CASTING:

Definition:

Casting is a manufacturing process by which a liquid material is usually poured into a mold, 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.

Casting materials are usually metals or various cold setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.

Types of casting:

Metal casting:

Metal casting is one of the most common casting processes. Metal patterns are more expensive but are more dimensionally stable and durable. Metallic patterns are used where repetitive production of castings is required in large quantities.

Plaster, concrete, or plastic resin:

Plaster and other chemical setting materials such as concrete and plastic resin may be cast using single-use waste molds, multiple-use ‘piece’ molds, or molds made of small rigid pieces or of flexible material such as latex rubber (which is in turn supported by an exterior mold). When casting plaster or concrete, the finished product is, unlike marble, unattractive, lacking in transparency, and so it is usually painted, often in ways that give the appearance of metal or stone. Alternatively, the first layers cast may contain colored sand so as to give an appearance of stone. By casting concrete, rather than plaster, it is possible to create sculptures, fountains, or seating for outdoor use. A simulation of high-quality marble may be made using certain chemically-set plastic resins (for example epoxy or polyester) with powdered stone added for coloration, often with multiple colors worked in. The latter is a common means of making attractive washstands, washstand tops and shower stalls, with the skilled working of multiple colors resulting in simulated staining patterns as is often found in natural marble or travertine.

Sand casting:

Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the mold material. The term “sand casting” can also refer to an object produced via the sand casting process. Sand castings are produced in specialized factories called foundries. Over 70% of all metal castings are produced via a sand casting process.

Sand casting is relatively cheap and sufficiently refractory even for steel foundry use. In addition to the sand, a suitable bonding agent (usually clay) is mixed or occurs with the sand. The mixture is moistened, typically with water, but sometimes with other substances, to develop strength and plasticity of the clay and to make the aggregate suitable for molding. The sand is typically contained in a system of frames or mold boxes known as a flask. The mold cavities and gate system are created by compacting the sand around models, or patterns, or carved directly into the sand.

Procedure to make sand mould:

1. Lay a flat-sided object on a piece of plywood.

2. Place a wooden box mold over this object, centering the article.

3. Sprinkle parting powder over the item to be molded.

4. Sift fine sand and dry clay through a window screen to remove any large particles. Use 5 percent clay to sand, as an initial ratio. This sifting motion is called riddling.

5. Sprinkle with water and mix. Continue to add water until the mixture resembles a wrung-out sponge. Allow the sand mixture to absorb this water for an hour.

6. Squeeze the sand mix and be certain that it holds its shape but does not stick to your hand. This is proof that the sand is the correct consistency for mold-making. Sprinkle more powdered clay into the mix, if need be, until it passes this test.

7. Press the sand around the object with hand pressure.

8. Use some wet sand and clay mixture that has not been sieved to fill the rest of the mold box. After the finest sand encases your casting article, you may fill the rest of the box with a coarser damp mix.

9. Compress the sand with a tamping ram. A ram is made of sturdy oak or other hardwood.

10. Drag the top edge of the box with a straight edge to flatten it. Make multiple passes, and exert pressure as you grade the top of the mix to level it with the molding box.

11. Turn the wooden box, called a drag, over to reveal the indentation to be cast. This is called a green mold and must dry completely before it can be used in hot metal casting. Plaster casts can be made in wet sand.

Core

A core is a device used in casting and molding processes to produce internal cavities and reentrant angles. The core is normally a disposable item that is destroyed to get it out of the piece. They are most commonly used in sand casting, but are also used in injection molding.

Types of core making:

There are many types of cores available. The selection of the correct type of core depends on production quantity, production rate, required precision, required surface finish, and the type of metal being used. For example, certain metals are sensitive to gases that are given off by certain types of core sands; other metals have too low of a melting point to properly break down the binder for removal during the shakeout.

Green-sand cores:

Green-sand cores are not a typical type of core in that it is part of the cope and drag, but still form an internal feature. Their major disadvantage is their lack of strength, which makes casting long narrow features difficult or impossible. Even for long features that can be cast it still leave much material to be machined. A typical application is a through hole in a casting.

 

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Fig: Green-sand cores

Dry-sand cores:

Dry-sand cores overcome some of the disadvantages of the green-sand cores. They are formed independently of the mold and then inserted into the core prints in the mold, which hold the core in position. They are made by mixing sand with a binder in a wooden or metal core box, which contains a cavity in the shape of the desired core.

