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Processing of Metals: Casting

Alessandro Anzalone, Ph.D.

Hillsborough Community College, Brandon Campus

Agenda

1. Introduction2. The Casting Process3. Patterns4. Sand Casting5. Evaporative Casting Process6. Shell Process7. Permanent Mold Casting8. Slush Casting9. Centrifugal Casting10. Investment Casting11. Shaw Process12 Die Casting12. Die Casting13. Furnaces and Metal Handling14. Molten Metal Safety15. Pouring Practice16. Casting Cleanup17. Casting Design and Problems18. References

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Introduction

Casting is one of the oldest methods of manufacturing metals. Prehistoric humans made tools by pouring molten metal into open molds made of stone or baked clay. Cast objects over 4000 years old have been found dating from ancient Assyrian, Egyptian, and Chinese cultures.dating from ancient Assyrian, Egyptian, and Chinese cultures.

The process of casting metals is accomplished by pouring or forcing molten metal into a mold cavity having a desired shape. When the metal has solidified the casting is removed from the mold. Virtually any shape can be produced by this method, in some cases with such precision that subsequent machining is not required.

The Casting Process

When designing a metal part to be manufactured an engineer must choose a method of production. The part can be made by one or more processes, including machining from solid metal, welding fabrication, powder metallurgy, pressing and cold forming, hot forging, or casting. The main metallurgy, pressing and cold forming, hot forging, or casting. The main advantage of casting over other manufacturing processes is that parts with intricate internal cavities/passages (e.g., faucets, exhaust manifolds) can be made. This geometric feature would be difficult or even impossible to produce with any other manufacturing process. Besides, the casting process results in parts with smooth, flowing designs, either for practical or decorative purposes. Also, the metal can be placed only where it is required. Thus, the economy of using less metal for a part (especially when a very expensive metal is used), the eye-pleasing appearance (such as in machinery housings), and the possibility of producing intricate shapes are the factors that set casting apart from other manufacturing processes.

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The Casting Process

Sand casting is typically not a rapid method of production, whereas die casting is a relatively rapid process. Other casting processes lend themselves to production, but none should be considered to have high production rates as compared with punch press work or powder production rates as compared with punch press work or powder metallurgy.

Casting processes involve a large segment of the metals industry. These range from the tiniest precision parts to huge castings for machinery sections weighing many tons. Some metals that are too difficult to machine, such as those used for aircraft turbine impeller blades, can be cast to a precision shape not requiring any subsequent machining. Other softer metals, such as aluminum, are used to form articles such as transmission cases and valve covers for automobiles, It would take many hours of machining to make a complicated carburetor part from solid metal in machine shop, but it takes only seconds in a die casting machine.

The Casting Process

Several general conditions must be met for the production of good castings, regardless of the method used:

1. A method of melting the metal to the correct temperature must be available.available.

2. A mold cavity of the desired shape must he formed, with sufficient strength to contain the metal without distorting or having too much restraint on the molten metal as it solidifies. Also, the mold must be designed to avoid porosity and cracking of the casting.

3. Molds must be arranged so that when molten metal is introduced into the mold, air and gases can escape so the casting will be free from gas-related defects.

4. As we will see, molds usually are split to facilitate the removal of the pattern and/or the casting. Depending on the cross-sectional area of the mold cavity, just the gravitational force of the liquid metal can he enough to separate the mold halves. Some casting methods apply additional pressure to the metal. In either case, a force adequate to hold the mold halves together must be provided.

5. Any mold material (or cores) in internal cavities must have a provision for its removal. Finishing operations are often required to remove any excess material from the casting.

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Patterns

The first requirement in making a casting is to design and make a pattern. Patterns are usually made of wood when only a few castings are needed. For larger quantities and when wear (due to an abrasion by the molding material) becomes a problem, patterns are made of metal, such as material) becomes a problem, patterns are made of metal, such as aluminum or bronze. Hard, tough plastics are also used for patterns. Patterns include several types — single-piece, split-piece, loose-piece, match-plate, and cope and drag. The type of pattern used in the mold depends on the required production. For example, small production is economical with a single-piece pattern: however, large production requires use of mechanized molding with application of a match-plate pattern. A previously made casting can also be used as a pattern if additional shrinkage is not a factor.

