Sand Casting - The most basic manufacturing process and all the related topics covered in this PDF

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The most basic manufacturing process and all the related topics covered in this paper....

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Sand Casting 1. Introduction Sand casting is also known as sand molded casting is metal casting process characterized by using sand as the mold material. The mold cavity is created by withdrawing the pattern that has been packed around from the sand. The pattern usually made by wood, plastic, or variety of other materials. By casting, it can be produced a product that cannot be produced by machining where in production of complex shapes, irregular curved surfaces and parts made from metal. Besides the cost for sand casting also cheaper compared to metal casting. There are six basic requirements to the casting process: ➢ ➢ ➢ ➢ ➢ ➢

Mold Cavity Melting Process Pouring Technique Solidification Process Removal Process Cleaning and Finishing Process

Sand casting is produced in specialized laboratory called foundry laboratory. Foundry is a workshop in which casing of metal is done. The objectives for this laboratory are: ➢ Learn various stages of casting ➢ Analyze the quality of mold and casting product The final products obtained from sand casting from laboratory session show porosity and shrinkage cavities. Porosity is caused by the factors of the temperature for melting is too high, the cooling rate is non uniform and sand has low permeability. Defects of shrinkage cavities are due to insufficient amount of molten metal to feed the shrinkage while impurities with the molten metal, sand and dirt from the mold that not properly cleaned caused inclusion.

2. History Casting is one of the earliest metal shaping technique known to human being. Around 1300 BC, the Shang Dynasty in China were the first to utilize sand casting by melting metals. Then around 500BC, the Zhou Dynasty introduced cast iron to the world, but it was mostly used by farmers. Casting generally means pouring a molten metal into a refractory mold with a cavity of the desired shape to be made and allowing it to solidify for some time before taking it out. When solidified the desired metal object is taken out from the refractory mold either by breaking the mold or taking the mold apart. The solidified object is called casting. This process is also called founding. Casting required a lot of processes and that includes fabrication of pattern, mold constructions and pouring of molten metal.

3. Process Cycle Sand casting process includes following stages: ➢ ➢ ➢ ➢ ➢ ➢

Pattern making Mold construction Clamping Melting the metal Pouring of metal Cooling

➢ Removal ➢ Trimming

4. Stages 4.1. Pattern Making In casting, pattern is the replica of the object to be casted, used to prepare the cavity into which the molten metal will be poured during the casting process. It provides the shape for object that need to be casted. Typically, patterns used in sand casting may be made of wood, metal, plastics or other material. Patterns are made to exacting standard of construction, so that they can last for a reasonable length of time, according to the quality grade of the pattern being built, and so that they will repeatedly provide a dimensionally acceptable casting. The patternmaker or foundry engineer decides where the sprues, gating systems, and risers are placed with respect to the pattern. Where a hole is desired in a casting, a core may be used which define a volume or location in a casting where metal will not flow into. Sometimes chills may be placed on a pattern surface prior to molding, which are then formed into the sand mold. Chills are heat sinks which enable localized rapid cooling. The chills can be reclaimed and reused.

Figure1. 1: Pattern

4.1.1. Types of Pattern Pattern is of ten types: ➢ Single/Solid Pattern: A single pattern or solid pattern is simplest. It is replica of the desired casting usually in a slightly larger size to offset the shrinkage of the intended metal. Gated patterns connect a number of loose patterns together with a series of runners that will detached after shake-out. ➢ Split Pattern: Split pattern or two-piece pattern is most widely used type of pattern for complex casting. It is split along the parting surface, the position of which is determined by the shape of the casting. One half of the pattern is molded in drag and the other half is cope. ➢ Match Plate Pattern: A match-plate pattern is similar to a split pattern, except that each half of the pattern is attached to opposite sides of a single plate. The plate is usually made from wood or metal. This pattern design ensures proper alignment of the mold cavities in the cope and drag and the runner system can be included on the match plate. Match-plate patterns are used for larger production quantities and are often used when the process is automated. ➢ Cope and Drag Pattern: A cope and drag pattern is similar to a match plate pattern, except that each half of the pattern is attached to a separate plate and the mold halves are made independently. Cope and drag patterns are often desirable for larger castings, where a match-

