現(xiàn)報(bào)告。
能力。 BPM 打印機(jī)可以用32個(gè)噴墨噴嘴并排排列在50/ m 的范圍內(nèi)每秒噴射10000個(gè)小液滴。最大的工作塊尺寸是 12 × 2 × 12"。
三維印刷(3D Printing)
三維空間的印刷技術(shù)是由麻薩諸塞州學(xué)會(huì)研發(fā)的。在3D立體印刷程序中，一個(gè)3D立體模型進(jìn)入計(jì)算機(jī)的2D 跨區(qū)段層之內(nèi)被切成薄片。在一個(gè)圓筒中,噴層的粉放在活塞的頂端和床子上, 然后噴墨頭按照程序把層粉噴到指定位置的模型上，那里的信息由計(jì)算機(jī)模型庫(kù)提供出噴涂所需要的相應(yīng)的信息。在一層完成之后, 活塞放下滑到預(yù)先定義的距離，一個(gè)新層按照上一步 的方法繼續(xù)建立出來(lái)。當(dāng)整個(gè)部份完成的時(shí)候，熱處理是提高層與層之間的粘結(jié)所必要的, 然后去除層粉。 圖 9 演示 3D立體印刷的工作程序的原理。
3D立體印刷有如下的特征:
材料。 3D立體印刷程序能使用鋁-氧化物和礬土-矽石陶瓷粉。 材料是無(wú)定形的或膠質(zhì)的碳化物支持。 由于 3D立體印刷技術(shù)，支持結(jié)構(gòu)的設(shè)計(jì)就顯得沒(méi)有必要了,因?yàn)樵趪娡砍绦蜻M(jìn)行時(shí)層粉在每一層都能保留它的自然形態(tài)。
模型準(zhǔn)確表現(xiàn)性。 小數(shù)量數(shù)據(jù)可從3D立體印刷中獲得因?yàn)樗匀辉跍y(cè)試的階段中。 對(duì)于測(cè)試的樣品，層厚度是 178 和最小尺寸是 0.017v。
能力。 3D立體印刷程序能用來(lái)生產(chǎn)功能的部份和加工原型。 現(xiàn)在的最大部份尺寸是 12×12×24。這種技術(shù)在構(gòu)筑模型是最大潛能是 20×21 ，厚度100nm，以每小時(shí)0.18m的速度構(gòu)建每層。當(dāng)一層完成以后，活塞下降一個(gè)提前設(shè)定好的距離，一新的粉層噴灑出來(lái)，并有選擇的粘結(jié)。當(dāng)整個(gè)零件完成的時(shí)候，為了提高粘結(jié)層的結(jié)合需要必要的熱處理，然后除去沒(méi)有粘結(jié)的層。圖9顯示了3維印刷的工作過(guò)程。
3維印刷的特點(diǎn)綜述如下：
材料. 三維印刷可以利用氧化鋁和鋁-矽陶瓷粉。黏合劑是無(wú)定形或者膠體的碳化矽。
支撐體。 利用三維印刷技術(shù)，零件的職稱(chēng)結(jié)構(gòu)是不需要設(shè)計(jì)的，因?yàn)槊繉記](méi)有粘結(jié)的形成了一個(gè)天然的支撐。
模型精度. 三維印刷適應(yīng)于數(shù)量較少的加工，因?yàn)樗€在測(cè)試階段。對(duì)于測(cè)試樣本，層的厚度是178μm，最小的尺寸是0。017’’。三維印刷能夠制造功能性零件或刀具的原型?，F(xiàn)在最大的工件尺寸是12×12×24’’。這項(xiàng)技術(shù)在大約每層2秒（0.18mh-1）生產(chǎn)率情況下，有能夠生產(chǎn)尺寸超過(guò)20×20’’零件的潛力。
結(jié)論
對(duì)于加工者保持競(jìng)爭(zhēng)力，生產(chǎn)特點(diǎn)，質(zhì)量，成本和推向市場(chǎng)的時(shí)間是重要因素。快速原型系統(tǒng)給制造產(chǎn)品更快，提供了機(jī)會(huì)，并且比傳統(tǒng)的加工方法成本低。因?yàn)镽P&M能夠極大的縮短產(chǎn)品的開(kāi)發(fā)周期，越來(lái)越多的商品利用了這個(gè)速度，由計(jì)算機(jī)設(shè)計(jì)的產(chǎn)品能夠生產(chǎn)味精準(zhǔn)的模型。而這些模型能夠被感知，看到，研究，測(cè)試和比較。
討論了幾種新的有發(fā)展前景的快速原型制造。他們都是基于一層一層堆積的原理。它們中的每一個(gè)考慮到精度，材料種類(lèi)和機(jī)器成本都有各自的特點(diǎn)。還討論了一些顯露出來(lái)的問(wèn)題和研究成果。這是一個(gè)快速發(fā)展的領(lǐng)域?？焖氽兄圃旒夹g(shù)的能力和潛力吸引著非常多的工廠為這項(xiàng)技術(shù)投資。
非常期待為了研究和發(fā)展那些技術(shù)能有更多的努力，這樣，它們可以廣泛的應(yīng)用于制造工業(yè)。
A review of rapid prototyping technologies and systems
Rapid Prototyping and Manufacturing (RP&M) technologies have emerged for quickly creating 3D products directly from computer-aided design systems. These technologies significantly improve the present prototyping practices in industry. This paper reviews the main technologies and applications of RP&M. The principles and the features of those RP&M technologies are presented. Some existing problems and research isles on these new technologies are introduced.
Keywords; raid prototyping, layered manufacturing
INTRODUCTION
