A mold is formed into the geometric shape of a
desired part. Molten metal is then poured into the mold, the
mold holds this material in shape as it solidifies. A metal
casting is created. Although this seems rather simple, the
manufacturing process of metal casting is both a science and an
art. Let's begin our study of metal casting with the mold.
First, molds can be classified as either open or closed. An open
mold is a container, like a cup, that has only the shape of the
desired part. The molten material is poured directly into the
mold cavity which is exposed to the open environment.
This type of mold is rarely used in manufacturing production,
particularly for metal castings of any level of quality. The
other type of mold is a closed mold, it contains a delivery
system for the molten material to reach the mold cavity, where
the part will harden within the mold. A very simple closed mold
is shown in figure 2. The closed mold is, by far, more important
in manufacturing metal casting operations.
There are many different metal casting processes used in the
manufacture of parts. Two main branches of methods can be
distinguished by the basic nature of the mold they employ. There
is expendable mold casting and permanent mold casting. As the
name implies, expendable molds are used for only one metal
casting while permanent molds are used for many. When
considering manufacturing processes, there are advantages and
disadvantages to both.
1). Expendable Mold
Can produce one metal casting only
Made of sand, plaster, or other similar material
Binders used to help material hold its form
Mold that metal solidifies in must be destroyed to remove
More intricate geometries are possible for casting
2). Permanent Mold
Can manufacture many metal castings
Usually made of metal or sometimes a refractory ceramic
Mold has sections that can open or close, permitting removal of
Need to open mold limits part shapes
Expendable molds require some sort of pattern. The interior
cavities of the mold, in which the molten metal will solidify,
are formed by the impression of this pattern.
Pattern design is crucial to success in manufacture by
expendable mold metal casting. The pattern is a geometric
replica of the metal casting to be produced. It is made slightly
oversize to compensate for the shrinkage that will occur in the
metal during the casting's solidification, and whatever amount
of material that will be machined off the cast part afterwards.
Although machining will add an extra process to the manufacture
of a part, machining can improve surface finish and part
dimensions considerably. Also, increasing the machine finish
allowance will help compensate for unknown variables in
shrinkage, and reduce trouble from areas of the metal casting
that may have been originally too thin or intricate.
The material from which the pattern is made is dependent upon
the type of mold and metal casting process, the casting's
geometry and size, the dimensional accuracy required, and the
number of metal castings to be manufactured using the pattern.
Patterns can be made from wood, like pine (softwood), or
mahogany (hardwood), various plastics, or metal, like aluminum,
cast iron, or steel. In most manufacturing operations, patterns
will be coated with a parting agent to ease their removal from
For metal castings with internal geometry cores are used. A core
is a replica, (actually an inverse), of the internal features of
the part to be cast. Like a pattern, the size of the core is
designed to accommodate for shrinkage during the metal casting
operation. Unlike a pattern, a core remains in the mold while
the metal is being poured. Hence, a core is usually made of a
similar material as the mold. Once the metal casting has
hardened, the core is broken up and removed much like the mold.
Depending upon the location and geometry of the core within the
casting, it may require that it is supported during the
operation to prevent it from moving or shifting. Structural
supports that hold the core in place are called chaplets. The
chaplets are made of a material with a higher melting
temperature than the casting's material, and become assimilated
into the part when it hardens. Note that when manufacturing a
metal casting with a permanent mold process, the core will be a
part of the mold itself.
A typical mold is shown in figure 3.
When manufacturing by metal casting, consideration of the mold
is essential. The pattern is placed in the mold and the mold
material is packed around it. The mold contains two parts, the
drag (bottom), and the cope (top). The parting line between the
cope and drag allows for the mold to be opened and the pattern
to be removed once the impression has been made.
The core is placed in the metal casting after the removal of the
pattern. Figure 5 shows the pattern impression with the core in
Now the impression in the mold contains all the geometry of the
part to be cast. This metal casting setup, however, is not
complete. In order for this mold to be functional to manufacture
a casting, in addition to the impression of the part, the mold
cavity will also need to include a gating system. Sometimes the
gating system will be cut by hand or in more adept manufacturing
procedures, the gating system will be incorporated into the
pattern along with the part. Basically, a gating system
functions during the metal casting operation to facilitate the
flow of the molten material into the mold cavity.
4. Elements Of A Gating System:
This is where the molten metal employed to manufacture the part
enters the mold. The pouring basin should have a projection with
a radius around it to reduce turbulence.
From the pouring basin, the molten metal for the casting travels
through the down sprue. This should be tapered so its
cross-section is reduced as it goes downward.
The down sprue ends at the sprue base. It is here that the
casting's inner cavity begins.
Once at the sprue base, the molten material must pass through
the ingate in order to enter the inner area of the mold. The
ingate is very important for flow regulation during the metal
Runners are passages that distribute the liquid metal to the
different areas inside the mold.
The impression of the actual part to be cast is often referred
to as the main cavity.
Vents help to assist in the escape of gases that are expelled
from the molten metal during the solidification phase of the
metal casting process.
Risers are reservoirs of molten material. They feed this
material to sections of the mold to compensate for shrinkage as
the casting solidifies. There are different classifications for
Top Risers: Risers that feed the metal casting from the top.
Side Risers: Risers that feed the metal casting from the side.
Blind Risers: Risers that are completely contained within the
Open Risers: Risers that are open at the top to the outside
Figure 6 shows a mold with all its features, ready for metal
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