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模具设计与制造专业英语翻译及原文
文章来源:www.biyezuopin.cc   发布者:毕业设计网站   发布时间:2018-01-13 10:54:46   阅读:2788

英文原文版出处:《模具设计与制造专业英语》,刘建雄 王家惠 廖丕博 主编,北京大学出版社2006年3月第1版

译文:
3.3压铸
压铸是通过压力作用下使熔融金属进入金属模具,是快速生产精密产品的一部分。这个术语同时适用于所得的铸件。压铸件因为具有有良好的表面而可以经济地应用于批量和大批量生产中。只需要相对小的加工,就可以实现很好的公差保证,这些原则在一些压铸操作中都得到了很好的检验。压铸模具是永久性的,不会由于金属的引入而影响到他们,除正常磨损或损耗。在相同的大小和形状的条件下,压铸模通常比塑料模和永久铸造更昂贵。
这种快速成型依靠快速把金属注入模具、冷却、开模、铸件取出和模具下次压铸的准备。

3.3.1模具压铸周期
在铸造周期里,首先模具闭合并且锁紧,熔化的金属在一个特定温度的熔炉中,然后进入注射缸,根据合金的类型,用于热压室或者冷压室金属浇注系统,这些将在后面描述。在注射阶段的压铸过程中,熔融金属在压力的作用下,快速通过模具浇注系统进入模具并排出模具里的空气,金属量必须足够大以充满型腔和溢流井,溢流井的设计是用来储存接收溢流出来的融溶金属液的,因为接触到模具型腔的空气容易氧化,同时也最先接触到模具其也可以快速冷却以便可以接下来进行第二次的压铸。一旦这模具型腔填满了,作用于金属液的压力会增大,保压一定的时间以便金属液凝固,模具的分离、工件的取出通常通过机器自动操作完成,打开模具进行必要的清理和润滑,然后下一轮压铸继续循环。
从模具取下的工件冷却后,操作工切除金属浇注填充时产生的披锋,同时去除溢流井和分型线,接着能进行二次加工和表面后处理。

3.3.2压铸合金
四类主要的压铸模具合金为锌、铝、镁和铜基合金,压铸工艺发展于19世纪铅/锡合金零件的制作,然而,铅和锡由于力学性能差,现在很少用来压铸。
最常见的压铸合金是铝合金,它们密度低,耐腐蚀性好,易于铸造,并具有良好的力学性能和尺寸稳定性。铝合金的缺点是用冷压室铸造时比热压室铸造需要更长的周期因为需要一个分离浇注操作。
锌基合金是最容易铸造的,它们也有很高的延展性和良好的抗冲击强度,因此可广泛用于产品上,铸件可以制造得很薄,并且表面平滑性良好,使其易于电镀和喷涂。然而,锌基合金容易被腐蚀,需要在零件各部分涂上保护膜。因此,锌合金的高比重导致每个单位体积的锌合金比压铸铝合金要更贵。
锌铝合金的铝含量(82.7%)比标准锌合金高,类似于标准锌合金,能获得薄壁和耐久的压铸件,但与铝合金,在冷压室机器中必须每个周期都需要浇注熔融金属,唯一例外的是ZA8(8%铝),这是铝锌系列中含铝量最低的。
镁合金有较低的密度、高的强度重量比、优越的阻尼能力和良好的机械加工性能。
铜基合金、黄铜和青铜,比任何压铸合金的力学性能都要好,但是它们更贵。黄铜具有高的强度和韧性、好的耐磨性,并具有优良的耐腐蚀性。
铜基合金铸造的一个主要缺点是在非常高的铸造温度下由热疲劳引起模具寿命缩短,合金浇注温度对模具寿命影响最为强烈,也正是因为这个原因模具寿命最长的是锌合金,最短的是铜合金。然而,铸件尺寸、壁厚和几何复杂性同样影响模具表面的磨损和最终的损坏也是差不多的。

