Casting Info Design and Practices

8/2/2019 Casting Info Design and Practices

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SPRIN G/ SUMMER1999 EN GIN EERED CASTING SOLUTIONS 57

Cost-Ef fective Casting Desig n:W ha t Ever y Com ponent Design er Should Know

Viewing these six key factors as a systemwhile sketching geometriesprovides

a workable methodology for consistently good casting designs.

Michael A. Gw yn

Pelton Casteel, Inc., Milwaukee

2c2a

Overall geometr y should be ex ploredw ith structural, cast-ing and dow nstream m anufacturing needs in mindbeforelocking in to a sol id model.

Fig. 1 . W hen considered as a comp lete sys-tem, these six par am eters drive cost-effec-tive casting design.

Casting Properties1. Fluid Life2. Solidification Shrinkage

Type (eutectic, directional and equiaxed)Volume (small, medium and large)

3. Slag/ Dross Formation Tendency4. Pouring Temperature

Structural Properties5. Section Modulus (stiffness of casting geometry)6. Modulus of Elasticity (stiffness of alloy itself)

tructural design engineers

who work successfully with

castings commonly design in a nar-

row group of casting types poured

from familiar alloys (like the fam-

ily of irons or the 300 series of alu-

minum) and molded from famil-

iar foundry processes (like greensand or nobake). Rules of thumb

have been developed over the years

for common design situations.

Close inspection of these rules

reveals that they sometimes recom-

mend conflicting geometry. For ex-

ample, the use of gusseting instead

of mass for stiffness might be la-

beled recommended in one set of

design rules and poor in another.

Further, when a design engi-

neer leaves a familiar casting de-

sign realm for an unfamiliar one,unexpected trouble may result. For

example, lets say we are moving from

ductile iron to aluminum bronze while

staying in a familiar foundry process,

nobake molding. No alarms are sounded

among the rules of thumb, but theres

likely trouble in the usual ductile iron-

style geometry. Good aluminum bronze

geometry is different than typical ductile

iron geometry, and the molding process

may need to supplement the different ge-

ometr y with heat transfer techniques. Not

suspecting this, the design engineers new

S casting design may suffer fromno-qu otes, h igher-than-ex-pected prices and foundry re-

quests for design changes.

How are design engineers sup-

posed to know that successfully

casting geometry for aluminium

bronze should somehow be differ-ent? And if a design engineer did

know that, what would be the

proper course of design action?

The answer lies in a better un-

derstanding of the relationship

among geometry, various foundry

alloys and structure. As shown in

Fig. 1, there are six parameters

(based on physics) that underlie

cost-effective casting design.

All six, applied as a system,

drive the geometry of casting de-

sign. Geometry is not only the re-sult of casting design but is also the

most powerful weapon in creating success-

ful casting design.

This six-faceted system is capable of op-

timizing geometry for castability, struc-

ture, downstream processing (machining

and assembly) and process geometry

(risering, gating, venting and heat trans-

fer patterns) in the mold. The process ge-

ometry forms the casting geometry.

Quickly sorting through possible cast-

ing and process geometries by marking up

blueprints or by making engineering

Fig. 2. Show n here is the origina l steel fab rication (2a), a carbon steel casting design featur ing g eometry that suits its four casting character-istics (2b), and a gr ay iron casting featurin g an entirely d iff erent geometr y tha t is also still ba sed on its four casting alloy chara cteristics (2c).


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58 EN GIN EERED CASTIN G SOLUTIONS SPRIN G/ SUMMER1999

sketches is the way to find optimal sys-

tem geometry. An elegant result of good

sketched brainstorming can be a solid

model of the casting and its process ge-

ometry, the basis of rapid prototyping and/

or computerized testing.

Ap ply ing th e System

Optimizing casting geometry using the

six-parameter system is not difficult. The

six casting and structural characteristics in

Fig. 1 influence important variables in de-

signing, producing and using metal cast-

ings. These variables include:

casting method;

design of casting sections;

design of junctions between casting

sections;

surface integrity;

internal integrity;

dimensional capability;

cosmetic appearance.

Both the designer and metalcaster

possess a vital ally to streamline any

casting design. Casting geometry is the

most powerful tool available to improve

castability of the alloy and mechanical

stiffness of the casting.

Carefully planned geometry can offset

alloy problems in fluid life, solidification

shrinkage, pouring temperature and slag/

dross forming tendency. Section modulus,

an attr ibute of structural geometry, has the

capability to increase stiffness and/or re-

duce stressa capability that can be very

important when applied to alloys with

lower strength and stiffness. Modulus of

elasticity, an alloys inherent stiffness, can

be combined with section modulus and

section length to limit or allow deflection

in a casting design.

To preview geometrys ability to influ-

ence the four characteristics of

castability, consider the simple steel fab-

rication in Fig. 2a that was converted into

carbon steel and gray iron casting designs,

Figs. 2b and 2c, respectively.

The fabrication is a guide block to con-

strain low velocity/low load sliding mo-

tion, and it was welded from rectangular

bar stock, subsequently milled, drilled andtapped. The geometries in 2b and 2c are

considerably different as a consequence of

differences among fluid life, solidification

shrinkage type and amount, pour ing tem-

perature and tendency to form nonmetal-

lic inclusions (See Junctions, Fig. 6).

CASTIN G PROPERTIES

1. Fluid Life

Fluid life more accurately defines the

alloys liquid characteristics than does the

traditional term fluidity. Molten metalsfluidity is a dynamic property, changing

Definitions:Eutectic-Type Solidification: Eutectic alloys or behaving like them. These alloys re-

main liquid in the mold for a br ief period, cool and then solidify very quickly all over. This

phenomenon minimizes internal shrinkage and the need for risers.

Directional Solidification: These alloys begin solidifying quickly, perpendicular to

molds walls. Solidification direction and pathways are predictable from casting geom-

etry and thermal patterns in the mold walls. Without proper pathway geometry, isolated

internal shrinkage can result.

