History of Casting

Schematic illustration of investment castingSchematic illustration of investment casting 1 WAX INJECTION 1 WAX INJECTION Wax replicas of the Wax replicas of the

desired castings are produced by desired castings are produced by injection molding These replicas are injection molding These replicas are called patternscalled patterns

2 ASSEMBLY 2 ASSEMBLY The patterns are attached The patterns are attached to a central wax stick called a sprue to a central wax stick called a sprue to form a casting cluster or assemblyto form a casting cluster or assembly

3 SHELL BUILDING 3 SHELL BUILDING The shell is built by immersing The shell is built by immersing the assembly in a liquid ceramic slurry and then the assembly in a liquid ceramic slurry and then into a bed of extremely fine sand Up to eight into a bed of extremely fine sand Up to eight layers may be applied in this mannerlayers may be applied in this manner

4 DEWAX 4 DEWAX Once the ceramic is dry the wax is Once the ceramic is dry the wax is melted out creating a negative impression of the melted out creating a negative impression of the assembly within the shellassembly within the shell

5 CONVENTIONAL CASTING5 CONVENTIONAL CASTINGIn the conventional process the shell is filled with In the conventional process the shell is filled with molten metal by gravity pouring As the metal molten metal by gravity pouring As the metal cools the parts and gates sprue and pouring cools the parts and gates sprue and pouring cup become one solid casting cup become one solid casting

6 KNOCKOUT6 KNOCKOUTWhen the metal has cooled and solidified the When the metal has cooled and solidified the ceramic shell is broken off by vibration or water ceramic shell is broken off by vibration or water blastingblasting

7 CUT OFF7 CUT OFFThe parts are cut away from the central sprue The parts are cut away from the central sprue using a high speed friction sawusing a high speed friction saw

8 FINISHED CASTINGS8 FINISHED CASTINGSAfter minor finishing operations the metal After minor finishing operations the metal castings--identical to the original wax patterns--castings--identical to the original wax patterns--are ready for shipment to the customerare ready for shipment to the customer

Advantages of Investment Casting

Intricate geometry 1048707 Close dimensional tolerance 1048707 Superior surface finish 1048707 High-melting point alloys

Casting DefectsCasting Defects

MisrunMisrun Cold shutCold shut Hot tears amp crackHot tears amp crack PorosityPorosity ShrinkageShrinkage InclusionsInclusions

Fig Investment casting of an integrally cast rotor for a gas turbine (a) Wax pattern assembly (b) Ceramic 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 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 (d) The cast rotor produced to net or near-net shape

Ceramic-Shell CastingCeramic-Shell Casting

ndash Variation of the investment-casting processVariation of the investment-casting processndash Uses same type of wax or plastic pattern as investment castingUses same type of wax or plastic pattern as investment castingndash Patten is then dipped into fluidized bed of Patten is then dipped into fluidized bed of

Fine- grained fused silicaFine- grained fused silica Zircon flourZircon flour

ndash Pattern is then dipped into coarser grained silica to build up additional Pattern is then dipped into coarser grained silica to build up additional coatings and proper thickness to withstand the thermal shock of pouringcoatings and proper thickness to withstand the thermal shock of pouring

ndash The rest of the procedure follows the investment casting processThe rest of the procedure follows the investment casting process

Vacuum CastingVacuum Casting

11 Mixture of fine sand and Mixture of fine sand and urethane is molded over urethane is molded over metal dies a cured with metal dies a cured with amine vaporamine vapor

22 The mold is partially The mold is partially immersed into molten immersed into molten metal held in an metal held in an induction furnaceinduction furnace

33 The metal is melted in air The metal is melted in air or in a vacuumor in a vacuum

44 The molten metal is The molten metal is usually 55 C above the usually 55 C above the liquidus temperature ndash liquidus temperature ndash begins to solidify within a begins to solidify within a fraction of a secondfraction of a second

5 Alternative to investment shell-5 Alternative to investment shell-mold and green-sand castingmold and green-sand casting