3.3.3压铸模具
压铸模具由两个主要部分组成——动模部分和定模部分,它们在分型线闭合,型腔和型芯通常加工后镶嵌到这两部分,定模固定在定模扳上,而动模固定在动模板上(见图3-10,3-11),型腔和型芯设计必须匹配以便能够顺利开模。
压铸模具的结构几乎与注塑成型模具相似,在注射成型术语中,动模包括型芯和喷射器壳体,定模包括型腔和支撑板。
压铸模铸件侧抽芯机构与外部相交特征可以非常精准地建立起来,这是压铸模和注塑模相同的特性。然而,熔模铸造合金在模具接触表面的粘性远远小于注射成型的。这种现象被称为“喷射”,容易堵塞模具,正是因为这个原因,结合飞边的高收缩力导致局部收缩非常困难,使得产品难以满足内部核心机制。因此,内螺纹或其他内部切削孔通常无法被铸造,必须由昂贵的额外加工生产。浇注系统在压铸和注塑模具中的建立是相同的。
“喷射”经常在定模和动模之间产生,在产品分型线上形成一些薄而不规则的披锋,有时,这些分型线会溅出合金,由于这个原因,压铸机必须安装安全装置以抑制这些溅出的材料。
在压铸过程中的一个区别主要在于溢流井通常围绕压铸型腔的周边,如前面所提到的,在铸造时它们可以减少氧化物的量,最先进入模具的合金,通过排气孔把型腔内的空气排出,随后注射的合金和模具有更高的温度,从而减少金属凝固过早的机会,这种过早凝固形成的表面缺陷称为冷料,是由于金属流动时还没有相遇熔接在一起就已经凝固所致,当压铸模较小时,溢流槽大幅增加熔融金属也可以使模具保持一定的温度。


3.3.4压铸机
1.热室压铸机
    一个典型的热室注射系统如图3-12所示,由气缸、柱塞、流道和喷嘴组成,注塑周期开始时的柱塞在相应的位置上,熔融金属流入保温炉内,通过进气口并进入压力缸,然后,当模具合模并锁紧时,液压柱塞移动到气缸和密封进气口,熔融金属通过流道和喷嘴到浇口、浇注系统和模具模腔,浇口是喷嘴通过进料系统进入定模的锥形扩流通道,锥形状提供从浇注点到浇注流道的平滑过渡,这使凝固后的产品容易脱出。在预设的金属凝固时间里,液压系统使柱塞返回,这个周期循环往复。
2.冷室压铸机
一个典型的冷室压铸机如图3-13所示,由水冷式柱塞,一个压射缸,和位于上方的水平式注射室和一个浇注孔组成。操作顺序如下:当模具关闭和锁紧并且气缸柱塞缩回,熔融金属通过浇注孔被浇进注射室,为了包紧型腔里的金属液,金属的浇注体积大于型腔和浇注系统、溢流井的体积,然后注射气缸加压,使活塞通过注射室,使熔融金属进入模具型腔,在金属凝固后,模具打开,柱塞返回到原来的位置。当模具打开,在注射筒端多余的金属,被称为料柄,是被逼出来的,因为它是连接到缸体铸件。在压铸周期中料柄是需要的,用于保持液态金属的压力铸造、凝固和收缩。


第4章  锻造模具
4.1  简介
锻造是一个通过各种模具和工具施加压力到工件加工的方法,这是最古老的金属加工方法,至少可以追溯到公元前4000年,也许早在公元前8000年,锻造通过用石头锤击金属用来制造珠宝、钱币和各种器具。
简单的锻造操作,传统上由铁匠用重锤子和砧进行。然而,大多数锻件需要一套模具和压力机或锻锤等设备完成。
典型的锻造产品有螺栓和铆钉、连杆、涡轮机轴、齿轮、有手柄的工具、机械构件、飞行器、铁路和各种其它运输设备。
通过控制金属的流动和晶粒结构,可以锻造具有良好的强度和韧性的零件;它们能够可靠地用于高强度工序(如图4-1)。锻造可在室温(冷锻)或在高温(热锻,取决于温度)进行。