Equiaxed Solidification:These alloys not only begin solidifying perpendicular to mold

walls, but also solidify in the midst of the liquid, forming equiaxed islands of solid. Solidi-

fication pathways are interrupted by the equiaxed islands, making these alloys difficult to

feed. Fine, dispersed microporosity is typical.

as the alloy is delivered from a pouring

ladle, die casting chamber, etc. into a gat-

ing system and finally into the mold or die

cavity. Heat transfer reduces the metals

temperature, and oxide films form on the

metal front as this occurs. Fluidity de-

creases most rapidly with temperature loss,

and it can decrease significantly from the

surface tension of oxide films.

The absolute value of temperature is

not the test of fluidity at a given moment.

For example, some aluminum alloys at

1200-1400F (650-750C) have excellent

fluid life. However, some molten steels at

3000F (1650C) have much shorter fluid

life. In other words, a molten alloys fluid

life also depends on chemical, metallurgi-

cal and surface tension factors.

Fluid life affects the design character-

istics of a casting, such as the minimum

section thickness that can be cast reliably,

the maximum length of a thin section, the

fineness of cosmetic detail (like lettering

and logos) and the accuracy with which

the alloy fills the mold extremities.

It is essential to understand that mod-

erate or even poor fluid life does not limit

the cost-effectiveness of design. Knowing

that an alloy has limited fluid life tells the

designer that the part should feature:

softer shapes and larger lettering;

finer detail in the bottom portion of the

mold, where metal arrives first, fastest

and generally hottest;

coarser detail in the upper por tions of

the mold where the metal is slower to

arrive and more affected by oxide films

and solidification skin formation.

Even an alloy with good fluidity, when

overexposed to oxygen, may form a

Tab le 1. Four Casting Chara cteristics of Comm on Foundr y A lloy s

Solidif ication Shrink age

Alloy Group Fluid Life Ty pe Am ount Pour Tem p . Slag/ Dross

FERROUS:

Gray Iron Excellent Eutectic- Very Small 2500-2600F Little Type (1371-1427C)

Ductile Iron Good Eutectic/ Small 2500-2600F Some Directional (1371-1427C)

Carbon & Low- Poor Directional Large 2850-3000F Moderate Alloy Steel (1566-1649C)

High Alloy Steels Fair 1Various 1Various 1Various Moderate

N ON FERROUS:

Aluminum 3 56 Excellent 2 Eutectic- 2 Little 1300-1400F Moderate Type (704-760C)

Aluminum 206 Fair/ Good Equiaxed Moderate/ 1300-1400F Moderate/ Large (704-760C) Large

Aluminum Bronze Fair Equiaxed Moderate/ 2000-2150F Large Large (1093-1177C)

Silicon Bronze Fair Eutectic- Little 1900-2050F Large Type (1038-1121C)

Magnesium ZE43 Excellent Directional Moderate 1300-1400F Little/ (704-760C) Moderate

Yellow Brass Poor/ Fair Eutectic- Moderate 1800-1950F Large Type (982-1066C)

Titanium Very Good Eutectic- Little 3200-3300F Very Large Type (1760-1816C)

Zirconium Fair Eutectic- Little 3300-3400F Very Large Type (1816-1871C)

1 Among martensitic, partly austenitic and fully austenitic grades, solidification shrinkage encompassesall three types. Shrinkage amount and pouring temperature vary also.

2 For premium structural castings, solidification is more complex. Depending on alloy modifications,

section sizes and specifics of liquid-to-solid transformation, directional and/ or equiaxed shrinkagemay be involved.


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Fig. 3. Directional solidif ication on a p late casting is illustrat ed here.Ex tensive risering and ta pering (bottom) a l low s for ex cellent inter-nal casting soundness.

Fig. 4. Eutectic-type solidification is the most forgiving of the alloyshrinka ge types. Risers may be much sma ller w ith these alloy s, asthe avenue of liquid feed metal remain s open through solidification.

Fig. 5. Designs for equiaxed sol id ify ing al loys areshow n here. The larg e riser design (second f rom bot-tom ) illustrates how not to f eed a section. W hile sucha taper and larg e riser w ork ed w ith directional so-l id if ication, using this approach here adds m ore heatto an area that needs to cool more uniformly, and

results in lar ger, coalesced shri nk ag e. The prop er cast-ing and process geometry (smaller r isers and a ther-mally neutral shape) is i l lustrated at bottom.

high surface tension oxide film that

makes the fluidity die, rounding off

of the leading metal front as it flows.

more taper toward thin sections.

Some alloys, like 356 aluminum, have

been specifically designed metallurgically

to enhance fluid life. In the case of 356,

the high heat capacity of silicon atoms re-

vive aluminum atoms as their fluid life

begins to wane.

2. Solidif ication Shrink age

There are three distinct stages of

shrinkage as molten metals solidify: liq-

uid shrinkage, liquid-to-solid shrinkage

and patternmakers contraction.

1. Liquid shrinkageisthe contrac-tion of the liquid before solidification

begins. It is not an important design

consideration.

2. Liquid-to-solid shrinkageistheshrinkage of the metal mass as it trans-

forms from the liquids disconnected

atoms and molecules into the struc-

tured building blocks of solid metal.

The amount of solidification shrink-

age varies greatly from alloy to alloy.

Table 1 provides a guide to the liquid-

to-solid shrinkage of common alloys.

As shown, shrinkage can vary from

low to high shrinkage volumes.

Alloys are further classified based

on their solidification type: direc-

tional, eutectic-type and equiaxed (see

definitions in Table 1). The type of so-

lidification shrinkage in a casting is

just as important as the amount of

shrinkage. Specific types of geometry

can be chosen to control internal in-

tegrity when solidification amount or

types are a problem.

