Casting is a manufacturing process by which a liquid material is usually poured into a mould, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process.
Since the evolution of mankind, man has used his intelligence and creative instinct to develop things that will reduce his labor. He shaped bowls, tools and weapons out of stones and wood which was naturally found in nature. With the passage of time he discovered other element in nature like gold, silver and copper. He melted and shaped these metal according to his desires.
Different Ages of Casting: According to Biblical records casting technology can be traced back to 5000 BC Copper Age (7700-3300BC): Stone age is followed by copper age in the prehistoric times. Mankind found copper in the fires from copper-bearing ore that he lined his fire pits. Copper found an instant liking with man because it can be melted and molded into any desired form The Bronze Age: (3300-1200 BC): Eventually, he learned that tin could be mixed with copper to produce a stronger, durable and attractive metal which was called bronze. Iron Age (1200 BC onwards): Iron age brought the systematic production of metals. The advent of this age in every culture was coincidental in changes in agricultural practice, religious beliefs and cultural beliefs Industrial Age (18th Century Onwards): Also known as the age of technical revolution this age saw rapid increased in the demand for casted products. Mass production was made possible by the invention of new machines in this age
Metal casting origin dates back to the period around 3000 BC. It is possible that metal casting technology, using moulds originated in the Middle East. However, there are suggestions that this process may have been developed in India and China.
The melting ovens of the early Iron Age can partly be traced back to ceramic burning ovens. The model and mould building was mastered very well from the beginning. Lost moulds made of loam and clay, wax models, single piece-work as well as permanent moulds made of stone and metal for the serial production of casting parts were already used.
During World War II, with urgent military demands overtaxing the machine tool industry, the art of investment casting provided a shortcut for producing near net shape precision parts and allowed the use of specialized alloys which could not be readily shaped by alternative methods. The Investment Casting process was found practical for many wartime needs--and during the postwar period it expanded into many commercial and industrial applications where complex metal parts were needed.
Evolution of casting process: Gold, silver, copper, iron, lead, mercury and tin are known as the magnificent metals since they were known to man from ancient times. The basic process of melting of metals in furnace, using patterns and solidifying the metal in mould has remained the same. Furnace: The earliest furnace were simple and easy to operate, with bee wax used for patterns and bellows for blowing air into the furnace. In the iron age probably ceramic ovens were used to melt the metals. Molds: Different types of mold made from clay, wax and loam were known from the early times. The lost form technique was also prevalently used from the early times. Patterns: The first patterns of casting were made probably 4000 years from bee wax. A frog casted in copper is the oldest living proof of intricate patterns used as early as 3200 B.C.
In metalworking, casting involves pouring liquid metal into a mould, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mould to complete the process. Casting is most often used for making complex shapes that would be difficult to make by other methods.
Casting is a solidification process, which means the solidification phenomenon controls most of the properties of the casting. Solidification occurs in two steps: nucleation and crystal growth.
In the Nucleation Stage solid particles form within the liquid. When these particles form their internal energy is lower than the surrounded liquid, which is followed by the crystal growth stage
All of the nucleations represent a crystal, which grows as the heat of fusion is extracted from the liquid until there is no liquid left. The direction, rate, and type of growth can be controlled to maximize the properties of the casting. Directional solidification is when the material solidifies at one end and proceeds to solidify to the other end; this is the most ideal type of grain growth because it allows liquid material to compensate for shrinkage.
Area of the casting which is cooled quickly will have a fine grain structure and an area which cools slowly will have a coarse grain structure. Cooling curve for pure metal & eutectic alloy.
Most castings are of alloys, which have a cooling curve shaped as shown below. There is no longer a Thermal arrest, instead there is a freezing range. The freezing range corresponds directly to the liquidus and solidus found on the phase diagram for the specific alloy.
Chvorinovs Rule: The local solidification time can be calculated using Chvorinovs rule, which is: Where t is the solidification time, V is the volume of the casting, A is the surface area of the casting that contacts the mold, n is a constant, and B is the mould constant. It is most useful in determining if a riser will solidify before the casting, because if the riser does solidify first then it is worthless.
The gating system: The gating system serves many purposes, the most important being conveying the liquid material to the mold, but also controlling shrinkage, the speed of the liquid, turbulence, and trapping dross. In especially large castings multiple gates or runners may be required to introduce metal to more than one point in the mold cavity.
The speed of the material is important because if the material is traveling too slowly it can cool before completely filling, leading to misruns and cold shuts. If the material is moving too fast then the liquid material can erode the mold and contaminate the final casting. The shape and length of the gating system can also control how quickly the material cools.
