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翻译部分英文原文Additive Manufacturing: Polymers applicable for Laser Sintering (LS)AbstractAdditive 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.IntroductionTechniques 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 PolymersA 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 111.2. Commercial situationFig. 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 1900 t/year is less than a niche market compared to total consumption of about 290 Mio-t/y; a relation of about 1: 200000! 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 PAs 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 marketTable 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 thesesignificant 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-ProcessingFig. 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 cant 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 - ParticleShape 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 1.4 means fluidization problems (cohesive properties): H R = tap / bulk ( loose = bulk density; 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 - PowderFor LS powders a certain particle size distribution (PSD) is necessary to be processable on LS equipment. This distribution is favorably between 20 m and 80 m 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 enhancedadhesion 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 distributionIt 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 dont 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 BehaviorIdentifying 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 temperatureHowever 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 fromsurrounding 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. Powderparticles 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 toSelective Laser Sintering.2.4. Intrinsic Properties Viscosity and Surface TensionA low zero viscosity ( 0 ) and a low surfaces tension () 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 ofimperfections 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 ofthose 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 viscosity2.5. Intrinsic Properties Optical PropertiesFig. 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 m) 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 m CO 2 -laser radiation.Fig. 10. Optical circumstances for LS processingHowever 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. ConclusionAdditive 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 onecomponent 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 togenerate 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 isoutlined. 