casting design 02 - casting process
Sunday, February 20, 2011
By
st_fanuc
The fundamental process of casting consists of five basic elements
• Molding—The mold cavity must be formed from a material that will withstand the operating temperatures and conditions of the chosen casting process and metal.
• Pouring—The molten metal is poured into the mold and travels through its passages to fill the mold cavity.
• Solidification—During the solidification process, the metal cools and becomes a solid shape.
• Mold Removal—The cooled casting is removed from the mold.
• Secondary Operations—The casting is trimmed, cleaned, heat-treated, machined, inspected, painted, etc.
These five basic elements are supported by design and fabrication of the patterns and cores for the mold, the fabrication of the mold cavity and the melting of the metal.
A key part of designing a mold involves the use of cores. Cores are preformed masses of bonded sand or some other material that are used to make the internal passageways of a casting. Castings may require a single core, a complex assembly of cores or no cores at all. Like castings, cores are made in a mold, called a coldbox. Typically these cores are made of sand and may be combined with other materials that bind the sand together. Metal cores are used in permanent mold and diecasting processes. The type of cores used in each metalcasting process will also be part of your decision making process.
Sand Casting
Fundamentally, a mold is produced by shaping a refractory material to form a cavity of a desired shape such that molten metal can be poured into the cavity. The mold cavity needs to retain its shape until the metal has solidified and the casting is removed. This sounds easy to accomplish, but depending on the choice of metal, certain characteristics are demanded of the mold.
Green Sand Molding
The most common method used to make metal castings is green sand molding. In this process, granular refractory sand is coated with a mixture of bentonite clay, water and, in some cases, other additives. The additives help to harden and hold the mold shape to withstand the pressures of the molten metal.
The green sand mixture is compacted through mechanical force or by hand around a pattern to create a mold. The mechanical force can be induced by slinging, jolting, squeezing or by impact/impulse.
The following points should be taken into account when considering the green sand molding process:
• for many metal applications, green sand processes are the most cost-effective of all metal forming operations;
• these processes readily lend themselves to automated systems for high-volume work as well as short runs and prototype work;
• in the case of slinging, manual jolt or squeeze molding to form the mold, 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, making it possible to use this process with all types of green sand molding equipment and for a majority of alloys poured.
Chemically Bonded Molding Systems
Several coldbox processes exist, including phenolic urethane/amine vapor, furan/SO2, acrylic/SO2 and sodium silicate/CO2. In general, coldbox processes offer:
• good dimensional accuracy of the cores because they are cured without the use of heat;
• excellent surface finish of the casting;
• short production cycles that are optimal for high production runs;
• excellent shelf life of the cores and molds.
Shell Process___In this process, sand is pre-coated with a phenolic novalac resin containing a hexamethylenetetramine catalyst. The resin-coated sand is dumped, blown or shot into a metal corebox or over a metal pattern that has been heated to 450-650F (232-343C). Shell molds 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 complicated applications, can be made of multiple pieces glued together.
Benefits of the shell process include:
• an excellent core or mold surface resulting in good casting finish;
• 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;
• selection of refractory material other than silica for specialty applications;
• a savings in materials usage through the use of hollow cores and thin shell molds.
Nobake or Airset Systems___In order to improve productivity and eliminate the need for heat or gassing to cure mold and core binders, a series of resin systems referred to as nobake or airset binders was developed.
In these systems, sand is mixed with one or two liquid resin components and a liquid catalyst component. As soon as the resin(s) and catalyst combine, a chemical reaction begins to take place that hardens (cures) the binder. The curing time can be lengthened or shortened based on the amount of catalyst used and the temperature of the refractory sand.
The mixed sand is placed against the pattern or into the corebox. Although the sand mixtures have good flowability, some form of compaction (usually vibration) is used to provide densification of the sand in the mold/core. After a period of time, the core/mold has cured sufficiently to allow stripping from the corebox or pattern without distortion. The cores/molds are then allowed to sit and thoroughly cure. After curing, they can accept a refractory wash or coating that provides a better surface finish on the casting and protects the sand in the mold from the heat and erosive action of the molten metal as it enters the mold cavity.
The nobake process provides the following advantages:
• 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 allow easy shakeout (the separation of the casting from the mold after solidification is complete);
• cores and molds can be stored indefinitely.
