Metal Casting Processes CHAPTER 11 PART II Metal Casting Process Investment Casting Vacuum Casting Permanent-Mold Casting Slush Casting Pressure Casting Die Casting Centrifugal Casting Investment Casting Also called lost-wax process First used 4000 – 3000 BC
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2. ASSEMBLY : The patterns are attached to a central wax stick, called a sprue, to form a casting cluster or assembly.
3. SHELL BUILDING : The shell is built by immersing the assembly in a liquid ceramic slurry and then into a bed of extremely fine sand. Up to eight layers may be applied in this manner.
4. DEWAX : Once the ceramic is dry, the wax is melted out, creating a negative impression of the assembly within the shell.Schematic illustration of investment casting
6. KNOCKOUTWhen the metal has cooled and solidified, the ceramic shell is broken off by vibration or water blasting.
7. CUT OFFThe parts are cut away from the central sprue using a high speed friction saw.
8. FINISHED CASTINGSAfter minor finishing operations, the metal castings--identical to the original wax patterns--are ready for shipment to the customer.
Fig : Investment casting of an integrally cast rotor for a gas turbine. (a) Wax pattern assembly. (b) Ceramic shell around wax pattern. (c) Wax melted out and the mold is filled under a vacuum, with molten super alloy. (d) The cast rotor produced to net or near-net shape.
The mold is partially immersed into molten metal held in an induction furnace
The metal is melted in air or in a vacuum
The molten metal is usually 55 C above the liquidus temperature – begins to solidify within a fraction of a second
5. Alternative to investment, shell-mold, and green-sand casting
6. Relatively low costVacuum Casting
2. The plunger pushes the molten metal into the die cavity where it is held under pressure until solidification.Cold-Die casting process
4. Ejector pins push the casting out of the ejector die and the plunger returns to its original position.
First used in the 1800’;s
Three types of centrifugal casting
True centrifugal casting
Semi centrifugal casting
Schematic illustration of the centrifugal casting process. Pipes, cylinder liners, and similarly shaped parts can be cast with this process.Centrifugal Casting
(a) Schematic illustration of the semi centrifugal casting process. Wheels with spokes can be cast by this process. (b) Schematic illustration of casting by centrifuging. The molds are placed at the periphery of the machine, and the molten metal is forced into the molds by centrifugal force.
(c) Close die and apply pressure
(a) Melt Metal
(d) Eject squeeze casting and charge melt stock and repeat cycle
Sequence of operations in the squeeze-casting process. This process combines the advantages of casting and forging.
(b) Pour molten metal into die
Fig : Methods of casting turbine blades. (a) directional solidification; (b) method to produce a single-crystal blade; and (c) a single-crystal blade with construction portion still attached.
Fig : Two methods of crystal growing: (a) crystal pulling and (b) the floating zone method. Crystal growing is especially important in the semiconductor industry.
Fig : Schematic illustration of melt-spinning to produce thin strips of amorphous metal.
Fig : Two types of melting furnaces are used in foundries (a) crucible and (b) cupola