Page 303 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

2 2 Chapter 11 Metal-Casting Processes and Equipment
(simple, small dies that can be combined in two or more units in a master holding
die). Typically, the ratio of die weight to part weight is 1000 to 1; thus, the die for a
casting weighing 2 kg would weigh about 2000 kg. The dies usually are made of
hot-work die steels or mold steels (see Section 5.7). Die wear increases with the tem-
perature of the molten metal. Heat checking of dies (surface cracking from repeated
heating and cooling of the die, discussed in Section 3.6) can be a problem. When die
materials are selected and maintained properly, dies may last more than a half mil-
lion shots before any significant die wear takes place.

I l.4.6 Centrifugal Casting
As its name implies, the centrifugal-casting process utilizes inertial forces (caused by
rotation) to distribute the molten metal into the mold cavities-a method that was
first suggested in the early 1800s. There are three types of centrifugal casting: true
centrifugal casting, semicentrifugal casting, and centrifuging.

True Centrifugal Casting. In true centrifugal casting, hollow cylindrical parts (such
as pipes, gun barrels, bushings, engine-cylinder liners, bearing rings with or without
flanges, and street lampposts) are produced by the technique shown in Fig. 11.22. In
this process, molten metal is poured into a rotating mold. The axis of rotation is usu-
ally horizontal, but can be vertical for short workpieces. Molds are made of steel,
iron, or graphite and may be coated with a refractory lining to increase mold life. The
mold surfaces can be shaped so that pipes with various external designs can be cast.
The inner surface of the casting remains cylindrical, because the molten metal is dis-
tributed uniformly by the centrifugal forces. However, because of density differences,
lighter elements (such as dross, impurities, and pieces of the refractory lining) tend to
collect on the inner surface of the casting. Consequently, the properties of the casting
can vary throughout its thickness.
Cylindrical parts ranging from 13 mm to 3 m in diameter and 16 m long can
be cast centrifugally with wall thicknesses ranging from 6 to 125 mm. The pres-
sure generated by the centrifugal force is high (as much as 150 g); such high pres-
sure is necessary for casting thick-walled parts. Castings with good quality,
dimensional accuracy, and external surface detail are produced by this process.
The capabilities of centrifugal casting are given in Table 11.2.

Semicentrifugal Casting. An example of semicentrifugal casting is shown in
Fig. 11.23a. This method is used to cast parts with rotational symmetry, such as a
‘ar a g »-ee ©
wheel with spokes.

Molten metal A
Mold

;_) gi Rollers SW
Driveshaft

(3) (b)

FIGURE II.22 (a) Schematic illustration of the centrifugal-casting process. Pipes, cylinder
liners, and similarly shaped parts can be cast with this process. (b) Side View of the machine.