由于该材料的强度较高,冷锻需要更大的力量,工件的材料在室温下必须具有足够的延展性,冷锻造件具有良好的表面光洁度和尺寸精度。热锻需要较小的力量,但它产生的尺寸精度和表面光洁度较差。
锻件通常需要额外的后处理操作,例如热处理以修改其性能,然后加工以得到准确的成品尺寸,这些操作可以通过精密锻造,是一个趋向最终形状或近终形状成型过程的重要例子,这种显著减少加工,降低产品制造成本的方法,是今后发展的趋势。
有几种形式的锻造,但还是有一些差异识别过程与名字在不同的引用。

4.2  开式模锻
开式模锻是最简单的锻造工艺,虽然大多数开放式模锻一般重达15公斤〜500公斤,但是也有锻重300吨的,大小的范围可以从非常小的零件到长达23米(如螺旋桨)。
开模锻过程可以描述为由固体工件放置在两个平面之间通过压缩它降低高度(如图4-2),这个过程也被称为镦粗或平锻。模具表面在开式模锻可以简单铸造,生产相对简单的锻件。在理想的条件下对工件的变形如图4-2(b)所示,因为锻件体积恒定,在高度方向减少,在直径方向就会增加。
需要注意的是,在图4-2(b)中,所述工件是均匀地变形,而在实际操作中,工件变成桶形(如图4-2 (c)),这种变形也被称为镦锻。快速移动造成主要由摩擦力形成接口,反的材料在这些接口对流。快速移动可以有效利用润滑剂。


把加热后的工件放在冷模之间锻造同样能变成圆桶形,在界面的部分迅速冷却,而工件的其余部分仍然相对较热,因此,在工件的端部的材料具有变形比在其中心处的材料高的阻力,所以在工件的中心部分沿横向扩展比端部大,可以通过热效应使用加热的金属模具减少或消除圆桶效应,热障如在模具和工件接触面之间使用玻璃纤维也是可行的。


引伸锻造也称为拔长,是一个基于开式模锻并通过间隔连续锻造工序减小棒料厚度的操作(如图4-3所示),因为每次行程的接触面积较小,一条长的棒料不需要很大的力或者机器就能减小其厚度。铁匠用锤子和砧加工加热的金属工件,各种设计的铁栅栏通常用这种方法制成。

4.3  模锻和闭式模锻
在模锻里,工件通过在两个有所需形状的模具型腔(锻模)中成形(如图4-4所示),需要注意的是一些材料向外流动,并形成一些飞边。飞边有一个显著的作用在流动的材料进入模锻时:薄的飞边快速冷却,并且由于其摩擦力,这使得材料在模具型腔中受到高的压力,从而促进模具型腔的填充。


坯件通过这些方法制备(a)切割或剪切一条挤压或拉伸的棒料(b)粉末冶金制作型坯(c)铸造或者(d)在锻造前预先成形毛坯。坯件放在下模,然后上模开始下降,坯件的形状逐渐改变,如图4-5(a)所示为一个连杆锻造。


预制坯锻造过程如拔长和滚压(如图4-5(b)和(c))通常用于将材料分配到不同坯件区域,就像它们是面团制作馅饼。拔长时,材料从一个区域分配,滚压时,材料聚集在一个局部区域里,然后将部件形成为连杆的粗糙形状称为预锻过程,使用预锻模,最后的操作是在锻造金属模具给予锻造件最终形状的精加工,飞边通常通过修整操作去除(如图4-6所示)。
如图4-4和4-5(a)所示的例子被称为闭式模锻。然而,真正的闭式模或者毛边锻造,飞边不形成并且工件完全填充模腔(如右图4-7(b)所示)。精确控制材料体积和适当的模具设计是为了闭式模锻获得所需尺寸和公差。尺寸不足的坯料阻碍完全填充模腔,相反尺寸有余的坯料产生过多的压力并且可能引起模具过早地失效或者卡住。