The simplest way to make dry-sand cores is in a dump core box, in which sand is packed into the box and scraped level with the top. A wood or metal plate is then placed over the box, and then the two are flipped over and the core segment falls out of the core box. The core segment is then baked or hardened. Multiple core segments are then hot glued together or attached by some other means. Any rough spots are filed or sanded down. Finally, the core is lightly coated with graphite, silica, or mica to give a smoother surface finish and greater resistance to heat.

Single-piece cores do not need to be assembled because they are made in a split core box. A split core box, like it sounds, is made of two halves and has at least one hole for sand to be introduced. For simple cores that have constant cross-sections they can be created on special core-producing extruders. The extrusions are then just cut to the proper length and hardened. More complex single-piece cores can be made in a manner similar to injection moldings and die castings.

Types of core boxes:

· half core box

· dump core box

· split core box

· left and right core box

· gang core box

· strickle core box

· loose piece core box

· Ghayoor

-Dangleberries

Lost cores

Core is used for complex injection moldings in the fusible core injection molding process. First, a core is made from a fusible alloy or low melting temperature polymer. It is then placed inside the injection mold’s dies and the plastic is shot into the mold. The molding is then removed from the mold with the core still in it. Finally, the core is melted or washed out of the molding in a hot bath.

Moulding tools:

The moulder’s tool kit is simple and includes articles of the following list, most of which are shown in Fig below:

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(1)Vent wire for sticking vent holes through the sand of the mould.

(2)Pattern lifter.

(3)Joint trowel

(4)heart trowel for smoothing and finishing the parting and flat surfaces of the mould.

(5) Gate cutter and pattern lifter.

(6) Slick and oval spoon for finishing mould surfaces.

(7) (8) Sand lifter sand slicks.

(9) Yankee heel lifter and flat slick.

(10)Flange and bead slick.

(11) Corner slick.

(12) Edge slick.

(13) Bound corner slick.

(14)Pipe slick.

(15) Button slick.

(16) Oval Slick.

Injection moulding machine:

An Injection molding machine, also known as an injection press, is a machine for manufacturing plastic products by the injection molding process. It consists of two main parts, an injection unit and a clamping unit.

Operation:

Injection molding machines can fasten the molds in either a horizontal or vertical position. The majority of machines are horizontally oriented, but vertical machines are used in some niche applications such as insert molding, allowing the machine to take advantage of gravity. Some vertical machines also don’t require the mold to be fastened. There are many ways to fasten the tools to the platens, the most common being manual clamps (both halves are bolted to the platens); however hydraulic clamps (chocks are used to hold the tool in place) and magnetic clamps are also used. The magnetic and hydraulic clamps are used where fast tool changes are required.

The person designing the mold chooses whether the mold uses a cold runner system or a hot runner system to carry the plastic from the injection unit to the cavities. A cold runner is a simple channel carved into the mold. The plastic that fills the cold runner cools as the part cools and is then ejected with the part as a sprue. A hot runner system is more complicated, often using cartridge heaters to keep the plastic in the runners hot as the part cools. After the part is ejected, the plastic remaining in a hot runner is injected into the next part.

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Fig: machine moulding

Co2 moulding process:

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Shell moulding:

Shell molding, also known as shell-mold casting, is an expendable mold casting process that uses a resin covered sand to form the mold. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labor requirements. It is used for small to medium parts that require high precision.

Shell mold 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.

Shell mold casting allows the use of both ferrous and non-ferrous metals, most commonly using cast iron, carbon steel, alloy steel, stainless steel, aluminum alloys, and copper alloys. Typical parts are small-to-medium in size and require high accuracy, such as gear housings, cylinder heads, connecting rods, and lever arms.

The shell mold casting process consists of the following steps:

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.

Mold 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.

Examples of shell molded items include gear housings, cylinder heads and connecting rods. It is also used to make high-precision molding cores.

The process of creating a shell mold consists of six steps:

1. Fine silica sand that is covered in a thin (3–6%) thermosetting phenolic resin and liquid catalyst is dumped, blown, or shot onto a hot pattern. The pattern is usually made from cast iron and is heated to 230 to 315 °C (450 to 600 °F). The sand is allowed to sit on the pattern for a few minutes to allow the sand to partially cure.

2. The pattern and sand are then inverted so the excess sand drops free of the pattern, leaving just the “shell”. Depending on the time and temperature of the pattern the thickness of the shell is 10 to 20 mm (0.4 to 0.8 in).

3. The pattern and shell together are placed in an oven to finish curing the sand. The shell now has a tensile strength of 350 to 450 psi (2.4 to 3.1 MPa).

