30-05-2012, 03:54 PM
CASTING
Casting1.ppt (Size: 2.81 MB / Downloads: 63)
The Industry
14 million pounds of castings are produced every year
The most common materials cast are gray iron, ductile iron, aluminum alloys, and copper alloys
35% of the market is in automotive and light truck manufacturing
Castings are used in applications ranging from agriculture to railroad equipment and heating and refrigeration
Process Steps
Six basic steps of casting
1. Mold cavity is produced having the desired shape and size of the part
Takes shrinkage into account
Single-use or permanent mold
2. Melting process
Provides molten material at the proper temperature
3. Pouring technique
Molten metal is poured into the mold at a proper rate to ensure that erosion and or defects are minimized
4. Solidification process
Controlled solidification allows the product to have desired properties
Mold should be designed so that shrinkage is controlled
5. Mold removal
The casting is removed from the mold
Single-use molds are broken away from the casting
Permanent molds must be designed so that removal does not damage the part
6. Cleaning, finishing, and inspection operations
Excess material along parting lines may have to be machined
Casting Terms
Pattern- approximate duplicate of the part to be cast
Molding material- material that is packed around the pattern to provide the mold cavity
Flask- rigid frame that holds the molding aggregate
Cope- top half of the pattern
Drag- bottom half of the pattern
Core- sand or metal shape that is inserted into the mold to create internal features
Mold cavity- combination of the mold material and cores
Riser-additional void in the mold that provides additional metal to compensate for shrinkage
Gating system- network of channels that delivers the molten metal to the mold
Pouring cup- portion of the gating system that controls the delivery of the metal
Sprue- vertical portion of the gating system
Runners- horizontal channels
Gates- controlled entrances
Parting line- separates the cope and drag
Draft- angle or taper on a pattern that allows for easy removal of the casting from the mold
Casting- describes both the process and the product when molten metal is poured and solidified
The Solidification Process
Molten material is allowed to solidify into the final shape
Casting defects occur during solidification
Gas porosity
Shrinkage
Two stages of solidification
Nucleation
Growth
Nucleation
Stable particles form from the liquid metal
Occurs when there is a net release of energy from the liquid
Undercooling is the difference between the melting point and the temperature at which nucleation occurs
Each nucleation event produces a grain
Nucleation is promoted (more grains) for enhanced material properties
Inoculation or grain refinement is the process of introducing solid particles to promote nucleation
Grain Growth
Occurs as the heat of fusion is extracted from the liquid
Direction, rate, and type of growth can be controlled
Controlled by the way in which heat is removed
Rates of nucleation and growth control the size and shape of the crystals
Faster cooling rates generally produce finer grain sizes
Cast Structure Cooling Zones
Three distinct regions or zones
Chill zone
Rapid nucleation that occurs when the molten metal comes into contact with the cold walls of the mold
Forms a narrow band of randomly oriented crystals on the surface of a casting
Columnar zone
Rapid growth perpendicular to the casting surface
Long and thin
Highly directional
Equiaxed zone
Crystals in the interior of the casting
Spherical, randomly oriented crystals
Process Factors
Molten metal problems
Reaction of the metal and its environment can lead to poor quality castings. Oxygen and molten metal react to form “slag” or “dross.” These impurities can become trapped in castings to impair surface finish, machinability, or reduce the mechanical properties of the castings.
Patterns
Shrinkage allowance
Typical allowances
Cast iron 0.8-1.0%
Steel 1.5-2.0%
Aluminum 1.0-1.3%
Magnesium 1.0-1.3%
Brass 1.5%
Amount of draft
Size and shape of pattern
Depth of mold cavity
Method used to withdraw pattern
Pattern material
Mold material
Molding procedure
Finish material allowance
Final dimensional accuracy of the casting