CENTRIFUGAL CASTING sometimes called rotocasting, is a
metal casting process that uses centrifugal force to form cylindrical parts.
This differs from most metal casting processes, which use gravity or pressure
to fill the mold. In centrifugal casting, a permanent mold made from steel,
cast iron, or graphite is typically used. However, the use of expendable sand
molds is also posible. The casting process is usually performed on a horizontal
centrifugal casting machine (vertical machines are also available) and includes
the following steps:
1. Mold preparation - The walls of a cylindrical mold are
first coated with a refractory ceramic coating, which involves a few steps
(application, rotation, drying, and baking). Once prepared and secured, the
mold is rotated about its axis at high speeds (300-3000 RPM), typically around
1000 RPM.
2. Pouring - Molten metal is poured directly into the
rotating mold, without the use of runners or a gating system. The centrifugal
force drives the material towards the mold walls as the mold fills.
3. Cooling - With
all of the molten metal in the mold, the mold remains spinning as the metal
cools. Cooling begins quickly at the mold walls and proceeds inwards.
4. Casting removal
- After the casting has cooled and solidified, the rotation is stopped and the
casting can be removed.
5. Finishing -
While the centrifugal force drives the dense metal to the mold walls, any less
dense impurities or bubbles flow to the inner surface of the casting. As a
result, secondary processes such as machining, grinding, or sand-blasting, are
required to clean and smooth the inner diameter of the part.
Centrifugal casting is used to produce axi-symmetric
parts, such as cylinders or disks, which are typically hollow. Due to the high
centrifugal forces, these parts have a very fine grain on the outer surface and
possess mechanical properties approximately 30% greater than parts formed with
static casting methods. These parts may be cast from ferrous metals such as low
alloy steel, stainless steel, and iron, or from non-ferrous alloys such as
aluminum, bronze, copper, magnesium, and nickel. Centrifugal casting is
performed in wide variety of industries, including aerospace, industrial,
marine, and power transmission. Typical parts include bearings, bushings,
coils, cylinder liners, nozzles, pipes/tubes, pressure vessels, pulleys, rings,
and wheels.
Capabilities
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Die casting is a
manufacturing process that can produce geometrically complex metal parts through
the use of reusable molds, called dies. The die casting process involves the
use of a furnace, metal, die casting machine, and die. The metal, typically a
non-ferrous alloy such as aluminum or zinc, is melted in the furnace and then
injected into the dies in the die casting machine. There are two main types of
die casting machines - hot chamber machines (used for alloys with low melting
temperatures, such as zinc) and cold chamber machines (used for alloys with
high melting temperatures, such as aluminum). The differences between these
machines will be detailed in the sections on equipment and tooling. However, in
both machines, after the molten metal is injected into the dies, it rapidly
cools and solidifies into the final part, called the casting. The steps in this
process are described in greater detail in the next section.
Capabilities
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Disclaimer: All process specifications
reflect the approximate range of a process's capabilities and should be viewed
only as a guide. Actual capabilities are dependent upon the manufacturer,
equipment, material, and part requirements.
The process cycle for die casting consists of five main
stages, which are explained below. The total cycle time is
very short, typically between 2 seconds and 1 minute.
- Clamping
- The first step is the preparation and clamping of the
two halves of the die. Each die half is first cleaned from the previous
injection and then lubricated to facilitate the ejection of the next part. The lubrication
time increases with part size, as well as the number of cavities and side-cores.
Also, lubrication may not be required after each cycle, but after 2 or 3
cycles, depending upon the material. After lubrication, the two die halves,
which are attached inside the die casting machine, are closed and securely
clamped together. Sufficient force must be applied to the die to keep it
securely closed while the metal is injected. The time required to close and
clamp the die is dependent upon the machine - larger machines (those with
greater clamping forces) will require more time. This time can be
estimated from the dry cycle time of the machine.
- Injection
- The molten metal, which is maintained at a set
temperature in the furnace, is next transferred into a chamber where it can be
injected into the die. The method of transferring the molten metal is dependent
upon the type of die casting machine, whether a hot chamber or cold chamber
machine is being used. The difference in this equipment will be detailed in the
next section. Once transferred, the molten metal is injected at high pressures
into the die. Typical injection pressure ranges from 1,000 to 20,000 psi. This
pressure holds the molten metal in the dies during solidification. The amount
of metal that is injected into the die is referred to as the shot.
The injection time is the time required for the molten metal to fill all of the
channels and cavities in the die. This time is very short, typically less than
0.1 seconds, in order to prevent early solidification of any one part of the
metal. The proper injection time can be determined by the thermodynamic
properties of the material, as well as the wall thickness of the casting. A
greater wall thickness will require a longer injection time. In the case where
a cold chamber die casting machine is being used, the injection time must also
include the time to manually ladle the molten metal into the shot chamber.
- Cooling
- The molten metal that is injected into the die will
begin to cool and solidify once it enters the die cavity. When the entire
cavity is filled and the molten metal solidifies, the final shape of the
casting is formed. The die can not be opened until the cooling time has elapsed
and the casting is solidified. The cooling time can be estimated from several
thermodynamic properties of the metal, the maximum wall thickness of
the casting, and the complexity of the die. A greater wall thickness will
require a longer cooling time. The geometric complexity of the die also
requires a longer cooling time because the additional resistance to the flow of
heat.
- Ejection
- After the predetermined cooling time has passed, the
die halves can be opened and an ejection mechanism can push the casting out of
the die cavity. The time to open the die can be estimated from the dry cycle
time of the machine and the ejection time is determined by the size of the
casting's envelope and should include time for the casting to
fall free of the die. The ejection mechanism must apply some force to eject the
part because during cooling the part shrinks and adheres to the die. Once the
casting is ejected, the die can be clamped shut for the next injection.
- Trimming
- During cooling, the material in the channels of the die
will solidify attached to the casting. This excess material, along with
any flash that has occurred, must be trimmed from the casting
either manually via cutting or sawing, or using a trimming press. The time
required to trim the excess material can be estimated from the size of the
casting's envelope. The scrap material that results from this trimming is
either discarded or can be reused in the die casting process. Recycled material
may need to be reconditioned to the proper chemical composition before it can
be combined with non-recycled metal and reused in the die casting process.
