Casting

• Solidification Processes


  Various ways of transforming molten parts into their solidified state which will be used further in various manufacturing operations
• Casting

  1. Pouring molten metal into a mold

  2. Allowing it to cool

  3. Removing the metal from the mold
• Foundry


  Factory equipped for making molds, melting and handling molten metal, performing the casting process, and cleaning the finished casting
• Foundrymen


  Workers who perform casting
• Molds


  • Made of a variety of materials, including sand, plaster, ceramic, and metal
  • Contain cavities whose geometry determines part shape
  • Actual size and shape of cavity must be slightly oversized to allow for shrinkage of metal during solidification and cooling
• Types of Molds

  • Expendable Molds (made of sand, plaster, ceramics, and similar refractory materials mixed with binders/ bonding agents)
  • Permanent Molds (made of metals which maintain their strength at high temperatures and can be used repeatedly)
  • Composite Molds (made of two or more different materials such as sand, graphite, and metals)
• Expendable Molds


  • After casting has solidified, the mold is broken up to remove the casting
  • Meant to be broken to get the product inside
• Permanent Molds

  • Designed to easily get the casting
  • Provide better heat conductivity to the process
• Sand Casting


  • Traditional method of casting, making it to be the most prevalent even up to this day
  • Most of the molding processes use Silica sand (SiO2) as the mold material because it resists high temperatures
• Types of Sand

  • Naturally bonded – bank sand
  • Synthetic – lake sand
• Mulling machines


  Used to uniformly and thoroughly mix sand with additives
• Factors to consider when selecting sand mold

  • Surface finish (fine grains form smooth mold surface)
  • Good permeability (allows steam and gases to escape)
  • Good collapsibility (allow shrinking while cooling)
• Types of Sand Molds based on Sand Type and Production method

  • Cold-box (organic/inorganic binders blended for better dimensional accuracy)
  • Green-sand (Sand + Clay + Water, green-sand is moist, least expensive)
  • No-bake molds (Synthetic liquid resin mixed with sand, hardens at room temperature)
• Sand Casting Process

  1. Heating the Metal

  2. Pouring the Molten Metal

  3. Solidification of Metals
• Heating the Metal

  • Heating furnaces are used to heat the metal to molten temperature sufficient for casting
  • The heat required is the sum of: heat to raise temperature to melting point, heat of fusion to convert from solid to liquid, heat to raise molten metal to desired temperature for pouring
• Pouring the Molten Metal

  • Metal must flow into all regions of the mold, most importantly to the main cavity, before solidifying
  • Factors that determine success: pouring temperature must be higher than solidification temperature, pouring rate, turbulence or the inconsistent and irregular variations in the flow
• Solidification of Pure Metals

  • A thin skin of solid metal is formed at the interface immediately after pouring
  • Skin thickness increases to form a shell around the molten metal as solidification progresses
  • Rate of freezing depends on heat transfer into mold, as well as thermal properties of the metal
  • Characteristic grain structure in a casting of a pure metal
• Solidification of Alloys

  • Most alloys freeze over a temperature range rather than at a single temperature
  • Characteristic grain structure in an alloy casting
• Chvorinov's Rule


  The solidification time is a function of the volume of a casting and its surface area
• Solidification Shrinkage


  • Exception: cast iron with high C content (gray iron, 2.5% – 4% Carbon)
  • Occurs in nearly all metals because the solid phase has a higher density than the liquid phase
  • Graphitization during final stages of freezing causes expansion that counteracts volumetric decrease associated with phase change.
  • Patternmakers account for solidification shrinkage and thermal contraction by making the mold cavity oversized
  • Amount by which mold is made larger relative to final casting size is called pattern shrinkage allowance
• Riser


  • A waste material separated from the casting and remelted to make more castings
  • Must have longer freezing time (small surface area per unit volume) than the casting
  • Vertical channels that provide a continuous flow of molten metal to eliminate shrinkage as solidification occurs during the casting process
  • To minimize waste in the unit operation, it is desirable for the volume of metal in the riser to be a minimum.
• Defects in Casting

  • Segregation Defects (concentration of the solute is not constant throughout the casting)
  • Porosity Defects (caused by shrinkage, trapped gases, or both and is detrimental to ductility and fatigue life)
• Shell Mold Casting


  • The box contains fine sand, mixed with a thermosetting binder (phenol-formaldehyde) that coats the sand particles
  • The sand will coat the pattern
• Investment Casting

  • AKA LOST-WAX process
  • "Investment" is derived from the fact that the pattern is invested with the refractory material
  • Refractory has an unusually high melting point and that maintains its structural properties at very high temperatures
  • Wax can still be recovered and reused
• Capabilities and Advantages of Casting

  • Can create complex part geometries
  • Can create both external and internal shapes
  • Some casting processes are net shape; others are near net shape
  • Can produce very large parts
  • Some casting methods are suited to mass production
• Disadvantages of Casting

  • Limitations on mechanical properties
  • Poor dimensional accuracy and surface finish for some processes; e.g., sand casting
  • Safety hazards to workers due to hot molten metals
• • Binders / Bonding agents are used to provide cohesiveness and structural stability.
• Open mold – simply a container in the shape of the desired part
• Closed mold – mold geometry is more complex and requires a gating system (passageway) leading into the cavity
• Solidification causes a reduction in volume per unit

  weight of metal.
• Segregation can be reduced by homogenization- the process of making things uniform or similar. The homogenization can

  be achieved through faster cooling rate.
• Shrinkage induced porosity occurs when molten metal is trapped inside already solidified metal. They can be reduced by proper mold design
• Gas porosity is caused by gas bubbles. Initially, the gases are dissolved in the molten metal. Since the solubility of the gas in the metal decreases drastically on solidification, gas atoms are rejected. Such porosity is generally spherical
• Typical shrinkage of metals ranges between 4 and 7%