4. The hardened shell is then stripped from the pattern.

5. Two or more shells are then combined, via clamping or gluing using a thermoset adhesive, to form a mold. This finished mold can then be used immediately or stored almost indefinitely.

6. For casting the shell mold is placed inside a flask and surrounded with shot, sand, or gravel to reinforce the shell.

The machine that is used for this process is called a shell molding machine. It heats the pattern, applies the sand mixture, and bakes the shell.

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Investment casting

Investment casting is an industrial process based on and also called lost-wax casting, one of the oldest known metal-forming techniques. From 5,000 years ago, when beeswax formed the pattern, to today’s high-technology waxes, refractory materials and specialist alloys, the castings allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys. Lost-foam casting is a modern form of investment casting that eliminates certain steps in the process.

The process is generally used for small castings, but has been used to produce complete aircraft door frames, steel castings of up to 300 kg (660 lbs) and aluminium castings of up to 30 kg (66 lbs). It is generally more expensive per unit than die casting or sand casting, but has lower equipment costs. It can produce complicated shapes that would be difficult or impossible with die casting, yet like that process, it requires little surface finishing and only minor machining.

Process:

Casts can be made of the wax model itself, the direct method; or of a wax copy of a model that need not be of wax, the indirect method. The following steps are for the indirect process which can take two days to one week to complete.

Produce a master pattern: An artist or mould-maker creates an original pattern from wax, clay, wood, plastic, steel, or another material.

Mould making: A mould, known as the master die, is made of the master pattern. The master pattern may be made from a low-melting-point metal, steel, or wood. If a steel pattern was created then a low-melting-point metal may be cast directly from the master pattern. Rubber moulds can also be cast directly from the master pattern. The first step may also be skipped if the master die is machined directly into steel.

Produce the wax patterns: Although called a wax pattern, pattern materials also include plastic and frozen mercury. Wax patterns may be produced in one of two ways. In one process the wax is poured into the mold and swished around until an even coating, usually about 3 mm (0.12 in) thick, covers the inner surface of the mould. This is repeated until the desired thickness is reached. Another method is filling the entire mould with molten wax, and let it cool, until a desired thickness has set on the surface of the mould. After this the rest of the wax is poured out again, the mould is turned upside down and the wax layer is left to cool and harden. With this method it is more difficult to control the overall thickness of the wax layer.
If a core is required, there are two options: soluble wax or ceramic. Soluble wax cores are designed to melt out of the investment coating with the rest of the wax pattern, whereas ceramic cores remain part of the wax pattern and are removed after the work piece is cast.

Assemble the wax patterns: The wax pattern is then removed from the mould. Depending on the application multiple wax patterns may be created so that they can all be cast at once. In other applications, multiple different wax patterns may be created and then assembled into one complex pattern. In the first case the multiple patterns are attached to a wax sprue, with the result known as a pattern cluster, or tree; as many as several hundred patterns may be assembled into a tree. Foundries often use registration marks to indicate exactly where they go.The wax patterns are attached to the sprue or each other

by means of a heated metal tool. The wax pattern may also be chased, which means the parting line or flashing are rubbed out using the heated metal tool. Finally it is dressed, which means any other imperfections are addressed so that the wax now looks like the finished piece.

1. Investment: The ceramic mould, known as the investment, is produced by three repeating steps: coating, stuccoing, and hardening. The first step involves dipping the cluster into a slurry of fine refractory material and then letting any excess drain off, so a uniform surface is produced. This fine material is used first to give a smooth surface finish and reproduce fine details. In the second step, the cluster is stuccoed with a coarse ceramic particle, by dipping it into a fluidised bed, placing it in a rainfall-sander, or by applying by hand. Finally, the coating is allowed to harden. These steps are repeated until the investment is the required thickness, which is usually 5 to 15 mm (0.2 to 0.6 in). Note that the first coatings are known as prime coats. An alternative to multiple dips is to place the cluster upside-down in a flask and then liquid investment material is poured into the flask. The flask is then vibrated to allow entrapped air to escape and help the investment material fill in all of the details.

Common refractory materials used to create the investments are: silica, zircon, various aluminium silicates, and alumina. Silica is usually used in the fused silica form, but sometimes quartz is used because it is less expensive. Aluminium silicates are a mixture of alumina and silica, where commonly used mixtures have an alumina content from 42 to 72%; at 72% alumina the compound is known as mullite. During the primary coat(s), zircon-based refractories are commonly used, because zirconium is less likely to react with the molten metal. Chamotte is another refractory material that has been used.Prior to silica, a mixture of plaster and ground up old molds (chamotte) was used.

