Metal Casting - by MANIGANDAN

Introduction: Metal Casting is one of the oldest materials shaping methods known. Casting means pouring molten metal into a mold with a cavity of the shape to be made, and allowing it to solidify. When solidified, the desired metal object is taken out from the mold either by breaking the mold or taking the mold apart. The solidified object is called the casting. By this process, intricate parts can be given strength and rigidity frequently not obtainable by any other manufacturing process.  The mold, into which the metal is poured, is made of some heat resisting material. Sand is most often used as it resists the high temperature of the molten metal. Permanent molds of metal can also be used to cast products.

Melting and Pouring: Several types of furnaces are available for melting metals and their selection depends on the type of metal, the maximum temperature required and the rate and the mode of molten metal delivery. Before pouring provisions are made for the escape of dissolved gases. The gating system should be designed to minimize the turbulent flow and erosion of mould cavity.The other important factors are the pouring temperature and the pouring rate.

Solidification and Cooling: The properties of the casting significantly depends on the solidification time cooing rate. Shrinkage of casting, during cooling of solidified metal should not be restrained by the mould material, otherwise internal stresses may develop and form cracks in casting. Proper care should be taken at the design stage of casting so that shrinkage can occur without casting defects.

Removal, Cleaning, Finishing and Inspection: After the casting is removed from the mould it is thoroughly cleaned and the excess material usually along the parting line and the place where the molten metal was poured, is removed using a potable grinder. White light inspection, pressure test, magnetic particle inspection, radiographic test, ultrasonic inspection etc. are used

Schematic diagram of casting mould:

Steps involved in making a casting: Make the pattern out of Wood , Metal or Plastic. Prepare the necessary sand mixtures for mould and core making. Prepare the Mould and necessary Cores. Melt the metal/alloy to be cast. Pour the molten metal/alloy into mould and remove the casting from the mould after the metal solidifies. Clean and finish the casting. Test and inspect the casting. Remove the defects, if any. Relieve the casting stresses by Heat Treatment. Again inspect the casting. The casting is ready for shipping.

Advantages of metal casting: Casting is one of the most versatile manufacturing process. Casting provides the greatest freedom of design in terms of shape, size and the product quantity. Casting imparts uniform directional properties and better vibration capacity to the cast parts. A product may be cast as one piece, there by eliminating the need of metal joining processes.

Limitations of casting: Dimensional accuracy and surface finish of the castings made by sand casting processes are a limitation to this technique. Many new casting processes have been developed which can take into consideration the aspects of dimensional accuracy and surface finish. Some of these processes are die casting process, investment casting process, vacuum-sealed molding process, and shell molding process. The metal casting process is a labor intensive process

Applications of Casting: Transportation vehicles Turbine vanes Power generators Railway crossings Agricultural parts Aircraft jet engine parts Sanitary fittings

Different Sections in Foundry: Pattern making Sand mixing & preparation Mould and core making Mould assembly & handling Melting Pouring Shaking out Fettling & finishing Heat treatment Inspection & testing

Metals and alloys commonly used in Foundries: 1. Ferrous 2. Non-Ferrous FERROUS: Cast irons Steels NON-FERROUS: Copper alloys Aluminium alloys Magnesium alloys Zinc alloys Nickel alloys

Fuels for Melting Metals: Coal Coke Gas Oil electricity

Fig: Single piece pattern

Fig: split piece pattern

Fig: Match plate pattern

Fig: Cope and drag pattern

2 types of moulding flask designs

Sand mold - opened

Sand mold - closed

Mixing moulding sand with binders & adhesives

Filling sand in moulding flasks

Melting furnace

Pouring molten liquid

Knock out

Heat treatment

Machining

final products of casting

Reference: http://nptel.iitm.ac.in/ A text book of Production Technology Vol. I / O.P. Khanna / Dhanpat Rai Publications A text book of Production Technology (Manufacturing Processes) / P. C. Sharma / S. Chand & Company Ltd Manufacturing Technology (Second Edition) / P N Rao / Tata McGraw-Hill Publishing Company Ltd & http://nptel.iitm.ac.in/

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