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INTRODUCTION:

Sand casting is the most widely used metal casting process in manufacturing. Almost all casting metals can be sand cast. Sand castings can range in size from very small to extremely large. Some examples of items manufactured in modern industry by sand casting processes are engine blocks, machine tool bases, cylinderheads, pump housings, and valves, just to name a few.

Most sand casting operations use silica sand (SiO2). A great advantage of sand in manufacturing applications is that sand is inexpensive. Another advantage of sand to manufacture products bymetal casting processes, is that sand is very resistant to elevated temperatures. In fact, sand casting is one of the few processes that can be used for metals with high melting temperatures such as steels, nickel, and titanium. Usually sand used to manufacture a mold for the casting process is held together by a mixture of water and clay. A typical mixture by volume could be 89% sand, 4% water, 7% clay. Control of all aspects of the properties of sand is crucial when manufacturing parts by sand casting, therefore a sand laboratory is usually attached to the foundry.

Sand casting is a widely used method of producing rough metal castings. Unfinished, these castings have an easily recognizable,grainy texture, although most foundries will perform finishing services that yield a smoother surface.Sand castings are made by first starting with a wood or metal representation of the part, called a pattern. The pattern is made slightly larger than the desired size of the final casting to account for shrinkage duringcooling. This pattern is placed in a molding box, which is then half filled with a mixture of sand (crushed rock), clay and water. The sand is tightly packed around the pattern, and then the box is turned and the process is repeated with the top half of the pattern. The two mold halves are then separated and the pattern is removed. This pattern can then be used to generate additional molds, if necessary. A hole, called a sprue, is

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punched into the top of the mold to allow molten metal to be poured into the mold. Additional holes, known as risers, are alsopunched into the pattern to allow gas to escape..

HISTORY:

"Whereas in former times the kings my forefathers had created bronze statues imitating real-life forms to put on display insidetheir temples, but in their method of work they had exhausted allthe craftsmen, for lack of skill and failure to understand the principles they needed so much oil, wax and tallow for the work that they caused a shortage in their own countries - I, Sennacherib, leader of all princes, knowledgeable in all kinds ofwork, took much advice and deep thought over doing that work. Great pillars of bronze, colossal striding lions, such as no previous king had ever constructed before me, with the technical skill that Ninushki brought to perfection in me, and at the prompting of my intelligence and the desire of my heart I invented a technique for bronze and made it skillfully. I createdclay moulds as if by divine intelligence....twelve fierce lion-colossi together with twelve mighty bull-colossi which were perfect castings... I poured copper into them over and over again; I made the castings as skillfully as if they had only weighed half a shekel each.

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In 1924, the Ford automobile company set a record by producing 1 million cars, in the process consuming one-third of the total casting production in the U.S. As the automobile industry grew the need for increased casting efficiency grew. The increasing demand for castings in the growing car and machine building industry during and after World War I and World War II, stimulated new inventions in mechanization and later automation of the sand casting process technology.There was not one bottleneck to faster casting production but rather several. Improvements were made in molding speed, molding sand preparation, sand mixing, core manufacturing processes, and the slow metal melting rate in cupola furnaces. In 1912, the sandslinger was invented by the American company Beardsley & Piper. In 1912, the first sand mixer with individually mounted revolving plows was marketed by the Simpson Company. In 1915, thefirst experiments started with bentonite clay instead of simple fire clay as the bonding additive to the molding sand. This increased tremendously the green and dry strength of the molds. In 1918, the first fully automated foundry for fabricating hand grenades for the U.S. Army went into production. In the 1930s the first high-frequency coreless electric furnace was installed in the U.S. In 1943, ductile iron was invented by

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adding magnesium to the widely used grey iron. In 1940, thermal sand reclamation was applied for molding and core sands. In 1952,the "D-process" was developed for making shell molds with fine, pre-coated sand. In 1953, the hotbox core sand process in which the cores are thermally cured was invented. In 1954, a new core binder - water glass (sodium silicate) hardened with CO2 from theambient air, came into use

TYPES OF SAND MOLDS:

There are different preparations of the sand for the mold, whichcharacterize the following four unique types of sand molds. Greensand mold - Greensand molds use a mixture of sand, water,

and a clay or binder. Typical composition of the mixture is90% sand, 3% water, and 7% clay or binder. Greensand molds arethe least expensive and most widely used.