The binders used to hold the refractory material in place include: ethyl silicate (alcohol-based and chemically set), colloidal silica (water-based, also known as silica sol, set by drying), sodium silicate, and a hybrid of these controlled for pH and viscosity.

2. Dewax: The investment is then allowed to completely dry, which can take 16 to 48 hours. Drying can be enhanced by applying a vacuum or minimizing the environmental humidity. It is then turned upside-down and placed in a furnace or autoclave to melt out and/or vaporize the wax. Most shell failures occur at this point because the waxes used have a thermal expansion coefficient that is much greater than the investment material surrounding it, so as the wax is heated it expands and induces great stresses. In order to minimize these stresses the wax is heated as rapidly as possible so that the surface of the wax can melt into the surface of the investment or run out of the mold, which makes room for the rest of the wax to expand. In certain situations holes may be drilled into the mold beforehand to help reduce these stresses. Any wax that runs out of the mold is usually recovered and reused.

3. Burnout & preheating: The mold is then subjected to a burnout, which heats the mold between 870 °C and 1095 °C to remove any moisture and residual wax, and to sinter the mold. Sometimes this heating is also used as the preheat, but other times the mold is allowed to cool so that it can be tested. If any cracks are found they can be repaired with ceramic slurry or special cements. The mold is preheated to allow the metal to stay liquid longer to fill any details and to increase dimensional accuracy, because the mold and casting cool together.

4. Pouring: The investment mold is then placed cup-upwards into a tub filled with sand. The metal may be gravity poured, but if there are thin sections in the mold it may be filled by applying positive air pressure, vacuum cast, tilt cast, pressure assisted pouring, or centrifugal cast.

5. Removal: The shell is hammered, media blasted, vibrated, water jeted, or chemically dissolved (sometimes with liquid nitrogen) to release the casting. The sprue is cut off and recycled. The casting may then be cleaned up to remove signs of the casting process, usually by grinding.

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The investment shell for casting a turbo charger rotor

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A view of the interior investment shows the smooth surface finish and high level of detail

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The completed work piece

Advantage of investment casting:

· Many Intricate forms with undercuts can be cast.

· A very smooth surface is obtained with no parting line.

· Dimensional accuracy is good.

· Certain unmachinable parts can be cast to preplanned shape.

· It may be used to replace die-casting where short runs are involved.

Disadvantage of investment casting:

· This process is expensive, is usually limited to small casting, and presents some difficulties where cores are involved.

· Holes cannot be smaller than 1/16 in. (1.6mm) and should be no deeper than about 1.5 times the diameter.

· Investment castings require very long production-cycle times versus other casting processes.

· This process is practically infeasible for high-volume manufacturing, due to its high cost and long cycle times.

· Many of the advantages of the investment casting process can be achieved through other casting techniques if principles of thermal design and control are applied appropriately to existing processes that do not involve the shortcomings of investment castings.

Permanent mold casting:

Permanent mold casting is metal casting process that employs reusable molds (“permanent molds”), usually made from metal. The most common process uses gravity to fill the mold, however gas pressure or a vacuum are also used. A variation on the typical gravity casting process, called slush casting, produces hollow castings. Common casting metals are aluminum, magnesium, and copper alloys. Other materials include tin, zinc, and lead alloys and iron and steel are also cast in graphite molds.

Typical parts include gears, splines, wheels, gear housings, pipe fittings, fuel injection housings, and automotive engine pistons.

Process:

There are four main types of permanent mold casting: gravity, slush, low-pressure, and vacuum.

Gravity process

The gravity process begins by preheating the mold to 150-200 °C (300-400 °F) to ease the flow and reduce thermal damage to the casting. The mold cavity is then coated with a refractory material or a mold wash, which prevents the casting from sticking to the mold and prolongs the mold life. Any sand or metal cores are then installed and the mold is clamped shut. Molten metal is then poured into the mold. Soon after solidification the mold is opened and the casting removed to reduce chances of hot tears. The process is then started all over again, but preheating is not required because the heat from the previous casting is adequate and the refractory coating should last several castings. Because this process is usually carried out on large production run work pieces automated equipment is used to coat the mold, pour the metal, and remove the casting.

The metal is poured at the lowest practical temperature in order to minimize cracks and porosity. The pouring temperature can range greatly depending on the casting material; for instance zinc alloys are poured at approximately 700 °F (371 °C), while gray iron is poured at approximately 2,500 °F (1,370 °C).

