3Investment Casting Also called lost-wax process First used 4000 – 3000 BCThe pattern is made of wax or of a plastic by molding or rapid prototyping techniquesTerm investment derives from the fact that the pattern is invested with the refractory materialNeed careful handling because they are not strong enough to withstand the forces involved in mold makingWax can be recovered and reused
5Investment Casting One-piece mold Dried in the air Heated to 90 – 175 CHeld inverted for 12 hrs to melt out waxThe mold is then heated to 650 – 1150 C for about 4 hrs depending on the metal to be cast to drive off the water of crystallizationAfter the metal has been poured the mold is broken up and the cast is removedA number of patterns can be joined to make one mold called a tree which increases production rate
6One-piece mold - con’t Small parts Not a cheap process The tree can be inserted on to a flask and filled with slurry investmentThe investment is then placed into a chamber and evacuated to remove air bubblesNext it is placed in a vacuum drawing machine to produce fine detailNot a cheap processProduces fine detailsGood surface finishFew or no finishing operationsCan produce intricate parts from parts weighing 1g – 35KgEx : Investment die casting examples
7Schematic illustration of investment casting 1. WAX INJECTION : Wax replicas of the desired castings are produced by injection molding. These replicas are called patterns.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.
85. CONVENTIONAL CASTING In the conventional process, the shell is filled with molten metal by gravity pouring. As the metal cools, the parts and gates, sprue and pouring cup become one solid casting.6. KNOCKOUT When the metal has cooled and solidified, the ceramic shell is broken off by vibration or water blasting.7. CUT OFF The parts are cut away from the central sprue using a high speed friction saw.8. FINISHED CASTINGS After minor finishing operations, the metal castings--identical to the original wax patterns--are ready for shipment to the customer.
9Ceramic-Shell Casting Variation of the investment-casting processUses same type of wax or plastic pattern as investment castingPatten is then dipped into fluidized bed ofFine- grained fused silicaZircon flourPattern is then dipped into coarser grained silica to build up additional coatings and proper thickness to withstand the thermal shock of pouringThe rest of the procedure follows the investment casting processFig : 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.
10Vacuum CastingMixture of fine sand and urethane is molded over metal dies a cured with amine vaporThe mold is partially immersed into molten metal held in an induction furnaceThe metal is melted in air or in a vacuumThe molten metal is usually 55 C above the liquidus temperature – begins to solidify within a fraction of a secondAlternative to investment, shell-mold, and green-sand castingRelatively low cost
11Permanent-Mold Casting Called hard-mold castingTwo halves of a mold are made from materials such as iron, steel, bronze, or other alloysThe mold cavity and gating system are machined in to the moldSand aggregate are placed in to the mold prior to casting for producing cavitiesTypical core materials areOil-bonded or resin-boned sandPlasterGraphiteGray ironLow-carbon steelHot-worked die steelMold cavity surfaces are coated with refractory slurry to increase the life of the mold every few castingsMechanical ejectors are used to remove complex partsCan produce high production ratesGood surface finish
12Slush Casting Pressure Casting Molten metal is poured into the metal moldA desired thickness of the solidified skin is obtainedThe remaining metal is poured outThe mold halves are then opened and the casting is removedUsed a graphite or metal moldMolten metal is forced into the mold by gas pressureThe pressure is maintained until the metal solidifies in the moldUsed for high-quality castingsPressure Casting
13Pressure Casting(a) The bottom-pressure casting process utilizes graphite molds for the productin of steel railroad wheels. (b) Gravity pouring method of casting a railroad wheel. Note that the pouring basin also serves as a riser.
14Die Casting Further example of permanent-mold casting Molten metal is forced into the die cavity at pressures ranging from .7MPa – 700MPaParts made from here range from:Hand toolsToysAppliance componentsThere are two basic types of die casting machinesHot-chamber - involves the use of a piston to push molten metal in to the die cavityCold-chamber – molten metal is poured in to the injection chamber & the shot chamber is not heated
15Hot chamber Die-casting process 1. The die is closed and the piston rises, opening the port and allowing molten metal to fill the cylinder.2. The plunger moves down and seals the port pushing the molten metal through the gooseneck and nozzle into the die cavity, where it is held under pressure until it solidifies.
163. The die opens and the cores, if any, retract 3. The die opens and the cores, if any, retract. The casting remains in only one die, the ejector side. The plunger returns, allowing residual molten metal to flow back through the nozzle and gooseneck.4. Ejector pins push the casting out of the ejector die. As the plunger uncovers the filling hole, molten metal flows through the inlet to refill the gooseneck, as in step (1).
17Cold-Die casting process 1. The die is closed and the molten metal is ladled into the cold-chamber shot sleeve.2. The plunger pushes the molten metal into the die cavity where it is held under pressure until solidification.
183. The die opens and the plunger advances, to ensure that the casting remains in the ejector die. Cores, if any, retract.4. Ejector pins push the casting out of the ejector die and the plunger returns to its original position.
19Process Capabilities and Machine Selection Dies are rated according to their clamping force that is neededFactors involved in selection of die cast machines areDie sizePiston strokeShot pressureCostDie-casting diesSingle cavityMultiple-cavityCombination-cavityUnit diesRatio of Die weight to part weight is 1000 to 1Surface cracking is a problem with dies due to the hot metal that is poured in to themHas ability to produce strong high- quality parts with complex shapesGood dimensional accuracy and surface details
20Various types of cavities in a die casting die. Single – cavity dieMultiple – cavity diec) Combination died) Unit die
21800 ton hot chamber die casting machine, DAM 8005 800 ton hot chamber die casting machine, DAM This is the largest hot chamber machine in the world and costs about $1.25 million.
22Centrifugal CastingUtilizes the inertial forces caused by rotation to distribute the molten metal in to the mold cavitiesFirst used in the 1800’;sThree types of centrifugal castingTrue centrifugal castingSemi centrifugal castingCentrifugingSchematic illustration of the centrifugal casting process. Pipes, cylinder liners, and similarly shaped parts can be cast with this process.
23Semi centrifugal Casting Process (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.
24Squeeze Casting(c) Close die and apply pressure(a) Melt Metal(d) Eject squeeze casting and charge melt stock and repeat cycleSequence of operations in the squeeze-casting process. This process combines the advantages of casting and forging.(b) Pour molten metal into die
25Single Crystal Casting of Turbines blades 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.
26Single Crystal-Casting Fig : Two methods of crystal growing: (a) crystal pulling and (b) the floating zone method. Crystal growing is especially important in the semiconductor industry.
27Melt SpinningFig : Schematic illustration of melt-spinning to produce thin strips of amorphous metal.
28Types of Melting Furnaces Fig : Two types of melting furnaces are used in foundries (a) crucible and (b) cupola