Shrinkage There are three types of shrinkage: Shrinkage of the liquid, Solidification shrinkage Patternmakers shrinkage. The shrinkage of the liquid is rarely a problem because more material is flowing into the mold behind it. Solidification shrinkage occurs because metals are less dense as a liquid than a solid, so during solidification the metal density dramatically increases. Patternmakers shrinkage refers shrinkage that occurs when the material is cooled from the solidification temperature to room temperature, which occurs due to thermal contraction.
Risers and riser aids Risers, also known as feeders, are the most common way of providing directional solidification. It supplies liquid metal to the solidifying casting to compensate for solidification shrinkage.
For a riser to work properly the riser must solidify after the casting, otherwise it cannot supply liquid metal to shrinkage within the casting. Risers add cost to the casting because it lowers the yield of each casting; i.e. more metal is lost as scrap for each casting.
Risers are classified by three criteria. The first is if the riser is open to the atmosphere, if it is then its called an open riser, otherwise its known as a blind type. The second criterion is where the riser is located; if it is located on the casting then it is known as a top riser and if it is located next to the casting it is known as a side riser. Finally, if riser is located on the gating system so that it fills after the molding cavity, it is known as a live riser or hot riser, but if the riser fills with materials thats already flowed through the molding cavity it is known as a dead riser or cold riser.
Contraction allowances: The pattern needs to incorporate suitable allowances for shrinkage; these are called contraction allowances, and their exact values depend on the alloy being cast and the exact sand casting method being used. Draft allowance-When the pattern is to be removed from the sand mold, there is a possibility that any leading edges may break off, or get damaged in the process. To avoid this, a taper is provided on the pattern, so as to facilitate easy removal of the pattern from the mold, and hence reduce damage to edges.
Finishing allowance: The surface finish obtained in sand castings is generally poor (dimensionally inaccurate), and hence in many cases, the cast product is subjected to machining processes like turning , grinding in order to improve the surface finish. During machining processes, some metal is removed from the piece. To compensate for this, a machining allowance should be given in the casting. Shake allowance: Usually during removal of the pattern from the mold cavity, the pattern is rapped all around the faces, in order to facilitate easy removal. In this process, the final cavity is enlarged. To compensate for this, the pattern dimensions need to be reduced.
Distortion allowance : During cooling of the mold, stresses developed in the solid metal may induce distortions in the cast. This is more evident when the mold is thinner in width as compared to its length. This can be eliminated by initially distorting the pattern in the opposite direction.
Fluid Life-A molten metals fluid life is more than its ability to fill the mould cavity. The fluid life also determines how easily and how long the metal flows through narrow channels to form thin sections, and how readily it conforms to fine surface detail. Fluid life will affect the design characteristics of a casting. By understanding the nature of an alloys fluid life, the designer will recognize several important design criteria. Some of these are: Minimum section thickness that can be attained; The maximum length of a thin section; The fineness of cosmetic detail that is possible;
Solidification Shrinkage There are three distinct stages of shrinkage as molten metal alloys solidify: Liquid shrinkage Liquid-to-solid shrinkage Solid shrinkage
Liquid Shrinkage is the contraction of the liquid before solidification begins. While important to metal casters, it is not an important design consideration. Liquid-to-Solid Shrinkage is the shrinkage of the metal as it goes from the liquids disconnected atoms and molecules to the formation of crystals of atoms and chemical compounds, the building blocks of solid metal. Liquid-to-Solid shrinkage is an extremely important consideration for the design engineer. In some alloys, disregard for this type of shrinkage results in voids in the casting. Both the design and foundry engineer have the tools to combat this problem, but the designer has the most cost-effective tool, that is geometry.
Solid Shrinkage (often called patternmakers shrink) occurs after the metal has completely solidified and is cooling to ambient temperature. Solid shrinkage changes the dimension of the casting from those in the mould to those dictated by the rate of solid shrinkage for the alloy. In other words, as the solid casting shrinks away from the mould walls, it assumes final dimensions that must be predicted by the patternmaker. This variability of patternmakers shrink is a very important design consideration.
Slag / Dross Formation Slag is usually is associated with the higher melting point metals (ferrous metals) and is composed of liquid non-metallic compounds , products of alloying and products of oxidation in air. Dross, on the other hand, usually is associated with lower melting point metals (non-ferrous alloys) and often means the non-metallic compounds produced primarily by the molten metal reacting with air.