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 less common V-process.
Lost Foam Casting___In this process, the pattern is made of expendable polystyrene (EPS) beads. For high-production runs, the patterns can be made by injecting EPS beads into a die and bonding them together using a heat source—usually steam. For shorter runs, pattern shapes are cut from sheets of EPS using conventional woodworking equipment and then assembled with glue. In either case, internal passageways in the casting, if needed, are not formed by conventional sand cores but are part of the mold itself.
The polystyrene pattern is coated with a refractory coating, which covers both the external and internal surfaces. With the gating and risering system attached to the pattern, the assembly is suspended in a one-piece flask, which then is 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 densify. The sand flows around the pattern and into the internal passageways of the pattern.
As the molten metal is poured into the mold, it replaces the EPS pattern, which vaporizes. After the casting solidifies, the unbonded sand is dumped out of the flask, leaving the casting with an attached gating system.
With larger castings, the coated pattern is covered with a facing of chemically bonded sand. The facing sand is then backed up with more chemically bonded sand.
The lost foam process offers the following advantages:
• no size limitations for castings;
• improved surface finish of 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___In the V-process, the cope and drag halves of the mold are formed separately by heating a thin plastic film to its deformation point. It then is vacuum-formed over a pattern on a hollow carrier plate.
The process uses dry, free-flowing, unbonded sand to fill the special flask set over the film-coated pattern. Slight vibration 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 on the flask, and the sand between the two plastic sheets becomes rigid.
The cope and drag then are assembled to form a plastic-lined mold cavity. Sand hardness is maintained by holding the vacuum within the mold halves at 300-600 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.
Permanent Mold
At least three families of molding and casting processes can be categorized as permanent mold processes. These include diecasting (high-pressure diecasting), low-pressure permanent mold casting and permanent mold 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 molten metal is injected into it. Premeasured amounts of molten metal are forced from a shot chamber into the permanent mold or die under extreme pressure (greater than 15,000 psi). This allows for high production rates.
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.
The diecasting process is 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 0.005-0.01 in.;
• recommended machining allowances of 0.01-0.03 in.;
• thin-section castings.
• Molding—The mold cavity must be formed from a material that will withstand the operating temperatures and conditions of the chosen casting process and metal.
• Pouring—The molten metal is poured into the mold and travels through its passages to fill the mold cavity.
• Solidification—During the solidification process, the metal cools and becomes a solid shape.
• Mold Removal—The cooled casting is removed from the mold.
• Secondary Operations—The casting is trimmed, cleaned, heat-treated, machined, inspected, painted, etc.
These five basic elements are supported by design and fabrication of the patterns and cores for the mold, the fabrication of the mold cavity and the melting of the metal.
A key part of designing a mold involves the use of cores. Cores are preformed masses of bonded sand or some other material that are used to make the internal passageways of a casting. Castings may require a single core, a complex assembly of cores or no cores at all. Like castings, cores are made in a mold, called a coldbox. Typically these cores are made of sand and may be combined with other materials that bind the sand together. Metal cores are used in permanent mold and diecasting processes. The type of cores used in each metalcasting process will also be part of your decision making process.
Sand Casting
Fundamentally, a mold is produced by shaping a refractory material to form a cavity of a desired shape such that molten metal can be poured into the cavity. The mold cavity needs to retain its shape until the metal has solidified and the casting is removed. This sounds easy to accomplish, but depending on the choice of metal, certain characteristics are demanded of the mold.
Green Sand Molding
The most common method used to make metal castings is green sand molding. In this process, granular refractory sand is coated with a mixture of bentonite clay, water and, in some cases, other additives. The additives help to harden and hold the mold shape to withstand the pressures of the molten metal.
The green sand mixture is compacted through mechanical force or by hand around a pattern to create a mold. The mechanical force can be induced by slinging, jolting, squeezing or by impact/impulse.
The following points should be taken into account when considering the green sand molding process:
• for many metal applications, green sand processes are the most cost-effective of all metal forming operations;
• these processes readily lend themselves to automated systems for high-volume work as well as short runs and prototype work;
• in the case of slinging, manual jolt or squeeze molding to form the mold, 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, making it possible to use this process with all types of green sand molding equipment and for a majority of alloys poured.
