Guide to Casting and Molding Processes

Transcription

1 Guide to Casting and Molding Processes Understanding the metalcasting basics can help you design for manufacturability and utilize processes that meet your specific requirements. Fred Schleg (retired), Cast Metals Institute, Des Plaines, Illinois David P. Kanicki, Publisher The versatility of metalcasting is demonstrated by the number of casting and molding processes currently available. This wide range of choices offers design engineers and component users enormous flexibility in meeting their metalforming needs. Each process offers distinct advantages and benefits when matched with the proper alloy and application. When reviewing these processes and determining which best suit your needs, consider the following: required quality of the casting surface; required dimensional accuracy of the casting; number of castings required per order; type of pattern and corebox equipment needed; cost of making the mold(s); how the selected casting process will affect the design of the casting. Molding processes also can be broken into three general categories: sand casting processes; processes that use permanent molds; ceramic, plaster and special processes. (1 de 23)16/10/ :36:10 p.m.

2 Molten metal is manually poured from a ladle into a green sand mold on a conveyor line. SAND CASTING PROCESSES Fundamentally, a mold is produced by shaping a suitable refractory material to form a cavity of the desired shape, such that liquid metal can be introduced into this cavity. The mold cavity has to retain its shape until the molten metal has solidified and the casting is separated from the mold. This sounds easy enough to accomplish, but depending on the choice of metal, certain definite characteristics are demanded of the mold. When granular refractory material, such as silica, zircon, olivine or chromite sands are used, the mold must be: strong enough in its construction to sustain the weight of the metal; constructed to permit any gases formed within the mold and mold cavity to escape into the air; resistant to the erosive action of a rapidly moving stream of molten metal during pouring and the high heat of the metal until the casting is solid; collapsible enough to permit the metal to contract without undue restraint during solidification; able to cleanly strip away from the casting after the casting has sufficiently cooled; economical, since large amounts of refractory material are used. Green Sand Molding (2 de 23)16/10/ :36:10 p.m.

3 The most common molding process used to make metal castings is the green sand process. In this process, a granular refractory mineral is coated with a mixture of bentonite clay, water and, in some cases, other additives. When the coated grains of refractory are compacted around the pattern, they are held together by the clay and water "glue." Thus, when the pattern is removed, the mold cavity retains the shape of the pattern surfaces (Fig. 1). Fig. 1. This schematic shows the main features of a green sand mold, the most common molding process used to make metal castings. The granular refractory mineral most commonly used in green sand molding is silica sand. Silica sand is used most often because of its abundance and availability in the U.S. and its cost effectiveness when compared to other granular refractory minerals. More costly refractory minerals, such as zircon, olivine, chromite, mullite and carbon sands are often used for special applications. Because the surface of the metal castings is in immediate contact with the sand mold, the quality of the casting (especially its surface) will reflect the quality of the molding sand. For this reason, the goal of every foundry is consistent and close control of the sand mixture. Most foundries use extensive sand test procedures and automated sand preparation systems to aid in attaining this goal. Following mixing or mulling, the green sand is ready for molding. There are various methods used to compact the molding sand around the pattern. Method selection is dependent upon the desired mold rigidity, which also determines its ability to hold casting dimensions. The material used to build the pattern is determined, in part, by the method of compaction. In hand molding, the molder physically compacts the sand around the pattern or uses pneumatic hand tools to compact the sand into a mold. Wood and plastic patterns can be used with this method of compaction. In some cases, the pattern is loose that is, it is not attached to a pattern board or plate. This type of molding usually is done for one-of-a-kind or larger castings. If the pattern is mounted on a pattern board or plate, the molding process can be sped up. A mechanical force on the molding sand will produce better compaction than hand molding. The mechanical force can be induced by slinging, jolting, squeezing or, a more recent innovation, by (3 de 23)16/10/ :36:10 p.m.

4 impact/impulse. A sand slinger uses centrifugal force to throw the green sand against the pattern. The operator manipulates the slinger over the pattern, which is in the flask, and builds up layers of molding sand. This action compacts the sand. The sand slinger is used primarily for large castings, which are made in a flask or, in some cases, in a pit. Machine molding can be of several types: jolt or squeeze, jolt and squeeze or impact/impulse. With these types, a molding machine is used to make the molds. The casting size is dependent largely on the size of the molding machine. In the case of smaller castings, multiple castings can be made in one mold. The term "high-density" molding refers to sand molds that are compacted with pressure equal to or greater than 100 psi. These pressures usually are achieved consistently on automatic molding machines (Fig. 2). These machines may use a jolting and then a squeezing action or may rely solely on squeezing for consistent compaction. More recently, a form of compaction called "impact/impulse molding" has been developed and is used in some foundries. It is said to offer better uniformity of compaction and density in green sand molds. Fig. 2. In this high-pressure, squeeze molding machine, a mechanical force compacts sand with pressure equal to or greater than 100 psi. In impact/impulse molding, the molding sand is blown into the flask and compacts over the pattern surface. The pressure under which this is accomplished can be increased to improve the molding sand s compaction. After the flask has been filled with molding sand, an impact/impulse pressure (40-75 psi) is exerted against the molding sand, which, in turn, further compacts the sand. Due to the permeability of the molding sand, this pressure is distributed throughout and develops a more uniformly compacted mold. In addition to the systems already discussed, some high-pressure molding machines are capable of (4 de 23)16/10/ :36:10 p.m.

