Page 287 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

2 Chapter 11 Metal-Casting Processes and Equipment

Sandslingers fill the flask uniformly with sand under a high-pressure stream;
they are used to fill large flasks and are operated typically by machine. An impeller
in the machine throws sand from its blades (or cups) at such high speeds that the
machine not only places the sand, but also rams it appropriately.
In impact molding, the sand is compacted by a controlled explosion or instan-
taneous release of compressed gases. This method produces molds with uniform
strength and good permeability.
In vacuum molding (also known as the V process), the pattern is covered tightly
with a thin sheet of plastic. A flask is placed over the coated pattern and is filled with
dry, binderless sand. A second sheet of plastic then is placed on top of the sand, a vac-
uum action compacts the sand, and the pattern can then be withdrawn. Both halves
of the mold are made in this manner and subsequently assembled. During pouring,
the mold remains under vacuum, but the casting cavity does not. When the metal has
solidified, the vacuum is turned off and the sand falls away, releasing the casting.
Vacuum molding produces castings with high-quality surface detail and dimensional
accuracy; it is suited especially well for large, relatively flat (plane) castings.

The Sand-casting Operation. After the mold has been shaped and the cores have
been placed in position, the two halves (cope and drag) are closed, clamped, and
weighted down to prevent the separation of the mold sections under the pressure ex-
erted when the molten metal is poured into the mold cavity. A complete sequence of
operations in sand casting is shown in Fig. 11.8.
After solidification, the casting is shaken out of its mold, and the sand and
oxide layers adhering to the casting are removed by vibration (using a shaker) or by
sand blasting. Castings also are cleaned by blasting with steel shot or grit (shot
blasting; Section 26.8). The risers and gates are cut off by oxyfuel-gas cutting, saw-
ing, shearing, or abrasive wheels; or they are trimmed in dies. Gates and risers on
steel castings also may be removed with air carbon-arc cutting (Section 30.8) or
torches. Castings may be cleaned further by electrochemical means or by pickling
with chemicals to remove surface oxides.
The casting subsequently may be heat treated to improve certain properties re-
quired for its intended use; heat-treatment is particularly important for steel castings.
Finishing operations may involve machining, straightening, or forging with dies (siz-
ing) to obtain final dimensions. Inspection is an important final step and is carried
out to ensure that the casting meets all design and quality-control requirements.
Rammed-graphite Molding. In this process, rammed graphite (Section 8.6) is used
to make molds for casting reactive metals, such as titanium and zirconium. Sand
cannot be used because these metals react vigorously with silica. The molds are
packed like sand molds, air dried, baked at 175°C, fired at 870°C, and then stored
under controlled humidity and temperature. The casting procedures are similar to
those for sand molds.
I I.2.2 Shell Molding

Shell molding was first developed in the 19405 and has grown significantly because
it can produce many types of castings with close dimensional tolerances and a good
surface finish at low cost. Shell-molding applications include small mechanical parts
requiring high precision, such as gear housings, cylinder heads, and connecting rods.
The process also is used widely in producing high-precision molding cores. The
capabilities of shell-mold casting are given in Table 11.2.
In this process, a mounted pattern made of a ferrous metal or aluminum is
(a) heated to a range of 175° to 37O°C, (b) coated with a parting agent (such as sil-
icone), and (c) clamped to a box or chamber. The box contains fine sand, mixed