40裝載機橋殼減速器殼體加工工藝及銑專機設計含3張CAD圖,40,裝載,機橋殼,減速器,殼體,加工,工藝,專機,設計,cad 快速成型技術和系統(tǒng)的回顧 快速成型制造技術已經(jīng)為很快地直接地從計算機輔助設計系統(tǒng)創(chuàng)造 3D立體產(chǎn)品出現(xiàn)。這些技術在工業(yè)成型制造中起著重要的作用。這一篇文章回顧了 RP&M 的主要技術和應用程序。RP&M的原則和技術在這里被展現(xiàn)。在這些新的技術方面的一些現(xiàn)有的問題和研究議題被介紹。我們也在使用快速設計原型作為較進一步的例證。 關鍵字：快速成型，分層堆積制造 簡介 產(chǎn)品制造工業(yè)正在面對二個重要的挑戰(zhàn)性的任務:(1) 縮減產(chǎn)品發(fā)展時間的; (2) 制造產(chǎn)品的小整批尺寸產(chǎn)品和多種類型產(chǎn)品在柔性方面的改進，CAD和CAM已經(jīng)著重改良傳統(tǒng)的生產(chǎn)設計和制造形式。然而, 對于新產(chǎn)品的迅速發(fā)展在和計算機輔助制造的計算機輔助設計的真實整合中有一些障礙。 雖然可觀的研究已經(jīng)在過去被為計算機輔助設計和制造業(yè)整合, 像功能辨識, CNC 程序規(guī)劃和程序計劃，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)建復雜模型表面產(chǎn)品的成本問題 這是一篇和快速成型有關的指導性文章 RP& M 能做什么 為了從實質(zhì)上短縮發(fā)展中的圖案，鑄模和原形的時間,一些制造業(yè)的企業(yè)已經(jīng)啟動使用迅速設計成型方法作為制造復雜圖案和元件設計方法。在過去幾年內(nèi), 多種新的快速制造業(yè)的技術, 通常叫做快速成型制造 (RP&M)的技術 已經(jīng)出現(xiàn); 這項技術發(fā)展包含 Stereolithography;SLS;FDM;LOM;BPM和3D印刷。這些技術能夠直接地產(chǎn)生來自 CAD 數(shù)據(jù)庫的實際的物件。他們通常都有一個的重要功能: 加工零件模型時用堆積成型方法要多于材料切除法。這就把3D模型的加工簡化到2D書屋堆積成型，因此便能直接的調(diào)用計算機模型數(shù)據(jù)庫中的模型直接加工。 RP&M 的基本程序 如圖 1 所示，一個零件首先被模型制造者做出一個幾何模型, 例如一個實體模型。零件進入一系列的平行截面塊之內(nèi)然后算術地被區(qū)分之。 對于每個模塊, 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。在一個層被建造之后, 一個新的層被用同樣的方法建在先前的那個層上。因此，模型是從底層一步步建到高層的。在摘要中, 迅速成型設計的犯法包含兩個方面：: 數(shù)據(jù)準備和模型生產(chǎn)。 圖-1 固體模型實例 圖-2 切片掃描 RP&M 的目前應用領域 雖然 RP&M 技術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機公司已經(jīng)從技術改良下列三個方面的他們的產(chǎn)品發(fā)展獲益。 工程學設計 視覺化。概念上的模型在產(chǎn)品設計非常重要。設計者使用 計算機中的CAD程序產(chǎn)生他們的設計觀念。然而，無論工程師用CAD能夠多么好的解釋藍圖和復雜工件的圖像，它仍然非常困難的完全地展示實際的復雜產(chǎn)品將會看起來像什么。一些錯誤可能仍然從工程師和設計者的檢查中逃脫。實際物件的觸覺有時能揭露未預料到的問題。閃爍一個較好的設計。由RP&M產(chǎn)生一個較好的設計思路，復雜工件的成型加工能夠在短時間內(nèi)建造, 因此工程師能非?？斓卦u估一個設計。 證明和最佳化。改良產(chǎn)品質(zhì)量總是制造加工中的一個重要議題。由傳統(tǒng)的方法, 模型的研發(fā)或者一個最佳化的有效設計是要耗費大量時間和資金的。在此對比, RP&M 成型能被很快地設計加工再沒有現(xiàn)成的用工具工作和勞動力的情況下。結果證明設計觀念變的很簡單; 產(chǎn)品質(zhì)量能被在有限制的時間里和可負擔成本的前提下得到改善。反復。就像汽車工業(yè)，廠家經(jīng)常把性產(chǎn)品投放到市場中。產(chǎn)品投放市場的時間快慢是在今天的競爭市場中區(qū)分勝利者和失敗者的主要因素之一。由于 RP&M 技術存在，在一個短時間內(nèi)完成多樣化設計是可能的。 總體上減少模型的研發(fā)時間。 制造業(yè) 我們能使用 RP&M 成型技術作為可生產(chǎn)研究。 在產(chǎn)品的設計初期提供一個實物模型，我們能加速程序計劃和工具工作設計的程序。 除此之外，由于正確地描述復雜的幾何形狀，這種方法能幫助在工場上解釋藍圖方面減少問題的出現(xiàn)。 另外的一個應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 為了協(xié)助產(chǎn)品售賣，一個原型能用來示范觀念，設計主意,同時也代表了一個公司的生產(chǎn)能力。實際模型的真實舉例說明設計的可行性。同時, 原型能用來修正設計得到客戶的回應，以便最后的產(chǎn)品將會符合客戶的需求。及時的領會客戶的想法, 方法在90年代是市場競爭的關鍵。 