Pattern

allowances in

metal casting Presentado por:

Diana Gasca

Andrés Escárraga

Fundición de Metales

Profesor: Dr. Héctor Sánchez S.

Universidad del Valle

Pattern is a model or the replica of the object to be cast. It is a larger in size as compared to the final casting, because it carries certain allowances due to metallurgical and mechanical reasons for example, shrinkage allowance is the result of metallurgical phenomenon where as machining, draft, distortion, shale, and other allowances are provided on the patterns because of mechanical reasons.

Top center is the clay original, then the two

part plaster mold used for casting the lead at

above, and wax cast from mold, sprued for

better brass casting, not yet cast. 2008-01-12.

homepages.waymark.net/mikefirth/tapper688

1b.jpg

Functions of Patterns:

• A Pattern prepares a mould cavity for the purpose of making a casting.

• A Pattern may contain projections known as core prints (corazón) if the casting requires a core and need to be made hollow.

• Patterns properly made and having finished and smooth surfaces reduce casting defects.

• Properly constructed patterns minimize overall cost of the casting.

The pattern material should be:

1. 2. 3.

4.

5. 6. 7.

Easily worked,shaped and joined.

Light in weight.

Strong,hard and durable.

Resistant to wear and abrasion .

Resistant to corrosion,and to chemical

reactions.

Dimensionally stable and unaffected by

variations in temperature and humidity.

Available at low cost.

Pattern Material Characteristics

(a) Split pattern

(b) Follow-board

(c) Match Plate

(d) Loose-piece

(e) Sweep

(f) Skeleton

pattern

Types of Patterns:

©2007 John Wiley & Sons, Inc. M P

Groover, Fundamentals of Modern

Manufacturing 3/e

Types of Patterns Figure 11.3 Types of patterns used in sand casting:

(a) solid pattern

(b) split pattern

(c) match-plate pattern

(d) cope and drag pattern

Fig:Single piece pattern

castings

Gating system

6.Gated pattern:

Fig:Cope and drag pattern

Reason for allowances:

Solidification Shrinkage

Most metals undergo

noticeable volumetric

contraction when cooled

Three principle stages of

shrinkage:

Shrinkage of liquid as it

cools from the solidification

temperature

Solidification shrinkage as

the liquid turns into solid

Solid metal contraction as

the solidified metal cools to

room temperature

Figure Dimensional changes experienced by a

metal column as the material cools from a

superheated liquid to a room-temperature solid.

Note the significant shrinkage that occurs upon

solidification.

Prediction of porosity after casting

Minuto 1:12. Se observa la contracción del metal, no hay tolerancia para

compensarla, genera porosidad residual.

Volumetric Shrinkage

2% Residual Shrinkage after casting

Dimensional Allowances

Typical allowances Cast iron 0.8-1.0% Steel 1.5-2.0% Aluminum 1.0-1.3% Magnesium 1.0-1.3% Brass 1.5%

Shrinkage allowances are incorporated into the pattern using shrink rules

Thermal contraction might not be the only factor for determining pattern size

Surface finishing operations (machining, etc.) should be taken into consideration

Solidification Shrinkage

Amount of liquid metal contraction depends on the

coefficient of thermal contraction and the amount of

superheat

As the liquid metal solidifies, the atomic structure

normally becomes more efficient and significant

amounts of shrinkage can occur

Cavities and voids can be prevented by designing the

casting to have directional solidification

Hot tears can occur when there is significant tensile

stress on the surface of the casting material

Types of Pattern Allowances: THE VARIOUS PATTERN ALLOWANCES ARE:

1.

2.

3.

4.

Shrinkage or contraction allowance.

Machining or finish allowance.

Draft of tapper allowances.

Distortion or chamber allowance.

5. Shake or rapping allowance.

1.ShrinkageAllowance: All most all cast metals shrink or contract

volumetrically on cooling.

1.Liquid Shrinkage:

it refers to the reduction in volume when the metal changes from liquid state to solid state at the solidus temperature.To account for this shrinkage;riser,which feed the liquid metal to the casting,are provided in the mold.

2.Solid Shrinkage:

it refers to the reduction in volume caused when metal loses temperature in solid state.To account for this,shrinkage allowance is provided on the patterns.