5 making molds that are parted vertically (Fig. 3). These machines are automated and compact the molding sand by squeezing. Normally these machines are used for high-production runs but can be used for short runs if necessary. Fig. 3. Vertically parted molding machines, normally run for highproduction runs, are automated and compact molding sand by squeezing. Following are some important points to review when considering the green sand molding process: for many metal applications, green sand processes are the most cost-effective of all metalforming methods; these processes readily lend themselves to automated systems for high-volume work as well as short runs and prototype work; in the case of hand molding, sand slinging, manual jolt or squeeze molding, wood or plastic pattern materials can be used. High-pressure, high-density molding methods almost always require metal pattern equipment; high-pressure, high-density molding normally produces a well-compacted mold, which yields better surface finishes, casting dimensions and tolerances; the properties of green sand are adjustable within a wide range, which makes it possible to use this process with all types of green sand moldmaking equipment and for the majority of alloys poured. Titanium and manganese-steel castings cannot be produced in silica sand molds. Skin-Dried Molding One variation of green sand molding is the "skin-dried" mold. In this type of molding, the skin of the mold cavity or parting line is heated to a temperature in which the water in the molding sand is driven back into the mold. This movement of the water, in effect, leaves a dry (5 de 23)16/10/ :36:10 p.m.

6 sand layer on the parting line. Many times, a special refractory coating is brushed or sprayed onto the mold surfaces that will be in contact with the molten metal. The refractory used in this coating is usually a finely crushed silica, zircon, chromite or mullite. This coating provides the mold surface with high-temperature resistance to the liquid metal and enhances the separation of the molding sand from the casting. Skin-dried molds are preferred for the production of large, heavy castings produced in ferrous alloys. Dry Sand Molding Another variation of green sand molding is "dry sand" molding. This method is similar to skin-dried molding except the mold is placed into an oven, and the water is baked out of the entire mold. Sometimes a petroleum binder is used, resulting in a high-strength mold after oven baking. Dry sand molds are very durable and may be stored for a relatively long time before pouring. Again, this molding process is normally used for larger castings, and mold size is limited to the size of ovens used to dry the molds. Because of the oven baking, the process is limited by its poor productivity. A refractory coating can be applied to the surface of the mold cavity for the same reasons as mentioned in skin-dried molding. Points to consider when using these latter two processes include the following: both offer a good surface finish on castings; many mold-related casting surface defects can be eliminated due to the removal of the water from the surface of the mold; both of these molding processes provide a stable mold, thus dimensional tolerances are improved; wood or plastic patternmaking materials can be used; when producing large ferrous alloy castings, both processes provide a relatively inexpensive molding method with good results. However, these processes generally cannot be considered for high- or medium-production runs; with both processes, especially dry sand molding, energy costs used to dry the sand add to the overall cost. Floor and Pit Molding Floor and pit molding techniques are used when the casting is too large to be made in a conventional jolt-squeeze or automatic molding machine. Pit molding is used when the castings are too large to be made in flasks. Many times these molds can take up to days and weeks to be made. In this type of molding, a pit is dug in the foundry floor. The mold is formed by assembling mold sections outside of the pit. These mold sections can be made of dry sand or chemically bonded systems. These sections are held in place by anchors and backing sand rammed behind them. When the mold is finished, the pit mold is covered with a cope flask. These two molding processes are used only to make large ferrous (iron and steel) castings that can weigh more than 200 tons. Chemical Setting Systems This category of sand casting and molding processes is used widely throughout the foundry industry (6 de 23)16/10/ :36:10 p.m.

