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CHAPTER 11
DESIGN FOR CASTING
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11.2.7
11.3
11.3.1
11.3.2
11.3.2
11.4
11.4.1
Wear of Cores, Slides, and Pins
Design Factors for Castings
Effect of Grain Size
Hot Spots
Details of Design for Steel Castings
Case Study #1 – A Simple Cover Plate
Material and Overhead Costs
11.1.6
11.2
11.2.1
Permanent Molding
Tooling for Casting in Metal Molds
Design for Permanent Molding
Gating System Design
Risers
Vents
Mold Material
Venting Details for Low-Pressure
Permanent Molding
Die Casting
Minimum Section Thickness
11.4.2
11.4.3
11.5
11.2.2
11.2.3
11.2.4
11.2.5
11.2.6
Undercuts and Inserts
Cored Holes
Draft Requirements
Dimensional Tolerance
Die Materials
11.6
11.7
11.8
11.9
Auxiliary Operations
Purchasing Castings
Case Study #2 – Comparison of Cast
and Forged Connecting Rods
Summary
Questions
Problems
References
11.1
11.1.1
11.1.2
11.1.2.1
11.1.3
11.1.4
11.1.5
In Chapter 9, the fundamentals of casting processes were presented. Chapter 10
continued the discussion of casting by introducing the reading to the material science of
casting and further discussing the principles of casting. This chapter continues the
discussion of casting by looking at more advanced casting techniques and then examining
casting design principles. Casting methods are only successful when the product and
production requirements are fully integrated into the process. That is, a designer must
understand what the process requirements are before he/she can effectively design a
component that will meet or exceed the functional requirements. We begin the chapter
by looking at additional casting process details for permanent (mold) casting activities.
This discussion is followed by a detailed discussion of design considerations.
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11.1
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PERMANENT MOLDING
Sand casting and investment casting are used for a variety of activities. Sand
casting is the most common casting process and produces parts economically for low to
high volume runs. In both sand and investment casting a new mold is made for each item
that is cast. In some cases (geometry and material dependent), it is possible to use and
reuse a metal mold for casting. For certain products greater than 50,000 castings can be
made in a single “permanent” mold. In most of these cases, the permanent mold is made
from either: cast iron, steel, bronze, or graphite. The mold cavity is typically machined in
the shape of the part to be cast. Gates, vents and risers are also machined into the
permanent mold. The general classification name given to casting processes where the
mold is reused several times is permanent mold casting or permanent molding. When
used properly permanent molding can reduce the cost of cast products. For use in this
book, we will refer to the general process of reusing molds as “permanent molding”.
“Permanent mold casting” will be defined as a special set of permanent molding in the
following sections.
11.1.1 TOOLING FOR CASTING IN METAL MOLDS
A "permanent mold” can yield 50,000 or more pieces of aluminum or magnesium
parts before failure. In typical permanent mold applications, the mold surfaces are coated
with a refractory slurry to increase mold life.
Permanent molding is most likely to be successful if:
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1. Production volume is high (normally more than several thousand pours), and
2. Shape is basically simple and suitable to the process.
The mold cost for permanent molding and the equipment required is more costly
than casting with sand, but the advantages of permanent molding include lower cost
(material and labor), less entrained gas, finer grain structure, smoother surfaces, closer
dimensional tolerances, and lower machining costs. Tooling for permanent molds is
more challenging; the designer must thoroughly understand metal flow, solidification,
and shrinkage. Certain casting problems demand more careful attention. Fillet design is
more critical; mold-wall thickness becomes significant in terms of the cavity volume
enclosed, the alloy cast, proximity to the sprue, and other variables. New problems such
as cyclical thermal balance and casting removal appear.
Tooling for permanent molding, which utilizes gravity feeding, is treated here.
Die casting is distinguished from permanent molding when the molten cast material is
introduced to the mold via pressure injection, and is the subject of the following section.
Parts can be more economically made by die casting up to a certain production rate;
thereafter, permanent-mold casting becomes more attractive (Figure 11.1).
In practice, permanent-mold castings usually weighs less than 20 lb. Die castings
can weigh up to 100 lb., and sand castings can frequently weigh many tons. In many
cases, shapes too complex for metallic molds can be cast in sand. The strength of
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permanent-mold castings exceeds that of die castings partly because correctly designed
gating systems in the former entrap little or no gas. The economics of the process will
depend on production volume (Figure 11.1).
11.1.2 DESIGN FOR PERMANENT MOLDING
A product that is suitable for permanent-mold casting must be carefully studied,
and modified if needed, before the permanent mold can be considered in detail. The
basic features of such a mold design include:
1. Simplicity-to minimize the cost of the mold, turbulence, and the need of
machining
2. Liberal tolerances
3. Foresight in the choice of the parting plane, which largely establishes the
location of the gating and risers
4. Progressive solidification toward the riser from thinner sections remote from it
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Figure 11.1 Cost comparisons for aircraft castings. (Redrawn from
American Society of Metals, Metals Handbook, Metals Park, Ohio, 1970.)
Dimensional standards and tolerances for permanent molding (Tables 11.1 and 11.2)
must also be included in the production design of the casting. Figure 11.2 illustrates
some of the principal design fundamentals.