4.3.1精密锻造
出于经济原因,当今锻造加工的趋势是朝着更大的精确度发展,从而降低了额外精加工操作的数量。其中形成的部分接近于所需零件的最终尺寸称为近净形状或者净形锻造,在这样的过程中,有少量过量的材料在锻造部件上,并且它随后被去除(通常通过修整或磨削)。
在精密锻造,特别模具生产零件比模锻精度更高并要求更少的加工,该方法需要更高性能的设备因为得到精细部件需要更大的力。由于它们需要相对低的锻造负载和温度,铝和镁的合金特别适用于精密锻造,同时发生模具磨损小,表面光洁度好,钢和钛也可以精密锻造。典型精密锻造产品有齿轮、连杆、外壳和涡轮叶片。
精密锻造需要特殊和更复杂的模具,精密控制坯料的体积和形状,坯料在模腔内精确定位,因此投入较高。然而更少的材料被浪费,并且需要更小后续加工,因为该工件接近最终所需的形状。因此,以往的锻造与精密锻造之间的选择需要一个经济分析,特别是在考虑到生产数量时。

4.3.2 精压
精压基本上是一个闭式模锻过程,通常用来铸造硬币、奖章和珠宝(如图4-8(a)(b)所示),锻造毛坯在一个完全封闭的模腔里精压。为了生产精细所需的压力可以是五或六倍的材料强度,例如,在新铸币的细致部分。对于某些工件可能需要几个精压操作,润滑油不能用于精压中,因为它们能陷入模具型腔中,并且是不能压缩的,阻止充分再现模具表面细节。
在精压过程中也使用锻件和其他的产品,以改善表面光洁度和赋予所需的尺寸精度,这一过程被称为按尺寸加工,涉及高压的同时在工件形状按尺寸加工使用小的变形。制造带有字母和数字的工件能够类似于精压过程快速完成。


原文:
3.3 Die Casting
Die casting is the art of rapidly producing accurately dimensioned parts by forcing molten metal under pressure into metal dies. The term also applies to the resultant casting. Die castings can be used economically in designs having moderate to large activity because the completed piece has a good surface, requires relatively little machining, and can be held to close tolerances. The principles of die casting follow those of good practice in any casting operation. The steel dies are permanent and should not be affected by the metal introduced into them, except for normal abrasion or wear. Die-casting dies are usually more expensive than those used in plastic or permanent molding of a part of similar size and shape.
The rapidity of operation depends upon the speed with which the metal can be forced into the die, cooled, and ejected; the casting removed; and the die prepared for the next shot.

3.3.1 The Die Casting Cycle
In the casting cycle, first the die is closed and locked. The molten metal, which is main tained by a furnace at a specified temperature, then enters the injection cylinder. Depending on the type of alloy, either a hot-chamber or cold-chamber metal-pumping system is used. These will be described later. During the injection stage of the die casting process, pressure is applied to the molten metal, which is then driven quickly through the feed system of the die while air escapes from the die through vents. The volume of metal must be large enough to overflow the die cavities and fill overflow wells. These overflow wells are designed to receive the lead portion of the molten metal, which tends to oxidize from contact with air in the cavity and also cools too rapidly from initial die contact to produce sound castings. Once the cavities are filled, pressure on the metal is increased and held for a specified dwell time during which solidification takes place. The dies are then separated, and the part extracted, often by means of automatic machine operation. The open dies are then cleaned and lubricated as needed, and the casting cycle is repeated.
Following extraction from the die, parts are often quenched and then trimmed to remove the runners, which were necessary for metal flow during mold filling. Trimming is also necessary to remove the overflow wells and any parting-line flash that is produced. Subsequently, secondary machining and surface finishing operations may be performed.

