metal casting lectures 2

7/29/2019 Metal Casting Lectures 2

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Part Two:

Metal casting Processes


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SyllabusPart A(Sand casting)1. Patterns and cores

2. Molds and mold making3. The casting operations

Part B (Other expandable mold casting processes)1. S hell molding2. Vacuum molding3. Expanded polystyrene process4. Investment molding5. Plaster mold and ceramic mold casting

Part C(Permanent mold casting processes)1. The basic permanent mold process2. Variations of permanent mold casting3. Die casting4. Centrifugal casting

Part D(Foundry Practice)1. Furnaces2. Pouring, Cleaning and heat treatment

Part E(Casting quality)


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Part A(Sand casting)

1. Patterns and cores2. Molds and mold making3. The casting operations


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Two Categories of Casting Processes

1-Expendable mold processes

uses an expendable mold which must bedestroyed to remove castingMold materials

1. sand,

2. plaster,3. and similar materials, plus binders

2-Permanent mold processesuses a permanent mold which can be used over and over to produce many

castingsMade of metal (or, less commonly, a ceramic refractory material


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Advantages and Disadvantages

More intricate geometries are possible

with expendable mold processes

Part shapes in permanent mold processes

are limited by the need to open the mold

Permanent mold processes are more

economic in high production operations


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Types of casting

Casting

Conventional Methods Unconventional Methods

Green sand mouldDry sand mould

CO2 Moulding (Strong mould)Permanent (Metal mould)

Shell Moulding (Thin mould)

Investment casting (Precision)

Centrifugal ( without core)

Continuous Casting (Open)


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Casting

Casting is one of the oldest manufacturing process and eventoday is the first step in manufacturing most of the products

It is based on the property of a liquid to take up the shape of

vessel containing it.

Products ranging from a few mm to several m anda few grams to several tons

Foundry is the place where metal is melted

and casting are produced

Similar to that of making ice cubes in a refrigerator


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Sand Casting Sand casting is the process of pouring molten metal in to

a mould cavity of required shape & size and allowing forcooling and the breaking up the mold to remove the casting

Six Basic Steps of Casting1. Pattern making

2. Core making

3. Mould making

4. Melting

5. Pouring6. Cleaning and inspection


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Six Basic Steps of Casting


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1-The Pattern

A full-sized model of the part, slightly enlarged to account for shrinkageand machining allowances in the casting

Pattern materials:

Wood- common material because it is easy to work, but it warps

Metal- more expensive to make, but lasts much longer

Plastic- compromise between wood and metal

Waxprecision casting


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Types of Patterns(pattern geometry)

(a) Solid pattern ( single piece)

(b) Split pattern ( Two piece)

(c) Match plate pattern

(d) Cope and drag pattern


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Pattern Allowances

1. Shrinkage or contraction allowance

2. Draft or taper allowance

3. Machining or finish allowance 4. Distortion or camber allowance

5. Rapping allowance


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1-Shrinkage or Contraction Allowance

All most all cast metals shrink or contract volumetrically on cooling. The

metal shrinkage is of two types:

i. Liquid Shrinkage: it refers to the reduction involume when the metal changes from liquid state to

solid state at the solidustemperature. To account for this shrinkage; riser, which

feed the liquid metal to the casting, are provided in the

mold.

ii. Solid Shrinkage: it refers to the reduction involume caused when metal loses temperature in

solid state. To account for this, shrinkage allowance

is provided on the patterns


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2. Draft or taper allowance

By draft is meant the taper provided by the pattern

maker on all vertical surfaces of the pattern so that it can

be removed from the sand without tearing away the

sides of the sand mold and without excessive rapping by

the molder. Figure 3 (a) shows a pattern having no draftallowance being removed from the pattern. In this case,

till the pattern is completely lifted out, its sides will

remain in contact with the walls of the mold, thus tending

to break it. Figure 3 (b) is an illustration of a patternhaving proper draft allowance. Here, the moment the

pattern lifting


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Pattern Having Draft on Vertical Edges

Draft

Pattern Having No Draft on Vertical Edges


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3-Machining or finish allowance

The finish and accuracy achieved in sand

casting are generally poor and therefore when

the casting is functionally required to be of good

surface finish or dimensionally accurate, it isgenerally achieved by subsequent machining.

Machining or finish allowances are therefore

added in the pattern dimension.


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2-Core in Mold Acore consists of two portions: the body of the core and one or more extensions

(called prints)

Cores are used to create internal cavities. Core is a separate entity placed in a mould to produce a corresponding cavity hole

or undercut in the casting

Cores for sand casting are manufactured by packing specially prepared sand in coreboxes

Chaplets: is used to the support the core


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Major part of Moulding material in sand casting are

1. 85-90% silica sand (SiO2)

2. 3-7% bonding material e.g., clay cereal etc.

