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资料来源：文章名：The injection molding and machine书刊名：Professional English for mould作 者：王浩钢 曹艳清出版社：人民邮电出版社章 节：Chapter 1 - 3页 码：P1P18文 章 译 名： 模具专业英语 外文原文:Lesson 1 The injection molding and machine1.The injection molding Injection molding is principally used for the production of the thermoplastic parts,although some progress has been made in developing a method for injection molding some thermosetting materials. The problem of injecting a melted plastic into a mold cavity from a reservoir of melted material has been extremely difficult to solve for thermosetting plastics which cure and harden under such conditions within a few minutes.The principle of injection molding is quite similar to that of die-casting.The process consists of feeding a plastic compound in powdered or granular form from a hopper through metering and melting stages and then injecting it into a mold?.After a brief cooling period,the mold is opened and the solidified part ejected.Injection-molding machines can be arranged for manual operation,automatic single-cycle operation,and full automatic operation.The advantage of injection molding are: (i) a high molding speed adapted for mass production is possible; (ii) there is a wide choice of thermoplastic materials Providing a variety of useful propertics; (iii)it is possible to mold threads,undercuts,side holes,and large thin sections. 2.The injection-molding machine Several methods are used to force or inject the melted plastic into the mold. The most commonly used system in the larger machines is the in-line reciprocating screw.The screw acts as a combination injection and plasticizing unit.As the plastic is fed to the rotating screw, it passes through three zones as shown: feed, compression, and metering.After the feed zone,the screw-fight depth is gradually reduced,forcing the plastic to compress.The work is converted to heat by shearing the plastic,making it a semifluid mass.In the metering zone, additional heat is applied by conduction from the barrel surface. As the chamber in front of the screw becomes filled,it forces the screw back,tripping a limit switch that activates a hydraulic cylinder that forces the screw forward and injects the fluid plastic into the closed mold.An antiflowback valve prevents plastic under pressure from escaping back into the screw flights. The clamping force that a machine is capable of exerting is part of the size designation and is measured in tons. A rule-of-thumb can be used to determine the tonnage required for a particular job. It is based on two tons of clamp force per square inch of projected area. If the flow patter is difficult and the parts are thin, this may have to go ta three or four tons. Many reciprocating-screw machines are capable of handing thermosetting plastic materials.Previously these materials were handled by compression or transfer molding.Thermosetting materials cure or polymerize in the mold and are ejected hot in the range of 375 C 410 C. Thermoplastic partsmust be allowed to cool in the mold in order to remove them without distortion. Thus thermosetting cycles can be faster.Of course the mold must be heated rather than chilled,as with thermoplastics.Reading material A competent mould designer must have a thorough knowledge of the principles of mould making as the design of the various parts of the mould depends on the technique adopted for its manufacture. This chapter is included primarily for the beginner who does not have a background knowledge of the various machining and other mould making techniques.To cover the topic of mould making thoroughly would require a companion work equal in size to this monograph and therefore this introduction to the subject must,of necessity,be superfcial. However,we hope that very fact that it is included in a monograph on design,will emphasize the importance ofmould making as a subject and will also encourage the beginner to a further and more complete study in this field. The majority of moulds are manufactured by the use of conventional machine tools found in most modem toolrooms. From the manufacturing viewpoint we classify the mould into two parts (i)the cavity and core, and (ii)the remainder of the mould. The lattet part is commonly referred to as bolster work. The work on the cavity and core is by far the most important as it is from these members that the plastics moulding takes its form. The work on the cavity and core can further be classified depending upon whether the form is of a simple or a complex nature.For example,the cavity and core for a circular or rectangular box-type moulding is far simpler to make than a cavity and core to produce a telephone handset moulding.The mould parts for the simple form are produced on such machine tools as the lathe and the milling machine,whereas the more complex form requires the use of some kind of copying machine. The bolster work is not as critical as the manufacture of the cavity and core forms but nevertheless,accuracy in the manufacture of the various parts necessary to ensure that the mold can be assembled by the fitter without an undue amount of bench-work, Now,while the bolster work is always produced on conventional machine tool,the cavity and core,particularly the former,can be produced by one of a number of other techniques.These include investment casting,electro-deposition,cold hobbing,pressure casting and spark machining.1.Machine tools The purpose of any machine tool is to remove metal.Each machine tool removes metal in a different way.For example,in one type (the lathe) metal is removed by a single point tool as the work is rotated,whereas in another type(the milling machine)a cutter is rotated and metal is removed as the work is progressed beneath it. Which machine tool is to be used for a particular job depends to a large extent upon the type of machining required.There is,however,a certain amount of overlapping and some machine tools can be utilized for several different operations. The machine tools which will be found in the modern toolroom are as follows:(i)Lathes for turning, boring and screwcutting, etc. (ii)Cylindrical grinding machines for the production of precision cylindrical surfaces. (iii) Shaping and planing machines for the reduction of steel blocks and plates to the required thickness and forsquaring upthese plates (iv)Surface grinding machines for the production of precision flat surfaces. (v) Milling machines for the rapid removal of metal, for machining slots,recesses,boring holes machining splines,etc. (vi) Tracer-controlled milling