Pouring Temperature Metal castings are produced in moulds that must withstand the extremely high temperature of liquid metals. Interestingly, there really are not many choices of refractors to do the job. As a result, high molten metal temperatures are very important to casting geometry as well as what casting process should be used. The following is a summary of common foundry alloys and their pouring temperatures:
For practical purposes, sand and ceramic materials with their refractory limits of 3,000 - 3,330°F (1650-1820°C) are the most common mould materials used today. As the temperature of the molten metal alloy increases, design consideration must be given to heat transfer problems and thermal abuse of the mould itself. Metal moulds, such as those used in die-casting and permanent moulding, also have temperature limitations. In fact, most of the alloys on the list are beyond the refractory capability of metal moulds (except for special thin geometry designs, alloys from the copper-base group and up require sand or ceramic moulds).
The chill zone is named so because it occurs at the walls of the mold where the wall chills the material. Here is where the nucleation phase of the solidification process takes place. As more heat is removed the grains grow towards the center of the casting. These are thin, long columns that are perpendicular to the casting surface, which are undesirable because they have anisotropic properties. Finally, the center zone contains spherical, randomly oriented crystals. These are desirable because they have isotropic properties. The creation of this zone can be promoted by using a low pouring temperature, alloy inclusions, or inoculants
Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster, and investment (lost-wax technique) moldings. This method of mold casting involves the use of temporary, non-reusable moulds. Non-expendable mold casting differs from expendable processes in that the mold need not be reformed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting.
Sand casting is one of the most popular and simplest types of casting that has been used for centuries. The most widely used casting process, utilizes expendable sand molds to form complex metal parts. Because the sand mold must be destroyed in order to remove the part, called the casting.
Sand casting typically has a low production rate. he sand casting process involves the use of a furnace, metal, pattern, and sand mold. The metal is melted in the furnace and then ladled and poured into the cavity of the sand mold, which is formed by the pattern.
Advantages : Can produce very large parts. Can form complex shapes. Many material options. Low tooling and equipment cost. Scrap can be recycled. Short lead time possible. Disadvantages: Poor material strength. High porosity possible. Poor surface finish and tolerance. Secondary machining often required. Low production rate. High labor cost. Applications: Engine blocks and manifolds, machine bases, gears, pulleys
Mould-making - The first step in the sand casting process is to create the mold for the casting. In an expendable mold process, this step must be performed for each casting. A sand mold is formed by packing sand into each half of the mold. Clamping - Once the mold has been made, it must be prepared for the molten metal to be poured, the cores are positioned and the mold halves are closed and securely clamped together. Pouring - The molten metal is maintained at a set temperature in a furnace. After the mold has been clamped, the molten metal can be ladled from its holding container in the furnace and poured into the mold. Cooling - The molten metal that is poured into the mold will begin to cool and solidify once it enters the cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. Removal - After the predetermined solidification time has passed, the sand mold can simply be broken, and the casting removed. This step, sometimes called shakeout, is typically performed by a vibrating machine that shakes the sand and casting out of the flask. Trimming - During cooling, the material from the channels in the mold solidifies attached to the part. This excess must be trimmed from the casting either manually via cutting.
The quality of the sand that is used also greatly affects the quality of the casting and is usually described by the following five measures: Strength - Ability of the sand to maintain its shape. Permeability - Ability to allow venting of trapped gases through the sand. A higher permeability can reduce the porosity of the mold, but a lower permeability can result in a better surface finish. Permeability is determined by the size and shape of the sand grains. Thermal stability - Ability to resist damage, such as cracking, from the heat of the molten metal. Collapsibility - Ability of the sand to collapse, or more accurately compress, during solidification of the casting. If the sand can not compress, then the casting will not be able to shrink freely in the mold and can result in cracking. Reusability - Ability of the sand to be reused for future sand molds.
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. Lost -wax processes are one-to-one (one pattern 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
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. 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. 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.
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. 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 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.
Advantages: Can form complex shapes and fine details Many material options High strength parts Very good surface finish and accuracy Little need for secondary machining Disadvantages: Time- Consuming process High labor cost High tooling cost Long lead time possible Applications: Turbine blades, pipe fittings, lock parts, hand tools, jewelry.
Shell Mould casting is a metal casting process similar to sand casting, in that molten metal is poured into an expendable mold. However, in shell mold casting, the mold is a thin-walled shell created from applying a sand-resin mixture around a pattern. The pattern, a metal piece in the shape of the desired part, is reused to form multiple shell molds. A reusable pattern allows for higher production rates, while the disposable molds enable complex geometries to be cast. Shell mold casting requires the use of a metal pattern, oven, sand-resin mixture, dump box, and molten metal.
