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Die Casting (Permanent Mold)
Die Casting (Permanent Mold) The die casting process is a name given to metal casting processes that utilize metal molds or dies. There are really several distinct processes included under the general name. The die casting process actually has three main sub-processes. These are: (i) permanent mold casting, also called gravity die casting, (ii) low-pressure die casting, and (iii) high-pressure die casting. In North America, the term die casting is used to mean high-pressure die casting, but the general term embraces all three subcategories. The three processes differ mainly in the amount of pressure that is used to force the molten metal into the die. In permanent mold casting, the molten metal is poured into the mold and flows only at the force of gravity. The low-pressure process normally utilizes air pressures up to 1.5 MPa to force the molten metal into the die, although for special products the pressure can be as high as 7 MPa. The high-pressure die casting process subjects the molten metal to hydraulic pressures as high as 140 MPa. As this pressure on the metal is increased there is a corresponding reduction in the time required for the molten metal to fill the die. The die casting process involves pouring or injecting molten metal into hardened steel dies, where the metal solidifies to the desired casting shape. In the casting processes that utilize sand or plaster for molds, the mold is destroyed by the molten metal heat or when the casting is extracted from the mold. In all of the die casting processes, the dies can withstand the casting heat and are constructed with movable sections that allow easy removal of the solidified casting. Therefore, these molds are reusable and can be used for producing many thousands, or even millions, of castings. The advantages of such a process include high production rates, exceptional dimensional repeatability, low part costs, and less machining due to reduced casting finish stock. The disadvantages are design limitations due to steel dies, higher initial die cost, and longer lead time for die construction and for changes to the die caused by a casting design change.
1. Permanent Mold Casting In permanent mold casting, the molten metal is poured into the steel die and flows only at the force of gravity. For the most part, permanent mold castings are produced by pouring the molten metal into the top of a die that has been made in the desired shape of the casting. There are many deviations from this simple approach in actual casting practice. Sometimes, the metal is poured into the mold at the top, but a runner is cut into the die that conveys the molten metal to the side or bottom of the casting cavity itself, so that the metal flow during filling is from the bottom or side of the mold. This is done to optimize casting conditions
for the part being produced. Other times the metal may be poured into a basin beside the die and then the entire unit is tilted to achieve controlled filling of the mold. The flow of metal into the die cavity and the flow of heat from the metal to the die during solidification are two of the main criteria for successful casting production by the permanent mold process. Dies for the permanent mold process are produced from hardened steel. The heat transfer rate is higher than from sand or ceramic molds used in other processes and this provides advantages in casting cycle time. Often, a ceramic coating is applied to the steel to protect it from the molten metal and to control the heat transfer rate from the molten metal to the die at only a slight increase in cycle time. The solidification of the molten metal must be controlled to prevent solidification shrinkage cavities in the final product. Also, the solidification rate is managed to optimize the microstructure of the resulting casting to achieve the design objectives. For high production castings, the dies are often water-cooled, further decreasing the cycle time and reducing the cost for each part. Sometimes the casting design requires undercuts or hollow cavities in the casting that cannot be produced with conventional steel dies. In these cases, the die designer may choose a loose piece or an expendable core for the permanent mold process. Both of these options increase die and casting costs, but often the function of the product requires these features in the casting. When expendable cores are used, the process is called semipermanent mold casting. This subcategory of permanent mold casting refers to processes that utilize one or more expendable cores that are placed in the steel die before the production of each casting. The expendable core is then removed by vibration or heat treatment after the casting is removed from the die. Expendable cores for semipermanent mold casting are normally produced from sand with a binder to give them strength, similar to the cores utilized in sand casting processes. The advantages of the permanent mold and semipermanent mold casting processes are: reasonable piece costs resulting from the high production rates achieved with metal molds (especially water-cooled molds) compared to sand and investment casting, and lower investment required for equipment when compared to low-pressure and high-pressure die casting. In addition, the use of expendable cores in semipermanent mold casting permits great design flexibility for castings. One of the disadvantages of the permanent mold casting process is that metal dies are more expensive than patterns for sand casting or investment casting so the process is not economical for short runs. At low volume, it is difficult to overcome the high initial tooling cost and compete based on casting cost. In addition, since castings are filled with liquid metal under only the pressure of gravity, castings sections tend to be thicker in permanent mold casting than in 1
Die Casting (Permanent Mold) the low-pressure and high-pressure die casting processes. Also, where material properties are critical, emerging concepts in high-pressure die casting, such as squeeze casting and semisolid metal processing, are creating new competition for permanent mold casting. Typical parts produced in the permanent mold process include automotive parts such as aluminum pistons, steering knuckles, brackets, wheels, and pump impellers. Parts are also produced in zinc, brass, copper, lead, and even gray iron. Since the process has great design flexibility and is compatible with so many metals, the types of products that can be produced are almost unlimited.