3.3.2 Die Casting Alloys
The four major types of alloys that are die-cast are zinc, aluminum, magnesium, and copper-based alloys. The die casting process was developed in the 19th century for the manu- facture of lead/tin alloy parts. However, lead and tin are now very rarely die-cast because of their poor mechanical properties.
The most common die casting alloys are the aluminum alloys. They have low density, good corrosion resistance, are relatively easy to cast, and have good mechanical properties and dimensional stability. Aluminum alloys have the disadvantage of requiring the use of cold-chamber machines, which usually have longer cycle times than hot-chamber machines owing to the need for a separate ladling operation.
Zinc-based alloys are the easiest to cast. They also have high ductility and good impact strength, and therefore can be used for a wide range of products. Castings can be made with very thin walls, as well as with excellent surface smoothness, leading to ease of preparation for plating and painting. Zinc alloy castings, however, are very susceptible to corrosion and must usually be coated, adding significantly to the total cost of the component. Also, the high specific gravity of zinc alloys leads to a much higher cost per unit volume than for aluminum die casting alloys.
Zinc-aluminum (ZA) alloys contain a higher aluminum content (82.7%) than the standard zinc alloys. Thin walls and long die lives can be obtained, similar to standard zinc alloys, but as with aluminum alloys, cold-chamber machines, which require pouring of the molten metal for each cycle, must usually be used. The single exception to this rule is ZA8 (8% Al), which has the lowest aluminum content of the zinc-aluminum family.
Magnesium alloys have very low density, a high strength-to-weight ratio, exceptional damping capacity, and excellent machinability properties.
Copper-based alloys, brass and bronze, provide the best mechanical properties of any of the die casting alloys; but they are much more expensive. Brasses have high strength and toughness, good wear resistance, and excellent corrosion resistance.
One major disadvantage of copper-based alloy casting is the short die life caused by thermal fatigue of the dies at the extremely high casting temperatures. Die life is influenced most strongly by the casting temperature of the alloys, and for that reason is greatest for zinc and shortest for copper alloys. However, this is only an approximation since casting size, wall thickness, and geometrical complexity also influence the wear and eventual breakdown of the die surface.

3.3.3 Die Casting Dies
Die casting dies consist of two major sections——the ejector die half and the cover die half——which meet at the parting line. The cavities and cores are usually machined into inserts that are fitted into each of these halves. The cover die half is secured to the stationary platen, while the ejector die half is fastened to the movable platen (see Figs. 3-10, 3-11). The cavity and matching core must be designed so that the die halves can be pulled away from the solidified casting.
The construction of die casting dies is almost identical to that of molds for injection molding. In injection molding terminology, the ejector die half comprises the core plate and ejector housing, and the cover die half comprises the cavity plate and backing support plate.
Side-pull mechanisms for casting parts with external cross-features can be found in exactly the same form in die casting dies as in plastic injection molds. However, molten die casting alloys are much less viscous than the polymer melt in injection molding and have a great tendency to flow between the contacting surfaces of the die. This phenomenon, referred to as “flashing”, tends to jam mold mechanisms, which must, for this reason, be robust. The combination of flashing with the high core retraction forces due to part shrinkage makes it extremely difficult to produce satisfactory internal core mechanisms. Thus, internal screw threads or other internal undercuts cannot usually be cast and must be produced by expensive additional machining operations. Ejection systems found in die casting dies are identical to the ones found in injection molds.
“Flashing” always occurs between the cover die and ejector die halves, leading to a thin, irregular band of metal around the parting line. Occasionally, this parting line flash may escape between the die faces. For this reason, full safety doors must always be fitted to manual die casting machines to contain any such escaping flash material.
One main difference in the die casting process is that overflow wells are usually designed around the perimeter of die casting cavities. As mentioned earlier, they reduce the amount of oxides in the casting, by allowing the first part of the shot, which displaces the air through the escape vents, to pass completely through the cavity. The remaining portion of the shot and the die are then at a higher temperature, thereby reducing the chance of the metal freezing prematurely. Such premature freezing leads to the formation of surface defects called cold shuts, in which streams of metal do not weld together properly because they have partially solidified by the time they meet. Overflow wells are also needed to maintain a more uniform die temperature on small castings, by adding substantially to the mass of molten metal.

 

