Page 298 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 11.4 Permanent-mold Casting Processes 277

ll.4 Permanent-mold Casting Processes

Permanent-mold casting processes have certain advantages over other casting
processes.

I l.4.l Permanent-mold Casting
In permanent-mold casting (also called hard-mold casting), two halves of a mold are
made from materials with high resistance to erosion and thermal fatigue, such as cast
iron, steel, bronze, graphite, or refractory metal alloys. Typical parts made are auto-
mobile pistons, cylinder heads, connecting rods, gear blanks for appliances, and
kitchenware. Parts that can be made economically generally weigh less than 25 kg, al-
though special castings weighing a few hundred kilograms have been made using this
process. The capabilities of permanent-mold casting are given in Table 11.2.
The mold cavity and gating system are machined into the mold and thus become
an integral part of it. To produce castings with internal cavities, cores made of metal
or sand aggregate are placed in the mold prior to casting. Typical core materials are
oil-bonded or resin-bonded sand, plaster, graphite, gray iron, low-carbon steel, and
hot-work die steel. Gray iron is used most commonly, particularly for large molds for
aluminum and magnesium casting. Inserts also are used for various parts of the mold.
In order to increase the life of permanent molds, the surfaces of the mold cavity
usually are coated with a refractory slurry (such as sodium silicate and clay) or
sprayed with graphite every few castings. These coatings also serve as parting agents
and as thermal barriers, thus controlling the rate of cooling of the casting. Mechanical
ejectors (such as pins located in various parts of the mold) may be required for the re-
moval of complex castings; ejectors usually leave small round impressions.
The molds are clamped together by mechanical means and heated to about
150° to 200°C to facilitate metal flow and reduce thermal damage to the dies due to
high-temperature gradients. Molten metal is then poured through the gating system.
After solidification, the molds are opened and the casting is removed. The mold often
incorporates special cooling features, such as a means of pumping cooling water
through the channels located in the mold and the use of cooling fins. Although the
permanent-mold casting operation can be performed manually, it is often automated
for large production runs. The process is used mostly for aluminum, magnesium, and
copper alloys, as well as for gray iron, because of their generally lower melting points,
although steels also can be cast using graphite or heat-resistant metal molds.
Permanent-mold casting produces castings with a good surface finish, close dimen-
sional tolerances, uniform and good mechanical properties, and at high production
rates.
Although equipment costs can be high because of high die costs, labor costs
are kept low through automation. The process is not economical for small produc-
tion runs and is not suitable for intricate shapes, because of the difficulty in remov-
ing the casting from the mold. However, easily collapsable sand cores can be used,
which are then removed from castings, leaving intricate internal cavities. This
process then is called semipermanent-mold casting.

I l.4.2 Vacuum Casting

A schematic illustration of the vacuum-casting process, or countergraz/ity low-
pressure (CL) process (not to be confused with the vacuum-molding process described
in Section 11.2.1) is shown in Fig. 11.18. Vacuum casting is an alternative to invest-
ment, shell~mold, and green-sand casting and is suitable particularly for thin-walled
(0.75 mm) complex shapes with uniform properties. Typical parts made are superal-
loy gas-turbine components with walls as thin as 0.5 rnm.