Cavity
Air inlet Stalk Molten metal Furnace
Crucible
2. Low-pressure Die Casting Low-pressure die casting was developed in an attempt to eliminate hand ladling of the hot metal. This process also utilizes metal molds to produce castings, but the molten metal is pressurized to achieve faster or bettercontrolled filling of the mold. The process utilizes pressures up to 7 MPa for special products such as automotive wheels, but typically the pressure used is below 0.5 MPa. In low-pressure casting, the molten metal is contained in an insulated crucible or furnace which is pressure sealed. A tube passes vertically down through the furnace, with its lower end immersed in molten metal and its top flange sealed against the furnace lid. The die is mounted on the machine, over the furnace, and sealed to the open end of the tube. When air pressure is applied to the furnace, it displaces the molten metal, causing it to travel up the tube and into the die cavity as seen in Fig. 1. The cooling process takes place in a sequence starting at the extremities of the die, working back to the feed head, which is the mouth of the tube. Upon solidification of the casting, the unwanted metal is returned to the furnace by relaxing the air pressure; the die is opened, the casting is removed, and the casting cycle is repeated. Sometimes a vacuum is applied to the mold cavity to cause the metal to flow. Atmospheric pressure on the metal then forces the molten metal up into the bottom of the die and fills the part. Using pressure to fill the die cavity permits faster filling of the die than by gravity casting. Faster fill times are important because the molten metal loses temperature to the die as it flows past the die steel and the hotter the metal the thinner the section thickness that can be filled. For some products, the fill rate must be varied depending on the casting section thickness and very sophisticated computer-controlled, low-pressure machines are utilized. This additional capability is not free, however, and is not cost effective for every product. In that sense, low-pressure die casting is an enhanced process that is generally used for parts with 2
Figure 1 Low-pressure die casting.
premium requirements. This is the case for many automotive engine components such as heads, lower crankcases, and manifolds that require expendable cores because of the intricate internal passageways that are required. These parts must also be leak free. Many of these parts are produced by the low-pressure die casting process because of their size, complexity, and volume. Gravity and low-pressure, semipermanent mold casting are the primary methods used to produce these complex aluminum automotive components throughout the world. The decision to use gravity or low-pressure casting depends on product requirements and casting volume. Dies for low-pressure die casting are also produced from hardened steel, but they require more complex design and engineering to assure that pressurized molten metal is contained within the casting system. Detailed metal feed systems and connections to the molten metal bath are required as well as seals in the die where a vacuum is used. These features increase the cost and lead-time of dies when compared to gravity permanent mold dies. The dies are still less costly than those used in high-pressure die casting, however. The casting machines for the low-pressure process can also become fairly sophisticated as well, requiring higher capital investment for sophisticated pressure or vacuum control systems. These higher costs are offset by the higher value of complex castings that can be produced, as well as by higher production rates and improved casting fill capability, and they still cost much less than machines for high-pressure die casting. The advantages of low-pressure die casting are that casting quality is very good, especially in heavy wall
Die Casting (Permanent Mold) sections, and casting yield is 90–95% since no runners, risers, or gates are used. This results in much less trimming to remelt and corresponding savings in fuel cost. There is a much greater alloy choice than with high-pressure die casting, permitting the use of heat treatable alloys. The use of expendable cores in lowpressure die casting allows this process to cast parts that are impossible to produce in the high-pressure process. The disadvantages of low-pressure die casting are that it has a lower production rate than high-pressure casting and it is not normally suitable for casting smaller parts. The minimum wall thickness, while smaller than permanent mold casting, is still larger than for high-pressure die casting. Tooling cost is also higher than for the permanent mold process.
