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1、精选优质文档-倾情为你奉上外文翻译原文：Injection Molding Many different processes are used to transform plastic granules, powders, and liquids into product. The plastic material is in moldable form, and is adaptable to various forming methods. In most cases thermosetting materials require other methods of forming. Thi
2、s is recognized by the fact that thermoplastics are usually heated to a soft state and then reshaped before cooling. Theromosets, on the other hand have not yet been polymerized before processing, and the chemical reaction takes place during the process, usually through heat, a catalyst, or pressure
3、. It is important to remember this concept while studying the plastics manufacturing processes and polymers used. Injection molding is by far the most widely used process of forming thermoplastic materials. It is also one of the oldest. Currently injection molding accounts for 30% of all plastics re
4、sin consumption. Since raw material can be converted by a single procedure, injection molding is suitable for mass production of plastics articles and automated one-step production of complex geometries. In most cases, finishing is not necessary. Typical products include toys, automotive parts, hous
5、ehold articles, and consumer electronics goods.Since injection molding has a number of interdependent variables, it is a process of considerable complexity. The success of the injection molding operation is dependent not only in the proper setup of the machine hydraulics, barrel temperature variatio
6、ns, and changes in material viscosity. Increasing shot-to-shot repeatability of machine variables helps produce parts with tighter tolerance, lowers the level of rejects, and increases product quality (i.e., appearance and serviceability).The principal objective of any molding operation is the manuf
7、acture of products: to a specific quality level, in the shortest time, and using repeatable and fully automatic cycle. Molders strive to reduce or eliminate rejected parts in molding production. For injection molding of high precision optical parts, or parts with a high added value such as appliance
8、 cases, the payoff of reduced rejects is high.A typical injection molding cycle or sequence consists of five phases;1. Injection or mold filling2. Packing or compression3. Holding 4. Cooling5. Part ejectionPlastic granules are fed into the hopper and through an in the injection cylinder where they a
9、re carried forward by the rotating screw. The rotation of the screw forces the granules under high pressure against the heated walls of the cylinder causing them to melt. As the pressure building up, the rotating screw is forced backward until enough plastic has accumulated to make the shot. The inj
10、ection ram (or screw) forces molten plastic from the barrel, through the nozzle, sprue and runner system, and finally into the mold cavities. During injection, the mold cavity is filled volumetrically. When the plastic contacts the cold mold surfaces, it solidifies (freezes) rapidly to produce the s
11、kin layer. Since the core remains in the molten state, plastic follows through the core to complete mold filling. Typically, the cavity is filled to 95%98% during injection. Then the molding process is switched over to the packing phase.Even as the cavity is filled, the molten plastic begins to cool
