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 • The pattern is made of wax or of a plastic by molding or rapid prototyping techniques • Term investment derives from the fact that the pattern is invested with the refractory material • Need careful handling because they are not strong enough to withstand the forces involved in mold making • Wax can be recovered and reused
Investment Casting • One-piece mold • Dried in the air • Heated to 90 – 175 C • Held inverted for 12 hrs to melt out wax • The 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 crystallization • After the metal has been poured the mold is broken up and the cast is removed • A number of patterns can be joined to make one mold called a tree which increases production rate
One-piece mold - con’t • Small parts • The tree can be inserted on to a flask and filled with slurry investment • The investment is then placed into a chamber and evacuated to remove air bubbles • Next it is placed in a vacuum drawing machine to produce fine detail • Not a cheap process • Produces fine details • Good surface finish • Few or no finishing operations • Can produce intricate parts from parts weighing 1g – 35Kg • Ex : Investment die casting examples
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. Schematic illustration of investment casting
5. CONVENTIONAL CASTINGIn 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. 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. Ceramic-Shell Casting • Variation of the investment-casting process • Uses same type of wax or plastic pattern as investment casting • Patten is then dipped into fluidized bed of • Fine- grained fused silica • Zircon flour • Pattern is then dipped into coarser grained silica to build up additional coatings and proper thickness to withstand the thermal shock of pouring • The rest of the procedure follows the investment casting process
Mixture of fine sand and urethane is molded over metal dies a cured with amine vapor 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 cost Vacuum Casting
Permanent-Mold Casting • Called hard-mold casting • Two halves of a mold are made from materials such as iron, steel, bronze, or other alloys • The mold cavity and gating system are machined in to the mold • Sand aggregate are placed in to the mold prior to casting for producing cavities • Typical core materials are • Oil-bonded or resin-boned sand • Plaster • Graphite • Gray iron • Low-carbon steel • Hot-worked die steel • Mold cavity surfaces are coated with refractory slurry to increase the life of the mold every few castings • Mechanical ejectors are used to remove complex parts • Can produce high production rates • Good surface finish
Slush Casting • Molten metal is poured into the metal mold • A desired thickness of the solidified skin is obtained • The remaining metal is poured out • The mold halves are then opened and the casting is removed • Used a graphite or metal mold • Molten metal is forced into the mold by gas pressure • The pressure is maintained until the metal solidifies in the mold • Used for high-quality castings Pressure 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. Pressure Casting
Die Casting • Further example of permanent-mold casting • Molten metal is forced into the die cavity at pressures ranging from .7MPa – 700MPa • Parts made from here range from: • Hand tools • Toys • Appliance components • There are two basic types of die casting machines • Hot-chamber - involves the use of a piston to push molten metal in to the die cavity • Cold-chamber – molten metal is poured in to the injection chamber & the shot chamber is not heated
Hot 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.
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).
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. Cold-Die casting process
3. 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.
Process Capabilities and Machine Selection • Dies are rated according to their clamping force that is needed • Factors involved in selection of die cast machines are • Die size • Piston stroke • Shot pressure • Cost • Die-casting dies • Single cavity • Multiple-cavity • Combination-cavity • Unit dies • Ratio of Die weight to part weight is 1000 to 1 • Surface cracking is a problem with dies due to the hot metal that is poured in to them • Has ability to produce strong high- quality parts with complex shapes • Good dimensional accuracy and surface details
Single – cavity die Multiple – cavity die c) Combination die d) Unit die Various types of cavities in a die casting die.
800 ton hot chamber die casting machine, DAM 8005. This is the largest hot chamber machine in the world and costs about $1.25 million.
Utilizes the inertial forces caused by rotation to distribute the molten metal in to the mold cavities First used in the 1800’;s Three types of centrifugal casting True centrifugal casting Semi centrifugal casting Centrifuging Schematic illustration of the centrifugal casting process. Pipes, cylinder liners, and similarly shaped parts can be cast with this process. Centrifugal Casting
Semi 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.
Squeeze Casting (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
Single 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.
Single 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.
Melt Spinning Fig : Schematic illustration of melt-spinning to produce thin strips of amorphous metal.
Types of Melting Furnaces Fig : Two types of melting furnaces are used in foundries (a) crucible and (b) cupola