Die cast part
http://www.custompartnet.com/wu/die-casting
Equipment
The two types of die casting machines
are a hot chamber machine and cold chamber machine.
- Hot
chamber die casting machine
- Hot chamber machines are used for
alloys with low melting temperatures, such as zinc, tin, and lead. The
temperatures required to melt other alloys would damage the pump, which is in
direct contact with the molten metal. The metal is contained in an open holding
pot which is placed into a furnace, where it is melted to the necessary
temperature. The molten metal then flows into a shot chamber
through an inlet and a plunger, powered by hydraulic pressure, forces the
molten metal through a gooseneck channel and into the die. Typical injection
pressures for a hot chamber die casting machine are between 1000 and 5000 psi.
After the molten metal has been injected into the die cavity, the plunger
remains down, holding the pressure while the casting solidifies. After
solidification, the hydraulic system retracts the plunger and the part can be
ejected by the clamping unit. Prior to the injection of the molten metal, this
unit closes and clamps the two halves of the die. When the die is attached to
the die casting machine, each half is fixed to a large plate, called a platen.
The front half of the die, called the cover die, is mounted to a stationary
platen and aligns with the gooseneck channel. The rear half of the die, called
the ejector die, is mounted to a movable platen, which slides along the tie
bars. The hydraulically powered clamping unit actuates clamping bars that push
this platen towards the cover die and exert enough pressure to keep it closed
while the molten metal is injected. Following the solidification of the metal
inside the die cavity, the clamping unit releases the die halves and
simultaneously causes the ejection system to push the casting out of the open
cavity. The die can then be closed for the next injection.
 Hot chamber die casting machine - Opened |
 Hot chamber die casting machine - Closed |
- Cold
chamber die casting machine
- Cold chamber machines are used for
alloys with high melting temperatures that can not be cast in hot chamber
machines because they would damage the pumping system. Such alloys include aluminum,
brass, and magnesium. The molten metal is still contained in an open holding
pot which is placed into a furnace, where it is melted to the necessary
temperature. However, this holding pot is kept separate from the die casting
machine and the molten metal is ladled from the pot for each casting, rather
than being pumped. The metal is poured from the ladle into the shot chamber
through a pouring hole. The injection system in a cold chamber machine
functions similarly to that of a hot chamber machine, however it is usually
oriented horizontally and does not include a gooseneck channel. A plunger,
powered by hydraulic pressure, forces the molten metal through the shot chamber
and into the injection sleeve in the die. The typical injection pressures for a
cold chamber die casting machine are between 2000 and 20000 psi. After the
molten metal has been injected into the die cavity, the plunger remains
forward, holding the pressure while the casting solidifies. After
solidification, the hydraulic system retracts the plunger and the part can be
ejected by the clamping unit. The clamping unit and mounting of the dies is
identical to the hot chamber machine. See the above paragraph for details.
 Cold chamber die casting machine - Opened |
 Cold chamber die casting machine - Closed |
Machine specifications
Both hot chamber and cold chamber die
casting machines are typically characterized by the tonnage of the clamp
force they provide. The required clamp force is determined by
the projected area of the parts in the die and the pressure
with which the molten metal is injected. Therefore, a larger part will require
a larger clamping force. Also, certain materials that require high injection
pressures may require higher tonnage machines. The size of the part must also
comply with other machine specifications, such as maximumshot volume, clamp
stroke, minimum mold thickness, and platen size.
Die cast parts can vary greatly in size and therefore require these measures to cover a very large range. As a result, die casting machines are designed to each accommodate a small range of this larger spectrum of values. Sample specifications for several different hot chamber and cold chamber die casting machines are given below.
Die cast parts can vary greatly in size and therefore require these measures to cover a very large range. As a result, die casting machines are designed to each accommodate a small range of this larger spectrum of values. Sample specifications for several different hot chamber and cold chamber die casting machines are given below.
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Type
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Clamp force (ton)
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Max. shot volume (oz.)
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Clamp stroke (in.)
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Min. mold thickness (in.)
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Platen size (in.)
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Hot chamber
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100
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74
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11.8
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5.9
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25 x 24
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Hot chamber
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200
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116
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15.8
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9.8
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29 x 29
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Hot chamber
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400
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254
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21.7
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11.8
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38 x 38
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Cold chamber
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100
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35
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11.8
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5.9
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23 x 23
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Cold chamber
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400
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166
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21.7
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11.8
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38 x 38
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Cold chamber
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800
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395
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30.0
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15.8
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55 x 55
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Cold chamber
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1600
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1058
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39.4
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19.7
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74 x 79
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Cold chamber
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2000
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1517
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51.2
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25.6
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83 x 83
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Tooling
The dies into which the molten metal
is injected are the custom tooling used in this process. The dies are typically
composed of two halves - the cover die, which is mounted onto a
stationary platen, and the ejector die, which is mounted onto a
movable platen. This design allows the die to open and close along its parting
line. Once closed, the two die halves form an internal part cavity which is
filled with the molten metal to form the casting. This cavity is formed by two
inserts, the cavity insert and the core insert, which are inserted into the
cover die and ejector die, respectively. The cover die allows the molten metal
to flow from the injection system, through an opening, and into the part
cavity. The ejector die includes a support plate and the ejector box, which is
mounted onto the platen and inside contains the ejection system. When the
clamping unit separates the die halves, the clamping bar pushes the ejector
plate forward inside the ejector box which pushes the ejector pins into the
molded part, ejecting it from the core insert. Multiple-cavity dies are
sometimes used, in which the two die halves form several identical part
cavities.
Die channels
The flow of molten metal into the part
cavity requires several channels that are integrated into the die and differs
slightly for a hot chamber machine and a cold chamber machine. In a hot chamber
machine, the molten metal enters the die through a piece called a sprue bushing
(in the cover die) and flows around the sprue spreader (in the ejector die).
The sprue refers to this primary channel of molten metal entering the die. In a
cold chamber machine, the molten metal enters through an injection sleeve.
