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外文翻譯
外文翻譯
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. This 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. 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 resin 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, household 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 variations, 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 manufacture 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 cases, the payoff of reduced rejects is high.
A typical injection molding cycle or sequence consists of five phases;
1. Injection or mold filling
2. Packing or compression
3. Holding
4. Cooling
5. Part ejection
Plastic granules are fed into the hopper and through an in the injection cylinder where they are 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 injection 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 skin 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. 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 cavity 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 compensates 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 the part thickness. Typically, the part temperature must cool below the material’s 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 made, 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 naturally, 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 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 controversy.
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 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 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 1920’s. Staudinger’s work was primarily fundamental. Carother’s achievements led to the development of our present huge plastics industry by causing an awakening of interest in polymer chemistry, an interest which is still strongly apparent today.
The Nature of Thermodynamics
Thermodynamics 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, 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 about 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 provides 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 thermodynamics 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 technological 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 energy 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 this 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 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.
The 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 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 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 hotter 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, the 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 external agency is necessary to reverse the natural flow of heat and cool the interior at the expense of further heating the warmer surroundings.
譯文：
塑料注射成型
許多不同的加工過程習(xí)慣于把塑料顆粒、粉末和液體轉(zhuǎn)化成最終產(chǎn)品。塑料材料用模具成型，并且適合用多種方式成型。在大多數(shù)情況下，熱塑性材料可以用許多方法成型，但熱固性塑料需要用其他方法成型。對于熱塑性材料有這種事實的認(rèn)識，它常常被加熱成為另一種柔軟狀態(tài)，然后在冷卻以前成型。對于熱固性塑料，換句話說，在它加工以前還沒有形成聚合物，在化學(xué)反應(yīng)加工過程中發(fā)生變化，如通過加熱、催化劑或壓力處理。記住這個概念在學(xué)習(xí)塑料加工過程和聚合物的形成是很重要的。
塑料注射成型越來越廣泛地運(yùn)用于熱塑性材料的成型工藝。它也是最古老的一種方式。突然間，塑料注射成型材料占所有成型材料消費(fèi)的30%。塑料注射成型適合于大批量生產(chǎn)，當(dāng)原材料被成單一的步驟轉(zhuǎn)換成為塑料物品和單步自動化的復(fù)雜幾何形狀制品。在大多數(shù)情況下，對于這樣的制品，精加工是不需要的。所生產(chǎn)的各種各樣的產(chǎn)品包括：玩具、汽車配件、家用物品和電子消費(fèi)物品。
因為塑料注射模具有很多易變的相互影響，那是一種復(fù)雜的虛慎重考慮的加工過程。塑料注射模具設(shè)備的成功是不依賴于機(jī)器變化到恰當(dāng)?shù)牟襟E，只有淘汰了需要注射變化的機(jī)器，才會導(dǎo)致適應(yīng)液壓變化、料筒溫度變化和材料黏度變化的機(jī)器的產(chǎn)生。增加機(jī)器重復(fù)注射的能力的變化可以幫助減少公差，降低次品等級和增加產(chǎn)品質(zhì)量。
對于任何模具注射設(shè)備的操作人員目的是制造產(chǎn)品，成為特等品、用最短的時間、用重復(fù)精度和全自動化生產(chǎn)作為周期。模塑人員在生產(chǎn)過程中總是想盡辦法降低或消除不合格產(chǎn)品。對于塑料注射模具有高要求的光學(xué)制品，或者有高附加值的制品如：家用電器制品，它的利潤大大降低。
一種塑料注射模具的生產(chǎn)周期或順序由五個階段組成：
注射或填充模具
補(bǔ)料或壓縮
保壓
冷卻
局部注射
塑料顆粒被投入料斗并且打開塑料注射料筒，在那里顆粒被旋轉(zhuǎn)螺桿帶動進(jìn)入料筒。螺桿的旋轉(zhuǎn)強(qiáng)迫塑料顆粒在高壓下擠壓料筒筒壁導(dǎo)致它變成熔體。隨著壓力的增加，旋轉(zhuǎn)螺桿被迫后退直到有足夠的塑料被注射成為儲料。塑料螺桿強(qiáng)迫熔融的塑料從料筒流到噴嘴、主流道經(jīng)澆注系統(tǒng)，最終進(jìn)入模具型腔。當(dāng)注射模具型腔容積被充滿。當(dāng)塑料接觸冷的模具表面，它被固化以減少表層。當(dāng)模具保持熔融狀態(tài)，塑料沿著模芯充滿整個模具。，利用率特別高，在注射時型腔被充滿95%~98%。接著成型過程進(jìn)入補(bǔ)料階段。
當(dāng)型腔被充滿，熔融塑料便開始冷卻。冷卻塑料的收縮，就增加了諸如凹痕、孔洞和尺寸不穩(wěn)定等制品缺陷的發(fā)生。為了補(bǔ)償收縮，增加塑料壓入型腔。當(dāng)型腔被封裹，為防止的熔融狀態(tài)塑料從型腔內(nèi)流向出口，把壓力應(yīng)用于熔體。這種壓力必須應(yīng)用直到出口為固態(tài)。這種加工可分為兩步（補(bǔ)料和保壓）或可能包含成為一步（保壓或第二階段）。在補(bǔ)料時，熔體被補(bǔ)料壓力收縮補(bǔ)償壓入型腔。在保壓時，壓力僅僅防止聚合物回流。
在保壓階段完成以后，冷卻階段開始。在冷卻時，是制品在型腔內(nèi)保持需具體說明的一個階段。在冷卻持久的階段主要依靠材料的特性和制品的收縮率。典型的，制品溫度必須冷卻到材料的注射溫度。在冷卻制品時，這種機(jī)器塑料熔體被冷卻到下一個周期。聚合物是以剪切作用為主題的，如同加熱圈獲得能量一樣。當(dāng)注射開始，到塑料注射終止。聚合物會立刻出現(xiàn)在冷卻階段以前，直到模具打開和制品被注射。
當(dāng)聚合物被編制成有用的文章，它們被稱為：塑料、橡膠和纖維。許多聚合物，例如棉花和羊毛來自自然，但是絕大多數(shù)商業(yè)的產(chǎn)品都是人造的，都來源于此。一系列眾所周知的材料包括酚醛塑料，滌綸，尼龍，聚硅氧烷，有機(jī)玻璃，纖維素，聚丙乙烯和特氟隆。