6 Relatively low cost6 Relatively low cost

Permanent-Mold CastingPermanent-Mold Casting Called hard-mold castingCalled hard-mold casting Two halves of a mold are made from materials such as iron Two halves of a mold are made from materials such as iron

steel bronze or other alloyssteel bronze or other alloys The mold cavity and gating system are machined in to the moldThe mold cavity and gating system are machined in to the mold Sand aggregate are placed in to the mold prior to casting for Sand aggregate are placed in to the mold prior to casting for

producing cavitiesproducing cavities Typical core materials areTypical core materials are

ndash Oil-bonded or resin-boned sandOil-bonded or resin-boned sandndash PlasterPlasterndash GraphiteGraphitendash Gray ironGray ironndash Low-carbon steelLow-carbon steelndash Hot-worked die steelHot-worked die steel

Mold cavity surfaces are coated with refractory slurry to increase Mold cavity surfaces are coated with refractory slurry to increase the life of the mold every few castingsthe life of the mold every few castings

Mechanical ejectors are used to remove complex partsMechanical ejectors are used to remove complex parts Can produce high production ratesCan produce high production rates Good surface finishGood surface finish

Slush CastingSlush Casting Molten metal is poured into the metal moldMolten metal is poured into the metal mold A desired thickness of the solidified skin is obtainedA desired thickness of the solidified skin is obtained The remaining metal is poured outThe remaining metal is poured out The mold halves are then opened and the casting is removedThe mold halves are then opened and the casting is removed

Used a graphite or metal moldUsed a graphite or metal mold Molten metal is forced into the mold by gas pressureMolten metal is forced into the mold by gas pressure The pressure is maintained until the metal solidifies in the moldThe pressure is maintained until the metal solidifies in the mold Used for high-quality castingsUsed for high-quality castings

Pressure CastingPressure Casting

(a) The bottom-pressure casting process utilizes graphite molds for the (a) The bottom-pressure casting process utilizes graphite molds for the productin of steel railroad wheels (b) Gravity pouring method of casting a productin of steel railroad wheels (b) Gravity pouring method of casting a railroad wheel Note that the pouring basin also serves as a riser railroad wheel Note that the pouring basin also serves as a riser

Pressure CastingPressure Casting

Die Casting Part Example

Die CastingDie Casting

Further example of permanent-mold castingFurther example of permanent-mold casting Molten metal is forced into the die cavity at pressures Molten metal is forced into the die cavity at pressures

ranging from 7MPa ndash 700MParanging from 7MPa ndash 700MPa Parts made from here range fromParts made from here range from

ndash Hand toolsHand toolsndash ToysToysndash Appliance componentsAppliance components

There are two basic types of die casting machinesThere are two basic types of die casting machinesndash Hot-chamber - involves the use of a piston to push Hot-chamber - involves the use of a piston to push

molten metal in to the die cavitymolten metal in to the die cavityndash Cold-chamber ndash molten metal is poured in to the Cold-chamber ndash molten metal is poured in to the

injection chamber amp the shot chamber is not heated injection chamber amp the shot chamber is not heated

Hot chamber Die-casting processHot chamber Die-casting process

11 The die is closed and The die is closed and the piston rises opening the piston rises opening the port and allowing the port and allowing molten metal to fill the molten metal to fill the cylinder cylinder

22 The plunger moves The plunger moves down and seals the port down and seals the port pushing the molten metal pushing the molten metal through the gooseneck through the gooseneck and nozzle into the die and nozzle into the die cavity where it is held cavity where it is held under pressure until it under pressure until it solidifies solidifies

33 The die opens and the The die opens and the cores if any retract The cores if any retract The casting remains in only one casting remains in only one die the ejector side The die the ejector side The plunger returns allowing plunger returns allowing residual molten metal to residual molten metal to flow back through the flow back through the nozzle and gooseneck nozzle and gooseneck

44 Ejector pins push the Ejector pins push the casting out of the ejector casting out of the ejector die As the plunger die As the plunger uncovers the filling hole uncovers the filling hole molten metal flows through molten metal flows through the inlet to refill the the inlet to refill the gooseneck as in step (1) gooseneck as in step (1)