size of the cavity into which the molten metal is reusable mold, or an expendable mold and

poured. Gates and risers may be attached to the pattern are employed in the process.
pattern or molded in separately. The casting
Each casting process has certain inherent
manufacturer generally uses tapered sections,
advantages and limitations. Size and shape of the
chills, risers, insulation, and hot tops to provide
casting, dimensional accuracy and tolerance,
adequate soundness through directional
surface finish, metallurgical properties, choice of
solidification.
alloys, production quantities, and cost all enter
Internal surfaces are formed by casting against into the choice of a molding and casting process.
cores. Cores are generally made from a similar Many sand castings, for example, have low labor
mold material, but sometimes are made from a and finishing costs; others are cleaned but are not
material that offers a greater resistance to the ground or machined. A comparison of the molding
physically harsh and chemically reactive processes with respect to these and other factors
environment in the mold cavity before and during is given in Table 16-3, and a comparative rating of
solidification. Hollow parts may also be made by the processes is given in Table 16-4. For the
one of the coreless casting techniques, such as highest quality, most cost-effective application of
centrifugal casting and slush casting. a casting method, advantages of the method must
be fully used and its limitations must be
After the binder hardens sufficiently, the pattern
recognized.
is removed. Sometimes special mold coatings are
applied to improve the casting surface finish or Effective operation of a foundry requires careful
reduce cleaning costs. A variation of this practice attention to the basics-part geometry, molding,
is found in evaporative-pattern (full-mold) casting, gating, heat transfer, melt and mold materials,
in which a shape made from low-density, metal chemistry, cleanliness, and safety.
expanded polystyrene is left in the sand to
Process control is vitally important, because the
evaporate when hot metal is poured into the
foundry has more process variables than most
mold. Another variation is the use of permanent
other manufacturing operations. A good process
molds that may be reused several thousand times.
control system forces the identification and
The mold is such an important aspect of most understanding of key process parameters and
casting processes that the name of the molding their interrelationships. Productivity and
process and the type of mold media are consistent casting quality result from knowing and
commonly used to identify the processes used to controlling key parameters within critical ranges.
make castings. Examples include no-bake sand- Experience indicates that a well-controlled
molded steel castings, green sand molded brass foundry process affords an opportunity to reduce
castings, and investmentmolded nickel-based basic material costs by 25-30% while maintaining
superalloy castings. casting quality at the desired level.

PROCESS SELECTION COMPUTERIZED OPERATIONS

The casting processes most often used are Computer-based systems are gaining acceptance
identified broadly as sand-mold casting, metal- in virtually all aspects of foundry operations.
mold casting, and plaster and ceramic-mold Computer use extends into all functional areas,
casting. As illustrated by the format used for Table from sand control to final quality inspection; and
16-2, the major metal casting processes also can the computer now has an integral role in the
be characterized and grouped based on whether a entire process, from design to the manufacture of
reusable pattern with expendable mold, a castings.
 processing. The near-net-shape must be defined.
The casting process can then render its best
Computer-Aided Design
reproduction of that shape, and further
processing by machining can be most
economically performed. Accurate and precise 3D
The capability for producing intricate shapes, both representation of complex casting geometries is a
internal and external, is an important advantage requirement that is not generally available to
of casting processes. A number of different small and medium-sized foundries at an
casting processes may be used to make castings affordable cost. The 2D blueprint limitation is
with complex geometric configurations. omnipresent. To resolve this problem, a
Investment casting, die casting, gravity metal- computer-aided design/computer-aided
mold casting, and sand casting processes present manufacturing (CAD/CAM) technique called solid
and array of possibilities for near-net-shape geometric modeling is being developed to
component manufacturing. While each of the facilitate computer-aided casting design and
casting processes has advantages and limitations, process engineering. Representing casting
the extent to which the complexity of the dimensions in 3D through solid geometric
geometry affects the capability for near-net-shape modeling is expected to quantify near-net-shape
processing is a common factor. Generally, the casting design definitions for further processing or
more complex the shape the more likely the raw machining.
shape will be different from the final shape. For
complex parts, the near-net-shape casting The key role of solid geometric modeling in
processes offer a significant potential for saving computer-aided casting design and in the
material and lessening the need for secondary University of Wisconsin-Madison's "cast metals
operations. program" (CMP) project is shown on the
information flow diagram ilustrated in Fig. 16-2.
Another common problem with near-net-shape The top area represents the component design
processing of complex geometries stems from utilized by a foundry customer and presented
representing three-dimensional (3D) shapes on a either on a conventional 2D drawing or blueprint
two-dimensional (2D) medium, the drawing or or on magnetic tape. The drawing or tape
blueprint. The blending of external and internal represents the finished component design (net
joining sections is commonly done on the drawing shape) without processing allowances.
by notation alone and must later be more fully
defined by the craftsman., who is the Computer input. When the blueprint or tape is
patternmaker in the case of castings. Subsequent received by the foundry, process engineering may
layouts of the component frequently do not deal begin after the solid geometric database is
with questions of near-net-shape processing, but developed, as illustrated in the middle area of Fig.
rather whether the casting reasonably represents 16-2. Wire frame data from computerized drafting
the drawing and whether the important systems may be converted to a solid geometric
dimensions to be machined are as required. database via a translator program. Or, the
Process capabilities may be ignored or accepted at blueprint may be converted to the solid geometric
this stage rather than used to their limits. It is database either by digitizing or by using a
here that the possibilities that come from computer-aided design terminal. After digitizing
computer-aided engineering can bring about the blueprint geometry, the translator program
substantial improvements in near-net-shape produces the solid geometry database. Or at the
casting of components. CAD workstation, the solid geometric modeling
software program permits the operator to directly
Net shape. Accurate and precise dimensional model the casting from the blueprint.
representation of casting geometry, both as cast
and finished, is a necessity for near-net-shape
This is done by inputting rudimentary solid
geometric subshapes, such as plates, cylinders,
cubes, cones, etc., and joining them as specified
on the blueprint. Figure 16-3 illustrates how an
air-cooled cylinder casting can be modeled. An
assembly of disks (1) is joined with a cylinder (3)
to produce the finned shape (2). Next, the bore is
removed, bosses are added (4 and 5), and boss
holes are removed (6), and a sector of fins is
removed to produce the air-cooled cylinder model
(7). The model can be rotated and blend lines
removed (7). Cross sectioning can also be done.

This geometric assembly process is familiar to