3. 3-6% water

Requirements of molding sand are:

(a) Refractoriness

(b) Cohesiveness

(c) Permeability

(d) Collapsibility

The cavityin the sand mold is formed by packing sand around a pattern,

then separating the mold into two halves and removing the pattern

3-Moulds and mould making


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Mould Material

Major part of Moulding material in sand castingare

1. 70-85% silica sand (SiO2)

2. 10-12% bonding material e.g., clay etc.

3. 3-6% water

The performance of mould depends on following

factors:

(a) Permeability

(b) Green strength

(c) Dry strength


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Requirements of molding

sand

1. Refractoriness(strength)

2. Permeability

3. Collapsibility4. Thermal stability

5. Reusability


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Requirements of molding sand

(1) Refractoriness(strength)

Is the ability to remain solid at high temp and resist

erosion caused by flow of molten metal

(2) Permeability

Is the ability gas flowthrough mould

(3) Collapsibility

Is theability of the mold to give a way and allow the castingto shrink without cracking the casting,i,e to permit metal

to shrink after solidification


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Requirements of molding sand

(4)Thermal stability

Is the ability of sand at the surface of the mold cavity to

resist cracking and buckling upon contact with the

molten metal.

(5)Reusability

Is the ability of sand to be used for making other mold


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4-Melting (next)

Furnaces most commonly used in foundries:

Cupolas

Direct fuel-fired furnaces

Crucible furnaces Electric-arc furnaces

Induction furnaces


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5-Pouring the Molten Metal(next)

For this step to be successful, metal must flow into

all regions of the mold, most importantly the main

cavity, before solidifying

Factors that determine success:

Pouring temperature

Pouring rate

Turbulence


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Sections of foundry

Metal

Melting

Handling

molten metal

Sand

Sand mixing & preparation

Moulding

Pouring

Shaking out

Finishing

Heat treatment

Inspection & Testing

Melting Section Moulding Section

Sand additives


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General layout of foundry

Foundry store

Melting

Sand

Moulds ready for pouring

Furnaces

Inspection Bench Core

making

Grinding

Muller


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Preparation of sand mould (Sand Mold Making Procedure)(

Before any casting can take place a wooden pattern is made precisely. This iscalled pattern making and in industry this is a very skilful job. Any inaccuracy

at this stage will result in the final cast being wrong or even failing.

Drag is placed inverted on the mouldfloor and pattern is placed at the center

of the box

SAND CASTING


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Special casting sand will soon bepacked around the pattern for easy

removal of pattern from

parting powder is sprinkled over and

around it. It stops the casting sand

sticking to the pattern and pulling away

with it when the pattern is finallyremoved from the sand.

Casting sand is then shaken through a sieve (called

riddled sand) so that only fine particles fall around

the pattern. This is called facing sand and it must befine so that detail on the pattern shows up on the

final casting.

Preparation of sand mould


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The drag is then packed with more casting sand

and then ram it down firmly using a ramming tool.The tool has two ends, one is cylindrical and is

used for general packing down of the sand. The

other end is quite pointed and this can be used for

packing sand close up to the pattern.

When the drag is packed fully it is levelled

off (called strickled off ) using a straightsteel bar.



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The entire drag and its contents are then turned over so that the base of

the pattern can be seen

A top box called a cope is then placed on top of the drag and locating pins

are put in position so that the casting boxes cannot move sideways.



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Sprue pins are positioned. One usually on

the back of the pattern and the other to the side.

These will eventually provide an entrance and

exit for the molten aluminium when it is poured

into the sand.

The sand is packed/rammed into the cope in the same way as the drag.



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The top box (the cope) is then removed and if all is well the cope with the

sand inside should lift off the drag (bottom box) without the sand falling out.A small gate is cut below the position of one of the sprue pins. This will

help the molten metal to flow into the cavity left by the mould. Small

tools are available or can easily be made to dig a variety of shapes in the

casting sand. They are similar to small trowels



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The pattern is removed using a spike. Before

removing the pattern it is a good idea to gentlytap the spike so that it loosens the pattern from

the sand. It can then be lifted away from the

casting box (drag).

The cope (top casting box) is placed back on top of the drag and thelocating pins put in position. Before this is done vents can be created using a

thin piece of welding rod, pushing it through the sand . This allows gases to

escape once the molten metal is poured.



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The molten metal is poured with great care. The molten metal is poured

down the hole left by the first sprue pin (now called the runner). As it runs

down the runner it flows through the gate cut by the trowel, into the cavity

left by the pattern and up the riser (the hole left by the second sprue pin).

The casting should be left for at least an hour before removal from the sand

When removed from thesand, the runner and riser are

cut away and the casting is

ready for machining



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metal casting lectures 2

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