Ceramic Mold Casting

The manufacturing process of ceramic mold casting is like the process of plaster mold casting but can cast materials at much higher temperatures. Instead of using plaster to create the mold for the metal casting, ceramic casting uses refractory ceramics for a mold material. In industry, parts such as machining cutters, dies for metalworking, metal molds, and impellers may be manufactured by this process.

Process

The first step in manufacture by ceramic mold casting is to combine the material for the mold. A mixture of fine grain zircon (ZrSiO₄), aluminum oxide, fused silica, bonding agents, and water, creates a ceramic slurry . This slurry is poured over the casting pattern and let set. The pattern is then removed and the mold is left to dry. The mold is then fired. The firing will burn off any unwanted material and make the mold hardened and rigid. The mold may also need to be baked in a furnace as well. The firing of the mold produces a network of microscopic cracks in the mold material. These cracks give the ceramic mold both good permeability and collapsibility for the metal casting process.



Once prepared, the two halves of the mold are assembled for the pouring of the metal casting. The two halves,(cope and drag section), may be backed up with fireclay material for additional mold strength. Often in manufacturing industry, the ceramic mold will be preheated prior to pouring the molten metal. The metal casting is poured, and let solidify. In ceramic mold casting, like in other expendable mold processes, the ceramic mold is destroyed in the removal of the metal casting.

Properties And Considerations Of Manufacturing By Ceramic Mold Casting

• Manufacturing by ceramic mold casting is similar to plaster mold casting in that it can produce parts with thin sections, excellent surface finish, and high dimensional accuracy. Manufacturing tolerances between .002 and .010 inches are possible with this process.
  


• To be able to cast parts with high dimensional accuracy eliminates the need for machining, and the scrap that would be produced by machining. Therefore precision metal casting processes like this are efficient to cast precious metals, or materials that would be difficult to machine.
  


• Unlike the mold material in the plaster metal casting process, the refractory mold material in ceramic casting can withstand extremely elevated temperatures. Due to this heat tolerance, the ceramic casting process can be used to manufacture ferrous and other high melting point metal casting materials. Stainless steels and tool steels can be cast with this process.
  


• Ceramic mold casting is relatively expensive.
  


• The long preparation time of the mold makes manufacturing production rates for this process slow.
  


• Unlike in plaster mold casting, the ceramic mold has excellent permeability due to the microcrazing, (production of microscopic cracks), that occurs in the firing of the ceramic mold.

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Plaster Mold Casting

Plaster mold casting is a manufacturing process having a similar technique to sand casting. Plaster of Paris is used to form the mold for the casting, instead of sand. In industry parts such as valves, tooling, gears, and lock components may be manufactured by plaster mold casting.

The Process

Initially plaster of Paris is mixed with water just like in the first step of the formation of any plaster part. In the next step of the manufacture of a plaster casting mold, the plaster of Paris and water are then mixed with various additives such as talc and silica flour. The additives serve to control the setting time of the plaster and improve its strength. The plaster of Paris mixture is then poured over the casting pattern. The slurry must sit for about 20 minutes before it sets enough to remove the pattern. The pattern used for this type of metal casting manufacture should be made from plastic or metal. Since it will experience prolonged exposure to water from the plaster mix, wood casting patterns have a tendency to warp. After striping the pattern, the mold must be baked for several hours, to remove the moisture and become hard enough to pour the metal casting. The two halves of the mold are then assembled for the casting process.

Properties and Considerations of Manufacturing by Plaster Mold Casting

• When baking the casting mold just the right amount of water should be left in the mold material. Too much moisture in the mold can cause metal casting defects, but if the mold is too dehydrated, it will lack adequate strength.
  


• The fluid plaster slurry flows readily over the pattern, making an impression of great detail and surface finish. Also due to the low thermal conductivity of the mold material the casting will solidify slowly creating more uniform grain structure and mitigating casting warping. The qualities of the plaster mold enable the process to manufacture parts with excellent surface finish, thin sections, and produces high geometric accuracy.
  