Skin-dried mold - A skin-dried mold begins like a greensand mold,but additional bonding materials are added and the cavitysurface is dried by a torch or heating lamp to increase moldstrength. Doing so also improves the dimensional accuracy andsurface finish, but will lower the collapsibility.  Dry skinmolds are more expensive and require more time, thus loweringthe production rate.

Dry sand mold - In a dry sand mold, sometimes called a cold boxmold, the sand is mixed only with an organic binder. The moldis strengthened by baking it in an oven. The resulting moldhas high dimensional accuracy, but is expensive and results ina lower production rate.

No-bake mold - The sand in a no-bake mold is mixed with aliquid resin and hardens at room temperature.

Definitions of different parts:

1. POURING CUP: This is where the metal is poured into the mold.

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2. SPRUE: The vertical channel from the top of the mold to the gating and riser system. Also, a generic term used to cover all gates, runners and risers.

3. RUNNER: The portion of the gate assembly that connects the sprue to the casting in gate or riser.

4. GATE: The end of the runner in a mold where molten metal enters the mold cavity.

5. RISER: A reservoir of molten metal provided to compensate forthe contraction of the metal as it solidifies.

6. MOLD CAVITY: The impression in a mold produced by the removalof the pattern. When filled with molten metal it forms a casting.

7. COPE: Upper or top most section of a flask, mold or pattern. 8. PARTING LINE: A line on a pattern or casting corresponding to

the separation between the parts of a mold. 9. DRAG: Lower or bottom section of a flask, mold or pattern.

Patterns :

The cavity in the sand is formed by using a pattern (an approximate duplicate of the real part), which are typically madeout of wood, sometimes metal. The cavity is contained in an aggregate housed in a box called the flask. Core is a sand shape inserted into the mold to produce the internal features of the part such as holes or internal passages. Cores are placed in the

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cavity to form holes of the desired shapes. Core print is the region added to the pattern, core, or mold that is used to locateand support the core within the mold. A riser is an extra void created in the mold to contain excessive molten material. The purpose of this is feed the molten metal to the mold cavity as the molten metal solidifies and shrinks, and thereby prevents voids in the main casting.

Process Cycle:The process cycle for sand casting consists of six main stages,which are explained below.

1. Mold-making - The first step in the sand casting process is tocreate the mold for the casting. In an expendable moldprocess, this step must be performed for each casting. A sandmold is formed by packing sand into each half of the mold. Thesand is packed around the pattern, which is a replica of theexternal shape of the casting. When the pattern is removed,the cavity that will form the casting remains. Any internalfeatures of the casting that cannot be formed by the patternare formed by separate cores which are made of sand prior tothe formation of the mold. Further details on mold-making willbe described in the next section. The mold-making timeincludes positioning the pattern, packing the sand, andremoving the pattern. The mold-making time is affected by thesize of the part, the number of cores, and the type of sandmold. If the mold type requires heating or baking time, themold-making time is substantially increased. Also, lubricationis often applied to the surfaces of the mold cavity in orderto facilitate removal of the casting. The use of a lubricantalso improves the flow the metal and can improve the surfacefinish of the casting. The lubricant that is used is chosenbased upon the sand and molten metal temperature.

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Process of sand casting

2. Clamping - Once the mold has been made, it must be prepared forthe molten metal to be poured. The surface of the mold cavityis first lubricated to facilitate the removal of the casting.Then, the cores are positioned and the mold halves are closedand securely clamped together. It is essential that the moldhalves remain securely closed to prevent the loss of anymaterial.

3. Pouring - The molten metal is maintained at a set temperaturein a furnace. After the mold has been clamped, the moltenmetal can be ladled from its holding container in the furnaceand poured into the mold. The pouring can be performed

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manually or by an automated machine. Enough molten metal mustbe poured to fill the entire cavity and all channels in themold. The filling time is very short in order to prevent earlysolidification of any one part of the metal.

