Comprehensive lecture notes on Manufacturing Processes I

Lecture 13 Casting Process

Design of Molds, Patterns, Riser, Runner,

Gates, Cores & Cost Analysis

13.1 Introduction:
As discussed earlier, a foundry is a factory equipped for making molds,
melting and handling metal in molten form, establishing the casting support
systems, performing the casting process, and cleaning to make a finished
product called casting. In the casting operation, the selected 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 sections.

13.2 Molds
Sand casting is by far the most important casting process. A sand-casting
mold is the representative of the basic features of all expandable mold. Many
of these features and terms are common to the molds used in other casting
processes.
There are two types of molds, depending on the mold cavity. These are-
(i) Open mold, and
(ii) Closed mold.

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Fig. 13.1 Two forms of mold: (a) open mold, simply a container, and (b) closed
mold, that is more complex and requires a gating system. (Courtesy:
Custom part).

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In an open mold (Fig. 13.1 a), the liquid metal is simply poured until it fills
the open cavity. In a closed mold (Fig. 13.1 b), a passageway, called the
gating system, is provided to permit the molten metal to flow from outside
the mold into the cavity. The closed mold is by far the more important form
in production casting operations.
The mold consists of two halves: cope and drag. The cope is the upper half
of the mold, and the drag is the bottom half, which meets along a parting
line. These two mold parts are contained in a box called a flask, which is
also divided into two halves, one for the cope and the other for the drag. The
two halves of the mold separate at the parting line.
Flask: A flask is a wood or metal frame in which a mold is made. It is made
of two principal parts, the cope part (top section) and the drag part (bottom
section). When more than two sections of a flask are necessary to increase
the depth of the flask, an intermediate part is added between the cope and
drag, and this section is known as cheek, shown in Fig. 13.2.

Fig. 13.2 Sand casting flasks: A steel flask (left), and a wooden flask (right).

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The flask must be strong and rigid so as not distort when it is handled or
when sand is rammed into it. It must also resist the pressure of the molten
metal during casting.
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.

13.3 Patterns
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. The mold cavity is the impression of a pattern, which is an
approximate replica of the exterior of the desired casting. The pattern is
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.
The pattern can be reused to create the cavity for many molds of the same
part. 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 is sometimes more expensive, but will
last longer and has higher tolerances. Plastic, on the other hand, inexpensive
to produce, excellent wear resistance properties, ease of modification, and
close dimensional tolerances are the positive sides of the plastic; therefore
suitable for high-volume production. A pattern that lasts longer will reduce

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tooling costs, therefore, a large extent on the total quantity of castings to be

made.
A pattern for a part can be made of many ways, which are classified into the
following four types:
(i) solid pattern,
(ii) split pattern,
(iii) match plate pattern, and
(iv) cope-and-drag pattern, as illustrated in Fig. 13.3.

Fig. 13.3 Types of patterns used in sand casting: (a) Solid pattern, (b) Split pattern,
(c) Match-plate pattern, and (d) Cope-and-drag pattern.

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▪ Solid pattern: The simplest is made of one piece, called a solid pattern,
as shown in Fig. 13.3 a, with the same geometry as the casting, adjusted
in size for shrinkage and machining. Although it is the easiest pattern to
fabricate, it is not the easiest to use in making the sand mold.
Determining the location of the parting line between the two halves of
the mold for a solid pattern can be a problem, and incorporating the
gating system and sprue into the mold is left to the judgment and skill of
the foundry worker. Consequently, solid patterns are generally limited to
very low production quantities.
▪ Split pattern: A split pattern, as shown in Fig. 13.3 b, 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.
▪ Match-plate pattern: A match-plate pattern, as illustrated in Fig. 13.3 c,
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, usually 2‒4 holes and pins, 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.
▪ Cope and drag pattern: A cope and drag pattern, as illustrated in Fig.
13.3 d, 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

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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.

13.4 Riser
A riser, also known as a feeder, shown in Fig. 13.4, is a reservoir built into
a metal casting mold connected to the main cavity to prevent cavities due
to shrinkage. Most metals are less dense as a liquid than as a solid, so
castings shrink upon cooling, which can leave a void at the last point to
solidify. Risers prevent this by providing molten metal to the casting as it
solidifies so that the cavity forms in the riser and not the casting. Besides,
o a riser mitigates the hydraulic ram effect of the metal entering the
mold and vents the mold,
o the volume of the riser must be large enough to supply all the metal
needed.

Fig. 13.4 A schematic illustration of Riser used in the casting process.