7 because of the economics and improved productivity each offers. Each process uses a unique chemical binder and catalyst to cure or harden the mold. They do not use heat, reducing the overall cost of these processes. Chemical setting systems are currently being used to produce both cores and molds, which form the external casting surfaces. In many cases, the same process can be used to produce the entire casting. Very intricately shaped castings can be produced with self-setting systems. Shell Process Shortly after World War II, a process requiring heat to cure a core or mold composed of sand grains coated with a thermosetting plastic resin was brought to this country. This molding method was originally called the Croning Process, but today it is better known as the shell process. Sand coated with the thermosetting resin is dumped, blown or shot into a metal corebox or over a metal pattern, which has been heated to at least 450F (232C). Shell molds (Fig. 4) are made in halves that are glued or clamped together before pouring. Cores, on the other hand, can be made whole, or, in the case of very complicated coring applications, are glued together. Fig. 4. Shell molds, which provide an excellent surface and dimensional accuracies in castings, are made in halves that are glued or clamped together before pouring. Pictured, a worker conducts final quality checks on molds. (7 de 23)16/10/ :36:10 p.m.

8 The term "shell" came from the fact that, in most cases, the core is hollow, or, in the case of the shell mold, has a relatively thin mold wall. The thickness of the core or mold wall is dependent on the temperature and amount of molten metal that will surround the core or must be contained by the mold. The shell thickness is determined by the length of time that the coated sand fills the entire corebox or is kept over the pattern. At the proper time, the corebox or pattern is inverted, allowing the coated sand not affected by the heat to drain out of the center of the core or away from the mold. This action leaves the shell adhering to the corebox or pattern. In the shell process, cores and molds can be used together or in conjunction with other molding and coremaking processes. Shell molds are widely used throughout the foundry industry, and despite the required energy and metal tooling, their use continues to grow. Benefits of the shell process include: an excellent core or mold surface, resulting in good casting surfaces; good dimensional accuracy in the casting because of mold rigidity; storage for indefinite periods of time, which improves just-in-time delivery; high-volume production; versatility in the use of refractory materials other than silica sand; a savings in materials through the use of hollow cores and thin shell molds. In order to improve productivity and eliminate the need for heat-to-cure mold and core binders, foundry binder manufacturers have developed a series of binders, which many call nobake binders. As with the shell systems, the nobake binder systems can be used to produce either cores, core molds or just molds (Fig. 5). (8 de 23)16/10/ :36:10 p.m.

9 Fig. 5. Combined with sand, nobake binders improve productivity and eliminate the need for heat-to-cure mold and core binders. Pictured, a continuous mixer fills a flash with chemically bonded sand. The nobake binder systems can be divided into two main categories: those in which a gas catalyst is used to cure the binder and those in which the binder is cured by a liquid catalyst. Gas Catalyst Systems The oldest of these systems is the sodium silicate/carbon dioxide (CO2) process. In this system, the refractory material is coated with a sodium silicate-based binder. In some cases, other materials that enhance the collapse of cores and molds have been added to the mixture. For cores, this mixture can be hand-rammed, blown or shot into the corebox. For molds, the sand mixture can be compacted manually, jolted or squeezed around the pattern in the flask. After compaction, CO2 gas is passed through the core or mold. The CO2 chemically reacts with the sodium silicate to cure, or harden, the binder. This cured binder then holds the refractory in place around the pattern. After curing, the cores are removed from the corebox or the pattern is withdrawn from the mold. There are several less frequently used variations of the sodium silicate processes that are selfhardening without the use of CO2 gas. These methods require either ferrosilicon fines, Portland cement or an ester to harden the mold. (9 de 23)16/10/ :36:10 p.m.

10 From the metalcasters point of view, the CO2 process is one of the most environmentally acceptable of the chemical processes available. Despite this, the process presents some production difficulties that have limited its use. Chief among these is short bench life, because the binder is very hygroscopic and readily absorbs water, which weakens it. Also, because the binder creates such a hard, rigid mold wall, shakeout and collapsibility characteristics can slow down production. Despite these obstacles, metalcasters continue to overcome these production shortcomings because the process offers the following significant benefits: when used for making cores, the CO2 process can be automated for high-speed and long production runs; a hard, rigid core and mold are typical of the process, which gives the casting good dimensional tolerances; good casting surface finishes are readily obtainable; wooden and plastic pattern and corebox materials can be used, but metal tooling generally is used for high-production runs; a wide range of core and mold sizes can be produced with the CO2 process; this binder is one of the least gas-generating of the core and mold processes, reducing the possibility of defects. Coldbox and Sulfur Dioxide (SO2 ) Processes Two other binder systems that use a gas as the catalyst are the coldbox and SO2 processes. Although these are more often used as core processes, they are covered here because both systems have and can be used to produce molds as well. Both of these processes use a phenolic urethane resin as the binder each uses a different gas as the catalyst. In the coldbox process, an amine gas is used to catalyze the binder. Because these processes were developed to reduce overall costs, improve productivity and meet other casting challenges, both offer the following benefits for the casting user: very good dimensional accuracy of cores and molds made with both coldbox and SO2 systems; excellent casting finishes of both the external and cored surfaces of the castings; adaptability for high-production runs since the production cycles are very short; excellent shelf life of cores and molds. Liquid Catalyst Systems Also within the chemical, self-setting family of molding methods is a series of nobake binder systems in which a liquid catalyst is used. The refractory sand is coated with the binder and then the catalyst is added. As soon as the binder and catalyst combine, a chemical reaction takes place, which hardens (cures) the binder. The hardening action can be shortened or lengthened based on the amount of catalyst used, the temperature of the refractory sand or the binder and catalyst. The nobake, air-set sand systems are mixed in a continuous, high-intensity mixer and deposited directly from the mixer into the corebox or into a flask and over the pattern. Although these sand systems have good flowability, some form of compaction or vibratory device is used to get good (10 de 23)16/10/ :36:10 p.m.

