10 rules for cost effective casting design

10 rules for cost effective casting design

CAST METALS INSTITUTE, INC

10 RULES FOR COST EFFECTIVE CASTING DESIGN

ETAL CASTING DESIG

METAL CASTING DESIGN

At the conception of any metal casting design three

people should be present: designer, patternmaker and

foundryman The designer knows what he wants his

casting to do in regard to mechanical and physical prop-

erties The patternmaker and foundryman can help him

design the casting so it can achieve its goals This is

the ideal situation However, in most instances the first

contact these three people have is when a problem

arises and this first encounter many times becomes

negative Should these three individuals meet at the

conception of the casting, many of the problems that

usually arrive later on will be eliminated Simply put

“It's easier to erase lines on a sketch or drawing than

rebuild the tooling!”

Pefore we move into some basic principles in casting

design, let's spend a few moments discussing toler-

ances The key point to remember from a designer's

standpoint is, “Do | really need these close tolerances?”

If the stringent tolerances stated can be justified the

team of designer, patternmaker and foundryman must

map out the strategy to obtain these tolerances

Variations occur on dimensions of parts made by any

metal shaping process and this includes metal

castings Dimensional tolerances for any metal shaping

process are functions of: 1) characteristic behavior of

the meta! used; 2) the design of the part; 3) variables

in the manufacturing process, and 4) the rigidity and

precision of the equipment employed in manufacturing

the part

For example, the casting design, mold restraint and

mold accuracy all have an effect on the contraction and

resulting dimensions on a casting Tolerance values

given in handbooks usually apply only to production

runs after necessary pattern adjustments have been

made In these cases the tolerances do not apply to

the first sample or to a small number of parts

Requirements for concise tolerances significantly affect

the cost and delivery time of castings In fact, the special

tolerances could result in the need for speciai casting

processes Realistic tolerance requirements are of crit-

ical importance when consideration is to be given to

obtaining economic costs Each metaicaster knows his

shop's capability in meeting tolerances and the casting

designer should discuss this matter with him

The only answer to concise tolerances is value analysis

if value analysis indicates that the concise tolerance is

not worth its cost the obvious answer Is not to spend

the extra money in controlling a tighter tolerance Or,

going to higher cost pattern and corebox equipment

An important factor to be considered is also the “run”

of the castings That is, will only a few castings be

required or will there be thousands On the other hand,

a close tolerance may well save the customer much

more than the foundry charges him to maintain the

tolerance

The design of the casting determines the manner in which the mold will be parted and, thus affects the dimensions on the mold parting Tolerances must al- ways be greater on dimensions across the parting line because of the fact that the cope and drag parting surfaces in the case of a sand mold will not close together precisely at all points from mold to mold Re- peating what was said earlier, the best advice that can

be offered in regard to casting tolerances is to contact the foundry with whom you are planning to do business and discuss this matter with them early in the stages

of the casting’s design

Many of the rules in designing a metal casting take into account the areas of heat transfer and solidification of the casting These two areas are intertwined Therefore when designing a casting, the designer should look for those areas where the mold into which the molten metal

iS going to be poured can become very hot These hot- spots in the mold can cause a delay in the solidification

of the casting in those areas and thus be a source of casting defects that are visible on the casting or can set up internal stresses that could later cause the cast- ing to fail When you study the following material con- cerning casting design, you will see that many of these rules take into account how the casting will solidify and also how to eliminate the possibility ot developing these hot spots in the mold

NEW DESIGNS

Rule No 1 — Before issuing the final drawings, consult a competent foundryman or patternmaker relative to the following priorities:

General Design Type of pattern needed Metal shrinkage How the casting is to be molded Locations of parting line

Design of a gating system to ensure proper filling

of the mold cavity

7 Placement of risers to provide molten metal as the casting solidifies

8 Number of castings to be produced in each moid

9 Feasibility of making on a molding machine

10 Accessibility of parts for cleaning

11 Locations of machined surfaces

12 Dimensional tolerances which can be held

GENERAL DESIGN

Rule No 2 — Construct a small model or visualize

the casting in the mold:

A three-dimensional model of the casting, if feasible,

will aid in determining parting lines, hot spots (section(s)

of casting last to solidify), design of gating system and

location of risers if needed

The model will aiso help visualize the “thermal” design

of the casting The designer, patternmaker and found-

ryman will be able to determine those areas in the

casting where parts of the mold or core are surrounded

by farge sections of molten metal These are the areas

of anticipated problems, which if known, precautionary

measures can be taken or a change in the design of

the casting can be made

CASTING DESIGN

Foundry technology encompasses many process vari-

ables from coremaking, metallurgy, molding to final

cleaning and grinding When castings are being de-

signed, each of these variables must be considered to

arrive at the optimum design for an efficient cost effec-

tive end product This section outlines basic recommen-

dations to consider in the design of castings It is recom-

mended that any change to existing equipment or the

development of new designs be reviewed with your

foundry to assist you in these areas

Rule Na 3 — Avoid sharp angles and abrupt section

changes:

One of the best things a casting designer can do is to

throw away his right triangles and “T” squares to avoid

acute angles Molten metal will take the shape of the

moid cavity and thus the designer is not limited to only

acute angles Probably the best drawing instrument to

be used in designing a casting is a “French Curve.”

Metal structure is affected by shape of casting section

Solidification of molten metal always proceeds from the

mold face, forming unbalanced crystal grains that pen-

etrate into the mass at right angles to the plane of

cooling surface A simple section presents uniform cool-

ing and greatest freedom from mechanical weakness

When two or more sections conjoin, mechanical weak-

ness is induced at the junction and free cooling 1s inter-

rupted, creating a “hot spot.”

Metal structure is also influenced by the freezing or

solidification range of the alloy poured Freezing range

is the difference in temperature from when the alloy

begins to solidify until it is completely solid The de-

signer should be aware of this when selecting an alloy

to be used for the casting if the designer is not familiar

with freezing ranges of alloys he should contact the

foundryman with whom he is working

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Iustrations of rounded corners avoiding local structural weak- ness

Fair Best in Some Cases

Avoid abrupt section changes: eliminate sharp corners at adjoining sections

The difference in the relative thickness of adjoining sections should be a minimum and not exceed a ratio of 2:1 Where

a greater difference is unavoidable, consider design with de- tachable parts: for example the ways of machine tool beds can be bolted, etc

When a change of thickness is less than 2:1, it may take the form of a fillet: where the difference is greater, the form recom- mended !s that of a wedge

Wedge-shaped changes in wail thickness are to be desig- nated with a taper not exceeding 1:4

Where light and heavy sections are unavoidable, use proper fillets or tapering sections or both

If blending is not permissible, use fillets of fairly large size at junctions

Replace sharp angles and corners with radii

Poor Design

V

Local Shrink Weakness

Local Structural

Weakness

Figure C

Be

Too Large Fillet Causing

Weak Metal Structure or

Shrinkage Defect

Improved Design

In designing adjoining sections, avoid acute angles Replace

all sharp angles with radii and minimize heat and stress con-

centration Fugures A B, and C illustrate poor designs that

result in local structural weakness Figures D and E show

recommended designs which assure improved strength and

solidity Figure F portrays a common defect involving a “T”

section The improved design as shown in Figure G removes

the hot-spot and stress concentration

Stress

Concentration

Figure F

Incorrect

Figure G

Correct

Cylinder Castings

Hot Spot

Incorrect

Engine cylinder parts are often designed to cause local casting weakness

Avoid core design which does not present a cooling surtace

Correct

Incorrect

Correct

Incorrect

Correct

Streamlining of exteriors resulted in heavy section at Y Thin elongated point of core at A resulted in heat concentration which accentuated heavy section Result: Hot spot causing leakage due to shrink effect

Re-design eliminated defect and resulted in a solid casting

of uniform strength properties

Rule No 4 — Fillet all sharp angies:

Fillets have three functional purposes:

1 To reduce stress concentration in the casting in service

To eliminate cracks, tears, and draws at re-entry

angles

To make corners more moldable and to eliminate

hot-spots

HOT SPOT

Poor

Improved

Number of radii or fillets in one pattern should be the

minimum possible, preferably only one To fulfill en-

gineering stress requirements and reduce stress con-

centration, relatively large fillets are used with radius

equaling or exceeding casting section

Where this dimension fillet is used, casting thickness

is increased at joint and tends to cause structural weak-

ness as shown in Figure A

“R= T to 2T

es Figure B

ein Figure A

Âm ho OF =~ 3

T Draw |

Uniform Cooling Causes Shrinkage or

Rate Obtained Weak Metal Structure

Where large fillets are required, design as shown in Figure B

Where this is not possible, consideration must be given

as to whether the engineering design or the foundry

casting problem is most vital From the foundryman’s

viewpoint, too large fillets are undesirable and the

radius of the fillet should not exceed one-half of the

thickness of the section joined

4"

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Other examples where fillets can help solve casting problems are those of V and Y section

Causes Structural of Heat Effect

Weakness

Weakness Rule No 5 — Bring the minimum number of sec-

tions together:

A well designed casting brings the minimum nurnber

of sections together avoiding acute angles Also, the sections are no thicker than necessary to achieve the desired strength and are evenly proportioned to avoid

local slow cooling

Figure A

A.MMMM.-

Solidifies in 3 Minutes

Simple section cools freely from all surfaces

Figure B

TL VL

f solid in Solid in

Adding a second section creates a hot spot at Y and area inside circle cools at rate of section 50% larger

Figure C

When two sections cross, only material outside dotted circle | | |

represents true properties Area inside circle solidifies at the

Ju

ii 9 Minutes

Adding too large fillets aggravates defect

Staggered ribs reduce distortion caused by shrinkage

Rule No 6 — Design ail sections as uniform in thickness as possible

Design all sections as uniform in thickness as possible Failing this, all heavy sections should be accessible for

Cylinder with Lugs

Avoid concentration of metal by staggering

` ~

X

N "`

N

Bore Incorrect

Porous Spots

where a number of sections are joined

Corrected

Design

Incorrect Correct

Design for uniform section Saves weight material, machining Circular web with adjoining sections is preferred costs and results in a stronger casting

The hydraulic coupling was originally designed with a

2-in cylindrical core through its center This produced

excessive metal and caused localized porosity Rede-

signing with sections of reasonable uniformity of section

thickness corrected the problem reduced the weight of

the casting and lowered the cost of manufacture

Incorrect

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Correct

Hydraulic Coupling

Rule No 7 — Design for casting soundness:

Most alloys shrink as they solidify The design of the

casting should be such that all sections progresively

increase in thickness to one or more suitable locations

where risers can be placed If this is done the risers

(which are reservoirs of molten metal) will be able to

supply molten metal to offset the shrinkage

The figures below illustrate correct and incorrect methods of

designing to assure soundness of section

——

Shank

Shrink Defect Incorrect

C annot Correct Light Sect Incorrect T

Be Fed ight section at 10p

Prevents Feeding

“Ricer

at R will not feed Y

Incorrect

“Tmprov

ed Design

Hub Casting

Rule No 8 — Dimensions of inner walls should be correctly proportioned:

Inner sections of castings, resulting from complex cores, cool much slower than outer sections and cause variations in strength properties A good rule is to re- duce inner sections to 9/10th of the thickness of the outer wall Avoid rapid section changes and sharp ang- les Wherever complex cores must be used, design for unormity of section to avoid local heavy masses of

When the diameter of the core equals the section thick- ness of the metal surrounding it the danger point of casting defects is being reached Core diameter to

sec-tion thickness ratios of this proporsec-tion causes the core

to become overheated slowing down the solidification

rate and can cause defects to occur

Rule No 9 — Bosses, lugs and pads should not be

used unless absolutely necessary:

Bosses, pads and lugs increase metal thickness creat-

ing hot spots which can be a cause of casting defects

When used these items should be blended by tapering

or flattening the fillets

WA WIRY WITT

Hot Spot

Incorrect

Thickness of bosses and pads preferably should be

less than the thickness of the casting section they are

attached to However, they must be thick enough to

allow machining without touching the casting wall

Where the casting section is light and does not allow

following this rule, the following recommended heights

will serve as a guide:

When several bosses or pads are on one surface they

should be joined if possible, to facilitate machining

This aids in removing possible hot spots and also allows for moving hole locations

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Continous Rib Instead of A Series of Bosses Permits Shifting Hole Location

Incorrect Lugs Spread Our

at Center ~ Solidification

Imoproved Designs for Bolting or Bearing Bosses

Bosses on Large Castings Metal Section to Heavy

at Bosses Difficult to Feed Solid

Uniform Metal Section Assures Sound Casting Less Weight, Lower Machining Costs

Undercuts Should be Eliminated

Wherever Possible

Omit Outside Bosses to Obtain

Straight Draft

Rule No 10 — Design ribs and brackets for

maximum effectiveness:

Ribs and brackets when incorrectly used can create

molding problems, shrink defects and localized hot

spots which in turn can create weakness Ribs have

two basic functions: 1) Improve stiffness and 2) Reduce

weight

Design preference normally used calls for ribs to have

greater depth than thickness Ribs in compression in

general offer a greater factor of safety than ribs in ten-

sion However, castings having thin ribs or webs in

compression may require design changes to give

necessary stiffing to avoid buckling Thickness of ribs

should approximate 0.8 of the casting thickness

LLL Li LLM ne Lif Cll

Ribs to Shallow

4 <<

Too Widely Spaced

Correct rib depth and spacing

is a matter of engineering design

⁄

Undesirable from a Foundry Viewpoint

⁄⁄⁄ yo 2

Cross-coupled Ribs Should Preferably be Designed as Double T Forms

Avoid complex ribbing when possible to simplify mold- ing procedure and assure more uniform solidification conditions This procedure will also reduce hot spots Stress and stress distribution in the casting wall favors omission of ribbing if it can be made of ample strength

and stiffness itself

Brackets carrying offset loads introduce bending mo-

ments both local and in the body of the casting Make

length of contact with the main casting as ample as possible Brackets may frequently be cast separately and then attached to the casting This method simplifies molding and reduces cost of manufacture Ribbed brac-

kets offer advantages as to stiffness

Local Stress High

"

Improved

Severe Stress Concentration

⁄

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Correct

Avoid concentration of heat by providing cored open-

ings in webs and ribs These openings shouid be as

iarce as possible, consistent with strength and stiffness

Avoid rectangular shaped cored holes in ribs or webs

Use oval shaped cored holes with the longest dimen-

sion ir the direction of the stresses

Cored Hole

Rib or Web

Incorrect

Correct

Rule No 11 — Avoid casting strain:

Casting design creating a variation in cooling rates be- tween its different parts is the commonest cause of casting strain Cracks, distortion and structural weak- ness usually result from differential or hindered solid contraction When a casting cools from its solidification

to room temperature it contracts In the case of cast irons, having a hypereutectic carbon content just at the point of complete solidification there may occur a marked expansion followed by normal contraction To reduce or prevent casting strain:

1 Avoid sudden changes of shape which produce

a corresponding change in the directio of shrink- age

2 Avoid re-entrant angles

3 Avoid multiplicity of cores—these expand under the influence of heat and offer resistance to free’ shrinkage

4 Avoid widely differing section sizes—especially those in close justaposition—which might cause different rates of cooling

5 Where internal casting strain must be a minimum

or where complete stability is vital, provide for a stress relief anneal

When designing gears flywheels and spoked wheels:

1 Use odd number and curved spokes

2 Provide for all cross-sections to cool.as evenly as possible by avoiding excessive sectional vari- ation

3 Blend sections of varying sizes carefully

Correct Incorrect

Material used in this section is exerpted from:

1 Casting Engineering & Foundry World Continen- tai Communications Inc., Bridgeport, CT

2 Customer's Foundry Orientation Manual, Robin- son Foundry, Inc., Alexander City, AL