Designing castings for performance

© 2009 ASM International. All Rights Reserved.
Casting Design and Performance (#05263G)

www.asminternational.org

Casting Design
and Performance

Materials Park, Ohio 44073-0002
www.asminternational.org


© 2009 ASM International. All Rights Reserved.
Casting Design and Performance (#05263G)

www.asminternational.org

Copyright # 2009
by
ASM InternationalW
All rights reserved
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First printing, November 2009

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Prepared under the direction of the ASM International Technical Book Committee (2008–2009), Lichun L. Chen, Chair.
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© 2009 ASM International. All Rights Reserved.
Casting Design and Performance (#05263G)

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Contents
Design Problems Involving Junctions . . . . . . . . . . . . . . . . . . 147
Design Problems Involving Distortion . . . . . . . . . . . . . . . . . 155

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
Part I: Casting Design Principles and Practices
Casting Design Issues and Practices . . . . . . . . . . . . . . . . . . . . . 1
Casting Design and Processes . . . . . . . . . . . . . . . . . . . . . . . . . 9
Modeling of Casting and Solidification Processing . . . . . . . . . 37
Part II: Process Design
Riser Design . . . . . . . . . . . . . . . . . .
Gating Design . . . . . . . . . . . . . . . . .
Design for Economical Sand Molding
Design for Economical Coring . . . . .

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Part III: Design and Geometry
Casting Design and Geometry . . . . . . . . . . . .
Design Problems Involving Thin Sections. . . .
Design Problems Involving Uniform Sections .
Design Problems Involving Unequal Sections .

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101
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Part IV: Casting Performance
Corrosion of Cast Irons . . . . . . . . . . . . . . . . . .
Corrosion of Cast Carbon and Low-Alloy Steels .
Corrosion of Cast Stainless Steels . . . . . . . . . . .
Fatigue and Fracture Properties of Cast Irons . . .
Fatigue and Fracture Properties of Cast Steels. . .
Fatigue and Fracture Properties of Aluminum
Alloy Castings . . . . . . . . . . . . . . . . . . . . . . .
Friction and Wear of Cast Irons . . . . . . . . . . . .

Friction and Wear of Aluminum-Silicon Alloys .
Failure Analysis of Castings . . . . . . . . . . . . . . .
Inspection of Castings . . . . . . . . . . . . . . . . . . .

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209
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Appendix: Classification of Casting Defects . . . . . . . . . . . . . . 251
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

iii


© 2009 ASM International. All Rights Reserved.
Casting Design and Performance (#05263G)

www.asminternational.org


© 2009 ASM International. All Rights Reserved.
Casting Design and Performance (#05263G)

www.asminternational.org

Preface
Component geometry is a powerful aspect of casting design in terms of both effective production
and the function of a cast part. Many tools have been developed for casting design, and the examples of
past designs, even those of years ago, provide an important baseline in producing effective castings.
Computer modeling and simulation has greatly facilitated the design process, and the article “Modeling
of Casting and Solidification Processing” provides an extensive review of the subject. In addition, the
complex aspects of configuration design are detailed in a series of articles in the sections on “Process
Design” and “Design and Geometry.” Several of these articles are based on the ASM publication
Casting Design Handbook (1962), which has been out-of-print for many years. Nonetheless, the lessons

are still relevant today, as the basic fundamentals of geometry, metallurgy, and physics
remain unchanged (even within the view of new modern perspectives and the advent of more
powerful analytical or numerical tools).
It is noted that the distinct sections on “Process Design” and “Design and Geometry” are a
somewhat artificial division of topics, because really both process and design are intertwined in
complex ways, especially for castings. In a sense, the “design” is like the fulcrum that leverages these
two important aspects of castings into effective products. It is hoped that this collection of articles
provides a useful reference on casting design. Finally, the performance of cast products is covered in a
series of articles in the last section. Ultimately, the performance of product determines its success.

S. Lampman
February 2009

v


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information. As with any material, evaluation of the material under end-use conditions prior to
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Casting Design and Performance
Pages 18

Copyright â 2009 ASM Internationalđ
All rights reserved.
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Casting Design Issues and Practices*
H.W. Stoll, Northwestern University

DESIGN is the critical first step in the development of cost effective, high quality castings.
Designing a successful casting requires an
integrated, concurrent engineering approach. It
also requires systematic and structured use of
sophisticated computer-aided design software
and casting analysis and simulation software.
In this paper, we discuss these and other issues
impacting good casting design. In particular,
design decisions that drive casting performance

and cost are identified and used as the basis for
a proposed holistic approach to casting design.
This new design philosophy and methodology
is aimed at optimizing both the structural performance and producibility of the casting while
minimizing design time and effort.
In casting, the part being produced and the
tooling used to produce the part interact in complex ways, which effect both the quality and cost
of the casting. This suggests that the design of a
successful casting requires an integrated
approach that considers both functional and
process requirements simultaneously. In the traditional setting, however, the design engineer
typically first decides the geometry of a casting
and then a casting engineer independently develops the mold and process. This decoupled
arrangement often results in more costly castings
with larger safety factors than necessary. In addition, lead times for casting development using
this approach can be excessive.
As manufactures seek to reduce weight and
cost of products, casting has re-emerged as the
manufacturing process of choice in many situations. This is because casting offers the important advantage of being able to produce highly
complex functional shapes quickly and easily.
Cost is reduced because numerous parts and
complex construction and processing typically
associated with built up structures and weldments can be replaced by a single cast part.
Weight is reduced because material can be
distributed to where it is needed and because
sections can be thinner since load does not
transfer across part interfaces (i.e., through
fasteners or welds).
To take advantage of these unique capabilities, castings must be properly conceived and
designed from the start. This means that design

engineers and casting engineers will need to
change the casting design and development process from the historical sequential, decoupled
approach to a more integrated and concurrent
approach. This new approach must be based
on the recognition that the part geometry not
only affects the load carrying functionality of
the casting, but also the mold construction,
mold filling and material solidification processes involved in producing the casting. These
processes in turn affect cycle time, casting
quality, and material properties such as yield
strength, ultimate strength, and fatigue resistance. Casting geometry must therefore be
determined based on both functional and processing requirements. Hence, design engineers
must become more knowledgeable of the casting process, and casting engineers must have a
better understanding of the functional requirements that drive the design. In essence, the new
design philosophy must not only facilitate good
part and process design, it must also teach it.
This paper discusses the issues and practices
associated with good casting design. The focus
of the paper is on casting design in general, and
on sand and permanent mold aluminum casting
in particular. We begin by examining the casting
design process from a variety of design and processing perspectives. Strategies for casting design
process improvement are then proposed which
provide the basis for a new casting design philosophy and methodology. Two possible implementations are presented. The first is a structured
team approach that is intended as a possible means
for quickly improving traditional casting design
practice by integrating the casting geometry and
process design. The second is a longer-term
approach involving the development of casting

design guidelines for the design of lightweight,
high quality, and high performance structural
castings.

Geometry/Material/Process
Interactions
Carefully planned geometry is the secret to
efficient load carrying capacity and acceptable
component performance. It is also the secret

*Reprinted from Proceedings from Materials Solutions Conference ’98 on Aluminum Casting Technology

for achieving high quality (i.e., uniform properties, soundness, etc.) and for avoiding costly
and time consuming problems associated with
pouring and solidification of the alloy. Understanding how casting geometry, the material in
both its liquid and solid phases, and the casting
process interact provides the insight needed to
specify the best casting geometry from a function, form, and fabrication point of view. In
the following, we explore several geometry/
material/process interactions that are pivotal to
good casting design. Much of this discussion
is based on the paper “Cost Effective Casting
Design — Developing a Conceptual Framework for Designing Metal Castings” by
Michael Gwyn (Ref 1). The reader is referred
to this paper for a more in-depth discussion of
many of the topics briefly referred to here.
Fluid life is the ability of the molten alloy to
fill the mold cavity, flow through thin narrow
channels to form thin walls and sections, and
conform to fine surface detail. In addition to

temperature of the molten metal, fluid life also
depends on chemical, metallurgical, and surface
tension factors. Therefore, the fluid life of each
alloy is different. For example, aluminum 356
is considered to have excellent fluid life
whereas the fluid life of aluminum 206 is only
fair to good. Fluid life determines the minimum
wall thickness and maximum length of a thin
section. It also determines the fineness of cosmetic detail that is possible. Hence, knowing
that an alloy has limited fluid life suggests that
the part should feature softer shapes (i.e., generous radii, etc.), larger lettering, finer detail in
the bottom portion of the mold, coarser detail
in the upper portions of the mold, more taper
leading to thin sections, and so forth.
Solidification Shrinkage. Shrinkage occurs
in three distinct stages: liquid shrinkage,
liquid-to-solid shrinkage, and solid shrinkage.
Liquid shrinkage is the contraction of the liquid
before solidification. Liquid-to-solid shrinkage
or solidification shrinkage is the shrinkage that
occurs as the liquid’s disconnected atoms
and molecules form into the crystals of atoms
and chemical compounds that comprise the
solid metal. Solid shrinkage is the shrinkage
that occurs as the solid metal casting cools to


2 / Casting Design and Performance
ambient temperature. Although liquid shrinkage
is important to the metal caster, it is not an

important design consideration. Solidification
shrinkage and solid shrinkage, on the other
hand, are extremely important and must be
carefully considered during casting design.
Different alloys have differing amounts of
liquid-to-solid shrinkage (e.g., aluminum 356
has little while aluminum 206 has moderate to
large). Most importantly, there are three different types of solidification shrinkage: directional, eutectic, and equiaxed. In alloys such
as malleable iron and carbon steel, which solidify directionally, solidification moves along
predictable pathways determined by the casting
geometry and thermal gradients in the mold.
For example, solidification will typically begin
at the mold wall and move perpendicularly
toward the center of the part. This is called progressive solidification. Solidification will also
begin in cooler regions where the mold surface
area to metal volume ratio is large and travel
toward the hotter regions of the casting. This
is called directional solidification. The key is
to configure the part geometry so that directional solidification can occur before progressive solidification shuts off the source of
liquid metal supply (the riser). Without proper
pathway geometry (e.g., risering and tapering),
voids or pores due to isolated internal shrinkage
can result.
In eutectic-type solidification, the liquid
metal cools and then solidifies very quickly all
over. This behavior minimizes internal shrinkage and the need for risers and makes this type
of alloy the most forgiving of the three. Eutectic-type materials that have very little solidification shrinkage like gray iron often require no
risering at all. The key geometric concern for
eutectic-type solidifying alloys like aluminum
356 which have small but appreciable solidification shrinkage is to ensure that the avenue of

liquid metal supply stays open and functioning
all the way to final solidification.
In addition to solidifying both progressively
and directionally from the mold walls, alloys
that exhibit an equiaxed solidification behavior
also begin to solidify throughout the liquid,
forming mushy regions consisting of equiaxed
islands of solid. These equiaxed islands can
block the avenues of liquid metal supply
making these alloys difficult to feed. To offset
this tendency, regions that solidify in an
equiaxed-type manner should be designed to
have small thermal gradients, that is, to be as
thermally neutral as possible. Therefore, thermal mass in these regions should be spread
out and distributed uniformly throughout the
region. This causes the shrinkage to be
distributed as microscopic pores throughout
the volume of the casting. Although the thought
of having microscopic holes in the casting is
disturbing, the effect on mechanical properties
is greatly minimized by the small size, rounded
shape, and uniform dispersion produced by
using the proper geometry for this type of alloy.
Also, a uniform dispersion of very small pores