machines for accurate reproduction of complex cavity and core forms. In addition to the above list of major machine tools there is,of course,ancillary equipment without which no toolroom would be complete. This includes power saws, drilling machines, toolpost grinders,hardening and polishing facilities,etc.2.Castings The manufacture of cavities and cores in steel by the conventional casting method using sand moulds is not satisfactory owing to the poor finish obtained and to the porosity which occurs on,or just below the surface of the casting.The expenditure involved in plugging,machining and finishing these conventional castings makes this method of mould making uneconomic. The Shaw investment casting process docs not,however,share the disadvantages associated with sand casting and is therefore applicable to the manufacture of cavities and cores.The process is carried out by specialists and the mould maker supplies the company with a pattern of the required mould part.As the final casting will be an accurate reproduction of the pattern supplied,this must be manufactured to close tolerances and have a good surface finish.To allow for the contraction of the steel on cooling the pattern is made approximately 0.020 mm/mm(in/ in) oversize.3.Electro-deposition Electro-deposition is an electrochemical process used to reproduce accurately a cavity or core form from a given pattern.The pattern can be made in an easily worked material and is the reverse form to that required. That is, a male pattern is required for a cavity and a female pattern for a core. Normally it is much easier to machine a male pattern than the reverse cavity form and it is for this reason that most applications for this technique are for intricate cavity work.4.Cold hobbing Cold hobbing is a process in which a hardened steel master hob is forced into a soft steel blank under condiderable pressure. Hobbing is used for the production of cavities which by virtue of their shape would be difficult to die-sink on conventional machine tools.5.Pressure casting Beryllium-copper is a material which is increasingly being used in mould construction because it possesses several desirable characteristics.In particular it has a high thermal conductivity combined with a reasonable hardness (Brinell Hardness Number of about 250),which makes it suitable for certain types of cavity and core,and for other mould parts,such as hot runner unit secondary nozzles. Its high thermal conductivity means that when beryllium-copper is used fora cavity or a core,the heat from the melt will be transferred away from the impression faster than if a corresponding steel cavity and core are used,and this often results in a shorter moulding cycle. Berylium-copper can be machined, in which case the conventional machine tools are used, and it can be cold-hobbed,hot-hobbed or pressure-cast,The last technique offers certain advantages over the hobbing methods,in that cold or hot hobbing of beryllium-copper tends to work harden the material which results in the development of stress concentrations. Pressure casting (or liquid hobbing) is used mainly for the production of cavities but if can be used,where applicable,for the production of the cores as well,As the terms suggest,it is basically a process which combines the casting and hobbing techniques.6.Spark machiningThis is one of more recent additions to mould making methods and strictly speaking it should come under the machine tool section.However,as the principle of operation is different from that of all other basic machine tools it is preferable to discuss this technique separately. Spark machining is a process in which steel or other metals can be machined by the application of an electrical discharge spark.The spark is localized and metal is progressively removed in small quantities over a period of time.7.Bench fitting Irrespective of the machine tool or technique used to manufacture the various parts of the mould,the final responsibility for the finishing of the individual parts and for fitting them together lies with the bench fitter.The mould parts finishing and assembly procedure adopted by the bench fitter varies quite often from toolroom to individual toolmakers working in the same work; it is therefore impossible to set down a standard pattern for the work.In consequence,we intend only to indicate the general approach to this problem without going into details.We will do this by considering the various stages in the bench fitting involved in the manufacture of a simple mould.Lesson 2 Ejection A molding is formed in mould by injeting a plastic melt,under pressure,into an impression via a feed system.It must therefore be removed manually.Furthermore,all thermoplastic materials contract as they solidify,which means that the molding will shrink on to the core which forms it.This shrinkage makes the molding dificult to remove. Facilities are provided on the injection machine for automatic actuation of an ejector system,and this is situated behind the moving platen.Because of this,the moulds ejector system will be most effectively operated if placed in the moving half of the mould,i.e.the half attached to the moving platen. We have stated previously that we need to eject the molding from the core and it therefore follows that the core ,too, will most satisfactorily be located in the moving half. The ejector system in a mould will be discussed under three headings, namely: (i) the ejector grid; (ii) the ejector plate assembly; and (iii)the method of ejection. Ejector grid The ejector grid is that part of the mould which supports the mould plate and provides a space into which the ejector plate assembly can be fitted and operated.The grid normally consists of a back plate on to which is mounted a number of conveniently shapedsupport blocks.1.Ejector plate assembly The ejector plate assembly is that part of the mould to which the ejector element is attached. The assembly is contained in a pocket, formed by the ejector grid, directly behind the mould plate.The assembly consists of an ejector plate,a retaining plate and an ejector rod.One end of this latter member is threaded and it is screwed into the ejector plat.In this particular design the ejector rod functions not only as an actuating member but also as a method of guiding the assembly.Note that the parallel portion of the ejector rod passes through an ejector rod bush fitted in the back plate of the mould.2.Ejection techniques When a molding cools,it contracts by an amount depending on the material being processed.For a molding which has no internal