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塑料
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李冠男
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塑料笔筒注射模具设计(李冠男),塑料,笔筒,注射,模具设计,李冠男
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Special casting processesThe process used for making a casting depends on the quantity to be produced, the metal to be cast, and the intricacy of the part. All metals can be cast in sand molds, and there is no restriction as to size. However, sand molds are single-purpose molds and are completely destroyed after the metal has solidified. Quite obviously, the use of a permanent mold effects considerable saving in labor cost. The development of permanent molds is seen in the historic photograph in Figure 6.1.workmen manually poured the molten steel into permanent molds; they released the metal from the molds by releasing steel binding rings. Nowadays this hard toil has been all but eliminated. A summary of the various special casting methods that will be discussed as follow:Methods of casting in metallic molds Permanent molds must be made of metals capable of withstanding high temperatures. Because of their high cost they are recommended only when many castings are to be produced. Although permanent molds are impractical for large castings and alloys of high-melting temperatures, they can be used advantageously for small-and medium-size castings that are manufactured in large quantities.Die casting Die casting is a process in which molten metal is forced by pressure into a metal mold known as a die. Because the metal solidifies under a pressure from 0.6 to 275MPa , the casting conforms to the die cavity in both shape and surface finish. The usual pressure is from 10.3 to 14MPa.Die casting is the most widely used of any of the permanent-mold processes. There are two methods employed:1. hot-chamber2. cold-chamber The principal distinction between the two is determined by the location of the melting pot. In the hot-chamber method, a melting pot is included with the machine, and the injection cylinder is immersed in the molten metal at all times. The injection cylinder is actuated either by air or hydraulic pressure which forces the metal into the dies to complete the casting. Machines using the cold-chamber process have a separate melting furnace, and metal is introduced into the injection cylinder by hand or mechanical means. Hydraulic pressure then forces the metal into the die. The process is rapid, since both the dies and cores are permanent. The smooth surface not only improves the appearance but also minimizes the work required to prepare the castings for plating or other finishing operations. The wall thickness can be more uniform than in sand castings and, consequently, less metal is required. The optimum production quantity ranges from 1000to2000 pieces. The maximum weight of a brass die casting is about 2.3 kg, but aluminum die casting of over 50kg are common. Small to medium-size castings can be made at a cycle rate of 100 to 800 die fillings per hour. The size is so accurately controlled that little or no machining is necessary. The scrap loss is low, since the sprue, runners, and gates can be remelted. The process eliminates such machining operations as drilling and certain types of threading. Die-casting tolerances vary according to the size of the casting and the kind of metal used. For small castings the tolerance rangs from +0.3 to 0.25mm. The closest tolerances are obtained when zinc alloys are die-cast. One of the limitations of die casting is the high cost of the equipment and dies. This is not an important factor in mass production, but it does limit its use in shout-run jobs. There is a rapid decrease in the life of the dies as the metal temperatures increase. In some cases there is an undesirable chilling effect on the metal unless high temperatures are maintained. Metals having a high coefficient of contraction must be removed from the mold as soon as possible because of the inability of the mold to contract with the casting. Although there are certain limitations in shape, the process can produce very large castings. Die castings have been limited to low-melting alloys, but with a gradual improvement of heat-resisting metals for dies, this process can now be used for numerous alloys. Gray cast iron, and alloy steels have been produced in dies made of unalloyed, sintered molybdenum, but the process is commercially limited to nonferrous alloys. DiesDies for both the hot and cold chamber machines are similar in construction because there is little difference in the method of holding and operating them. They are made in two sections to provide a means of removing the castings and are usually equipped with heavy dowel pins to keep the halves in proper alignment. Metal enters the stationary side when the die is locked in closed position. As the die opens, the ejector plate in the movable half of the die is advanced so that pins project through the die half and force the casting from the cavity and fixed cores. The dies are provided with a separate mechanism for moving the ejector plate or movable cores. The life of these molds depends on the metal cast and may range from 10000 fillings, if brass castings are made, to several million if zinc