is clearly preferable to large, irregular pores
concentrated in possibly critical regions of the
casting that could result from less appropriate
part geometries.
Solid shrinkage is often called pattern

maker’s shrink because the tooling must be
properly sized so that the part will shrink to
the desired final size and shape upon cooling.
Solid shrinkage is critical for two important
reasons. First, the shrinkage must be predicted
and then built into the patterns/dies and corebox
dimensions. If this is not done correctly, then
the tooling will need to be modified iteratively
to achieve an acceptable production casting.
This adds time and cost to the design cycle
and introduces quality risk in the final product.
Second, as the casting cools, it may not be able
to shrink uniformly because some regions are
stiffer than others. This can result in undesirable residual stresses and/or undesirable warpage. Creating geometry’s that make shrinkage
predictable and that avoid residual stress and
warping is therefore highly desirable.
Slag/Dross Formation. Slag is typically
composed of liquid nonmetallic compounds
(usually fluxed refractories), products of alloying, and products of oxidation in air. Dross
refers to nonmetallic compounds produced
primarily by the molten metal reacting with
air. Some molten metal alloys are much more
sensitive to slag/dross formation than others.
Castings made from these alloys are much more
likely to contain nonmetallic inclusions.
In addition to process and quality control techniques, part geometry can be used to dramatically reduce the likelihood of nonmetallic
inclusions. For example, for castings made
from alloys that have buoyant slag/dross, the
probability of having an inclusion in a critical
machined surface can be reduced by designing

the part so those surfaces are in the lower portion of the mold. Similarly, rigging design
(i.e., configuration of sprues, runners, and
gates) can be designed to control the amount
of oxidation that occurs due to turbulent flow
and entrained air.
Pouring Temperature. Molds used in the
casting process must withstand the extremely
high temperature of molten metal. Often,
proper casting geometry can help make the
mold robust against this thermal abuse. Also,
recognizing the undesirable effects of high
pouring temperature on casting quality can
help. For example, high pouring temperatures
can lead to poor as-cast surface finish due to
metal penetration into small sand cores. Therefore, when pouring temperature is high, it is
often advisable to machine rather than core
small holes and other small features.
Fluid Flow. Another key geometry/process
interaction involves the flow of molten metal
into the mold cavity. As mentioned previously,
turbulent flow through gates and other channels
can effect the amount of oxidation and consequent dross formation that occurs. Another consideration is the force generated by the molten
metal as it flows into the mold cavity and the

turbulence of the flow in the cavity since both
of these effects can displace cores and erode
mold walls, especially sharp edges and high
detail features. Steep thermal gradients can also
arise due to fluid flow. If the flow separates to
pass around cores and other features and the

joins together again, weld lines, nonmetallic
inclusions, and other flaws can occur due to
cooling and oxidation of the flow front. In order
to minimize undesirable effects of fluid flow,
the casting must be poured slowly. Unfortunately, this gives the molten metal more time
to oxidize and increases process cycle time.
Undesirable interactions due to fluid flow
effects can often be reduced or eliminated by
designing the casting geometry and rigging as
a system.
Heat Transfer Considerations. The geometry must also be selected with an understanding
of the heat transfer conditions involved. High
pouring temperatures mean that large amounts
of heat must be transferred into the mold. If
the geometry is such that the heat cannot
escape, a hot spot is likely to occur. For example, narrow peninsulas or tight corners of mold
material surrounded by molten metal will get
hot very quickly and as a result, solidification
of the molten metal in these regions will be
slower than surrounding regions. This creates
the possibility of hot tears or shrinkage pulls
because the hotter material will have less tensile strength and is therefor less able to resist
internal forces that develop due to solidification
and solid shrinkage. Voids can also form
because liquid metal supply paths close off
before the material in the region of the hot spot
is fully solidified.
Just the opposite situation occurs when sharp
corners or narrow peninsulas of molten metal
are surrounded by mold material. In these

cases, the molten metal cools and solidifies very
quickly. This is generally a desirable situation.
However, if cooling is too rapid, it can cause
cold cracking due to stressing of the solidified
skin or thin region by solidification shrinkage
occurring at a slower rate in more massive adjacent regions. Also, difficult to machine or undesirable material properties may result from to
rapid cooling of some alloys.
Geometry/Alloy Interactions. In most
cases, it is the combination of material properties possessed by a particular alloy that determines the most desirable casting geometry.
For example, gray iron has a moderately high
pouring temperature, excellent fluid life, and
very small, eutectic type solidification shrinkage. As a result, accept for the danger of hot
spots due to its relatively high pouring temperature, gray iron is very casting friendly. It’s
excellent fluid life permits fine detail and thin
sections and its low solidification shrinkage
provides considerable geometry latitude.
Aluminum 356 has a low pouring temperature, excellent fluid life, and more eutectic type
than directional type solidification. The low
pouring temperature makes it an excellent candidate for precision castings. Also, its excellent


Casting Design Issues and Practices / 3
fluid life permits fine detail and thin walls everywhere. However, although still relatively small,
solidification shrinkage is significant enough to
warrant consideration especially with respect
to risering, section size, and feeding pathways.
Also of concern is the sensitivity of aluminum
to dross formation. Dross can be a particular
problem because the specific gravity of aluminum oxide is close to that of molten aluminum
and hence buoyancy does not aid separation.

Carbon steel is at the opposite end of the spectrum. Steel has a very high pouring temperature,
poor fluid life, and large, directional type solidification shrinkage. This combination of material
properties makes steel very casting unfriendly.
As a result, careful attention to casting geometry
is essential. Because of its poor fluid life and high
pouring temperature, fine detail and thin sections
are difficult. Most importantly, because of its
large solidification shrinkage, feeding of the
casting is a great concern. Risers need to be large
and the geometry must be carefully designed
to ensure proper feeding of the casting. Because
of its unfavorable combination of properties,
steel and materials like it, require softer shapes
(i.e., large radii, rounded shape, large lettering,
no sharp detail) compared to casting friendly
materials such as gray iron.

Cost Drivers
A second way to look at the design of a casting is to understand how design decisions
regarding casting geometry drive total cost of
the casting. By total cost, we mean the sum of
all costs, both the direct and indirect, that result
from the design, production, distribution, use,
and salvage of the casting over its lifetime.
Although all components of total cost are
important, we are particularly concerned in this
paper with how design decisions drive both the
direct cost associated with production of the
casting and the cost of designing, building,
and proving out the tooling. This cost can be

calculated on a per unit basis as follows,
CT
C0 tcycle
Cost ẳ
ỵ Cc ỵ V Cm ỵ
ỵ Cs
N
Y

(Eq 1)

where:
CT = total tooling cost ($)
N = lifetime number of castings
CC = cost of coring ($/unit)
V = total casting volume (in3)
Cm = alloy cost ($/in3)
C0 = casting equipment and labor cost ($/hr)
tcycle = total casting lead time (hr)
Y = yield (useable castings/N)
Cs = cost of secondary processing ($/unit)
It is important to note that total tooling cost (CT)
includes all cost associated with tooling including
the cost of pattern and corebox construction, the
cost of producing and inspecting the first article,
and the cost of iteratively modifying the tooling
to meet specifications. Also, material volume (V)

includes not only the volume of the casting, but
also the volume of the risers, runners, and sprues

used to feed the casting. Total casting cycle time
(tcycle) is given by the following,
tcycle ẳ tnp ỵ tbuild ỵ tcast ỵ tcool ỵ ttrim

(Eq 2)

where:
tnp = non-productive time (hr)
tbuild = mold build time including core placement (hr)
tcast = time to pour the casting (hr)
tcool = time to cool to ambient temperature (hr)
ttrim = time to remove gates, risers, etc. (hr)
Since many castings involve more than one
core, the per unit cost of coring (CC) is calculated as,
Cc ẳ


nc 
X
C00 t0cycle
0
V 0 Cm
ỵ
0
YC i
iẳ1

(Eq 3)

where:

nC = number of cores
V0 = volume of core material (in3)
0
= cost of core material ($/in3)
Cm
0
C0 = core making equipment and labor cost
($/hr)
0
tcycle
= core making cycle time (hr)
YC0 = yield (useable cores/lifetime number of
cores)
Similarly, secondary processing may involve
more than one process such as machining, heat
treating, welding, painting, and plating. In addition, processes such as machining might involve
several different operations (e.g., drilling,
milling, grinding, etc.). The per unit cost of
secondary processing is therefore calculated as
Cs ẳ


ns 
X
C000 t00cycle
CT00 ỵ
YS00 i
iẳ1

(Eq 4)

where:
ns = number of secondary processes
CT00 = secondary process tooling cost ($/unit)
C000 = secondary process equipment and labor
cost ($/hr)
00
tcycle
= secondary process cycle time (hr)
YS00 = secondary process yield (useable castings/N)
Equations 1 through 4 are very clear as to
what should be done to reduce the per unit
production and design cost:






Design to minimize tooling cost
Design to minimize material cost
Design to minimize process cycle time
Design to maximize yield
Minimize the number of cores and secondary
processes

By looking at how geometry decisions effect
the sources of cost in the above equations, it is
possible to make geometry decisions that reduce
cost. For example, both tooling cost and

production cost will be reduced by selecting the
parting plane early in the design and then creating
geometry that minimizes the number of undercuts
and other features that must be cored. Locating
riser and gate contacts at easy to access areas on
the casting will reduce trimming time. Also,
locating riser and gate contacts so that they don’t
interfere with machining fixture targets will
reduce trimming time, the cost of the fixture,
and possibly machining cycle time because fixturing will be easier and more consistent. Designing
so that critical dimensions do not cross the parting
line will decrease build time and increase yield
since the positional accuracy required between
the cope and drag is reduced.

Shape Optimization
Metal casting offers two unique and very
desirable design advantages: (1) metal mass
can be located exactly where it is needed and
(2) complex, three-dimensional geometry is
readily created. By properly capitalizing on these
advantages, part geometry can be created that
minimize both weight and cost of the part. For
example, the use of continuously varying section
geometry that fully utilizes the material strength
while also satisfying deflection requirements is
readily achievable. In addition, many parts can
be consolidated into one part, thereby eliminating piece part fabrication cost, assembly cost,
and all the indirect costs, interfacing information, quality risk, and manufacturing complexity

associated with built-up parts and weldments.
Shape optimization is the design perspective
that seeks to leverage these advantages. This
practice has been greatly facilitated by the
development of powerful engineering workstations and solid modeling software that significantly enhances the engineer’s ability to
visualize complex three-dimensional geometry
and to analyze stress levels and deflections of
complex three-dimensional shapes.