Product manufacturing industry is facing two important challenging tasks: (I) substantial reduction of product development time; and (2) improvement on flexibility for manufacturing small batch size products and a variety of types of products, Computer-aided design and manufacturing (CAD and CAM) have significantly improved the traditional production design and manufacturing. However, there arc a number of obstacles in true integration of computer-aided design with computer-aided manufacturing for rapid development of new products. Although substantial research has been done in the past for computer-aided design and manufacturing integration, such as feature recognition, CNC programming and process planning, the gap between CAD and CAM remains unfilled in the following aspects~:
(1) rapid creation of 3D models and prototypes.
(2) (2) cost-effective production of patterns and moulds with complex surfaces.
This is a tutorial paper of rapid prototyping and manufacturing (RP& M).
What RP& M can do
To substantially shorten the time for developing patterns, moulds, and prototypes, some manufacturing enterprises have started to use rapid prototyping methods for complex patterns making and component prototyping. Over the past few years, a variety of new rapid manufacturing technologies, generally called Rapid Prototyping and Manufacturing (RP & M), have emerged; the technologies developed include Stereolithography; Selective Laser Sintering (SLS), Fused Deposition Manufacturing (FDM), Laminated Object Manufacturing(LOM), Ballistic Particle Manufacturing (BPM), and Three Dimensional Printing (3D Printing). These technologies are capable of directly generating physical objects from CAD databases. They have a common important feature: the prototype part is produced by adding materials rather than removing materials. This simplifies the3D part producing processes to 2D layer adding processes such that apart can be produced directly from its computer model.
The basic process of RP& M
As shown in Figure 1 a part is first modelled by a geometric modeller such as a solid modeller. The part is then mathematically sectioned (sliced) into a series of parallel cross-section pieces. For each piece, the curing or binding paths are generated, shown in Figure2. These curing or binding paths are directly used to instruct the machine for producing the part by solidifying or binding a line of material. After a layer is built, anew layer is built on the previous one is the same way. Thus, the model is built layer by layer from the bottom to top. In summary, the rapid prototyping activities consist of two parts: data preparation and model production.