is used. It is always desirable to provide vents and small overflow wells on one side of a die to facilitate the escape of air and to catch surplus metal that has passed through the die cavity. In spite of this provision, there is a certain amount of flash metal that must be trimmed off in the finishing operation. For large or complex castings, a single-cavity mold is used. The casting and gate from such a mold are shown in figure 6.2. the part shown is cast with a steel insert from magnesium in a 5.3-MN machine. If the quantity of castings to be produced is large and they are relatively small in size, a multiple-cavity die can be used. Figure 6.3 shows a number of castings with the flash,gates, and sprue from such a die. The parts are produced from aluminum in a 3.6-MN machine. A combination die is one that has two or more cavities, each of which is different. They are frequently made up of insert blocks that can be removed so that other die blocks can be substituted. Most dies are provided with channels for water cooling to keep the die at the correct temperature for rapid production. Hot-Chamber Die Casting Low-melting alloys of zinc, tin, and lead are the most widely used materials cast in hot-chamber machines. In chapter 3 will be found a discussion of these alloys. Most other materials either have too high a melting point, an affinity for iron, or else create other problems that will reduce the life of the machine. Hot-chamber castings vary in size from 20kgto 40kg, although in the case of very small castings they are usually cast in multiple-mold dies. In this method, metal is forced into the mold and pressure maintained during solidification, either by a plunger or by compressed air. The plunger-type machine, illustrated in figure 6.4is hydraulically operated for both the metal plunger and the mechanism for opening and closing the die. In this machine the plunger operates in one end of a gooseneck casting which is submerged in the molten metal. With the plunger in the upper position, metal flows by gravity into this casting through several holes just below the plunger. On the down stroke these holes are closed by the plunger, and pressure is applied on the entrapped metal, causing it to be forced into the die cavity. Pressures over 35MPa are used in some machines of this type, resulting in castings of dense structure. As soon as the casting is solidified, the pressure is relieved, the dies are forces open , and the casting is ejected by means of knockout pins. The sprue is removed with the runner and the castings. The air-operated machines have a gooseneck operated by a lifting mechanism. In the starting position, the casting is submerged in the molten metal and is filled by gravity. It is then raised, so that the nozzle is in contact with the die opening, and locked in position. Compressed air, at pressures ranging from 0.5to4.0MPa, is applied directly on the metal, thus forcing it into the die. When solidification is about complete, the air pressure is turned off and the gooseneck lowered withdrawing cores, and ejecting the castings is the same as for the plunger-type machine.Cold-Chamber Die Casting The die casting of brass, aluminum, and magnesium requires higher pressures and melting temperatures and necessitates a change in the melting procedure from that previously described. Chapter 4has a discussion of the alloys used in cold-chamber casting. These metals are not melted in a self-contained pot, since the life of the pot would be very short. The usual procedure is to heat the metal in an auxiliary furnace and ladle it to the plunger cavity next to the dies. It is then forced into the dies under hydraulic pressure. Machines operating by this method are built to be very strong and rigid to withstand the heavy pressures exerted on the metal as it is forces into the dies. Of the machines in general use, one has the plunger in a vertical position; the other, in a horizontal position. A diagrammatic sketch illustrating the operation of horizontal-plunger cold-chamber machines is shown in Figure 6.5. in the first figure the dies are shown closed, with cores in position, and the molten metal ready to the ladled in. as soon as the ladle is emptied, the plunger moves to the left and forces the metal into the mold. After the metal solidifies, the cores are withdrawn, and then the dies are opened. In the third figure the dies are opening, and the casting is ejected from the stationary half. To complete the process of opening, an ejector rod comes into operation and ejects the casting from the movable half of the die. This operating cycle is used in the a variety of machines that operate at pressures ranging from 40 to 150MPa. These machines are fully hydraulic and semiautomatic. After the metal is ladled in, the rest of the operations are automatic. The 22-MN, hydraulically operated, cold-chamber, die-casting machine, shown in figure 6.6, is for making die castings up to 38 kg of aluminum, brass, or magnesium. Machine of this type are used to cast lager parts, such as automobile engine blocks and rotary power mower housings, as well as smaller castings when more than one is cast per mold. This particular machine has a furnace and gooseneck attachment that fits onto the injection end, thus converting