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Risers are not effective on materials that have a large freezing range, because
directional solidification is not possible. They are also not needed for casting
processes that utilized pressure to fill the mold cavity. A feeder operated by
a treadle is called an underfeeder.
A riser is categorized based on three criteria:

• where it is located,
• whether it is open to the atmosphere, and
• how it is filled.
There are different types of risers used in the casting process.

• If the riser is located on the casting, then it is known as a top riser,

but if it is located next to the casting, it is known as a side riser.
• If the riser is open to the atmosphere, it is known as an open riser,
but if the riser is completely contained in the mold, it is known as
a blind riser.
Top risers are advantageous because they take up less space in the flask than
a side riser, plus they have a shorter feeding distance. An open riser is usually
bigger than a blind because the open riser loses more heat to mold through
the top of the riser.

• If the riser receives material from the gating system and fills before
the mold cavity, it is known as a live riser or hot riser.
• If the riser fills with material that has already flowed through the
mold cavity, it is known as a dead riser or cold riser.
Live risers are usually smaller than dead risers.
The shape of the riser is an important consideration: the area of the
connection of the riser to the casting must be large enough not to freeze too
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soon; on the other hand, the connection must not be so large that the solid
riser is difficult to remove from the casting. Experience has shown that the
most effective height of a riser is one and one-half times its diameter to
produce maximum feeding, with a minimum amount of metal. As the area
over volume ratio of a molten mass decreases, less chance is offered for the
escape of heat, and the solidification rate decreases.

13.5 Gating System

A gating system refers to all passageways through which the molten metal
passes to enter the mold cavity. The basic components of a simple gating
system are pouring basin, sprue, runner, gates.
A proper method of gating system is that it leads the pure molten metal to
flow through a ladle to the casting cavity, which ensures proper and smooth
filling of the cavity. The main function of the gating system in a mold is to
deliver the liquid metal to the mold cavity. The gating system must be
designed so that the following are ensured:
• introduce the molten metal into the mold with as little turbulence as
possible;
• a continuous and uniform flow of molten metal into the mold cavity,
i.e., regulate the rate of entry of the metal;
• permit complete filling of the mold cavity;
• promote a temperature gradient within the casting to help the casting
to solidify with the least conflict between sections; and
Minimizing turbulence plays a vital role in the quality of the casting. A
turbulence metal flow tends to form dross in the mold. Avoid sudden or
right- angle changes in the flow direction. A proper thermal gradient should
be maintained so that the casting is cooled without any shrinkage cavities or

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distortions. System design should be economical to maximize the yield and
should be easy to implement and remove after casting solidification.

13.5.1 Pouring basin

Pouring basin is the first part of the gating system to handle the molten metal
and maintain pressure head over sprue. A good design of the pouring basin
eliminates the slag entering the down sprue.
Two types of pouring basins are generally used in the sand casting process:
conical basin (conical cup), and offset basin (offset cup). Conical pouring
cups are the most popular cups almost everywhere in the foundry industry
because it is easy to fabricate and inexpensive. There are some drawbacks
too:

• Molten metal enters at high, unchecked velocity, and it results in

undesired turbulence.
• Contaminants that enter along with the melt and not filtered and are
necessarily taken directly down the sprue.
• Extremely susceptible to vortex formation and air entrapment.
On the other hand, the offset step basin offers a horizontal jet across the top
of a sprue. Other advantages are:

• The offset blind end of the basin is important in bringing the vertical
downward velocity to a stop.
• Velocity of the molten metal that enters the sprue is checked, and
unwanted components like slag, dross can be separated.
However, the drawbacks of the offset cup are: it is expensive and difficult to
keep the down runner full from the beginning to the end of the pouring.

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13.5.2 Sprue
A sprue, also called downsprue, is a large diameter channel through which
the material enters the mold. As illustrated in Fig. 13.5, it is a simple cone-
shaped funnel aids in getting the melt down to the lowest level of the mold
while introducing a minimum of defects despite the high velocity of the
stream. There are some design requirements, these are:

• Sprue should be designed as narrow as possible so that the metal has

minimal opportunity to break and entrain its surface during the fall.
• To reduce air entrapment and oxide formation, sprues should be
tapered so as to mimic the taper that the falling stream adopts
naturally as a result of its acceleration due to gravity.

Fig. 13.5 A schematic illustration of a runner used in the casting process.