After entering the die, in either type of machine, the molten metal flows
through a series of runners and enters the part cavities
through gates, which direct the flow. Often, the cavities will
contain extra space called overflow wells, which provide an additional source
of molten metal during solidification. When the casting cools, the molten metal
will shrink and additional material is needed. Lastly, small channels are
included that run from the cavity to the exterior of the die. These channels
act as venting holes to allow air to escape the die cavity. The molten metal
that flows through all of these channels will solidify attached to the casting
and must be separated from the part after it is ejected. One type of channel
that does not fill with material is a cooling channel. These channels allow
water or oil to flow through the die, adjacent to the cavity, and remove heat
from the die.
 Die assembly - Open (Hot chamber) |
 Die assembly - Closed (Hot chamber) |
 Die assembly - Exploded view (Hot chamber) |
 Die assembly - Opened (Cold chamber) |
 Die assembly - Closed (Cold chamber) |
 Die assembly - Exploded view (Cold chamber) |
Die Design
In addition to these many types of
channels, there are other design issues that must be considered in the design
of the dies. Firstly, the die must allow the molten metal to flow easily into
all of the cavities. Equally important is the removal of the solidified casting
from the die, so a draft angle must be applied to the walls of
the part cavity. The design of the die must also accommodate any complex
features on the part, such as undercuts, which will require
additional die pieces. Most of these devices slide into the part cavity through
the side of the die, and are therefore known as slides, or side-actions.
The most common type of side-action is a side-core which
enables an external undercut to be molded. Another important
aspect of designing the dies is selecting the material. Dies can be fabricated
out of many different types of metals. High grade tool steel is the most common
and is typically used for 100-150,000 cycles. However, steels with low carbon
content are more resistant to cracking and can be used for 1,000,000 cycles.
Other common materials for dies include chromium, molybdenum, nickel alloys,
tungsten, and vanadium. Any side-cores that are used in the dies can also be
made out of these materials.
Materials
Die casting typically makes use of
non-ferrous alloys. The four most common alloys that are die cast are shown
below, along with brief descriptions of their properties. (Follow the links to
search the material library).
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Materials
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Properties
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The selection of a material for die
casting is based upon several factors including the density, melting point,
strength, corrosion resistance, and cost. The material may also affect the part
design. For example, the use of zinc, which is a highly ductile metal, can
allow for thinner walls and a better surface finish than many other alloys. The
material not only determines the properties of the final casting, but also
impacts the machine and tooling. Materials with low melting temperatures, such
as zinc alloys, can be die cast in a hot chamber machine. However, materials
with a higher melting temperature, such as aluminum and copper alloys, require
the use of cold chamber machine. The melting temperature also affects the
tooling, as a higher temperature will have a greater adverse effect on the life
of the diez
Possible Defects
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Defect
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Causes
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Flash
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Unfilled sections
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Bubbles
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Hot tearing
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Ejector marks
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Many of the above defects are caused
by a non-uniform cooling rate. A variation in the cooling rate can be caused by
non-uniform wall thickness or non-uniform die temperature
Design Rules
Maximum wall thickness
- Decrease
the maximum wall thickness of a part to shorten the cycle
time (injection time and cooling time specifically) and reduce
the part volume
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INCORRECT
 Part with thick walls |
CORRECT
 Part redesigned with thin walls |
- Uniform
wall thickness will ensure uniform cooling and reduce defects
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INCORRECT
 Non-uniform wall thickness (t1 ≠ t2) |
CORRECT
 Uniform wall thickness (t1 = t2) |
Corners
- Round
corners to reduce stress concentrations and fracture
- Inner radius
should be at least the thickness of the walls
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INCORRECT
 Sharp corner |
CORRECT
 Rounded corner |
Draft
- Apply
a draft angle to all walls parallel to the parting
direction to facilitate removing the part from the die.
- Aluminum:
1° for walls, 2° for inside cores
- Magnesium:
0.75° for walls, 1.5° for inside cores
- Zinc: 0.5°
for walls, 1° for inside cores
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INCORRECT
 No draft angle |
CORRECT
 Draft angle (q) |
Undercuts
- Minimize
the number of external undercuts
- External
undercuts require side-cores which add to the tooling
cost
- Some simple
external undercuts can be cast by relocating the parting line
 Simple external undercut |
 Die cannot separate |
 New parting line allows undercut |
- Redesigning
a feature can remove an external undercut
 Part with hinge |
 Hinge requires side-core |
 Redesigned hinge |
 New hinge can be cast |
- Remove
all internal undercuts that require lifters -
Jamming of these devices often occurs in die casting
- Designing
an opening in the side of a part can allow a side-core to form an internal
undercut
 Internal undercut accessible from the side |
- Redesigning
a part can remove an internal undercut
 Part with internal undercut |
 Die cannot separate |
 Part redesigned with slot |
 New part can be cast |
- Minimize
number of side-action directions
- Additional
side-action directions will limit the number of possible cavities in the
die
Cost Drivers
Material cost
The material cost is
determined by the weight of material that is required and the unit price of
that material. The weight of material is clearly a result of the part volume and material density; however, the
part's maximum
wall thickness can
also play a role. The weight of material that is required includes the material
that fills the channels of the die. A part with thinner walls will require a
larger system of channels to ensure that the entire part fills quickly and
evenly, and therefore will increase the amount of required material. However,
this additional material is typically less than the amount of material saved
from the reduction in part volume, a result of thinner walls. Therefore,
despite the larger channels, using thinner walls will typically lower the
material cost.
Production cost
The production cost is
primarily calculated from the hourly rate and
the cycle time.
The hourly rate is proportional to the size of the die casting machine being
used, so it is important to understand how the part design affects machine
selection. Die casting machines are typically referred to by the tonnage of the clamping force they provide. The required clamping
force is determined by the projected area of
the part and the pressure with which the molten metal is injected. Therefore, a
larger part will require a larger clamping force, and hence a more expensive
machine. Also, certain materials that require high injection pressures may
require higher tonnage machines. The size of the part must also comply with
other machine specifications, such as clamp stroke, platen size,
and shot capacity. In addition to the size of
the machine, the type
of machine (hot
chamber vs. cold chamber) will also affect the cost. The use of materials with
high melting temperatures, such as aluminum, will require cold chamber machines
which are typically more expensive.