Mold

Molds for the casting process consist of two halves. Casting molds are usually formed from gray cast iron because it has about the best thermal fatigue resistance, but other materials include steel, bronze, and graphite. These metals are chosen because of their resistance to erosion and thermal fatigue. They are usually not very complex because the mold offers no collapsibility to compensate for shrinkage. Instead the mold is opened as soon as the casting is solidified, which prevents hot tears. Cores can be used and are usually made from sand or metal.

As stated above, the mold is heated prior to the first casting cycle and then used continuously in order to maintain as uniform a temperature as possible during the cycles. This decreases thermal fatigue, facilitates metal flow, and helps control the cooling rate of the casting metal.

Venting usually occurs through the slight crack between the two mold halves, but if this is not enough then very small vent holes are used. They are small enough to let the air escape but not the molten metal. A riser must also be included to compensate for shrinkage. This usually limits the yield to less than 60%.

Mechanical ejectors in the form of pins are used when coatings are not enough to remove casts from the molds. These pins are placed throughout the mold and usually leave small round impressions on the casting.

Slush:

Slush casting is a variant of permanent molding casting to create a hollow casting or hollow cast. In the process the material is poured into the mold and allowed to cool until a shell of material forms in the mold. The remaining liquid

is then poured out to leave a hollow shell. The resulting casting has good surface detail but the wall thickness can vary. The process is usually used to cast ornamental products, such as candlesticks, lamp bases, and statuary, from low-melting-point materials. A similar technique is used to make hollow chocolate figures for Easter and Christmas.

The method was developed by William Britain in 1893 for the production of lead toy soldiers. It uses less material than solid casting, and results in a lighter and less expensive product. Hollow cast figures generally have a small hole where the excess liquid was poured out.

Low-pressure

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Fig: Schematic of the low-pressure permanent mold casting process

Low-pressure permanent mold (LPPM) casting uses a gas at low pressure, usually between 3 and 15 psig (20 to 100 kPag) to push the molten metal into the mold cavity. The pressure is applied to the top of the pool of liquid, which forces the molten metal up a refractory pouring tube and finally into the bottom of the mold. The pouring tube extends to the bottom of the ladle so that the material being pushed into the mold is exceptionally clean. No risers are required because the applied pressure forces molten metal in to compensate for shrinkage. Yields are usually greater than 85% because there is no riser and any metal in the pouring tube just falls back into the ladle for reuse.

The vast majority of LPPM castings are from aluminum and magnesium, but some are copper alloys. Advantages include very little turbulence when filling the mold because of the constant pressure, which minimizes gas porosity and dross formation. Mechanical properties are about 5% better than gravity permanent mold castings. The disadvantage is that cycles times are longer than gravity permanent mold castings.

Vacuum

Vacuum permanent mold casting retains all of the advantages of LPPM casting, plus the dissolved gases in the molten metal are minimized and molten metal cleanliness is even better. The process can handle thin-walled profiles and gives an excellent surface finish. Mechanical properties are usually 10 to 15% better than gravity permanent mold castings. The process is limited in weight to 0.2 to 5 kg (0.44 to 11 lb).

Advantages:

The main advantages are the reusable mold, good surface finish, and good dimensional accuracy. Typical tolerances are 0.4 mm for the first 25 mm (0.015 in for the first inch) and 0.02 mm for each additional centimeter (0.002 in per in); if the dimension crosses the parting line add an additional 0.25 mm (0.0098 in). Typical surface finishes are 2.5 to 7.5 μm (100–250 μin) RMS. A draft of 2 to 3° is required. Wall thicknesses are limited to 3 to 50 mm (0.12 to 2.0 in). Typical part sizes range from 100 g to 75 kg (several ounces to 150 lb). Other advantages include the ease of inducing directional solidification by changing the mold wall thickness or by heating or cooling portions of the mold. The fast cooling rates created by using a metal mold results in a finer grain structure than sand casting. Retractable metal cores can be used to create undercuts while maintaining a quick action mold.

Disadvantages:

There are three main disadvantages: high tooling cost, limited to low-melting-point metals, and short mold life. The high tooling costs make this process uneconomical for small production runs. When the process is used to cast steel or iron the mold life is extremely short. For lower melting point metals the mold life is longer but thermal fatigue and erosion usually limit the life to 10,000 to 120,000 cycles. The mold life is dependent on four factors: the mold material, the pouring temperature, the mold temperature, and the mold configuration. The pouring temperature is dependent on the casting metal, but the higher the pouring temperature the shorter the mold life. A high pouring temperature can also induce shrinkage problems and create longer cycle times. If the mold temperature is too low misruns are produced, but if the mold temperature is too high then the cycle time is prolonged and mold erosion is increased. Large differences in section thickness in the mold or casting can decrease mold life as well.

