manufacturing: chapter 3 casting chapter 3 casting mdp024: prepared by: dr. mohamed ahmed awad page...

Manufacturing: Chapter 3Casting

MDP024:Prepared by: Dr. Mohamed Ahmed Awad Page - 27 -

CHAPTER THREEMetal Casting

Casting, shown in Fig. 3.1, is the process of pouring

molten metal into a mould containing a cavity ,

which represents the required product shape. It is one of the

most commonly used processes for making parts is casting.

Fig. 3.1: Casting process

Steps in casting are three:1. Melt the metal

2. Pour it into a mold

3. Let it freeze



Advantages of castingCasting process most often is selected over other

manufacturing methods because the following reasons:

Can create complex part geometries

Can create both external and internal shapes

Some casting processes are net shape; others are

near net shape

Can produce very large parts

Some casting methods are suited to mass production.

It is economical to use.

Disadvantages of castingDifferent disadvantages for different casting processes:

– Limitations on mechanical properties

– Poor dimensional accuracy and surface finish for

some processes; e.g., sand casting

– Safety hazards to workers due to hot molten

metals

– Environmental problems

Parts Made by Casting• Big parts: engine blocks and heads for automotive

vehicles, wood burning stoves, machine frames,

railway wheels, pipes, church bells, big statues, and

pump housings

• Small parts: dental crowns, jewelry, small statues, and

frying pans



• All varieties of metals can be cast, ferrous and

nonferrous

1. Types of Casting

Casting classification is related to:

Mold material;

Molding processes

Methods of feeding the mold with molten metal

The mold in casting

Contains cavity whose geometry determines part

shape

Actual size and shape of cavity must be slightly

oversized to allow for shrinkage of metal during

solidification and cooling

Made of a variety of materials, including sand, plaster,

ceramic, and metal

The mold can be in two forms as shown in Fig. 3.2 :

open mold, simply a container in the shape of the

desired part; and

closed mold, in which the mold geometry is more

complex and requires a gating system

(passageway) leading into the cavity



Fig. 3.2: Forms of molds

The two major categories are ; expandable and permanent

mold casting.

3.1.1 Expandable moldsExpandable molds are made of sand, plaster, ceramics and

similar material. After the casting has solidifies, the molds in

these molds are broken up to remove the casting.

– Advantage: more complex shapes possible

– Disadvantage: production rates often limited by time

to make mold rather than casting itself

3.1.2 Permanent moldsPermanent molds are made of metals that maintain their

strength at high temperature. The casting can be easily

removed and the mold used for the next casting.

– Advantage: higher production rates

– Disadvantage: geometries limited by need to open

mold



The choice of casting types depends on several factors such

as complexity, surface finish and cost.

2. Solidification of MetalsSolidification is the transformation of molten metal back

into solid state. Solidification differs depending on whether

the metal is a pure element or an alloy. Solidification

process is shown in Fig. 3.3.

Fig. 3.3: Solidification process

Solidification shrinkageA casting with a higher volume-to-surface area ratio cools

and solidifies more slowly than one with a lower ratio.

Since riser and casting mold constants will be equal, design

the riser to have a larger volume-to-area ratio so that the

main casting solidifies first. This minimizes the effects of

shrinkage. Shrinkage effect is shown in Fig. 3.4.



Fig. 3.4: Shrinkage of casting product during solidification

The shrinkage shape due to further reduction in height and

diameter due to thermal contraction is shown in Fig. 3.5.

Fig. 3.5: Cavity caused due to shrinkage effect

3. Sand Casting

This process relies on the mold maket creating a cavity

in a block of sand. This cavity must have the desired

configuration so that when molten metal is poured into it and



allowed to solidify, the solid metal will take on the desired

shape and have dimensions that match, or can be machined

to match, a finished parts.

Steps in Sand Casting

1. Pour molten metal into sand mold

2. Allow metal to solidify

3. Break up the mold to remove casting

4. Clean and inspect casting

5. Heat treatment of casting is sometimes required to

improve metallurgical properties

3.3.1 Properties required in sandPermeability posrosity of the sand enables the :النفاذیة

escape of gas and steam formed in the mold.

Strength: sand must be cohesive to the extent that it has

sufficient bond; both water and clay content affect the

cohesive properties.

Refractoriness: Sand must resist high temperatures

without fusing.

Grain size and shape: Sand must have a grain size

commensurate with the surface to be produced, and

grains must be irregular to the extent that they will have

sufficient bonding strength.

