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翻譯部分 英文原文 Additive Manufacturing: Polymers applicable for Laser Sintering (LS) Abstract Additive Manufacturing (AM) is close to become a production technique changing the way of part fabrication in future. Enhanced complexity and personalized features are aimed. The expectations in AM for the future are enormous and betimes it is considered as kind of the next industrial revolution. Laser Sintering (LS) of polymer powders is one component of the AM production techniques. However materials successfully applicable to Laser Sintering (LS) are very limited today. The presentation picks up this topic and gives a short introduction on the material available today. Important factors of polymer powders, their significance for effective LS processing and analytical approaches to access those values are presented in the main part. Concurrently the exceptional position of polyamide 12 powders is this connection is outlined. 1.Introduction Techniques capable to transfer CAD data directly into physical objects are specified today as Additive Manufacturing (AM) [1]. AM is opposite to subtractive technologies, where material is removed by drilling, milling or grinding to achieve a desired geometry. In a recently published ASTM standard (ASTM F2792-12a) the following definition of AM is established: “Additive manufacturing (AM), – Processes of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing fabrication methodologies.” One element of the ‘layer upon layer’ based additive production techniques is Laser Sintering (LS) of polymers [2]. Space-resolved consolidation of polymer powders by means of laser energy opens innumerable options to yield custom-built parts with freedom of complexity [3]. Amongst all presently existing AM-techniques LS is considered as the most promising approach to become a sincere production technique for plastic parts appropriate for industry. 1.1. LS Basic Polymers A problem obstructing LS in a wider prospect is the limited variety of applicable polymers. Whilst traditional polymer processing techniques e.g. injection molding or extrusion have access to thousands of different formulas composed of several dozen basic polymers [4], for LS treatment just a handful different formulations are provided so far. Moreover, almost all of them are based on two basic polymers: polyamide 12 (PA12) and its near relative polyamide 11 (PA11) [5]. Fig. 1 presents the chemical formula of the two basic polymer structures. There similarity is obvious.Fig. 1. Chemical formula of the two basic polymers for LS powders: PA 12 and PA 11 1.2. Commercial situation Fig. 2 illustrates the situation for the global and the LS market today regarding consumption, price and market share. It can clearly be seen, that LS-market with a share of approximately 1’900 t/year is less than a niche market compared to total consumption of about 290 Mio-t/y; a relation of about 1: 200’000! This means in words, as 1 kg LS-powder is sold about 200 t other polymeric material is sold in the same time. Even when the relation with the total amount of only PA’s is used a consumption of 1.5 t to 1 kg LS-material exists. So, the market of LS needs further development and new materials urgently to develop more weight. Fig. 2. Market overview and comparison between global and LS market Table 1 provides additionally an outline of the main commercially available LS materials based on PA12 and PA 11. The differences between the global polymer market and the LS market are obvious. An application rate of around 900 tons/year the LS share is not even a fraction related to 260 million tons of worldwide plastic use. However the price of LS polymers is around factor 10 and more higher compared to their respective plastics in the standard pyramid. Table 1. Commercial LS Polymers based on PA12 and PA 11 (most important ones in bold letters). In addition noticeable is the lack of standard polymers of the bottom of the pyramid for LS. Almost no materials from these so-called commodity plastics: PE, PP, PVC and others are available so far for LS processing. What are the reasons for these significant differences in polymer distribution for “standard” use and LS adoption? Besides some business and consumption arguments the main reason is the very sophisticated combination of polymer properties necessary for successful application. 