Castings of high detail and section thickness as low as .04 - .1 inch, (2.5 - 1 mm), are possible when manufacturing by plaster mold casting

• There is a limit to the casting materials that may be used for this type of manufacturing process, due to the fact that a plaster mold will not withstand temperature above 2200F (1200C). Higher melting point metals can not be cast in plaster. This process is typically used in industry to manufacture castings made from aluminum, magnesium, zinc, and copper based alloys.
  


• Manufacturing production rates for this type of metal casting process are relatively slow, due to the long preparation time of the mold.
  


• The plaster mold is not permeable, which severely limits the escape of gases from the casting. 

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Gases In Metal Casting

Gases During The Manufacture Of A Casting

The molten metal used during the casting process may trap and contain gases. There are various reasons that gases are absorbed into the metal melt during manufacture. Turbulent flow of the casting material through the system may cause it to trap gas from the air. Gases may be trapped from material or the atmosphere in the crucible when the melt is being prepared. Gases may be trapped from reactions between the molten metal and the mold material.
Since liquid metal has a much higher solubility than solid metal, as the casting solidifies these gases are expelled. If they can not escape they may form vacancies in the material, causing porosity in the metal casting.
Whether a vacancy in a cast material is a result of gases or shrinkage is sometimes hard to tell. If the vacancies are spherical and smooth they are most likely a result of gases. Angular and rough vacancies are most likely a result of shrinkage. Gross absences of material within the metal casting are a result of shrinkage.

Prevention Of Gas Defects When Manufacturing A Part By Casting

• Gases being expelled by the material during solidification can be eliminated by a proper venting system in the mold. This can be planned out during the manufacturing design phase of the metal casting process.
• Mitigating the amount of turbulence in the fluid flow will reduce gas absorption into the metal.
• Removal of slag will help eliminate gases and other impurities in the casting.
• Gases may be removed by flushing a metal melt with inert gas.
• Elimination of gases may also be accomplished by pouring the metal casting in a vacuum.

Material Selection

The selection of proper materials is important in the design of a metal casting process. Here are a few things to remember when selecting manufacturing materials.

Metals, when in a molten state, may react a certain way with other materials they encounter during the casting process. This should always be a consideration. For example, liquid aluminum will react readily with iron. Iron ladles and surfaces contacting the molten aluminum can be covered with a spray on ceramic coating to prevent this. When selecting a type of manufacturing process, remember that some materials may be more applicable to different metal casting techniques than others. Knowing the specific heat of the mold and that of the metal used for the casting will be influential in controlling the thermal gradients in the system. Section of casting metal will factor heavily on the melt's fluidity. A material with a high heat of fusion will take longer to solidify and may improve flow characteristics within the casting. When manufacturing a casting with a metal alloy that freezes over a temperature range, problems may occur due to the solid phase interfering with the liquid phase, both of which will be present within the temperature range. To help reduce this problem, a metal alloy with a shorter solidification temperature range may be selected to manufacture the casting. Or select a mold material with a high thermal conductivity, which could reduce the time spent in the solidification range by increasing the cooling rate.  Other Topics IC engine, Air Standard Cycles, Method of Ignition, Gear Types, mechanical Engineering, Gears, spur gears,Worm gears,English books,Photoshop tutorials,Physics files,Thamizh,Manufacturing,Gears pdf,Computer science,Harry potter,Best 100 english books,Face Gears,IC engine,Metal Casting,Sand casting,SAND Casting Quiz,Casting patterns quiz,Sand Casting Process,Patterns,Mechnical Previous Years Gate Question Papers ,Mechanical-old-question-paper,Core and Core Box,Shell Moulding,Permanent mold casting,Investment Casting,Die Casting,Centrifugal Casting,Workshop Technology Quiz,Milling Quiz,Metal Quiz,Mechanical Process Quiz,Chemically Bonded Molding Systems,Unbonded Sand Processes,Green Sand Molding,

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.

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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.

 

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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.
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.