Figures 3-5 illustrate what is im-

plied by the three solidificationshrinkage types defined in Table 1. In

each case, a simple plate casting is shown

with attached risering (a riser is a reser-

voir of liquid metal attached to a casting

section to feed solidification shrinkage).

Cross sections of the plate and riser(s)

show conceptually how solidification takes

place; metallurgical reality is similar, but

microscopic.

Figure 3 shows solidification on and

perpendicular to the casting surfaces,

known as progressive solidification. At

the same time, solidification moves at a

faster rate from the ends of the section(s)

toward the source of feed metal (r isers)

this is known as directional solidification.Directional solidification moves faster

from the ends of the sections because of

the greater amount of surface area through

which the solidifying metal can lose its

heat. The objective is for directional so-

lidification to beat out progressive solidi-

fication before it can close the door to

the source of the feed metal. As shown,

directionally solidifying alloys require ex-

tensive risering and tapering, but they also

have the capability for excellent internal

soundness when solidification patterns are

designed properly.

Figure 4 illustrates the eutectic-typeal-loy, the most forgiving of the three. Such

alloys typically have less solidification

shrinkage volume. Risers are much smaller,

and in special cases can be eliminated by

strategically placed gates. The key feature

with these alloys is the extended time that

the metal feed avenue stays open. The plate

solidifies more un iformly all over and all

at once, similar to eutectic solidification.

Eutectic-type alloys are less sensitive

to shrinkage problems from abrupt ge-

ometry changes.

Alloys that exhibit equiaxed solidi-ficationrespond the most dramati-cally to differences in geometry (Fig.

5). Shrinkage in these alloys tends to

be widely distributed as micropores,

typically along the center plane of a

casting section. The reason is that so-

lidification occurs not only progres-

sively from casting surfaces inward and

directionally from high surface area

extremities toward lower surface area

sections, but also equiaxially via is-

lands in the middle of the liquid.

These islands of solidification inter-

rupt the liquid pathway of directional

solidification. Gradually, the pathways

freeze off, leaving micropores of

shrinkage around and behind the is-

lands that grew in the middle of the

pathway. Larger r isers, thicker sections

and tapering (shown at center of Fig.5) are counterproductive, causing


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micropores to coalesce into larger pores

across more of the casting cross section.

As illustrated at the bottom of Fig. 5,

microporosity is kept small and confined

to a narrow mid-plane in the casting sec-

tion by more thermally neutral geom-

etry with smaller, further-spaced risers.

As illustrated in Fig. 3-5, there is a

significant bilateral and reciprocal re-

lationship between solidification

shrinkage and geometry. Most simply,

eutectic-type solidification is tolerant of

a wide variety of geometries; the least

reciprocity is required. Most complex,

equiaxed solidification requires the

most engineering foresight in the choice

of geometry and may require supple-

mental heat tr ansfer techniques in the

mold process. In the middle lies direc-

tional solidification, while capable of

the worst shrinkage cavities, it is the

most capable of very high internal in-

tegrity when the geometry is properly

designed. Well-planned geometry in a

directionally solidifying alloy can elimi-

nate not only shrinkage but the need for

any supplemental heat transfer tech-

niques in the mold.

In fact, the real mechanism behind the

bilateral and reciprocal relationship be-

tween solidification shrinkage and geom-

etry is heat transfer. All three modes of heat

transfer, radiation, conduction and con-

vection are involved in solidification of

castings, and all three depend on geom-

etry for transfer efficiency. Convection and

conduction, are very impor tant in casting

solidification, and transfer rates are highly

affected by geometry.

3. Patternmakers Contractionis thecontraction that occurs after the metal has

completely solidified and is cooling to am-

bient temperature. This contraction

changes the dimensions of the casting

from those of liquid in the mold to those

dictated by the alloys rate of contraction.So, as the solid casting shrinks away from

the mold walls, it assumes final dimensions

that must be predicted by the pattern- or

diemaker. This variability of contraction

is another impor tant casting design con-

sideration, and it is critical to dimensional

accuracy. Tooling design and construction

must compensate for it.

Achieving dimensions that are just like

the blueprint require the foundrys pat-

tern- and/or diemaker to be included. The

unpredictable nature of patternmakers

contraction makes tooling adjustments in-

evitable. For example, a highly recom-

mended practice for critical dimensions

and tolerances is to build the patterns/dies/

coreboxes with extra material on critical

surfaces so that the dimensions can be

fine-tuned by removing small amounts of

tooling stock after capability castings have

been made and measured.

3. Slag / Dross Form ation

Among foundrymen, the terms slag

and dross have slightly different meanings.

Slag typically refers to high-temperature

fluxing of refractory linings of furnaces/

ladles and oxidation products from alloy-

ing. Dross typically refers to oxidation or

reoxidation products in liquid metal from

reaction with air during melting or pour-

ing, and can be associated with either high

or low pouring temperature alloys.

Some molten metal alloys generate

more slag/dross than others and are more

prone to contain small, round-shaped

nonmetallic inclusions trapped in the cast-

ing. Unless a specific application is exceed-

ingly critical, a few small rounded inclu-

sions will not affect casting structure sig-

nificantly. In most commercial applica-

tions, nonmetallic inclusions are only a

problem if they are encountered during

machining or appear in a functional as-

cast cosmetic surface.

The best defense against nonmetallic

inclusions is to inhibit their formationthrough good melting, ladling, pouring

and gating practices. Ceramic filters, which

can be used with alloys that have good fluid

life, have advanced the foundrys ability to

eliminate nonmetallics. Vacuum melting

and pouring are applied in extremely

dross-prone alloys, like titanium.

4. Pouring Temp erature

Even though molds must withstand ex-

tremely high temperatures of liquid met-

als, interestingly, there are not many

choices of materials with refractory char-

acteristics. When pouring temperature

approaches a mold material refractory

limit, the heat transfer patterns of the cast-

ing geometry become important.