Pressure Die Casting:

Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process. Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter andtin based alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.

The casting equipment and the metal dies represent large capital costs and this tends to limit the process to high volume production. Manufacture of parts using die casting is relatively simple, involving only four main steps, which keeps the incremental cost per item low. It is especially suited for a large quantity of small to medium sized castings, which is why die casting produces more castings than any other casting process. Die castings are characterized by a very good surface finish (by casting standards) and dimensional consistency.

Two variants are pore-free die casting, which is used to eliminate gas porosity defects; and direct injection die casting, which is used with zinc castings to reduce scrap and increase yield.

Equipment:

There are two basic types of die casting machines: hot-chamber machines and cold-chamber machines. These are rated by how much clamping force they can apply. Typical ratings are between 400 and 4,000 st (2,500 and 25,000 kg).

Hot-chamber machines

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Fig: Schematic of a hot-chamber machine

Hot-chamber machines, also known as gooseneck machines, rely upon a pool of molten metal to feed the die. At the beginning of the cycle the piston of the machine is retracted, which allows the molten metal to fill the “gooseneck”. The pneumatic or hydraulic powered piston then forces this metal out of the gooseneck into the die. The advantages of this system include fast cycle times (approximately 15 cycles a minute) and the convenience of melting the metal in the casting machine. The disadvantages of this system are that high-melting point metals cannot be utilized and aluminum cannot be used because it picks up some of the iron while in the molten pool. Due to this, hot-chamber machines are primarily used with zinc, tin, and lead based alloys.

Cold-chamber machines

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Fig: A schematic of a cold-chamber die casting machine.

These are used when the casting alloy cannot be used in hot-chamber machines; these include aluminum, zinc alloys with a large composition of aluminum, magnesium and copper. The process for these machines starts with melting the metal in a separate furnace. Then a precise amount of molten metal is transported to the cold-chamber machine where it is fed into an unheated shot chamber (or injection cylinder). This shot is then driven into the die by a hydraulic or mechanical piston. This biggest disadvantage of this system is the slower cycle time due to the need to transfer the molten metal from the furnace to the cold-chamber machine.

Centrifugal casting:

Centrifugal casting or rotocasting is a casting technique that is typically used to cast thin-walled cylinders. It is noted for the high quality of the results attainable, particularly for precise control of their metallurgy and crystal structure. Unlike most other casting techniques, centrifugal casting is chiefly used to manufacture stock materials in standard sizes for further machining, rather than shaped parts tailored to a particular end-use.

Process:

In centrifugal casting, a permanent mold is rotated continuously about its axis at high speeds (300 to 3000 rpm) as the molten metal is poured. The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling. The casting is usually a fine-grained casting with a very fine-grained outer diameter, owing to chilling against the mould surface. Impurities and inclusions are thrown to the surface of the inside diameter, which can be machined away.

Casting machines may be either horizontal or vertical-axis. Horizontal axis machines are preferred for long, thin cylinders, vertical machines for rings.

Most castings are solidified from the outside first. This may be used to encourage directional solidification of the casting, and thus give useful metallurgical properties to it. Often the inner and outer layers are discarded and only the intermediary columnar zone is used.

Centrifugal casting was the invention of Alfred Krupp, who used it to manufacture cast steel tyres for railway wheels in 1852.

Features of centrifugal casting:

· Castings can be made in almost any length, thickness and diameter.

· Different wall thicknesses can be produced from the same size mold.

· Eliminates the need for cores.

· Resistant to atmospheric corrosion, a typical situation with pipes.

· Mechanical properties of centrifugal castings are excellent.

· Only cylindrical shapes can be produced with this process.

· Size limits are up to 3 m (10 feet) diameter and 15 m (50 feet) length.

· Wall thickness range from 2.5 mm to 125 mm (0.1 – 5.0 in).

· Tolerance limit: on the OD can be 2.5 mm (0.1 in) on the ID can be 3.8 mm (0.15 in).

· Surface finish ranges from 2.5 mm to 12.5 mm (0.1 – 0.5 in) rms.