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11.1.2.1
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Gating System Design
Successful casting once depended largely on an operator's skill. Today, the
product engineer uses fundamental fluid-flow principles for gating design, as shown in
gating for sand casting.
In order to attain better molten metal flow a gating ratio of 1:2:2 or 1:2:1.5 with a
pouring basin, conical sprue, and sprue base well that are proportioned as in sand casting,
is used for reducing heat loss and turbulence. Most molds are gated to fill from the
bottom to the top (Figure 11.3). The metal flows from the sprue base well along the
runner bottom, feeds the riser, and passes through a slot gate along the vertical side of the
casting into the cavity. Note that a horizontal extension of the base runner receives the
initial dross.
Table 11.1 Comparison of sand- and metal-mold casting processes.
Note to
WyskNeed to
add
investm
ent
casting
here
Metals cast
ferrous & non-ferrous non ferrous non ferrous
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Figure 11.2 Design fundamentals for permanent-mold castings.
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Table 11.2 Typical dimensional standards for permanent molds and cores.
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Figure 11.3 A typical gating system for a permanent mold.
11.1.3 Risers
All castings must have directional solidification from thin remote sections toward
the riser in order to achieve proper material feeding. The riser in turn is fed last with hot
metal and has a surface-area-to-volume ratio such that it will freeze more slowly than the
casting.
The concept of casting yield may be used to determine a reasonable estimate of
the riser size for permanent molds. Yield refers to the ratio of the casting weight divided
by the total weight of metal poured. Metals that exhibit greater shrinkage characteristics
require larger risers to fill voids during solidification. Low-shrinkage alloys such as a
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eutectic alloy of aluminum with 12 percent silicon, require little feed metal for a sound
casting. The figures given are once again approximations, because the weight of the
gating system must be estimated and the surface-area-to-volume ratio is not considered.
11.1.4 Vents
Vents are used to discharge the gas in the gating system and mold cavity as fast as
the metal enters the mold. Natural venting along sliding members and at the parting line
is usually inadequate. Additional venting may be added as follows.
1. Cut slots as deep as 0.011 in. (.3 mm) and of suitable width across the parting
seal surface.
2. Drill small clusters of holes 0.008 to 0.011 in. (.2 - .3 mm) in diameter in the
mold wall at a point where venting is needed.
3. Drill holes and install slotted plugs (plug vent).
11.1.5 Mold Material
The mold material is chosen on the basis of three criteria: material cost, the
expected total number of pours required, and the casting alloy. Most common and
suitable for a permanent mold is high-quality pearlitic gray iron, inoculated at the ladle to
achieve uniform grain size and highly dispersed fine graphite has the composition given
in Table 11.3.
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Table 11.3 Typical gray iron composition for permanent molds and cores.
Element
Carbon
Silicon
Manganese
Phosphorus
Sulfur
Composition, %
3-3.5 (with 0.4-0.5%
combined C)
1.6-2
0.5-0.8
0.2 max.
0.05 max
For longer life add:
Element
Composition, %
Chromium
Molybdenum
Nickel
0.6
0.4
0.4
Large castings or high pouring temperatures may require cores of alloy cast iron
or H11 die steel. Undercuts or complex internal features can be formed with sand cores.
11.1.6 VENTING DETAILS FOR LOW-PRESSURE PERMANENT MOLDING
In low-pressure permanent molding, ejection of the casting and venting are the
major problems. The use of ejector pins that bear on overflow wells can reduce the
damage to a casting in any metallic-mold process, as is shown in Figure 11.4. Several
means of venting large flat areas include the use of ribbon vents (Figure 11.4b) and plug
vents (Figure 11.4c).
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Figure 11.4 Ejection and venting details for dies to make low-pressure
permanent-molded parts. (a) Ejector pins must be balanced to provide for uniform
part removal; (b) Typical plug and ribbon vents; (c) plug vent details.
11.2
DIE CASTING
Die casting produces accurate castings in large quantities and with good die life
for non ferrous alloys. Thus, product and process engineers are able to incorporate
features that minimize or eliminate subsequent machining and finishing operations. The
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extent of incorporating features that would otherwise require machining operations, such
as holes and external and internal threads, in the casting depends on the ingenuity of the
designer and the additional die and operating expense.
If inserts are used in the casting process, the time for placing them in the die will
increase the molding-machine operation time. If it is necessary to use slides and cores
operated by separate mechanisms other than those opening the die, these slow down the
operation and increase die and machine expense.
When observing the production requirements for a part that is die cast, it is often
discovered that more labor is expended in machining, and cleaning the casting of fins and
burrs than in the casting operation itself. Therefore, the engineer must consider all
production operations for making the part, whether by the toolmaker, in his own shop, or
elsewhere in determining the economics of using a die-casting process. For example, a
cored hole may be made by a two-part core that leaves a fin in the hole. This fin must
then be removed. Since a drilling or punching operation is required, the mold can be
designed without the metal core in the casting die, making the die less expensive and
possibly speeding up the operation.