Rigging System Design
The rigging system includes the system of
sprues, runners, gates, risers, and chills that
channel and control the flow of liquid metal into
the mold cavity, feed the casting as it solidifies,
and control the heat transfer and rate of solidification in critical regions. Rigging system design
specifies the size, dimensions and location of
sprues, runners, gates, risers, and chills that comprise the system. In the traditional approach, an
expert casting engineer designs the rigging system, usually after the geometry of the casting has
been specified. Rigging design decisions typically
include selection of the following: orientation of
the cast part, parting line, potential sites for chills
and chill types, sprue height and location, runner
types and configuration, ingate sites, choke area
(smallest cross-section area present in the flow
system), riser sites and configuration, and pouring
rate and temperature.


4 / Casting Design and Performance

Casting Process Simulation
In casting process simulation, comprehensive
modeling of the intended production process is
performed in order to determine the size and
shape of sprues, runners, gates, and risers. A
variety of simulation software for performing
this type of simulation is available. In addition,
methodologies have been developed to understand and predict the size and location of process related defects (microporosity, etc.). See
for example references [2–6]. Using these
methodologies, the rigging system design can
be varied in the foundry system simulation to
evaluate how defect size and location are to
be controlled and/or eliminated.
Using computer simulation early in the
design process can greatly reduce the amount
of guess work involved in specifying cost effective and functionally acceptable casting geometry. Computer based casting process simulation
offers two important advantages: (1) design
iterations and what-if analysis are much easier
to perform and (2) the physics engine underlying the simulation software provides a consistent and predictable science base for casting
design. This allows the casting geometry and
rigging system to be specified and optimized
as a coordinated system. Most importantly, it
allows evaluation of the overall design before
tools are cut and the design is irreversibly committed to hardware. When used properly, the
result is a substantial reduction in design time
and tooling iterations. It is extremely important
to note however, that the use of casting process
simulation software is not, in itself, a viable
substitute for early input of experienced tooling
and foundry engineers. Rather, it is a very

powerful tool that helps leverage and assist
the team approach.

Casting Design Improvement
Strategies
Casting design is an iterative process (Fig. 1).
The problem of design is typically formulated in
terms of functional requirements and constraints

that must be satisfied. Functional requirements
relate to the functions the part must provide
while constraints relate to the form (shape, size,
surface finish, precision, etc.) and processing
(parting line, draft, section thickness, etc.)
requirements that constrain the geometry that
can be selected. Based on the problem formulation, an initial design is created. This design is
then evaluated and modified iteratively until an
acceptable design is achieved. Typically, the
redesign is guided by the design information,
insight, and understanding developed in the evaluation step. To be acceptable, the design must
satisfy all functional requirements and
constraints.
When the traditional approach to casting
design is examined, we see that the iterative
process characterized by Fig. 1 is essentially
repeated at least two and perhaps several times
as shown in Fig. 2. First the design engineer
goes through the iterative design process to
specify the casting geometry. This geometry is
then passed on to the casting engineer who

repeats the iterative design process to specify
the rigging system and apply pattern maker
shrink to the casting dimensions. Problems discovered during rigging system design can generate additional iterations if casting geometry
changes are required. Additional iterations to
the casting geometry and rigging system design
may also be required during tool fabrication
and preparation for production of the first article. Finally, iterative changes to the tooling
and perhaps the casting and rigging system
geometry may be necessary to tweak the design
to meet production requirements.
Excessive design iterations can adversely
impact the casting design in two important ways.
First, design iterations significantly increase
design cost and time. Second, design iterations,
especially those performed late in the process,
can lead to suboptimal design. The result is a
casting that falls short of cost, weight, and
performance targets. Such designs give casting
a bad reputation and are a disappointment to all
concerned.
Several strategies for improving the traditional casting design process are possible based
on the design perspectives discussed above.
These are summarized as follows:
1. Design the casting geometry and casting
process as a coordinated system by integrating shape optimization and rigging system
design into one concurrent process. Consider
geometry, material, and process interactions

Fig. 1

Iterative model of the design process

Fig. 2

Traditional casting design process

2.

3.

4.
5.

and design related cost drivers from the
beginning as part of the process.
Develop a thorough understanding of all
customer needs including downstream processing constraints before beginning the
design.
Focus on creating an acceptable initial
design. By spending the time up-front to create the best possible initial design, a large
number of lengthy analyze-redesign iterations
are avoided. The evaluation phase should
confirm the design rather than create it.
Use casting process simulation and other
modern computer-aided analysis and inspection methods to quickly optimize the design.
Develop a consistent, well defined science
base for casting design in the form of
casting design guidelines and structured
methodologies.

The goal of these strategies is to shorten the
design cycle and help ensure that the best possible casting design is created. In the sections that
follow, we propose some possible approaches
for implementing these strategies.

Structured Team Approach
Strategies 1 through 3, and to some extent,
strategy 4 can be implemented very easily and
quickly by adopting a structured team
approach. By a structured team approach, we
mean that the casting design is performed using
a multi-disciplinary team and a structured
design methodology. The goal of the team
approach is to have all required product and
process knowledge available when the key
early design decisions are being made. In a
structured design methodology, the overall
problem of design is broken down into a series
of sequential, easier to perform steps that proceed from the general to the specific. Excessive
iteration and long design times are avoided by
performing each step in a thorough and disciplined manner. In general, each step in the process can be further subdivided into steps to
create a hierarchy of structured methodologies.
To illustrate the structured team approach,
we propose the simple methodology shown
schematically in Fig. 3. This approach recognizes that not all members of the team can be
available for designing the casting on a continuous basis. Team meetings are therefore scheduled at which all salient aspects of the design
are reviewed and discussed. All members of


Casting Design Issues and Practices / 5

a casting engineer, a tooling design engineer,
and perhaps one or more specialists who are
familiar with casting process simulation
software, finite element analysis, fracture
mechanics, non-destructive evaluation (NDE)
techniques, and so forth. In addition, it is essential that the end customer, the foundry that is to
make the casting, and others concerned with
secondary processing that is to be farmed out
be properly represented on the team. This not
only helps ensure that all customer and processing needs are appropriately considered, it also
makes it possible to rapidly negotiate changes
to the design specification when necessary,
and to quickly assess cost consequences of
design decisions.
Meeting 1: Clarify the Design Problem.
Clarifying the problem consists of developing
a general understanding of the cost, performance, and manufacturing goals and constraints
of the design. A typical agenda for this meeting
might include the following:

Fig. 3

Simple structured team approach

the team must be present at these meetings. The
purpose of the meetings is to establish design
direction, make key design decisions that
require input and consensus from all team
members, make sure that all process constraints
and requirements are being properly considered, and resolve conflicts and impediments to

the proposed design. The outcome of each
meeting is a set of action steps to be implemented by individual team members. In this way,
all team members are kept informed and participate in the design decision making process. At
the same time, the actual detail work of creating
the design is delegated to specific team members according to the skills and knowledge
required. In the following, we briefly discuss
each step of the methodology and imagine
how a casting would be designed using this
methodology.
Step 1: Form Team. This is the pivotal first
step in the methodology. Unless the arrangement is formalized in some way, it often is
difficult to get effective collaboration between
the design and casting engineer early in the
casting design process, especially before the
casting geometry is defined. By being formally
assigned to a team, each individual team member takes personal responsibility for the design
from the beginning. This fosters and facilitates
the kind of collaborative attitude that is essential for good casting design.
All stakeholders who have an interest in the
casting should be represented on the team. A
typical team might include a design engineer,

1. Review product background.
2. Customer requirements and design objectives.
3. Expected annual production volumes and
target costs.
4. Geometry concepts and alternatives.
5. Material and processing options.
6. Foundry and secondary processing locations.
7. Potential geometry/material/process interactions.

8. Develop a preliminary configuration design.
9. Make assignments to team members to
create the initial design.
As mentioned above, it is essential that all
team members be present at each team meeting.
For example, although the analyst may not be
actively involved with the design before the
casting geometry is fully defined, it is
extremely important that he or she participate
in the early design decisions that lead to the
proposed geometry. In this way, the analyst
knows all the needs of the design problem and
is familiar with the reasoning behind the particular geometry that will eventually be analyzed
and optimized.
Step 2: Create the Initial Design. The initial design establishes the detail layout of the
casting geometry. It includes the configuration
and parametric design of the part together with
the rigging design required to fill the mold cavity and feed the solidifying casting. Configuration design involves the determination of what
features such as walls, holes, ribs, etc. will be
present and how these features will be
connected to provide the desired form, fit, function, and manufacturability (e.g., parting line,
coring, and draft for low tooling cost, short
cycle time, and minimal trimming and secondary processing). Parametric design involves
the determination of dimensions, tolerances,
and exact material specifications needed to
meet durability, stiffness, and/or natural frequency targets. As discussed previously, rigging design involves the location and sizing of
the sprues, runners, gates, and risers.

As a general approach, a preliminary configuration and rigging design might be proposed in
Meeting 1 by the team as a whole. Using this as

a starting point, the design engineer and casting
engineer would work together to develop the
details of the configuration design, seeking
input and consensus from various team members as necessary. Once the initial casting and
rigging configuration has been firmed up, a
preliminary parametric design would be
performed. The goal of this task is to quickly
determine section dimensions, secondary processing requirements, and material property
requirements using simple strength of materials
methods or, if necessary, a rough finite element
analysis. Once the approximate parametric
design is complete, the overall design is evaluated and modified to minimize cost. This is
easy and straightforward to perform with a
minimum of analysis and iteration because the
casting geometry, secondary processing, and
rigging system have been conceived and developed as a coordinated system with input from
all team members. Hence, all of the information
needed to make quality design decisions is
readily available.
Meeting 2: Refine and Approve the Initial
Design. The goal of this meeting is to react as a
team to the initial design and to make any
adjustments or modifications deemed necessary
by general consensus of the team. This is the
time when all design and processing issues
should be discussed and resolved. If there are
significant impediments to the design as proposed, these should be resolved before proceeding to Step 3. A typical agenda for this meeting
might include the following:
1. Review the initial design.
2. Identify impediments, potential undesirable

interactions
and
performance
and
processing concerns.
3. Discuss all design-related costs to ensure
that the best casting geometry from a total
cost standpoint has been identified.
4. Make assignments to team members to work
out solutions to various impediments and
schedule a follow on meeting, or
5. Approve the proposed initial design and
make assignments to team members to refine
and optimize the design.
Step 3: Refine and Optimize the Design.
Once the team is confident that the initial casting
geometry and rigging design is the best solution
possible, the effort required to optimize details
of the casting geometry and rigging system by
computer analysis can be justified. The goal of
this step is therefore to computer model the
design and iteratively improve it until all aspects
have been appropriately optimized. The amount
of effort expended on this step will depend on
how important it is to optimize the casting. For
example, if weight and/or material cost is critical, extensive effort to minimize the amount of
material used can be justified. Similarly, if safety
is an important issue, comprehensive analysis to

6 / Casting Design and Performance
ensure acceptable fatigue life and reliable detection of flaws can be justified. The key to this step
is to start with a casting geometry that is close to
the optimum. This will minimize the time,
analysis effort, and number of iterations required
to converge to the optimum design.
Meeting 3: Approve the Final Design. The
result of Step 3 will be a fully specified casting
design including the detailed casting geometry,
parting plane, draft, coring, riser locations and
sizes, and plumbing system design. In addition,
the finished part design including machining,
heat treating, and so forth will be fully
specified. The purpose of Meeting 3 is to formally review the finished design as a team and
approve the design for release to manufacturing.
When the structured team approach is performed
properly, the final design will almost always be
approved. However, if the team decides that the
design is not ready to be released, then appropriate action plans for correcting design deficiencies
must be developed and implemented. One or
more follow-on meetings may then be required
before the design is released.
Although painful at times, not releasing the
part until it is ready helps insure a minimum
number of tooling changes and tweaks and, in
the long run, is the most cost effective policy.
By strictly adhering to this policy, the casting
design should proceed quickly and smoothly
to first article and production with little or no
modification. When this is the case, the team

knows that it has done its job well.