13.5.3 Runner
Runners, as illustrated in Fig. 13.4 and 13.5, are passages that distribute
molten metal from the sprue to gates or risers around the cavity inside a

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mold. Runners slow down and smooth out the flow of liquid metal and are
designed to provide approximate uniform flow rates to the various parts of
the mold cavity.
The main difference between a runner and a riser in casting is that runner is
a horizontal pathway into the mold cavity, whereas riser is a vertical
pathway. However, both are used to identify and ensure the filling of the
mold and is taking care of liquid solidification. A well-designed running
system can:
o directs the molten metal towards the individual part (particularly
common when casting multiple parts at once),
o regulate the speed of the molten metal,
o avoid shrinkage during the liquid solidification, and
o minimize turbulence.
It is vital that the running system does not have any sharp corners or edges
which could restrict the flow of metal and reduce its effectiveness.
Additionally, the runner uses a filter that assists in the removal of any
impurities that enter the channels and also controls the speed of the molten
metal flow, and it sawn off the casting process when the has been completed.

13.5.4 Gates
In simple words, molds are filled with molten metal by means of channels,
called gates (also called ingates). Gating systems having sudden changes in
direction cause slower filling of the mold cavity, are easily eroded and cause
turbulence in the liquid metal resulting in the gas pick-up. Right angle turn
should be avoided. Depending upon the orientation of the parting plane,
gates are: (i) Horizontal gating, and (ii) Vertical gating system.

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Pouring basin

Gates

Sprue
Casting

Runner

Fig. 13.6 A typical gating system used in the metal casting process.

The gating system is shown in Fig. 13.6. The horizontal gating is most
widely used for flat casting, which is filled under gravity. This type is
normally applied in ferrous metal's sand casting and gravity die-casting of
non-ferrous metals. However, the vertical gate is applied in tall castings
where high-pressure sand mold, shell mold, and die-casting processes are
done.
But, depending upon the position of gates, horizontal gating systems are:
a. Top gate,
b. Parting gate, and
c. Bottom gate, as illustrated in Fig. 13.7.

a. Top gate: This type of gate is applied in places where liquid metal enters
the cavity directly from the bottom of sprue at atmospheric pressure. The
velocity of liquid metal which enters into the mold cavity is very high. It
helps directional solidification of the casting from top to bottom.

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 Therefore, it suits only flat castings to limit the damage of the metal
during the initial filling. However, there is a possibility of turbulence,
splashing of the liquid metal and mold erosion. Time taken to fill the
cavity will be less.

b. Parting gate: Gate is provided along the parting line between the cope
and drag part of the flask such that cavity above the parting line can be
filled by assuming the bottom gate and the cavity below the parting line
can be filled by assuming top gate. To get advantages of both top and
bottom gate, it is the most commonly used type of gating. The parting
gate is the easiest and fastest for the molder to make. However, common
disadvantages of parting gates are― the metal drops into the drag cavity
and may cause erosion or washing of the mold, and in the case of non-
ferrous metals, this drop aggregates the dross and entraps air in the metal,
which makes an inferior casting.

c. Bottom gate: This gate lies below the drag part of the mold cavity. The
velocity of liquid metal in the cavity will be very less and will become
zero. There is no possibility of turbulence, splashing liquid metal and
mold and core erosion. However, the main disadvantage of the bottom
gate is that it creates an unfavorable temperature gradient. The metal is
introduced into the bottom of the mold cavity and rises quietly and
evenly. It cools as it rises, and the result is a condition of cold metal and
cold metal near the riser and hot metal and hot mold near the gate.
Therefore, the riser should contain the hottest metal in the hottest part of
the mold so it can feed metal into the mold until all the casting has
solidified. In addition, the time taken to fill the mold cavity will be
maximum.

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Fig. 13.7 Various types of gates used in the casting; (a) Parting gate, (b) Top gate,
and (c) Bottom gate.

Gating through side riser should be done whenever possible. Gating directly
into the casting results in hot spots, because all the metal enters the casting
through the gate, and the sand around the gate becomes hot.
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13.6 Core, Chaplets, etc.
Patterns define the external shape of the cast part. If the casting is to have
internal surfaces, a core is required. A core is a full-scale model of the
interior surfaces of the part. It is inserted into the mold cavity prior to pouring
so that the molten metal will flow and solidify between the mold cavity and
the core to form the casting’s external and internal surfaces.

Fig. 13.8 Core used in the mold cavity.

Cores, as shown in Fig. 13.8, are additional pieces that form the internal
holes and passages of the casting. Cores are typically made of green sand or
dry sand so that they can be shaken out of the casting, rather than require the
necessary geometry to slide out. They are generally made of green sand cores
that have relatively low strength. The dry sand core contains dry sand and
binders (clay, organic, or inorganic), so they develop strength on baking. The
types of sand cores are: Horizontal, Vertical, Balanced, and Drop cores. The
metal cores are generally made of cast iron or steel. The selection of the
correct type of core depends on:
o production quantity,
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o production rate,
o required precision,
o required surface finish, and
o the type of metal being used.
As a result, sand cores allow for the fabrication of many complex internal
features. The various properties required of good cores are: (1)
refractoriness, (2) some green strength, (3) high dry strength, (4) good
collapsibility, (5) a minimum amount of gas generation by the core during
casting, (6) good permeability, and (7) high density.
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, as shown in Fig. 13.9.
A chaplet is a metal location piece is inserted in a mold to provide extra
support to the core and prevent shifting from its position. The chaplet melts
as it comes in contact with the molten metal and forms part of the cast
material.