The cycle time can be broken down into the injection time, cooling time, and resetting time. By reducing any of these times, the production cost will be lowered. The injection time can be decreased by reducing the maximum wall thickness of the part. Also, certain materials can be injected faster than others, but the injection times are so short that the cost saving are negligible. Substantial time can be saved by using a hot chamber machine because in cold chamber machines the molten metal must be ladled into the machine. This ladling time is dependent upon the shot weight. The cooling time is also decreased for lower wall thicknesses, as they require less time to cool all the way through. Several thermodynamic properties of the material also affect the cooling time. Lastly, the resetting time depends on the machine size and the part size. A larger part will require larger motions from the machine to open, close, and eject the part, and a larger machine requires more time to perform these operations. Also, the use of any side-cores will slow this process.
The cycle time can be broken down into the injection time, cooling time, and resetting time. By reducing any of these times, the production cost will be lowered. The injection time can be decreased by reducing the maximum wall thickness of the part. Also, certain materials can be injected faster than others, but the injection times are so short that the cost saving are negligible. Substantial time can be saved by using a hot chamber machine because in cold chamber machines the molten metal must be ladled into the machine. This ladling time is dependent upon the shot weight. The cooling time is also decreased for lower wall thicknesses, as they require less time to cool all the way through. Several thermodynamic properties of the material also affect the cooling time. Lastly, the resetting time depends on the machine size and the part size. A larger part will require larger motions from the machine to open, close, and eject the part, and a larger machine requires more time to perform these operations. Also, the use of any side-cores will slow this process.
Tooling cost
The tooling cost has two
main components - the die set and the machining of the cavities.
The cost of the die set is primarily controlled by the size of the part's envelope.
A larger part requires a larger, more expensive, die set. The cost of machining
the cavities is affected by nearly every aspect of the part's geometry. The
primary cost driver is the size of the cavity that must be machined, measured
by the projected area of the cavity (equal to the projected area of the part
and projected
holes) and its depth. Any other elements that will require
additional machining time will add to the cost, including the feature count, parting surface, side-cores, tolerance,
and surface
roughness.
The quantity of parts and material used will affect the tooling life and therefore impact the cost. Materials with high casting temperatures, such as copper, will cause a short tooling life. Zinc, which can be cast at lower temperatures, allows for a much longer tooling life. This effect becomes more cost prohibitive with higher production quantities.
One final consideration is the number of side-action directions, which can indirectly affect the cost. The additional cost for side-cores is determined by how many are used. However, the number of directions can restrict the number of cavities that can be included in the die. For example, the die for a part which requires 3 side-core directions can only contain 2 cavities. There is no direct cost added, but it is possible that the use of more cavities could provide further savings.
The quantity of parts and material used will affect the tooling life and therefore impact the cost. Materials with high casting temperatures, such as copper, will cause a short tooling life. Zinc, which can be cast at lower temperatures, allows for a much longer tooling life. This effect becomes more cost prohibitive with higher production quantities.
One final consideration is the number of side-action directions, which can indirectly affect the cost. The additional cost for side-cores is determined by how many are used. However, the number of directions can restrict the number of cavities that can be included in the die. For example, the die for a part which requires 3 side-core directions can only contain 2 cavities. There is no direct cost added, but it is possible that the use of more cavities could provide further savings.
Investment casting is one of the oldest manufacturing
processes, dating back thousands of years, in which molten metal is poured into
an expendable ceramic mold. The mold is formed by using a wax pattern - a
disposable piece in the shape of the desired part. The pattern is surrounded,
or "invested", into ceramic slurry that hardens into the mold.
Investment casting is often referred to as "lost-wax casting" because
the wax pattern is melted out of the mold after it has been formed. Lox-wax
processes are one-to-one (one pattern creates one part), which increases
production time and costs relative to other casting processes. However, since
the mold is destroyed during the process, parts with complex geometries and
intricate details can be created.
Investment casting can make use of most metals, most commonly using aluminum alloys, bronze alloys, magnesium alloys, cast iron, stainless steel, and tool steel. This process is beneficial for casting metals with high melting temperatures that can not be molded in plaster or metal. Parts that are typically made by investment casting include those with complex geometry such as turbine blades or firearm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries.
Investment casting requires the use of a metal die, wax, ceramic slurry, furnace, molten metal, and any machines needed for sandblasting, cutting, or grinding. The process steps include the following:
Investment casting can make use of most metals, most commonly using aluminum alloys, bronze alloys, magnesium alloys, cast iron, stainless steel, and tool steel. This process is beneficial for casting metals with high melting temperatures that can not be molded in plaster or metal. Parts that are typically made by investment casting include those with complex geometry such as turbine blades or firearm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries.
Investment casting requires the use of a metal die, wax, ceramic slurry, furnace, molten metal, and any machines needed for sandblasting, cutting, or grinding. The process steps include the following:
- Pattern creation -
The wax patterns are typically injection molded into a metal die and are
formed as one piece. Cores may be used to form any internal features on
the pattern. Several of these patterns are attached to a central wax
gating system (sprue, runners, and risers), to form a tree-like assembly.
The gating system forms the channels through which the molten metal will
flow to the mold cavity.
- Mold creation -
This "pattern tree" is dipped into a slurry of fine ceramic
particles, coated with more coarse particles, and then dried to form a
ceramic shell around the patterns and gating system. This process is
repeated until the shell is thick enough to withstand the molten metal it
will encounter. The shell is then placed into an oven and the wax is
melted out leaving a hollow ceramic shell that acts as a one-piece mold,
hence the name "lost wax" casting.
- Pouring - The
mold is preheated in a furnace to approximately 1000°C (1832°F) and the
molten metal is poured from a ladle into the gating system of the mold,
filling the mold cavity. Pouring is typically achieved manually under the
force of gravity, but other methods such as vacuum or pressure are
sometimes used.