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plate pattern would be too heavy and cumbersome. They are also used for larger production quantities and are often used when the process is automated. Sweep Pattern: A sweep is a section or wooden board of proper contour that is rotated about one edge to shape mold cavities having shapes of rotational symmetry. This type of pattern is used when a casting of large size is to be produced in a short time. The wooden rod is fixed to metal rod and rotated about the metal rod as the axis of rotation, for getting a complete mold cavity. It is economical. Gated Pattern: A gated pattern consists of casting patterns, pattern and stripping plates, core boxes, patterns of parts of the gating, molding and control patterns, jigs, and drying plates. When large number of small castings are required, gated patterns are used. Loose Piece Patten: Loose piece pattern has more than two pieces of the patten. Small pieces of the pattern are attached to the main piece of pattern using wire. When molding is over, the main piece of pattern (large pieces) is removed from the mold, then loose and with draw the small pieces through the gap generated by the main piece. Loose piece pattern is expansive, and molding with these patterns requires highly skilled workers. Shell Pattern: This type of pattern is generally made of metals and fitted on a plate. It is like a split pattern; one half of the pattern is placed in the upper part of the plate and another one is fitted on the bottom of the plate. Skeleton Pattern: Pattern is the Skeleton of desired shape, generally mounted on the metal base. Skeleton is made from wooden strips and is filled with loam sand and rammed. Extra sand is removed by stickle. Cores are required if necessary. It is generally used for casting very huge parts. Segmental Pattern: It is used for preparing circular castings. In this type it does not revolve continuously like sweep pattern, instead prepares the mold by parts. It completes one portion of the mold and then moves to next position to make the next part of the mold and so on till the mold is completed.

4.2. Mold Construction Mold construction required some criteria. The choice of molding materials is based on their process properties. Generally, the properties are refractoriness, green strength, dry strength, hot strength, hot strength and permeability. Refractoriness is an ability of molding material to withstand the high temperature of the metal so that it does not cause fusion. The molding the sand that contains moisture is termed as green sand. The sand selected should have enough strength so that the constructed mold retains its shape. In the meantime, when the moisture in the molding sand is completely expelled, it is called dry sand. While the strength of the sand that is required to hold the shape of the mold cavity then is called hot strength. Finally, permeability is a term used to measure gas evolution capability during solidification of casting.

4.3. Clamping Once the mold has been made, it must be prepared for the molten metal to be poured. The surface of the mold cavity is first lubricated to facilitate the removal of the casting. Then, the cores are positioned and the mold halves are closed and securely clamped together. It is essential that the mold halves remain securely closed to prevent the loss of any material.

4.4. Melting the Metal The metal is melted in the furnace. The molten metal is maintained at a set temperature in a furnace. A furnace is device used for heating and melting.

4.5. Pouring of Metal The molten metal is maintained at a set temperature in a furnace. After the mold has been clamped, the molten metal can be ladled from its holding container in the furnace and poured into the mold. The pouring can be performed manually or by an automated machine. Enough molten metal must be poured to fill the entire cavity and all channels in the mold. The filling time is very short in order to prevent early solidification of any one part of the metal.

4.6. Cooling The molten metal that is poured into the mold will begin to cool and solidify once it enters the cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. The mold cannot be opened until the cooling time has elapsed. The desired cooling time can be estimated based upon the wall thickness of the casting and the temperature of the metal. Most of the possible defects that can occur are a result of the solidification process. If some of the molten metal cools too quickly, the part may exhibit shrinkage, cracks, or incomplete sections. Preventative measures can be taken in designing both the part and the mold and will be explored in later sections.

4.7. Removal After the predetermined solidification time has passed, the sand mold can simply be broken, and the casting removed. This step, sometimes called shakeout, is typically performed by a vibrating machine that shakes the sand and casting out of the flask. Once removed, the casting will likely 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 surface roughness.

