MANUFACTURING TECHNOLOGY

MANUFACTURING TECHNOLOGYBy,ARVIND PASUPARTHY1021310090MECHANICAL-B1

THE CASTING PROCESSCasting the forming technology that fascinates the world2

CASTING-TERMINOLOGYCasting 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.3

HISTORY OF CASTINGHuman being has first encountered with metals back in 5,000-6,000 B.C. when people were processing natural gold/silver/copper by hand beating. Thus, the very first processing method of metals was forging. The prideful Japanese sword could be said as the most advanced forging technology in the world.The start of casting technology which forms casting by pouring melted metal into a mold and solidifying it, origins back to around 3,600 B.C. at Mesopotamia. It was approximately 5600 years before from today. Back then, bronze was melted and poured into a mold.4

On a papyrus from about 1500 B.C. of Egypt, we can see a drawing of people using their legs on bellows to send air (figure:-1). By inventing bellows, human being has achieved to obtain higher heating technology. This was how the Bronze Age arrived. By the way, the melting point of bronze is approximately 800 with Cu-25%Sn. Considering casting, it could be assumed that the melting temperature could have been exceeded 1000 deg C.For modern cast iron, there are two significant developments.One of it is the manufacturing method of spheroidal graphite cast iron based on inoculation technology which was developed by G. F. Meehan and O. Smalley in 1940s. By this technology, it became possible to produce high strength cast iron with tensile strength more than 300N/mm2 stably.The second is spheroidal graphite cast iron which was discovered by Morrogh in England back in 1947. By this discovery, the manufacturing of cast iron with even higher strength of more than 800N/mm2 realized.Figure 1: Casting of bronze door (BC1500)5

SAND-MOULD CASTINGSand casting, the most widely used casting process, utilizes expendable sand molds to form complex metal parts that can be made of nearly any alloy. Because the sand mold must be destroyed in order to remove the part, called the casting, sand casting typically has a low production rate. The sand casting process involves the use of a furnace, metal, pattern, and sand mold. The metal is melted in the furnace and then ladled and poured into the cavity of the sand mold, which is formed by the pattern. The sand mold separates along a parting line and the solidified casting can be removed. 6

Figure 2: Casting mold-opened

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Sand casting is used to produce a wide variety of metal components with complex geometries. These parts can vary greatly in size and weight, ranging from a couple ounces to several tons. Some smaller sand cast parts include components as gears, pulleys, crankshafts, connecting rods, and propellers. Larger applications include housings for large equipment and heavy machine bases. Sand casting is also common in producing automobile components, such as engine blocks, engine manifolds, cylinder heads, and transmission cases.The process of sand casting can be classified as,1) Mold-making - The first step in the sand casting process is to create the mold for the casting. In an expendable mold process, this step must be performed for each casting. A sand mold is formed by packing sand into each half of the mold. The sand is packed around the pattern, which is a replica of the external shape of the casting. When the pattern is removed, the cavity that will form the casting remains. Any internal features of the casting that cannot be formed by the pattern are formed by separate cores which are made of sand prior to the formation of the mold. Further details on mold-making will be described in the next section. The mold-making time includes positioning the pattern, packing the sand, and removing the pattern. The mold-making time is affected by the size of the part, the number of cores, and the type of sand mold. If the mold type requires heating or baking time, the mold-making time is substantially increased. Also, lubrication is often applied to the surfaces of the mold cavity in order to facilitate removal of the casting. The use of a lubricant also improves the flow the metal and can improve the surface finish of the casting. The lubricant that is used is chosen based upon the sand and molten metal temperature.

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Figure 3: Casting mold-closed

2) 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.3) Pouring - The molten metal is maintained at a set temperature in a furnace. After the mold has been clamped, the molten metal can be ladled from its holding container in the furnace and poured into the mold. 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) 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 can not 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.5) 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.6) 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.8

CASTING TERMS1. Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed. Depending upon the position of the flask in the molding structure, it is referred to by various names such as drag lower molding flask, cope upper molding flask, cheek intermediate molding flask used in three piece molding.2. Pattern: It is the replica of the final object to be made. The mold cavity is made with the help of pattern.3. Parting line: This is the dividing line between the two molding flasks that makes up the mold.4. Molding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a mixture of silica sand, clay, and moisture in appropriate proportions.5. Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to give a better surface finish to the castings.