Patterns

Shrinkage Allowance When metal solidifies from the liquid state, it shrinks. Thus, in order to reproduce casting of a desired dimension, a shrinkage allowance must be added to the pattern. Each metal has a different shrinkage. The following are shrinkages in in/ft for some metals:different shrinkage. The following are shrinkages in in/ft for some metals:

Cast iron 1/8Steel 1/4Brass 3/16Aluminum 5/32Magnesium 5/32

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Patterns

Draft Patterns must have draft or taper to permit removal from the mold. Draft is usually about 2° to 3°. Without draft on the pattern, parts of the sand mold would be broken as the pattern is pulled out.

Other Allowances If there are areas on the casting that will have machined surfaces, the pattern needs to provide machining allowance that will leave additional metal for removal. The amount of machining allowance depends on the roughness and accuracy of the finished casting. Enough material must be left for subsequent machining in order to cut under sand inclusions on the surface of the casting.

Sand Casting

In sand casting, a specially prepared sand that is mixed with different binders and additives is used as a mold material. The appropriately conditioned sand is then compacted around a pattern that has the shape of a desired casting. Sand has the advantage of being highly refractory of a desired casting. Sand has the advantage of being highly refractory (can resist high temperatures without melting), so metals like cast iron and steel can be cast easily in sand molds. Although there are many different molding materials, sand casting accounts for the greatest tonnage of all castings produced.

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Sand Casting

Molding Sands

The basic characteristics of sand are that it is (1) easily molded and capable of holding accurate detail, (2) reusable, and (3) inexpensive. Sand of holding accurate detail, (2) reusable, and (3) inexpensive. Sand molding processes can be classified into the following groups: green sand molding, heat-cured resin binder processes, cold-box resin binder processes, no- bake resin binder processes, and silicate and phosphate bonds. The most common type of sand molding process is green sand molding. Green refers to the fact that the molding sand contains moisture. Other sand molding processes, because of their higher strength, are often used for cores that form holes and hollow spaces in green sand molds.

The most common molding sand is a natural silica sand (SiO2); however, other sand types such as zircon (ZrSiO4)and olivine are also used in special applications. The sands can be classified into two groups: natural and synthetic. Natural sand is the sand in its natural form. The synthetic sands are natural sands that have been washed, screened, classified, and blended to meet the requirements of a particular application. The synthetic sands are most commonly used in foundries.

Sand Casting

General Characteristics of Molding Sands

Molding sand must have several characteristics:

1. Cohesiveness The ability to be packed (rammed) in a mold and retain its shape. This property is achieved by adding various sand additives (e.g., clay, water, resins).

2. Refractoriness The ability to withstand high temperatures.

3. Permeability Porosity that allows gases to escape through the mold.

4. Collapsibility The ability to allow freedom for the solidifying, shrinking metal to move without fracturing, and to allow the cast part to be removed easily from the mold. This property is also achieved by adding appropriate sand additives (e.g., corn flour, dextrin).

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Sand Casting

Preparation of Green Sand Mix

The green sand mixture (sand, clay, and water) is placed in a muller , where the ingredients are thoroughly mixed in order to obtain the proper the ingredients are thoroughly mixed in order to obtain the proper consistency. A typical muller consists of a large tub in which an arm with rollers swings around, forcing the rollers over the sand.

To achieve required molding properties, the sand grains must be of the right size, and clay and water must be added in the right proportion. To obtain consistently good castings, the mold must be of appropriate hardness, strength, and permeability. Some standard tests are commonly applied in the foundries to control process variables (e.g., grain size and grain distribution, moisture content, green strength, hardness). These tests are performed both during sand preparation as well as during mold making in order to assure high-quality molds and castings.

Sand Casting

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Sand Casting

Molding

A series of steps in manually producing castings are necessary. Although many kinds of patterns are used, such as split-piece, single-piece, and many kinds of patterns are used, such as split piece, single piece, and loose-piece types, patterns for green sand molding are usually made in two halves and are called match-plate patterns, or in the case of larger castings cope-and-drag patterns. Cope refers to the top half and drag refers to the bottom half.