Cores for small-diameter holes are normally expensive to maintain in die-casting
molds, and often holes can be drilled less expensively. It is the prerogative of the product
and process engineer to determine which is the most economic production method for a
given design.
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Die-cast parts made of ductile material can be formed, bent, spun, or joined to
another part after casting. They can be pierced, twisted, embossed, swaged, and staked;
and lugs can be used as rivets. Such operations make it practical for the product and
process engineer to consider the use of die casting for a broad variety of assembly
components.
The cleaning of flash depends largely on the design of the part and the die.
Polishing and finishing costs can also be reduced when the designer eliminates re-entrant
corners, avoids flat surfaces, and places ejector pins where marks can be removed or
hidden.
Slide cores are an asset in die casting; nevertheless, their use should be carefully
questioned in the light of subsequent operations and die maintenance costs. Cores save
weight, keep sections more uniform, and thus create sound castings. They help carry the
heat away and reduce the casting cycle of the machine. Often the dies can be vented
through the core. Cored holes can reduce machining, time depending on their accuracy
(size and location).
11.2.1 Minimum Section Thickness
Good casting design dictates the use of as uniform a wall thickness as possible or
one which tapers slightly from the thinnest section remote from the gate to the heaviest
section at the gate. In addition, alloy fluidity dictates the minimum practical wall
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thickness for each die-castable metal (Table 11.4). Walls must be thick enough to permit
proper filling but sufficiently thin for rapid chilling to obtain maximum mechanical
properties.
Table 11.4 Minimum wall thickness for die castings.
Surface area,
Up to 3.9
4.0 – 15.5
15.6 – 77.5
Over 77.6
11.2.2
in.2
Base casting alloy
Aluminum, magnesium
Copper
0.0315 – 0.0471
0.0589 – 0.0787
0.0472 – 0.0707
0.0788 – 0.0982
0.0708 – 0.0982
0.0983 – 0.118
0.0983 – 0.118
0.119 – 0.157
Undercuts and Inserts
Wherever possible, one should redesign a part to eliminate undercuts. In most
cases, such part modification is more economical than the labor required to handle a
loose piece during each casting cycle. In one case, redesigning to reduce undercuts saved
$1.50 per casting on a part that was in production for more than 15 years. Over 4.5
million castings have been made during that time.
Inserts such as bearings, wear plates, bushings, and shafts can be incorporated in
die castings, but they must be easily and precisely located in the die. If they are not
securely placed, they may slip between the dies during the casting cycle and cause great
damage. Inserts must be provided with properly knurled, crimped, or grooved surfaces to
ensure a good mechanical bond between the insert and casting. If the insert is large
relative to the casting, better results will be obtained if the insert is preheated, e.g.,
cylinder liners for automotive engines.
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11.2.3 Cored Holes
Holes can be readily cast in all alloys according to the data given in Table 11.5.
Table 11.5 Maximum depths of cored holes in die castings.
Alloy
Zinc
Aluminum
Magnesium
Copper-based
Minimum
diameter
castable
0.039
0.098
0.078
0.118
1/8
5/32
3/8
5/16
5/16
--
9/16
1/2
1/2
--
Hole diameter, in.
3/16 1/4 3/8 1/2
3/4
5/8
5/8
--
1
1
1
1/2
1 1/2
1 1/2
1 1/2
1
2
2
2
1 1/4
5/8
3/4
1
3 1/8
3 1/8
3 1/8
2
4 1/2
4 1/2
4 1/2
3 1/2
6
6
6
5
11.2.4 Draft Requirements
The amount and location of draft in a die-cast part depends upon how it is located
in the die and whether it is an external surface or cored hole. Draft on the die surfaces
normal to the parting line permits the parts to be ejected without galling or excessive
wear of the die impression (Figure 11.5). The values shown represent normal production
practice at the most economic level. Greater accuracy involving extra close work or care
in production should be specified only when necessary because it may involve extra cost.
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(1) Draft on cored holes is the total for both sides.
(2) Draft for outside wall is 50% of inside wall value.
(3) Values given herein represent normal production practice. Greater accuracy should be
specified only when absolutely required because it usually involves extra cost.
Figure 11.5. Draft allowance for die-casting dies (wall: solid lines: cored holes: dashed lines).
Note that (1) draft on cored holes is the total for both sides; (2) draft for outside walls is 50% of
inside wall value; and (3) values given represent normal production practice.
11.2.5 Dimensional Tolerance
The dimensional tolerance that can be achieved in die casting depends on several
factors.
1. The accuracy to which the die cavity and cores are machined.
2. The thermal expansion of the die during operation
3. The injection temperature and shrinkage of the alloy being cast
4. Normal wear and erosion of the die cavity and cores
5. Position of the movable parts relative to each other during casting
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Surface finish-50 to 125  in. is common, a better finish can be achieved if
needed.
The tolerances for basic linear dimensions in one-half of the die are summarized
in Table 11.6. Another tolerance must be added to the basic tolerance if the linear
dimension is for a part that extends across a movable die section. More tolerance must be
added to dimensions that extend across the parting line.
Table 11.6 Tolerance for dimensions of die castings.