11 compaction or densification of the sand. After a period of time, the core or mold has cured sufficiently to allow them to be stripped from the corebox or pattern without distortion. The cores and molds are then allowed to sit and cure thoroughly. After curing, the cores and molds can accept a refractory wash or coating to help protect the refractory sand from the heat and erosion action of molten metal as it enters the mold cavity. One of the advantages of these nobake processes can be seen when making intricate core assemblies. In many applications, because of the complexity of the internal passageways of a casting, highly configured core assemblies require much time-consuming handwork. Today, if the size of the assembly permits, "take-down" coreboxes can be used, which allow these core assemblies to be produced as one integral piece. Nobake air-set systems do not always lend themselves to high production because of the time necessary to thoroughly cure the core or mold. But in many applications, castings of all sizes and complexities can and are being made using these processes. In the case of complex casting shapes, the mold can be assembled with cores to shape the outside surfaces and the internal passageways of the casting at the same time. Along with the advantages already highlighted, these nobake systems provide other positive features such as: wood and, in some cases, plastic patterns and coreboxes can be used; due to the rigidity of the mold, good casting dimensional tolerances are readily achievable; casting finishes are very good; most of the systems have excellent shakeout; cores and molds can be stored indefinitely. Vacuum Processes Two unique processes use a vacuum to assist the molten metal into the mold. These are patented processes, and their use is limited to those foundries holding the patent or license to use the process. The first uses a silicon carbide tube, which is attached to a shell or nobake mold. The mold and one end of the tube are held in a sealed metal container. The container with the mold in it is placed over a furnace that contains molten metal. The end of the silicon carbide tube not attached to the mold is submerged into the molten metal bath. A vacuum is drawn in the container, and the molten metal is literally "sucked" up into the mold. The vacuum is maintained until the casting has solidified, after which it is released, and the remaining liquid in the tube drains back into the furnace. The second, and most recently developed process, uses the same vacuum- assist, mold-filling operation. The main difference is that, instead of using the silicon carbide tube, a portion of the mold is submerged into the bath of liquid metal. This process is called the CLAS process. With the CLAS process, the mold cross-sectional area is a circle. The two halves of the mold are glued together, and once again, the shell molding or nobake molding process is used. The drag half of the mold has openings that are connected with the casting cavities in the mold. The completed mold is then "screwed" into a round metal chamber. The metal chamber has internal threads, which, as the (11 de 23)16/10/ :36:10 p.m.

12 chamber rotates, cut into the circumference of the mold, usually just below the parting line of the mold. The chamber and mold are then swung over and lowered into the furnace holding the liquid metal. The container and mold are lowered until a portion of the drag half of the mold is submerged into the bath of liquid metal. A vacuum is then drawn in the container and the liquid metal is pulled up through the openings into the mold cavities. When the castings have solidified, the vacuum is released and the mold withdrawn from the bath. The container is then rotated in reverse, and the mold is "unthreaded" from the container. A normal shakeout then takes place. A variation of the process that uses a ceramic shell mold also was developed. While this process is best used for long production runs with small- to medium-sized castings, CLAS offers the following benefits: flow rate into the mold cavity is accurately controlled by the amount of vacuum drawn, improving overall casting soundness; only clean metal is drawn into the mold cavities, reducing the potential for inclusions in the castings; microstructure of the castings is said to be better, thus imparting better mechanical properties; castings have good surface finishes; dimensional tolerances are good. Unbonded Sand Processes Unlike the sand casting processes that use various binders to hold the sand grains together, two unique processes use unbonded sand as the molding media. These include the lost foam process and the V-process. Lost Foam Casting (also known as expendable pattern casting, or EPC) In this process (Fig. 6), the pattern is made of expendable polystyrene (EPS) beads. For high-production runs, the patterns can be made in a die by injecting EPS beads into a die and bonding them together using a heat source, usually steam. For shorter runs, sheets of EPS can be made in the pattern shop. Pattern shapes are cut out of the sheets using conventional woodworking equipment, and then assembled with glue. In either case, internal passageways in the casting, if needed, are not formed by conventional cores but are part of the mold itself. (12 de 23)16/10/ :36:10 p.m.