facilitate design for damage tolerance,
design for manufacture, and design for
inspection.
2. Capture and reduce to practice the latest
research results quantifying the relationships
between casting processes, microstructure
characteristics, cyclic (fatigue) properties,
and component function for cast aluminum
and magnesium alloys.
3. Facilitate the development and implementation of design strategies that seek to leverage
part family concepts (i.e., group technology)
and to standardize casting geometry, rigging,
and tooling features.
4. Help the design engineer make better casting
design decisions when input from experienced
casting engineers is not readily available.
This goal recognizes that many companies
may not have deep in-house casting expertise.
As shown in Fig. 4, the guidelines would be
organized in a manner similar to the structured
team approach discussed previously. The guidelines associated with each step in the process
are briefly discussed as follows.
Step 1: Clarify the Problem. The purpose of
these guidelines would be to help the casting
design team to properly understand the problem
of design by considering all customer needs

including downstream tooling and processing

needs. These guidelines might consist of a stepby-step structured methodology for gathering
and evaluating needs together with one or more
check lists formulated to ensure that all design
requirements and constraints are considered.
They would also include details regarding design
strategy and standardization objectives. Ideally,
the design process would not continue until all
items have been checked off and appropriately
evaluated.
Step 2: Create the Initial Design. This step
includes the iterative development of the configuration design, parametric design, and rigging
design. Guidelines for configuration design would
include high level design for light metal casting
guidelines and recommended practices. Such
guidelines will recognize the important metallurgical characteristics of the foundry alloy and
mechanical engineering structural characteristics
that drive the geometry of good casting design.
These will include the geometry, material, and
process interactions discussed earlier as well as
best practices for communicating design information to the foundry and design for guidelines pertaining to downstream considerations such as
casting inspection, heat treating, and machining.
Guidelines for parametric design would
include best dimensioning and tolerancing

Casting Design Guidelines
Casting design guidelines make needed alloy
and process information readily available to the
design and casting engineer in a timely fshion
and in a form that is easy to understand and
apply. Such guidelines typically consist of

design rules suggested best practices, structured
design methods, check lists look-up tables,
graphs and charts, and computerized design tools
that can be used by the casting design team to
perform each step of the design process.
In this section, we present a vision for the development of comprehensive casting design guidelines. The goal of the envisioned guidelines is to
facilitate the design of high performance castings
that are also easily manufactured in large production quantities for a reasonable cost. Specific
development objectives include the following.
1. Provide the sophisticated design information
needed to design light metal cast structural
component systems for use in safety critical
automotive applications. Such castings
must be as light as possible and also manufacturable in large production quantities for
a reasonable cost. This implies that every
section must be optimized to use an absolute
minimum amount of material while satisfying durability, stiffness, natural frequency,
and other structural and performance targets.
The design guidelines must therefore

Fig. 4

Structure of guidelines for light metal casting design


Casting Design Issues and Practices / 7
practices, material properties including those
relevant to the geometry, material, and process
interactions discussed earlier as well as structural properties needed for proper consideration
of damage tolerance, etc. The guidelines would

follow existing best practices such as those
given in standard texts on automotive component design and machine design (Ref 7 and 8).
The major difference would be in the way
material properties such as finite life fatigue
strength would be estimated. Rather than using
the traditional casting factor, the design guidelines would provide charts and property estimation procedures based on recent and developing
research results (Fig. 5 and Ref 9 and 10).
Rigging design would include the approximate configuration and parametric design of
the risers, gates, runners, chills, insulation, etc.
to feed the casting, prevent porosity formation,
and minimize cycle time. As discussed previously, rigging design plays an extremely important role in the development of a successful
casting design. As such, it must be considered
early in the design process to achieve the goals
outlined above. In addition, many of the computer simulations to be applied in the computer
analysis step (see Fig. 4) require that the rigging system be modeled along with the part.
The inclusion of as a part of the casting design
represents a major departure from traditional
casting design practice. Hence, the development of comprehensive guidelines that both
teach and ensure good practice is essential.
One approach that might be considered is use
of computer-aided rigging design tools such as
that described in Ref (11).
Once an approximate rigging system design
has been developed, it becomes possible to consider the casting geometry and rigging system
as a coordinated system. This represents an
important opportunity to improve the overall
initial design quickly with a minimum of analysis and iteration. Development of guidelines for
this step might include extensive use of a computer automated rapid prototyping (CARP)
environment such as that shown schematically
in Fig. 6.

This graph illustrates how material properties
might be expressed as a function of section
geometry. Developing such graphs will obviously
require a focused research effort.

Computer Analysis. This step is necessary
to meet the stringent weight, performance, and
cost goals described above. It would be performed using computer based analysis and
redesign methods to facilitate rapid convergence to the optimum coordinated casting
geometry and rigging system design. As discussed in Ref 12 the computer analysis might
begin with a casting process evaluation to estimate local material properties and to determine
if and where flaws (porosity, etc.) will form.
This would be followed by a casting inspectability evaluation to determine the smallest flaw
that can be reliably detected in critical using
modern NDE inspection techniques. Results of
these evaluations would then be used in a damage tolerance evaluation to predict component
durability and fatigue characteristics.
Using component weight as the objective
function and performance, cost, and quality targets as the constraints, these evaluations would
be incorporated in an iterative optimization procedure that would modify component and rigging system geometry until an acceptable
minimum weight design is achieved. It is
expected that the guidelines for this step would
consist of a procedure for using standard, commercially available finite element and casting
process analysis software together with special
computer programs and math based simulation
techniques to perform the optimization see Ref
12 and 14.

Summary

A broad range of casting design issues and
practices has been reviewed. Based on this, a
new casting design philosophy and methodology is proposed. The structured team approach
is intended as a possible means for quickly
improving traditional casting design practice
by including early consideration of downstream
tooling and processing constraints early in the
design process. The development of casting
design guidelines represents a longer-term
approach for reducing design cycle time and

Fig. 5

Fig. 6

Computer automated rapid prototyping
environment (CARP) Source: Ref 13

improving the quality and performance of
lightweight structural castings.
The methodology used for both approaches is
to comprehensively understand the problem of
design, quickly define an initial casting geometry and rigging system design that is close to
target requirements, and then refine the total
design iteratively as a coordinated system using
computer based analysis tools. Both approaches
are predicted on the underlying assumption
that, given appropriate design and processing
information and guidance at each step, the
design will converge quickly to the best possible

result.
It is important to note that development of
design guidelines such as those suggested in
this paper require a substantial research and
development undertaking. It is also important
to point out that the proposed methodologies
represent starting points for casting design
process improvement. It is expected that the
effectiveness and usefulness of these methods
will increase as they are used and improved
over time by casting design professionals and
the foundry industry.

REFERENCES
1. M.A. Gwyn, “Cost Effective Casting
Design,” presented at the AFS 101 Casting
Congress, Preprint No. 97–147, American
Foundrymen’s Society, Inc., Des Plaines,
IL, 1997
2. E. Niyama, et.al., “A Method of Shrinkage
Prediction and Its Application to Steel
Casting Practice,” 49th International
Foundry Congress, 1982
3. K. Tynelius, J. Major, and D. Apelian, “A
Parametric Study of Microporosity in the
A356 Casting Alloy System,” AFS Transactions, 1993, p 401–413
4. H. Huang, and J. Berry, “Evaluation Criteria
Functions to Minimize Microporosity Formation in Long-Freezing Range Alloys,’
AFS Transactions, 1993, p 669–675
5. N. Tsumagari, et.al., “Construction and

Application of Solidification Maps for
A356 and D357 Aluminum Alloys,” AFS
Transactions, 1993, p 335–341
6. J. Huang, “Study of Criteria Function for
Porosity Prediction in A356 Castings,”
Masters Thesis, Mechanical Engineering,
Northwestern University, Dec. 1995
7. Fatigue Design Handbook, 2nd Ed., SAE
International. Warrendale, PA, 1988
8. J. Shigley, and C. Mischke, Mechanical
Engineering Design, 5th Ed., McGraw-Hill,
1989
9. A.J. Hinkle, J.R. Brockenbrough, and
J.T. Burg, “Microstructural Material Models
for Fatigue Design of Castings,” SAE Paper
960161, SAE International, Warrendale, PA,
1996
10. J.M. Boileau, J.W. Zindel, and J.E. Allison,
“The Effect of Solidification Time on the
Mechanical Properties in a Cast A356-T6


8 / Casting Design and Performance
Aluminum Alloy,” SAE Paper 970019,
SAE International. Warrendale, PA, 1997
11. N. Nanda, et.al., “Feature-Based Design of
Gates and Risers in a Casting,” p 75–84,
Knowledge-Based Applications in Materials Science and Engineering, Ed. by
J.K. McDowell and K.J. Meltsner, The

Minerals, Metals, and Materials Society,
1994
12. J. Conley, B. Moran, and J. Gray, “A New
Paradigm for the Design of Safety Critical
Castings,” SAE Paper 980455, SAE International, Warrendale, PA, 1998
13. J. Conley, Private communication.

14. N. Palle, S. Singh, S. Mahadeva, R.J. Yang,
and J.A. Dantzig, 1997, “An Optimization
Based Methodology for Casting Process
Design,” presented at the McNU ’97 Joint
ASME, ASCE, and SES Summer Meeting,
Northwestern University, June 29–July 2,
1997


Casting Design and Performance
Pages 936

Copyright â 2009 ASM Internationalđ
All rights reserved.
www.asminternational.org

Casting Design and Processes
SHAPE CASTING, as a metal forming process, offers considerable flexibility and wide
scope in producing parts. Almost all metals and
alloys can be cast with few restrictions on part
weight or size, and casting is capable of producing highly reliable, cost-effective components
ranging from low-volume, single-part prototype
production runs to economies of scale for

millions of parts. In terms of component design,
metal casting is very flexible in terms of configuration design. Casting permits the formation of
streamlined, intricate, integral parts of strength
and rigidity that are not obtainable by other
methods of fabrication. For example, the flexibility of metal casting, particularly sand molding, may permit the use of difficult design
techniques, such as undercuts and curved or
reflex contours that are not possible with other
high-production processes. Tapered sections
with thickened areas for bosses and generous
fillets are routine. The inherent design freedom
of metal casting allows the designer to combine
what would otherwise be several parts of a fabrication into a single piece. This is significant
when exact alignment must be held, as in highspeed machinery, machine tool parts, or engine
end plates and housings that carry shafts.
Nonetheless, there are a number of challenges
in the design of castings. To begin, component
geometry and properties are closely interrelated
in casting design. Most castings involve complex configurations with sections of varying
thicknesses, which influences solidification rates
and thus properties within various sections of a
casting. For example, concave sections, or reentrant angles, solidify more slowly than fins or
protrusions, thus affecting the resultant local
structure and the properties. In other words,
property variations occur within a casting due
to differences in cooling rates. These variations
are predictable and need to be taken into
account during component design.
Designers also need to be aware that different casting alloys have different levels of “castability,” meaning that development of reliable
casting geometries and properties may be more
difficult in some alloys than others. A suitable

casting geometry for ductile iron may not be
the same as that of cast aluminum bronze depending on key casting parameters such as pouring
temperature, liquid-metal fluid life; solidification shrinkage, and formation of inclusions,
slag or dross, in the melt. Design engineers thus

need to know how suitable casting geometries
vary for the wide variety of casting alloys.
Moreover, designers need to know how differences in suitable casting geometries can be
utilized to achieve more optimum design configurations in terms of strength and rigidity.
Traditionally, for sand-casting analysis, the
use of geometric methods has been known as
the section modulus approach. The technique
uses the well-known Chvorinov’s rule, which
is commonly used to compare the solidification
times of simple casting shapes. Although originally developed for the solidification of pure
metals and alloys solidifying over a very narrow temperature interval, the concept is more
broadly applicable and states that the total
solidification time of a casting (or casting section) is proportional to the square of the volume-to-area ratio of the casting (or casting
section). This rule can be stated mathematically
as follows:


Vc
Ac

tf ¼ K 

2
(Eq 1)

where tf is the total solidification time for the
casting or casting section, Vc is the volume of
the casting or casting section, Ac is the surface
area of the casting or casting section, and K is
a constant for a given metal and mold
combination.
For simple shapes, the modulus in Eq 1 can
be calculated from the ratio of volume and
surface area involved in cooling. However, for
complex shapes discretized in a three-dimensional grid, the continuous distribution of modulus can be determined using the concept of
distance from the mold. The modulus at each
point in the casting is determined by the
relation:
M¼

2
N
P

(Eq 2)

1=di

i¼1

The concept of the section modulus approach
has been extended to other casting processes.
The modulus approach is an approximate
analysis scheme that uses geometry-based considerations to provide valuable insights into the
solidification times and, therefore, the propensity for defect formation during solidification.

The next stage is to design the rigging system
for the casting, which includes the design of
the gate, risers, downsprue, and so forth. This
has been based on the rules of thumb of foundry
experts and empirical charts. Once the rigging
design is established, the solidification behavior
can be evaluated. Here, heat, mass, and momentum transfer determine the cooling history of
the casting. The factors ultimately influence
microstructure evolution and the development
of stresses in the casting
This article provides a general introduction on
casting processes and design. The next chapter
describes computer modeling and simulation,
which continues to be an important tool in solidification modeling and casting design. The
remaining chapters describe in more detail the
role of design in meeting specific process or
component requirements. Some of these chapters
reflect more traditional or conventional solutions
from the rules of thumb developed by foundries
over years past. This retrospective view of casting design solutions may provide perspective in
this age of computer–based engineering and
perhaps an appreciation of the continuity in the
basic principles of casting as both a science and
a processing art.

Casting Methods
As one of the oldest manufacturing methods,
casting involves pouring molten metal into a
mold cavity that is configured to the shapes and

dimensions of the finished form. The methods
of shape casting can be divided into several
broad categories, as illustrated in Fig. 1. The
main categories are:
 Expendable molds with permanent patterns
 Expendable molds with expendable patterns
 Metal or permanent mold processes

In the case of an expendable mold, made with
bonded sand or other loose granular mold material, the patterns may be permanent as is typical
in sand casting or expendable as in lost foam
and investment casting. When patterns are permanent, the mold must be separable into two or
more parts in order to permit withdrawal of the
permanent pattern (Fig. 2). The tapered ends of
the pattern permit it to be removed from the
sand mold without restriction. Cores are separate


10 / Casting Design and Performance

Fig. 1

Chart of shape casting process

shapes that are placed in the mold to provide
castings with contours, cavities, and passages
that are not practical or obtainable with molds.
Permanent molds must be separable into two or
more parts in order to permit withdrawal of the
raw casting from the mold or die.

With expendable patterns, the limitations of
two or more separable parts of the mold is not
necessary, the molding media must only surround the expendable pattern and maintain its
shape during molten metal pouring and solidification. After the shape has solidified, the sand
or other molding media is shaken off and out of
part. Casting parts using expendable mold processes with either permanent patterns or expendable patterns is a very versatile molding method
that provides tremendous freedom of design in
terms of size, shape, and product quality.
Table 1 summarizes the differences in the steps
of casting a part between the permanent-pattern
versus the expendable-pattern methods.
In terms of capability, each shape casting
process has its niche, depending on the metal
or metals being cast, productivity; component
size, dimensional tolerances, and configuration
details. The choice of pattern materials can
affect dimensional tolerances. Tables 2 and 3
briefly compare some of the typical capabilities
of shape casting processes. The ratings and data
are necessarily typical and approximate, as
many material and design factors can influence
the economical feasibility of a casting process.

The tables only provide a brief overview only
for general approximate comparison. Ferrous
metals, steel, gray and ductile iron, account for
nearly 75% of all metals cast. On the nonferrous
side, high-pressure die casting (HPDC) is the
dominant process, largely because it readily
accommodates scrap-based secondary alloys,

and among metal-mold processes it enjoys the
highest productivity because of short cycle times.
Success depends, to a large extent, on the
skill of the mold designer in understanding the
very strong influences of the metal alloy and
mold material. Even with the best design, solidification on the mold walls closes passage of
melt at a certain distance, and this leads to a
minimum allowable section thickness. There
are large differences in the fluidity of alloys,
and therefore minimum allowable section thickness depends on the alloy being cast. Lower
minimum thickness can be allowed with zinc,
aluminum, and cast iron than with steel.
Streamlined flow is helpful; thus sharp corners
are avoided. Thicker sections solidify last and
must be fed adequately.

General Principles of Casting
Design
Regardless of the casting method, good casting designs generally follow the same basic
guidelines. The basic objective of good casting

design is to concentrate liquid-solid contraction
to the last portion(s) of the casting to solidify.
Almost all molten alloys, with some important
exceptions, undergo shrinkage during solidification. One exception is cast iron, both gray and
ductile, because graphite expands during solidification. This expansion of graphite often, but
not always, can compensate for the shrinkage
of the iron.
During alloy solidification, small nuclei of
solid grains or dendrites form and grow as the

temperature in the casting falls. As these grains
grow, a mush of liquid and solid occurs within
the freezing range of the alloy. Because each
small dendrite forms a site of shrinkage, the
shrinkage allows metal to flow between the
dendrites to the location where solidification
and shrinkage is taking place. As the dendrites
grow, however; the paths for molten-metal flow
become smaller, thus restricting complete filling
of the cavity. Thus, an overarching principle of
good casting design is that casting sections
should freeze progressively, so that contraction
results in a sound part.
There are several ways to achieve the goal of
progressive solidification and liquid-solid contraction. One basic technique of metalcasters
is to design reservoirs of molten metal risers
that feed liquid metals to the last portion of
the casting to solidify. The risers supply liquid
metal to feed shrinkage that occurs during
solidification, so that casting sections freeze


Casting Design and Processes / 11
Table 1 Differences in casting process steps between the permanent pattern and
expendable pattern methods of the expendable molding process
Expendable mold process
Steps in casting a part

Permanent patterns

Design the part
and
Pick the casting process to be
used for the part
Design of tooling to make the part
Make the tooling
Make the patterns for the part
Make the part the first time
Assemble the pattern sections, negative or
positive shape of the part, with its
gating system into an expendable mold
Cast (create/make) the part

Simultaneous engineering with the product
designer is a key to making a quality part.
Once designer and caster have reached consensus on the casting
process, the design can be finalized to meet functional requirements
of both the final part and the chosen foundry process.
Design tooling to make the negative Design tooling to make the positive shape
shape of the part
of the part
Permanent pattern tooling to make Expendable pattern tooling to make the
the negative shape of the part
positive shape of the part
Negative shape of the part
Positive shape of the part
Not applicable
If necessary, assemble the positive-shaped
patterns into the part to be cast.
Although pattern sections may be either a negative or positive shape,

they must be assembled into a final expendable mold assembly ready
for casting.
Introduce molten metal to the part in the selected casting process

and slag/dross forming tendency. However, systems for feeding and filling have evolved
largely from the experience and capabilities of
individual foundries. New tools and computer
simulation have lead to the design of improved
filling systems, but filling system designs for
castings is not always systematic. Key objectives and principles that should be addressed
for gating and risering include (Ref 1 and 2):

 Design runners and gates, if two or more


 Establish nonturbulent metal flow.
 Systematically fill the mold cavity with

metal of minimally degraded quality.

 In conjunction with the selection of an






Major components of a sand mold (a) pattern
assembly for cope and drag sections. (b) Cross
section of mold with core

Fig. 2

progressively. The need for risers depends on
the alloy being cast and the geometry of the
mold cavity. For example, cast iron often needs
very little in the way of risers, because graphite
expansion during solidification may compensate for the shrinkage of the iron.
Adequate filling depends on the configuration of the mold assembly and alloy castability
factors such as fluidity, freezing range, and
shrinkage during solidification. Thin sections,
for example, may require risers, so that all portions of the mold cavity are properly filled with
molten metal. Carefully planned geometry also
can offset alloy limitations in terms of fluid life,
solidification shrinkage, pouring temperature












Expendable patterns

appropriate pouring temperature, provide

conditions for mold filling consistent with
misrun avoidance.
Establish thermal gradients within the cavity
to promote directional solidification and to
enhance riser effectiveness.
Design riser size and geometry, and locate
risers and riser inlets to minimize the ratio
of gross weight to net weight.
Minimize to the extent possible the vertical
distance the metal must travel from the lowest position of metal entry to the base of the
sprue.
Taper the sprue or use a sprue geometry
other than cylindrical to minimize vortexing
and aspiration.
Keep the sprue continuously filled during
pouring.
Avoid abrupt changes in the direction of metal
flow; gate and runner passages should be
streamlined for minimum induced turbulence
at angles or points of divergence in the system.
Provide contoured transitions in gate, runner, and infeed cross sections at points of
cross-sectional area changes.
Employ multiple gates to improve thermal
distribution and to reduce metal velocity at
entry points.
Avoid molten metal impingement on mold
surfaces or cores by appropriate gate location.












sprues are used, to prevent the turbulence
associated with the collision of flow patterns.
Design risers to be of sufficient size and
effectiveness to compensate for volumetric
shrinkage. Riser position, shape, and filling
from the gating system relative to the casting
cavity are interrelated and critical considerations. In general, risers should be placed to
achieve the maximum pressure differential
and, when possible, should be open to the
mold surface. Blind or enclosed risers must
be adequately vented.
Observe the principles of directional solidification. The use of chills, riser insulation, and
casting design changes may be required. The
effects of inadequate gating and riser design
can in some cases be corrected only by complete redesign.
Provide runner overruns, dross traps, or insystem filtration to avoid the impact of
degraded metal on casting quality.
Locate the runners in the drag and locate the
gates in the cope for horizontal mold orientation. This rule is subject to intelligent variation by the uniqueness of each part.
Place the riser cavities in the gate path for
maximum effectiveness whenever possible.