Sand Casting

The cope-and-drag pattern is placed in a flask that is made in two halves (cope and drag) . First, the drag half of the flask is filled with sand. The sand is then rammed into place and afterward struck off (leveled off) even with the top of the flask with a straightedge. The drag half of the flask and pattern is also g g g prammed in the same manner. The pattern also has an extension called a core print if a core is used. This space in the sand mold provides a support for the ends of the sand core. One or more vertical holes are provided for pouring the metal. This hole, called a sprue, has a pouring cup on the top. The role of the pouring cup is to make pouring easy and to keep the sprue constantly filled with metal. At its bottom the sprue is connected to the runner, which is connected to the mold cavity through the gate. When the molten metal is poured it fills the mold cavity and other holes in the top of the mold, called risers. The risers are used as reservoirs to feed liquid metal to the mold cavity risers. The risers are used as reservoirs to feed liquid metal to the mold cavity as it shrinks while solidifying. They also provide for escaping gases and allow impurities to float to the top of the riser and out of the casting. The cope-and-drag pattern is made in segments that can be separated, such as the riser and downsprue, which are removed from the pouring side of the cope half of the flask, and the pattern is removed from the parting line side after it is turned over. Finally, the patterns are removed from the mold sections. Cores are installed, the two halves of the flask are assembled, and the mold is ready for casting.

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Sand Casting

http://www.youtube.com/watch?v=oRIbaGRB6tI&feature=related

Sand Casting

The Advantages and Disadvantages of Sand Casting

The greatest advantages of sand casting are that almost any metal can be poured in the sand mold, and there is almost no limit on size, shape, or weight of the , , p , g fpart. Sand casting pro- vides the most direct route from pattern to casting. Tooling costs are low and the gravity-casting process is economical. Among the limitations involved in sand casting is the need for machining in order to finish the castings, especially large ones having rough surfaces. Other disadvantages are that it is not practical in the green sand process to cast parts with long and thin sections, and a new mold is required for every pour.

Sand castings are extensively used for machine tool housings, bases, slideways, and other parts, and they are also extensively used in the automotive industry and other parts, and they are also extensively used in the automotive industry for making engine blocks.

Castings must be defect free, and their quality must be consistent from casting to casting. Statistical process control (SPC) techniques are used often to improve quality and reduce rejection of parts.

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Evaporative Casting Process

In the evaporative casting process, the pattern, sprue, and riser are made of foamed polystyrene. They can be made as a single piece, or they can be made separately and then glued together. The completed polystyrene pattern is then coated with a thin layer of refractory material. The coated pattern is then coated with a thin layer of refractory material. The coated pattern is placed in the mold and dry sand is compacted/vibrated around the pattern. When the metal is poured, the heat vaporizes the polystyrene pattern almost instantaneously, leaving the mold shape intact as it is being filled with metal. This process can be used for casting parts of any size and shape, since the patterns do not need to be removed prior to casting, thus eliminating the need for draft, on the pattern. Also, problems related to mold shift are eliminated, since the entire pattern is assembled before being covered with sand, allowing for corrections prior to pouring. Another advantage of the process is the low cost of sand preparation, since no additives or binders are used for sand conditioning. The process is economical only for prototyping or large production, that is, only if the polystyrene patterns themselves can be mass produced in a die.

Evaporative Casting Process

http://www.productionnavigator.com/ventura/productnav/productie/Verlorenschuimgieten1.gif

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Shell Process

The shell molding process is a type of sand casting process that provides a finer detail and smoother finish because the sand is finer and it is combined with plastic resin to make a smooth mold surface. The process can be automated for mass production.can be automated for mass production.

http://www.custompartnet.com/wu/images/shell-mold-casting/shell-mold-casting-small.png

Permanent Mold Casting

The greatest disadvantage of sand casting is that a new mold must be made for each casting. In addition, some inherent dimensional inaccuracies are present in the sand casting. These disadvantages gave rise to the development of a permanent mold. Despite the name, permanent molds development of a permanent mold. Despite the name, permanent molds can be reused at most for several thousand pours, after which they lose their true shape and must be scrapped. Most permanent molds are made of gray cast iron or steel. Graphite molds are often used for casting higher-temperature metals. The molds are made by machining processes and are hand finished or polished. A refractory wash is applied to the mold prior to casting in order to prolong its life. When cores are needed, they can be made from metal, in which case they are reused, or they can be made from sand, in which case they cannot be reused. The mold halves may be hinged or mounted on a casting machine so they can be opened and closed quickly and accurately.