12、. Since the cooling plastic contracts or shrinks, it gives rise to defects such as sink marks, voids, and dimensional instabilities. To compensate for shrinkage, addition plastic is forced into the cavity. Once the cavity is packed, pressure applied to the melt prevents molten plastic inside the cav
13、ity from back flowing out through the gate. The pressure must be applied until the gate solidifies. The process can be divided into two steps (packing and holding) or may be encompassed in one step(holding or second stage). During packing, melt forced into the cavity by the packing pressure compensa
14、tes for shrinkage. With holding, the pressure merely prevents back flow of the polymer malt.After the holding stage is completed, the cooling phase starts. During, the part is held in the mold for specified period. The duration of the cooling phase depends primarily on the material properties and th
15、e part thickness. Typically, the part temperature must cool below the materials ejection temperature. While cooling the part, the machine plasticates melt for the next cycle.The polymer is subjected to shearing action as well as the condition of the energy from the heater bands. Once the short is ma
16、de, plastication ceases. This should occur immediately before the end of the cooling phase. Then the mold opens and the part is ejected. When polymers are fabricated into useful articles they are referred to as plastics, rubbers, and fibers. Some polymers, for example, cotton and wool, occur natural
17、ly, but the great majority of commercial products are synthetic in origin. A list of the names of the better known materials would include Bakelite, Dacron, Nylon, Celanese, Orlon, and Styron.Previous to 1930 the use of synthetic polymers was not widespread. However, they should not be classified as
18、 new materials for many of them were known in the latter half of the nineteenth century. The failure to develop them during this period was due, in part, to a lack of understanding of their properties, in particular, the problem of the structure of polymers was the subject of much fruitless controve
19、rsy.Two events of the twentieth century catapulted polymers into a position of worldwide importance. The first of these was the successful commercial production of the plastic now known as Bakelite. Its industrial usefulness was demonstrated in1912 and in the next succeeding years. Today Bakelite is
20、 high on the list of important synthetic products. Before 1912 materials made from cellulose were available, but their manufacture never provided the incentive for new work in the polymer field such as occurred after the advent of Bakelite. The second event was concerned with fundamental studies of
21、the nature polymers by Staudinger in Europe and by Carohers, who worked with the Du Pont company in Delaware. A greater part of the studies were made during the 1920s. Staudingers work was primarily fundamental. Carothers achievements led to the development of our present huge plastics industry by c
22、ausing an awakening of interest in polymer chemistry, an interest which is still strongly apparent today. The Nature of ThermodynamicsThermodynamics is one of the most important areas of engineering science used to explain how most things work, why some things do not the way that they were intended,
23、 and why others things just cannot possibly work at all. It is a key part of the science engineers use to design automotive engines, heat pumps, rocket motors, power stations, gas turbines, air conditioners, super-conducting transmission lines, solar heating systems, etc. Thermodynamics centers abou