3.3.2 Elements of Sand mouldThe elements of sand casting, shown in Fig. 3.6 can be

summarized as follows:-



Fig. 3.6: Elements of sand mould

Molten metal is poured into the mould cavity.

Sand is packed into the casting boxes around the

pattern.

Pattern an exact copy of the required casting

made from wood and sometimes from metal. It is

removed to leave the mould cavity.

Casting removed from sand after cooling.

Cores الدلیك are placed in the cavity to form holes.

Riser is an extra void created to contain excessive

molten metal. The purpose of this is to feed the molten

metal to the mold cavity as the molten metal solidifies

and shrinks.

The cavity in the sand is formed using a pattern.

In the two-part mould, which is typically of sand

casting, the upper half, flask is called cope and

the lower half is called drag.

Parting line is the surface that separates the mould

parts.



3.3.2.1 PatternThe pattern is a full-sized model of the part, slightly enlarged

to account for shrinkage and machining allowances in the

casting

A. Types of patternsAccording to the type of pattern used, there are two different

methods by which sand casting can be produced

Removable : The mold is made in a flask that

has two parts, or in a cheek that has three or more

parts. The top part of the flask is called a cope while

the lower part is called a drag.

Fig. 3.7: Types of patterns used in sand casting:

(a) solid pattern (b) split pattern (c) match-plate pattern

(d) cope and drag pattern

Disposable : In this case, the pattern is usually one

piece including the gate. The pattern is placed on a

follow board and the drag is molded in the conventional

way.



B. Pattern allowanceThree pattern allowances, shown in Fig. 3.8, should be

considered during pattern making; shrinkage, draft and

machining.

Shrinkage : When any pure metal and most alloys

cool, they shrink. To compensate for shrinkage

a shrink rule must be used in laying out the

measurements for the pattern.

Draft : When a removable pattern is drawn from a

mold, the tendency to tear away the edges of the mold

in contact with the pattern is greatly decreased if the

surfaces of the pattern, parallel to the direction it is being

withdrawn, are slightly tapered. This tapering of the

sides of the pattern, known as draft

Machining (Finish): The finish of the cast surface is

fairly bad. In most cases these surface have to be

machined. The Machining allowance is added to

compensate the loss of material as chip during machining

process.

Fig. 3.8: Pattern allowances



2.4.2.2 Core

Core as a 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

Fig. 3.9: Core

Core can be place in different orientation as shown in Fig.

3.10.

Horizontal Vertical Complex

Fig. 3.10: Different orientation of core



4. Preparation of mould

The steps as shown in Fig. 3.11 include not only the casting

operation but also pattern-making and mold-making

Fig. 3.11: Mould preparation steps

Steps required to cast the following sample product

Pattern making Core Making

Drawing Drawing



Pattern made with its three

allowances; shrinkage,

machining and draft. A core

prints are added to hold the

core in place.

Core boxes produce core.

The core will be used to

produce the hollow area

of the part shown.

Molding Preparation

1 – Drag preparation

Drawing Description

Half of the pattern have

been mounted on plate

inside the drag. The drag is

filled with casting sand.

2- Drag inverted

Drawing Description

The drag is inverted.

The half pattern is removed

3- cope preparation

Drawing Description

Drag is filled with sand.

The full pattern is inserted

The cope is ready for sand



4- hole; riser, sprue and vent holes; making

Drawing Description

.

Cope and drag are filled

with sand.

The pattern is ithdrawn.

The riser, sprue and vent

holes are made inside the

sand in the cope

5. Cope and drag assembly

Drawing Description

Core is set in place.

Cope and drag assembled

ready for pouring.

5 . Die Casting السباكة باسطمباتDesigned to hold and accurately close two mold halves and

keep them closed while liquid metal is forced into cavity

Die-casting, shown in Fig. 3.12, is employed where many

items of the same form are to be manufactured. In this

process, molten metal if forced into the cavity between dies

under high pressure. After the metal has been injected, the

pressures is held for a short time whilst the metal solidifies.

The die blocks are then opened and the casting is injected.