- Cooling -
After the mold has been filled, the molten metal is allowed to cool and
solidify into the shape of the final casting. Cooling time depends on the
thickness of the part, thickness of the mold, and the material used.
- Casting removal -
After the molten metal has cooled, the mold can be broken and the casting
removed. The ceramic mold is typically broken using water jets, but
several other methods exist. Once removed, the parts are separated from
the gating system by either sawing or cold breaking (using liquid
nitrogen).
- Finishing -
Often times, finishing operations such as grinding or sandblasting are
used to smooth the part at the gates. Heat treatment is also sometimes
used to harden the final part.
Investment Casting
Capabilities
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Permanent mold casting is a metal casting process that
shares similarities to both sand casting and die casting. As in sand casting,
molten metal is poured into a mold which is clamped shut until the material
cools and solidifies into the desired part shape. However, sand casting uses an
expendable mold which is destroyed after each cycle. Permanent mold casting,
like die casting, uses a metal mold (die) that is typically made from steel or
cast iron and can be reused for several thousand cycles. Because the molten
metal is poured into the die and not forcibly injected, permanent mold casting
is often referred to as gravity die casting.
Permanent mold casting is typically used for high-volume production of small, simple metal parts with uniform wall thickness. Non-ferrous metals are typically used in this process, such as aluminum alloys, magnesium alloys, and copper alloys. However, irons and steels can also be cast using graphite molds. Common permanent mold parts include gears and gear housings, pipe fittings, and other automotive and aircraft components such as pistons, impellers, and wheels.
The permanent mold casting process consists of the following steps:
Permanent mold casting is typically used for high-volume production of small, simple metal parts with uniform wall thickness. Non-ferrous metals are typically used in this process, such as aluminum alloys, magnesium alloys, and copper alloys. However, irons and steels can also be cast using graphite molds. Common permanent mold parts include gears and gear housings, pipe fittings, and other automotive and aircraft components such as pistons, impellers, and wheels.
The permanent mold casting process consists of the following steps:
- Mold preparation -
First, the mold is pre-heated to around 300-500°F (150-260°C) to allow
better metal flow and reduce defects. Then, a ceramic coating is applied
to the mold cavity surfaces to facilitate part removal and increase the
mold lifetime.
- Mold assembly -
The mold consists of at least two parts - the two mold halves and any
cores used to form complex features. Such cores are typically made from
iron or steel, but expendable sand cores are sometimes used. In this step,
the cores are inserted and the mold halves are clamped together.
- Pouring - The
molten metal is poured at a slow rate from a ladle into the mold through a
sprue at the top of the mold. The metal flows through a runner system and
enters the mold cavity.
- Cooling - The
molten metal is allowed to cool and solidify in the mold.
- Mold opening -
After the metal has solidified, the two mold halves are opened and the
casting is removed.
- Trimming -
During cooling, the metal in the runner system and sprue solidify attached
to the casting. This excess material is now cut away.
Permanent Mold Casting
Using these basic steps, other variations on permanent
mold casting have been developed to accommodate specific applications. Examples
of these variations include the following:
- Slush Casting -
As in permanent mold casting, the molten metal is poured into the mold and
begins to solidify at the cavity surface. When the amount of solidified
material is equal to the desired wall thickness, the remaining slush
(material that has yet to completely solidify) is poured out of the mold. As
a result, slush casting is used to produce hollow parts without the use of
cores.
- Low Pressure Permanent
Mold Casting - Instead of being poured, the molten metal is
forced into the mold by low pressure air (< 1 bar). The application of
pressure allows the mold to remain filled and reduces shrinkage during
cooling. Also, finer details and thinner walls can be molded.
- Vacuum Permanent Mold
Casting - Similar to low pressure casting, but vacuum pressure is
used to fill the mold. As a result, finer details and thin walls can be
molded and the mechanical properties of the castings are improved
Capabilities
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Sand casting, the most widely used casting process,
utilizes expendable sand molds to form complex metal parts that can be made of
nearly any alloy. Because the sand mold must be destroyed in order to remove
the part, called the casting, sand casting typically has a low production
rate. The sand casting process involves the use of a furnace, metal,
pattern, and sand mold. The metal is melted in the furnace and then ladled and
poured into the cavity of the sand mold, which is formed by the pattern. The
sand mold separates along a parting line and the solidified casting can be
removed. The steps in this process are described in greater detail in the next
section.
 |
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Sand casting overview
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Sand casting is used to produce a wide variety of metal
components with complex geometries. These parts can vary greatly in size and
weight, ranging from a couple ounces to several tons. Some smaller sand cast
parts include components as gears, pulleys, crankshafts, connecting rods, and
propellers. Larger applications include housings for large equipment and heavy
machine bases. Sand casting is also common in producing automobile components,
such as engine blocks, engine manifolds, cylinder heads, and transmission
cases.
Capabilities
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Process Cycle
The process cycle for sand casting consists of six main
stages, which are explained below.
- Mold-making -
The first step in the sand casting process is to create the mold for the
casting. In an expendable mold process, this step must be performed for
each casting. A sand mold is formed by packing sand into each half of the
mold. The sand is packed around the pattern, which is a replica of the
external shape of the casting. When the pattern is removed, the cavity
that will form the casting remains. Any internal features of the casting
that cannot be formed by the pattern are formed by separate cores which
are made of sand prior to the formation of the mold. Further details on
mold-making will be described in the next section. The mold-making time
includes positioning the pattern, packing the sand, and removing the
pattern. The mold-making time is affected by the size of the part, the
number of cores, and the type of sand mold. If the mold type requires
heating or baking time, the mold-making time is substantially increased.
Also, lubrication is often applied to the surfaces of the mold cavity in
order to facilitate removal of the casting. The use of a lubricant also
improves the flow the metal and can improve the surface finish of the
casting. The lubricant that is used is chosen based upon the sand and
molten metal temperature.
- Clamping -
Once the mold has been made, it must be prepared for the molten metal to
be poured. The surface of the mold cavity is first lubricated to
facilitate the removal of the casting. Then, the cores are positioned and
the mold halves are closed and securely clamped together. It is essential
that the mold halves remain securely closed to prevent the loss of any
material.
