CENTRIFUGAL CASTING (video) POINTS TO REMEMBER: TRUE CENTRIFUGAL Applying ceramic slurry to the mold wall, drying and baking. Rotation of the mold at a predetermined speed(300-3000 rpm) Pouring a molten metal directly into mold(no gating system is employed). The mold is stopped after the casting has solidified. Extraction of the casting from the mold.

SEMI-CENTRIFUGAL CASTING POINTS TO REMEMBER: For more complicated but axi-symmetric shapes Rotating speed is not as high as true centrifugal casting Symmetrical objects like wheel having arms like flywheel, gears

CENTRIFUGING CASTING POINTS TO REMEMBER: For producing non-axisymmetrical small castings having intricate details. Small jobs connected by radial runners and central sprue Centrifugal force aids proper feeding

Casting yield= (WT.OF THE CASTING/WT.OF THE POURED METAL)X100 Advantages… Formation of hollow cavities in cylinders without cores. Non-mettalic and slag inclusions and gas bubbles are forced to the inner surface of the casting by the centrifugal force. No gating system. Hence casting yield is high. Fine outside details can be successfully cast. Fettling costs are reduced. Easy to inspect the casting. Casting yield= (WT.OF THE CASTING/WT.OF THE POURED METAL)X100

Disadvantages… Equipments are expensive and thus is suitable or large quantity production. Skilled workers are required for operation. Inaccurate internal diameter.

INVESTMENT CASTING Known as lost Wax Process or Precision Casting Common pattern material: Wax (others are Frozen mercury,plastic,tin) Fine material is used first to give a smooth surface finish and reproduce fine details

INVESTMENT CASTING STEP: ONE Pattern creation: The wax patterns are typically injection molded into a metal die. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow to the mold cavity.

INVESTMENT CASTING contd…. STEP: TWO Mould creation: The "pattern tree“ dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried. This process repeated until the shell is thick enough. The shell then placed into an oven and the wax is melted out leaving a hollow ceramic shell. Also known as "lost wax" casting.

Finally, the coating is allowed to harden. ►video\sander_01.wmv Finally, the coating is allowed to harden. These steps are repeated until the investment is the required thickness, which is usually 5 to 15 mm The investment is then allowed to completely dry, which can take 16 to 48 hours

INVESTMENT CASTING contd…. STEP: THREE Pouring and Solidification: The mould is preheated in a furnace to approximately 1000°C to harden it further to remove any moisture and residual wax The molten metal poured from ladle into the gating system of the mold, filling the mold cavity. Pouring achieved manually under the force of gravity, but vacuum or pressure sometimes used. After solidification mould is broken (sometimes by water jet) and cast is taken out VIDEO:LOST WAX

Investment Casting Advantages: Limitations: Shell thickness typically 9 mm is used Surface of shell mold cavity is smoother than sand mold Easy flow of molten metal, good surface quality Finish is of the order of 2.5 micrometer Can be mechanized for mass production and is very economical Limitations: Expensive metal pattern Difficult to justify for small quantities Possible on small to medium size Suitable for steel castings<10 kg. Preferred for casting weight less than 5 kg, maximum dimension less than 300 mm, Thickness is usually restricted to 15mm Applications: Small size, complex parts (art pieces, jewelry, dental fixtures), gas turbine blades, pinion gears

Slush Casting Used where only eternal features of the casting are important. Good for making open hollow parts

Step by step procedure Fill the mould with molten metal. Solidification is allowed to proceed until desired wall thickness is reached. Pour out the excess molten metal from the mould. Eject the solidified casting from the mould.

Solidified skin on a steel casting Solidified skin on a steel casting. the remaining molten metal is poured out at the times indicated in the figure.

Limitations of slush casting Low volume production. Thickness cannot be controlled accurately. Not preferred for engineering applications. Skilled workers are required. Applicable only for relatively pure metals. (alloys do not form strong solid skin).