11.2.6 Die Materials
Die-casting die materials must be resistant to thermal shock, softening, and
erosion at elevated temperatures because of the environment in which they are used. Of
lesser importance are heat treatability, machinability, weldability, and resistance to heat
checking. Tool steels of increasing alloy content are required as the injection temperature
of the molten alloy, the thermal gradients within the die, and the production cycle
increase. Dies for use with zinc can be prehardened by the manufacturer in a range of RC
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29 to 34. The higher-melting alloys require hot-work tool steels (Tables 11.7 to 11.8).
11.2.7 Wear of Cores, Slides, and Pins
Cores, slides, and pins must provide abrasion resistance in addition to heat
resistance. Wear can be reduced by the following procedures.
1. Use of contacting materials of differing hardness
2. Use of nitride on one or both surfaces in contact
3. Use of a lubricant on the areas of contact (avoid contamination of the
molten metal)
4. Establishing and maintaining proper clearance between mating parts
5. Polishing the wear surfaces of the mating parts.
Materials for such moving parts are listed in Table 11.9.
Table 11.7. Recommended materials for die-casting dies.
Table 11.8 Normal composition of die-casting steels.
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Table 11.9 Materials for cores, slides, and ejector pins.
11.3
Design Factors for Castings
The part to be made from a casting may be intricate or simple. Intricate castings
can be economical because they may combine many parts into one piece and thus save
the cost of the fabrication and joining of several separate pieces. Sand castings weighing
several tons (such as a locomotive frame) have been made economically used because
several parts are combined into one piece. The more complicated the casting, the more
ingenuity and control required. The simpler the part, the less the cost of the mold and
pattern equipment and hence the less expensive the part. Variations in size and strength
may be more difficult to control in more complex parts.
The design of a part to be cast depends upon the behavior of the material as it
cools, the construction of the mold, and the functions of the part in service. The art of
casting and molding has progressed to such an extent that practically anything can be cast
that is within the size range of the equipment available. It might not be economical to
cast all parts, but technology exists to convert many parts from sheet metal, forged, or
welded design into a casting. To make a casting simple and easy to cast demands the
highest skill and the best judgment on the part of the product engineer and the
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foundryman engineer.
Design factors to be considered include the following:
1. For maximum strength and stiffness, material should be kept away from
the neutral axis. This is important to all designs and is related to the
moment of inertia I, which is a measure of the ability of a cross section to
resist rotation or bending about the axis passing through its center.
The modulus of rupture, which designates the load (pounds per
square inch or newton per square meter) imposed on a beam, is given by
S
MC
I
where S is the modulus of rupture or maximum fiber stress, (lb/in.2
or NT/M2) imposed on metal at greatest distance from the neutral axis, M is the
bending moment, or load times distance from concentrated load
to point under study and C is the distance from netural axis to outer
surface.
2. Keep plates in tension and ribs in compression. This is desirable because
the compressive strength of most cast materials, particularly cast iron, is much
greater than the same materials intensity. The plate distributes the
load over its entire surface and is more effective when placed in tension
rather than compression. Ribs, on the other hand, are most effective when
placed in compression.
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3. Insure that the pattern or part can be easily removed from the mold. This
factor is probably violated more than any other and causes the use of loose
pieces and complicated cores, gates, and risers.
4. Ensure that cores can be removed.
5. Use smoothly tapered sections to eliminate high stress concentrations.
6. Sharp corners and abrupt section changes at adjoining sections should be
eliminated by employing fillets and blending radii (see Figures 11.6 to 11.7).
7. Determine the best location(s) where material should be fed into the part.
8. Does the design avoid the development of hot tears and unsound sections?
9. Does the necessary draft on the pattern interfere with the part design?
10.Can parts be clamped and located easily for machine operations without
interference from fins or excess material at parting lines and junctions of
gates and feeders?
11. Are locating points for chucking or holding indicated on the drawing for
foundry or molding-shop and machine-shop information?
11.3.1 Effect of Grain Size
A relatively large dendrite grain size may be developed in the process of
solidification as a result of the freezing rate or section size of the casting. Large castings
and ingots freeze with coarse grains. Thin-sectioned castings, or castings made in metal
molds, develop a fine grain size owing to rapid freezing.
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Figure 11.6 Stress conditions around hot spot.
Figure 11.7 Design of a corner joint to produce soundness.
Normally, a fine grain size is desirable, since higher ductility and impact strength
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values are obtained at a given tensile strengths level with fine grain size.
Figure 11.8 Design for changes in section thickness.
11.3.2 Hot Spots
Hot spots are the last portions of the casting to solidify. They usually occur at
points where one section joins another, or where a section is heavier than that adjoining
it-at a square corner for instance. The outside of the corner should be rounded to reduce
the section change (Figure 11.7).
Hot spots are subject to unsoundness and to hot tearing. Figure 11.6 shows stress
conditions around the hot spot within the circle. In casting steel and other hightemperature metals, the spot enclosed by the circle must be fed to avoid cavities.
