1. Metal Casting Processes
CHAPTER 11
PART II

2. Metal Casting Process Investment Casting Vacuum Casting
Permanent-Mold Casting
Slush Casting
Pressure Casting
Die Casting
Centrifugal Casting

3. 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

4. Investment Casting Process

5. 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

6. One-piece mold - con’t Small parts Not a cheap process
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

7. Schematic 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.

8. 5. 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.

9. 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
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.

10. Vacuum Casting
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
Alternative to investment, shell-mold, and green-sand casting
Relatively low cost

11. 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

12. Slush Casting Pressure 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

13. 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.

14. 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

15. 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.

16. 3. 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).

17. Cold-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.

18. 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.

19. 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

20. Various types of cavities in a die casting die.
Single – cavity die
Multiple – cavity die
c) Combination die
d) Unit die

21. 800 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.

22. Centrifugal Casting
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.

23. 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.

24. 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

25. 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.

26. 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.

27. Melt Spinning
Fig : Schematic illustration of melt-spinning to produce thin strips of amorphous metal.

28. Types of Melting Furnaces
Fig : Two types of melting furnaces are used in foundries (a) crucible and (b) cupola

29. THE END