24、t the notions of energy, the idea that energy is conserved is the first low of thermodynamics. It is starting point for the science of thermodynamics is entropy; entropy provides a means for determining if a process is possible.This idea is the basis for the second low of thermodynamics. It also pro
25、vides the basis for an engineering analysis in which one calculates the maximum amount of useful that can be obtained from a given energy source, or the minimum amount of power input required to do a certain task.A clear understanding of the ideas of entropy is essential for one who needs to use the
26、rmodynamics in engineering analysis. Scientists are interested in using thermodynamics to predict and relate the properties of matter; engineers are interested in using this data, together with the basic ideas of energy conservation and entropy production, to analyze the behavior of complex technolo
27、gical systems.There is an example of the sort of system of interest to engineers, a large central power stations. In this particular plant the energy source is petroleum in one of several forms, or sometimes natural gas, and the plant is to convert as much of this energy as possible to electric ener
28、gy and to send this energy down the transmission line.Simply expressed, the plant does this by boiling water and using the steam to turn a turbine which turns an electric generator.The simplest such power plants are able to convert only about 25 percent of the fuel energy to electric energy. But thi
29、s particular plant converts approximately 40 percent; it has been ingeniously designed through careful application of the basic principles of thermodynamics to the hundreds of components in the system.The design engineers who made these calculations used data on the properties of steam developed by
30、physical chemists who in turn used experimental measurements in concert with thermodynamics theory to develop the property data.Plants presently being studied could convert as much as 55 percent of the fuel energy to electric energy, if they indeed perform as predicted by thermodynamics analysis. Th
31、e rule that the spontaneous flow of heat is always from hotter to cooler objects is a new physical idea. There is noting in the energy conservation principle or in any other law of nature that specifies for us the direction of heat flow. If energy were to flow spontaneously from a block of ice to a
32、surrounding volume of water, this could occur in complete accord with energy conservation. But such a process never happens. This idea is the substance of the second law of thermodynamics.Clear, a refrigerator, which is a physical system used in kitchen refrigerators, freezers, and air-conditioning
33、units must obey not only the first law (energy conservation) but the second law as well.To see why the second law is not violated by a refrigerator, we must be careful in our statement of law. The second law of thermodynamics says, in effect, that heat never flows spontaneously from a cooler to a ho
34、tter object.Or, alternatively, heat can flow from a cooler to a hotter object only as a result of work done by an external agency. We now see the distinction between an everyday spontaneous process, such as the flow of heat from the inside to the outside of a refrigerator.In the water-ice system, th