2. Polymer Properties for LS-Processing Fig. 3 summarizes the most important factors to transfer a polymer into a LS powder and distinguish between extrinsic and intrinsic properties. Accepting Fig. 3 it is obvious that a complex system of interconnected powder features exists. The different properties can be divided into intrinsic (thermal, optical and rheological) and extrinsic properties (particle and powder). Intrinsic properties are typically determined form the molecular structure of the polymer itself and can’t be influenced easily, whereas production of powder controls extrinsic properties. This mandatory property combination is not easy to achieve from new powders and will be discussed following. Fig. 3. Combination of important properties of LS-powders (intrinsic and extrinsic); 2.1. Extrinsic Properties - Particle Shape and surface of single particles regulate the behavior of the resulting powder to a great extent. In case of LS powders the particles should be at least as feasible formed spherical. This is in order to induce an almost free flowing behavior and is necessary as LS powders are distributed on the part bed of an LS machine by roller or blade systems and will not be compacted additionally. A simple approach to access the flowability of powders is the determination of bulk and tap density. Determination of bulk and tap density gives a good indication on the one hand regarding powder density which is correlated with the final part density and on the other hand regarding the flowability by calculation of the so called Hausner ratio H R . Regarding literature a H R < 1.25 means free flowing powder behavior and a H R > 1.4 means fluidization problems (cohesive properties): H R = ρ tap /ρ bulk (ρ loose = bulk density; ρ tap = tapped density) The LS part density achieved during processing is openly linked to powder density in part bed and is thus coupled to the shape of particles and their free flowing behavior. Fig. 4 illustrates some particle forms attained from different powder generation processes. Spherical particles are usually received from co-extrusion processes with soluble/non-soluble material mixtures, like oil droplets in water. Potato-shaped particles are typical for the today available commercial PA12 powder from precipitation process. Particles obtained from cryogenic milling are inadequate in the majority of cases and fail for LS processing. The poorer powder flowability generates poor part bed surface in LS machine and a reduced powder density as well. Thus, cryogenic milled powders finally end in weak, less condensed LS parts with low density and poor properties usually. Fig. 4. Particle shapes attainable by different production technologies; 2.2. Exrinsic Properties - Powder For LS powders a certain particle size distribution (PSD) is necessary to be processable on LS equipment. This distribution is favorably between 20 μm and 80 μm for commercial system. The PSD is usually measured by laser diffraction systems. However, with this measurement the fraction of small particles is frequently neglected. But particularly the amount of small units is often responsible if a powder depicts a reasonable LS processing behavior or not. Fig. 5 illustrates such a case. Both, ‘Powder 1’ and ‘Powder 2’ have some good and acceptable PSD looking at volume distribution (Fig. 5, middle column). From that point of view both powder should be processable on LS equipment. However, in reality, the trial to do so with ‘Powder 2’ failed. The reason can be recognized form number distribution (Fig. 5, right column).