Never place filters (if used) between riser
and cavity.
Design the gates so that metal entry occurs
near the lowest surface of the casting cavity.
Geometrically contour the runners to maintain
uniform fluid pressure throughout. Formulas
applicable to all gravity casting methods have
been developed for this purpose.

General Guidelines of Geometry in Casting Design. In terms of general geometry, good
casting designs generally follow some basic
guidelines:
 Limit drastic changes of thickness to mini-

mize stresses and aid in the casting process.


12 / Casting Design and Performance
Table 2 Shape casting processes ratings chart
Capabilities
Mold method

Applicable metals

Expendable mold with permanent patterns
Bonded sand processes
Green sand
CO2 sand
Cold box
Hot box

Shell
Slurry processes
Plaster
Ceramic
Rammed graphite
Nonbonded sand processes
Vacuum molding
Magnetic molding
Expendable mold with expendable patterns
Lost foam
Investment
Replicast
Centrifugal casting
Metal mold
Sand mold
Permanent mold and semipermanent mold Gravity
Static top pour

Tilt pour
Low-pressure permanent mold (LPPM)
Standard LPPM
Vacuum riserless casting/pressure riserless casting
Counterpressure casting/pressure-counterpressure casting
Countergravity casting
High-pressure die casting (HPDC)
Conventional HPDC
Hot chamber
Cold chamber
Vacuum die casting
Squeeze casting

Indirect
Direct
Semisolid processing
Thixocasting (billet)
Rheocasting (slurry)
Thixomolding (granular magnesium)

Productivity(a)

Disposable cores

Casting size

Dimensional control

All
All
All
All
All

5
2
2
2
3

Yes
Yes
Yes

Yes
Yes

Unlimited
Unlimited
Unlimited
Unlimited
Small to medium

Nonferrous
All
Reactive

1
1
1

Yes
Yes
Yes

Unlimited
Unlimited
Unlimited

Outstanding
Outstanding
...

Aluminum

Ferrous

1
1

NA
NA

Unlimited
Unlimited

...
...

Aluminum
All
All

3
2
1

NA
NA
NA

Medium to large
Small to medium
Medium to large

...
Outstanding
...

Nonferrous
All

3
2

Yes
Yes

Small to medium
Small to medium

...
...

Nonferrous and
metal-matrix
composites (MMCs)
Nonferrous

3

Yes

Medium to large

...

3

Yes

Medium to large

...

Light metals
Aluminum
Aluminum
All

3
3
3
3

Yes
Yes
Yes
...

Medium
Medium
Medium
Small

...
...
...
...

Zn and Mg
Nonferrous
Aluminum

5
5
4

No
No

Small
Small to medium

Outstanding
Outstanding

Al and MMCs
Nonferrous and MMCs

4
1

No
No

Small to medium
Small to medium

...
...

Aluminum
Nonferrous
Magnesium

4
5
5

No
No
No

Small to medium
Small to medium
Small

Outstanding
Outstanding
Outstanding

...
...
...

...
...

NA, not applicable. (a) Productivity rankings: 1 = low productivity; 5 = high productivity

 Taper sections as liberally as possible

Table 3 Comparison of several casting methods
Approximate and depending on the metal



toward risers to help control directional
solidification.
Do not interpose thin sections between thick
sections and access to risers.
Avoid extensive horizontal, flat surfaces.
Avoid isolated thick sections that create hot
spots that are difficult to feed.
Critical dimensions should not cross parting
lines in molds or injection or core dies.

Parameter

Green and casting

Permanent
mold cast

Die casting

CO2-core casting

Investment casting

Relative cost in quantity
Relative cost for small
number
Permissible weight of casting

Low
Lowest

Low
High

Lowest
Highest

Medium-high
Medium-high

Highest
Medium




Up to approx.
900kg (1ton)

45kg
(100lb)

40kg (85lb)

g(oz)–45kg
(100lb)



Thinnest section castable,
mm (in.)
Typical dimensional
tolerance, mm (in.)
(not including parting
lines)
Relative surface finish

2.5(1/10)

3.0(1/8)

0.8(1/32)

Shell: g(oz)–115kg
(250lb)
CO2 0.22kg
(½ lb)–Mg (tons)
2.5(1/10)

1.5(1/16)

0.3 (0.012)

0.75 (0.03)

0.25 (0.01)

0.25 (0.01)

0.25 (0.01)

Fair to good

Good

Best

Very good

Good

Good

Very good

Shell: good
CO2: fair
Good

Fair

Fair to good

Fair

Good

Good

Best

Best

Poor

Poorest

Fair

Fair

Designers also should have a good knowledge
of the casting characteristics of available alloys,
as this influences design approaches to work
around limitations of alloy castability. See
Chapter 8 “Casting Design and Geometry” in
this book.
In general, designing for progressive solidification requires tapering walls so that they

freeze from one end to the other (Fig. 3a, 3b),
thus avoiding situations where two heavy sections are separated by a thin section. This is a
poor design because metal must feed one heavy

Relative mechanical
properties
Relative ease of casting
complex design
Relative ease of changing
design in production


Casting Design and Processes / 13
section through the thin section; when the thin
section freezes before the heavy section, the
flow path will be cut off and shrinkage may
form in the heavy section. Tapered configurations also facilitate removal of patterns from
sand molds (Fig. 3c).
Junctions also concentrate heat, leading to areas
in the casting where heat is retained. These areas
solidify more slowly than others, thus having a
coarser structure and different properties from
other sections, and solidify after the rest of the
casting has solidified, so that shrinkage cannot be
fed. Minimizing the concentration of heat in junctions, therefore, aids in improving casting properties. Examples of this are shown in Fig. 4.
Concave corners concentrate heat, so they
freeze later and more slowly than straight sections, while convex corners lose heat faster,
and freeze sooner and more quickly than
straight sections. In designing a casting, the
designer should use properties from test bars

that have solidified at the actual cooling rate
in that section of the casting. This cooling rate
can be determined by instrumenting a casting
and measuring the cooling rate in various sections or by simulating its solidification using a
commercial solidification simulation program.
The hollow spaces in castings are formed by
cores, refractory shapes placed in the molds and

around which the casting freezes. These cores
are later removed from the casting, usually by
thermal or mechanical means. However, each
core requires tooling to form it and time to
place it in the mold. Casting designs that minimize cores are preferable to minimize costs.
Some examples are given in Fig. 5.
The designer also must provide surfaces for
the attachment of gates and risers. The casting
must solidify toward the riser in order to be
sound, and gates must be located so that the
mold fills from the bottom to the top so that
oxide films that form are swept to the top surface of the casting or into risers where they will
not affect casting properties. Gate and riser
locations must be accessible for easy removal
to minimize processing costs. The position of
gate and riser contacts may also add costs if
they are placed where subsequent machining
will be required to remove gate stubs or riser
pads in the finished component.

Expendable-Mold Casting
Expendable mold processes are widely used

in casting. The choice of molding method
depends on several factors such as part size
and shape, quantity, tooling, and the molten
metal being poured into mold. The basic characteristics of foundry molds must address four
basic requirements:
 Be formable into the desired shape around

some type of pattern

 Be able to hold that shape while molten

metal is introduced

 Be able to break down and become strippable

after the metal solidifies
The first requirement is that the mold material
must flow or be pliable enough to encapsulate
the surface around the shape of the pattern. Patterns can be either expendable; as in lost-foam
casting and investment or lost-wax casting; or
permanent. Permanent patterns are made from
wood, steel, hard plastic, where numerous molds
can be duplicated before significant wear would
alter the shape of the part being cast. Once the
mold material is tightly bonded around the pattern, then the mold must be stripped and
removed from that pattern, while not damaging
either the permanent pattern material or the
mold shape established.
The mold shape, which is typically the reverse
or mirror image of the part being cast, must then

be strong enough to be handled and manipulated,
so that it can be combined with other mold
shapes such as copes, drags, cores, gatings, etc.,
that produce the empty cavity of the negative
shape of the part being cast. Typically the plumbing system—the sprues, runners, gates—that
allows the molten metal to be introduced into
the cast part shape are also made from these same
molding materials, and becomes part of the mold
assembly. After complete assembly of this mold
package, the molten metal is introduced in some
manner into this mold assembly.
The second requirement is the molding material must be able to withstand the erosion action
of the molten metal as it flows through the gating system and the part geometry without deteriorating or losing its shape. Any portion of the
molding media that may break away could be

 Be able to maintain that shape while the

molten metal solidifies

Redesign with tapered sections. Tapered walls
provided progressive solidification in (a) elbow
and (b) valve fitting. (c) Taper allows easier removal of
pattern from mold.

Fig. 3

Redesign of casting to minimize heat
concentration. (a) Design has numerous hot
spots (X junctions) that will cause the casting to distort.
(b) Improved design using Y junctions.

Fig. 4

Redesign of castings to eliminate cores. (a)
Casting redesigned to eliminate outside cores.
(b) Simplification of a base plate design to eliminate a
core. (c) Redesign of a bracket to eliminate a core and
to decrease stress problems.

Fig. 5


14 / Casting Design and Performance
washed up in the molten metal, and end up as a
defect within the solidified part being cast. The
molten metal could also infiltrate the molding
media at the mold/metal interface producing a
nonhomogenous scab of solidified metal and
molding media on the part being cast.
The third requirement for molding media is
to maintain shape while the molten metal solidifies. This is a critical requirement to ensure
that the cast part in the mold accurately reproduces the outline of the pattern used to make
the mold. During solidification, the thermal conductivity of the molding medium is also important in meeting the material properties of the
metal being cast. Foundries must constantly balance the correct thermal conductivity of the
molding media. If heat conducts too quickly
from the mold cavity, freezing of the metal
before filling the entire mold cavity could result.
Conversely, slow heat conduction can result
in slow cycle times or improper microstructure
of the solidifying metal, which can lead to

incorrect material properties for the part being
cast. To assist solidification, the mold may
include regions with metal chill plates, which
are added to assist in quicker solidification in
certain sections of cast parts. Most metals can
be heat treated after casting to assist in meeting
the material properties of any particular part. Of
course this adds cost to the product, so any
compromises in the heat conductance of the
molding media that can be implemented would
save this cost.
Finally, the fourth basic attribute of the
molding media is being able to be removed
from the cast part. The expendable mold must
be broken away and stripped from the solidified
part. More cast parts, especially those with
intricate internal passages, are rendered unusable during this shake out step of the casting
process than in any other place in the foundry.
This is why some of the expendable pattern
practices, for example lost-foam and investment casting, may have an advantage over the
permanent pattern methods of the expendable
molding processes.
Mold Materials. Sand is the most prevalent
molding medium, and various types of binders
are used to bond the sand into useable molds.
Besides sand, other materials can meet the aforementioned requirements for molding media.
Slurry molding uses a plaster and/or ceramic
material to form the shape around either a permanent pattern or an expendable pattern.
Graphite molding materials rammed around a
pattern have also been utilized.