3. High-pressure Die Casting High-pressure die casting is a process in which molten metal is forced under pressure into a securely locked metal die cavity, where it is held by a powerful press until the metal solidifies. After solidification of the metal, the die is unlocked, opened, and the casting ejected. After removal of the casting, the die is closed and locked again for the next cycle. The injection of metal into the die cavity is completed in a fraction of a second. Often, while the molten metal is still held in the die, extremely high pressure is applied (called intensification pressure). This high pressure compresses any gas entrapped in the metal and feeds additional metal into the cavity to compensate for the shrinkage of the metal as it solidifies. Two types of systems are used for injecting the molten metal into the die. The hot chamber system is used with metals such as zinc, magnesium, and lead. The injection system of a hot chamber machine is immersed in the molten metal bath of the melting furnace. As the shot plunger moves, it forces metal through the nozzle and into the die. The cold chamber system is used for metals that melt at high temperatures, such as aluminum, brass, and magnesium. Magnesium parts can be produced using both systems, though usually small parts are produced in hot chamber machines and large parts in cold chamber machines since hot chamber machines are limited in size. There are also two injection systems used in the cold chamber process, horizontal and vertical injection. In the cold chamber process, the molten metal is poured, by hand or by automatic means, into a port of the cold chamber sleeve. A hydraulically operated plunger advances through this steel sleeve, sealing off the port, and forcing the metal into the die at high speed and pressure. After solidification of the casting, the plunger is retracted, the die opened, the casting
ejected, and the system is then ready for the next shot cycle. Higher pressure is used in this system than the hot chamber process. The production rate of a hot chamber machine is higher than that of a cold chamber machine because of the shorter time required during the pour operation. Typical zinc castings produced by the high-pressure die casting process are shown in Fig. 2. In addition to the conventional high-pressure die casting processes, several enhancements to the process have been developed in recent years. These enhancements include the use of vacuum systems to reduce entrapped gas, slower fill processes to eliminate turbulence during fill and permit the use of heat treatment to enhance mechanical properties of the castings, and the application of semisolid metal processing to produce pressure tight parts not normally able to be produced in high-pressure die casting. Each of these processes utilizes the fundamentals of high-pressure die casting, but with additional capabilities to produce high integrity parts. They also have developed unique names for the purpose of distinguishing themselves from the conventional processes of hot chamber and cold chamber die casting. Vacuum die casting utilizes a vacuum system on the die cavity to remove gas from the cavity prior to injection of the molten metal. The result is a high integrity part with very low levels of porosity and high mechanical properties. The process is used to produce critical components in light alloys, such as structural and safety components for motor vehicles. The process has higher costs than conventional die casting, but the parts produced justify the additional cost and they cannot be produced successfully in conventional die casting. Slow fill die casting is often called squeeze casting. This process is widely practiced throughout the world today for producing parts that must be heat treated to achieve the required mechanical properties. Many automotive components that have been converted from iron castings or weldments of castings and\or stampings are now produced in light alloys in the squeeze casting process. This process utilizes the advantages of the low-pressure die casting process, controlled filling, and directional solidification of the molten metal, as well as the advantages of highpressure die casting, high-pressure solidification, and fast cycle times. Parts produced in the squeeze casting process include steering and suspension components, e.g., alloy wheels, steering knuckles, and control arms, and air conditioning parts, e.g., compressor scrolls. Parts are produced with both vertical and horizontal shot systems. A schematic diagram of a vertical shot horizontal squeeze casting machine is shown in Fig. 3. Semisolid metal processing (SSM) is beginning to share some of the same markets as squeeze castings, as well as some of the smaller automotive components such as link arms, fuel rail parts, and drivetrain parts. In this process, the metal injected into the die is only 3
Die Casting (Permanent Mold)
Figure 2 Typical zinc die castings.