13 Fig. 6. At top, the major steps in producing a lost foam casting include foam pattern molding, assembling the foam patterns on a cluster, coating the foam pattern with a refractory, placing the cluster in a flask and filling sand or other media around the cluster, metal pouring and shakeout of the solidified casting. At right, a diesel engine component produced at Willard Industries, Inc., Cincinnati, illustrates the stages of the process from pattern (r) to machined casting (l). The pattern is coated with a refractory wash, which covers both the external and internal surfaces of the pattern. With the gating and risering system attached to the pattern, the assembly is suspended in a one-piece flask. The flask is then placed onto a compaction or vibrating table. As the dry, unbonded sand is poured into the flask and pattern, the compaction and vibratory forces cause the sand to flow and become rigid. The sand flows around the pattern and into the internal passageways of the pattern. After compaction, the mold is moved to the pouring area. As the molten metal is poured into the mold, it replaces the EPS pattern, which vaporizes. The sand stays in place due to its rigidity. After the casting has solidified, the mold is moved to the shakeout area where the unbonded sand is dumped out of the flask leaving the casting with an attached gating system. The sand forming the internal passageways in the casting drains out at the same time. In the case of large castings, the coated pattern is first covered with a facing of a chemically bonded sand. The facing sand is then backed up with a weaker chemically bonded sand or green sand. The lost foam process offers the following advantages: no size limitations for castings; improved surface finish of metal castings due to the pattern s refractory coating; no fins around coreprints or parting lines; in most cases, separate cores are not needed; excellent dimensional tolerances. V-Process The primary differences between the V-Process and conventional sand casting are that the V-Process uses a thin plastic film heated to its deformation point and then vacuum-formed over a (13 de 23)16/10/ :36:10 p.m.

14 pattern on a hollow carrier plate (Fig. 7). Like the lost foam process, the V-Process uses dry, freeflowing, unbonded sand to fill the special flask set over the film-coated pattern. The sand contains no water or organic binders and is kept under a vacuum during the molding process. Fig. 7. This vacuum-molding unit at Frog, Switch & Manufacturing Co., Carlisle, Pennsylvania, is the largest in the world. Because permeability of the sand is not a concern, as in green sand, finer sand can be used to achieve improved casting surfaces. Slight vibration quickly compacts the fine grain sand to its maximum bulk density. The flask is then covered with a second sheet of plastic film. The vacuum is drawn, and the sand becomes rigid. Releasing the vacuum originally applied to the pattern permits easy stripping. The other half of the mold is fashioned in the same manner. The cope and drag are then assembled forming a plastic-lined mold cavity. Sand hardness is maintained by holding the vacuum within the mold halves at mm Hg. As molten metal is poured into the mold, the plastic film melts and is replaced immediately by the metal. After the metal solidifies and cools, the vacuum is released and the sand falls away. The V-Process offers the following benefits: smooth surface finish; excellent dimensional accuracy; zero draft; thin-wall capabilities; excellent reproduction of details; (14 de 23)16/10/ :36:10 p.m.

15 low tooling costs; unlimited pattern life because only plastic contacts the pattern there is no sand to cause wear, reduce surface finish or to open up the tolerances; "user-friendly" patterns easy revisions to patterns, no metal tooling, good for prototypes; fast turnarounds and short lead times. Permanent Mold Casting At least four families of molding and casting processes can be categorized as "permanent mold" processes. These include diecasting, permanent mold casting, squeeze casting, graphite mold and centrifugal casting. Unlike sand casting processes in which a mold is destroyed after pouring to remove the casting, permanent mold casting uses the mold repeatedly. Diecasting Diecasting is used to produce small- to medium-sized castings at high-production rates. The metal molds are coated with a mold surface coating and preheated before being filled with molten metal. A premeasured amount of liquid metal is forced under extreme pressure from a shot chamber into the permanent mold or die. Castings of varying weights and sizes can be produced. Nearly all die castings are produced in nonferrous alloys with limited amounts of cast iron and steel castings produced in special applications. Die castings and the diecasting process (Fig. 8) are suitable for a wide variety of applications in which high part volumes are needed. Benefits include: excellent mechanical properties and surface finish; dimensional tolerances of in.; recommended machining allowances of in.; thin section castings. Fig. 8. This schematic (l) shows the diecasting process, in which metal is ladled into shot sleeves and forced under extreme pressure into the permanent mold or die (r). Permanent Molding Another form of permanent molding is where the molten metal is poured into the mold, either directly or by tilting the mold into a vertical position. In this process, the mold is made in two halves, male and female, from cast iron or steel. If cores are to be used, they can be (15 de 23)16/10/ :36:10 p.m.