Sand is an abundant raw material, although
the sand for foundry use means something much
more than a layman’s concept of sand. In casting, sand has numerous properties that need to
be carefully controlled and monitored. In terms
of sand molding, the basic requirements of the
aggregate material include:
 Dimensional and thermal stability at eleva-

ted temperatures

 Suitable particle size and shape

 Chemically unreactive with molten metals
 Not readily wetted by molten metals
 Freedom from volatiles that produce gas

upon heating

 Economical availability
 Consistent cleanliness, composition, and pH
 Compatibility with binder systems

Many minerals possess some of these features,
but few have them all. The most prevalent mold
base is silica sand, due to its wide availability
and lower cost relative to other types of sands
such as zircon, olivine, and chromite. Manufactured ceramics (such as mullite pellets) are also
used as mold-base aggregates.

Expendable Molds Produced with

Permanent Patterns
Various methods are used to fabricate
expendable molds from permanent patterns.
The methods include:
 Molding of sand with a clay-water binder










and mechanical compaction green-sand
molding
Molding of sand with chemically thermal
activated binders made from inorganic compounds such as silicates or organic resins
Shell molding of sand with a thin resinbonded shell that is baked
No-bond vacuum molding of sand, where
molding media is held together with a vacuum source
Plaster-mold casting
Ceramic-mold casting
Rammed graphite molding
Magnetic or no-bond molding of ferrous
shot

Green sand molding is perhaps the most popular molding process. Green sand mold; which
are usually never green in color, but black; use

natural or synthetic clays with other additives to
bond the sand grains together. Green sand molding may be done either with flasks or without
flasks. The resin-bonded mold processes called
cold-box, hot-box, warm-box, and shell molding
were originally based on organic resin binders,
although lately even inorganic binders are mixed
with the sand and then hardened or cured by
chemical or thermal reactions to fixate the shapes.
Often these expendable resin bonded processes
are used to produce cores that are placed in permanent molds. This combination of expendable
cores with permanent molds is referred to as semipermanent molding. This is used extensively in
the nonferrous metal casting area.
Virtually all sand processes are suitable for
casting both ferrous and nonferrous metals,
but green sand excels over other sand processes
because it is also the most productive. Green
sand is by far the most often employed casting
process, simply because it is the dominant
process for casting ferrous metals. No other
sand process can boast hundreds of molds per
hour, with the potential for numerous cavities

per mold. Percentage of casting tonnage by
molding process is
All
All
All
All

Sand Casting

Permanent & Semi Permanent Mold
Die Casting
“other”

75%
>5
19
<1

But every dominant process has its down side.
Green sand does not provide the dimensional
accuracy or surface finish of the other bondedsand processes, thus they find their niche. The
properties of many nonferrous alloys are sensitive to solidification rate; when strength and
ductility of those alloys is important, green sand
gives way to metal mold processes. High speed
green sand lines make the employment of chills
somewhat difficult, so when localized high cooling rates are required in a casting, CO2 or cold
box sand processes may provide better local
chill placement opportunity.
High-pressure die casting (HPDC) is noted
for spraying molten metal into the die at very
high velocity, creating turbulence and entrapping cavity air and gasses in the solidified casting. Thus, HPDC is not suitable for castings
that must be heat treated or welded or subjected
to other elevated-temperature treatments like
porcelain enameling. Variations on HPDC such
as squeeze casting semisolid processing and
high-vacuum die casting have emerged to overcome those shortcomings, but at a premium cost
that renders them somewhat less competitive
than HPDCs, so there must be a trade-off.
The slurry processes provide exceptional

dimensional accuracy and surface finish for precision parts having complex contours that would be
difficult and expensive to machine. High-pressure
die casting and its variations also provide nearnet-shape castings but require much more expensive tooling so are generally warranted only for
high annual volume requirements.
The sand and permanent mold processes
readily accommodate disposable internal cores,
usually made from sand. The HPDC processes,
on the other hand, have very limited ability to
accommodate disposable cores. The high metal
flow velocities, and the high pressures applied
during solidification destroy most cores and
those robust enough to withstand HPDC processing parameters cannot be easily removed from
the solidified part. Some salt cores are routinely
used in HPDC as are some low-melting-temperature metal cores, but HPDC must be considered, at this time, very limited with respect to
application of disposable cores such as those
applied to the sand and permanent mold
processes.
By assembling simple-to-mold shapes, the
lost foam process has the ability to consolidate
several complex cast configurations into one,
sometimes enabling a cast part that would be
impossible to machine. Lost foam requires no
separate disposable internal cores to create
internal shape that would require cores in other
processes.


Casting Design and Processes / 15
While conventional high-pressure die casting
dominates the nonferrous casting industry, it

has been losing ground in recent years to other
metal mold processes and to variations on itself
that are better able to meet the demands for
light weight structural castings such as those
in the chassis and suspension systems of automobiles and light trucks. Squeeze casting was
the first variation on HPDC to prove capable
of structural aluminum castings such as steering
knuckles. Now the low pressure casting process
(LPPM) and its variations such as VRC/PRC
and CPC/PCPC are equally able to provide
crash-worthy products and can achieve better
cavity counts than squeeze casting because
metal is not injected or solidified under pressure. But those processes are best suited for
thicker-walled parts and semisolid processing
and high-vacuum die casting are now emerging
as the processes for large, very thin walled
2mm structural parts.

Expendable Molds Produced with
Expendable Patterns.
Casting with expendable molds is a very versatile metal-forming process, and the complexity and tolerance of cast products can be
enhanced further when expendable patterns are
used. Depending on the size and application,
castings manufactured with the expendable
mold process and with expendable patterns
increase the tolerance from 1.5 to 3.5 times that
of the permanent pattern methods. The two
major expendable pattern methods are lost
foam and investment or lost-wax casting. A
hybrid of these two methods is the Replicast

casting process, which involves patternmaking
with polystyrene, like lost foam, but with a
ceramic shell mold, like investments casting.
One major difference between the permanent
pattern casting methods and the expendable
pattern methods is that the expendable pattern
is typically always the positive shape of the part
(Fig. 6). In contrast, permanent patterns are the

Fig. 6

negative or mirror image of the part to be cast.
When using the expendable pattern method, the
part is typically made twice—once in an expendable or disposable form of part, and then as the
actual functional metal form of the part. Some
sort of tooling is needed to replicate many
expendable patterns for production. This extra
set of tooling or patterns may not be needed if
only one part or just a few parts are needed. If
this is the case, then expendable patterns can
be hand crafted or machined from some type of
expendable pattern material. In contrast, higher
production volumes of parts require repetitive
fabrication of expendable patterns. In this case,
tooling is needed to make the patterns.
Investment casting uses an expendable/disposable pattern, typically made of wax and then
coated with some type of slurry molding media.
The wax positive shape of the part is usually
made from tooling mounted in some type of
wax injection machine. Again for very few

parts, the wax could be hand crafted or shaped
from a billet of the material. The positive shape
of part, or more typically multiple parts, are
combined with a gating system and made into
a cluster (Fig. 7).
The wax cluster is then typically coated with
a ceramic slurry and dried. This process is
repeated until a sufficiently thick enough shell
is produced. This cluster with the wax disposable pattern is then fired in an oven until all
the wax is melted out, and the ceramic shell is
rigid enough to support the introduction of the
molten metal. Thus the term lost wax is used
to refer to investment casting. The metal is then
introduced into the empty cavity created by the
wax shape that has now been melted out.
The crux of the molding problem in investment casting involves withdrawal of the wax
or plastic pattern from the metal die, and the
withdrawal of cores if used from the pattern.
See Chapter 7 “Design for Economical Coring”
in this book. In other respects, the principles
applicable to sand molding; as described in
Chapter 6 “Design for Economical Sand Molding”, apply to investment casting.

Cast manifold with positive pattern for lost foam casting. Courtesy of the Vulcan Group

The lost foam casting process uses a disposable pattern typically made of expanded polystyrene (EPS). One previous name of this process
was EPS casting. The EPS material is very similar
to the material used for foam coffee cups, and
plates, etc. The foam pattern embodies the positive shape of the part to be cast (Fig. 6). The foam
pattern can be produced by hand crafting or

machining the shape from a billet of the material.
This method of making the expendable foam patterns is typically not the best way to produce a
good lost-foam casting, but it is acceptable for
prototypes or one-of-a-kind applications. More
typically, especially for high volumes of parts,
the foam pattern is produced from tooling
installed on a foam blowing machine of some
type. The lost foam foundries have discovered
numerous EPS additives and foam molding techniques to produce the best foam pattern that
results in the best casting with the least number
of defects.
Lost Foam is especially suitable for fairly
complex castings with many internal passages.
Cylinder blocks and cylinder heads for internal
combustion engines are fairly good candidates
for this process, where internal oil, water, and
gas passages can be integrally cast as one piece.
Typically these complex internal passages are
made possible by gluing together numerous pattern sections and then attaching the final gating
system to this positive shape of part to be cast
(Fig. 8). After the positive shape of part is
assembled into a cluster, the cluster is coated
with a thin ceramic slurry, dried, and placed
into a flask (Fig. 9) for investing of a casting
mold consisting of unbonded sand or other
molding media. Some fairly sophisticated sand
investment and compaction methods for lost

Fig. 7

Wax cluster


16 / Casting Design and Performance
decompositions to escape too quickly, allowing
the unsupported sand to cave in and either
mix with the molten metal stream, or loose the
shape of the part being cast. To be successful,
the lost foam foundryman must determine the
key coating characteristics for making a particular part with a particular molten metal. Many of
the coating / slurry suppliers to the industry have
learned these lessons, and can be of invaluable
service to the lost foam foundryman.
A major attraction of lost foam casting is the
next step—shakeout of the part from the
expendable mold material (Fig. 10). In lost
foam casting, the unbonded sand or other molding material just flows out of part; no violent
shaking and/or heating of the mold is needed
to break up mold like in green-sand or resinset sand mold and cores.
Replicast casting, a patented process exclusive to the Casting Technology International,
UK; is a hybrid method between lost-foam and
investment or lost-wax castings. The expendable
pattern is made from some type of foam and/or
plastic, but unlike lost foam, the pattern is
removed from the mold cavity during firing of
the ceramic that surrounds the pattern. See the
section “Replicast Molding” in this chapter.
Replicast is; therefore, more like the investment
casting process, where the disposable pattern
material is removed from the mold shape before

the introduction of the molten metal. Replicast
was originally developed for steel casting; especially low-carbon steels; as the pattern removal
before pouring eliminated carbon introduction
in the metal as EPS is comprised mainly of carbon. The process is especially valuable for large
one of a kind parts, where the caster can hand
craft and/or machine the shape from a billet of
foam material, and then mold it with some type
of expendable molding media that will set-up
and become the empty cavity for the molten
metal to fill after the plastic expendable pattern
has been removed either by heat or some
chemical means.