http://www.youtube.com/watch?v=owgcK0TswBM&feature=related

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Permanent Mold Casting

http://www.custompartnet.com/wu/images/permanent-mold-casting/permanent-mold-casting.png

Slush Casting

Slush casting is used with permanent molds to make a shell of metal in the mold. The molten metal is poured into the mold and allowed to solidify to a certain wall thickness against the mold, and the remainder of the molten metal is dumped out, thus producing a shell. Toys, lamp bases, and metal is dumped out, thus producing a shell. Toys, lamp bases, and ornamental objects are made with this process.

http://www.carhobby.com/55Mscc.JPG

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Centrifugal Casting

Centrifugal casting is a process in which molten metal is poured into a rapidly revolving mold. The liquid metal is forced to conform to the shape of the mold by centrifugal forces many times the force of gravity. In this process the mold rotates about either a horizontal or a vertical axis. No process the mold rotates about either a horizontal or a vertical axis. No core is needed to make an inner surface, since this process naturally produces a hollow shape such as pipe. The thickness of the mold controls the cooling rate and therefore the grain structure of the cast part. Also, if a hard outer wear surface is needed with an inner machinable soft metal, two dissimilar metals can be used—the outer one to harden when solidified and the inner one to remain relatively soft.

http://www.efunda.com/processes/metal_processing/images/casting_centrifugal.gif

Centrifugal Casting

http://www.centrifugal.net/images/index_bottomright.jpg http://www.hycast.com.au/images/CentrifugalCasting.jpg

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Investment Casting

One of the oldest methods of casting metals is the investment casting (i.e., lost wax) process. The pattern is made of wax coated (invested) with a thick layer of refractory material. After the refractory material has hardened, the wax is melted and poured out of the mold and reused for another pattern. The mold p pis then preheated close to pouring temperature, and molten metal is then poured. This method is used for dentistry, arts and crafts, and for any type of very accurate near-net-shape casting. When it is used for industrial purposes, significant production rates are possible.

http://www.youtube.com/watch?v=WXFRRg8YMT0

Shaw Process

In this process, a rubbery jelling agent and a slurry of refractory aggregate is poured over the pattern. This rubbery mold hardens sufficiently to be easily stripped off the pattern and will return to the exact shape of the pattern. The mold is ignited to burn off the volatile elements, and it is then placed in a g , pfurnace and brought to a high temperature. The mold is then ready for pouring. The advantage of this process is good permeability and good collapsibility, which allow for production of delicate and intricate shapes with fine detail and higher- quality castings.

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Die Casting

Die casting is similar to permanent molding in that a metal mold made in two halves is used. The difference is that the metal is not gravity poured into the mold (die), but instead the metal is injected under high pressures ranging from 1000 to 100,000 PSI. This requires massive machines that are generally , q g yoperated hydraulically to exert the hundreds of tons of force necessary to hold the two halves of the die together when the molten metal is being injected at such high pressures.

http://www.azom.com/work/gmQ9Dmtd0mw9jnoTHN6z_files/image008.gif

Die Casting

http://www.welsonmold.com/images/02/Die-CastingParts/Die-Casting_4.jpg http://www.lucidhardware.com/images/Die_Casting.jpg

http://www.youtube.com/watch?v=yQlrc4pBGkw&feature=related

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Furnaces and Metal Handling

The cupola furnace has been of primary importance in the melting of cast iron for foundry work. Coke (produced from coal) is used for producing heat in the cupola furnace. A typical cupola furnace is a circular steel shell lined with a refractory material such as fire- brick. It is equipped with a blower, air duct, y q pp , ,and wind box with tuyeres for admitting air into the cupola. A sand bottom keeps the molten metal from burning through and is sloped so that the iron may flow out when the tap hole is opened.

http://www.theworkshop.ca/casting/course/MTB70/2/cupola_furn.jpg

http://www.industrialmetalcasting.com/gifs/cupola-furnace.gif

Furnaces and Metal Handling

http://www.indiamart.com/foundmaticengineers/ladles.html

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Molten Metal Safety

The hazards inherent in the handling of molten metal require thoughtful planning and engineering to ensure they are avoided. Molten metal is a threat to the safety of people and equipment if it is mishandled; therefore, the handling process, for example, cranes, ladles, fork trucks, and lifting devices, must be p , p , , , , g ,reliable and operate with a minimum of difficulty. The floor space or aisleways in which such equipment is used must be designed so they are free of obstructions and (if possible) people. Auxiliary equipment such as troughs, molds, and stirring and skimming equipment must be engineered to avoid spilling or splashing of metal.