16 metal inserts, which operate mechanically in the mold, or sand cores, which are placed in the molds before closing. If sand cores are used, the process is called "semi-permanent molding." The mold halves are preheated and the internal surfaces are then coated with a refractory. If static pouring is to be used, the molds are closed and set into the vertical position for pouring; thus, the parting line is in the vertical position. In the case of tilt pouring, the mold is closed and placed in the horizontal position at which point molten metal is poured into a cup(s) attached to the mold. The mold is then tilted to the vertical position, allowing the molten metal to flow out of the cup(s) into the mold cavity. The various permanent mold techniques gravity pour, tilt pour, semi-permanent molding offer a variety of advantages for a variety of metalforming applications. Benefits include: a casting with superior mechanical properties is produced because the metal mold acts as a chill; the castings are uniform in shape and have excellent dimensional tolerances because the molds are made of metal; excellent surface finishes are obtainable; the process lends itself to high-production runs; sections of the mold can be selectively insulated or cooled, which helps control the solidification and improves overall casting properties. Low-Pressure Permanent Molding (LPPM) The term "low pressure" means that the liquid metal is forced into the mold rather than poured. The amount of pressure, from 3-15 psi, is dependent on the casting configuration and the quality of the casting desired. When pressure is used to fill the mold cavity, this pressure also is used to feed shrinkage in the casting. Metal can be fed directly into the casting or through a gating system. When internal passageways are required, they can be made by either mechanically actuated metal inserts or sand cores. A low-pressure permanent mold machine is depicted in Fig. 9. Fig. 9. This schematic illustrates the principal components of a low-pressure (16 de 23)16/10/ :36:10 p.m.

17 permanent mold machine. "Low pressure" means that the liquid is forced into the mold rather than poured. Nearly all of the LPPM castings produced are made of aluminum, other light alloys and, to a lesser extent, some copper-base alloys. Because it is a highly controllable process, LPPM offers the following advantages: when liquid metal is fed directly into the casting, excellent yields are realized; odd casting configurations and tooling points for machining can be placed in areas where gates and risers normally would be placed; when liquid metal is fed directly into the casting, it reduces the need for additional handwork; the solidification rate in various sections of the casting can be controlled through selective heating or cooling of the mold sections, thus offering excellent casting properties; surface finish of castings is good to excellent. It should be mentioned that the processes using metal permanent molds are used primarily for casting aluminum and magnesium alloys. However, copper-base alloys also are diecast and poured statically in permanent molds. Some small, thin-section steel castings also are diecast. Graphite Mold Casting Another form of permanent molding uses molds constructed of graphite. This process is used mostly for specialized types of castings, such as railroad car wheels, and is usually coupled with a special pouring operation, such as pressure pouring. In addition, the geometry of the casting must be such that solidification shrinkage takes away from the graphite mold to prevent hot tearing of the casting and damage to the mold. Graphite molds have been used effectively with the family of zinc-aluminum alloys in certain applications. Graphite permanent molds offer the following significant advantages in specialized applications including: the chilling effect of the graphite mold minimizes risering; this pronounced chilling effect enhances the physical and mechanical properties of the casting; dimensional accuracy is excellent, and machining is not required on many of the castings produced in graphite molds; casting surface is excellent. Squeeze Casting/Semi-Solid Casting (SSM) These two processes are relatively new to the metalcasting family. Squeeze casting is a technique in which molten metal is metered into a metal permanent mold die cavity and, as the metal solidifies, pressure is applied. Pressure of 8000 psi or more is required for molding in this process. Generally, the squeeze casting process is used for high-production runs in aluminum alloys. If a particular application calls for relatively small castings with well-defined geometries, squeeze casting provides the following advantages: reduction of shrinkage or gas voids; (17 de 23)16/10/ :36:10 p.m.

18 elimination of dimensional shrinkage; enhancement of mechanical properties; excellent surface finishes; significantly less metal is required compared to hot forging or conventional casting. Semi-solid metalcasting is similar to high-pressure diecasting in that metal is injected into a reusable steel die under pressure (Fig. 10). However, rather than using liquid metal, SSM uses metal that is about 40% liquid and 60% solid (Fig. 11). Currently, aluminum is the major alloy used with this process, and the major users are automakers. Fig. 10. This schematic depicts the various stages of the semi-solid casting process, in which metal is injected into a reusable steel die under pressure. Fig. 11. An aluminum billet is heated to the consistency of ice cream for use in the semi-solid process. Centrifugal Casting Generally speaking, the centrifugal casting process can be categorized as a (18 de 23)16/10/ :36:10 p.m.