Lost foam pattern for cylinder head. (a) Four
different sections glued together. (b) Foam
cluster with its gating system and the casting after base
cubing

Fig. 8

foam casting are utilized within the lost foam
casting industry. The key sand compaction
attribute is that the sand flow into all the cavities of the part being cast and that it become
fully dense behind the thin ceramic coating on
the foam pattern.
After the sand is fully compacted around this
foam expendable pattern, the part is ready for
molten metal introduction with the foam still
in the mold assembly. The molten metal must
now not only become the shape of the final part,

it must also decompose the foam pattern, thus
the term lost foam. This process has also been
referred to as full mold casting, because the
mold cavity is not empty but rather filled with
foam. A typical defect in lost foam casting is
that this foam is not 100% lost, if it does not
escape from the mold entirely, the liquid or gaseous remains can mix with the metal and
become defects within the final cast part.
A key casting attribute of the ceramic slurry is
that it acts as barrier and/or valve between the
foam pattern and the sand during casting. The
thin ceramic shell allows the foam decomposition materials to escape through it and into the
sand at the proper rate. If it is too impermeable
for the foam decomposition materials to pass
through it, the molten metal could freeze off
and the part would fail to fill out. If the coating
is too permeable, it would allow the foam

Permanent Mold Casting
Permanent mold processes involve the production of castings by pouring molten metal
into permanent metal molds using gravity, low
pressure, vacuum or centrifugal pressure. Simple reusable cores are usually made of metal.
More complex cores are made of sand, plaster,
ceramics or even salt. When nonmetal destructible cores are used, the process is called semipermanent mold casting. the production of
metal objects by placing molten metal in a
durable mold of iron, steel, or solid graphite
using gravity or low pressure.
The general process of permanent mold casting process involves the following steps:
Lost foam cluster being (a) dipped into ceramic
slurry, (b) dried, and (c) inserted into sand-cast

mold. Courtesy of the Vulcan Group

Fig. 9

1. A refractory wash or coating is applied onto
the surfaces of the preheated mold that will


Casting Design and Processes / 17

Fig. 10

2.
3.
4.
5.
6.

Dumping and extraction of casting in lostfoam operation.

be in direct contact with the molten metal
alloy.
Cores, if applicable, are inserted, and the mold
is closed either manually or mechanically.
The alloy, heated to above its melting temperature, is introduced into the mold through
the gating system.
After the casting has solidified, metal cores
and loose mold members are withdrawn, the
mold is opened, and the casting removed.
Steps 2 to 4 are repeated until repair of the

refractory coating is required, at which time
Step 1 is repeated to the extent necessary.
The usual foundry practice is followed for
trimming gates and risers from the castings.

The vast majority of permanent mold castings
are of aluminum or its alloys. Other metals that
can be cast in permanent molds include magnesium, zinc, copper alloys and hypereutectic gray
iron. Permanent mold casting is particularly
suitable for the high-volume production of castings with fairly uniform wall thickness and limited undercuts or intricate internal coring. The
process can also be used to produce complex
castings, but production quantities should be
high enough to justify the cost of the molds.
Practical sizes of permanent mold castings differ
according to materials cast, part configuration,
and number of parts needed.
Compared to sand casting, permanent mold
casting permits the production of more uniform
castings with closer dimensional tolerances and
finer surface finish (Fig. 11) and improved
mechanical properties. In many applications,

Fig. 11

Approximate values of surface roughness and tolerance on dimensions typically obtained with different
manufacturing processes. ECM, electrochemical machining; EDM, electrical discharge machining.

the higher mechanical properties can justify
the cost of a permanent mold when production
volume does not. Permanent mold casting has

the following limitations:

 Complicated and undercut internal sections

 Not all alloys are suitable for permanent

mold casting

 Because of relatively high tooling costs, the

process can be prohibitively expensive for
low production quantities
 Some shapes cannot be made using permanent mold casting, because of parting line
location, undercuts, or difficulties in removing the casting from the mold
 Coatings are required to protect the mold
from attack by the molten metal
Good designs of permanent mold casting
generally follow the same guidelines as for
any casting, as previously outlined in the section “General Principles of Casting Design” in
this chapter. A number of factors must be considered when designing and building molds
for cost-effective production of sound permanent mold castings. These factors include the
following:











are usually made more easily with destructible cores than with metal cores, although
collapsible steel cores or loose metal pieces
can sometimes be used instead of expendable cores.
Proper selection of mold material and wall
thickness for economy and consistent mold
temperature profiles.
The gating system, which carries the metal into
the mold cavity at the proper location and fill
rate to promote directional solidification.
The riser or feeding system, which contributes to proper temperature gradients and
availability of molten metal to feed the solidifying metal front.
Venting must be placed advantageously to
allow air and gas to escape ahead of the
poured metal.
Chills which are sometimes inserted in the
mold to initiate and accelerate solidification
in desired locations and directions must be
properly sized and located. Antichills are
occasionally needed. Use of both must be
judicious.

 Cavity dimensions must be compensated for

the shrinkage that occurs as the casting
cools:
 Undercuts on the outside of a casting complicate mold design and increase casting cost
because additional mold parts or expendable

cores are needed.

Methods of Permanent Mold Casting
Methods of permanent mold casting include:
 Gravity casting with static top pour or tilt

pour


18 / Casting Design and Performance
 Low-pressure casting with permanent mold
 Counter gravity casting
 High-pressure die casting

These methods are distinguished by the method
of injecting molten metal into the mold cavity.
Gravity casting is common, but the dynamics
of gravity-fed systems such as turbulence with
environmental reactions can introduce inclusion
in the melt. The entrainment of inclusions such
as oxides in molten aluminum is extremely difficult to eliminate from gravity poured castings.
Therefore, the latter three methods have advantages in reducing inclusions and improving
properties. See ASM Handbook, Volume 15,
Casting for more details on these and other
casting methods.
Low Pressure Casting with Permanent
Molds. Low pressure metal casting has been utilized since the 1950s to produce high-volume,
high-integrity castings in alloys ranging from aluminum to zinc. Low-pressure casting, as it is most
widely practiced, is a variation of permanent mold
casting, although it is used to a limited degree to

fill sand or plaster molds too. It might also be
referred to as low pressure die casting, perhaps
deriving from the European practice of referring
to all metal mold processes as die casting.
Low pressure casting is a process where molten
metal is introduced to the mold by the application
of pressure to a hermetically-sealed metal bath
forcing the molten metal up through a narrow
diameter fill stalk tube from a furnace usually
residing below the casting machine; although,
there is a version using electromagnetic forces to
lift metal into the mold and then the furnace might
be an open hearth located beside the casting
machine. The process can be considered for low
to high volumes of castings from 5 to 100g and
usually incorporates the use of iron or steel permanent molds. Recent developments in sand molding
technology have made precision sand molds a viable choice for high-volume, low pressure casting
as well. A wide range of casting core options such
as expendable sand and shell cores and mechanical single or multipiece permanent cores are successfully used in the low pressure process.
The large majority of castings produced in
low pressure are aluminum, however iron, magnesium and copper-based alloys are successfully
cast with this process also. The mechanical and
fatigue properties of low pressure cast components are typically 5 to 6% greater than those
of gravity cast components of a similar alloy.
Controlled application of the pressure yields a
smooth, nonturbulent cavity fill from the bottom to the top which is highly desirable when
casting light alloys such as aluminum and magnesium. The single point entry of metal to the
casting and minimal or nonexistent risers minimizes subsequent processing to remove these
items reducing overall manufacturing costs.
There are three techniques on low-pressure

casting:

to vertically open and close a casting mold
situated over a ready supply of molten metal
under low pressure Fig. 12
 Counter-pressure casting, which is similar to
low-pressure casting but with a die surrounded
by an air tight mold chamber (Fig. 13)
 Vacuum riserless/pressure riserless casting
(VRC/PRC), which incorporates the use of
vacuum and pressure to bottom fill multiple
mold cavities from a hermetically sealed
molten metal source
More details on these processes are in ASM
Handbook, Volume 15, Casting (Ref 3). These
processes are used to producing high-integrity
aluminum casting and have been used to replace
aluminum forgings and squeeze castings.
High-pressure die casting. (HPDC) is a fast
(or perhaps the fastest) method for net shape
manufacturing of parts from nonferrous alloys.
It is ideal for the economical production of high
volume metal parts with moderately accurate
dimensions. The rapid cycle time of die casting
provides for economical production with the
capability for a high degree of automation.
The process is adaptable to the use of metal
saving designs and can also replace an assembly of separate parts with a single casting.
One basic process limitation is that part’s

geometry must allow removal from the die
cavity.
Die casting is generally limited to metals
with low melting points. Aluminum is the most
commonly used, followed by zinc, magnesium,
copper, tin and lead. Zinc, tin and lead alloys
are considered to be low melting point alloys,
while aluminum and magnesium alloys are considered as alloys with moderate melting points.
Copper is considered as a high melting point
alloy, and there are some copper-base alloys
for die casting. Suitable high-temperature die
materials are critical for economical die casting
of copper alloys.

Mold Complexity
Although casting solidification, discussed in
subsequent sections, is the first major factor in
casting design, the other major factor is mold
complexity. This usually is related to a requirement for an excessive number of cores. That is,
an important rule in economical casting design
is to eliminate as many design features requiring cores as possible.
The use of cores in the casting process provides a unique feature that is not available in
most other methods of manufacture, yet each

 Conventional low-pressure casting, which

incorporates a hydraulically operated machine

Fig. 12

Schematic of low-pressure casting machine with an electric resistance crucible furnace


Casting Design and Processes / 19

Fig. 13

Counterpressure casting process steps

core required adds to the final casting cost.
Thus, only those cores that are absolutely necessary for producing the desired shape should
be used if design is to be directed toward lower
cost castings.
The first principle that must be understood to
eliminate cores is the method of mold manufacture. Even investment and expendable pattern
or lost foam molds require that the molds or
tooling used to make the patterns be fabricated
as separate pieces or mold halves. For example,
in sand processes, a boxlike device called a
flask is set over the pattern and filled with sand.
The sand is hardened either through chemical
or mechanical means, and the pattern is then
removed. The pattern must be removed by
drawing it away from the mold in a direction
perpendicular to the parting line; therefore, the
design factors related to this common practice
must be considered by the design or casting
engineers.
As noted, taper or draft in the configuration
of a permanent pattern (Fig. 3c) facilitates withdrawal of the pattern from the mold. Although

pattern draft is usually not a problem, the
requirement of removing the pattern by withdrawing it in a direction 90 from the parting
line does restrict total design freedom if casting
costs are to be minimized.
Because the pattern is to be withdrawn
straight out of the mold, no protrusions that
restrict this movement can be allowed in the
construction of the pattern. If the geometry of
the part requires such protrusions, there are
only two alternatives for the casting engineer:
 use a loose piece that remains in the mold or

core after the pattern is drawn and then is
removed separately, or;
 provide a portion of the pattern that creates a
cavity for the setting of a core to create the
geometry.
The first alternative has limitations in that sufficient room and access to remove the loose
pieces must exist. Loose pieces can be used
only in moderately low volume casting production because they limit productivity in high

production. Furthermore, because of the added
handwork and potential for mold breakage during extraction, the use of loose pieces can be
expected to increase casting costs even in low
production. The second alternative involves
the addition of a core; therefore, increases casting costs. The problem and some proposed
solutions to undercuts are shown in Fig. 14.
Finally, because part geometry can require
more complex patterns due to irregular parting

lines, the final rule for reducing casting costs
is to design straight parting lines whenever
possible. This rule can be seen in practice in
Fig. 15. Because much thicker patterns are
required for offset parting lines, the cost of the
pattern is also greater.
The factors that affect the resulting cost of
castings and that have sufficient common traits
to be independent of the casting process
selected include:
 Requirements for surface finish and dimen-

sional tolerances

 The number of castings to be produced