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Mechanical process Quiz

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Metal Quiz

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Milling QUIZ

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Workshop Technology

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Flask Technology

This casting technology was originally developed for the jewellery and dental industries, for the production of comparatively small parts. As the production time is relatively short compared to conventional shell building, it is also being applied more and more for Rapid Prototyping.  In the embedding machine a chalky powder is mixed with water under vacuum and poured over the wax part, housed in a small metal container, the so-called flask.


This casting technology was originally developed for the jewellery and dental industries, for the production of comparatively small parts. As the production time is relatively short compared to conventional shell building, it is also being applied more and more for Rapid Prototyping.  In the embedding machine a chalky powder is mixed with water under vacuum and poured over the wax part, housed in a small metal container, the so-called flask.This casting technology was originally developed for the jewellery and dental industries, for the production of comparatively small parts. As the production time is relatively short compared to conventional shell building, it is also being applied more and more for Rapid Prototyping.  In the embedding machine a chalky powder is mixed with water under vacuum and poured over the wax part, housed in a small metal container, the so-called flask.

After solidification of the plaster, the flask is heated to a casting temperature of 300 – 500 °C, step by step. It is then cooled down depending on the material.

After the cast the part is de-bedded by means of a high-pressure water-jet to release the part from the plaster. The part is then sent for finishing.
Advantages:
Very good surface quality
Material is only mixed if required
No cores are necessary
Easy de-bedding by water
Disadvantages:
Only small and medium-sized parts are possible
Only low melting alloys e.g. aluminium can be cast, no steel
High material usage and lot of waste
No perfect casting quality because of the long solidification process, depending on the size of the flask
Burning takes up to several days, depending on flask size
Oven is blocked for many hours / days with (one) flask, as soon as the tempering process has been started

After solidification of the plaster, the flask is heated to a casting temperature of 300 – 500 °C, step by step. It is then cooled down depending on the material.

After the cast the part is de-bedded by means of a high-pressure water-jet to release the part from the plaster. The part is then sent for finishing.
Advantages:
Very good surface quality
Material is only mixed if required
No cores are necessary
Easy de-bedding by water
Disadvantages:
Only small and medium-sized parts are possible
Only low melting alloys e.g. aluminium can be cast, no steel
High material usage and lot of waste
No perfect casting quality because of the long solidification process, depending on the size of the flask
Burning takes up to several days, depending on flask size
Oven is blocked for many hours / days with (one) flask, as soon as the tempering process has been started

After solidification of the plaster, the flask is heated to a casting temperature of 300 – 500 °C, step by step. It is then cooled down depending on the material.

After the cast the part is de-bedded by means of a high-pressure water-jet to release the part from the plaster. The part is then sent for finishing.



Advantages:
Very good surface quality
Material is only mixed if required
No cores are necessary
Easy de-bedding by water


Disadvantages:
Only small and medium-sized parts are possible
Only low melting alloys e.g. aluminium can be cast, no steel
High material usage and lot of waste
No perfect casting quality because of the long solidification process, depending on the size of the flask
Burning takes up to several days, depending on flask size
Oven is blocked for many hours / days with (one) flask, as soon as the tempering process has been started


This casting technology was originally developed for the jewellery and dental industries, for the production of comparatively small parts. As the production time is relatively short compared to conventional shell building, it is also being applied more and more for Rapid Prototyping.  In the embedding machine a chalky powder is mixed with water under vacuum and poured over the wax part, housed in a small metal container, the so-called flask.

After solidification of the plaster, the flask is heated to a casting temperature of 300 – 500 °C, step by step. It is then cooled down depending on the material.