‘Powder 2’ consists of an extreme high portion of small particles which may induce stickiness in powders. The enhanced adhesion between particles reduces the free flowing powder behavior and prevents LS processing. As especially milled powder represents often a high amount of fine particles this is another reason why these powders are frequently unsuccessful in LS processing. Fig. 5. Distribution of powders with similar volume distribution and dissimilarnumber distribution It is also interesting to recognize, that even for the most often used commercial powders for LS-processing: PA 2200 (Company EOS) and Duraform PA (company 3D-systems) the powder distribution is not equal. Fig. 6 indicates the distribution. It can be clearly identified, that PA 2200 exhibits an almost mono-modal distribution in contrast to Duraform PA, where the distribution consists of several powder fractions. Even form the particle photos in the right side of Fig. 6 it can be identified that Duraform PA powder has a much broader distribution with a higher amount of fine particles. If the fine particles don’t influence the flowability too much in a negative sense, the smaller particles can help to enlarge the powder density and consequently the part density as well. Fig. 6. Powder distribution of commercial LS-Powders (PA12) 2.3. Intrinsic Properties – Thermal Behavior Identifying the challenging aspects of the desired thermal properties it is necessary to understand the course of action during LS processing. In a LS system essentially a CO 2 laser beam is used to selectively fuse or melt the polymer particles deposited in a thin layer. Locally full coalescence of polymer particles in the top powder layer is necessary as well as an adhesion with previous sintered layers. For semi crystalline polymers usually used in LS processing this implies that crystallization (T c ) should be inhibited during processing as long as possible, at least for several sintered layers. Thus, processing temperature must be precisely controlled in-between melting (T m , red line, Fig. 7) and crystallization (T c , blue line, Fig. 7) of the given polymer. This meta-stable thermodynamic region of undercooled polymer melt is called ‘sintering window’ of LS processing for a given polymer. Fig. 7 shows a DSC run (DSC = Differential Scanning Calorimetry) for commercial PA 12 LS-powder. The nature of sintering window between onset points of T c and T m is obvious. Fig. 7. Typical DSC-Thermogram with nature of ‘sintering window’ as LS process temperature However it must be indicated, that the scheme in Fig. 7 is just an idealized representation of thermal reality as it is received with fixed heating and cooling rates (10C/min) never existing during LS processing. In fact there are undefined and hardly controllable temperature change rates and especially the sintering temperature (T s = process temperature during sintering) close to crystallization onset means that stimulation of crystallization shifts to higher temperatures for LS processing. Fig. 8 indicates what can occur usually for polymer powders with a too small sintering window. If T s is too close to crystallization (left side in Fig. 8) curling due to premature crystallization is induced and parts are distorted after releasing from surrounding powder bed. If temperature is just slightly higher during processing (right side of Fig. 3) an early crystallization can be avoided but in this case the temperature is too close to melting and leads to a loss of exact definition of part features. Powder particles in the direct neighborhood of the laser trace stick on the molten surfaces (lateral growth) and prevent desired resolution of part topography. Fig. 8. LS Processing problems for too small ‘sintering window’: curling or lateral growth; Additionally to the very critical point of suitable thermal transitions (T m , T c ) there are farther intrinsic factors like optical properties, melt viscosity and surface tension that needs to be very specific for successful application of polymer powders to Selective Laser Sintering. 