Page 299 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

2 8 Chapter 11 Metal-Casting Processes and Equipment
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O ,,.= Vacuum

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Mold _, __ Castmg
Gate
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O O O O Molten metal
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Induction furnace
(8) (b)

FIGURE l l.l 8 Schematic illustration of the vacuum-casting process. Note that the mold has
a bottom gate. (a) Before and (b) after immersion of the mold into the molten metal.
Source: After R. Blackburn.

In the vacuum-casting process, a mixture of fine sand and urethane is molded
over metal dies and cured with amine vapor. The mold is then held with a robot arm
and immersed partially into molten metal held in an induction furnace. The metal
may be melted in air (CLA process) or in a vacuum (CLV process). The vacuum re-
duces the air pressure inside the mold to about two-thirds of atmospheric pressure,
thus drawing the molten metal into the mold cavities through a gate in the bottom of
the mold. The metal in the furnace is usually at a temperature of 55°C above the liq-
uidus temperature of the alloy. Consequently, it begins to solidify within a very short
time. After the mold is filled, it is withdrawn from the molten metal.
The process can be automated, and production costs are similar to those for
green-sand casting. Carbon, low- and high-alloy steel, and stainless steel parts
weighing as much as 70 kg have been vacuum cast by this method. CLA parts are
made easily at high volume and relatively low cost. CLV parts usually involve reac-
tive metals, such as aluminum, titanium, zirconium, and hafnium.
I l.4.3 Slush Casting

It was noted in Fig. 10.11 that a solidified skin develops in a casting and becomes
thicker with time. Hollow castings with thin walls can be made by permanent-mold
casting using this principle: a process called slush casting. This process is suitable for
small production runs and generally is used for making ornamental and decorative
objects (such as lamp bases and stems) and toys from low-melting-point metals such
as zinc, tin, and lead alloys.
The molten metal is poured into the metal mold. After the desired thickness of
solidified skin is obtained, the mold is inverted (or slung) and the remaining liquid
metal is poured out. The mold halves then are opened and the casting is removed.
Note that this operation is similar to making hollow chocolate shapes, eggs, and
other confectionaries.

I I.4.4 Pressure Casting
In the two permanent-mold processes described previously, the molten metal flows
into the mold cavity by gravity. In pressure casting (also called pressure pouring or
lou/-pressure casting), the molten metal is forced upward by gas pressure into a