1. Casting Processes

2. Sand casting
consists of placing a pattern (having the shape of the desired casting) in sand to make an imprint, incorporating a gating system, filling the resulting cavity with molten metal, allowing the metal to cool until it solidifies, breaking away the sand mold, and removing the casting
in the USA about 15 million tons of metal are cast by sand casting
used for machine tool bases, engine blocks, cylinder heads, and pump houses

3. Sands silica sands (SiO2)
fine round grains can be closely packed and forms a smooth mold surface
good permeability of molds and cores allows gases and steam evolved during casting to escape easily
the mold should have good collapsibility to avoid defects in the casting (tearing and cracking)
ability to with stand high temperatures
ability to retain shape under the action of metal flow
permeability
collapsibility

4. Three basic types of sand molds
green sand (a mixture of sand, clay, and water)
cold box (various organic and inorganic binders are blended into the sand to bond the grains chemically for greater strength)
no-bake molds (a synthetic liquid resin is mixed with sand, and the mixture hardness at the room temperature

5. Major components of sand molds
The mold is supported by a flask. Two piece molds consist of a cope on top and a drag on the bottom. The seam between them is the parting line.
A pouring basin or cup, into which the molten metal is poured.
A sprue, through which the molten metal flows downward.
A gate, which is located at the base of the sprue. Molds typically contain a system of gates constructed to minimized turbulence in the molten metal and control flow so that metal is supplies at a rate to adequately supply the critical section thickness of the casting. Gating systems often include passageways called runners.
Risers, which supply additional metal to the casting as it shrinks during solidification.
Cores, which are inserts made from sand. They are placed in the mold to form hollow regions.
Vents, which are placed in molds to carry off gases produced when the molten metal comes into contact with the sand in the molds.

6. Cores used to form internal cavities or passages
they ar placed in the mold cavity before casting and are removed from the finished part during shakeout and further processing
they must possess strength, permeability, ability to withstand heat, and collapsibility
they are typically made of sand aggregates
the core is anchored by core prints (they are recesses that are added to the pattern to support the core and to provide vents for the escape of gasses)
or by metal supports, known as chaplets

17. Shell mold casting
produces many types of castings with close tolerances and good surface finishes at a low cost
a mounted pattern, made of a ferrous metal or aluminum, is heated to C, coated with a parting agent such as silicone, and clamped to a box or chamber containing a fine sand coated with a % thermosetting resin binder
the sand mixture is blown over the heated pattern, coating it evenly
the assembly is placed in an oven to complete the curing of the resin
the shell is formed by removing the pattern
two half shells are made and are clamped together in preparation for pouring

18. Thin walls (5 - 10mm)
gases easy to escape
walls are smooth, less resistance to flow of the molten metal, produce castings with sharper corners, thinner section
more economical
the high quality of the finished casting can reduce cleaning, machining, and other finishing costs
can produce complex shapes
it is also used in producing high precision cores

20. Composite molds made of two or more different materials
utilized in casting complex shapes (impellers for turbines)
molding materials are: shells, plaster, sand with binder, metal and graphite
they increase the strength of the mold, improve the dimensional accuracy and surface finish, and may reduce overall costs

21. Expandable pattern casting (lost foam)
uses a polystyrene pattern which evaporates upon contact with molten metal to form a cavity for the casting
one of the most important casting processes for the automobile industry
the polystyrene pattern is coated with a waterbase refractory slurry, dried, and placed in a flask. The flask is filled with loose fine fine sand. The molten metal is poured into the mold.

22. Advantages:
it is relatively simple process (there are no parting lines, cores, or riser systems)
inexpensive flasks are sufficient for the process
polystyrene is inexpensive (complex shapes and various sizes)
the casting requires minimum finishing and cleaning operations
the process is economical for long production runs
the process can be automated

24. Plaster mold casting
the mold is made of plaster of paris (gypsum, or calcium sulfate) with addition of talc and silica flour to improve strength and control the time required for the plaster to set
these components are mixed with water and the resulting slurry is poured over the pattern
after the plaster is set, the pattern is removed and the mold is dried
the mold halves are then assembled to form the mold cavity and preheated to about 120 C for 16 hours
the molten metal is then poured into the mold

25. Low permeability, the molten metal is poured either in a vacuum or under pressure
material for patterns are: aluminum alloys, magnesium, zinc, and some copper-base alloys
because of low thermal conductivity, the castings are cooled slowly, yielding more uniform grain structure with less warpage and better mechanical properties
high precision casting
used for casting gears, lock components, valves, fittings, tooling, and ornaments