Fig. 3.12: Die casting

3.4.1 Molds for Die Casting• Usually made of tool steel, mold steel

• Tungsten and molybdenum (good refractory qualities)

used to die cast steel and cast iron

• Ejector pins required to remove part from die when it

opens

• Lubricants must be sprayed into cavities to prevent

sticking

3.4.2 Die Casting StepsFig 3.13 shows the steps of die casting process.

Fig. 3.13: Die casting steps



• Advantages:

– Economical for large production quantities

– Good dimensional accuracy and surface finish

– Thin sections are possible

– Rapid cooling provides small grain size and good

strength to casting

• Disadvantages:

– Generally limited to metals with low metal points

– Part geometry must allow removal from die cavity



Review Questions

Q1: Define the casting process?

Q2: Describe three properties required in casting sand?

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Q3: State three differences between final product and the pattern

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Q4: State three properties required in casting sand

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Q5: State two disadvantages of casting process

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Q7: Discuss the correctness of the following statements. ( Statetrue of false, then describe the reasons in detail)

1 The riser has to be solidified before the product solidification.

2 Sand casting is required when good surface quality is required

3 Draft allowance is added to facilitate product extraction

4 In die casting, the metal fills the cavity under gravity force.

5 Core is used in die casting to form the internal cavities



Q8: Draw the sand mould?

9. Identify each part of the following sand mould

10. Identify the function of each of the following sand mouldelements:CoreFlaskDragCopeRiserPouring basinventcavity



11. Compare between sand and die mouldComparison Item Sand Die

Mould material

No. of product permouldMould cost

Product cost

Product accuracyProductcomplexityProduct volumeForce required

Q12: State the three pattern allowances

1-

2-

3-

Q13: Write the allowance name which match each of the followingpurposesNo. Pattern

allowance typePurpose

1 To facilitate pattern withdrawal

2 To compensate volume reduction due toshrinkage during solidification

3 To compensate volume loss due to chipformation



Q14: Discuss the correctness of the following statements. (State true of false, then describe the reasons in detail)

1 The riser has to be solidified before the product solidification.

2 Sand casting is required when good surface quality is required

3 Draft allowance is added to facilitate product extraction

4 In die casting, the metal fills the cavity under gravity force.



7 Complex shapes could be produced by casting technique.

8 Core is positioned in the mould cavity during pouring of the melt

9 Core must be made from the same material of the cast.

10 Pattern may be made from one, or two, or three pieces

11 Pattern is positioned in the mould cavity during pouring of the melt

12 Pattern must be made from the same material of the cast



Q15: Explain the reasons of each of the following:

Using pattern instead of final product in sand molding

Adding draft in sand molding

Using die mould to produce high no of products

Q16: Identify the steps required to cast the following sample

product (fill the missing fields)

Drawing Drawing

Molding Preparation



Drawing Description

Drawing Description

Drawing Description

Drawing Description

Drawing Description



Q17: Write the Suitable Word(s) in the shown spaces:

1. Sand casting mold is made from a mixture of

_____________,____________, and ____________.

2. The process of cleaning the cast, and removing the unwanted

projection is called_____________

3. The process of extracting the cast from the mold is

called_____________

4. To produce a hollow cast, a ____________ must be positioned in

the mold during pouring of the melt.

5. The rise of molten metal in the ___________ indicates that the mold

is completely filled.

6. Riser is made to compensate for _____________ during

solidification of the cast.

Q18: Choose the best answer and make a circle around the

number:

1) Comparing the size of the pattern and the cast, the pattern should

be made----

a. Smaller than the cast

b. Exactly equal to the cast.

c. Bigger than the cast.

2) The last part of the mould filled with molten metal is called-----------

b. Mold cavity.

c. Riser.

d. Pouring cup.

3) -------------------------is better for products made from cast iron.

a. Machining technique b. Forming technique

c. Casting technique d. Welding technique



Q19: Put a check mark (√) against the correct answer, and

cross mark (×) against the wrong answer:

1 ( ) Complex shapes could be produced by casting technique.

2 ( ) Core is positioned in the mould cavity during pouring of the melt.

3 ( ) Pattern is positioned in the mould cavity during pouring of the melt.

4 ( ) Pattern is used to generate the mould cavity.

5 ( ) Core is used to generate the mould cavity.

6 ( ) Pattern is used to generate the cast cavity.

7 ( ) Core is used to generate the cast cavity.

8 ( ) Pattern must be made from the same material of the cast.

9 ( ) Core must be made from the same material of the cast.

10 ( ) Pattern may be made from one, or two, or three pieces.

manufacturing: chapter 3 casting chapter 3 casting mdp024: prepared by: dr. mohamed ahmed awad page...

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