4.8. Trimming During cooling, the material from the channels in the mold solidifies attached to the part. This excess material must be trimmed from the casting either manually via cutting or sawing, or using a trimming press. The time required to trim the excess material can be estimated from the size of the casting's envelope. A larger casting will require a longer trimming time. The scrap material that results from this trimming is either discarded or reused in the sand-casting process. However, the scrap material may need to be reconditioned to the proper chemical composition before it can be combined with non-recycled metal and reused.

5. Raw Materials and Process Tools 5.1. Mold In sand casting, the primary piece of equipment is the mold, which contains several components. The mold is divided into two halves. The cope (upper half) and the drag (bottom half), which meet along a parting line. Both mold halves are contained inside a box, called a flask, which itself is divided along this parting line. The mold cavity is formed by packing sand around the pattern in each half of the flask. The sand can be packed by hand, but machines that use pressure or impact ensure even packing of the sand and require far less time, thus increasing the production rate. After the sand has been packed and the pattern is removed, a cavity will remain that forms the external shape of the casting. Some internal surfaces of the casting may be formed by cores. Cores are additional pieces that form the internal holes and passages of the casting. Cores are typically made out of sand so that they can be shaken out of the casting, rather than require the necessary geometry to slide out. As a result, sand cores allow for the fabrication of many complex internal features. Each core is positioned in the mold before the molten metal is poured. In addition to the external and internal features of the casting, other features must be incorporated into the mold to accommodate the flow of molten metal. The molten metal is poured into a pouring basin,

which is a large depression in the top of the sand mold. The molten metal funnels out of the bottom of this basin and down the main channel, called the sprue. The sprue then connects to a series of channels, called runners, which carries the molten metal into the cavity. At the end of each runner, the molten metal enters the cavity through a gate which controls the flow rate and minimizes turbulence. Often connected to the runner system are risers. Risers are chambers that fill with molten metal, providing an additional source of metal during solidification. When the casting cools, the molten metal will shrink and additional material is needed. A similar feature that aids in reducing shrinkage is an open riser. The first material to enter the cavity is allowed to pass completely through and enter the open riser. This strategy prevents early solidification of the molten metal and provides a source of material to compensate for shrinkage. Lastly, small channels are included that run from the cavity to the exterior of the mold. These channels act as venting holes to allow gases to escape the cavity. The porosity of the sand also allows air to escape, but additional vents are sometimes needed. The molten metal that flows through all of the channels (sprue, runners, and risers) will solidify attached to the casting and must be separated from the part after it is removed.

Figure1. 2: Sand Mold Opened

Figure1. 3: Sand Mold Closed

5.2. Types of Sand Mold There are four types of sand mold: ➢ Green Sand Mold: Greensand molds use a mixture of sand, water, and a clay or binder. Typical composition of the mixture is 90% sand, 3% water, and 7% clay or binder. Greensand molds are the 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 cavity surface is dried by a torch or heating lamp to increase mold strength. Doing so also improves the dimensional accuracy and surface finish, but will lower the collapsibility. Dry skin molds are more expensive and require more time, thus lowering the production rate. ➢ Dry Sand Mold: In a dry sand mold, sometimes called a cold box mold, the sand is mixed only with an organic binder. The mold is strengthened by baking it in an oven. The resulting mold has high dimensional accuracy, but is expensive and results in a lower production rate. ➢ No-Bake Mold: The sand in a no bake mold is mixed with liquid resins and hardens at room temperature. Some major components of sand molds are: ➢ The mold itself, which is supported by a flask. The flask consists of two pieces of molds that are known as the cope located on the top and a drag on the bottom. ➢ A pouring basin, into which molten metal is poured.

➢ A sprue, the part where molten metal flows through. ➢ A riser, the section that supplies additional metal to the casting while metal shrinks in the solidification stage. ➢ Vents are placed in molds for the main purpose of releasing existing gases when the molten metal comes into contact with the sand. They also exhaust air from the mold cavity as the molten metal flows into the mold.