Sand and ceramic materials with re-

fractory limits of 3000-3300F (1650-

1820C) are the most common mold ma-

terials. Metal molds, such as those used

in diecasting and permanent molding,

have temperatu re limitations. Except for

special thin designs, all alloys that have

pouring temperatures above 2150F

(1180C) are beyond the refractory ca-

pability of metal molds.

Its also important to recognize the dif-

ference between heat and temperature; tem-

perature is the measure of heat concentra-

tion. Lower temperature alloys also can pose

problems if heat is too concentrated in a

small areabetter geometry choices allow

heat to disperse into the mold.

Design of Junctions

A junction is a region in which differ-

ent section shapes come together within

an overall casting geometry. Simply stated,

junctions are the intersection of two or

more casting sections. Figure 6 illustrates

both L and T junctions among the four

junction types, which also include X and

Y designs.

Designing junctions is the first step to

finding castable geometry via the six-faceted

system for casting design. Figure 6 illustratesthat there are major differences in allowable

Fig . 6 . Junct ion geometry is importa nt to a l loy s w i th cons iderab le shr inkage and / or pour ing temperature . The cas t ing geometry a t le f t show s L and T junctions. The i l lustrations at r ight show the consequences of junction design and geom etry in creasingly dif f icultcombinat ions o f shr inkage am ount and/ or temperature . In rev iew ing Fig . 2 , the gray i ron junc tions (2c) are simi la r to type 1 a bove, andsteel (2b) are s imilar to t yp e 3.

1 2 3 1 2 3

2 3 2 3

33

Alloy Solidification Shrink age1 . Ver y l it t le 2 . M o d er a te 3 . Sign if i can t

L T


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Fig. 8. Show n here is an ex amp le of w hat a GD&Tdraw ing shou ld look l ike , complete w i th datum def i -nitions and geometric tolerance zones.

junction geometry, de-

pending on alloy shrink-

age amount and pouring

temperature. Alloy 1 al-

lows abrupt section

changes and tight geom-

etry, while alloy 3 re-

quires considerable ad-

justment of junction ge-

ometry, such as

radiusing, spacing, dim-

pling and feeding. Figure

7 illustrates a very high

form of the foregoing

principles in a critical au-

tomotive application.

Considerationsof SecondaryOpera tions inDesign

System-wide think-

ing also must include

the secondary opera-

tions, such as machin-

ing, welding and joining, heat treating,

painting and plating.

One aspect that affects geometr y is the

use of fixturing to hold the casting dur ing

machining. Frequently, the engineers who

design machining fixtures for castings are

not consulted by either the design engi-

neer or the foundry engineer as a new cast-

ing geometry is being developed. Failure

to do so can be a significant oversight that

adds machining costs. If the casting geom-

etry has been based on the four casting

characteristics of the alloy, then the de-

signer knows the likely surfaces for riser

contacts and may have some idea of likely

parting lines and core match lines. These

surfaces and lines will be irregularities on

the casting geometry and will cause prob-

lems if they contact fixturing targets.

It is best to define the casting dimen-

sional datums as the significant installa-

tion surfaces, in order of function prior-

ity, based on how the casting is actu-

ally used. Targets for machining fix-

tures should be consistent with these

datum principles.There is nothing more significant

in successful CNC and transfer line

machining of castings than the reli-

gious application of these datum fix-

ture and targeting principles.

Draw ings andDimensions

The tool that has had the most dra-

matic positive impact on the manu-

facture of parts that reliably fit to-

gether is geometric dimensioning and

tolerancing (GD&T), as defined byANSI Y14.5M1994. When com-

Fig. 7. This premium A356 aluminum casting for a cri t ical structural applica-t ion on a min ivan saved 14 lb o ver i ts stam ped steel w eldment pr edecessorand of fered nine addit ional m ounting locations. Close inspection show s extr ageometr y at junctions and sur faces w here heat transfer must be enhanced forhigh structural integr ity. The perm anent m old pr ocess featur es add it ional g e-ometry and heat transfer techniques to augment the castings geometry forstructural integrity.

PhotocourtesyofCMIInternatio

nal.

pared to traditional (coordinate) methods,

GD&T:

considers tolerances, feature-by-feature;

minimizes the use of the title block

tolerances and maximizes the appli-

cation of tolerances specific to the re-

quirement of the feature and its

function;

is a contract for inspection, rather than

a recipe for manufacture. In other words,

GD&T specifies the tolerances required

feature-by-feature in a way that does not

specify or suggest how the feature should

be manufactured. This allows casting

processes to be applied more creatively,

often reducing costs compared to other

modes of manufacture, as well as finish

machining costs.

GD&T encourages the manufacturer to

be creative in complying with the

drawings dimensional specifications be-

cause the issue is compliance with toler-

ance, not necessarily com-

pliance with a manufac-

turing method. By forcing

the designer to consider

tolerances feature-by-fea-

ture, GD&T often results

in broader tolerances in

some features, which

opens up consideration of

lower cost manufacturing

methods, like castings.

Figure 8 illustrates GD&T

principles applied to a de-

sign made as a casting.

Note the use of installa-

tion surfaces as datums

and the use of geometric

zones of tolerance.

Factors thatControl CastingTolera nces

How a cast feature is

formed in a mold has a

significant effect on the

features tolerance capability. The follow-

ing six parameters control the tolerance ca-

pability of castings. In order of preference,

they are:

Molding ProcessThe type of mold-ing process (such as green sand, shell, in-

vestment, etc.) has the greatest single in-

fluence on tolerance capability. How a

given molding process is mechanized and

the sophistication of its pattern or die

equipment can refine or coarsen its base

tolerance capability.

Casting Weight and Longest Dimen-sionLogically, heavier castings withlonger overall dimensions require more

tolerance. These two parameters have been

defined statistically in tolerance tables for

some alloy families.

Mold Degrees of FreedomThis pa-rameter is least understood. Just as some

molding processes have more mold com-

ponents (mold halves, cores, loose pieces,

chills, etc.) than others, some casting de-

signs require more mold components.