After the cast the part is de-bedded by means of a high-pressure water-jet to release the part from the plaster. The part is then sent for finishing.
Advantages:
Very good surface quality
Material is only mixed if required
No cores are necessary
Easy de-bedding by water
Disadvantages:
Only small and medium-sized parts are possible
Only low melting alloys e.g. aluminium can be cast, no steel
High material usage and lot of waste
No perfect casting quality because of the long solidification process, depending on the size of the flask
Burning takes up to several days, depending on flask size
Oven is blocked for many hours / days with (one) flask, as soon as the tempering process has been started

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Centrifugal Casting

Centrifugal casting, sometimes called rotocasting, is a metal casting process that uses centrifugal force to form cylindrical parts. This differs from most metal casting processes, which use gravity or pressure to fill the mold. In centrifugal casting, a permanent mold made from steel, cast iron, or graphite is typically used. However, the use of expendable sand molds is also possible. The casting process is usually performed on a horizontal centrifugal casting machine (vertical machines are also available) and includes the following steps:

1. Mold preparation - The walls of a cylindrical mold are first coated with a refractory ceramic coating, which involves a few steps (application, rotation, drying, and baking). Once prepared and secured, the mold is rotated about its axis at high speeds (300-3000 RPM), typically around 1000 RPM.
2. Pouring - Molten metal is poured directly into the rotating mold, without the use of runners or a gating system. The centrifugal force drives the material towards the mold walls as the mold fills.
3. Cooling - With all of the molten metal in the mold, the mold remains spinning as the metal cools. Cooling begins quickly at the mold walls and proceeds inwards.
4. Casting removal - After the casting has cooled and solidified, the rotation is stopped and the casting can be removed.
5. Finishing - While the centrifugal force drives the dense metal to the mold walls, any less dense impurities or bubbles flow to the inner surface of the casting. As a result, secondary processes such as machining, grinding, or sand-blasting, are required to clean and smooth the inner diameter of the part.

Centrifugal casting is used to produce axi-symmetric parts, such as cylinders or disks, which are typically hollow. Due to the high centrifugal forces, these parts have a very fine grain on the outer surface and possess mechanical properties approximately 30% greater than parts formed with static casting methods. These parts may be cast from ferrous metals such as low alloy steel, stainless steel, and iron, or from non-ferrous alloys such as aluminum, bronze, copper, magnesium, and nickel. Centrifugal casting is performed in wide variety of industries, including aerospace, industrial, marine, and power transmission. Typical parts include bearings, bushings, coils, cylinder liners, nozzles, pipes/tubes, pressure vessels, pulleys, rings, and wheels.

Centrifugal Casting

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Die Casting

Die casting is a manufacturing process that can produce geometrically complex metal parts through the use of reusable molds, called dies. The die casting process involves the use of a furnace, metal, die casting machine, and die. The metal, typically a non-ferrous alloy such as aluminum or zinc, is melted in the furnace and then injected into the dies in the die casting machine. There are two main types of die casting machines - hot chamber machines (used for alloys with low melting temperatures, such as zinc) and cold chamber machines (used for alloys with high melting temperatures, such as aluminum). The differences between these machines will be detailed in the sections on equipment and tooling. However, in both machines, after the molten metal is injected into the dies, it rapidly cools and solidifies into the final part, called the casting. The steps in this process are described in greater detail in the next section.

Die casting hot chamber machine overview

Die casting cold chamber machine overview

The castings that are created in this process can vary greatly in size and weight, ranging from a couple ounces to 100 pounds. One common application of die cast parts are housings - thin-walled enclosures, often requiring many ribs and bosses on the interior. Metal housings for a variety of appliances and equipment are often die cast. Several automobile components are also manufactured using die casting, including pistons, cylinder heads, and engine blocks. Other common die cast parts include propellers, gears, bushings, pumps, and valves.

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Investment Casting

Investment casting is one of the oldest manufacturing processes, dating back thousands of years, in which molten metal is poured into an expendable ceramic mold. The mold is formed by using a wax pattern - a disposable piece in the shape of the desired part. The pattern is surrounded, or "invested", into ceramic slurry that hardens into the mold. Investment casting is often referred to as "lost-wax casting" because the wax pattern is melted out of the mold after it has been formed. Lox-wax processes are one-to-one (one pattern creates one part), which increases production time and costs relative to other casting processes. However, since the mold is destroyed during the process, parts with complex geometries and intricate details can be created.