The shape of the part should be such as to enable directional solidification of the
molten metal in unbroken sequence from the farthest part of the casting to the point of
entry. If the casting should "freeze" somewhere along the line ahead of its turn, the
sections between it and the farthest end, which are still liquid, would be cut off from the
supply of feeding liquid metal; unsoundness or hot tears, would result. In view of this,
the casting should be designed to avoid abrupt changes from a heavy to a thin section
(Figure 11.6). Where light and heavy sections join, the thickness of the lighter section
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should be increased gradually as it approaches the heavier section.
Steel is difficult to cast. The design rules for steel castings can be modified for
other ferrous materials. If they are followed in principle, better castings will be obtained
regardless of the material being cast. Six design rules for casting provide important
guidelines for the product engineer.
1. In designing unfed joining sections in L or V shapes, it is suggested that all
sharp corners at the junctions be replaced by larger radius corners, so that these
sections become slightly smaller than those of the arms (Figure 11.8). Reason:
Outside corners of joining sections are positions of extra mass and result in hot
spots. If these positions are not fed from outside sources (by risers) they will be
the location of shrinkage cavities. Cutting off the outside corner is similar to
producing a uniform section and is a fundamental of good design.
Figure 11.9 Unfed joining section.
2. In designing members that join in an X section, it is suggested that two of the arms
be offset considerably (Figure 11.10). Reason: The center of an X section that
cannot be fed by a riser is a hot spot and will result in the formation of a shrinkage
cavity. Offsetting the arms permits the use of external chills by the foundryman to
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produce a section free from cavities. If the design does not permit offsetting the
arms at the X section, then the possibility of placing a core in the middle of the
junction should be considered. This will permit uniform sections with small
stresses through the center (Figure 11.11).
Figure 11.10 Offset the arms of an X section to produce two T sections, thereby
reducing the hot spot.
Figure 11.11 Reduction of a hot spot by coring.
3. Isolated masses not fed directly by risers are details of poor design. They should be
hollowed out with a core construction or constructed of a lighter section. If this
cannot be done, then internal chills should be used. Reason: Areas of heavy metal
attached on all sides to members of much smaller thicknesses are considered
isolated masses. When these areas are so located that the foundry has no
opportunity to feed the heavy portions properly by means of conveniently placed
risers, shrinkage cavities occur within the section.
4. The use of webs, brackets, and ribs at joining sections should be kept to a
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minimum. If they are required as stiffeners for a casting, the poor design effect can
be remedied by the extensive coring of the web, bracket, or rib in the region of the
adjoining section (Figure 11.12). Small brackets will help prevent hot tears and
consequently are frequently used. Reason: Stiffening members at joining sections
are sources of considerable trouble with regard to the formation both of hot tears
and of shrinkage cavities. They increase the mass at the intersection of the
adjoining members. Coring will not impair the stiffening features of these webs,
brackets, or ribs.
Figure 11.12 Cored construction in stiffening members.
5. When the design of a one-piece cast steel structure is very complicated or intricate
or when section thickness variations are large, the piece can sometimes be broken
up into parts so that they may be cast separately and then assembled by welding,
riveting, or bolting. Reason: Very complicated designs often lead to enclosed
stress-active systems and any mold relieving that may be employed by the foundry
would not be sufficient to produce the casting without cracking or failure. In such
cases, appendages, cast-in baffles, and other extraneous members should be
removed from the design and cast or prepared separately, and then welded into
place.
6. Cast members that are parts of an enclosed stress-active system should be designed
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slightly waved or curved (Figure 11.13). Reason: Curve construction or wave
design reduces distortion resulting from contraction of the steel during cooling. A
good example of this type of design is the use of curved spokes in many classes of
wheels (Figure 11.13). In such castings the rim, hub, and spokes may cool at
different rates, thus subjecting the casting to considerable internal stress, Spokes
designed with a wave in them will, under stress, tend to flex, thus preventing
tearing or distortion. Residual stresses can be relieved through heat treatment.
11.3.2 Details of Design for Steel Castings
The greater the shrinkage and the higher the temperature of the metal, the more
difficult the problem of casting. While iron castings are common, and iron is relatively
easy to cast, steel casting is much more difficult, and the principles established for steel
Figure 11.13 Wave construction. Design (A)produced cracked spokes. It is
corrected by the use of five curved spokes as in (B) or the alternative design as in
(C).
castings are described as a review and as an illustration of the more severe conditions.
The practices described may be modified for other kinds of materials.
1. The casting and mold should be so designed that solidification stresses are as small
as possible when the casting is at a temperature just below the solidification
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temperature. It is at this point that steel has poor strength and ductility
characteristics so that it is likely to hot-tear.
2. Steel has low fluidity compared with other metallic alloys. Thus, thin sections
should be avoided, especially when located remotely from the gating system.
3. A poured steel casting will solidify from the outside of the casting inward and the
rate of solidification is about the same regardless of section size; therefore, heavier
sections will take proportionally longer to solidify than lighter sections.
The principal defects in steel castings that result from poor design are hot tears,
cold shuts, shrinkage cavities, misruns, and sand inclusions.
Hot tears. Hot tears are solidification cracks at various points in a casting brought
about by internal stresses resulting from restricted contraction. Sharp angles and abrupt
changes in cross section contribute to large temperature differences within a casting,
which may result in hot tears.