35、e exchange of energy takes place spontaneously and the flow of heat always proceeds from the water to the ice. The water gives up energy and becomes cooler while the ice receives energy and melts.In a refrigerator, on the other hand, the exchange of energy is not spontaneous. Work provided by an ext
36、ernal agency is necessary to reverse the natural flow of heat and cool the interior at the expense of further heating the warmer surroundings. 译文：塑料注射成型许多不同的加工过程习惯于把塑料颗粒、粉末和液体转化成最终产品。塑料材料用模具成型，并且适合用多种方式成型。在大多数情况下，热塑性材料可以用许多方法成型，但热固性塑料需要用其他方法成型。对于热塑性材料有这种事实的认识，它常常被加热成为另一种柔软状态，然后在冷却以前成型。对于热固性塑料，换句话说，在
37、它加工以前还没有形成聚合物，在化学反应加工过程中发生变化，如通过加热、催化剂或压力处理。记住这个概念在学习塑料加工过程和聚合物的形成是很重要的。塑料注射成型越来越广泛地运用于热塑性材料的成型工艺。它也是最古老的一种方式。突然间，塑料注射成型材料占所有成型材料消费的30%。塑料注射成型适合于大批量生产，当原材料被成单一的步骤转换成为塑料物品和单步自动化的复杂几何形状制品。在大多数情况下，对于这样的制品，精加工是不需要的。所生产的各种各样的产品包括：玩具、汽车配件、家用物品和电子消费物品。因为塑料注射模具有很多易变的相互影响，那是一种复杂的虚慎重考虑的加工过程。塑料注射模具设备的成功是不依赖于机器
38、变化到恰当的步骤，只有淘汰了需要注射变化的机器，才会导致适应液压变化、料筒温度变化和材料黏度变化的机器的产生。增加机器重复注射的能力的变化可以帮助减少公差，降低次品等级和增加产品质量。对于任何模具注射设备的操作人员目的是制造产品，成为特等品、用最短的时间、用重复精度和全自动化生产作为周期。模塑人员在生产过程中总是想尽办法降低或消除不合格产品。对于塑料注射模具有高要求的光学制品，或者有高附加值的制品如：家用电器制品，它的利润大大降低。一种塑料注射模具的生产周期或顺序由五个阶段组成：注射或填充模具补料或压缩保压冷却局部注射塑料颗粒被投入料斗并且打开塑料注射料筒，在那里颗粒被旋转螺杆带动进入料筒。螺
39、杆的旋转强迫塑料颗粒在高压下挤压料筒筒壁导致它变成熔体。随着压力的增加，旋转螺杆被迫后退直到有足够的塑料被注射成为储料。塑料螺杆强迫熔融的塑料从料筒流到喷嘴、主流道经浇注系统，最终进入模具型腔。当注射模具型腔容积被充满。当塑料接触冷的模具表面，它被固化以减少表层。当模具保持熔融状态，塑料沿着模芯充满整个模具。，利用率特别高，在注射时型腔被充满95%98%。接着成型过程进入补料阶段。当型腔被充满，熔融塑料便开始冷却。冷却塑料的收缩，就增加了诸如凹痕、孔洞和尺寸不稳定等制品缺陷的发生。为了补偿收缩，增加塑料压入型腔。当型腔被封裹，为防止的熔融状态塑料从型腔内流向出口，把压力应用于熔体。这种压力必须
40、应用直到出口为固态。这种加工可分为两步（补料和保压）或可能包含成为一步（保压或第二阶段）。在补料时，熔体被补料压力收缩补偿压入型腔。在保压时，压力仅仅防止聚合物回流。在保压阶段完成以后，冷却阶段开始。在冷却时，是制品在型腔内保持需具体说明的一个阶段。在冷却持久的阶段主要依靠材料的特性和制品的收缩率。典型的，制品温度必须冷却到材料的注射温度。在冷却制品时，这种机器塑料熔体被冷却到下一个周期。聚合物是以剪切作用为主题的，如同加热圈获得能量一样。当注射开始，到塑料注射终止。聚合物会立刻出现在冷却阶段以前，直到模具打开和制品被注射。当聚合物被编制成有用的文章，它们被称为：塑料、橡胶和纤维。许多聚合物，
41、例如棉花和羊毛来自自然，但是绝大多数商业的产品都是人造的，都来源于此。一系列众所周知的材料包括酚醛塑料，涤纶，尼龙，聚硅氧烷，有机玻璃，纤维素，聚丙乙烯和特氟隆。在1930年以前，商业用的聚合物没有广泛应用。然而它们本应该作为新材料在19世纪下半叶出名，却没有成功。在该期间，它们所以未能发展，部分原因是不了解它们的性质，特别是，聚合物结构曾是许多无结果争论的主题。二十世纪的两次事件使聚合物声名雀起，并且在世界范围内占据了很重要的地位。第一次是成功的商业塑料产品叫做酚醛塑料。它有用的工业价值在1912年表现得近乎疯狂，并且在以后许多年发挥着巨大的价值。今天，酚醛塑料仍然在一系列的人造的产品中占有
42、一席之地。在1912年以前，由塑料制造的材料是有用的，但是那种材料的制造从未提供像发明了酚醛塑料以后，形成新聚合物的动力那样有价值。第二次事件与基础学科的自然聚合物有关，被欧洲的史涛丁格和美国的卡罗瑟夫发现，他们在特达华州的杜邦公司工作。一些重要的研究在20世纪20年代被开展，史涛丁格主要从事基础工作。卡罗瑟夫的成功导致了我们目前巨大塑料工业的发展，引起了对化学聚合物的关注，并且在今天仍然引起了强烈而明显的关注。热力学的性质热力学是工程科学最重要的领域之一。这门科学是用来解释大多数东西是如何做功的，有些东西为什么不按所预期的那样做功，另外一些东西又为什么根本不做功。热力学是工程师在设计汽车发动
43、机、热泵、火箭发动机、发电站燃汽轮机、空气调节器、超导电输电线，太阳能加热系统等所用的科学知识的关键部分。热力学以能的各种概念为中心，能量守恒这一概念是热力学的第一定律。这是热力学以及工程分析的起点，热力学的第二个要领是熵；熵提供一种用以确定某一过程是否可行的手段。产生熵的过程是可行的，消灭熵的过程是不可行的，这个要领是热力学第二定律的基础。他还为一种工程分析奠定了基础，在这种工程分析中，人们可以算出从给定的能源中所能获得的有用功率的最大值，或算出做某种工作所能获得的有用功率的最小值。若要在工程分析中应用热力学，就必须对能和熵这些概念有一个清楚的了解。科学家关心的是利用这些数据，结合能量守恒及
44、熵的产生这些基本概念来分析复杂系统性能。举一个工程师感兴趣的例子一个大型中心发电站。在该发电站，能源是某种形式的石油，有时是天然气；该发电站的作用是把燃料能尽可能地转化成电能，并把电能沿输电线输送出去。简单的说，该发电站的发电方式是：使水沸腾，利用蒸汽转动汽轮机，汽轮机再转动发电机。这类发电站中最简单的只能把大约25%的燃料转化成电能。但该发电站却能把大约40%的燃料转化成电能，这是因为该发电站是经过精心设计的结果，把热力学的基本原理仔细的用于该系统内的数百个零部件。进行这些计算的设计工程师，利用了由物理学家研究出来的有关蒸汽特性的数据；而物理学家则是利用实验测得的数据，结合热力学理论，研究出
45、这种特性的数据的。目前在研究中的一些发电站，如果说的确按热力学分析所预测的那样工作，可以将多达55%的燃料能转化成电能。热始终是自发的从较热的物体流向较冷的物体，这一规律是一种新的物理概念。在能量守恒原理中或其他任何一种自然规律中，没有给我们规定热的方向。如果能量能自发的从冰块流向周围的水中，这可能和能量的守恒完全一致，但这一过程决不发生。这一概念是热力学第二定律的实质。很明显，冷冻机是一种物理系统，用于厨房的电冰箱、冷场库和空调装置，它不仅必须遵从第一定律（能量守恒）也必须遵从第二定律。为了弄清冷冻机为什么没有违背第二定律，必须对这一定律加以说明，热力学第二定律实质上是说：热不会自发地从较冷的物体流向较热的物体。换句话说，热之所以能从较冷的物体流向较热的物体，是外界力量做功的结果，现在我们弄清了某一日常的自然过程。如水和冰之间的热流动和冷冻机热从里面向外面流动之间的区别。在水、冰系统中，能量的交换是自发产生的，因而热的流动是水流向冰。水放出了能量从而变冷，而冰吸收热量从而融化。另一方面，在冷冻机中，能量交换不是自发产生的，而需要改变热的流动方向，并通过进一步加热较暖的周围环境而使冷冻机内部变冷，就必须依靠外力做功。专心-专注-专业