3.3.4 Die Casting Machines
1. Hot-Chamber Machines
A typical hot-chamber injection or shot system, as shown in Fig. 3-12, consists of a cylinder, a plunger, a gooseneck, and a nozzle. The injection cycle begins with the plunger in the up position. The molten metal flows from the metal-holding pot in the furnace, through the intake ports, and into the pressure cylinder. Then, with the dies closed and locked, hydraulic pressure moves the plunger down into the pressure cylinder and seals off the intake ports. The molten metal is forced through the gooseneck channel and the nozzle and into the sprue, feed system, and die cavities. The sprue is a conically expanding flow channel that passes through the cover die half from the nozzle into the feed system. The conical shape provides a smooth transition from the injection point to the feed channels and allows easy extraction from the die after solidification. After a preset dwell time for metal solidification, the hydraulic system is reversed and the plunger is pulled up. The cycle then repeats.
2. Cold-Chamber Machines
A typical cold-chamber machine, as shown in Fig. 3-13, consists of a horizontal shot chamber with a pouring hole on the top, a water-cooled plunger, and a pressurized injection cylinder. The sequence of operations is as follows: when the die is closed and locked and the cylinder plunger is retracted, the molten metal is ladled into the shot chamber through the pouring hole. In order to tightly pack the metal in the cavity, the volume of metal poured into the chamber is greater than the combined volume of the cavity, the feed system, and the overflow wells. The injection cylinder is then energized, moving the plunger through the chamber, thereby forcing the molten metal into the die cavity. After the metal has solidified, the die opens and the plunger moves back to its original position. As the die opens, the excess metal at the end of the injection cylinder, called the biscuit, is forced out of the cylinder because it is attached to the casting. Material in the biscuit is required during the die casting cycle in order to maintain liquid metal pressure on the casting while it solidifies and shrinks.

 

Chapter 4 Forging Die
4.1 Introduction
Forging is a process in which the workpiece is shaped by compressive forces applied through various dies and tools. It is one of the oldest metalworking operations, dating back at least to 4000 B.C.——perhaps as far back as 8000 B.C. Forging was first used to make jewelry, coins, and various implements by hammering metal with tools made of stone.
Simple forging operations can be performed with a heavy hand hammer and an anvil, as was traditionally done by blacksmiths. Most forgings, however, require a set of dies and such equipment as a press or a forging hammer.
Typical forged products are bolts and rivets, connecting rods, shafts for turbines, gears, hand tools, and structural components for machinery, aircraft, railroads, and a variety of other transportation equipment.
Metal flow and grain structure can be controlled, so forged parts have good strength and toughness; they can be used reliably for highly stressed and critical applications (Fig. 4-1). Forg- ing may be done at room temperature (cold forging) or at elevated temperatures (warm or hot forging, depending on the temperature).

Because of the higher strength of the material, cold forging requires greater forces, and the workpiece materials must have sufficient ductility at room temperature. Cold-forged parts have good surface finish and dimensional accuracy. Hot forging requires smaller forces, but it produces dimensional accuracy and surface finish that are not as good.
   Forgings generally require additional finishing operations, such as heat treating, to modify properties, and then machining to obtain accurate finished dimensions. These operations can be minimized by precision forging, which is an important example of the trend toward net-shape or near-net shape forming processes. This trend significantly reduces the number of operations required, and hence the manufacturing cost to make the final product.
There are several forms of forging, but there is some disparity identifying processes with names in different references.

4.2 Open-Die Forging
Open-die forging is the simplest forging process. Although most open-die forging generally weighs 15 kg~500 kg, forging as heavy as 300 tons have been made. Sizes may range from very small parts up to shafts some 23 m long (in the case of ship propellers).
The open-die forging process can be depicted by a solid workpiece placed between two flat dies and reduced in height by compressing it (Fig. 4-2). This process is also called upsetting or flat-die forging. The die surfaces in open-die forging may have simple cavities, to produce relatively simple forgings. The deformation of the workpiece under ideal conditions is shown in Fig. 4-2 (b). Because constancy of volume is maintained, any reduction in height increases the diameter of the forged part.
Note that, in Fig. 4-2 (b), the workpiece is deformed uniformly. In actual operations, the part develops a barrel shape (Fig. 4-2 (c)); this deformation is also known as pancaking. Barreling is caused primarily by frictional forces at the die-workpiece interfaces that oppose the outward flow of the materials at these interfaces. Barreling can be minimized if an effective lubricant is used.

Barreling can also occur in upsetting hot workpieces between cold dies. The material at and near the interfaces cools rapidly, while the rest of the workpiece remains relatively hot. Thus, the material at the ends of the workpiece has higher resistance to deformation than the material at its center. Consequently, the central portion of the workpiece expands laterally to a greater extent than do its ends. Barreling from thermal effects can be reduced or eliminated by using heated dies; thermal barriers such as glass cloth at the die-workpiece interfaces are also used.

