Mold Castings: Pathway to Quality"
as reprinted from MetlFax
casting approach enables metal to be put where it is required
by part function, in contrast to other processes that produce
shapes more rigidly constrained by the limitations of the
a method of producing parts, many factors must be taken
into account. A major reason for the selection of castings
is their close approximation to the shape desired by the
designer. Also, the wide range of casting methods and castable
metals has earned the process a key role in parts manufacture.
performer in the medium production range is the permanent
mold process. For quantities from a few hundred, up to hundreds
of thousands, permanent mold aluminum castings often provide
the maximum combination of economy and high quality. A look
at the nature of the
process and the requirements of mechanical equipment reveals
the specific reasons that favor the selection of permanent
mold aluminum castings.
Properties Chart (below) shows the relative ratings of the
major aluminum casting methods. "A" signifies the highest
value or degree of flexibility provided the user of castings.
iron castings are usually thought of as being the cheapest
castings available. However, when cost of providing a function
is considered, aluminum castings often prove to be more
economical or as economical as gray iron castings. When
considering the economics of competitive aluminum casting
processes, permanent mold casting occupies a unique place.
Usually, the cost of a permanent mold casting is less than
a sand casting (often substantially less), and usually higher
production rates can be obtained. However, the cost of tooling
for sand casting is less than that for permanent mold.
compared to die casting, permanent mold tooling is dramatically
lower than dies cast tooling, but piece prices are somewhat
higher. It is the combination of these two cost factors,
piece price and tooling costs, that enables permanent mold
castings to be extremely competitive over a wide range of
medium quantity orders.
of the casting processes has a particular combination of
properties in addition to the cost factors just mentioned.
When quality (strength and soundness), is the key consideration,
permanent mold aluminum casting stands out as clearly superior
to either sand or die casting. This is so because strength
in castings (alloy and heat treating being held constant),
depends on two considerations:
of metallurgical grain structure.
of voids caused by shrinkage or the presence of gas or
grain structure is obtained when castings are cooled rapidly
from a molten to a solid state. The iron of permanent molds
draws away heat rapidly from the aluminum, which creates
a fine grain structure in the aluminum casting. In contrast,
aluminum cast in sand requires much more time to solidify,
which allows a coarser grain structure to form. Hence, sand
castings are not as strong as those of the same alloy cast
in permanent molds.
example, 356T6 aluminum alloy sand cast will typically provide
an ultimate tensile strength of 33,000 psi. The same alloy
cast in permanent mold will have an ultimate tensile strength
of 38,000 psi. Yield strength in sand will be 24,000 psi
compared to 27,000 psi in permanent mold. Elongation of
the alloy sand cast is 3.5%, while the permanent mold value
is 5%. Compressive strength values are 25,000 psi and 27,000
psi, while shear strengths are 26,000 psi and 30,000 psi.
metal dies are used in the production of die castings, comparable
strengths cannot be achieved by the process. The metal near
the die cast surface does exhibit a fine grain structure.
However, when molten metal is injected into the die cavity
under pressure, air often becomes entrapped in the metal.
Also, after the metal is injected into the die, additional
metal cannot flow into the cavity as cooling and shrinkage
result, die cast parts have good strength and soundness
near the surface, but the more central portions of the castings
often contain voids caused either by entrapped air or gas,
or by metal shrinkage. This lowers the mechanical properties
of the castings and may also cause blistering during heat
of molten metal (called risers), are used in both sand and
permanent mold casting to supply additional metal as cooling
and metal contraction take place. This enables both sand
and permanent mold castings to be made without shrinkage
voids of the type often present in die castings.
of air is not a problem in pouring of either sand or permanent
mold castings. However, sand molds can generate gasses that
can be entrapped in the metal.
are several major reasons why aluminum castings have replaced
iron castings (and are likely to continue to do so).
savings is often the first reason designers look to aluminum.
Density of aluminum is about 39% that of gray iron. or conversely,
a shape weighing 10lbs in aluminum will weigh about 26lbs
in gray iron. In practice, however, substitution of one
material for another will not necessarily follow the 1.00
to 2.58 density ratio. Sometimes the aluminum casting must
have ribs added, sections made thicker, or inserts used
in order for the desired functions to be accomplished. Such
changes will reduce the weight savings below that anticipated
by following the weight ratio.
other hand, many casting designs are dictated by the needs
of the foundry, often causing heavier designs in iron than
are needed to meet the needs of the application. In these
cases, the better castability of aluminum enables thinner
sections (well ribbed for rigidity), to be used. Weight
savings can be increased to as much as 1 to 5 or 1 to 6
compared to iron.
the value of less weight in the finished part is usually
well understood, as in the trucking industry where additional
payloads are made possible with weight savings on truck
parts, sometimes savings are neglected. For example, freight
costs to the point of manufacture may be documented, but
freight costs of the completed product to the end user may
receive scant consideration. Or the savings made when replacement
parts are shipped by air may be well understood, but the
manufacturing savings made possible by lightweight aluminum
example, an aluminum casting weighing 30lbs can be moved
by hand through a machining line, while a 75lb iron casting
would call for the use of hoists. Similarly, the light weight
of aluminum often makes it possible to do equipment repairs
in the field, which would not be possible if gray iron castings
costs of aluminum castings are often the major factor in
making aluminum competitive with gray iron. Usually, aluminum
sand and permanent mold castings cost more than equivalent
iron castings before machining. However, machining costs
of aluminum are often substantially less (ranging to 40%
less), than the costs of machining equivalent gray iron
shapes. As a result, casting plus machining costs of aluminum
are often comparable to, or less than, gray iron costs.
better castability of aluminum castings makes possible closer
tolerance control and better surface finish, and as a result,
less machining stock (often, 50% less), is required. In
addition, aluminum alloys have better machinability ratings
and can be machined at higher rates with equal tool life
than can gray iron castings.
on the type of cutting operation, metal removal rates of
aluminum castings are two to seven times faster than those
of Class 20 gray iron. Considerably less energy is required
for machining aluminum than gray iron. Horsepower requirements
for removing an equal volume of aluminum are from 1/1 to
1/10 that of gray iron.
advantages of aluminum are often best demonstrated by examining
specific applications. For example, the good thermal conductivity
of aluminum castings makes them particularly suited for
use as transmission cases or for cooling system parts in
good ductility of aluminum leads to its use for hand tools
and similar applications. Also, the attractive appearance
and corrosion resistance of aluminum castings have led to
their use as control levers and equipment covers.
corrosion resistance, and thermal conductivity are the reasons
for the use of aluminum castings for radiator tanks and
reason for the use of aluminum castings in tractors and
construction equipment is their contribution to the lowering
of the center of gravity of such equipment. Other parts
that are made of aluminum include pistons, flywheel housings,
timing gear housings, oil pans, intake manifolds, torque
converter impellers, and turbo-charger compressor housings