2.4. Intrinsic Properties – Viscosity and Surface Tension A low zero viscosity (η 0 ) and a low surfaces tension (γ) of polymer melt are necessary for successful LS processing. This is indispensable to generate an adequate coalescence of polymer particles. Especially a low melt viscosity without shear stress is of high importance, as, unlike injection molding, LS cannot provide an additional compacting during part generation (holding pressure). Fig. 9 indicates the effect of inferior melt viscosity clearly visible. The right side image (Fig. 9) depicts a lot of imperfections in the part morphology and a poor surface quality as well. The required low zero viscosity is also the reason why attempts to process amorphous polymers with LS usually ends with brittle and instable parts. Due to the fact that viscosity of those polymers above glass transition (T g ) is still very high in general a proper coalescence does not take place usually. Fig. 9. Cross section of PA 12 parts made from PA 12 polymers with different melt viscosity 2.5. Intrinsic Properties – Optical Properties Fig. 10 depicts a scheme of the optical circumstances during LS processing. When a laser beam hits a polymer material three effects can occur in principle. Besides the absorption of the energy also (diffuse) reflexion and transmission is possible (Fig. 10a)). In case of energy absorption it is obvious that a sufficient capability of the material to absorb radiation of present laser wavelength (CO 2 -Laser: 10.6 μm) is necessary. This is apparent for most polymers as they consist of aliphatic compounds (C-H). Those polymers have, in the majority of cases, some group vibrations in the ‘fingerprint’ infrared (IR) region, sufficient to absorb relevant portions of 10.6 μm CO 2 -laser radiation. Fig. 10. Optical circumstances for LS processing However during the LS processing the effects of reflexion and transmission become relevant as well (see Fig. 10 b)). Transmission is desired to direct a sufficient portion of the radiation energy into deeper regions of the powder bed in order to induce an adequate layer adhesion. Only when the current powder layer is connected with the previous sintered layer in a satisfactory amount a LS part can be generated without layer delamination. In case of a poor absorption and transmission capability, an increase of laser energy power can compensate to a certain amount the effect. However an augmentation of laser power must be limited in order not to destroy the polymer by too high energy. 3. Conclusion Additive Manufacturing (AM) is close to become a production technique with the potential to change the way of producing parts in future. High complex parts in small series are targeted. Selective Laser Sintering (LS) of polymer powders is one component of the additive production techniques, which is regarded as one of the most promising ones for functional end products in the AM-area. However an analysis of the commercial situation reveals, that there is a problem with the small number of applicable polymer powders for this technology today. To understand this limitation, the paper summarizes the most important key factors materials which have to be fulfilled and their meaning for LS processing. It is highlighted the combination of intrinsic and extrinsic polymer properties necessary to generate a polymer powder likely for LS application. The thermal situation with a sufficient “sintering window” is presented as well as the requirements for a suitable viscosity and an appropriate optical behavior. The very specific requests regarding the powder distribution and for every single particle concerning sphericity and surface is outlined. Especially the point of high powder flowability connected with particle shape is very important, as it turns out that milled particles are unfavorable in connection with LS processing. This means the production of nearby spherical polymer particles providing a good flowability and a high powder density is a central point for the future development of LS-Technology. Especially a progress for polyolefin types (PP, PE, POM) with impact modified properties or flame retardancy should attract new markets (automotive, household, electronics, aviation) and enlarge the LS business drastically. References [1] I. Gibson, D.W. Rosen, Stucker, B. (1st ed.) Additive Manufacturing Technologies - Rapid