Each mold component has its own tol-

erances, and tolerances are stacked asthe mold is assembled. More mold com-

ponents mean more degrees of free-

dom; hence more tolerance. Good de-

sign for tolerance capability minimizes

degrees of freedom in the mold for fea-

tures with critical dimensions.

DraftIt is comm on for castingdesigns to ignore the certainty of

draft, including mold draft, draft on

wax and/or styrofoam patterns

made from dies, and core draft.

Since 1 of draft angle generates

0.017 in. of offset per in. of draw(about 0.5 mm/30 mm), draft can


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quickly use up all of a tolerance zone

and more.

Patternmakers ContractionTheuncertainty of patternmakers contraction

is why foundrymen normally recommend

producing first article and production pro-

cess verification castings (sometimes called

sample or capability castings) to estab-

lish what the dimensions really will be be-

fore going into production. A common

consequence of patternmakers contrac-

tion uncertainty is a casting dimension

that is out of tolerance, not because it var-

ies too much, but because its average value

is too far from nominal. In other words,

the dimension contracted more or less

than was expected.

Cleaning and Heat TreatingManycasting dimensions are touched by down-

stream processing. At the least, most cast-

ings are touched by abrasive cutt ing wheels

and grindingeven precision castings.

Many castings are heat- treated, which can

affect straightness and flatness.

When considering the breadth and

depth of geometrys importance in cast-

ing design, from its influence on castability,

the geometry of gating/risering, structural

form, cosmetic appearances and down-

stream fixturing, extensive brainstorming

of geometry is highly recommended. The

standard for optimal casting geometry

is high, but the possibilities for geometry

are limitless. Find ways of exploring ge-

ometry quickly, such as engineering

sketching, before committing to a print or

solid model.

STRUCTURAL PROPERTIES

In the preceding section, it was stated

that: 1) castability affects geometry but 2)

well-chosen geometryaffects castability. Inother words, a geometry can be chosen

that offsets the metallurgical nature of the

more difficult-to-cast alloys. Knowing how

to choose this proactive geometry is the

key to consistently good casting designs

in any foundry alloythat are economi-

cal to produce, machine and assemble into

a final product.

While the casting proper ties sectionwas the foundry engineeringspectrum ofgeometry for the benefit of design en-

gineers; the structural properties sec-

tion is the design engineeringspectrumof geometry for the benefit of foundry

engineers. Geometry found between

these two spectrums offers boundless

opportunity for castings.

Structural Geometry

Because castings can easily apply shape

to structural requirements, most casting

designs are used to statically or dynami-cally control forces. In fact, castings find

Fig. 9 . The m esh (l) show s the size of the f inite elements tha t ar e used for the FEA stressana lysis (righ t). The high-stress area s (red) could b e reduced w ith a geom etry change.

PhotocourtesyofGeneralMotors

their way into the most sophisticated ap-

plications because they can be so efficient

in shape, properties and cost. Examples are

turbine blades in jet engines, suspension

components (in automobiles, trucks and

railroad cars), engine blocks, airframe

components, fluid power components, etc.

When designing a component structur-

ally, a design engineer is generally inter-

ested in safely controlling forces through

choice of allowable stress and deflection.

Although choice of mater ial affects allow-

able stress and defection, the most signifi-

cant choice in the designers structural ar-

senal is geometry. As we will see, geom-

etry directly controls stiffness and stress in

a structure.

The casting processes are limitless in

their combined ability to allow variations

in shape. Not many years ago, efficient struc-

tural geometry was limited by the designers

ability to visualize in 3-D. Now, computer

generated solid models and rapid proto-

types are greatly enhancing the designers

ability to visualize structural shapes. This

technology often leads to casting designs.

Improved efficiency in solid modeling

software has led to an interesting design

dilemma. Solid models are readily appli-

cable to Finite Element Analysis (FEA) of

stress. FEA enables the engineer to quickly

evaluate stress levels in the design, and

solid models can be tweaked in shape via

the software so geometry can be optimized

for allowable, uniform stress. Figure 9 de-

picts a meshed solid model and a stress

analysis via the mesh elements.

However, optimum geometry for allow-

able, uniform stress may not be acceptable

geometry for castability. When a foundry

engineer quotes a design that considered

structural geometry only, requests for ge-

ometry changes are likely. At this point, the

geometry adjustments for castability may be

more substantial than the solid model soft-

ware can tweak. The result can be no-

quotes, higher-than-expected casting prices,

or starting over with a new solid model.

A practical solution to this problem is

to concurrently engineer geometry consid-

ering structural, foundry and downstream

manufacturing needs. The result can be

optimal casting geometry. The most effi-

cient technique is to make engineering

sketches or marked sections and/or views

on blueprints. The idea is to explore over-

all geometry before locking in to a solid

model too quickly. Engineering sketches

or mark-ups are easy and quick to

changeeven dramaticallyin the con-

current brainstorming process; solid mod-

els are not. A solid model should be the

elegant result, not the knee-jerk start.

The Obj ectiv e

Our objective is to explore geometry

possibilities, looking for an ideal shape

that is both castable in the chosen

foundry alloy and allowable in stress

and deflection for that alloy. As noted,

there is great variety in the four metal-

lurgical characteristics that govern al-

loy castability. Similarly, great variety

exists among metals in their allowable

stress and deflection. Therefore, an ideal

casting shape for all six of the casting

design factors in Fig. 1 is not necessar-

ily a tr ivial exercise. For alloys that have

good castability, choosing geometry for

allowable stress and deflection is the

best place to start. For alloys with less

than the best castability, it is better to

first find geometry that assists

castability, and then modify it for allow-

able stress and deflection.