26. Ceramic mold casting
the slurry is a mixture of fine grained zircon, aluminum oxide, and fused silica, which are mixed with bonding agents and poured over the pattern, which has been placed in a flask
because of the high temperature resistance, these molds can be used in casting ferrous and other high temperature alloys

27. Investment casting (lost wax process)
used during the period BC
the pattern is made of wax or plastics

30. Addition of nucleant to the molten metal
close control of superheat of the molten metal
control of pouring techniques
control of cooling rate

31. Vacuum casting
A schematic illustration of the vacuum casting process, or counter gravity low pressure process (not to be confused with the vacuum molding process) is shown in fig A mixture of the fine sand and urethane is molded over metal dies and cured with amine vapor. Then the mold is held with a robot arm and partially immersed into molten metal held in an induction furnace. The metal may be melted in air or in a vacuum. The vacuum reduces the air pressure inside the mold to about two thirds of atmospheric pressure, drawing the molten metal into the mold cavities through a gate in the bottom of the mold. The molten metal in the furnace is at a temperature usually 55C above the liquidus temperature; consequently it begins to solidify within a fraction of a second. After the mold is filled, it is withdrawn from the molten metal.

33. Permanent mold
better heat conduction than expendable mold (faster cooling that has affect on microstructure and grain size)
mold materials: steel, bronze, refractory metal alloys, graphite
core materials: shell or no-bake cores, gray iron, low carbon steel, hot work die steel
molds are preheated to facilitate metal flow and reduce thermal damage to the dies
process could be automated for large production runs
process is used mostly for aluminum, magnesium, and copper alloys
good surface quality, close tolerances, uniform and good mechanical properties and at high production rate
used to cast automobile pistons, cylinder heads, and connecting rods, gear blanks for appliance, and kitchenware

35. Slush casting Pressure casting
after the desired thickness of solidified skin is obtained, the mold is inverted or slung, and the remaining liquid metal is poured out
used for small production runs (ornamental and decorative objects and toys)
Pressure casting
the molten metal is forced upward by gas pressure (or by vacuum) into graphite or metal mold

36. Die casting Hot chamber process developed in the early 1900s
the molten metal is forced in the die cavity at pressures rangint from 0.7 to 700 MPa
typical parts: transmission housing, valve bodies, carburetors, motors, business machine and appliance components, hand tools, and toys
Hot chamber process

37. Cold chamber process

39. Process capabilities and machine selection
die has a tendency to part
rated according to clamping force
25 to 3000 tons
selected according to die size, piston stroke, shot pressure, and cost
single cavity, multiple cavity, or combined cavity
dies made of hot work die steel
dies may last half a million shots before wearing

41. True centrifugal casting
utilizes the inertial forces caused by rotation to distribute the molten metal into the mold cavities
True centrifugal casting
hollow cylindrical parts
molds are made of steel, iron, or graphite, and may be coated with a refractory lining to increase mold life
mold surfaces can be shaped so that pipes with various outer shapes, including square or polygonal, can be cast
inner surface remains cylindrical because the molten metal is uniformly distributed by centrifugal forces

44. Squeeze casting solidification of molten metal under high pressure
combination of casting and forging
high pressure promotes heat transfer, resulting in a fine microstructure with good mechanical properties and limited microporosity
made to near net shape with complex shapes and fine surface detail, from both ferrous and nonferrous alloys

45. Casting techniques for single-crystal components
used for gas turbine blades usually made of nickel-base superalloys
conventional casting of turbine blades (investment with ceramic mold)
polycrystalline grain structure makes it susceptible to creep and cracking along grain boundaries under centrifugal force
directionally solidified blades
uses a chill plate at one end of the mold
no transverse grain boundaries, only longitudinal
single crystal growing
seed crystal dipped into solution and pulled slowly out while being rotated
floating zone method involves polycrystalline silicon resting on a single crystal silicon, heated by an induction coil, the single crystal grows

48. Rapid solidification
involves cooling of molten metals at rates as high as 106 K/s
insufficient time to crystallize (amorphous alloys or metallic glasses)
typically contain iron, nickel, and chromium, alloyed with carbon, phosphorus, boron, aluminum, and silicon
amorphous alloys exhibit corrosion resistance, ductility, and high strength
useful magnetic properties make them attractive for magnetic cores