Chemically Bonded Molding Systems
This category of sand casting process is used widely throughout the metalcasting industry because of the economics and improved productivity each offers. Each process uses a unique chemical binder and catalyst to cure and harden the mold and/or core. Some processes require heat to facilitate the curing mechanism, though others do not.
Gas Catalyzed or Coldbox Systems___Coldbox systems utilize a family of binders where the catalyst is not added to the sand mixture. Catalysts in the form of a gas or vapor are added to the sand and resin component so the mixture will not cure until it is brought into contact with a catalyst agent. The sand-resin mixture is blown into a corebox to compact the sand, and a catalytic gas or vapor is permeated through the sand mixture, where the catalyst reacts with the resin component to harden the sand mixture almost instantly. Any sand mixture that has not come into contact with the catalyst is still capable of being cured, so many small cores can be produced from a large batch of mixed sand.
Gas Catalyzed or Coldbox Systems___Coldbox systems utilize a family of binders where the catalyst is not added to the sand mixture. Catalysts in the form of a gas or vapor are added to the sand and resin component so the mixture will not cure until it is brought into contact with a catalyst agent. The sand-resin mixture is blown into a corebox to compact the sand, and a catalytic gas or vapor is permeated through the sand mixture, where the catalyst reacts with the resin component to harden the sand mixture almost instantly. Any sand mixture that has not come into contact with the catalyst is still capable of being cured, so many small cores can be produced from a large batch of mixed sand.
Several coldbox processes exist, including phenolic urethane/amine vapor, furan/SO2, acrylic/SO2 and sodium silicate/CO2. In general, coldbox processes offer:
• good dimensional accuracy of the cores because they are cured without the use of heat;
• excellent surface finish of the casting;
• short production cycles that are optimal for high production runs;
• excellent shelf life of the cores and molds.
Shell Process___In this process, sand is pre-coated with a phenolic novalac resin containing a hexamethylenetetramine catalyst. The resin-coated sand is dumped, blown or shot into a metal corebox or over a metal pattern that has been heated to 450-650F (232-343C). Shell molds 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 complicated applications, can be made of multiple pieces glued together.
Benefits of the shell process include:
• an excellent core or mold surface resulting in good casting finish;
• 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;
• selection of refractory material other than silica for specialty applications;
• a savings in materials usage through the use of hollow cores and thin shell molds.
Nobake or Airset Systems___In order to improve productivity and eliminate the need for heat or gassing to cure mold and core binders, a series of resin systems referred to as nobake or airset binders was developed.
In these systems, sand is mixed with one or two liquid resin components and a liquid catalyst component. As soon as the resin(s) and catalyst combine, a chemical reaction begins to take place that hardens (cures) the binder. The curing time can be lengthened or shortened based on the amount of catalyst used and the temperature of the refractory sand.
The mixed sand is placed against the pattern or into the corebox. Although the sand mixtures have good flowability, some form of compaction (usually vibration) is used to provide densification of the sand in the mold/core. After a period of time, the core/mold has cured sufficiently to allow stripping from the corebox or pattern without distortion. The cores/molds are then allowed to sit and thoroughly cure. After curing, they can accept a refractory wash or coating that provides a better surface finish on the casting and protects the sand in the mold from the heat and erosive action of the molten metal as it enters the mold cavity.
The nobake process provides the following advantages:
• 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 allow easy shakeout (the separation of the casting from the mold after solidification is complete);
• cores and molds can be stored indefinitely.
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 less common V-process.
Lost Foam Casting___In this process, the pattern is made of expendable polystyrene (EPS) beads. For high-production runs, the patterns can be made by injecting EPS beads into a die and bonding them together using a heat source—usually steam. For shorter runs, pattern shapes are cut from sheets of EPS using conventional woodworking equipment and then assembled with glue. In either case, internal passageways in the casting, if needed, are not formed by conventional sand cores but are part of the mold itself.
The polystyrene pattern is coated with a refractory coating, which covers both the external and internal surfaces. With the gating and risering system attached to the pattern, the assembly is suspended in a one-piece flask, which then is 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 densify. The sand flows around the pattern and into the internal passageways of the pattern.