Shrinkage cavities. Shrinkage cavities are voids in the casting brought about by
insufficient metal to compensate for the volumetric contraction during the solidification
of the casting. Shrinkage cavities are more pronounced in areas fed by thin sections.
This results because the thin feeding section will solidify too rapidly to allow the
introduction of additional metal to the casting which has diminished in size because of
volumetric contraction.
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Misruns. When a section of a casting is incompletely filled with metal, it is
known as a misrun. This is usually brought about by solidification prior to the complete
filling of a mold cavity. Casting sections of less than 6mm must be designed with care.
Sand inclusions. When a portion of the mold breaks away or is eroded by the
metal stream a sand inclusion occurs. This may be brought about by abrupt turns and
complicated passages of flow of the metal as it is poured into the casting.
11.4
Case Study #1 – A Simple Cover Plate
The quantity of castings to be made within a period of time determines to a large
extent the cost of the casting. The greater the quantity, the more that can be invested in
pattern or die equipment and the less the casting will cost. Multiple patterns or dies
produce two or more castings in practically the same time it takes to produce one.
Increased quantities resulting in affording better equipment improve the quality of the
casting and permit the spreading of the cost of molds or patterns over more castings with
less development cost per unit.
Choosing between the various casting processes, as well as with other types of
processes requires careful scrutiny. Use of each process is determined by the following
factors: quantity to be produced; material required to meet strength, corrosion, weight,
and appearance conditions; accuracy required; complexity; and cost of subsequent
operations such as machining and finishing.
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For example, say a cover plate for a piece of equipment is required. As the
market for the product develops, improvements are made, quantities increase, and
functions change. In Figures 11.14 to 11.19 note the influence of quantity and function
on the type of material and operation used. For a brief period, rolled steel is used for the
product. Note how considerations of appearance and accuracy played an important part
in the choice of process. The metal molds (Figure 11.19) could not be used until the
activity of the part justified the expensive mold or pattern.
In looking at the figures, assume that a gasket is necessary to prevent oil leakage.
Assume also that the part comes from the foundry or molder with all operations-such as
removal of fins and marks due to parting lines, gates, pushout pins, and cores-performed
to give a piece that is finished except for machining and final finishing operations.
A permanent-mold aluminum cover plate in Figure 11.19 is being applied to
equipment that requires a better seal and a gasket that must be held in a gasket groove.
Figure 11.14 Cast-iron cover plate.
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Figure 11.15 Cast-iron cover plate molded two at a time. Quantity increases;
tools and dies can be paid for in 1 year.
Figure 11.16 Stamped-steel cover plate. Quality of apparatus demands better
appearance, and aluminum is chose. No other finish is required.
Better finish and greater accuracy are required. Bolt holes can be cast and better finish
and accuracy can be had by permanent metal molds. Figure 11.19 shows a die-cast zinc
cover plate. A lacquer finish was applied to harmonize with equipment. Countersink
head screws were necessary, and an insignia on the cover was added. Here the gasket is
cheaper, but sand blasting is required. This cover plate is used when activity has
increased and the accuracy of the part must be improved. The quantity required has also
increased. By using a zinc die-casting alloy, cost of material is reduced and less material
is required in die casting the part. Greater accuracy is obtained. The die-cast part
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Figure 11.17 Sand-cast aluminum cover plate. Cover plat is applied to other
equipment that requires a better seal and gasket that must be held in a gasket
groove. Better finish and greater accuracy are required. (Bolt holes can be cast,
and better finish and accuracy can be had using permanent metal molds.)
Figure 11.18 Die-cast aluminum cover plate.
Figure 11.19 Die-cast zinc cover plate.
requires no finishing operations other than cleaning and application of finish, as the
surface is better than with permanent-molded parts.
11.4.1 Material and Overhead Costs
The material cost is usually not the highest cost of casting. Material costs can be
reduced by using a standard alloys that is used frequently in the foundry. It may be
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expensive to prepare or to adapt equipment to melt special alloys in small quantities.
With the mechanization of foundries to handle sand, and raw and finished
material, by conveyers, special equipment, and molding machines, the overhead costing
rates have increased considerably over the days when sand was hand-molded. The
maintenance cost of such equipment is high because of the sand and dust in the air. In
spite of high overhead costs, the foundries have reduced the cost per pound of castings
through advanced mechanization.
Molding shops have high costs of maintaining molding equipment-with air,
steam, electric power, and water service. "Permanent" molds are also subject to wear and
are replaced after a given service life. This requires expensive machine equipment and
high-priced toolmakers to maintain the quality of the product.
An example of competition between materials and processes might be a single
automobile company die casting an engine block out of aluminum and at the same time
perfecting the sand molding and casting of thin-walled accurate lost foam casting for a
similar engine block.
11.4.2 Auxiliary Operations
The cost of molding is sometimes less than the auxiliary operations of removing
gates, risers, and parting-line material; trimming, cleaning, grinding, and polishing
surfaces; and inspection. Costs are reduced by eliminating these auxiliary operations.
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Weld repair and straightening operations are expensive and their cost can sometimes be
reduced by proper design and specification of material.