Not all alloys are like ductile iron,

which is both highly castable and rela-

tively resistant to stress, and m oderately

resilient against deflection. For ductile

iron, many geometries may be equally

acceptable. Martensitic high-alloy steel

has fair-to-poor castability, but can have

amazing resistance to stress and can tol-

erate very large deflections without

structural harm. Therefore, structural

geometry is easy to develop, but a coin-


7/10


cidental castable shape is more difficult

to design. Premium A356 aluminum has

good castability, but rather weak resis-

tance to stress and low tolerance for de-

flection . Carefully chosen structural ge-

ometry, however, combined with solidi-

fication enhancements in the molding

process, has resulted in extremely

weight-effective A356 structural com-

ponents for aircraft, cars and trucks.

5. Section Mo dulus

Playing with sketches before building

a solid model means that we have to find

another way to evaluate stress and deflec-

tion . This other way is the essence of ef-

ficient structural evaluation of geometry

in casting design.

The equivalent of FEA for the design

engineers structural analysis is computer-

ized mold filling and solidification

analysis for the foundry engineer;

the basis for both is a solid model.

The other way for the foundry

engineer is the manual calculation

of gating, solidification patterns and

riser sizes; these are established, rela-

tively simple mathematical tech-

niques used long before the advent

of solid models. (See references.)

This other way for the design

engineer is not so simple. To take

full advantage of engineering

sketching/pr int m arking as a way

to brainstorm geometry, we must

be able to quickly evaluate stress

and deflection at impor tant cross-

sections in the sketches. As the

design engineer well knows, the

classic formulas for bending

stress, torsional stress and deflec-

tion are relatively simple. Each,

however, contains the same pa-

rameter, Section Modulus, which

is a function of shape and diffi-

cult to compute. Therefore, a

quick, simple way to compute or

estimate Section Modulus (morespecifically, its foundational pa-

Fig. 11. The three significant parameters in deflection are

length (L), Area Mom ent of Iner tia (I) and Mod ulus of Elas-ticity (E). Simp ly, increasing L increases deflection, w hileincreasing E or I decreases deflection.

a b

x

LL

PaPb

L

Pb/L

P

+

V O

_

+

M O

-Pa/LPab/L

For O x a:

6LEI

Pbx

(L2 x2 b2)

DEFLECTIO N FORMULA FOR ON E TYPE

OF LOA D CON FIGURATION

Fig. 10. Show n here are the stress formula s for bending and tor sion. Also show n is apropor tionable (simplif ied) relationship for deflection.

Bending Mom ent; in- lb x Distance from Centroid; inBending Stress; lb/ in.2 = __________________________________________________________

(At a distance from Ax ial Area Moment of Iner t ia; in.4

the centroid)

Torq ue; in-lb x Distan ce fro m Centro id; inTorsional Shear; lb/ in.2 = __________________________________________________________

(At a distance from Polar Area M oment of Iner t ia; in.4

the centroid)

Bending Mom ent; in- lb x Section Length2; in.2

Deflection; in. __________________________________________________________(At any point along Modulus of Elasticity; lb/ in.2 x Area Moment Inertia; in.4

a section length)

rameter, Area Moment of Inertia) is

needed so that we can move from sketch

to improved sketch in our casting ge-

ometry brainstorming.

Interestingly, the difficulty in comput-

ing Area Moment of Inertia for casting

shapes is one of the hidden reasons for

the design and use of fabrications. Fabri-

cations are made from building blocks of

wrought shapes, like I-beams, rectangu-

lar bars, angles, channels and tubes. These

shapes, which are simple and constant

over their length, have Area Moments of

Inertia that are easy to calculate or are

available in handbooks. Consequently,

stress and deflection calculations are rela-

tively easy. Fabricated designs, however,

are heavy and nonuniform in stress com-

pared to a casting well-designed for the

same purpose.

Quick Method forEstima ting Ar ea Mo ment ofInertia from Sk etches

Although there are five kinds of stress

(tension, compression, shear, bending and

torsion), the interesting ones for complex

structures are bending and torsion, and

their equations are shown in Fig. 10. (If

more than one type of stress is involved in

the same section, the Principle of Super-

position allows the individual stress typesto be analyzed separately and then added

together; once again, the larger of the

stresses to be combined are usually from

bending or torsion.)

Equations for deflection are very com-

plex-looking and different for each type

of load geometry. An example of one of

these formulas is shown in Fig. 11. Al-

though it is not an equation, the simpli-

fied relationship that is propor tional to de-

flection is also shown in Fig. 10.

In all three cases, the relationships ap-

ply to a cross-section of the geometry. It iseasy to draw a scale cross-section, whether

it be from an engineering isometric sketch

or from a marked-up view on a blueprint.

If we can find a way to quickly estimate

Area Moment of Inertia, we can readily es-

timate stresses in our brainstormed

sketches as well as estimate whether de-

flection will increase or decrease. Note that

Area Moment of Inertia is in the denomi-

nator in each relationship, meaning that

increased Area Moment of Inertia reduces

stress and deflection.

Maximum tensile stress in bend-

ing is often most critical in struc-

tural design. Section Modulus is de-

fined as the Area Moment of Iner-

tia divided by the maximum dis-

tance from the center of bending

(centroid) to the outermost edge of

the casting cross-section. Section

Modulus is similar to a stiffness in-

dex because it considers not only

magnitude of Area Moment of In-

ertia, but also maximum section

depth. If maximum section depth

increases faster than Area Moment

of Inertia, a geometry change can

actually increase maximum tensile

stress, rather than reduce it. This

index termed Section Modulus

accounts for that potential problem.

The estimation method rec-

ommended is based on three

principles. One is intuitive and

the other two are from the math-

ematics of engineering mechan-

ics. The principles are:

1. The design engineerssense ofload magnitudes and componentsize/shapeEngineers routinely usethis sense to sketch sized shapes that


8/10


are in the ballpark of the fi-

nal design. Foundry engi-

neers can learn this sense,

and when they do, they be-

come effective concurrent

engineering partners in their

customers casting designs.