Cogging, also called drawing out, is basically an open-die forging operation in which the thickness of a bar is reduced by successive forging steps at specific intervals (Fig. 4-3). Because the contact area per stroke is small, a long section of a bar can be reduced in thickness without requiring large forces or machinery. Blacksmiths perform such operations with a hammer and an anvil using hot pieces of metal; iron fences of various designs are often made by this process.

4.3 Impression-Die and Closed-Die Forging
In impression-die forging, the workpiece acquires the shape of the die cavities (impressions) while being forged between two shaped dies (Fig. 4-4). Note that some of the material flows outward and forms a flash. The flash has a significant role in flow of material in impression-die forging: The thin flash cools rapidly, and, because of its frictional resistance, it subjects the material in the die cavity to high pressures, thereby encouraging the filling of the die cavity.

The blank to be forged is prepared by such means as (a) cutting or cropping from an extruded or drawn bar stock, (b) a preform in operations such as powder metallurgy, (c) casting, or (d) a preform blank in a prior forging operation. The blank is placed on the lower die and, as the upper die begins to descend, the blank’s shape gradually changes, as is shown for the forging of a connecting rod in Fig. 4-5 (a).

Preforming processes, such as fullering and edging (Figs. 4-5 (b) and (c)), are used to distribute the material into various regions of the blank, much as they are in shaping dough to make pastry. In fullering, material is distributed away from an area; in edging, it is gathered into a localized area. The part is then formed into the rough shape of a connecting rod by a process called blocking, using blocker dies. The final operation is the finishing of the forging in impression dies that give the forging its final shape. The flash is removed usually by a trimming operation (Fig. 4-6).
The examples shown in Figs. 4-4 and 4-5(a) are also referred to as closed-die forgings. However, in true closed-die or flashless forging, flash does not form and the workpiece completely fills the die cavity (right side of Fig. 4-7 (b)). Accurate control of the volume of material and proper die design are essential in order to obtain a closed-die forging of the desired dimensions and tolerances. Undersize blanks prevent the complete filling of the die cavity; conversely, oversize blanks generate excessive pressures and may cause dies to fail prematurely or to jam.

4.3.1 Precision Forging
For economic reasons the trend in forging operations today is toward greater precision, which reduces the number of additional finishing operations. Operations in which the part formed are close to the final dimensions of the desired component are known as near-net-shape or net-shape forging. In such a process, there is little excess material on the forged part, and it is subsequently removed (generally by trimming or grinding).
   In precision forging, special dies produce parts having greater accuracies than those from impression-die forging and requiring much less machining. The process requires higher-capacity equipment, because of the greater forces required to obtain fine details on the parts. Because of the relatively low forging loads and temperatures that they require, aluminum and magnesium alloys are particularly suitable for precision forging; also, little die wear takes place and the surface finish is good. Steels and titanium can also be precision-forged. Typical precision-forged products are gears, connecting rods, housings, and turbine blades.
   Precision forging requires special and more complex dies, precise control of the billet’s volume and shape, accurate positioning of the billet in the die cavity, and hence higher investment. However, less material is wasted, and much less subsequent machining is required, because the part is closer to the final desired shape. Thus, the choice between conventional forging and precision forging requires an economic analysis, particularly in regard to the production volume.

4.3.2 Coining
Coining essentially is a closed-die forging process typically used in minting coins, medallions, and jewelry (Fig. 4-8 (a), (b)). The slug is coined in a completely closed die cavity. In order to produce fine details the pressures required can be as high as five or six times the strength of the material, note, for example, the detail on newly minted coins. On some parts, several coining operations may be required. Lubricants cannot be applied in coining, because they can become entrapped in the die cavities and, being incompressible, prevent the full reproduction of die-surface details.
The coining process is also used with forgings and with other products, to improve surface finish and to impart the desired dimensional accuracy. This process, called sizing, involves high pressures, with little change in part shape during sizing. Marking of parts with letters and numbers can be done rapidly by a process similar to coining.

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