4. Cooling - The molten metal that is poured into the mold willbegin to cool and solidify once it enters the cavity. When theentire cavity is filled and the molten metal solidifies, thefinal shape of the casting is formed. The mold cannot beopened until the cooling time has elapsed. The desired coolingtime can be estimated based upon the wall thickness of thecasting and the temperature of the metal. Most of the possibledefects that can occur are a result of the solidificationprocess. If some of the molten metal cools too quickly, thepart may exhibit shrinkage, cracks, or incomplete sections.Preventative measures can be taken in designing both the partand the mold and will be explored in later sections.

5. Removal - After the predetermined solidification time haspassed, the sand mold can simply be broken, and the castingremoved. This step, sometimes called shakeout, is typicallyperformed by a vibrating machine that shakes the sand andcasting out of the flask. Once removed, the casting willlikely have some sand and oxide layers adhered to the surface.Shot blasting is sometimes used to remove any remaining sand,especially from internal surfaces, and reduce the surfaceroughness.

6. .Trimming - During cooling, the material from the channels inthe mold solidifies attached to the part. This excessmaterial must be trimmed from the casting either manually viacutting or sawing, or using a trimming press. The timerequired to trim the excess material can be estimated from thesize of the casting's envelope. A larger casting will requirea longer trimming time. The scrap material that results fromthis trimming is either discarded or reused in the sandcasting process. However, the scrap material may need to bereconditioned to the proper chemical composition before itcan be combined with non-recycled metal and reused.

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The Sand Casting Operation:The sand casting operation involves the pouring of the molten metal into the sand mold, the solidification of the casting within the mold, and the removal of the casting. The casting operation is covered in detail on the metal casting operation page.

Of specific interest to sand casting would be; the effect and dissipation of heat through the particular sand mold mixture during the casting's solidification, the effect of the flow of liquid metal on the integrity of the mold, (mold sand mixture properties and binder issues), and the escape of gases through the mixture. Sand usually has the ability to withstand extremely high temperature levels, and generally allows the escape of gasesquite well. Manufacturing with sand casting allows the creation of castings with complex geometry. Sand casting manufacture, however, only imparts a fair amount of dimensional accuracy to the cast part.

After the sand casting is removed from the sand mold it is shaken out, all the sand is otherwise removed from the casting, and the gating system is cut off the part. The part may then undergo further manufacturing processes such as heat treatment, machining, and/or metal forming. Inspection is always carried outon the finished part to evaluate the effectiveness and satisfaction of its manufacture.

Equipments:In sand casting, the primary piece of equipment is the mold,which contains several components. The mold is divided into twohalves - the cope (upper half) and the drag (bottom half), whichmeet along a parting line. Both mold halves are contained insidea box, called a flask, which itself is divided along this parting

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line. The mold cavity is formed by packing sand around thepattern in each half of the flask. The sand can be packed byhand, but machines that use pressure or impact ensure evenpacking of the sand and require far less time, thus increasingthe production rate. After the sand has been packed and thepattern is removed, a cavity will remain that forms the externalshape of the casting. Some internal surfaces of the casting maybe formed. Cores are additional pieces that form the internal holes andpassages of the casting. Cores are typically made out of sand sothat they can be shaken out of the casting, rather than requirethe necessary geometry to slide out. As a result, sand coresallow for the fabrication of many complex internal features. Eachcore is positioned in the mold before the molten metal is poured.In order to keep each core in place, the pattern has recessescalled core prints where the core can be anchored in place.However, the core may still shift due to buoyancy in the moltenmetal. Further support is provided to the cores by chaplets.These are small metal pieces that are fastened between the coreand the cavity surface.

 Sand Mold - Opened

 Sand Mold – Closed

The sand that is used to create the molds is typically silicasand (SiO2) that is mixed with a type of binder to help maintainthe shape of the mold cavity. Using sand as the mold material

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offers several benefits to the casting process. Sand is veryinexpensive and is resistant to high temperatures, allowing manymetals to be cast that have high melting temperatures. The quality of the sand that is used also greatly affects thequality of the casting and is usually described by the followingfive measures: Strength - Ability of the sand to maintain its shape.