- Pouring - The
molten metal is maintained at a set temperature in a furnace. After the
mold has been clamped, the molten metal can be ladled from its holding
container in the furnace and poured into the mold. The pouring can be
performed manually or by an automated machine. Enough molten metal must be
poured to fill the entire cavity and all channels in the mold. The filling
time is very short in order to prevent early solidification of any one
part of the metal.
- Cooling - The
molten metal that is poured into the mold will begin to cool and solidify
once it enters the cavity. When the entire cavity is filled and the molten
metal solidifies, the final shape of the casting is formed. The mold can
not be opened until the cooling time has elapsed. The desired cooling time
can be estimated based upon the wall thickness of the casting and the
temperature of the metal. Most of the possible defects that can occur are
a result of the solidification process. If some of the molten metal cools
too quickly, the part may exhibit shrinkage, cracks, or
incomplete sections. Preventative measures can be taken in designing both
the part and the mold and will be explored in later sections.
- Removal -
After the predetermined solidification time has passed, the sand mold can
simply be broken, and the casting removed. This step, sometimes called
shakeout, is typically performed by a vibrating machine that shakes the
sand and casting out of the flask. Once removed, the casting will likely
have some sand and oxide layers adhered to the surface. Shot blasting is
sometimes used to remove any remaining sand, especially from internal
surfaces, and reduce the surface roughness.
- Trimming -
During cooling, the material from the channels in the mold solidifies
attached to the part. This excess material must be trimmed from the
casting either manually via cutting or sawing, or using a trimming press.
The time required to trim the excess material can be estimated from the
size of the casting's envelope. A larger casting will require
a longer trimming time. The scrap material that results from this trimming
is either discarded or reused in the sand casting process. However, the
scrap material may need to be reconditioned to the proper chemical
composition before it can be combined with non-recycled metal and reused
Equipment
Mold
In sand casting, the primary piece of equipment is the mold, which contains several components. The mold is divided into two halves - the cope (upper half) and the drag (bottom half), which meet along a parting line. Both mold halves are contained inside a box, called a flask, which itself is divided along this parting line. The mold cavity is formed by packing sand around the pattern in each half of the flask. The sand can be packed by hand, but machines that use pressure or impact ensure even packing of the sand and require far less time, thus increasing the production rate. After the sand has been packed and the pattern is removed, a cavity will remain that forms the external shape of the casting. Some internal surfaces of the casting may be formed by cores.
Cores are additional pieces that form the internal holes and passages of the casting. Cores are typically made out of sand so that they can be shaken out of the casting, rather than require the necessary geometry to slide out. As a result, sand cores allow for the fabrication of many complex internal features. Each core is positioned in the mold before the molten metal is poured. In order to keep each core in place, the pattern has recesses called core prints where the core can be anchored in place. However, the core may still shift due to buoyancy in the molten metal. Further support is provided to the cores by chaplets. These are small metal pieces that are fastened between the core and the cavity surface. Chaplets must be made of a metal with a higher melting temperature than that of the metal being cast in order to maintain their structure. After solidification, the chaplets will have been cast inside the casting and the excess material of the chaplets that protrudes must be cut off.
In addition to the external and internal features of the casting, other features must be incorporated into the mold to accommodate the flow of molten metal. The molten metal is poured into a pouring basin, which is a large depression in the top of the sand mold. The molten metal funnels out of the bottom of this basin and down the main channel, called the sprue. The sprue then connects to a series of channels, called runners, which carries the molten metal into the cavity. At the end of each runner, the molten metal enters the cavity through a gate which controls the flow rate and minimizes turbulence. Often connected to the runner system are risers. Risers are chambers that fill with molten metal, providing an additional source of metal during solidification. When the casting cools, the molten metal will shrink and additional material is needed. A similar feature that aids in reducing shrinkage is an open riser. The first material to enter the cavity is allowed to pass completely through and enter the open riser. This strategy prevents early solidification of the molten metal and provides a source of material to compensate for shrinkage. Lastly, small channels are included that run from the cavity to the exterior of the mold. These channels act as venting holes to allow gases to escape the cavity. The porosity of the sand also allows air to escape, but additional vents are sometimes needed. The molten metal that flows through all of the channels (sprue, runners, and risers) will solidify attached to the casting and must be separated from the part after it is removed.
In sand casting, the primary piece of equipment is the mold, which contains several components. The mold is divided into two halves - the cope (upper half) and the drag (bottom half), which meet along a parting line. Both mold halves are contained inside a box, called a flask, which itself is divided along this parting line. The mold cavity is formed by packing sand around the pattern in each half of the flask. The sand can be packed by hand, but machines that use pressure or impact ensure even packing of the sand and require far less time, thus increasing the production rate. After the sand has been packed and the pattern is removed, a cavity will remain that forms the external shape of the casting. Some internal surfaces of the casting may be formed by cores.
Cores are additional pieces that form the internal holes and passages of the casting. Cores are typically made out of sand so that they can be shaken out of the casting, rather than require the necessary geometry to slide out. As a result, sand cores allow for the fabrication of many complex internal features. Each core is positioned in the mold before the molten metal is poured. In order to keep each core in place, the pattern has recesses called core prints where the core can be anchored in place. However, the core may still shift due to buoyancy in the molten metal. Further support is provided to the cores by chaplets. These are small metal pieces that are fastened between the core and the cavity surface. Chaplets must be made of a metal with a higher melting temperature than that of the metal being cast in order to maintain their structure. After solidification, the chaplets will have been cast inside the casting and the excess material of the chaplets that protrudes must be cut off.