about 50% liquid. The process offers distinct advantages in that the feed metal fills the cavity in a manner that is less turbulent than in conventional die casting. Furthermore, the casting is partially solidified at the onset of the process, thus the solidification ‘‘journey’’ is a much shorter one. The technical capabilities of the process include high integrity of the castings, high mechanical properties, the ability to cast thin walls, good dimensional accuracy, surface conditions similar to high-pressure die casting, and suitability for heat treatment and welding. Semisolid casting and SSM materials are covered in detail in Semisolid Processing, and Casting of Semisolid Metals: Engineering Applications The advantages of high-pressure die casting include a higher production rate than with gravity or lowpressure casting. Also, the ability to produce castings with close dimensional control greatly reduces machining operations. Die castings have good surface finish, which is a prime requirement for plating, and much thinner wall thickness is possible reducing overall casting weight. Dies have a long life, reducing unit part costs, and more complex parts can be produced, thereby reducing the number of components required in an assembly. The disadvantage of high-pressure die casting is that 4
it is best suited to high volume parts. High tooling costs make short production runs uneconomical. Also, the internal porosity prevalent in conventional highpressure die castings makes producing pressure tight parts difficult, often requiring the use of vacuum die casting, squeeze casting or SSM casting. There are a limited number of alloys suitable for die casting and this restricts the heat treatment or welding of the finished castings. Iron or steel alloys are normally not die castable. There are restrictions in die casting on the casting size and wall thickness which eliminate the possibility of die casting some parts, but this has been reduced with the development of the new high integrity processes of vacuum die casting, squeeze casting, and SSM casting. Die casting machine and maintenance costs are higher than for other casting processes.
4. Concluding Remarks The term die casting is used to describe processes that utilize metal dies, or molds, to produce parts from various metals. These processes include gravity permanent mold casting, in which the liquid metal is poured into the die; low-pressure die casting, in which
Die Casting (Permanent Mold)
Figure 3 Vertical shot horizontal squeeze casting machine.
the metal is forced into the mold with air pressure; and high-pressure die casting, in which a hydraulic ram is used to inject the molten metal into the die at extremely high pressures. In North America, the term die casting is used to describe high-pressure die casting, while in most of the world the term die casting may be used to describe all of these processes that utilize metal molds. The die casting processes provide a great deal of flexibility for the production of metal components. Permanent mold and low-pressure die casting can produce parts from almost any metal in almost any shape. In the case of extremely high volume parts, high-pressure die casting provides high production rates and good dimensional repeatability. In recent years, enhanced high-pressure die casting processes such as vacuum die casting, squeeze casting and semisolid metal processing have created new opportunities for utilizing the advantages of high-pressure die casting for structural or leak free components.
Bibliography Bradney D D 1994 The NFFS Guide to Aluminum Casting Design: Sand and Permanent Mold. Non-Ferrous Founders’ Society, Des Plaines, IL Corbit S, DasGupta R 1999 Squeeze cast automotive applications and squeeze cast aluminum alloy properties. In: SAE Technical Paper Series, International Congress and Exposition. Society of Automotive Engineers, Inc., Warrendale, PA. Herman E A 1982 Die Casting Handbook. North American Die Casting Association, Rosemont, IL Jorstad J 1999 Selecting parts and material for conversion to SSM. In: Nussbaum A I (ed.) Semi-Solid Metal World Users’ Conference. Ormet Primary Aluminum Corporation, Hannibal, OH Midson S P, Young K P 1999 Technical support for high integrity processes. Diecasting World (June), 16–18 Prince Machine Corporation 1980 Low-pressure Manual. Prince Machine, MI, USA
W. A. Butler
5
Die Casting (Permanent Mold)
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 2147–2152 6
1. Permanent Mold Casting In permanent mold casting, the molten metal is poured into the steel die and flows only at the force of gravity. For the most part, permanent mold castings are produced by pouring the molten metal into the top of a die that has been made in the desired shape of the casting. There are many deviations from this simple approach in actual casting practice. Sometimes, the metal is poured into the mold at the top, but a runner is cut into the die that conveys the molten metal to the side or bottom of the casting cavity itself, so that the metal flow during filling is from the bottom or side of the mold. This is done to optimize casting conditions