Permeability - Ability to allow venting of trapped gases throughthe sand. A higher permeability can reduce the porosity of themold, but a lower permeability can result in a better surfacefinish. Permeability is determined by the size and shape ofthe 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 moreaccurately compress, during solidification of the casting. Ifthe sand can not compress, then the casting will not be ableto shrink freely in the mold and can result in cracking.

Reusability - Ability of the sand to be reused for future sandmolds.

Tooling:The main tooling for sand casting is the pattern that is used tocreate the mold cavity. The pattern is a full size model of thepart that makes an impression in the sand mold. However, someinternal surfaces may not be included in the pattern, as theywill be created by separate cores. The pattern is actually madeto be slightly larger than the part because the casting willshrink inside the mold cavity. Also, several identical patternsmay be used to create multiple impressions in the sand mold.

Several different materials can be used to fabricate a pattern,including wood, plastic, and metal. Wood is very common becauseit is easy to shape and is inexpensive, however it can warp and

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deform easily. Wood also will wear quicker from the sand. Metal,on the other hand, is more expensive, but will last longer andhas higher tolerances. The pattern can be reused to create thecavity for many molds of the same part. Therefore, a pattern thatlasts longer will reduce tooling costs. A pattern for a part canbe made many different ways, which are classified into thefollowing four types: Solid pattern - A solid pattern is a model of the part as a

single piece. It is the easiest to fabricate, but can causesome difficulties in making the mold. The parting line andrunner system must be determined separately. Solid patternsare typically used for geometrically simple parts that areproduced in low quantities.

 Solid pattern

Split pattern - A split pattern models the part as two separatepieces that meet along the parting line of the mold. Using twoseparate pieces allows the mold cavities in the cope and dragto be made separately and the parting line is alreadydetermined. Split patterns are typically used for parts thatare geometrically complex and are produced in moderatequantities.

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 Split pattern

Match-plate pattern - A match-plate pattern is similar to a splitpattern, except that each half of the pattern is attached toopposite sides of a single plate. The plate is usually madefrom wood or metal. This pattern design ensures properalignment of the mold cavities in the cope and drag and therunner system can be included on the match plate. Match-platepatterns are used for larger production quantities and areoften used when the process is automated.

 Match-plate pattern

Cope and drag pattern - A cope and drag pattern is similar to amatch plate pattern, except that each half of the pattern isattached to a separate plate and the mold halves are madeindependently. Just as with a match plate pattern, the platesensure proper alignment of the mold cavities in the cope anddrag and the runner system can be included on the plates. Copeand drag patterns are often desirable for larger castings,where a match-plate pattern would be too heavy and cumbersome.

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They are also used for larger production quantities and areoften used when the process is automated.

 Cope and drag pattern

Another piece of tooling used in sand casting is a core-box. Ifthe casting requires sand cores, the cores are formed in theseboxes, which are similar to a die and can be made of wood,plastic, or metal just like the pattern. The core-boxes can alsocontain multiple cavities to produce several identical cores.

Use Of Binder In Sand Casting:

A mold must have the physical integrity to keep its shape throughout the casting operation. For this reason, in sand casting, the sand must contain some type of binder that acts to hold the sand particles together. Clay serves an essential purpose in the sand casting manufacturing process, as a binding agent to adhere the molding sand together. In manufacturing industry other agents may be used to bond the molding sand together in place of clay. Organic resins, (such as phenolic resins), and inorganic bonding agents, (such as phosphate and sodium silicate), may also be used to hold the sand together. In addition to sand and bonding agents, the sand mixture to create the metal casting mold will sometimes have other constituents added to it in order to improve mold properties. 

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Types Of Sand Used In Sand Casting:

There are two general types of sand used in the manufacturingprocess of sand casting.

Naturally Bonded- Naturally bonded sand is less expensive but itincludes organic impurities that reduce the fusion temperature ofthe sand mixture for the casting, lower the binding strength, andrequire a higher moisture content. 

Synthetic Sand- Synthetic sand is mixed in a manufacturing labstarting with a pure (SiO2) sand base. In this case, thecomposition can be controlled more accurately, which imparts thecasting sand mixture with higher green strength, morepermeability, and greater refractory strength. For these reasons,synthetic sand is mostly preferred in sand casting manufacture. 