it to a hot-chamber machine capable of high speed, large-shout operation with zinc, tin, or lead. The manufacture of brass die castings is an important achievement. The difficulties of the high temperatures involved and the resulting rapid oxidation of the steel dies have been largely overcome by improvements in die metals and casting at as low a temperature as possible. One machine is designed to use metal in a semiliquid or plastic state to permit operation at lower temperatures than those used for liquid metal. To protect the dies further from overheating, water is circulated through plates adjacent to the dies. Metal is maintained under close temperature used in this machine is 68MPa; 100 too 200 shots per hour can be made, depending on the size of the machine. Two variations of this process, each with the injection plunger in a vertical position, are diagrammatically illustrated in Figure 6.7. in the lower figure the compression chamber, into which the plastic metal is ladled, is separate from the dies. The metal is poured into this cavity onto a spring-backed plunger. As the ram descends, this plunger is forced down until the gate opening is exposed, permitting the metal to be ejector plunger also moves upward, carrying with it any surplus metal. As the die opens the casting is ejected. A variation of this machine, which the compression chamber a part of the dies, is shown in the upper part of the figure. Metal is poured into this chamber at the upper part of the die and forced by pressure into the die cavity as the ram descends. As soon as the ram moves up, the dies open, and the casting is ejected by means of the ejector pins. The sprue and excess metal are trimmed off in the finishing operation. Slush casting Slush casting is a method of producing hollow castings in metal molds without the use of cores. Molten metal is poured into the mold, which is turned over immediately so that the metal remaining liquid can run out. A thin-walled casting results, the thickness depending on the chilling effect from the mold and the time of the operation. The casting is removed by opening the halves of the mold. This method of casting is used only for ornamental objects, statuettes, toy, and other novelties. The metals used for these objects are lead, zinc, and various low-melting alloys. Parts cast in this fashion are either painted or finished to represent bronze, silver, or other more expensive metals.Pressed or Corthias Casting This method of casting resembles both the gravity and the slush processes but differs somewhat in procedure. A definite amount of metal is poured into an open-ended mold, and a close-fitting core is forced into the cavity, causing the metal to be forced into the mold cavities with some pressure. The core is removed as soon as the metal sets, leaving a hollow thin-walled casting. This process, developed in France by Corthias, is limited in use mainly to ornamental casting of open design.Nonmetallic molds are available that can be used with both high-and low-temperature alloys. Others have temperature limitations as to the kind of metal for which they are suitable. Nonmetallic molds are often used in precision casting where the dimensional accuracy obtained with accompanying smooth-surface finish tends to offset the higher costs. Centrifugal casting is included under this heading although casting are made by this process in both metal and nonmetallic molds. Electroslag Casting The electroslag casting process is unusual in that it does not employ a furnace. Instead, consumable electrodes melting or striking beneath a slag layer furnish molten metal to fill a water-cooled permanent mold. The molten metal continually drips or runs into the mold. It does not come in contact with the atmosphere because of the slag layer. No withdraw from the mold in concert with its filling from bottom to top. Studies indicate metal cast in this way may be superior to forgings. One interesting application comes about when the electrode material is changed in carbon content to effect a varying property in the casting.Centrifugal casting Centrifugal casting is the process of rotating a mold while the metal solidifies, so as to utilize centrifugal force to position the metal in the mold. Greater detail on the surface of the casting is obtained, and the dense metal structure has superior physical properties. Castings of symmetrical shape lend themselves particularly to this method, although many other types of castings can be produced. Centrifugal casting is often more economical than other methods. Cores in cylindrical shapes and rises or feedheads are both eliminated. The castings have a dense metal structure with all impurities forced back to the center where frequently they can be machined out. Because of the pressure exerted on the metal, thinner sections can be cast than would be possible in static casting. Permanent molds have been used successfully in the centrifugal casting of magnesium. Since the cast magnesium is forced against the mold, the casting cools more quickly because an air or gas gap is eliminated between the mold and the material. There is considerable difficulty in most cases in which metal molds are used because of the expansion of the mold due to heat and the contraction of the casting due to cooling. Although there are limitations on the size and shape of centrifugally cast parts, piston rings weighing a few grams and paper mill rolls weighing over 40 tonnes have been cast in this manner. Aluminum engine blocks utilize centrifugally cast iron liners. If a metal can be melted, it can be cast centrifugally, but in the case of a few alloys, the heavier elements tend to be separated from the base metal. This separation is known as gravity segregation. 