2. The equation for AreaMoment of Inertia (Fig.12)Although the calculusfor an interesting casting

cross-section can be very dif-

ficult, the relationship ex-

pressed between depth of

section (Y) and change in

cross-sectional area (dA) is

very simple.

The position and shape

of the two rectangles in Fig.

12 (top) clearly demon-

strates this simple yet pow-

erful relationship. The

change in shape of the inside

of the tube (at bottom of the

Fig. 12) is an even more dra-

matic illustration. Calcula-

tions werent made in either

case, but the qualitative im-

pact of Y2dA on stiffness and

stress is unmistakable.

3. Area Moment of Iner-tiaOnce the engineeringsense of structural size and

Y2dA have been applied

qualitatively to a sketched

cross-section, the Parallel

Axis Theorem can be ap-

plied to simple building

blocks in the cross-

section to estimate

Area Moment of In-

ertia quantitatively.

A numerical value

for Area Moment of

Inertia is required to

calculate the stress

level in the sketched

cross-section.

The Parallel

Axis Theorem is il-

lustrated in Fig. 13(see Appendix for

example equation) .

6. Modulusof Ela sticity

The measure of a

materials stiffness

(without regard to

material geometry)

is known as the

Modulus of Elastic-

ity. In the case of

metals, it is a func-tion of metallurgy,

Fig. 12 . Illustrated is the simp le relation ship betw een depth of section (Y)and change in cross section (dA). When dA increases rapidly aw ay from

the center, stif fness increases dram atically.

AREA M OM EN T OF IN ERTIA

Fig. 13. This relationship enables the quick estimation of Area Moment of Inert ia v ia

build ing b locks. The build ing b locks must be refer enced to the centroid. The centroid canbe calculated, but i t is easier and quicker to use a paper dol l of the cross-section andsimply f ind i ts balance point.

R1Centroid

h

1

and it is a mechanical prop-

erty of the metal alloy.

Modulus of Elasticity var-

ies widely among materials,

and it varies significantly

among metals; that is, some

metals are considerably

stiffer than others. Alloy

groups tend to have the

same modulus value; for

example the entire family

of steels (carbon , low alloy

and high alloy) all have the

same modulus value of 30

x 106 lb/in.2.

Modulus of Elasticity

is an impor tant parameter

in structu ral design, and it

is directly involved in the

relationship between cast-

ing geometry and deflec-

tion. A larger Modu lus of

Elasticity means less de-

flection. For example, a

steel casting would deflect

less than an aluminum

casting of identical geom-

etry simply because steel

is stiffer than aluminum.

As an aside,

foundrymen may know

more about Modulus of

Elasticity than they think

they do; it is simply the

elastic slope of the stress/

strain diagram created

when the foundrys met-

allurgical lab pulls a test

bar. Figure 14 il-

lustrates qualita-

tively the results of

pulled test bars for

common groups

of foundry alloys.

The steepness of

the elastic slope of

each graph indi-

cates the alloy

groups st iffness.

One subtlety

about Modulus ofElasticity is that it is

not affected by heat

treatment. How-

ever, heat treatment

can affect the

height of the elastic

slope. This is very

important because

the height at which

the elastic slope be-

gins to curve is

called the metals

yield stress. Thisis the stress level at


9/10


which plastic deformation be-

gins and the metal is perma-

nently affected. Stresses

should be designed below this

level so that deflections in the

casting under load do not

damage it.

For example, consider the

family of steels in Fig. 14; heat

treatment can considerably

raise the point at which an al-

loy steel yields. Although the

steel is no stiffer at higher

stress levels, it can withstand

the additional stress without

damage. The same is true for

heat-treatable aluminum al-

loys, but the magnitude of

heat treatment effect on yield

stress is considerably less than

that for steels.

Summary

Figure 15 and the Appen-

dix illustrate a hypothetical

casting design using the rec-

ommended six factors behind

good geometry selection. The

first four factors describe the

alloys castability. The final

two factors are from engineering mechan-

ics and are Modulus of Elasticity and Sec-

tion Modulus, an aspect of Area Moment

of Inertia.

As a ductile iron cast-

ing design (see castability

characteristics in Table 1),

the following example is

intended to illustrate

structural geometry more

than geometry for

castability. As noted pre-

viously, for alloys that are

highly castable like ductile

iron, it is convenient to fo-

cus first on geometry for

structure and let the

alloys friendly foundry

characteristics adapt to

the structural needs.

Briefly, as ductile iron,the casting could be made

in a horizontally-parted

sand mold with the cen-

ter cylindrical section

pointed down. One core

would form the tongue

and groove tabs, bolt

holes and hollowed center

of the cylinder. A second

core would form the top

side of the I-beam feature

and the corresponding

bottom side of the four-hole plate. Two risers

Fig. 15 . Show n here is a p rel iminar y engineering sketch of a structural castingdesigned to control tor sion and b ending f orces. Comp letion of th is design as aducti le iron casting is described in the appendix on pa ge 21.

BendingMomentDiagram

5000 lb

3000 lb

Tor sion

would feed solidification shrinkage in the

center section from the tab sides of the four

hole plate. A third r iser would follow the

side of the second core and

feed the cylindrical end of the

I-beam section.

The appendixon page 21

illustrates the main point:

This casting design is noth-

ing more than an engineer-

ing sketch with a sense of

size and proportion. Using

the quick method of

sketching cross-sectional ar-

eas, Area Moment of Iner-

tia can be estimated with

simple building blocks and

minimal calculation . Once a

value is known, stress can be

easily calculated for the cho-

sen cross-section. A relative

measure for deflection can

be easily calculated as well.

Final design would be a

solid model, based on at least

two or three sketched itera-

tions of combined structural

and castable geometry. De-

tailed structural evaluation

could then be done via FEA.

Any remaining stress prob-

lems could be easily solved by

tweaking the solid model,

which is already close to optimal geom-

etry. Finally, the solid model could be

modified to add r isers and a gating system

so that computer analysis

of solidification and

mold filling could verify

the geometry chosen for

castability.

