2 Sand castingconsists 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 castingin the USA about 15 million tons of metal are cast by sand castingused 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 surfacegood permeability of molds and cores allows gases and steam evolved during casting to escape easilythe mold should have good collapsibility to avoid defects in the casting (tearing and cracking)ability to with stand high temperaturesability to retain shape under the action of metal flowpermeabilitycollapsibility
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 processingthey must possess strength, permeability, ability to withstand heat, and collapsibilitythey are typically made of sand aggregatesthe 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 castingproduces many types of castings with close tolerances and good surface finishes at a low costa 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 binderthe sand mixture is blown over the heated pattern, coating it evenlythe assembly is placed in an oven to complete the curing of the resinthe shell is formed by removing the patterntwo half shells are made and are clamped together in preparation for pouring
18 Thin walls (5 - 10mm)gases easy to escapewalls are smooth, less resistance to flow of the molten metal, produce castings with sharper corners, thinner sectionmore economicalthe high quality of the finished casting can reduce cleaning, machining, and other finishing costscan produce complex shapesit 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 graphitethey 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 castingone of the most important casting processes for the automobile industrythe 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 processpolystyrene is inexpensive (complex shapes and various sizes)the casting requires minimum finishing and cleaning operationsthe process is economical for long production runsthe process can be automated
24 Plaster mold castingthe 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 setthese components are mixed with water and the resulting slurry is poured over the patternafter the plaster is set, the pattern is removed and the mold is driedthe mold halves are then assembled to form the mold cavity and preheated to about 120 C for 16 hoursthe 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 alloysbecause of low thermal conductivity, the castings are cooled slowly, yielding more uniform grain structure with less warpage and better mechanical propertieshigh precision castingused for casting gears, lock components, valves, fittings, tooling, and ornaments
26 Ceramic mold castingthe 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 flaskbecause 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 BCthe pattern is made of wax or plastics
30 Addition of nucleant to the molten metal close control of superheat of the molten metalcontrol of pouring techniquescontrol of cooling rate
31 Vacuum castingA 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 moldbetter heat conduction than expendable mold (faster cooling that has affect on microstructure and grain size)mold materials: steel, bronze, refractory metal alloys, graphitecore materials: shell or no-bake cores, gray iron, low carbon steel, hot work die steelmolds are preheated to facilitate metal flow and reduce thermal damage to the diesprocess could be automated for large production runsprocess is used mostly for aluminum, magnesium, and copper alloysgood surface quality, close tolerances, uniform and good mechanical properties and at high production rateused 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 outused for small production runs (ornamental and decorative objects and toys)Pressure castingthe 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 MPatypical parts: transmission housing, valve bodies, carburetors, motors, business machine and appliance components, hand tools, and toysHot chamber process
39 Process capabilities and machine selection die has a tendency to partrated according to clamping force25 to 3000 tonsselected according to die size, piston stroke, shot pressure, and costsingle cavity, multiple cavity, or combined cavitydies made of hot work die steeldies 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 cavitiesTrue centrifugal castinghollow cylindrical partsmolds are made of steel, iron, or graphite, and may be coated with a refractory lining to increase mold lifemold surfaces can be shaped so that pipes with various outer shapes, including square or polygonal, can be castinner 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 forginghigh pressure promotes heat transfer, resulting in a fine microstructure with good mechanical properties and limited microporositymade 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 superalloysconventional casting of turbine blades (investment with ceramic mold)polycrystalline grain structure makes it susceptible to creep and cracking along grain boundaries under centrifugal forcedirectionally solidified bladesuses a chill plate at one end of the moldno transverse grain boundaries, only longitudinalsingle crystal growingseed crystal dipped into solution and pulled slowly out while being rotatedfloating zone method involves polycrystalline silicon resting on a single crystal silicon, heated by an induction coil, the single crystal grows
48 Rapid solidificationinvolves cooling of molten metals at rates as high as 106 K/sinsufficient time to crystallize (amorphous alloys or metallic glasses)typically contain iron, nickel, and chromium, alloyed with carbon, phosphorus, boron, aluminum, and siliconamorphous alloys exhibit corrosion resistance, ductility, and high strengthuseful magnetic properties make them attractive for magnetic cores