Advancements in Sand Casting:

In 1924, the Ford automobile company set a record productionof 1 million cars, consuming the casting industry.

Experiments were conducted with types of clay to improvestrength in molding sand, updating cupola furnaces toelectric and moving the traditional foundry to a factorysetting improved process.

Now, sand casting is a fully automated factory system.

Advantages and Disadvantages of Sand Casting:Advantages:1)         Low capital investment means that short production

runs are viable;2)         Use of sand cores allows fairly complex shapes to be

cast;

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3)         Large components can be produced;4)         Suitable for small banch production( small production

rates)Disadvantages:1)         The process has a high unit cost, as it is labor

intensive and time consuming;2)    The sand mold leaves bad surface finish, due to sand

indentation and oxidizing medium, which often requiresfurther processing;

3)         Cannot make thin sections;4)         Not suitable for mass production, oftenly used to

produce few number of products compared with other casting proccesses which produce thousands and millions.

COSTING:Material cost 

The material cost for sand casting includes the cost of the metal, melting themetal, the mold sand, and the core sand. The cost of the metal is determinedby the weight of the part, calculated from part volume and material density,as well the unit price of the material. The melting cost will also be greaterfor a larger part weight and is influenced by the material, as some materialsare more costly to melt. However, the melting cost in typically insignificantcompared to the metal cost. The amount of mold sand that is used, and hencethe cost, is also proportional to the weight of the part. Lastly, the cost ofthe core sand is determined by the quantity and size of the cores used to castthe part.

Production cost 

The production cost includes a variety of operations used to cast the part, including core-making, mold-making, pouring, and cleaning. The cost of making the cores depends on the volume of the cores and the quantity used to cast thepart. The cost of the mold-making is not greatly influenced by the part geometry when automated equipment is being used. However, the inclusion of cores will slightly slow the process and therefore increase the cost. Lastly, the cost of pouring the metal and cleaning the final casting are both driven by the weight of the part. It will take longer to pour and to clean a larger and heavier casting.

Tooling cost 

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The tooling cost has two main components - the pattern and the core-boxes The pattern cost is primarily controlled by the size of the part (both the envelope and the projected area)

as well as the part's complexity. The cost of the core-boxes first depends on their size, a result

of the quantity and size of the cores that are used to cast the part. Much like the pattern, the complexity of

the cores will affect the time to manufacture this part of the tooling (in addition to the core size), and

hence the cost. The quantity of parts that are cast will also impact the tooling cost. A

larger production quantity will require the use of a tooling material, for both the pattern and core-boxes,

that will not wear under the required number of cycles. The use or a stronger, more durable, tooling

material will significantly increase the cost. 

Casting Defects:

1) Misruns (due to rapid solidification in the runner)

2) Cold shuts (due to rapid solidification before complete filling of the mold)

3) Cold shots (due to splattered globules of metal during pouring)

4) Shrinkage cavity (due to lack of riser system)

5) Microporosity (due to localized solidification shrinkage)

6) Hot tearing (due to the die's prevention of contraction)

Defects related with sand molds :

1) Sand blow

2) Pinholes

3) Sand wash

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4) Scabs

5) Penetration

6) Mold shift

7) Core shift

8) Mold crack

CONCLUSION:

The sand casting process is often far less expensive than other techniques, and is often one of the fastest methods available. Itdoes suffer from some disadvantages, however. First, it typicallyyields tolerances of .030 inches for the first foot, and then andadditional .010 for each additional three inches. It also produces a product that requires further machining. At the very least, the casting must be separated from the risers and the sprue. Most applications also require additional finishing services, such as shot and hammer peening, grinding, forging or plating. These processes create a much finer surface, and are particularly important any time that the casting will be used as a visible component

REFERENCES:

Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4.

Todd, Robert H.; Allen, Dell K.; Alting, Leo (1994), Manufacturing ProcessesReference Guide, Industrial Pre

http://www.industrialmetalcasting.com/sand-casting.html

http://www.efunda.com/processes/metal_processing/sand_casting_intro.cfm

http://en.wikipedia.org/wiki/Sand_casting

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www.google.com