In addition to the external and internal features of the casting, other features must be incorporated into the mold to accommodate the flow of molten metal. The molten metal is poured into a pouring basin, which is a large depression in the top of the sand mold. The molten metal funnels out of the bottom of this basin and down the main channel, called the sprue. The sprue then connects to a series of channels, called runners, which carries the molten metal into the cavity. At the end of each runner, the molten metal enters the cavity through a gate which controls the flow rate and minimizes turbulence. Often connected to the runner system are risers. Risers are chambers that fill with molten metal, providing an additional source of metal during solidification. When the casting cools, the molten metal will shrink and additional material is needed. A similar feature that aids in reducing shrinkage is an open riser. The first material to enter the cavity is allowed to pass completely through and enter the open riser. This strategy prevents early solidification of the molten metal and provides a source of material to compensate for shrinkage. Lastly, small channels are included that run from the cavity to the exterior of the mold. These channels act as venting holes to allow gases to escape the cavity. The porosity of the sand also allows air to escape, but additional vents are sometimes needed. The molten metal that flows through all of the channels (sprue, runners, and risers) will solidify attached to the casting and must be separated from the part after it is removed.
 Sand Mold - Opened |
 Sand Mold - Closed |
Sand
The sand that is used to create the molds is typically silica sand (SiO2) that is mixed with a type of binder to help maintain the shape of the mold cavity. Using sand as the mold material offers several benefits to the casting process. Sand is very inexpensive and is resistant to high temperatures, allowing many metals to be cast that have high melting temperatures. There are different preparations of the sand for the mold, which characterize the following four unique types of sand molds.
The sand that is used to create the molds is typically silica sand (SiO2) that is mixed with a type of binder to help maintain the shape of the mold cavity. Using sand as the mold material offers several benefits to the casting process. Sand is very inexpensive and is resistant to high temperatures, allowing many metals to be cast that have high melting temperatures. There are different preparations of the sand for the mold, which characterize the following four unique types of sand molds.
- Greensand mold -
Greensand molds use a mixture of sand, water, and a clay or binder.
Typical composition of the mixture is 90% sand, 3% water, and 7% clay or
binder. Greensand molds are the least expensive and most widely used.
- Skin-dried mold -
A skin-dried mold begins like a greensand mold, but additional bonding
materials are added and the cavity surface is dried by a torch or heating
lamp to increase mold strength. Doing so also improves the dimensional
accuracy and surface finish, but will lower the collapsibility. Dry
skin molds are more expensive and require more time, thus lowering the
production rate.
- Dry sand mold -
In a dry sand mold, sometimes called a cold box mold, the sand is mixed
only with an organic binder. The mold is strengthened by baking it in an
oven. The resulting mold has high dimensional accuracy, but is expensive
and results in a lower production rate.
- No-bake mold -
The sand in a no-bake mold is mixed with a liquid resin and hardens at
room temperature.
The quality of the sand that is used also greatly affects
the quality of the casting and is usually described by the following five
measures:
- Strength -
Ability of the sand to maintain its shape.
- Permeability -
Ability to allow venting of trapped gases through the sand. A higher
permeability can reduce the porosity of the mold, but a lower permeability
can result in a better surface finish. Permeability is determined by the
size and shape of the sand grains.
- Thermal stability -
Ability to resist damage, such as cracking, from the heat of the molten
metal.
- Collapsibility -
Ability of the sand to collapse, or more accurately compress, during
solidification of the casting. If the sand can not compress, then the
casting will not be able to shrink freely in the mold and can result in
cracking.
- Reusability -
Ability of the sand to be reused for future sand molds.
Packing equipment
There exists many ways to pack the sand into the mold. As mentioned above, the sand can be hand packed into the mold. However, there are several types of equipment that provide more effective and efficient packing of the sand. One such machine is called a sandslinger and fills the flask with sand by propelling it under high pressure. A jolt-squeeze machine is a common piece of equipment which rapidly jolts the flask to distribute the sand and then uses hydraulic pressure to compact it in the flask. Another method, called impact molding, uses a controlled explosion to drive and compact the sand into the flask. In what can be considered an opposite approach, vacuum molding packs the sand by removing the air between the flask and a thin sheet of plastic that covers the pattern.
The packing of the sand is also automated in a process known as flask-less molding. Despite the name of the process, a flask is still used. In conventional sand casting, a new flask is used for each mold. However, flask-less molding uses a single master flask in an automated process of creating sand molds. The flask moves along a conveyor and has sand blown against the pattern inside. This automated process greatly increases the production rate and also has many benefits to the castings. Flask-less molding can produce uniform, high density molds that result in excellent casting quality. Also, the automated process causes little variation between castings.
There exists many ways to pack the sand into the mold. As mentioned above, the sand can be hand packed into the mold. However, there are several types of equipment that provide more effective and efficient packing of the sand. One such machine is called a sandslinger and fills the flask with sand by propelling it under high pressure. A jolt-squeeze machine is a common piece of equipment which rapidly jolts the flask to distribute the sand and then uses hydraulic pressure to compact it in the flask. Another method, called impact molding, uses a controlled explosion to drive and compact the sand into the flask. In what can be considered an opposite approach, vacuum molding packs the sand by removing the air between the flask and a thin sheet of plastic that covers the pattern.
The packing of the sand is also automated in a process known as flask-less molding. Despite the name of the process, a flask is still used. In conventional sand casting, a new flask is used for each mold. However, flask-less molding uses a single master flask in an automated process of creating sand molds. The flask moves along a conveyor and has sand blown against the pattern inside. This automated process greatly increases the production rate and also has many benefits to the castings. Flask-less molding can produce uniform, high density molds that result in excellent casting quality. Also, the automated process causes little variation between castings.
Tooling
The main tooling for sand casting is the pattern that is
used to create the mold cavity. The pattern is a full size model of the part
that makes an impression in the sand mold. However, some internal surfaces may
not be included in the pattern, as they will be created by separate cores. The
pattern is actually made to be slightly larger than the part because the
casting will shrink inside the mold cavity. Also, several identical patterns
may be used to create multiple impressions in the sand mold, thus creating
multiple cavities that will produce as many parts in one casting.