在1930年以前，商業(yè)用的聚合物沒有廣泛應(yīng)用。然而它們本應(yīng)該作為新材料在19世紀(jì)下半葉出名，卻沒有成功。在該期間，它們所以未能發(fā)展，部分原因是不了解它們的性質(zhì)，特別是，聚合物結(jié)構(gòu)曾是許多無結(jié)果爭論的主題。
二十世紀(jì)的兩次事件使聚合物聲名雀起，并且在世界范圍內(nèi)占據(jù)了很重要的地位。第一次是成功的商業(yè)塑料產(chǎn)品叫做酚醛塑料。它有用的工業(yè)價值在1912年表現(xiàn)得近乎瘋狂，并且在以后許多年發(fā)揮著巨大的價值。今天，酚醛塑料仍然在一系列的人造的產(chǎn)品中占有一席之地。在1912年以前，由塑料制造的材料是有用的，但是那種材料的制造從未提供像發(fā)明了酚醛塑料以后，形成新聚合物的動力那樣有價值。第二次事件與基礎(chǔ)學(xué)科的自然聚合物有關(guān)，被歐洲的史濤丁格和美國的卡羅瑟夫發(fā)現(xiàn)，他們在特達(dá)華州的杜邦公司工作。一些重要的研究在20世紀(jì)20年代被開展，史濤丁格主要從事基礎(chǔ)工作。卡羅瑟夫的成功導(dǎo)致了我們目前巨大塑料工業(yè)的發(fā)展，引起了對化學(xué)聚合物的關(guān)注，并且在今天仍然引起了強(qiáng)烈而明顯的關(guān)注。
熱力學(xué)的性質(zhì)
熱力學(xué)是工程科學(xué)最重要的領(lǐng)域之一。這門科學(xué)是用來解釋大多數(shù)東西是如何做功的，有些東西為什么不按所預(yù)期的那樣做功，另外一些東西又為什么根本不做功。熱力學(xué)是工程師在設(shè)計汽車發(fā)動機(jī)、熱泵、火箭發(fā)動機(jī)、發(fā)電站燃汽輪機(jī)、空氣調(diào)節(jié)器、超導(dǎo)電輸電線，太陽能加熱系統(tǒng)等所用的科學(xué)知識的關(guān)鍵部分。
熱力學(xué)以能的各種概念為中心，能量守恒這一概念是熱力學(xué)的第一定律。這是熱力學(xué)以及工程分析的起點，熱力學(xué)的第二個要領(lǐng)是熵；熵提供一種用以確定某一過程是否可行的手段。產(chǎn)生熵的過程是可行的，消滅熵的過程是不可行的，這個要領(lǐng)是熱力學(xué)第二定律的基礎(chǔ)。
他還為一種工程分析奠定了基礎(chǔ)，在這種工程分析中，人們可以算出從給定的能源中所能獲得的有用功率的最大值，或算出做某種工作所能獲得的有用功率的最小值。
若要在工程分析中應(yīng)用熱力學(xué)，就必須對能和熵這些概念有一個清楚的了解?？茖W(xué)家關(guān)心的是利用這些數(shù)據(jù)，結(jié)合能量守恒及熵的產(chǎn)生這些基本概念來分析復(fù)雜系統(tǒng)性能。
舉一個工程師感興趣的例子———一個大型中心發(fā)電站。在該發(fā)電站，能源是某種形式的石油，有時是天然氣；該發(fā)電站的作用是把燃料能盡可能地轉(zhuǎn)化成電能，并把電能沿輸電線輸送出去。
簡單的說，該發(fā)電站的發(fā)電方式是：使水沸騰，利用蒸汽轉(zhuǎn)動汽輪機(jī)，汽輪機(jī)再轉(zhuǎn)動發(fā)電機(jī)。
這類發(fā)電站中最簡單的只能把大約25%的燃料轉(zhuǎn)化成電能。但該發(fā)電站卻能把大約40%的燃料轉(zhuǎn)化成電能，這是因為該發(fā)電站是經(jīng)過精心設(shè)計的結(jié)果，把熱力學(xué)的基本原理仔細(xì)的用于該系統(tǒng)內(nèi)的數(shù)百個零部件。
進(jìn)行這些計算的設(shè)計工程師，利用了由物理學(xué)家研究出來的有關(guān)蒸汽特性的數(shù)據(jù)；而物理學(xué)家則是利用實驗測得的數(shù)據(jù)，結(jié)合熱力學(xué)理論，研究出這種特性的數(shù)據(jù)的。
目前在研究中的一些發(fā)電站，如果說的確按熱力學(xué)分析所預(yù)測的那樣工作，可以將多達(dá)55%的燃料能轉(zhuǎn)化成電能。
熱始終是自發(fā)的從較熱的物體流向較冷的物體，這一規(guī)律是一種新的物理概念。在能量守恒原理中或其他任何一種自然規(guī)律中，沒有給我們規(guī)定熱的方向。如果能量能自發(fā)的從冰塊流向周圍的水中，這可能和能量的守恒完全一致，但這一過程決不發(fā)生。這一概念是熱力學(xué)第二定律的實質(zhì)。很明顯，冷凍機(jī)是一種物理系統(tǒng)，用于廚房的電冰箱、冷場庫和空調(diào)裝置，它不僅必須遵從第一定律（能量守恒）也必須遵從第二定律。
為了弄清冷凍機(jī)為什么沒有違背第二定律，必須對這一定律加以說明，熱力學(xué)第二定律實質(zhì)上是說：熱不會自發(fā)地從較冷的物體流向較熱的物體。
換句話說，熱之所以能從較冷的物體流向較熱的物體，是外界力量做功的結(jié)果，現(xiàn)在我們弄清了某一日常的自然過程。如水和冰之間的熱流動和冷凍機(jī)熱從里面向外面流動之間的區(qū)別。
在水、冰系統(tǒng)中，能量的交換是自發(fā)產(chǎn)生的，因而熱的流動是水流向冰。水放出了能量從而變冷，而冰吸收熱量從而融化。
另一方面，在冷凍機(jī)中，能量交換不是自發(fā)產(chǎn)生的，而需要改變熱的流動方向，并通過進(jìn)一步加熱較暖的周圍環(huán)境而使冷凍機(jī)內(nèi)部變冷，就必須依靠外力做功。