Pattern creation - A two-piece metal pattern is created in the shape of the desired part, typically from iron or steel. Other materials are sometimes used, such as aluminum for low volume production or graphite for casting reactive materials. Mold creation - First, each pattern half is heated to 175- 370°C (350-700°F) and coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand and a resin binder. The dump box is inverted, allowing this sand-resin mixture to coat the pattern. The heated pattern partially cures the mixture, which now forms a shell around the pattern. Each pattern half and surrounding shell is cured to completion in an oven and then the shell is ejected from the pattern.
Pattern creation - A two-piece metal pattern is created in the shape of the desired part, typically from iron or steel. Other materials are sometimes used, such as aluminum for low volume production or graphite for casting reactive materials. Mould creation - First, each pattern half is heated to 175- 370°C (350-700°F) and coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand and a resin binder. The dump box is inverted, allowing this sand-resin mixture to coat the pattern. The heated pattern partially cures the mixture, which now forms a shell around the pattern. Each pattern half and surrounding shell is cured to completion in an oven and then the shell is ejected from the pattern.
Mould assembly - The two shell halves are joined together and securely clamped to form the complete shell mold. If any cores are required, they are inserted prior to closing the mold. The shell mold is then placed into a flask and supported by a backing material. Pouring - The mold is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mold cavity.
Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting Casting removal - After the molten metal has cooled, the mold can be broken and the casting removed. Trimming and cleaning processes are required to remove any excess metal from the feed system and any sand from the mold.
Advantages: Can form complex shapes and fine details Very good surface finish High production rate Low labor cost Low tooling cost Little scrap generated Disadvantages: High equipment cost Applications: Cylinder heads, connecting rods
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 mould 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.
Mould 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). 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
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. Casting removal - After the casting has cooled and solidified, the rotation is stopped and the casting can be removed. 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.
Advantages: Can form very large parts. Good mechanical properties. Good surface finish and accuracy. Low equipment cost. Low labor cost. Little scrap generated. Disadvantages: Limited to cylindrical parts. Secondary machining is often required for inner diameter. Long lead time possible. Applications :Pipes, wheels, pulleys, nozzles
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). In both machines, after the molten metal is injected into the dies, it rapidly cools and solidifies into the final part, called the casting.
Clamping - The first step is the preparation and clamping of the two halves of the die. Each die half is first cleaned from the previous injection and then lubricated to facilitate the ejection of the next part Injection - The molten metal, which is maintained at a set temperature in the furnace, is next transferred into a chamber where it can be injected into the die. The method of transferring the molten metal is dependent upon the type of die casting machine, whether a hot chamber or cold chamber machine is being used.
Cooling - The molten metal that is injected into the die will begin to cool and solidify once it enters the die cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. The die can not be opened until the cooling time has elapsed and the casting is solidified. Ejection - After the predetermined cooling time has passed, the die halves can be opened and an ejection mechanism can push the casting out of the die cavity
Trimming - During cooling, the material in the channels of the die will solidify attached to the casting. This excess material, along with any flash that has occurred, must be trimmed from the casting either manually via cutting or sawing, or using a trimming press.
Two types of die casting machines are :- Hot chamber die casting machines Cold chamber die casting machines
Hot chamber machines are used for alloys with low melting temperatures, such as zinc, tin, and lead. The temperatures required to melt other alloys would damage the pump, which is in direct contact with the molten metal. The metal is contained in an open holding pot which is placed into a furnace, where it is melted to the necessary temperature. The molten metal then flows into a chamber through an inlet and a plunger, powered by hydraulic pressure, forces the molten metal through a gooseneck channel and into the die. Typical injection pressures for a hot chamber die casting machine are between 1000 and 5000 psi. After the molten metal has been injected into the die cavity, the plunger remains down, holding the pressure while the casting solidifies. After solidification, the hydraulic system retracts the plunger and the part can be ejected by the clamping unit. Prior to the injection of the molten metal, this unit closes and clamps the two halves of the die.
When the die is attached to the die casting machine, each half is fixed to a large plate, called a platen. The front half of the die, called the cover die, is mounted to a stationary platen and aligns with the gooseneck channel. The rear half of the die, called the ejector die, is mounted to a movable platen, which slides along the tie bars. The hydraulically powered clamping unit actuates clamping bars that push this platen towards the cover die and exert enough pressure to keep it closed while the molten metal is injected. Following the solidification of the metal inside the die cavity, the clamping unit releases the die halves and simultaneously causes the ejection system to push the casting out of the open cavity. The die can then be closed for the next injection.