As the molten metal is poured into the mold, it replaces the EPS pattern, which vaporizes. After the casting solidifies, the unbonded sand is dumped out of the flask, leaving the casting with an attached gating system.
With larger castings, the coated pattern is covered with a facing of chemically bonded sand. The facing sand is then backed up with more chemically bonded sand.
The lost foam process offers the following advantages:
• no size limitations for castings;
• improved surface finish of 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___In the V-process, the cope and drag halves of the mold are formed separately by heating a thin plastic film to its deformation point. It then is vacuum-formed over a pattern on a hollow carrier plate.
The process uses dry, free-flowing, unbonded sand to fill the special flask set over the film-coated pattern. Slight vibration 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 on the flask, and the sand between the two plastic sheets becomes rigid.
The cope and drag then are assembled to form a plastic-lined mold cavity. Sand hardness is maintained by holding the vacuum within the mold halves at 300-600 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.
Permanent Mold
At least three families of molding and casting processes can be categorized as permanent mold processes. These include diecasting (high-pressure diecasting), low-pressure permanent mold casting and permanent mold 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 molten metal is injected into it. Premeasured amounts of molten metal are forced from a shot chamber into the permanent mold or die under extreme pressure (greater than 15,000 psi). This allows for high production rates.
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.
The diecasting process is 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 0.005-0.01 in.;
• recommended machining allowances of 0.01-0.03 in.;
• thin-section castings.
Permanent Mold Casting (Gravity Diecasting)
Another form of permanent mold casting is when 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 from cast iron or steel. If cores are to be used, they can be metal inserts, which operate mechanically in the mold, or sand cores, which are placed in the molds before closing (semi-permanent molding).
The mold halves are preheated and the internal surfaces are 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 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 then is 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—static pour and tilt pour—offer a variety of advantages for a variety of metalforming applications. Benefits include:
• castings with superior mechanical properties because the metal mold acts as a chill;
• castings are uniform in shape and have excellent dimensional tolerances because molds are made of metal;
• excellent surface finishes;
• high-production runs;
• sections of the mold that can be selectively insulated or cooled, which helps control the solidification and improves overall casting properties.
Low-Pressure and/or Vacuum Permanent Mold Casting (LPPM)
In this process, low pressure is used to push the molten metal (and/or a vacuum is used to draw the metal) into the mold through a riser tube, as the furnace is below the mold cavity. The amount of pressure, from 3-15 psi, is dependent on the casting configuration and the quality of the casting desired. When internal passageways are required, they can be made by either mechanically actuated metal inserts or sand cores. The goal of this process is to control the molten metal flow as much as possible to ensure a tranquil fill of the mold cavity.
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 molten metal is fed directly into the casting, excellent yields are realized, and the need for additional handwork is reduced;
• odd casting configurations and tooling points for machining can be placed in areas where gates and risers normally would be placed;
• 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.
Another form of permanent mold casting is when 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 from cast iron or steel. If cores are to be used, they can be metal inserts, which operate mechanically in the mold, or sand cores, which are placed in the molds before closing (semi-permanent molding).
The mold halves are preheated and the internal surfaces are 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 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 then is 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—static pour and tilt pour—offer a variety of advantages for a variety of metalforming applications. Benefits include:
• castings with superior mechanical properties because the metal mold acts as a chill;
• castings are uniform in shape and have excellent dimensional tolerances because molds are made of metal;
• excellent surface finishes;
• high-production runs;
• sections of the mold that can be selectively insulated or cooled, which helps control the solidification and improves overall casting properties.
Low-Pressure and/or Vacuum Permanent Mold Casting (LPPM)
In this process, low pressure is used to push the molten metal (and/or a vacuum is used to draw the metal) into the mold through a riser tube, as the furnace is below the mold cavity. The amount of pressure, from 3-15 psi, is dependent on the casting configuration and the quality of the casting desired. When internal passageways are required, they can be made by either mechanically actuated metal inserts or sand cores. The goal of this process is to control the molten metal flow as much as possible to ensure a tranquil fill of the mold cavity.
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 molten metal is fed directly into the casting, excellent yields are realized, and the need for additional handwork is reduced;
• odd casting configurations and tooling points for machining can be placed in areas where gates and risers normally would be placed;
• 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.
