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WELDONG SCINECE AND TECHNOLOGY,CASTING PROCESSS

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Schematic of a typical sand mold 27
SAND MOLD CROSS-SECTION Ingate Runner
Flask: Box that contains molding aggregate Cope: Top half of the flask Drag: Bottom half of the flask Core: Sand shape that is inserted to produce holes, hollows Riser: Extra void filled with metal. Supplies metal to mold cavity to compensate for shrinkage solidification 29
Gating system: Network of channels used to deliver molten metal to the mold cavity. Comprises of Pouring cup Spruce Runners Ingates 30
Green Sand Mould Composition: Sand (70-85%); Clay (10-20%); Water (3-6%); Additives (wood flour, dextrin, sea coal) (1-6%) Shape and Size of Sand Grains: Vary widely The bulk density of a sand-mix is very low if the grains are of equal size with smooth round shape Result in increased voids and higher permeability Clay, together with water, acts as a bonding agent. Imparts tensile and shear strength to the moulding sand. The organic additives burn out at high temperatures making room for the moulding sand to expand and thus save the mould from crumbling. 31
Properties of Moulding Sand Properties of the Moulding Sand: Strength (Compressive Strength) Permeability (Gas flow rate through the specimen under a specified pressure difference across it) Deformation (Change in length of a standard specimen at the point of failure) Flowability (ability of the sand to flow around and over the pattern when the mould is rammed) Refractoriness (ability of the sand to remain solid as a function of temperature) 32
Effect of Water Content on moulding sand properties 33
Gating Design A good gating design ensures distribution of metal in the mould cavity at a proper rate without excessive temperature loss, turbulence and entrapping gases and slags. Bernoullis theorem states that the sum of the energies (head, pressure, kinetic, and friction) at any two points in a flowing liquid are equal 2 2 3 3 1 1 1 1 3 3 2 2 p v p v h F h F g g + + + = + + + Between points 1 and 3: (In a simple vertical gating) where, h is the head, cm, p is pressure on the liquid, N/cm2; is the density, g/cm3 ; v is the flow velocity; cm/s; g is gravitational acceleration constant, 981 cm/s/s ; and F is head losses due to friction, cm . Subscripts 1 and 2 indicate any two locations in the liquid flow. 34
In the figure, pressure at points 1 and 3 is equal ( ) Level 1 is maintained constant. Thus the velocity, = 0 Frictional losses are neglected The energy balance equation between points 1 and 3 gives: 1 3 p p = ｐ1 2 3 3 ; , 2 2 t t v h or v gh g = = Where, g is the acceleration due to gravity and is the velocity of the liquid metal at the gate. Time taken to fill up the mould is obtained as: ｐ3 3 f g V t A v = Where, and V are the cross-sectional area of the gate and the volume of the mould respectively. 基基 Gating Design (Contn.) 35

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WELDONG SCINECE AND TECHNOLOGY,CASTING PROCESSS

  • 1. Schematic of a typical sand mold 27
  • 3. Flask: Box that contains molding aggregate Cope: Top half of the flask Drag: Bottom half of the flask Core: Sand shape that is inserted to produce holes, hollows Riser: Extra void filled with metal. Supplies metal to mold cavity to compensate for shrinkage solidification 29
  • 4. Gating system: Network of channels used to deliver molten metal to the mold cavity. Comprises of Pouring cup Spruce Runners Ingates 30
  • 5. Green Sand Mould Composition: Sand (70-85%); Clay (10-20%); Water (3-6%); Additives (wood flour, dextrin, sea coal) (1-6%) Shape and Size of Sand Grains: Vary widely The bulk density of a sand-mix is very low if the grains are of equal size with smooth round shape Result in increased voids and higher permeability Clay, together with water, acts as a bonding agent. Imparts tensile and shear strength to the moulding sand. The organic additives burn out at high temperatures making room for the moulding sand to expand and thus save the mould from crumbling. 31
  • 6. Properties of Moulding Sand Properties of the Moulding Sand: Strength (Compressive Strength) Permeability (Gas flow rate through the specimen under a specified pressure difference across it) Deformation (Change in length of a standard specimen at the point of failure) Flowability (ability of the sand to flow around and over the pattern when the mould is rammed) Refractoriness (ability of the sand to remain solid as a function of temperature) 32
  • 7. Effect of Water Content on moulding sand properties 33
  • 8. Gating Design A good gating design ensures distribution of metal in the mould cavity at a proper rate without excessive temperature loss, turbulence and entrapping gases and slags. Bernoullis theorem states that the sum of the energies (head, pressure, kinetic, and friction) at any two points in a flowing liquid are equal 2 2 3 3 1 1 1 1 3 3 2 2 p v p v h F h F g g + + + = + + + Between points 1 and 3: (In a simple vertical gating) where, h is the head, cm, p is pressure on the liquid, N/cm2; is the density, g/cm3 ; v is the flow velocity; cm/s; g is gravitational acceleration constant, 981 cm/s/s ; and F is head losses due to friction, cm . Subscripts 1 and 2 indicate any two locations in the liquid flow. 34
  • 9. In the figure, pressure at points 1 and 3 is equal ( ) Level 1 is maintained constant. Thus the velocity, = 0 Frictional losses are neglected The energy balance equation between points 1 and 3 gives: 1 3 p p = ｐ1 2 3 3 ; , 2 2 t t v h or v gh g = = Where, g is the acceleration due to gravity and is the velocity of the liquid metal at the gate. Time taken to fill up the mould is obtained as: ｐ3 3 f g V t A v = Where, and V are the cross-sectional area of the gate and the volume of the mould respectively. 基基 Gating Design (Contn.) 35