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.References1 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, 730-7593 N. Hopkinson, Rapid Manufacturing An Industrial Revolution for the Digital Age, Wiley&Sons: New York, 20064 H. Dominighaus, Kunststoffe Eigenschaften und Anwendungen, Berlin, Heidelberg, Springer Verlag, 20125 M. Schmid, G. Levy, Lasersintermaterialien aktueller Stand und Entwicklungspotential. Fachtagung Additive Fertigung, Lehrstuhl fr Kunststofftechnik, Erlangen, Germany, 2009, 43-55.中文译文增材制造：适用于激光烧结的聚合物摘要增材制造即将成为未来改变零件制造方式的生产技术。以提高复杂性和个性化为目的。增材制造对于未来的期望是巨大的并且被认为会带来第三次“工业革命”。 聚合物粉末激光烧结是增材制造生产技术的一个组成部分。然而如今材料成功地应用于激光烧结是非常有限的，这份报告采用这个话题并简单地介绍一下如今可用的材料。聚合物粉末的重要因素，对于研究实际的激光烧结过程和分析方法的意义展现在主要的部分。同时聚酰胺12粉末与此的联系也占有优越的位置。简介增材制造是依据三维CAD 数据将材料累加制作实际物体的过程1。增材制造技术是有别于通过钻削，铣削或研磨等切削加工得到想要的几何形状的技术的。在最近发表的美国材料与试验协会( ASTM F2792-12a)标准对增材制造和3D 打印有明确的概念定义：增材制造是依据三维CAD 数据将材料连接制作物体的过程，相对于减法制造它通常是逐层累加过程。“自下而上”增材制造技术的一个重要因素是聚合物的激光烧结2。聚合物粉末的固化层是依靠激光能量产生的。在所有现有的增材制造技术中激光烧结被认为是最有前途的方法，成为一个真实的生产技术，适合工业塑料零件的生产。激光烧结的基本聚合物 阻碍激光烧结的更广阔的前景的因素是非常有限的适用聚合物。虽然传统的聚合物加工技术，如注射成型或挤出成型都有成千上万的不同的公式组成的几十个基本聚合物4，但对于激光烧结目前只有少量不同的公式可用。此外，几乎所有都是基于两个基体聚合物：聚酰胺12（PA12）及聚酰胺11（PA11） 5 。图1给出了两种基本聚合物结构的化学式。它们的相似之处很明显。商业情况图2说明了如今全球激光烧结市场的消费，价格和市场份额的情况。清晰可见，激光烧结所占市场份额约为1900吨/年，与约290吨/年的总消费量相比少了一个利基市场；比值约为1:200000！这意味着，每1公斤激光烧结粉末出售，约200吨其它聚合物材料也在同一时间出售。因此，激光烧结的市场需要进一步地发展，并且新材料也被迫切地需要提高更好的质量。表1特别指出了商业上主要可获得的激光烧结是以聚酰胺12（PA12）和聚酰胺11（PA11）为基础的。全球聚合物市场与激光烧结市场的差异是显而易见的。激光烧结的份额应用率约900吨/年，甚至不到全球相关塑料使用的2亿6000万吨的一小部分。然而，激光烧结聚合物的价格是围绕因子10和更高的标准金字塔相比，各自的塑料。然而，激光烧结聚合物的价格与金字塔上的各个塑料相比大约是它们的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烧结。几乎没有材料来源于这些所谓的有用塑料：目前为止聚乙烯，聚丙烯，聚氯乙烯和其它可用于激光烧结处理。是什么原因造成使用“标准”和激光烧结上聚合物分布的显著差异？此外一些商业和消费观点论证了其主要原因是使聚合物性能相互结合并成功地运用是十分复杂的。激光烧结处理的聚合物性能图3总结了将聚合物转化为激光烧结粉末以及区别其外在和内在属性的最重要的因素。从图3可以很明显的得出，相互关联的粉末特性的复杂系统是存在的。根据不同的性质可以将它们分为固有属性（热，光学和流变）和外在属性（颗粒和粉末）。固有属性通常由聚合物本身的分子结构决定，不会轻易地受影响，而粉末的生产会影响外在属性。这种强制性的属性组合是不容易从新的粉末中产生的，以下便是讨论的内容。外在属性-粒子单颗粒的形状和表面在很大程度上控制了粉末的的最终形态。在激光烧结粉末的情况下，颗粒无论如何应形成球形。这是为了诱导自由流动的行为，并且当激光烧结粉末分布在激光烧结机上这是必要的，也因此不会被压缩。获得流动性粉末的简单方法是由体积和液体密度决定的。松密度和压实密度的测定给出了一个很好的解释，一方面是与最终密度相关联的粉末密度，另一方面是用豪斯纳比计算的流动性。关于文献HR 表示存在流化问题（粘结性能）HR = tap/bulk (loose = 松密度; tap = 压实密度)激光烧结部分密度是在加工过程中实现的，这与机床上粉末密度相关联，从而耦合到颗粒的形状和它们的自由流动行为。图4示出了从不同的粉末生成过程中获得的粒子的形式。球形颗粒通常是由双挤压过程中的可溶性/非可溶性材料混合物得来的，如水里的油。土豆状颗粒是从商业PA12粉末沉淀的过程中得的典型形状。在大多数情况下，从低温铣削得到的颗粒是不够的，这种激光烧结处理过程是失败的。粉末流动性较差，在激光烧结机中产生较差的床面，降低了粉末密度。因此，低温球磨粉末最终会减弱，越少的浓缩激光烧结部分通常是低密度和低性能。exrinsic属性-粉对于激光烧结粉末来说，相位灵敏调解器在激光烧结设备上是可行的必要条件。介于20m和80m之间的这种分配对于商业系统是有利的。相位灵敏调解器通常是由激光衍射系统测量。然而，用这种方式测量小颗粒的一小部分经常被忽视。但是如果粉末通过了合理的激光烧结处理，那么个别小单位数量的通常是可以测量的。图5说明了下述情况。粉1 和粉2 的体积分布在相位灵敏调解器上是良好的和可观（图5，中柱）。从这一点来看，两种粉末都应在激光烧结设备上适当的进行加工处理。然而，在现实中，对粉末2的操作失败了。原因可以被归结为数量分布不均导致的（图5，右边）。“粉末2”由会导致粉末粘性的细小颗粒的大部分组成。增强颗粒的粘附力就是在降低了粉末的自由流动和防止激光烧结过程中获得的。特别是研磨粉通常是大多数细颗粒的代表，这就是为什么这些粉末激光烧结处理往往不成功。以下的认识也很有趣，即使是最常用的商业粉末进行激光烧结处理：PA 2200（公司EOS）和DuraformPA（公司3D系统）的粉末分布不均匀。图6显示的是它们的分布。图中可以清楚地发现，PA 2200与duraformPA对比几乎呈现单模态分布，它们的分布由几个粉末组成。甚至从图6右侧形成的粒子照片，也可以看出duraformPA粉末的细颗粒分布较广。细颗粒在一定程度上不影响流动性，较