form,for example,a solid rectangular block,the molding will shrink away from the cavity walls,thereby permitting a simple ejection technique to be adopted. However,when the molding has internal form,the molding,as it cools,will shrink onto the core and some positive type of ejection is necessary. The designer has several ejection techniques from which to choose,but in general,the choice will be restricted depending upon the shape of the molding.The basic ejection techniques are as follows:(i)pin ejection; (ii) sleeve ejection; (iii) stripper plate ejection and (iv) air ejection. Reading material In drop forging, a piece of metal, roughly or approximately of the desired shape, is placed between die faces having the exact form of the finished piece and forced to take this form by drawing he dies together.This method is widely used for the manufacture of parts both of steel and brass.Large drop hammers, since the work done by a press, goes deeper. Further, the press can take a cooler ingot to closer dimensions. The forging should be done at about the same temperature as rolling; the process improves the physical properties of the steel just as rolling does. In the final forging it is important not to have the steel too hot,for an overheated steel will have poor mechanical properties when cooled.In heating for forging the temperature is usually judged by the eye, but where large numbers of the same pattern are made. The piece to be forged are heated in furnaces in which the temperature is indicated by pyrometers,and often is automatically controlled. Metal in the form of hot-rolled or cold-rolled sheets or strips may be formed into many shapes by forcing the sheet into the impressions in metal dies.This process is known as stamping or pressing of metals,and involves many different operations,such as bending,drawing,punching,etc.These operations are successfully carried out through the proper die design and proper operation of the press in which the dies are placed.Sheets or strips of steel,brass,copper,aluminum,or other metal are placed between dies and a slow action of the power press makes the dies together forcing the metal to assume the required shape. Rolling is the process of shaping meta in a machine called rolling mill Ingots of metal are rolled by forcing them between two rollers rotating in opposite directions,thus pressing the metal into the required shape. If the rollers gave no grooves on their surfaces, the metal is rolled into the form of a sheet or plate.If the rollers have grooves of a certain shape. the metal will take the form of these grooves,thus it may be shaped into the form of bars or rods. There are two kinds of rolling: hot rolling and cold rolling. Before cold rolling, the scale covering the surface of the hot-rolled object should be removed. Cold rolling produces a higher surface finishing sheet and gives it a very exact size. The process has innumerable advantages. Many shapes may be manufactured in quantities at a rapid rate and at a relatively low cost.Lesson 3 Mould coolongOne fundamental principle of injection molding is that hot material enters the mould,where it cools rapidly to a temperature at which it solidifies sufficiently to retain the shape of the impression.The temperature of the mould is therefore important as it governs a portion of the overall molding cycle.While the melt flows more freely in a hot mould,a greater cooling period is required before the solidified molding can be ejected. Alternatively, while the melt solidifies quickly in a cold mould,it may not reach the extremities of the impression.A compromise between the two extremes must therefore be accepted to obtain the optimum molding cycle. The operating temperature for a particular mould will depend on a number of factors which include the following: type and grade of material to be molded; length of flow within the impression; wall section of the molding; length of the feed system,etc,It is often found advantagcous to use a slightly higher temperature than is required just to fill the impression,as this tends to improve the surface finish of the molding by minimizing weld lines,flow marks and other blemishes. To maintain the required temperature differential between the mould and plastic material water(or other fluid) is circulated through holes or channels within the mould These holes or channels are termed flow-ways or water-ways and the complete system of flow ways is tamed the circuit. During the impression filling stage the hottest material will be in the vicinity of the entry point,i.e.the gate,the coolest material will be at the point farthest from the entry.The temperature of the coolant fluid,however,increases as it passes through the mould.Therefore,To achieve an even cooling rate over the molding surface,it is necessary to locate the incoming coolant fluid adjacent to “hot”molding surfaces and to locate the channels containingheatedcoolant fluid adjacent to“cool molding surfaces.However,as will be seen from the following discussion,it is not always practicable to adopt the idealized approach and the designer must use a fair amount of common sense when laying out coolant circuits if unnecessarily expensive moulds are to be avoided.Units for the circulation of water (or other fluids) are commercially available.These units are simply connected to the mould via flexible hoses,with these units the moulds temperature can be maintained within close limits.Close temperature control is not possible using the alternative system in which the mould is connected to a cold water supply. It is the mould designers responsibility to provide an adequate circulating system within the mould.In general,the simplest systems are those in which holes are bored longitudinally through the mould plates. However,this is not necessarily the most efficient method for a particular mould. When using drillings for the circulation of the coolant, however, these mus not be positioned too close to the impression (closer than 16 mm) as this is likely to cause a marked temperature variation across the impression,with resultant molding problems. The layout of a circuit is often complicated by the fact that flow ways must not be drilled too close to any other hole in the same mould plate. It will be recalled that the mould plate has a large number ofholes or recesses, to accommodate ejector pins, guide pillars, guide bushes, sprue bush, inserts, etc. How close it is safe to position in a flow way adjacent to another hole depends to a large extent on the deptho
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