Benefits:

Cylinders and shapes with rotational symmetry are most commonly cast by this technique. “Tall” castings (in the direction of the settling force acting, usually gravity) are always more difficult than short castings. In the centrifugal casting technique the radius of the rotation, along which the centrifugal force acts, replaces the vertical axis. The casting machine may be rotated to place this in any convenient orientation, relative to gravity’s vertical. Horizontal and vertical axis machines are both used, simply to place the casting’s longest dimension conveniently horizontal.

Thin-walled cylinders are difficult to cast by other means, but centrifugal casting is particularly suited to them. To the rotation radius, these are effectively shallow flat castings and are thus simple.

Centrifugal casting is also applied to the casting of disk and cylindrical shaped objects such as railway carriage wheels or machine fittings where the grain, flow, and balance are important to the durability and utility of the finished product.

Providing that the shape is relatively constant in radius, noncircular shapes may also be cast.

Materials:

Typical materials that can be cast with this process are iron, steel, stainless steels, glass, and alloys of aluminum, copper and nickel.

Two materials can be cast together by introducing a second material during the process.

Applications:

Typical parts made by this process are pipes, boilers, pressure vessels ( flywheels, cylinder liners and other parts that are axi-symmetric. It is notably used to cast cylinder liners and sleeve valves for piston engines, parts which could not be reliably manufactured otherwise.

Continuous casting:

Continuous casting, also called strand casting, is the process whereby molten metal is solidified into a “semi finished” billet, bloom, or slab for subsequent rolling in the finishing mills. Prior to the introduction of continuous casting in the 1950s, steel was poured into stationary molds to form ingots. Since then, “continuous casting” has evolved to achieve improved yield, quality, productivity and cost efficiency. It allows lower-cost production of metal sections with better quality, due to the inherently lower costs of continuous, standardised production of a product, as well as providing increased control over the process through automation. This process is used most frequently to cast steel (in terms of tonnage cast). Aluminium and copper are also continuously cast.

Casting defects:

A casting defect is an irregularity in the metal casting process that is undesired. Some defects can be tolerated while others can be repaired otherwise they must be eliminated. They are broken down into five main categories: gas porosity, shrinkage defects, mold material defects, pouring metal defects, and metallurgical defects.

Types:

There are many types of defects which result from many different causes. Some of the solutions to certain defects can be the cause for another type of defect.[3]

The following defects can occur in sand castings. Most of these also occur in other casting processes.

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 macroporosity and microporosity (or microshrinkage), where macroporosity can be seen by the naked eye and microporosity 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.For casting that are a few kilograms in weight the pores are usually 0.01 to 0.5 mm (0.00039 to 0.020 in) in size. In larger casting they can be up to a millimeter (0.040 in) in diameter.

To prevent gas porosity the material may be melted in a vacuum, in an environment of low-solubility gases, such as argon or carbon dioxide, or under a flux that prevents contact with the air. To minimize gas solubility the superheat temperatures can be kept low. Turbulence from pouring the liquid metal into the mold can introduce gases, so the molds are often streamlined to minimize such turbulence. Other methods include vacuum degassing, gas flushing, or precipitation. Precipitation involves reacting the gas with another element to form a compound that will form a dross that floats to the top. For instance, oxygen can be removed from copper by adding phosphorus; aluminum or silicon can be added to steel to remove oxygen. A third source consists of reactions of the molten metal with grease or other residues in the mould.

Hydrogen is normally produced by the reaction of the metal with humidity or residual moisture in the mold. Drying the mold can eliminate this source of hydrogen formation.

Gas porosity can sometimes be difficult to distinguish from micro shrinkage because micro shrinkage cavities can contain gases as well. In general, micro porosities will form if the casting is not properly risered or if a material with a wide solidification range is cast. If neither of these are the case then most likely the porosity is due to gas formation.

clip_image001[6]

Fig Blowhole defect in a cast iron part.

Tiny gas bubbles are called porosities, but larger gas bubbles are called a blowholes or blisters. Such defects can be caused by air entrained in the melt, steam or smoke from the casting sand, or other gasses from the melt or mold. (Vacuum holes caused by metal shrinkage (see above) may also be loosely referred to as ‘blowholes’). Proper foundry practices, including melt preparation and mold design, can reduce the occurrence of these defects. Because they are often surrounded by a skin of sound metal, blowholes may be difficult to detect, requiring harmonic, ultrasonic, magnetic, or X-ray (i.e.,industrial CT scanning) analysis.

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 mould 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.