及時的領會客戶的想法在90年代是市場競爭的關鍵。RP& M 技術有潛力能確定提高產(chǎn)品質(zhì)量。主要有兩方面: 幾乎沒有幾何形狀方面的限制; 分層堆積制造方式允許從CAD到CAM的直接的簡單的接口，而完全忽略了計算機輔助設計程序。 快速原型制造和制造技術 正如以前被提到的。有幾種遵循“生長”和“添加制造”的原理的現(xiàn)代生產(chǎn)方式。在這些技術的主要不同點有二個方面: (1) 選用的材料(2) 零件制造技術。 以下部分將會在這兩方面詳細地說明快速原型制造。 光敏液相固化法 (SLA) SLA 被Charle Hull發(fā)明的3D立體系統(tǒng)Inc.2。它是首個商業(yè)化的快速原型制造技術并且應用的最廣泛。 用的材料是液體樹脂。在光的照射之下, 小的分子 (單體)聚合成較大的分子?；谶@個原則,零件在如圖 3 所顯示的一個盛放液體樹脂的桶中制造出來 SLA 機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)控機床得之字形切削路徑不同,探頭沿著平行線移動，然后再沿著垂直的方向。在液體下面有一個提升的支撐面。表面就在廣可以照射的深度。激光光線在檢流計驅動鏡子旁邊的 X 和 Y 軸中被水平地偏斜，以便它移動。樹脂的表面生產(chǎn)一個固體形狀。在一層被建造之后,支撐面下降一個用戶指定的距離，一層液體樹脂的新涂料覆蓋了原始層。一個掃帚幫助填充用來制造下一層的新的樹脂。激光在已經(jīng)做好的一層上畫新的層。這樣，模型從底部到頂端的逐層建造出來。當所有的層被完成的時候，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。這在一個充滿紫外線的烤箱中制做。SLA有一些值得提及的特點。 材料：有五種商業(yè)化得的原型材料。它們?nèi)菢渲? 支持：因為一個模型在液體被產(chǎn)生,在制造過程中，懸于零件(在下面不支持)之上的下垂或者飄浮離開。原型如此需要一些初步設計支持直到它被制造或固化。支持可能是柱子，橋和構架。有時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程中產(chǎn)生和必須在模型完成之后清除。 模型準確性和表現(xiàn)性。 被達成的準確性是大約 0.1% 的全部尺寸而且除了不超過 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 機器是在 RP& M 系統(tǒng)之中的最精確的機器。。 photopolymer 制作的原型是易碎的而且可能不夠結實來抵抗高的壓迫力測試。 同時, ，材料的收縮可能會使原型變形。 再循環(huán)。 Photopolymers 是thermoset材料和不能夠再一次為重復使用被融化。 選區(qū)激光繞結法(SLS) 動態(tài)同步傳輸套式公司 (奧斯汀 TX) 用在德克薩斯大學的機械工程部門被卡爾 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 使用對泉華的一個二氧化碳激光連續(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替換柜臺滾筒機制應用。 粉狀材料被些微地在它的熔點下面預先加熱到溫度。 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適當?shù)乇３忠暈閷Ψ巯聦拥闹С?這在減少扭曲方面是有幫助的。 當跨區(qū)段的層被完成的時候, 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 使用各類型的材料包括 polycarbonate ， pvc ，ABS ，尼龍，樹脂， 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器正在研發(fā)過程中。 支持。 SLS 系統(tǒng)通常不需要預先設計支持結構。 在建筑物期間，如支持在每個層上的為不融合的粉末處理。 做模型準確性和表現(xiàn)。 平均的準確性 " 為一個部份由從 -0.005 到 +0.015 達成了范圍直徑和 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, 但是在泉華點下面。 再循環(huán)。 原型能是 '粉碎' 成粉末以便重復使用。 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)用來的構造以被擠出的熱后可塑性的材料的沉淀為基礎的部份呼叫 FDM. 在一個 FDM 程序中，一個熱后可塑性細絲材料進入一個加熱的 FDM中之后被擠出。 FDM 頭的運動被計算機控制。 在飛的擠出頭內(nèi)，細絲進入設備內(nèi)被一個反抗加熱器融化。 噴頭追蹤部份的一個每跨區(qū)段層的精確部分。 當噴頭在空間的x軸和y軸移動時拉絲材料就在噴頭內(nèi)進行預熱然后再從噴頭擠出。當它工作的時候，材料以 1/10s的速度直接堆積在工作點上。在一層堆積完成后, 噴頭按照程序預先設計好的為堆積下一層在 z 方向移動。 每層的堆積都是對先前的堆積作加固。 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。 專有的尼龍 , 和投資投擲變大。 