Current application areas of RP&M
Although RP&M technologies are still at their early stage, a number of industrial companies such as Texas Instruments, Inc., Chrysler Corporation, Amp Inc. and Ford Motor Co. have benefited from applying the technologies to improve their product development in the following three aspects.
Figure 1 The solid model of an object
Design engineering
Visualization. Conceptual models are very important in product design. Designers use CAD to generate computer representations of their design concepts. However, no matter how well engineers interpret blue prints and how excellent CAD images of complex objects are, it is still very difficult to visualize exactly what the actual complex products will look like. Some errors may still escape from the review of engineers and designers. The touch of the physical objects can reveal unanticipated problems and sometimes .spark a better design. With RP& M, the prototype of a complex part can be built in short time, therefore engineers can evaluate a design very quickly.
Verification and optimization. Improving product quality is always an important issue of manufacturing. With the traditional method, developing of prototypes to validate or optimize a design is often time consuming and costly. In contrast, an RP&M prototype can be produced quickly without substantial tooling and labor cost. Consequently, the verification of design concepts becomes simple; the product quality can be improved within the limited time frame and with affordable cost. Iteration. Just like the automotive industry, manufacturers often put new product models into market. Time to market is one of the key features to separate winners from the losers in today's competitive market. With RP&M technology, it is possible to go through multiple design iterations within a short time and subs. tantially reduce the model development time.
Manufacturing
We can use the RP&M prototype for producibility studies. By providing a physical product at an earlier design stage, we can speed up process planning and tooling design. In addition, by accurately describing complex geometry, the prototype can help reduce problems in interpreting the blue prints on the shop floor. Another application is tooling development for moulds. The prototypes can also be used as master patterns for castings.
Marketing
To assist product sales, a prototype can be used to demonstrate the concept, design ideas, as well as the company's ability to produce it. The reality of the physical model illustrates the feasibility of the design. Also, the prototype can be used to gain customers' feedback for design modifications so that the final product will meet customers' requirements.
Meeting customers' demands in a timely, manner is the key to penetrating the market in the 1990s. RP& M technologies have the potential to ensure that quality products are developed quickly for two major reasons: there are almost no restrictions on geometrical shapes; and the layered manufacturing allows a direct and very simple interface from CAD to CAM which almost completely eliminates the need for computer-aided process planning.
RAPID PROTOTYPING AND
MANUFACTURING TECHNOLOGIES
As mentioned earlier, there are .several technologies available for model production based on the principle of "growing' or 'additive machining'. The major differences among these technologies are in two aspects: (1) materials used; and (2) part building techniques. The following sections will explain in detail these rapid prototyping technologies with respect to the above two aspects.
Stereolithography
Stereolithography apparatus (SLA)
SLA was invented by Charle Hull of 3D Systems Inc.2. It is the first commercially available rapid prototyper and is considered as the most widely used prototyping machine. The material used is liquid photo-curable resin, acrylate. Under the initiation of photons, small molecules (monomers) arc polymerized into large molecules. Based on this principle, the part is built in a vat of Liquid resin as shown in Figure 3.
The SLA machine creates the prototype by tracing layer cross-sections on the surface of the liquid photopolymer pool with a laser beam. Unlike the contouring or zig-zag cutter movement used in CNC machining, the beam traces in parallel lines, or vectorizing first in one direction and then in the orthogonal direction. An elevator table in the resin vat rests just below the liquid .surface whose depth is the light absorption limit. The laser beam is deflected horizontally in X and Y axes by galvanometer-driven mirrors so that it move. across the surface of the resin to produce a solid pattern. After a layer is built, the elevator drops a user-specified distance and a new coating of liquid resin covers the solidified layer. A wiper helps spread the viscous polymer over for building the next layer. The laser draws a new layer on the top of the previous one. In this way, the model is built layer by layer from bottom to top. When all layers are completed, the prototype is about 95% cured. Post-curing is needed to completely solidify the prototype. This is done in a fluorescent oven where ultraviolet light floods the object (prototype). There are several features worthy of mention of $LA.