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Figure 4: Casting

6. Core: A separate part of the mold, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings.7. Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten metal is poured.8. Sprue: The passage through which the molten metal, from the pouring basin, reaches the mold cavity. In many cases it controls the flow of metal into the mold.9. Runner: The channel through which the molten metal is carried from the sprue to the gate.10. Gate: A channel through which the molten metal enters the mold cavity.11. Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force.12. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as feed head.13. Vent: Small opening in the mold to facilitate escape of air and gases.

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PATTERNThe pattern is the principal tool during the casting process. It is the replica of the object to be made by the casting process, with some modifications. The main modifications are the addition of pattern allowances, and the provision of core prints. If the casting is to be hollow, additional patterns called cores are used to create these cavities in the finished product. The quality of the casting produced depends upon the material of the pattern, its design, and construction. The costs of the pattern and the related equipment are reflected in the cost of the casting. The use of an expensive pattern is justified when the quantity of castings required is substantial.11

Figure 5: Wooden pattern for a cast-iron gear with curved spokes

Functions of the Pattern1. A pattern prepares a mold cavity for the purpose of making a casting.2. A pattern may contain projections known as core prints if the casting requires a core and need to be made hollow.3. Runner, gates, and risers used for feeding molten metal in the mold cavity may form a part of the pattern.4. Patterns properly made and having finished and smooth surfaces reduce casting defects.5. A properly constructed pattern minimizes the overall cost of the castings.

12Figure 6: A typical pattern attached with gating and risering system

Pattern Material

Patterns may be constructed from the following materials. Each material has its own advantages, limitations, and field of application. Some materials used for making patterns are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins. To be suitable for use, the pattern material should be:1. Easily worked, shaped and joined2. Light in weight3. Strong, hard and durable4. Resistant to wear and abrasion5. Resistant to corrosion, and to chemical reactions6. Dimensionally stable and unaffected by variations in temperature and humidity7. Available at low costThe usual pattern materials are wood, metal, and plastics. The most commonly used pattern material is wood, since it is readily available and of low weight. Also, it can be easily shaped and is relatively cheap. The main disadvantage of wood is its absorption of moisture, which can cause distortion and dimensional changes. Hence, proper seasoning and upkeep of wood is almost a pre-requisite for large-scale use of wood as a pattern material.

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Pattern AllowancesPattern allowance is a vital feature as it affects the dimensional characteristics of the casting. Thus, when the pattern is produced, certain allowances must be given on the sizes specified in the finished component drawing so that a casting with the particular specification can be made. The selection of correct allowances greatly helps to reduce machining costs and avoid rejections. The allowances usually considered on patterns and core boxes are as follows:A. Shrinkage or contraction allowanceB. Draft or taper allowanceC. Machining or finish allowanceD. Distortion or camber allowanceE. Rapping allowance

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A. Shrinkage or contraction allowanceAll most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage is of two types: i. Liquid Shrinkage: it refers to the reduction in volume when the metal changes from liquid state to solid state at the solidus temperature. To account for this shrinkage; riser, which feed the liquid metal to the casting, are provided in the mold ii. Solid Shrinkage: it refers to the reduction in volume caused when metal loses temperature in solid state. To account for this, shrinkage allowance is provided on the patterns.The rate of contraction with temperature is dependent on the material. For example steel contracts to a higher degree compared to aluminum. To compensate the solid shrinkage, a shrink rule must be used in laying out the measurements for the pattern.15

A shrink rule for cast iron is 1/8 inch longer per foot than a standard rule. If a gear blank of 4 inch in diameter was planned to produce out of cast iron, the shrink rule in measuring it 4 inch would actually measure 4 -1/24 inch, thus compensating for the shrinkage. The various rate of contraction of various materials are given in Table 1.16

B. Draft or Taper AllowanceBy draft is meant the taper provided by the pattern maker on all vertical surfaces of the pattern so that it can be removed from the sand without tearing away the sides of the sand mold and without excessive rapping by the molder. Figure 3 (a) shows a pattern having no draft allowance being removed from the pattern. In this case, till the pattern is completely lifted out, its sides will remain in contact with the walls of the mold, thus tending to break it. Figure 3 (b) is an illustration of a pattern having proper draft allowance. Here, the moment the pattern lifting commences, all of its surfaces are well away from the sand surface. Thus the pattern can be removed without damaging the mold cavity17

Draft allowance varies with the complexity of the sand job. But in general inner details of the pattern require higher draft than outer surfaces. The amount of draft depends upon the length of the vertical side of the pattern to be extracted; the intricacy of the pattern; the method of molding; and pattern material. Table 2 provides a general guide lines for the draft allowance18

C. Machining or Finish Allowance

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