Ceramic and Plaster Molding
This family of casting processes is unique in that ceramic and plaster are used as molding media. These processes offer a high degree of precision in regard to dimensions, as well as excellent surface finishes.
Investment Casting
The investment casting process was one of the first processes used to produce 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.
In investment casting, the patterns are produced in dies via injection molding. For the most part, the patterns are made of wax; however, some patterns are made of plastic or polystyrene. Because the tooling cost for individual wax patterns is high, investment casting normally is used when high volumes are required. When cores are required, they are made of soluble wax or ceramic materials.
The ceramic shell is built around a pattern/gating assembly by repeatedly dipping the “tree” into a thin refractory slurry. After dipping, a refractory aggregate, such as silica, zircon or aluminum silicate 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 required thickness of this shell is dependent on the size of the castings and temperature of the metal to be poured. After the ceramic shell is complete, the entire assembly is placed into an autoclave oven to melt and remove a majority of the wax.
The majority of investment castings weigh less than 5 lbs., but there is a trend to produce larger castings in the 10-30-lb. range. Castings weighing up to 800 lbs. have been poured in this process. Some of the advantages of investment casting include:
• excellent surface finishes;
• tight dimensional tolerances;
• reduced or eliminated machining requirements;
• ability to cast titanium as well as the other superalloys.
Ceramic Molding
Generally, these processes employ a mixture of graded refractory fillers that are blended to a slurry consistency. Various refractory materials can be used as filler material. The slurry then is poured over a pattern that has been placed in a container.
First, a gel is formed in a pattern and stripped from the mold. The mold then is heated to a high temperature until it becomes rigid. After the molds cool, molten metal is 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 castings with excellent surface finish and good dimensional tolerances.
Plaster Molding
Plaster molding is used to produce castings of the lower melting temperature metals, such as the aluminum alloys. In the process, a slurry containing calcium sulfate, sometimes called gypsum, is poured into a flask that 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 cooled, the cores and mold are assembled. After assembly, most molds are preheated before pouring. Because these molds have very poor permeability, vacuum-assistance or pressure usually is required during pouring.The plaster mold processes are well-suited for short run and prototype work with the lower temperature alloys, particularly aluminum
This family of casting processes is unique in that ceramic and plaster are used as molding media. These processes offer a high degree of precision in regard to dimensions, as well as excellent surface finishes.
Investment Casting
The investment casting process was one of the first processes used to produce 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.
In investment casting, the patterns are produced in dies via injection molding. For the most part, the patterns are made of wax; however, some patterns are made of plastic or polystyrene. Because the tooling cost for individual wax patterns is high, investment casting normally is used when high volumes are required. When cores are required, they are made of soluble wax or ceramic materials.
The ceramic shell is built around a pattern/gating assembly by repeatedly dipping the “tree” into a thin refractory slurry. After dipping, a refractory aggregate, such as silica, zircon or aluminum silicate 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 required thickness of this shell is dependent on the size of the castings and temperature of the metal to be poured. After the ceramic shell is complete, the entire assembly is placed into an autoclave oven to melt and remove a majority of the wax.
The majority of investment castings weigh less than 5 lbs., but there is a trend to produce larger castings in the 10-30-lb. range. Castings weighing up to 800 lbs. have been poured in this process. Some of the advantages of investment casting include:
• excellent surface finishes;
• tight dimensional tolerances;
• reduced or eliminated machining requirements;
• ability to cast titanium as well as the other superalloys.
Ceramic Molding
Generally, these processes employ a mixture of graded refractory fillers that are blended to a slurry consistency. Various refractory materials can be used as filler material. The slurry then is poured over a pattern that has been placed in a container.
First, a gel is formed in a pattern and stripped from the mold. The mold then is heated to a high temperature until it becomes rigid. After the molds cool, molten metal is 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 castings with excellent surface finish and good dimensional tolerances.
Plaster Molding
Plaster molding is used to produce castings of the lower melting temperature metals, such as the aluminum alloys. In the process, a slurry containing calcium sulfate, sometimes called gypsum, is poured into a flask that 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 cooled, the cores and mold are assembled. After assembly, most molds are preheated before pouring. Because these molds have very poor permeability, vacuum-assistance or pressure usually is required during pouring.The plaster mold processes are well-suited for short run and prototype work with the lower temperature alloys, particularly aluminum
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