for the part being produced. Other times the metal may be poured into a basin beside the die and then the entire unit is tilted to achieve controlled filling of the mold. The flow of metal into the die cavity and the flow of heat from the metal to the die during solidification are two of the main criteria for successful casting production by the permanent mold process. Dies for the permanent mold process are produced from hardened steel. The heat transfer rate is higher than from sand or ceramic molds used in other processes and this provides advantages in casting cycle time. Often, a ceramic coating is applied to the steel to protect it from the molten metal and to control the heat transfer rate from the molten metal to the die at only a slight increase in cycle time. The solidification of the molten metal must be controlled to prevent solidification shrinkage cavities in the final product. Also, the solidification rate is managed to optimize the microstructure of the resulting casting to achieve the design objectives. For high production castings, the dies are often water-cooled, further decreasing the cycle time and reducing the cost for each part. Sometimes the casting design requires undercuts or hollow cavities in the casting that cannot be produced with conventional steel dies. In these cases, the die designer may choose a loose piece or an expendable core for the permanent mold process. Both of these options increase die and casting costs, but often the function of the product requires these features in the casting. When expendable cores are used, the process is called semipermanent mold casting. This subcategory of permanent mold casting refers to processes that utilize one or more expendable cores that are placed in the steel die before the production of each casting. The expendable core is then removed by vibration or heat treatment after the casting is removed from the die. Expendable cores for semipermanent mold casting are normally produced from sand with a binder to give them strength, similar to the cores utilized in sand casting processes. The advantages of the permanent mold and semipermanent mold casting processes are: reasonable piece costs resulting from the high production rates achieved with metal molds (especially water-cooled molds) compared to sand and investment casting, and lower investment required for equipment when compared to low-pressure and high-pressure die casting. In addition, the use of expendable cores in semipermanent mold casting permits great design flexibility for castings. One of the disadvantages of the permanent mold casting process is that metal dies are more expensive than patterns for sand casting or investment casting so the process is not economical for short runs. At low volume, it is difficult to overcome the high initial tooling cost and compete based on casting cost. In addition, since castings are filled with liquid metal under only the pressure of gravity, castings sections tend to be thicker in permanent mold casting than in 1
Die Casting (Permanent Mold) the low-pressure and high-pressure die casting processes. Also, where material properties are critical, emerging concepts in high-pressure die casting, such as squeeze casting and semisolid metal processing, are creating new competition for permanent mold casting. Typical parts produced in the permanent mold process include automotive parts such as aluminum pistons, steering knuckles, brackets, wheels, and pump impellers. Parts are also produced in zinc, brass, copper, lead, and even gray iron. Since the process has great design flexibility and is compatible with so many metals, the types of products that can be produced are almost unlimited.
Cavity
Air inlet Stalk Molten metal Furnace
Crucible
2. Low-pressure Die Casting Low-pressure die casting was developed in an attempt to eliminate hand ladling of the hot metal. This process also utilizes metal molds to produce castings, but the molten metal is pressurized to achieve faster or bettercontrolled filling of the mold. The process utilizes pressures up to 7 MPa for special products such as automotive wheels, but typically the pressure used is below 0.5 MPa. In low-pressure casting, the molten metal is contained in an insulated crucible or furnace which is pressure sealed. A tube passes vertically down through the furnace, with its lower end immersed in molten metal and its top flange sealed against the furnace lid. The die is mounted on the machine, over the furnace, and sealed to the open end of the tube. When air pressure is applied to the furnace, it displaces the molten metal, causing it to travel up the tube and into the die cavity as seen in Fig. 1. The cooling process takes place in a sequence starting at the extremities of the die, working back to the feed head, which is the mouth of the tube. Upon solidification of the casting, the unwanted metal is returned to the furnace by relaxing the air pressure; the die is opened, the casting is removed, and the casting cycle is repeated. Sometimes a vacuum is applied to the mold cavity to cause the metal to flow. Atmospheric pressure on the metal then forces the molten metal up into the bottom of the die and fills the part. Using pressure to fill the die cavity permits faster filling of the die than by gravity casting. Faster fill times are important because the molten metal loses temperature to the die as it flows past the die steel and the hotter the metal the thinner the section thickness that can be filled. For some products, the fill rate must be varied depending on the casting section thickness and very sophisticated computer-controlled, low-pressure machines are utilized. This additional capability is not free, however, and is not cost effective for every product. In that sense, low-pressure die casting is an enhanced process that is generally used for parts with 2
Figure 1 Low-pressure die casting.