所有的材料是無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 要加工一個鋼件工件需要按照預先設計好的程序來進行，然后激光在CAD的引導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。圖 7 演示LOM 的工作原理。 LOM 有如下特征: 材料。 事實上任何的箔 (材料) 都能被應用: 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先設計。支援結構。 模型準確表現(xiàn)性. 模型能準確的表現(xiàn)實物構造在 ±0.005之間。而且因為材料的使用不收縮或者扭曲。 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 機器— LOM-1015 使用一個 40瓦特二氧化碳激光器。LOM-1015 構造的原型范圍的大小是 15×10×15".因為只需要描繪切斷去代替診斷一個硬化區(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)技術是由一個感知系統(tǒng)使用一個 piezo 驅動的噴墨機制射擊融化的材料的小滴, 同時冷卻在先前存放的層之上。一個層由在移動 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距離，一個新的層在先完一個成的層的頂端產(chǎn)生。自動化動力學公司也獨立地研發(fā)了一部相似的機器。 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,自動化動力學公司使用鋁。 支持。 在構造模型程序時，對于突出部份和空虛結構的支持是必要的,支持的材料是水溶性的蠟的合成物質(zhì)。 當模型被完成的時候, 支持物被用溫水洗去。 模型準確表現(xiàn)性。 全部的準確性是 +0.004之間。 層厚度大約在0.0035之間。 沒有表現(xiàn)報告。 能力。 BPM 打印機可以用32個噴墨噴嘴并排排列在50/ m 的范圍內(nèi)每秒噴射10000個小液滴。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是由麻薩諸塞州學會研發(fā)的。在3D立體印刷程序中，一個3D立體模型進入計算機的2D 跨區(qū)段層之內(nèi)被切成薄片。在一個圓筒中,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，那里的信息由計算機模型庫提供出噴涂所需要的相應的信息。在一層完成之后, 活塞放下滑到預先定義的距離，一個新層按照上一步 的方法繼續(xù)建立出來。當整個部份完成的時候，熱處理是提高層與層之間的粘結所必要的, 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。 材料是無定形的或膠質(zhì)的碳化物支持。 由于 3D立體印刷技術，支持結構的設計就顯得沒有必要了,因為在噴涂程序進行時層粉在每一層都能保留它的自然形態(tài)。 模型準確表現(xiàn)性。 小數(shù)量數(shù)據(jù)可從3D立體印刷中獲得因為它仍然在測試的階段中。 對于測試的樣品，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。這種技術在構筑模型是最大潛能是 20×21 ，厚度100nm，以每小時0.18m的速度構建每層。當一層完成以后，活塞下降一個提前設定好的距離，一新的粉層噴灑出來，并有選擇的粘結。當整個零件完成的時候，為了提高粘結層的結合需要必要的熱處理，然后除去沒有粘結的層。圖9顯示了3維印刷的工作過程。 3維印刷的特點綜述如下： 材料. 三維印刷可以利用氧化鋁和鋁-矽陶瓷粉。黏合劑是無定形或者膠體的碳化矽。 支撐體。 利用三維印刷技術，零件的職稱結構是不需要設計的，因為每層沒有粘結的形成了一個天然的支撐。 模型精度. 三維印刷適應于數(shù)量較少的加工，因為它還在測試階段。對于測試樣本，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’’。這項技術在大約每層2秒（0.18mh-1）生產(chǎn)率情況下，有能夠生產(chǎn)尺寸超過20×20’’零件的潛力。 結論 對于加工者保持競爭力，生產(chǎn)特點，質(zhì)量，成本和推向市場的時間是重要因素。快速原型系統(tǒng)給制造產(chǎn)品更快，提供了機會，并且比傳統(tǒng)的加工方法成本低。因為RP&M能夠極大的縮短產(chǎn)品的開發(fā)周期，越來越多的商品利用了這個速度，由計算機設計的產(chǎn)品能夠生產(chǎn)味精準的模型。而這些模型能夠被感知，看到，研究，測試和比較。 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。它們中的每一個考慮到精度，材料種類和機器成本都有各自的特點。還討論了一些顯露出來的問題和研究成果。這是一個快速發(fā)展的領域。快速垸行制造技術的能力和潛力吸引著非常多的工廠為這項技術投資。 非常期待為了研究和發(fā)展那些技術能有更多的努力，這樣，它們可以廣泛的應用于制造工業(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 precis