Investment casting can make use of most metals, most commonly using aluminum alloys, bronze alloys, magnesium alloys, cast iron, stainless steel, and tool steel. This process is beneficial for casting metals with high melting temperatures that can not be molded in plaster or metal. Parts that are typically made by investment casting include those with complex geometry such as turbine blades or firearm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries.

Investment casting requires the use of a metal die, wax, ceramic slurry, furnace, molten metal, and any machines needed for sandblasting, cutting, or grinding. The process steps include the following:

1. Pattern creation - The wax patterns are typically injection molded into a metal die and are formed as one piece. Cores may be used to form any internal features on the pattern. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow to the mold cavity.
2. Mold creation - This "pattern tree" is dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mold, hence the name "lost wax" casting.
3. Pouring - The mold is preheated in a furnace to approximately 1000°C (1832°F) and the molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity. Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used.
4. Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mold, and the material used.
5. Casting removal - After the molten metal has cooled, the mold can be broken and the casting removed. The ceramic mold is typically broken using water jets, but several other methods exist. Once removed, the parts are separated from the gating system by either sawing or cold breaking (using liquid nitrogen).
6. Finishing - Often times, finishing operations such as grinding or sandblasting are used to smooth the part at the gates. Heat treatment is also sometimes used to harden the final part

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Permanent Mold Casting

Permanent mold casting is a metal casting process that shares similarities to both sand casting and die casting. As in sand casting, molten metal is poured into a mold which is clamped shut until the material cools and solidifies into the desired part shape. However, sand casting uses an expendable mold which is destroyed after each cycle. Permanent mold casting, like die casting, uses a metal mold (die) that is typically made from steel or cast iron and can be reused for several thousand cycles. Because the molten metal is poured into the die and not forcibly injected, permanent mold casting is often referred to as gravity die casting.

Permanent mold casting is typically used for high-volume production of small, simple metal parts with uniform wall thickness. Non-ferrous metals are typically used in this process, such as aluminum alloys, magnesium alloys, and copper alloys. However, irons and steels can also be cast using graphite molds. Common permanent mold parts include gears and gear housings, pipe fittings, and other automotive and aircraft components such as pistons, impellers, and wheels.

The permanent mold casting process consists of the following steps:

1. Mold preparation - First, the mold is pre-heated to around 300-500°F (150-260°C) to allow better metal flow and reduce defects. Then, a ceramic coating is applied to the mold cavity surfaces to facilitate part removal and increase the mold lifetime.
2. Mold assembly - The mold consists of at least two parts - the two mold halves and any cores used to form complex features. Such cores are typically made from iron or steel, but expendable sand cores are sometimes used. In this step, the cores are inserted and the mold halves are clamped together.
3. Pouring - The molten metal is poured at a slow rate from a ladle into the mold through a sprue at the top of the mold. The metal flows through a runner system and enters the mold cavity.
4. Cooling - The molten metal is allowed to cool and solidify in the mold.
5. Mold opening - After the metal has solidified, the two mold halves are opened and the casting is removed.
6. Trimming - During cooling, the metal in the runner system and sprue solidify attached to the casting. This excess material is now cut away.

Permanent Mold Casting

Using these basic steps, other variations on permanent mold casting have been developed to accommodate specific applications. Examples of these variations include the following:

• Slush Casting - As in permanent mold casting, the molten metal is poured into the mold and begins to solidify at the cavity surface. When the amount of solidified material is equal to the desired wall thickness, the remaining slush (material that has yet to completely solidify) is poured out of the mold. As a result, slush casting is used to produce hollow parts without the use of cores.
• Low Pressure Permanent Mold Casting - Instead of being poured, the molten metal is forced into the mold by low pressure air (< 1 bar). The application of pressure allows the mold to remain filled and reduces shrinkage during cooling. Also, finer details and thinner walls can be molded.
• Vacuum Permanent Mold Casting - Similar to low pressure casting, but vacuum pressure is used to fill the mold. As a result, finer details and thin walls can be molded and the mechanical properties of the castings are improved.