The author wishes tothank the following for theircontributions to this work:Mark Armstrong, DurironCo.; William F. Baker, Elec-tric Steel Castings Co.; LeoBaran, formerly of theAmerican FoundrymensSociety; Malcolm Blair,Steel Founders Society ofAmerica; Richard Heine,Univ. of Wisconsin-Madi-

son; Jay Janowak, GredeFoundries, Inc.; JohnJorstad, CMI International;Raymond Monroe, SteelFounders Society ofAmerica; Mark Morel, Mo-rel Industries; Tom Prucha,CMI International; FredSchleg, formerly of theAmerican FoundrymensSociety; and Jack Wright,consultant.

For more i nformation, see Re-

sources for Casting Designers &

Buyers, p. 67.

Fig. 14 . As show n, al loy famil ies var y considerably in sti f fness. Thesteepness of the elastic slope is the Mo dulus of Elasticity. Heat t reat-ment doesnt change the slope, but it can raise the yield point.

Stress;l b / i n .2

Strain; In/ In

M ODULUS OF ELASTICITY

Brass

Aluminums

Cast Iro ns

Steels


10/10


APPEN DIX: Qu ick -Metho d Stress Ca lcula tion

CROSS-SECTION 1 STRESS CALCULATIONArea Moment of Inertia:Top Rectangle

I = bh3/12 + bhYxx

2 = 2.50 x 1.063/12 +2.50 x 1.06x (1.34)2 = 5.01; in.4

Middle Rectangle = 0.50 x 2.123/12 +0.50 x 2.12 x (0.25)2 = 0.47; in.4

Bottom Rectangle = 2.50 x 0.753/12 +2.50 x 0.75 x( 1.69)2 = 5.44; in.4

IXX

= 10.92 ; in .4

Maximum Bending Stress in Tension:Bending Moment x Y

XX; max5000 lb x 10 in. x 1.87 in.

=Area Moment of Inertia 10.92 in.4

= 8600; lb/in.2

CROSS-SECTION 3C STRESS CALCULATIONArea Moment of Inertia:Bottom Outside Half CircleI = 0.1098r4/12 + (r2/2)(Y

xx2) = 0.1098(2.00)4 +

2 x (0.4244 x 2.00+0.12)2

= 7.65; in.4

(Bottom Inside Half Circle) = Similarly, = (0.58); in.4

Top Outside Half Ell ipse = 0.1098(2.00)3(2.50) +(2.50)(0.4244x2.00-0.12)2

= 6.31; in.4

(Top I ns id e Half Ell ips e) = Simi larly, = (0.05) ; in.4

IXX

= 13.33; in.4

Maximum Bending Stress in Tension:Bending Moment x Y

XX; max3000 lb x 6 in. x 1.87 in.

=Area Moment of Inertia 13.33 in.4

= 2500; lb/in.2

(Refer t o Fig . 15 )

CON CLUSION :The ductile iron alloy choice for this casting is 65/45/12, and the design safety factor is 4; therefore, the

design stress should be one-fourth of the yield stress, or about 11,000 lb/in.2. Although a fairly tough alloy,

the microstructure is not qu ite as strong in tension as it is in compression. It is easy to adjust casting geom-

etry to reduce tensile stress as illustrated by cross-sections 3A, B and C.

By the Principle of Superposition, the stress from torsion and bending in the center cylinder is addi-

tive (shear stress has been ignored for simplicity). The combined stress at cross-section 3C would be

roughly 12,500, slightly higher than the design stress. A final geometry iteration to slightly increase the

diameter of the base cylinder in the region of 3C toward the base would sufficiently reduce torsion str ess.

This would be a tapering diameter toward the base that also would assist the solidification feed path from

the plate through the cylinder to its end. Thus, geometry for castability and structure complement each

other in a good casting design.

Area Moment of InertiaBuilding Blocks forCross-Section 3B

References Basic Pr inc ip les o f Gat ing & Riser ing , AFS Cast Meta ls Inst i tu te Riser ing Stee l Cast ings (1973 ), Stee l Found ers Soc iety o f Am erica R.W. He ine, Com par ing th e Funct ion ing o f Risers to The ir Behav ior Pred icted by Comp uter Progr am s, AFS Tran sact ions ,vo l 19 85 p . 481 M .A . Gw yn , Cos t-Ef fect i ve Cas t ing Des ign , AFS R.W. He ine, Rise r ing P r incip les App l ied to Duct i l e I ron

Cast ings Mad e in Green Sand, AFS Tran sact ions 197 9, vo l . 8 7 , p . 6 5 R.W. He ine, Design M ethod f or Tapered Riser Feed ing o f Duct i le I rons , AFS Tran sact ions 19 82, vo l 9 0 , p . 14 7 AFS Riser ing SystemRiser S izer, Deve loped a t U.W.-Madison C.R. Loper,Jr., R.W. Heine a nd R.A. Rober ts, AFS Tra nsactions 19 68 , p. 37 3.

CROSS-SECTION 2 STRESS CALCULATIONArea Moment of Inertia:Outside Cylinder

I = r4/4 + r2Yxx

2 = 2.004/4 + 2.004(0.00)= 12.57; in.4

(Inside Cylinder) = 1.004/4 + 1.004(0.00)( 0.79; in.4)

IXX

= 11.78; in.4

Maximum Torsional Stress:Torque x Y

XX; max5000 lb x 12 in. x 2.00 in.

=Area Moment of Inertia 11.78; in4

= 10,200 lb/in.2

TOOLBOX: Bending Stress Formula : p. 63

Torsional Stress Formula: p . 63

Paral lel Ax is Theorem &

Paper Doll Centroid: Fig. 13

Principle of Superposition: p. 63

Sk etched-To-Scale Cross-Section s: See Righ t

Bui ld ing Blocks of Area

Mom ent of Iner tia: See Below

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