1. true centrifugal casting 2. semicentrifugal casting 3. centrifuging True centrifugal castingTrue centrifugal casting is used for pipe, liners, and symmetrical objects that are cast by rotating the mold about its horizontal or vertical axis. The metal is held against the wall of the mold by centrifugal force, and no core is required to from a cylindrical cavity on the inside. There are two types of horizontal axis molds used for producing cast-iron pipe. Massive, thick metal molds with a thin refractory coating allow the molten metal to begin solidification faster and for the solidification to proceed from the wall of the mold toward the inside of the cast pipe. Such a mold encourages a preferred solidification that assures a more solid casting with any impurities on the inside wall. Figure 6.10 illustrates such a casting machine. The mold is spinning rapidly at the time the molten metal is introduced and the spinning action is not stopped until solidification is complete. The wall thickness of the pipe produced is controlled by the amount of metal poured into the mold. Another type of horizontal centrifugal casting uses a thick, highly insulating sand interface between the mold and the casting. Such s sand lining is spun into the mold. When metal is introduced, the insulating nature of the sand prevents directional solidification, and hence the metal solidifies from the wall and from the inside pipe face at the same time. This can cause a spongy, less dense midsection that has entrapped inclusions.Another example of true centrifugal casting is shown in Figure 6.11which illustrates two methods that may be used for casting radial-engine cylinder barrels. The horizontal method of casting is similar to the process followed in the casting pipe lengths, and the inside diameter is a true cylinder requiring a minimum amount of machining. In vertical castings the inside cavity takes the form of a paraboloid as illustrated by the figure. The slope of the sides of the paraboloid depends on the speed of rotation, the dotted lines at A representing a higher rotational speed than shown by the paraboloid B, in order to reduce the inside diameter differences between the top and bottom of the cylinder, spinning speeds are higher for vertical than for horizontal casting. Semicentrifugal CastingIn semicentrifugal casting, the mole is completely full of metal as it is spun about its vertical axis and risers and cores may be employed. The center of the casting is not so dense and inclusions and entrapped air are often present. This method is normally used for parts in which the center of the casting will be removed by machining. The stack mold shown in Figure 6.12 can produce five semicentrifugal cast track wheels. The number of casting made in a mold depends on the size of the casting and the convenience in handling and assembling the molds. Rotational speeds for this form of centrifugal casting are not so great as for the true centrifugal process. The process produces a dense structure at the outer circumference, while the center metal is usually removed.Centrifuging In the centrifuge method, several casting cavities are located around the outer portion of a mold, and metal is fed to these cavities by radial gates ties are filled under pressure from the centrifugal force of the metal as the mold is rotated. In Figure 6.13 are shown five casting made in one mold by this process. The internal cavities of these casting are irregular in shape and are formed by dry-sand cores. The centrifugal method, not limited to symmetrical objects, cam produce castings of irregular shape, such as bearing caps or small brackets. The dental profession uses this process for casting gold inlays.PRECISION OR INVESTMENT CASTING Precision or investment casting employs techniques that enable very smooth, highly accurate castings to be made from both ferrous and nonferrous alloys. Figure 6.14 shows a small investment casting made from a chrome-molybdenum steel alloy. No other casting method, other than die casting, can assure production of so intricate a part. The process is useful in casting unmachinable alloys and radioactive metals. There are a number of processes employed, but all incorporate a sand, ceramic, plaster, or plastic shell made from an accurate pattern into which metal is poured. Although most castings with masses over 45kg. Advantages of precision or investment techniques are: (1) intricate forms with undercuts can be cast; (2) a very smooth surface is obtained with no parting line; (3) dimensional accuracy is good; (4) certain unmachinable parts can be cast to preplanned shape; and (5) it may be used to replace die casting where short runs are involved. On the other hand, the process is expensive, is limited to s
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