BME-C4

• Casting


  A process in which a metal is heated to the molten state and is poured into a cavity or a mould which is having the shape of the desired object. The molten metal fills the mould and on solidification gives the metal object of the same shape.
• Mould

  A cavity created in a compact sand mass which when filled with molten metal will produce the desired casting.
• Moulding


  1. Producing the cavity or mould

  2. Can be done by hand or using machines
• Types of moulding sand


  • Natural moulding sand or Green sand
  • Synthetic sand
  • Special sand
• Natural moulding sand (Green sand)


  • Contains sufficient amount of binding clay
  • Can be used directly for making moulds
  • Less refractory compared to synthetic sands
  • Cheap, easily available and have high operational flexibility
  • Used for casting cast iron and nonferrous metals

  
• Synthetic sand


  • High silica sands containing little or no clay
  • Binders and other ingredients added to make them suitable for casting
  • Allows greater flexibility in properties such as green and dry strength, permeability
  • More expensive than natural sand
  • Used for casting steel, ferrous and nonferrous castings
• Special sand


  • Specially made to acquire special characteristics and for some specific use
  • Different types: Parting sand, Facing sand, Backing sand, Loam sand and Core sand
• Moulding sand composition

  • Silica sand grains
  • Binder or clay
  • Water or moisture
  • Special additives
• Silica sand grains


  • Naturally occurring or crushed granular particles of sand that is silicon dioxide
  • Comprise nearly 75% of the total material in the moulding sand
  • Impart refractoriness, chemical resistivity and permeability
• Clay or Binder


  • Moulding sand contains about 8 to 15% of clay
  • With suitable water content clay imparts necessary bonding strength between the silica sand grains
  • In natural moulding sands clay and silica occur mixed in proper proportions
  • In other sands clay or any other binder such as bentonite must be added
  • Binders are less refractory and if the binder content increases the permeability of moulding sand decreases
• Water or Moisture

  • The amount of water in the moulding sand ranges from 6 to 8%
  • Responsible for the bonding action of clay
  • Activates the clay in the sand and the clay sand mixture develops necessary plasticity and strength
  • Acts as a lubricant and makes the sand more plastic and mouldable
• Special Additives


  • Added to enhance certain properties
  • Include limestone, soda potash, magnesia, certain oils etc.
• Properties of moulding sand

  • Permeability or Porosity
  • Refractoriness
  • Adhesiveness
  • Cohesiveness
  • Flowability
  • Collapsibility
• Permeability or Porosity


  Property that allows the gases and water vapour to escape through the sand mould

  When the molten metal is poured into the mould certain gases and steam is generated due to the heating of moisture, coal dust and other materials present in the mould

  sand should have sufficient porosity , otherwise blow holes

  permeability depends on sand grain size , % of clay , extent of ramming

  large grain size , softer ramming , lesser clay % -> increases permeability
• Refractoriness

  Property that enables the moulding sand to withstand the high temperature of the molten metal without fusing
• Adhesiveness


  Property due to which the sand is capable of adhering to the surfaces of other materials
• Cohesiveness


  • Ability of the sand particles to stick to one another
  • Provides green strength and dry strength to the mould
• Flowability


  Ability of the sand to behave like a fluid so that it flows to all portions of the mould and packs properly around the pattern
• Collapsibility


  Property due to which the sand mould automatically collapses after solidification of the casting to allow for the free contraction of the metal
• Pattern

  Replica or model of the desired casting used for producing the mould

  
• Pattern materials

  • Wood
  • Metals
  • Plastics
  • Plaster of paris
• Wood patterns


  • Most commonly used
  • Cheap and readily available
  • Easy to work with and can take up any shape
  • Light in weight
  • Readily affected by moisture and can change shape
  • Wear out quickly due to sand abrasion
• Metal patterns

  • Used when large number of castings are desired or when conditions are too severe for wooden patterns
  • Capable of withstanding jolts or sudden jerks
  • Used when close dimensional accuracy is required
  • High cost, greater weight and tendency to rust
  • Commonly used metals: cast iron, brass and aluminium
• Plastic patterns


  • Do not absorb moisture, are strong and dimensionally stable, resistant to wear, have a very smooth and glossy surface, and are light in weight
  • Can be withdrawn from the mould very easily without injuring the mould
• Plaster of paris patterns

  • Have high compressive strength and intricate shapes can be easily made
  • Affected by moisture and hard ramming
• Types of patterns

  • Single-piece or solid pattern
  • Split pattern
• Single-piece or solid pattern

  • A pattern made without any joints, partings or loose pieces in its construction
  • Simpler and cheaper
  • Used for large castings of simple shapes
• Split pattern

  • Made in two parts to form the lower and upper halves of the mould
  • Used for castings with contours or intricate shapes
  • Lower half is contained in the drag
  • Upper half in the cope.

  
• Pattern making allowances


  • Shrinkage allowance
  • Machining Allowance
  • Draft Allowance
  • Rapping or Shake Allowance
  • Distortion Allowance
  • Mouldwall Movement Allowance
• Shrinkage allowance

  Provided because the molten metal shrinks and contracts in size as it cools and solidifies
• Parting surface


  The surface formed at the line of separation of the two parts of a pattern, usually at the centerline of the pattern. It will also be the parting surface of the mould.
• Pattern Making Allowances

  • Shrinkage Allowance
  • Machining Allowance
  • Draft Allowance
  • Rapping or Shake Allowance
  • Distortion Allowance
  • Mouldwall Movement Allowance
• Shrinkage Allowance


  The increase in size which has to be incorporated in the pattern to compensate for the shrinkage of the metal as it cools and solidifies
• Machining Allowance


  The extra amount of metal which has to provided on the surfaces of a casting that is to be machined off to improve the surface finish and bring the casting to the required dimensions
• Draft Allowance


  The taper on the vertical surface of a pattern to facilitate easy withdrawal of the pattern from the mould
• Rapping or Shake Allowance


  A negative allowance provided in the pattern by making it smaller than the actual size to compensate for the slight increase in the size of the mould cavity when the pattern is rapped or struck lightly to free it from the sand
• Distortion Allowance


  The shape of the pattern is bent in the opposite direction to overcome the distortion that some castings tend to get during solidification due to thermal stresses
• Mouldwall Movement Allowance

  The allowance provided in the pattern to compensate for the movement of mould walls in sand moulds due to the excessive heat and static pressure exerted by the molten metal
• Two Box Moulding Procedure


  1. Place drag upside down, put bottom half of pattern in, fill with sand and ram

  2. Turn over, place cope, put top half of pattern, add sprue pins, fill cope with sand and ram

  3. Withdraw sprue pins, make pouring basin, withdraw pattern, cut gate

  4. If using core, make core separately and place in drag, close cope and clamp

  5. Pour molten metal, solidify, withdraw casting, remove runner and riser, clean and machine
• Core


  A body of sand prepared separately in a core box which is used to form cavity of desired shape and size in the casting