Personal protective equipment varies with the metal involved but usually includes safety glasses, face shields, hard hat, molder’s boots (or spats to cover shoelaces and shoe openings), heat-resistant leggings and aprons, and heat-shoelaces and shoe openings), heat resistant leggings and aprons, and heatresistant gloves; some plants require employees to wear flame- retardant cotton clothing and strictly forbid synthetic clothing. Some of these precautions are related to the heat involved in working around furnaces and with molten metals, but they all will also help protect the individual from coming in contact with molten metal in case there is an explosion.

Molten Metal Safety

A dangerous aspect of molten metal is the possible consequences of its coming in contact with water. The temperatures at which many metals are molten are many times the boiling point of water; if molten metal manages to trap or cover moisture or water, the water will rapidly transform to steam and explode , p y pas it expands. This explosion can hurl molten metal many feet, endangering people and equipment.

Molten metal can also explode with devastating force and consequences if it comes in contact with an oxide of another metal under just the right (or wrong) circumstances. This type of explosion very violent and depends on the fact that energy is required to free metals from their oxides, therefore if the reverse happens, that is, if a molten metal suddenly converts back to its oxide, there is a tremendous release of energy. This can happen with all metals, but there is a tremendous release of energy. This can happen with all metals, but aluminum is possibly the worst; it is estimated that the conversion of 1 lb of aluminum to aluminum oxide releases three times the energy of a pound of nitroglycerine! Some of these types of explosions have been known to level whole plants.

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Pouring Practice

Ladles are usually constructed of steel and lined with firebrick or other refractory material. The interior of the ladle is heated or kept hot while in use so as not to cool the molten metal. In small foundries, a handheld, shank-type ladle is often used to pour small quantities. This type requires two persons to use it. p q yp q pThe teapot ladle can contain more metal and is supported on an overhead monorail or crane. The handwheel is turned to tilt the ladle and make the pour. This and the bottom-pour types keep the slag and oxidized metal from going into the mold. In some operations the molds are placed on a pouring floor, whereas in other, automatic or semiautomatic, operations the molds are carried along a conveyor to the ladles where a measured amount of molten metal is poured into the mold.

Casting Cleanup

After the solidified castings are removed from the mold, they are cleaned in a shakeout, and small parts are often put in a tumbler. The risers and gates must be knocked off (often this can be done on cast iron because of its brittleness) or cut off by means of sawing or use of an abrasive cutoff wheel.y g

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Casting Design and Problems

Sufficient allowance of metal should always be provided when machining operations are to be performed. Size of the casting, and surface roughness, are involved in this decision. The location of the parting plane is also very important, since the pattern must be extracted at this plane without important, since the pattern must be extracted at this plane without disrupting the sand mold. The following are other factors to be considered:

1. Weight of the casting and mold strength2. Effective gating and sufficient riser3. Number of cores and their placement4. Required dimensional accuracy5. Radii, thickness of sections, and amount of shrinkage

Cracking can occur at sharp corners and where thick sections join with thin sections. Proper radii can reduce this problem. Also, the cooling rates are greater for the thin section than for the thick one; this also causes cracking at the juncture. Cooling rates may be increased in a thick section by using “chills,” metal sections embedded in the mold to absorb heat.

References

1. R Gregg Bruce, William K. Dalton, John E Neely, and Richard R Kibbe, , Modern Materials and Manufacturing Processes, Prentice Hall, 3rd edition, 2003, ISBN: 9780130946980

2. http://www.metmuseum.org/toah/ho/02/wae/ho_48.180.htm3. http://www.metmuseum.org/toah/ho/02/wai/ho_47.100.80.htm3 p // g/ / / / / 474. http://www.simpsongroup.com/images/mixmuller.jpg5. http://www.eriebronze.com/gallery/Pour014.jpg6. http://www.eriebronze.com/gallery/Autoline.jpg7. http://thelibraryofmanufacturing.com/metalcasting_sand.html