premium requirements. This is the case for many automotive engine components such as heads, lower crankcases, and manifolds that require expendable cores because of the intricate internal passageways that are required. These parts must also be leak free. Many of these parts are produced by the low-pressure die casting process because of their size, complexity, and volume. Gravity and low-pressure, semipermanent mold casting are the primary methods used to produce these complex aluminum automotive components throughout the world. The decision to use gravity or low-pressure casting depends on product requirements and casting volume. Dies for low-pressure die casting are also produced from hardened steel, but they require more complex design and engineering to assure that pressurized molten metal is contained within the casting system. Detailed metal feed systems and connections to the molten metal bath are required as well as seals in the die where a vacuum is used. These features increase the cost and lead-time of dies when compared to gravity permanent mold dies. The dies are still less costly than those used in high-pressure die casting, however. The casting machines for the low-pressure process can also become fairly sophisticated as well, requiring higher capital investment for sophisticated pressure or vacuum control systems. These higher costs are offset by the higher value of complex castings that can be produced, as well as by higher production rates and improved casting fill capability, and they still cost much less than machines for high-pressure die casting. The advantages of low-pressure die casting are that casting quality is very good, especially in heavy wall
Die Casting (Permanent Mold) sections, and casting yield is 90–95% since no runners, risers, or gates are used. This results in much less trimming to remelt and corresponding savings in fuel cost. There is a much greater alloy choice than with high-pressure die casting, permitting the use of heat treatable alloys. The use of expendable cores in lowpressure die casting allows this process to cast parts that are impossible to produce in the high-pressure process. The disadvantages of low-pressure die casting are that it has a lower production rate than high-pressure casting and it is not normally suitable for casting smaller parts. The minimum wall thickness, while smaller than permanent mold casting, is still larger than for high-pressure die casting. Tooling cost is also higher than for the permanent mold process.
3. High-pressure Die Casting High-pressure die casting is a process in which molten metal is forced under pressure into a securely locked metal die cavity, where it is held by a powerful press until the metal solidifies. After solidification of the metal, the die is unlocked, opened, and the casting ejected. After removal of the casting, the die is closed and locked again for the next cycle. The injection of metal into the die cavity is completed in a fraction of a second. Often, while the molten metal is still held in the die, extremely high pressure is applied (called intensification pressure). This high pressure compresses any gas entrapped in the metal and feeds additional metal into the cavity to compensate for the shrinkage of the metal as it solidifies. Two types of systems are used for injecting the molten metal into the die. The hot chamber system is used with metals such as zinc, magnesium, and lead. The injection system of a hot chamber machine is immersed in the molten metal bath of the melting furnace. As the shot plunger moves, it forces metal through the nozzle and into the die. The cold chamber system is used for metals that melt at high temperatures, such as aluminum, brass, and magnesium. Magnesium parts can be produced using both systems, though usually small parts are produced in hot chamber machines and large parts in cold chamber machines since hot chamber machines are limited in size. There are also two injection systems used in the cold chamber process, horizontal and vertical injection. In the cold chamber process, the molten metal is poured, by hand or by automatic means, into a port of the cold chamber sleeve. A hydraulically operated plunger advances through this steel sleeve, sealing off the port, and forcing the metal into the die at high speed and pressure. After solidification of the casting, the plunger is retracted, the die opened, the casting
ejected, and the system is then ready for the next shot cycle. Higher pressure is used in this system than the hot chamber process. The production rate of a hot chamber machine is higher than that of a cold chamber machine because of the shorter time required during the pour operation. Typical zinc castings produced by the high-pressure die casting process are shown in Fig. 2. In addition to the conventional high-pressure die casting processes, several enhancements to the process have been developed in recent years. These enhancements include the use of vacuum systems to reduce entrapped gas, slower fill processes to eliminate turbulence during fill and permit the use of heat treatment to enhance mechanical properties of the castings, and the application of semisolid metal processing to produce pressure tight parts not normally able to be produced in high-pressure die casting. Each of these processes utilizes the fundamentals of high-pressure die casting, but with additional capabilities to produce high integrity parts. They also have developed unique names for the purpose of distinguishing themselves from the conventional processes of hot chamber and cold chamber die casting. Vacuum die casting utilizes a vacuum system on the die cavity to remove gas from the cavity prior to injection of the molten metal. The result is a high integrity part with very low levels of porosity and high mechanical properties. The process is used to produce critical components in light alloys, such as structural and safety components for motor vehicles. The process has higher costs than conventional die casting, but the parts produced justify the additional cost and they cannot be produced successfully in conventional die casting. Slow fill die casting is often called squeeze casting. This process is widely practiced throughout the world today for producing parts that must be heat treated to achieve the required mechanical properties. Many automotive components that have been converted from iron castings or weldments of castings and\or stampings are now produced in light alloys in the squeeze casting process. This process utilizes the advantages of the low-pressure die casting process, controlled filling, and directional solidification of the molten metal, as well as the advantages of highpressure die casting, high-pressure solidification, and fast cycle times. Parts produced in the squeeze casting process include steering and suspension components, e.g., alloy wheels, steering knuckles, and control arms, and air conditioning parts, e.g., compressor scrolls. Parts are produced with both vertical and horizontal shot systems. A schematic diagram of a vertical shot horizontal squeeze casting machine is shown in Fig. 3. Semisolid metal processing (SSM) is beginning to share some of the same markets as squeeze castings, as well as some of the smaller automotive components such as link arms, fuel rail parts, and drivetrain parts. In this process, the metal injected into the die is only 3
Die Casting (Permanent Mold)
Figure 2 Typical zinc die castings.