Almost all cast metals shrink or contract volumetrically after solidification and therefore the pattern to obtain a particular sized casting is made oversize by an amount equal to that of shrinkage or contraction.

Different metals shrink at different rates because shrinkage is the property of the cast metal/alloy.

The metal shrinkage depends upon:

1. The cast metal or alloy.

2. Solidification temp.of the metal/alloy.

3. Casted dimensions(size).

4. Casting design aspects.

5. Molding conditions(i.e.,mould materials and molding methods employed)

Material Dimension Shrinkageallowance

(inch/ft)

Grey Cast Iron Up to 2 feet

2 feet to 4feet

Over 4feet

0.125

0.105

0.083

CastSteel Upto2feet

2feetto6feet

over6feet

0.251

0.191

0.155

Aluminum Upto4feet

4feetto6feet

over6feet

0.155

0.143

0.125

Magnesium Upto4feet

Over4feet

0.173

0.155

RATE OF CONTRACTION OF VARIOUS METALS :

i. ii. iii. iv. i. ii. iii.

2.MachiningAllowance: A CASTING IS GIVEN AN ALLOWANCE FOR MACHINING, BECAUSE: Castings get oxidized in the mold and during heat treatment;scales etc.,thus formed need to be removed. It is the intended to remove surface roughness and other imperfections from the castings. It is required to achieve exact casting dimensions.

Surface finish is required on the casting.

HOW MUCH EXTRA METAL OR HOW MUCH

MACHINING ALLOWANCE SHOULD BE

PROVIDED, DEPENDS ON THE FACTORS LISTED BELOW: Nature of metals. Size and shape of casting.

The type of machining operations to be employed for

Metal Dimension(inch) Allowance(inch)

Castiron

Upto12

12to20

20to40

0.12

0.20

0.25

Caststeel

Upto6

6to20

20to40

0.12

0.25

0.30

Nonferrous

Upto8

8to12

12to40

0.09

0.12

0.16

MACHINING ALLOWANCES OF

VARIOUS METALS:

3.Draft or TaperAllowance:

It is given to all surfaces perpendicular to parting line. Draft allowance is given so that the pattern can be easily removed from the molding material tightly packed around it with out damaging the mould cavity. The amount of taper depends upon:

i. Shape and size of pattern in the depth

direction in contact with the mould cavity.

ii. Moulding methods.

iii. Mould materials.

iv. Draft allowance is imparted on internal as well as external surfaces;of course it is more on internal surfaces.

©2007 John Wiley & Sons,

Inc. M P Groover,

Fundamentals of Modern

Manufacturing 3/e

Core Full-scale model of interior surfaces of part

It is inserted into the mold cavity prior to pouring

The molten metal flows and solidifies between the mold cavity and the core to form the casting's external and internal surfaces

May require supports to hold it in position in the mold cavity during pouring, called chaplets

Figure 11.4 (a) Core held in place in the mold cavity by chaplets, (b)

possible chaplet design, (c) casting with internal cavity.

©2007 John Wiley & Sons,

Inc. M P Groover,

Fundamentals of Modern

Manufacturing 3/e

Draft Minor changes in part design can reduce

need for coring

Figure 11.25 Design change to eliminate the need for

using a core: (a) original design, and (b) redesign.

©2007 John Wiley & Sons,

Inc. M P Groover,

Fundamentals of Modern

Manufacturing 3/e

Product Design Considerations

Draft Guidelines:

In expendable mold casting, draft facilitates removal of pattern from mold

Draft = 1 for sand casting

In permanent mold casting, purpose is to aid in removal of the part from the mold

Draft = 2 to 3 for permanent mold processes

Similar tapers should be allowed if solid cores are used

Pattern

material

Heightofthe

givensurface

(inch)

Draftangle

(External

surface)

Draftangle

(Internal

surface)

Wood

1

1to2

2to4

4to8

8to32

3.00

1.50

1.00

0.75

0.50

3.00

2.50

1.50

1.00

1.00

Metal and plastic

1

1to2

2to4

4to8

8to32

1.50

1.00

0.75

0.50

0.50

3.00

2.00

1.00

1.00

0.75

Table 2 : Draft Allowances of

Various Metals:

Fig:taper in design

4. Distortion or cambered allowance:

A CASTING WILL DISTORT OR WRAP IF :

i. It is of irregular shape,

ii. All it parts do not shrink uniformly i.e.,some parts shrinks while others are restricted from during so,

iii. It is u or v-shape,

iv. The arms possess unequal thickness, v. It has long,rangy arms as those of propeller strut for the ship,

vi. It is a long flat casting, vii. One portion of the casting cools at a faster rate

5.Shake allowance:

A patter is shaken or rapped by striking the same

with a wooden piece from side to side.This is done so that the pattern a little is loosened in the mold cavity and can be easily removed.

In turn,therefore,rapping enlarges the mould cavity which results in a bigger sized casting.

Hence,a ²ve allowance is provided on the pattern i.e.,the pattern dimensions are kept smaller in order to compensate the enlargement of mould cavity due to rapping.

The magnitude of shake allowance can be reduced by increasing the tapper.

Pattern Layout:

Steps involved:

Get the working drawing of the part for which the pattern is to be made.

Make two views of the part drawing on a sheet,using a shrink rule. A shrink rule is modified form of an ordinary scale which has already taken care of shrinkage allowance for a particular metal to be cast. Add machining allowances as per the

requirements.

Depending upon the method of molding, provide the draft allowance.

Pattern Construction:

Study the pattern layout carefully and establish,

a. Location of parting surface.

b. No.of parts in which the pattern will be made. Using the various hand tools and pattern making machines fabricate the different parts of the pattern. Inspect the pattern as regards the alignment of different portions of the pattern and its dimensional accuracy.

Fill wax in all the fillets in order to remove sharp corners.

Give a shellac coatings(3 coats) to pattern. impart suitable colors to the pattern for identification purposes and for other informations.

Design Considerations in Castings Location and orientation of the parting line is important

to castings

Parting line can affect:

Number of cores

Method of supporting cores

Use of effective and economical gating

Weight of the final casting

Final dimensional accuracy

Ease of molding

Design Considerations

Various allowances incorporated into a

casting pattern.

Two-part mold showing the parting line and the

incorporation of a draft allowance on vertical

surfaces.

Design Considerations

Figure 11-16 (Left) Elimination of a core by

changing the location or orientation of the

parting plane.

Figure 11-17 (Right) Elimination of a dry-

sand core by a change in part design.

Design Considerations It is often desirable to minimize the use of cores

Controlling the solidification process is important to producing quality castings

Thicker or heavier sections will cool more slowly, so chills should be used If section thicknesses must change, gradual is better

If they are not gradual, stress concentration points can be created

Fillets or radii can be used to minimize stress concentration points

Risers can also be used

Parting Line and Drafts

Figure 11-18 (Top left) Design where the location of the parting plane is specified by the

draft. (Top right) Part with draft unspecified. (Bottom) Various options to produce the top-

right part, including a no-draft design.

Section Thicknesses

(Above) Typical guidelines for section change transitions in castings.

Figure a) The “hot spot” at section r2 is cause by intersecting sections. B) An interior fillet and exterior

radius lead to more uniform thickness and more uniform cooling.

Design Modifications

Hot spots are areas of the material that cool more

slowly than other locations

Function of part geometry

Localized shrinkage may occur

Hot spots often result from intersecting sections of various thickness.

Design Modifications

Parts that have ribs may experience cracking due to

contraction

Ribs may be staggered to prevent cracking

An excess of material may appear around the parting

line

The parting line may be moved to improve appearance

Thin-walled castings should be designed with extra

caution to prevent cracking

Design Modifications

Figure 11-23 Using staggered ribs to prevent cracking during cooling.

References

Rao, P.N. (2003). Manufacturing Technology. New Delhi: Tata McGraw-Hill.

Pattern Allowances in casting, Vikrant Sharma, MITS Lakshmangarh 2008

Foundry technology. Peter Beeley, 2th edition, Reed Elsevier, 2001

Fundamentals of metal casting. Richard A. Flinn. Addison-Wesley, 1963