11.4.3 Purchasing Castings
Purchasing castings on the straight cost-per-pound basis is simple from the
accounting standpoint, but purchasing castings on a classified weight basis is less
misleading. The most economical arrangement is to deal with an organization that knows
the detailed costs of each operation required to make the casting, pattern, and tools. Then
these costs can be compared with detailed costs of other processes and the correct
decisions can be made.
If the true costs were allowed for, better pattern equipment would be ordered for
the production of the smaller castings. For example, small parts in small quantities may
be more economically made as weldments than as castings. The possible savings
resulting from using the best process, on an overall cost basis, are shown in Table 11.10.
Table 11.10 Actual cost cast versus fabricated at several production levels.
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11.5
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Case Study #2 - Comparison of Cast and Forged Connecting Rods
A major automotive company introduced the cast connecting rod. Since then it
has become the standard of the industry because of its economy and reliability. This
development is a triumph of engineering perseverance and casting design. The success of
this design depends on the use of the inherent strength of ductile iron and its
machinability. The presence of graphite in the matrix reduces the cost of cutting
operations by acting as an internal lubricant and eliminating deburring.
Why cast a connecting rod? The casting process permitted greater freedom of
design because the engineer could put the metal where it was needed. But stress analyses
were needed to determine where the greatest stresses occurred.
In their original work, engineers took conventional forged connecting rods and
ran exhaustive stress analyses. First static stress loads were calculated as follows:
Compressive load =
Pmax  A
Where Pmax , is the maximum gas pressure in cylinder, lb/in.2 and A is the area of cylinder
bore, in.2
S
S
Tensile load = KW   N 2 1  
2

2L 
where K is a constant = 2.84 x 10-5, W is a reciprocating weight, lb, S is the stroke, in., n
is the engine speed, rev/min and L length, pin center to crank center, in.
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Analysis of stresses in a forged rod. Brittle-laquer-stress-coat patterns showed
the location of highly stressed areas in the connecting rod (Figure 11.20). Strain gages
attached to those areas gave the quantitative values of stresses in the rod. The largest
stress was found to be in the center of the column of all rods studied. Similar stress
analysis can now be simply performed using finite element based software programs for
stress analysis.
Next, fatigue testing was carried out with alternating axial loading to produce a
Goodman diagram (Figure 11.21), which represents fatigue strength under combined
alternating loading and a static load. If no failure occurred in 11 x 11' cycles, the rod was
considered to be acceptable.
On the basis of the Goodman diagram the stresses in three particular areas were
plotted (Figure 11.21).
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Figure 11.20 Brittle-lacquer stress analysis.
Figure 11.21 Goodman diagram of fatigue stress levels at three sections of a
forged connecting rod.
Area 1. Compressive loading should result in high tensile stresses normal to the
long axis of the rod on the underside of the wrist pin.
Area 2. Compressive and tensile loads each produce moderate stresses in
the column in the direction of the long axis, but combined they produce a high
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alternating stress.
Area 3. The tensile load should induce high tensile stresses in the rail at
the crankpin end, but, in fact, relatively low stress was induced there because
of the compressive loading.
The validity of the fatigue tests was revealed when actual failure occurred as
predicted when the rods were subjected to engine tests. Failures took the form of
transverse fractures in the center of the column and longitudinal fractures below the
wrist-pin bosses (Figure 11.22). Although the fatigue tests were based on the combined
maximum compression and tension loads, in actual practice the rod never reached such a
load combination at any speed (Table 11.11).
Figure 11.22 Typical connecting-rod fatigue-test failures.
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Table 11.11 Rod loading as a function of engine speed.
The design must be such that the alternating stress in all areas falls within the
limits of the Goodman diagram.
Design of a cast connecting rod of pearlitic malleable iron. After the basic
design parameters had been determined, the design of the then pearlitic malleable-iron
connecting rod was tailored to the casting process. The column was specified to have a
minimum and uniform cross section consistent with good foundry practice (Figure
11.23). Wherever the stress level was low, metal was removed, and it was added where
more stress was found. The ability to redistribute the metal permitted the weights of the
cast and forged rods to be equalized (Figure 11.24).
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Figure 11.23 Typical cross section of a cast connecting rod.
Figure 11.24 Typical design details for the crank-pin and wrist-pin ends of the
cast connecting rod.
Design of gates and risers. To determine the best foundry practice, gating
studies were made using high speed photography and Lucite models. On the basis of the
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tests, the casting was gated at only one end because the meeting of the streams from dual
end gates could cause defects in the highly stressed center of the rod arm.
Risering was designed to achieve directional solidification by tapering the central
web so that solidification began at a point remote from the risers, in this case the center of
the thin arm. Thermocouples placed in test castings at critical points indicated the
sequence of freezing and helped achieve proper riser sizes for both ends of the connecting
rod. Directional solidification was achieved from the center of the arm to each end
despite gating from the small end. The I-beam section of the arm provided additional
chilling, so that solidification began at the arm center where it was critically stressed and
traveled to the risers. Cooling fins were added to the boss and column rail to provide
better local solidification. A match plate showing the gates, risers, and other details of
the arrangement reveals the care that was taken in maximizing the product yield when
using the casting process (Figure 11.25).