19 permanent mold method of casting, though variations of the process use other materials. It has been used for many years as an economical method for producing cylinders and tubes. With centrifugal casting, a permanent metal mold revolves at very high speeds in a horizontal, vertical or inclined position as the molten metal is being poured. Centrifugal castings can be made in almost any required length, thickness and diameter. Because the mold forms only the outside surface and length, castings of many different wall thicknesses can be produced from the same size mold. The centrifugal force of this process keeps the casting hollow, eliminating the need for cores. Horizontal centrifugal casting machines are used for the production of pipe and tubing up to 40 ft long. The length and outside diameter are fixed by the mold cavity dimensions while the inside diameter is determined by the amount of molten metal poured into the mold. Castings other than cylinders and tubes also can be produced in vertical casting machines. Castings such as controllable pitch propeller hubs, for example, are made using this variation of the centrifugal casting process. Molds for centrifugal castings generally are divided into three classifications. One is a permanent mold made of steel, iron or graphite. This type of mold usually is coated on the inside surface with a thin refractory wash to increase mold life. The mold is preheated before the coating application to dry the coating and improve its adherence to the mold surface. A second type of mold is a rammed mold. It consists of a metal flask, usually steel, lined with a layer of refractory molding mix, which has been rammed into place. The lining is coated with a refractory wash and then baked until dry and hard. A third type of mold is the spun or centrifugally cast mold. It consists of a metal flask into which a predetermined weight of refractory material in slurry form is poured. The flask is rotated rapidly and the refractory material is centrifuged onto the wall of the flask. The flask rotation is then stopped, and the liquid part of the slurry is drained off. This leaves a mold with a refractory coating, which is then baked until dry prior to use. Molten metal is then poured into a rotating mold where it is accelerated to mold speed. Centrifugal force causes the metal to spread over and cover the mold surface. Continued pouring of the molten metal increases the thickness to the intended cast dimensions. Rotational speeds vary but sometimes reach more than 150 times the force of gravity on the outside surface of the castings. Once the metal has been distributed over the mold surface, solidification begins immediately. Most of the heat in the molten metal is extracted through the mold. This induces progressive solidification. During solidification, the liquid head of metal feeds the solid-liquid interface as it progresses toward the bore. This, combined with the centrifugal pressure being applied, results in a sound, dense structure across the wall with impurities generally being confined near the inside surface. The inside layer of metal can be removed by boring if an internal machined surface is required. For specialized engineered shapes, centrifugal casting offers the following distinct benefits: (19 de 23)16/10/ :36:10 p.m.

20 any alloy common to static pouring can be produced centrifugally; mechanical properties of centrifugal castings are excellent; cleaner, denser metal is found on the outside of the casting, and the impurities are on the inside surface, where they can be bored out. CERAMIC, PLASTER & SPECIAL CASTING PROCESSES This family of casting processes is unique in that alternative materials are used as molding media, most noticeably ceramic and plaster. These processes offer a high degree of precision in regard to dimensions as well as excellent surface finishes. Investment Casting The investment casting process (Fig. 12) was one of the first processes used to make metal castings. The process has been described as the lost wax process, precision casting and investment casting. The latter name generally has been accepted to distinguish the present industrial process from artistic, medical and jewelry applications. The basic steps of the investment casting process are: 1. Production of heat-disposable wax or plastic patterns; 2. Assembly of these patterns onto a gating system; 3. "Investing," or covering the pattern assembly with ceramic to produce a monolithic mold; 4. Melting the pattern assembly to leave a precise mold cavity; 5. Firing the ceramic mold to remove the last traces of the pattern material while developing the high-temperature bond and preheating the mold ready for casting; 6. Pouring; 7. Knockout, cutoff and finishing. (20 de 23)16/10/ :36:10 p.m.

