Permanent Mold Castings
Aluminum and Zinc Permanent Mold Casting and Gravity Die Casting
Permanent Mold Castings, while not as flexible as sand castings in
allowing the use of different patterns (different part designs), lower the cost
of producing a part. At a production run of 1000 or more parts, permanent mold
castings produce a lower cost per part. Of course, the break-even point depends
on the complexity of the part. More complex parts being favored by the use of
permanent molds. Instead of using sand as the mold material, a metal is used as
a mold. Typically cast iron or Meehanite (a dense cast iron) is used as the mold
material and the cores are made from metal or sand. Cavity surfaces are coated
with a thin layer of heat resistant material such as clay or sodium silicate.
Permanent Mold or Gravity Die-Casting is mainly used for
nonferrous metals and alloys. The mold (or die) is usually iron, steel, or
graphite, and is cooled by water channels or by air jets on the outer surfaces.
Cavity surfaces in metal dies are coated with a thin layer of heat-resistant
material. The mold or die design is usually in two halves, although many
multiple-part molds are in use, with loose sand or metal cores to form
"undercut" surfaces.
The cast metal is simply poured into the bowls on the side of the mold. During
the period after solidification and before ejection, cooling continues but
shrinkage of the casting(s) is restricted by the die. The alloy being cast must
be sufficiently ductile to accommodate these restrictions without fracturing.
Dimensions of the casting(s) at shop temperatures will be related to the die
temperature and the dimensions at ejection.
Tilting mechanisms are used to control the passage of metal into (and emergence
of air from) the remote portions of die cavities. Because the die temperature
varies during the casting cycle, its dimensions vary correspondingly. The die is
opened and ejectors push the casting(s) out as soon as their temperature is low
enough for sufficient strength to build up.
The permanent mold process employs gravity pouring of metals using steel, cast
iron or graphite molds. The castings produced have excellent grain structure and
extremely low porosity which result in superior ductility and outstanding
strength. The castings can be designed with thinner walls and less weight with
good dimensional accuracy. Used primarily for low melting temperature metals
such as aluminum & zinc, permanent mold castings produce excellent finishes &
pressure tightness when compared to sand castings.
Savings in reduced machining operations can also be realized due to the
dimensional uniformity of the castings and details that can be cast in.
Secondary operations such as polishing are also reduced due to the excellent
surface finish. Permanent molds have a production life of 10,000-50,000 parts,
casting sizes range from a few ounces to 50lbs.
Learn more about Permanent Mold Casting from CIF Metal.
The Permanent Mold Casting Process
Permanent steel mold is CNC machined in two halves, similar to the hardened tool
steels used for die-casting molds thus creating a vertical or horizontal parting
line.
We have found that the most superior casting results come from using the latest
semi-automated pouring machines like one shown below. They fill molds from the
bottom and minimize turbulence of molten metal inside the mold. The machine then
tilts, allowing the molten metal to pour into the tooling.
Process controllers maximize density and minimize casting porosity by
simultaneously controlling fill rates, cycle times, and temperatures. The
results are high quality, machine-able, repeatable castings that can be made out
of zinc or aluminum.
A fine 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.
For 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.
Although 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 take place.
As a 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 treatment.
Reservoirs 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.
Entrapment 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.
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| CAD Model of Steel Mold. |
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Permanent Steel Tool Being Machined. |
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| Metal is Melted and Ready to Pour. |
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Molten metal is poured into a
gravity feed tilting machine. |
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| Machine is slowly tilted allowing molten
metal to
pour into the tool
at a precise rate. |
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The machine pauses in the horizontal position
allowing the metal to harden
in the tooling. |
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| The tooling is separated and the new mold is
exposed, ready for machining. |
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A fully horizontal tilting machine with the tooling
separated. |
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| Another view of tooling separated to show part. |
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Pulling the part out of the permanent mold tool. |
Additional Comparisons
Click here to view cost comparison charts
There are several major reasons why aluminum castings have replaced iron
castings (and are likely to continue to do so).
Weight 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.
On the 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.
While 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 are ignored.
As an 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 were
involved.
Machining 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.
The 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.
Depending 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.
The 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 truck engines.
The 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.
Strength, corrosion resistance, and thermal conductivity are the reasons for the
use of aluminum castings for radiator tanks and side-frame supports.
A major 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 and wheels.
Permanent Steel Mold Tooling Description
Permanent mold casting is a precision technique produced by pouring molten metal
into CNC machined steel molds under gravity, centrifugal force, or low pressure.
A permanent steel mold tool is usually made out of H-13 tool steel which allows
for a life of @ 70,000 units if properly maintained. The permanent steel mold
tool is created from 3D CAD files to generate a precise, solid tool. By using
H-13, tool steel, the unit remains extremely stable and doesn't warp, twist, or
check when molten aluminum or zinc is poured into it.
A permanent mold steel tool permits designs with thinner walls , less weight and
is not subject to shrinkage . A permanent steel mold does not contain the
entrapped gas often found in die castings. This reduces porosity & in some cases
eliminates it altogether. All of this results in a better quality casting.
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