Match-plate features. Use of rounded corners, generous fillets, and uniform
section thickness with increasing taper toward the direction of solidification provided for
sound castings. Other general requirements include:
1.
2.
3.
4.
Draft, 4
Stock for machining, 0.060 in.
Fillet radius, 0. 12 in.; corner radii, 0. 06 in.
Dimensional tolerances: connecting rod length 0.01 in.; other dimensions,
0.02 in.; gate removal, +0.06 to -0.03 in.
Inspection features. Of course, not only must the proper melting procedures and
foundry practice be followed but also hot trimming and mechanized inspection
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procedures must be provided. All connecting rods pass through conveyorized sonic,
ultrasonic, and Magnaglow inspection stations, where defective castings are immediately
Figure 11.25 Match plate for the cast connecting rod. Note that 12 are cast in one
mold, gating is from one end, and 3 risers are used on each rod.
removed from the line. Provision for such elaborate inspection procedures is needed in
order to guarantee to the customer that the quality of cast product is equal to or exceeds
the quality of rods produced by any other process.
11.6
SUMMARY
In this chapter, advanced casting techniques and the fundamentals of sound
casting design have been presented. Castings, in some form, represent the best way to
produce many components. If the engineer designs parts that are simple and easy to
produce, there is no doubt the end product will be able to meet international competition.
The principal cost reductions and cost avoidances are those that result from sound
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product design. The engineering student is urged to become familiar with good casting
design for the various casting processes. Since castings are used to a large extent in
practically all of the metal trade industries, it is highly probable that practicing engineers
will be confronted with problems relating to casting design on many occasions no matter
whether their efforts be centered in the functional design of products or in manufacturing,
where the principal concern is the utilization of the optimum process.
11.7
QUESTIONS
11.1. What precision is characteristic of investment castings?
11.2. What is the approximate size of the largest casting that is economically die cast
today?
11.3. From an economic standpoint, for a precision casting of 1 in.3 in volume and daily
production requirements of 2000 pieces, would you recommend permanent
molding or die casting. Explain your answer.
11.4. How could die life be prolonged for the die casting of zinc-based parts?
11.5. If porosity were causing trouble in the die casting department of your plant, what
steps would you take to minimize this complaint?
11.6. Why are the weight and area limitations of zinc-based die casting greater than for
aluminum-based die castings?
11.7. When would you recommend incorporating internal threads on die-cast parts?
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PROBLEMS
11.1The part shown in Figure P11.1 is to be made as a permanent-mold casting. Suggest
what changes in design should be made and explain why. Make a careful sketch to
scale to show the details of the part redesigned for permanent molding.
Figure P 11.1.
11.2Derive an equation for the injection velocity of an alloy in a die-casting operation as
a function of the maximum plunger pressure p. Assume that the shot chamber is large
in cross section compared with the ingate, the cavity is vented through the entire
injection process, the energy at any point in the molten metal stream is a constant,
and all friction and orifice coefficients are neglected.
11.9
REFERENCES
1.
American Foundrymen's Society: Metal Casting and Molding Processes,
DesPlaines, IL., 1981.
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2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
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American Society for Metals' Metals Handbook, 8th ed., vol. 5, Forging and
Casting, Metals Park, Ohio, 1970.
ASM Specialty Handbook, Cast Irons, Edited by J.R. Davis, ASM International,
Materials Park, OH, 1996.
Casting Design & Application, 1997 Reference Handbook, Edited by D.M.
Peters, Cleveland, OH, 1996.
Dantzig, J. A., and Berry, J. T. (eds.): Modeling of Casting and Welding
Processes,II, Metallurgical Society of AIME, Warrendale, Pa., 1984.
Flemings, M. C.: Solidification Processing, McGraw-Hill, New York, 1974.
Gray and Ductile Iron Founders' Society: Gray and Ductile Iron Castings
Handbook, Rocky River, Ohio, 1971.
Heine, R. W., Loper, C. R., and Rosenthal, C.: Principles of Metal Casting, 2nd
ed., McGraw-Hill, New York, 1967.
MacLaren, John L.: "Die Castings," in Handbook of Product Design for
Manufacturing, edited by James G. Braila, McGraw-Hill, New York, 1986.
Malleable Founders' Society: Malleable Iron Castings, Rocky River, Ohio, 1960.
Minkoff, L.: The Physical Metallurgy of Cast Iron, Wiley, New York, 1983.
Spinosa, Robert J.: "Investment Castings", In Handbook of Product Design for
Manufacturing, edited by James G. Braila, McGraw-Hill, New York, 1986.
Steel Founders' Society of America: Steel Castings Handbook, 5th ed., Rocky
River, Ohio, 1980.
Walton, C. F., and Opar, T. J. (eds.): Iron Castings Handbook, 3d ed., Iron
Castings Society, DesPlaines, Ill., 1981.
Zuppann, Edward C.: "Castings Made in Sand Molds", In Handbook of Product
Design for Manufacturing, edited by James G. Braila, McGraw-Hill, New York,
1986.
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