5.3. Sand The sand that is used to create the molds is typically silica sand (SiO2) that is mixed with a type of binder to help maintain the shape of the mold cavity. Using sand as the mold material offers several benefits to the casting process. Sand is very inexpensive and is resistant to high temperatures, allowing many metals to be cast that have high melting temperatures. The quality of the sand that is used also greatly affects the quality of the casting and is usually described by the following measures: ➢ Strength: Ability of the sand to maintain its shape. ➢ Permeability: Ability to allow venting of trapped gases through the sand. A higher permeability can reduce the porosity of the mold, but a lower permeability can result in a better surface finish. Permeability is determined by the size and shape of the sand grains. ➢ Thermal Stability: Ability to resist damage, such as cracking, from the heat of the molten metal. ➢ Collapsibility: Ability of the sand to collapse, or more accurately compress, during solidification of the casting. If the sand cannot compress, then the casting will not be able to shrink freely in the mold and can result in cracking. ➢ Reusability: Ability of the sand to be reused for future sand molds.

5.4. Process Tools When working with casing sand the caster needs a few basic tools to achieve good casting results. Casting tools can be expansive. ➢ Rammer: A rammer is a wood or metal tool used for ramming or packing the sand in the molding box. It has two parts peen and butt. Rammers are available in different designs and constructions. ➢ Vent wire: Vent wire is a thin rod or wire carrying a pointed edge at one end and a wooden handle at the other end. Vent wire is used to make small holes called vents in the sand mold. ➢ Roller: To smoother the surface. ➢ Cutting Scale: To remove extra sand. ➢ Lifter: Lifter is used to remove sand from edges. ➢ Slick: A slick is used for repair and finishing the mold surface after the removal of pattern. ➢ Chalk Powder or Clay: To avoid stickiness.

5.5. Machines and Their Applications ➢ Sand Separator: Sand separator is used to remove grit and other rocky particles from sand.

Figure1. 4: Sand Separator

➢ Ladle: In metallurgy, a ladle is vessel used to transport and pour out molten metals. Ladles are often used in foundries and range in size from small hand carried vessels to large scale. Ladle is made of ceramics material.

Figure1. 5: Ladle

➢ Sand Mixer Machine: A machine for mixing sand and binders by a kneading and squeezing action for use in sand molds. The mixture is usually sand, clay, and water, but synthetic chemical binders may be used.

Figure1. 6: Sand Mixer Machine

➢ Pneumatic Press: A forming press, commonly shortened to press, is a machine tool that changes the shape of a work piece by the application of pressure. The operator of a forming press is known as a press-tool setter, often shortened to tool setter.

Figure1. 7: Pneumatic Press and Compressor

➢ Gas furnace: Gas furnace is used to convert solid metal into liquid form at high temperature.

Figure1. 8: Gas Furnace

6. Defects ➢ Porosity shrinkage defect: Shrinkage defects include dispersed shrinkage, micro-shrinkage and porosity. For large porosity on the surface, you could see them easily, but for small dispersed shrinkage, you may see them after machining.

Figure1. 9: Porosity Defect

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Blowhole and Pinhole: This is a kind of cavities defect, which is also divided into pinhole and subsurface blowhole. Pinhole is very tiny hole; some could be seen on the surface. Subsurface blowhole only can be seen after machining or grinding.

Figure1. 10: Blowhole Defect

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Shrinkage Cavities Defect: These are also called as shrinkage holes, which is a type of serious shrinkage defect, you can see these holes easily on the rough surface of the metal castings. Foundries could improve their gating and venting system, then could solve these shrinkage problems.

Figure1. 11: Cavity Defect

➢ Hot Tears: Hot tears, also known as hot cracks, show up when the cooling metal contracts. In this state, when the metal is still weak, residual stress in the metal or poor mold design can cause the metal to pull apart, resulting in branching, irregular cracks. Sometimes these are difficult to see with the naked eye.

Figure1. 12: Hot Tears

➢ Slag Inclusion: Slag inclusion happens when metal isn’t properly cleared of slag before being poured. It simply means the finished castin...