Several different materials can be used to fabricate a pattern, including wood, plastic, and metal. Wood is very common because it is easy to shape and is inexpensive, however it can warp and deform easily. Wood also will wear quicker from the sand. Metal, on the other hand, is more expensive, but will last longer and has higher tolerances. The pattern can be reused to create the cavity for many molds of the same part. Therefore, a pattern that lasts longer will reduce tooling costs. A pattern for a part can be made many different ways, which are classified into the following four types:
Several different materials can be used to fabricate a pattern, including wood, plastic, and metal. Wood is very common because it is easy to shape and is inexpensive, however it can warp and deform easily. Wood also will wear quicker from the sand. Metal, on the other hand, is more expensive, but will last longer and has higher tolerances. The pattern can be reused to create the cavity for many molds of the same part. Therefore, a pattern that lasts longer will reduce tooling costs. A pattern for a part can be made many different ways, which are classified into the following four types:
- Solid pattern -
A solid pattern is a model of the part as a single piece. It is the
easiest to fabricate, but can cause some difficulties in making the mold.
The parting line and runner system must be determined separately. Solid
patterns are typically used for geometrically simple parts that are
produced in low quantities.
 Solid pattern |
- Split pattern -
A split pattern models the part as two separate pieces that meet along the
parting line of the mold. Using two separate pieces allows the mold
cavities in the cope and drag to be made separately and the parting line
is already determined. Split patterns are typically used for parts that
are geometrically complex and are produced in moderate quantities.
 Split pattern |
- Match-plate pattern -
A match-plate pattern is similar to a split pattern, except that each half
of the pattern is attached to opposite sides of a single plate. The plate
is usually made from wood or metal. This pattern design ensures proper
alignment of the mold cavities in the cope and drag and the runner system
can be included on the match plate. Match-plate patterns are used for
larger production quantities and are often used when the process is
automated.
 Match-plate pattern |
- Cope and drag pattern -
A cope and drag pattern is similar to a match plate pattern, except that
each half of the pattern is attached to a separate plate and the mold
halves are made independently. Just as with a match plate pattern, the
plates ensure proper alignment of the mold cavities in the cope and drag
and the runner system can be included on the plates. Cope and drag
patterns are often desirable for larger castings, where a match-plate
pattern would be too heavy and cumbersome. They are also used for larger
production quantities and are often used when the process is automated.
 Cope and drag pattern |
Another piece of tooling used in sand casting is a
core-box. If the casting requires sand cores, the cores are formed in these
boxes, which are similar to a die and can be made of wood, plastic, or metal
just like the pattern. The core-boxes can also contain multiple cavities to
produce several identical cores.
Materials
Sand casting is able to make use of almost any alloy. An
advantage of sand casting is the ability to cast materials with high melting
temperatures, including steel, nickel, and titanium. The four most common
materials that are used in sand casting are shown below, along with their
melting temperatures.
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Materials
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Melting temperature
|
|
Aluminum alloys
|
1220 °F (660 °C)
|
|
Brass alloys
|
1980 °F (1082 °C)
|
|
Cast iron
|
1990-2300 °F (1088-1260 °C)
|
|
Cast steel
|
2500 °F (1371 °C)
|
Possible Defects
|
Defect
|
Causes
|
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Unfilled sections
|
|
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Porosity
|
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Hot tearing
|
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Surface projections
|
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Design Rules
Maximum wall thickness
- Decrease the maximum
wall thickness of a part to shorten the cycle time (cooling
time specifically) and reduce the part volume
|
INCORRECT
Part with thick walls |
CORRECT
Part redesigned with thin walls |
- Uniform wall thickness
will ensure uniform cooling and reduce defects. A thick section, often
referred to as a hot spot, causes uneven cooling and can result in
shrinkage, porosity, or cracking.
|
INCORRECT
Non-uniform wall thickness (t1 ≠ t2) |
CORRECT
Uniform wall thickness (t1 = t2) |
Corners
- Round corners to reduce
stress concentrations and fracture
- Inner radius should be at
least the thickness of the walls
|
INCORRECT
Sharp corner |
CORRECT
Rounded corner |
Draft
- Apply a draft angle
of 2° - 3° to all walls parallel to the parting direction to facilitate
removing the part from the mold.
|
INCORRECT
No draft angle |
CORRECT
Draft angle (q) |
Machining allowance
- Add 0.0625 - 0.25 in.
(0.16 - 0.64 mm) to part dimensions to allow for machining to obtain a
smooth surface.
Cost Drivers
Material cost
The material cost for sand casting includes the
cost of the metal, melting the metal, the mold sand, and the core sand. The
cost of the metal is determined by the weight of the part, calculated from
part volume and material density, as well the unit price of
the material. The melting cost will also be greater for a larger part weight
and is influenced by the material, as some materials are more costly to melt.
However, the melting cost in typically insignificant compared to the metal
cost. The amount of mold sand that is used, and hence the cost, is also
proportional to the weight of the part. Lastly, the cost of the core sand is
determined by the quantity and size of the cores used to cast the part.
Production cost
The production cost includes a variety of
operations used to cast the part, including core-making, mold-making, pouring,
and cleaning. The cost of making the cores depends on the volume of the cores
and the quantity used to cast the part. The cost of the mold-making is not
greatly influenced by the part geometry when automated equipment is being used.
However, the inclusion of cores will slightly slow the process and therefore increase
the cost. Lastly, the cost of pouring the metal and cleaning the final casting
are both driven by the weight of the part. It will take longer to pour and to
clean a larger and heavier casting.
Tooling cost
The tooling cost has two main components - the
pattern and the core-boxes. The pattern cost is primarily controlled by the
size of the part (both the envelope and the projected
area) as well as the part's complexity. The cost of the core-boxes first
depends on their size, a result of the quantity and size of the cores that are
used to cast the part. Much like the pattern, the complexity of the cores will
affect the time to manufacture this part of the tooling (in addition to the
core size), and hence the cost.
The quantity of parts that are cast will also impact the tooling cost. A larger production quantity will require the use of a tooling material, for both the pattern and core-boxes, that will not wear under the required number of cycles. The use or a stronger, more durable, tooling material will significantly increase the cost.
The quantity of parts that are cast will also impact the tooling cost. A larger production quantity will require the use of a tooling material, for both the pattern and core-boxes, that will not wear under the required number of cycles. The use or a stronger, more durable, tooling material will significantly increase the cost.