Misruns and cold shuts are closely related and both involve the material freezing before it completely fills the mold cavity. These types of defects are serious because the area surrounding the defect is significantly weaker than intended. The castability and viscosity of the material can be important factors with these problems. Fluidity affects the minimum section thickness that can be cast, the maximum length of thin sections, fineness of feasibly cast details, and the accuracy of filling mold extremities. There are various ways of measuring the fluidity of a material, although it usually involves using a standard mould shape and measuring the distance the material flows. Fluidity is affected by the composition of the material, freezing temperature or range, surface tension of oxide films, and, most importantly, the pouring temperature. The higher the pouring temperature, the greater the fluidity; however, excessive temperatures can be detrimental, leading to a reaction between the material and the mold; in casting processes that use a porous mould material the material may even penetrate the mould material.

The point at which the material cannot flow is called the coherency point. The point is difficult to predict in mold design because it is dependent on the solid fraction, the structure of the solidified particles, and the local shear strain rate of the fluid. Usually this value ranges from 0.4 to 0.8.

An inclusion is a metal contamination of dross, if solid, or slag, if liquid. These usually are metal oxides, nitrides, carbides, calcides, or sulfides; they can come from material that is eroded from furnace or ladle linings, or contaminates from the mold. In the specific case of aluminium alloys, it is important to control the concentration of inclusions by measuring them in the liquid aluminium and taking actions to keep them to the required level.

There are a number of ways to reduce the concentration of inclusions. In order to reduce oxide formation the metal can be melted with a flux, in a vacuum, or in an inert atmosphere. Other ingredients can be added to the mixture to cause the dross to float to the top where it can be skimmed off before the metal is poured into the mold. If this is not practical, then a special ladle that pours the metal from the bottom can be used. Another option is to install ceramic filters into the gating system. Otherwise swirl gates can be formed which swirl the liquid metal as it is poured in, forcing the lighter inclusions to the center and keeping them out of the casting If some of the dross or slag is folded into the molten metal then it becomes an entrainment defect.

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.

Processes specific defects:
Die casting

In die casting the most common defects are misruns and cold shuts. These defects can be caused by cold dies, low metal temperature, dirty metal, lack of venting, or too much lubricant. Other possible defects are gas porosity, shrinkage porosity, hot tears, and flow marks. Flow marks are marks left on the surface of the casting due to poor gating, sharp corners, or excessive lubricant.

Continuous casting

A longitudinal facial crack is a specialized type of defect that only occurs in continuous casting processes. This defect is caused by uneven cooling, both primary cooling and secondary cooling, and includes molten steel qualities, such as the chemical composition being out of specification, cleanliness of the material, and homogeneity.

Sand casting

Sand casting has many defects that can occur due to the mold failing. The mold usually fails because of one of two reasons: the wrong material is used or it is improperly rammed.

The first type is mold erosion, which is the wearing away of the mold as the liquid metal fills the mold. This type of defect usually only occurs in sand castings because most other casting processes have more

robust molds. The castings produced have rough spots and excess material. The molding sand becomes incorporated into the casting metal and decreases the ductility, fatigue strength, and fracture toughness of the casting. This can be caused by a sand with too little strength or a pouring velocity that is too fast. The pouring velocity can be reduced by redesigning the gating system to use larger runners or multiple gates. A related source of defects are drops, in which part of the molding sand from the cope drops into the casting while it is still a liquid. This also occurs when the mold is not properly rammed.

The second type of defect is metal penetration, which is when the liquid metal penetrates into the molding sand. This causes a rough surface finish. This caused by sand particles that are too coarse, lack of mold wash, or pouring temperatures that are too high.

If the pouring temperature is too high or a sand of low melting point is used then the sand can fuse to the casting. When this happens the surface of the casting produced has a brittle, glassy appearance.

A run out is when the liquid metal leaks out of the mold because of a faulty mold or flask.

Scabs are a thin layer of metal that sits proud of the casting. They are easy to remove and always reveal a buckle underneath, which is an indentation in the casting surface. Rattails are similar to buckles, except they are thin line indentations and not associated with scabs. Another similar defect is a pull downs, which are buckles that occur in the cope of sand castings. All of these defects are visual in nature and no reason to scrap the work piece. These defects are caused by overly high pouring temperatures or deficiencies of carbonaceous material.

A swell occurs when the mold wall gives way across a whole face, and is caused by an improperly rammed mold.

Burn-on occurs when metallic oxides interact with impurities in silica sands. The result is sand particles embedded in the surface of the finished casting. This defect can be avoided by reducing the temperature of the liquid metal, by using a mold wash, and by using various additives in the sand mixture.

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