Fig. 13.9 Core and chaplets used in the sand casting.

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These are small metal supports that bridge the gap between the mold surface
and the core. Since the chaplets become part of the casting, the chaplets must
be of the same or similar material as the metal being cast. Moreover, their
design must be optimized because if they are too small, they will completely
melt and allow the core to move, but if they are too big, then their whole
surface cannot melt and fuse with the poured metal. After solidification, the
chaplets will have been cast inside the casting, and the excess material of the
chaplets that protrudes must be cut off. Therefore, the use of chaplets should
be minimized because they can cause casting defects or create weak spots in
the casting.
Vents: As the metal flows into the mold, the air that previously occupied the
cavity, as well as hot gases formed by reactions of the molten metal, must be
evacuated so that the metal will completely fill the empty space. In sand
casting, for example, the natural porosity of the sand mold permits the air
and gases to escape through the walls of the cavity, in permanent metal
molds, small holes are drilled into the mold or machined into the part in line
to permit removal of air and gases. Vents are used to escape the gases from
the mold.
Vent holes, as the name suggests, are vents for the routing of air /gases from
the cavity of the die to the outside environment. Therefore, venting is a
process that allows the escaping of gases from the molten metal during the
solidification. For the venting process, a vent is provided in the casting
system through which gases escape out. Proper mold venting is very
important and necessary for producing quality casting parts.
Experience has shown that round vents are large enough to evacuate mold
gases at a proper rate will frequently reveal fine shrinkage cavity in the
casting when they are removed.

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13.7 Cost Analysis

Bangladesh’s metal casting industry is booming at a rapid pace, and looking
at the present scenario has gained its popularity in the past couple of years.
The competition has grown at a tremendous rate, and in order to survive and
compete at a global platform, the metal casting industry has to meet ever-
increasing customers' expectations in terms of quality and pricing.
Casting process generally consists of the proper choice of the suitable casting
process and various materials. Product cost refers to the costs incurred to
create a product. These costs include direct labor, direct materials,
consumable production supplies, and factory overhead. Product cost can also
be considered the cost of the labor required to deliver a service to a customer.
The cost of a product on a unit basis is typically derived by compiling the
costs associated with a batch of units that were produced as a group and
dividing by the number of units manufactured. The calculation is:
Material cost + Production cost + Tooling cost + Total overhead
Product unit cost =
Number of units produced

Material cost: The material cost for sand casting includes the cost of the
metal, melting the metal, the molding 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 the unit price of the material.
The melting cost is also high for larger part weight and is influenced by the
material, as some materials are more costly to melt. However, the melting
cost is less 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.

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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 slightly slows the process and therefore
increases the cost. Lastly, the cost of pouring the metal and cleaning the final
casting are both driven by the weight of the part. It takes 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 depends on their size and the quantity of the cores
that are used to cast the part. Much like the pattern, the complexity of the
cores affects the time to manufacture this part of the tooling, and hence the
cost increases.
The quantity of parts used has also impacted the tooling cost. A larger
production quantity requires the use of huge tooling material, for both the
pattern and core-boxes. The use or a stronger, more durable, tooling material
significantly increases the tooling cost.

Total overhead: Factory overhead, also known as manufacturing overhead,

factory burden, production overhead, involves a company's manufacturing
operations. It includes the costs incurred in the manufacturing facilities other
than the costs of direct materials and direct labor. Hence, manufacturing
overhead is referred to as an indirect cost.

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Some examples of manufacturing overhead costs include the following:

o depreciation, rent and property taxes on the manufacturing facilities;
o depreciation on the manufacturing equipment;
o managers and supervisors in the manufacturing facilities;
o repairs and maintenance employees in the manufacturing facilities;
o electricity and gas used in the manufacturing facilities;
o indirect factory supplies, and much more.
Because manufacturing overhead is an indirect cost, accountants are faced
with the task of assigning or allocating overhead costs to each of the units
produced. However, the expenses that are outside of the manufacturing
facilities, such as selling, general and administrative expenses, are not
product costs, and are not inventoriable.
The use of factory overhead is mandated by accounting standards but does
not bring real value to the understanding of overhead costs, so the best
practice is to minimize the cost of factory overhead.

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