Material. There are five commercially available photopolymers. All of them are a kind of acrylate. Support. Because a model is created in liquid, the overhanging regions of the part (unsupported below) sag or float away during the building process. The prototype thus needs some predesigned support until it is cued or solidfied. The support can be pillars, bridges and trusses. Sometimes posts or internal honeycomb sections are needed to add rigidity to tall thin-walled shapes during the process. These additional features are built on the model parts and have to be trimmed after the model building is completed.
Model accuracy and performance. The accuracy achieved is about 0.1% of the overall dimension and deteriorates with larger sizes but no more than 0.5%. The layer thickness is between 0.004 and 0.03". Presently, the SLA machines made by 3D Systems Inc. are the most accurate machines among the RP& M systems.
The photopolymer-made prototype is brittle and may not be strong enough to withstand high stress testing. Also, the shrinkage of the material may make the prototype deform.
Recycling. Photopolymers are thermoset material and cannot be melted again for reuse.
Selective laser sintering (SLS)
DTM Corp. (Austin TX) offers an alternative to liquid-curing systems with its SLS systems which were developed by Carl Deckard and Joseph Beaman at the Mechanical Engineering Department of University of Texas at Austin4"5. SLS uses a carbon dioxide laser to sinter successive layers of powder instead of liquid. In SLS processes a thin layer of powder is applied by a counter-rotating roller mechanism onto the work place. The powder material is preheated to a temperature slightly below its melting point. The laser beam traces the cross-section on the powder surface to heat up the powder to the sintering temperature .so that the powder. scanned by the laser is bonded. The powder that is not scanned by the laser will remain in place to serve as the support to the next layer of powder, which aids in reducing distortion. When a layer of the cross-section is completed, the roller levels another layer of powder over the sintered one for the next pass. Figure 5 shows the working principle of SIS. SIS has several features. Material. SIS uses a wide range of materials for model production including polycarbonate, PVC (polyvinyl chloride), ABS (acrylonirile butadine styrene), nylon, resin, polyster, polypropane, polyurethane and investment casting wax. The machine that is capable of using metal and ceramic powder is in the process of development.
Support. The SLS systems usually do not need pre-designed support structures. The unfused powder on every layer acts as a support during the building process. Model accuracy and performance. The average accuracy achieved ranges from +0.005 to+0.015" for a part with 12" diameter and 15" height. The layer thickness is between 0.003 and 0.02".
The product may suffer shrinkage and war page due to sintering and cooling. Those two factors can be partly eliminated by choosing powder particles which have a small size, and a high aspect ratio and air flow temperature above the softening point of the powder, but below the sintering point.
Recycling. The prototype can be 'crushed' into powder for reuse.
Fused deposition modelling (FDM)
Rapid prototyping system--3D modeler developed by Stratasys Inc.--constructs parts based on deposition of extruded thermoplastic materials called FDM process6. In an FDM process, a spool of thermoplastic filament feeds into a heated FDM extrusion head. The movement of the FDM head is controlled by computer. Inside the flying extrusion head, the filament is melted into liquid by a resistant heater. The head traces an exact outline of each cross-section layer of the part. As the head moves horizontally in x and y axes the thermoplastic material is extruded out a nozzle by a precision pump. The material solidifies in 1/10 s as it is directed on to the workplace. After one layer is finished, the extrusion head moves up a programmed distance in z direction for building the next layer. Each layer is bonded to the previous layer through thermal heating. Figure 6 shows the working principle of FDM.
The FDM has the following main features:
Material. The FDM technology allows a variety of modeling materials and colors for model building. Available materials are wax-filled plastic adhesive material,
proprictay nylon, and investment casting wax. Proprietary nylon, and investment casting wax. All the materials are non-toxic and can be in different colors. There is minimum material wastage in the method. No post-curing is required.
Support. In many cases, the FDM process does not need support to produce part. The FDM extrusion head forms a