21 Fig. 12. Investment casting, or the lost wax process, offers a high degree of precision in regard to dimensions as well as excellent surface finishes. The steps include: producing heat-disposable wax or plastic patterns and assembling them onto a gating system, or tree (a); covering the pattern assembly with ceramic to produce a monolithic mold (b); melting the assembly to leave a precise mold cavity; firing the mold to remove any traces of the pattern material while developing the bond and preheating the mold ready for casting; pouring (c); and knockout, cutoff and finishing. The patterns are produced in dies. For the most part, the patterns are made of wax, however, there are patterns that are made of plastic or polystyrene. In all cases, the patterns are made in injection molding machines. When cores are required, they are made of soluble wax or ceramic materials. In the case of soluble wax cores, they are removed from the pattern before the pattern is "invested." Ceramic cores, on the other hand, stay in the mold during the entire casting process and are removed during the casting cleaning process. There are two types of molding processes: the block (solid) mold process and the ceramic shell process. The more common of these processes is the ceramic shell. The ceramic shell is built around a tree assembly by repeatedly dipping a pattern into a slurry. After dipping, a refractory aggregate, such as silica or zircon sand, is rained over the wet slurry coating. After each dipping and stuccoing is completed, the assembly is allowed to thoroughly dry before the next coating is applied. Thus, a shell is built up around the assembly. The thickness of this shell is (21 de 23)16/10/ :36:10 p.m.

22 dependent on the size of the castings and temperature of the metal to be poured. After the ceramic shell is completed, the entire assembly is placed into an autoclave or flash fire furnace at a high temperature. The shell is heated to about 1800F (982C) to burn out any residual wax and to develop a high-temperature bond in the shell. The shell molds can then be stored for future use or molten metal can be poured into them immediately. If the shell molds are stored, they have to be preheated before molten metal is poured into them. The vast majority of investment castings weigh less than 5 lb, but there is a distinct trend to produce larger castings in the lb range. Castings weighing up to 800 lb have been poured in this process. Some of the advantages of investment casting are: excellent surface finishes; tight dimensional tolerances; machining can be reduced or completely eliminated; lends itself to the production of titanium castings as well as the other superalloys. Because the tooling cost for the wax patterns is high, investment casting is normally used when high volumes are required. Ceramic Molding Another method of molding in which a ceramic material is used is simply referred to as ceramic molding. This process and its offshoots also are known as the Shaw Process, the Unicast Process, the Osborn-Shaw Process and the Ceramicast Process. Generally, these processes employ a mixture of graded refractory fillers (in some cases, hydrolyzed ethyl silicate and a liquid catalyst), which are blended to a slurry consistency. Various refractory materials can be used as filler material. The slurry is then poured over a pattern that has been placed in a container. First, a gel is formed in a pattern, and it is stripped from the mold. The mold is then heated to a high temperature until it becomes rigid. After the molds cool, molten metal is then poured into them, with or without preheating. The ceramic molding processes have proven effective with smaller size castings in short- and medium-volume runs. At the same time, these processes offer several advantages including: excellent surface finish; good casting dimensional tolerances; adaptability to intricate castings. Plaster Molding Plaster molding is used to produce castings of the lower melting temperature metals, such as the aluminum alloys. The four generally recognized plaster molding processes are: conventional plaster molding; matchplate-pattern plaster mold casting; Antioch process; (22 de 23)16/10/ :36:10 p.m.

23 foamed-plaster process. A slurry containing calcium sulfate, sometimes called gypsum, is poured into a flask, which contains the pattern. After the slurry has set, the pattern and flask are removed, and the drying cycle to remove the moisture from the mold begins. After the mold has been allowed to cool, the cores and mold are assembled. Most molds, after assembly, are preheated before pouring. Because these molds have very poor permeability, in many cases vacuum-assist or pressure usually is required during pouring. The plaster mold processes are especially suited for short run and prototype work with the lower temperature alloys, particularly aluminum. In addition to these benefits, plaster molding offers the following advantages: castings have especially smooth surfaces, and intricate designs and details are readily obtainable in plaster molds; dimensional accuracy of castings is good; because of the mold material and vacuum-assist, thinner wall castings can be produced; slow cooling of plaster molds minimizes warpage and promotes uniformity of structure and mechanical properties in the casting. Rheocasting and Thixomolding In 1976, a new metalforming process called Rheocasting was developed at the Massachusetts Institute of Technology. This process uses a phenomenon called thixotropy that involves the vigorous agitation of a semisolid metal to produce a highly fluid, diecastable alloy. Advantages of the process are reportedly longer die and chamber life, finer grained castings with fewer defects and greater economy (less loss) of metal fed to the diecasting machine. The use of nonmetallic materials to produce composites also was developed. Today, the principles of this process are used in the thixomolding process. The major difference between Rheocasting and thixomolding is that, in Rheocasting, the metal alloy completely liquifies and then cools. In thixomolding, the metal alloy is heated only to a "mushy" state between the liquidus and solidus states. Some advantages of thixomolding include: cast parts have less porosity than conventionally diecast parts; lower porosity levels allow parts to be heat treated to improve mechanical properties; there is improved material flow in thin-wall sections; component warpage is greatly reduced after the part is removed from the mold. For more information, see "Resources For Casting Designers & Buyers," p. 67. ECS (23 de 23)16/10/ :36:10 p.m.