about 50% liquid. The process offers distinct advantages in that the feed metal fills the cavity in a manner that is less turbulent than in conventional die casting. Furthermore, the casting is partially solidified at the onset of the process, thus the solidification ‘‘journey’’ is a much shorter one. The technical capabilities of the process include high integrity of the castings, high mechanical properties, the ability to cast thin walls, good dimensional accuracy, surface conditions similar to high-pressure die casting, and suitability for heat treatment and welding. Semisolid casting and SSM materials are covered in detail in Semisolid Processing, and Casting of Semisolid Metals: Engineering Applications The advantages of high-pressure die casting include a higher production rate than with gravity or lowpressure casting. Also, the ability to produce castings with close dimensional control greatly reduces machining operations. Die castings have good surface finish, which is a prime requirement for plating, and much thinner wall thickness is possible reducing overall casting weight. Dies have a long life, reducing unit part costs, and more complex parts can be produced, thereby reducing the number of components required in an assembly. The disadvantage of high-pressure die casting is that 4
it is best suited to high volume parts. High tooling costs make short production runs uneconomical. Also, the internal porosity prevalent in conventional highpressure die castings makes producing pressure tight parts difficult, often requiring the use of vacuum die casting, squeeze casting or SSM casting. There are a limited number of alloys suitable for die casting and this restricts the heat treatment or welding of the finished castings. Iron or steel alloys are normally not die castable. There are restrictions in die casting on the casting size and wall thickness which eliminate the possibility of die casting some parts, but this has been reduced with the development of the new high integrity processes of vacuum die casting, squeeze casting, and SSM casting. Die casting machine and maintenance costs are higher than for other casting processes.
4. Concluding Remarks The term die casting is used to describe processes that utilize metal dies, or molds, to produce parts from various metals. These processes include gravity permanent mold casting, in which the liquid metal is poured into the die; low-pressure die casting, in which
Die Casting (Permanent Mold)
Figure 3 Vertical shot horizontal squeeze casting machine.
the metal is forced into the mold with air pressure; and high-pressure die casting, in which a hydraulic ram is used to inject the molten metal into the die at extremely high pressures. In North America, the term die casting is used to describe high-pressure die casting, while in most of the world the term die casting may be used to describe all of these processes that utilize metal molds. The die casting processes provide a great deal of flexibility for the production of metal components. Permanent mold and low-pressure die casting can produce parts from almost any metal in almost any shape. In the case of extremely high volume parts, high-pressure die casting provides high production rates and good dimensional repeatability. In recent years, enhanced high-pressure die casting processes such as vacuum die casting, squeeze casting and semisolid metal processing have created new opportunities for utilizing the advantages of high-pressure die casting for structural or leak free components.
Bibliography Bradney D D 1994 The NFFS Guide to Aluminum Casting Design: Sand and Permanent Mold. Non-Ferrous Founders’ Society, Des Plaines, IL Corbit S, DasGupta R 1999 Squeeze cast automotive applications and squeeze cast aluminum alloy properties. In: SAE Technical Paper Series, International Congress and Exposition. Society of Automotive Engineers, Inc., Warrendale, PA. Herman E A 1982 Die Casting Handbook. North American Die Casting Association, Rosemont, IL Jorstad J 1999 Selecting parts and material for conversion to SSM. In: Nussbaum A I (ed.) Semi-Solid Metal World Users’ Conference. Ormet Primary Aluminum Corporation, Hannibal, OH Midson S P, Young K P 1999 Technical support for high integrity processes. Diecasting World (June), 16–18 Prince Machine Corporation 1980 Low-pressure Manual. Prince Machine, MI, USA
W. A. Butler
5
Die Casting (Permanent Mold)
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 2147–2152 6