Cold chamber die casting machine are used for alloys with high melting temperatures that can not be cast in hot chamber machines because they would damage the pumping system. Such alloys include aluminum, brass, and magnesium. The molten metal is still contained in an open holding pot which is placed into a furnace, where it is melted to the necessary temperature. However, this holding pot is kept separate from the die casting machine and the molten metal is ladled from the pot for each casting, rather than being pumped. The metal is poured from the ladle into the shot chamber through a pouring hole. The injection system in a cold chamber machine functions similarly to that of a hot chamber machine, however it is usually oriented horizontally and does not include a gooseneck channel.
A plunger, powered by hydraulic pressure, forces the molten metal through the shot chamber and into the injection sleeve in the die. The typical injection pressures for a cold chamber die casting machine are between 2000 and 20000 psi. After the molten metal has been injected into the die cavity, the plunger remains forward, holding the pressure while the casting solidifies. After solidification, the hydraulic system retracts the plunger and the part can be ejected by the clamping unit. The clamping unit and mounting of the dies is identical to e hot chamber machine.
Advantages: Can produce large parts. Can form complex shapes. High strength parts. Very good surface finish and accuracy. High production rate. Low labor cost. Scrap can be recycled. Disadvantages: Trimming is required. High tooling and equipment cost. Limited die life. Long lead time Applications: Engine components, pump components, appliance housing.
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.
Mould preparation - First, the mould is pre-heated to around 300-500°F (150-260°C) to allow better metal flow and reduce defects. Mould assembly - The mould 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
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. Cooling - The molten metal is allowed to cool and solidify in the mould. Mold opening - After the metal has solidified, the two mold halves are opened and the casting is removed. Trimming - During cooling, the metal in the runner system and sprue solidify attached to the casting. This excess material is now cut away.
Advantages: Can form complex shapes. Good mechanical properties. Many material options. Low porosity. Low labor cost Scrap can be recycled. Disadvantages: High tooling cost. Long lead time possible. Applications: Gears, wheels, housings, engine components.
For any Metal Casting Process, selection of right alloy, size, shape, thickness, tolerance, texture, and weight, is very vital. Special requirements such as, magnetism, corrosion, stress distribution also influence the choice of the Metal Casting Process. Views of the Tooling Designer; Foundry / Machine House needs, customers exact product requirements, and secondary operations like painting, must be taken care of before selecting the appropriate Metal Casting Process. Tool cost. Economics of machining versus process costs. Adequate protection / packaging, shipping constraints, regulations of the final components, weights and shelf life of protective coatings also play their part in the Metal Casting process.
Shrinkage defects Shrinkage defects occur when feed metal is not available to compensate for shrinkage as the metal solidifies. Shrinkage defects can be split into two different types: open shrinkage defects and closed shrinkage defects. Open shrinkage defects are open to the atmosphere , therefore as the shrinkage cavity forms air compensates. There are two types of open air defects: pipes and caved surfaces. Pipes form at the surface of the casting and burrow into the casting, while caved surfaces are shallow cavities that form across the surface of the casting. Closed shrinkage defects, also known as shrinkage porosity, are defects that form within the casting. Isolated pools of liquid form inside solidified metal, which are called hot spots. The shrinkage defect usually forms at the top of the hot spots. They require a nucleation point, so impurities and dissolved gas can induce closed shrinkage defects. The defects are broken up into macro porosity and micro porosity (or micro shrinkage), where macro porosity can be seen by the naked eye and micro porosity cannot.
Gas porosity Gas porosity is the formation of bubbles within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it cools. Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal,[which reduces strength in that vicinity. Nitrogen, oxygen and hydrogen are the most encountered gases in cases of gas porosity. In aluminum castings, hydrogen is the only gas that dissolves in significant quantity, which can result in hydrogen gas porosity.
Pouring Metal defects Pouring metal defects include misruns, cold shuts, and inclusions. A misrun occurs when the liquid metal does not completely fill the mold cavity, leaving an unfilled portion. Cold shuts occur when two fronts of liquid metal do not fuse properly in the mold cavity, leaving a weak spot. Both are caused by either a lack of fluidity in the molten metal or cross-sections that are too narrow. The fluidity can be increased by changing the chemical composition of the metal or by increasing the pouring temperature. Another possible cause is back pressure from improperly vented mold cavities
Metallurgical defects There are two defects in this category: hot tears and hot spots. Hot tears, also known as hot cracking, are failures in the casting that occur as the casting cools. This happens because the metal is weak when it is hot and the residual stresses in the material can cause the casting to fail as it cools. Proper mold design prevents this type of defect. Hot spots are areas on the surface of casting that become very hard because they cooled more quickly than the surrounding material. This type of defect can be avoided by proper cooling practices or by changing the chemical composition of the metal.