Prototyping to Direct Digital Manufacturing. New York, Berlin,Springer, 2010. [2] J. P. Kruth, G. Levy et al., Consolidation phenomena in laser and powder-bed based layered manufacturing CIRP Annals - Manufacturing Technology, 56(2),2007, 730-759 [3] N. Hopkinson, Rapid Manufacturing – An Industrial Revolution for the Digital Age, Wiley&Sons: New York, 2006 [4] H. Dominighaus, Kunststoffe – Eigenschaften und Anwendungen, Berlin, Heidelberg, Springer Verlag, 2012 [5] M. Schmid, G. Levy, Lasersintermaterialien – aktueller Stand und Entwicklungspotential. Fachtagung Additive Fertigung, Lehrstuhl fr Kunststofftechnik, Erlangen, Germany, 2009, 43-55. 中文譯文 增材制造：適用于激光燒結(jié)的聚合物 摘要 增材制造即將成為未來改變零件制造方式的生產(chǎn)技術(shù)。以提高復(fù)雜性和個性化為目的。增材制造對于未來的期望是巨大的并且被認為會帶來第三次“工業(yè)革命”。 聚合物粉末激光燒結(jié)是增材制造生產(chǎn)技術(shù)的一個組成部分。然而如今材料成功地應(yīng)用于激光燒結(jié)是非常有限的，這份報告采用這個話題并簡單地介紹一下如今可用的材料。聚合物粉末的重要因素，對于研究實際的激光燒結(jié)過程和分析方法的意義展現(xiàn)在主要的部分。同時聚酰胺12粉末與此的聯(lián)系也占有優(yōu)越的位置。 簡介 增材制造是依據(jù)三維CAD 數(shù)據(jù)將材料累加制作實際物體的過程[1]。增材制造技術(shù)是有別于通過鉆削，銑削或研磨等切削加工得到想要的幾何形狀的技術(shù)的。在最近發(fā)表的美國材料與試驗協(xié)會( ASTM F2792-12a)標準對增材制造和3D 打印有明確的概念定義：增材制造—是依據(jù)三維CAD 數(shù)據(jù)將材料連接制作物體的過程，相對于減法制造它通常是逐層累加過程。 “自下而上”增材制造技術(shù)的一個重要因素是聚合物的激光燒結(jié)[2]。聚合物粉末的固化層是依靠激光能量產(chǎn)生的。在所有現(xiàn)有的增材制造技術(shù)中激光燒結(jié)被認為是最有前途的方法，成為一個真實的生產(chǎn)技術(shù)，適合工業(yè)塑料零件的生產(chǎn)。 激光燒結(jié)的基本聚合物 阻礙激光燒結(jié)的更廣闊的前景的因素是非常有限的適用聚合物。雖然傳統(tǒng)的聚合物加工技術(shù)，如注射成型或擠出成型都有成千上萬的不同的公式組成的幾十個基本聚合物[4]，但對于激光燒結(jié)目前只有少量不同的公式可用。此外，幾乎所有都是基于兩個基體聚合物：聚酰胺12（PA12）及聚酰胺11（PA11）[ 5 ]。圖1給出了兩種基本聚合物結(jié)構(gòu)的化學(xué)式。它們的相似之處很明顯。 商業(yè)情況 圖2說明了如今全球激光燒結(jié)市場的消費，價格和市場份額的情況。清晰可見，激光燒結(jié)所占市場份額約為1900噸/年，與約290噸/年的總消費量相比少了一個利基市場；比值約為1:200000！這意味著，每1公斤激光燒結(jié)粉末出售，約200噸其它聚合物材料也在同一時間出售。因此，激光燒結(jié)的市場需要進一步地發(fā)展，并且新材料也被迫切地需要提高更好的質(zhì)量。 表1特別指出了商業(yè)上主要可獲得的激光燒結(jié)是以聚酰胺12（PA12）和聚酰胺11（PA11）為基礎(chǔ)的。全球聚合物市場與激光燒結(jié)市場的差異是顯而易見的。激光燒結(jié)的份額應(yīng)用率約900噸/年，甚至不到全球相關(guān)塑料使用的2億6000萬噸的一小部分。然而，激光燒結(jié)聚合物的價格是圍繞因子10和更高的標準金字塔相比，各自的塑料。然而，激光燒結(jié)聚合物的價格與金字塔上的各個塑料相比大約是它們的10倍或者更高。 此外，值得注意的是缺乏聚合物的在金字塔底部的激光燒結(jié)。幾乎沒有材料來源于這些所謂的有用塑料：目前為止聚乙烯，聚丙烯，聚氯乙烯和其它可用于激光燒結(jié)處理。是什么原因造成使用“標準”和激光燒結(jié)上聚合物分布的顯著差異？此外一些商業(yè)和消費觀點論證了其主要原因是使聚合物性能相互結(jié)合并成功地運用是十分復(fù)雜的。 激光燒結(jié)處理的聚合物性能 圖3總結(jié)了將聚合物轉(zhuǎn)化為激光燒結(jié)粉末以及區(qū)別其外在和內(nèi)在屬性的最重要的因素。從圖3可以很明顯的得出，相互關(guān)聯(lián)的粉末特性的復(fù)雜系統(tǒng)是存在的。根據(jù)不同的性質(zhì)可以將它們分為固有屬性（熱，光學(xué)和流變）和外在屬性（顆粒和粉末）。固有屬性通常由聚合物本身的分子結(jié)構(gòu)決定，不會輕易地受影響，而粉末的生產(chǎn)會影響外在屬性。這種強制性的屬性組合是不容易從新的粉末中產(chǎn)生的，以下便是討論的內(nèi)容。 外在屬性-粒子 單顆粒的形狀和表面在很大程度上控制了粉末的的最終形態(tài)。在激光燒結(jié)粉末的情況下，顆粒無論如何應(yīng)形成球形。這是為了誘導(dǎo)自由流動的行為，并且當激光燒結(jié)粉末分布在激光燒結(jié)機上這是必要的，也因此不會被壓縮。 獲得流動性粉末的簡單方法是由體積和液體密度決定的。松密度和壓實密度的測定給出了一個很好的解釋，一方面是與最終密度相關(guān)聯(lián)的粉末密度，另一方面是用豪斯納比計算的流動性。關(guān)于文獻HR < 1.25表示為自由流動的粉末，HR > 表示存在流化問題（粘結(jié)性能） HR = ρtap/ρbulk (ρloose = 松密度; ρtap = 壓實密度) 激光燒結(jié)部分密度是在加工過程中實現(xiàn)的，這與機床上粉末密度相關(guān)聯(lián)，從而耦合到顆粒的形狀和它們的自由流動行為。圖4示出了從不同的粉末生成過程中獲得的粒子的形式。球形顆粒通常是由雙擠壓過程中的可溶性/非可溶性材料混合物得來的，如水里的油。土豆狀顆粒是從商業(yè)PA12粉末沉淀的過程中得的典型形狀。在大多數(shù)情況下，從低溫銑削得到的顆粒是不夠的，這種激光燒結(jié)處理過程是失敗的。粉末流動性較差，在激光燒結(jié)機中產(chǎn)生較差的床面，降低了粉末密度。因此，低溫球磨粉末最終會減弱，越少的濃縮激光燒結(jié)部分通常是低密度和低性能。 exrinsic屬性-粉 對于激光燒結(jié)粉末來說，相位靈敏調(diào)解器在激光燒結(jié)設(shè)備上是可行的必要條件。介于20μm和80μm之間的這種分配對于商業(yè)系統(tǒng)是有利的。相位靈敏調(diào)解器通常是由激光衍射系統(tǒng)測量。然而，用這種方式測量小顆粒的一小部分經(jīng)常被忽視。但是如果粉末通過了合理的激光燒結(jié)處理，那么個別小單位數(shù)量的通常是可以測量的。 圖5說明了下述情況。粉1 和粉2 的體積分布在相位靈敏調(diào)解器上是良好的和可觀（圖5，中柱）。從這一點來看，兩種粉末都應(yīng)在激光燒結(jié)設(shè)備上適當?shù)倪M行加工處理。然而，在現(xiàn)實中，對粉末2的操作失敗了。原因可以被歸結(jié)為數(shù)量分布不均導(dǎo)致的（圖5，右邊）。“粉末2”由會導(dǎo)致粉末粘性的細小顆粒的大部分組成。增強顆粒的粘附力就是在降低了粉末的自由流動和防止激光燒結(jié)過程中獲得的。特別是研磨粉通常是大多數(shù)細顆粒的代表，這就是為什么這些粉末激光燒結(jié)處理往往不成功。 以下的認識也很有趣，即使是最常用的商業(yè)粉末進行激光燒結(jié)處理：PA 2200（公司EOS）和DuraformPA（公司3D系統(tǒng)）的粉末分布不均勻。圖6顯示的是它們的分布。圖中可以清楚地發(fā)現(xiàn)，PA 2200與duraformPA對比幾乎呈現(xiàn)單模態(tài)分布，它們的分布由幾個粉末組成。甚至從圖6右側(cè)形成的粒子照片，也可以看出duraformPA粉末的細顆粒分布較廣。細顆粒在一定程度上不影響流動性，較小的顆粒有利于增大粉末的密度，從而提高零件的密度。 內(nèi)在特性-熱反應(yīng) 識別具有挑戰(zhàn)性的熱學(xué)性能方面是必要的，以此來了解在激光燒結(jié)處理過程中的反應(yīng)。在激光燒結(jié)系統(tǒng)中，本質(zhì)上用CO2激光束選擇性地熔化或熔化沉積在薄層中的聚合物顆粒。在頂部粉末層中的聚合物顆粒的局部充分合并是必要的，跟之前的具有粘附性的燒結(jié)層類似。對于半結(jié)晶聚合物通常用于激光燒結(jié)處理