casting [compatibility mode].pdf

7/27/2019 CASTING [Compatibility Mode].pdf

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CASTINGIt is the process of producing metallic parts by pouring

molten metal in to the mould and allowing the metal to

solidify.

Eg:- Automobile engines, Pistons, machine tool

components, wheels & housings of steam turbines,

sanitary fittings etc.,

Mould is a void/cavity created in the compact sand masswhich, when filled with molten metal produce a casting.

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Steps in a Casting process

Pattern making Sand preparation and testing

Core making

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Moulding Metal melting

Metal pouring

Solidification & Cooling to room temperature Finishing operations


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Dept. of Mech & Mfg. Engg. 4


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Green sand mold DragPattern

DragDragPattern

FIRST OP

Drag

Molding board

Riser pin Sprue pinCo e

Pouringbasin

Weight LifterVents

6SECOND OPERATION

Parting

line

COMPLETE MOLD

Riser

Parting

line

Core

Runner

GateDrag


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Sand Casting Here a suitable foundry sand is

rammed around a wooden pattern

The two halves of the moulding boxbeing capable of separation

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a mould cavity into which the moltenmetal can be poured

MOULDING SAND- SAND CASTING

METAL MOULD- DIE CASTING


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Casting process

Metal is heated above melting

Liquid metal poured into mold

Metal solidifies into shape of mold

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Part removed from mold Post Casting Operations

Clean Up

Machining


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Properties/ Requirements of a molding sand

Porosity (permeability) Sand must be porous to allow the gases/ moisture present

within the molds to be removed freely/escape when moltenmetal is poured

Adhesiveness sand particles must be capable of adhering to sides of

molding boxes

h iv n / tr n th

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Ability of sand particles to stick/bind together firmly Should be sufficient to permit the mold to be formed to the desired

shape even after the hot metal is poured in the mold

Refractoriness

-enables to withstand high temperature of the moltenmetal without fusing thus facilitating a clean casting.

Molding sands with a poor refractoriness may burn on to thecasting.


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Collapsibility

Mold made out of sand has to automatically collapse as the

molten metal gets solidified in it

Flowability/plasticity

Ability of the sand to behave like a fluid so that, when rammed

Properties of a molding sand

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take up the required shape/packs properly around the pattern

It increases as clay & water content increase

High flowability is required of a molding sand of uniform density

To obtain good impression of the pattern in the mold.


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Molding sand classification

They can be classified as:

1. Natural molding sand (Green sand)

- Taken from river beds & contain major amount of clay

- easily available at low cost

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- contain little or no binder (clay)

- strength & bonding property can be easily controlled

- more expensive than natural sand


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Sand preparation

Very few natural sands have all the qualities

required by a molding sand

It is mixed with other sands which possess the

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Sufficient moisture must be added to the sand

Sand must be properly conditioned before use


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PATTERN

A pattern is a replica of a casting to be made.

Using pattern, impressions (mould cavity) can beformed in the damp sand to make a mould.

Difference b/n pattern & casting.

Pattern size is slightly larger than the casting

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allowances.

Pattern may not have all holes and slots likecasting.

Pattern may be single /two piece.


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Pattern materials-Any material which will retain exact outline of the designfor required number of molds

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Wood Most commonly used, easy to work, readilyavailable, light in weight, cheap & abundantly available. Can be cut in to required shapes. Used when small number of castings is required.

Disadvantages- Wears out quickly, absorb moisture contentand hence changes dimension in long run/life is less withgood dimensional accuracy.

Pattern materials

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-

moist condition, surface finish, dimensional accuracy isgood compared with wood

- used when large number of castings is required with

close dimensional accuracy.


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Plastics do not absorb moisture,smooth , can be withdrawn from the moldeasily,without damaging the mold

Plaster

WAX



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Types of patterns

-Single piece pattern

-Split pattern

-Cope & Drag pattern

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- Match plate pattern

-Loose-piece pattern

-Gated pattern


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Types of patterns

Dowel

pins

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Made from one piece & does not contain loose pieces

Inexpensive & are of simplest type

Single piece pattern

Split pattern

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Here the pattern is split in to 2 parts so that one part is in

the cope & other is in the drag

Two halves of the pattern is aligned properly by usingdowel pins


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Gated pattern

Match Plate pattern

Cope plat

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Gated pattern

Sand

Cope & Drag pattern

Drag plat


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Similar to split patterns

The cope & drag halves of the pattern along with the gating

and risering systems

But they are attached separately to the metal or wooden

plate along with alignment pins

Cope and Drag pattern

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Match plate pattern

An extension of cope and drag pattern

Here the cope & drag patterns along with the gating &

risering are mounted on a single matching or metal plate

on either side


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Used when contour of the part is such that withdrawing the

pattern from the mold is not possible

Obstructing part is held as a loose piece

Loose piece pattern

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SEGMENTAL PATTERN:-FOLLOW BOARD PATTERN:-

SKELETON PATTERN:-


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Usually made of metal whichincreases their strength andreduces tendency to warp

Section connecting differentpatterns serve as runner andgates

Gated pattern

Gated pattern

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ga e pa ern can manu ac uremany castings at one time

Used in mass productionsystems

Gated pattern

Sand


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Sweeps are used for

preparing the mouldsOflarge symmetricalcastings.

The sweep is rotated about

the spindle to form thecavity.

Once the cavity is formed,

WOODEN BOARD/ SWEEP

Sweep pattern

t e o e ma e y t eremoval of the spindle ispatched-up by filling thefoundry sand.

Eg: Bells,cast iron kettles areprepared by this method.

Dept. of Mech & Mfg. Engg.24


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Components of a gatingsystem


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Runner:- serves as a passage through which the molten

metal is poured.Riser (sprue, Pin) :- indicates that mould is filled up ornot. Both runner and riser Supplies the metal to the

casting to compensate the shrinkage In the first stagesof cooling.

Vents :- Holes are rovided to esca e ases and

Dept. of Mech & Mfg. Engg.26

vapours from the mould

Gate :- one of the channel which leads molten metal

into mould cavity


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Pattern Allowances

1. Shrinkage allowance

2. Machining or Finishing allowance

3. Draft or Taper allowance

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4. Shake or Rapping allowance

5. Camber or distortion allowance

6. Mould wall movement allowance


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Pattern Allowances

1. Shrinkage allowance

Most of the metals used in casting contract during

cooling from pouring temperature to room temperature

Provided to compensate for solid contraction by

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Amount of contraction varies with different metals


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Pattern Allowances

2. Machining allowance

Given on the surface of the pattern by increasing the metal

thickness there to compensate for loss of metal due to

machining on these surfaces

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Depends mainly upon method of machining to be employed

Given in addition to the shrinkage allowance


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Pattern Allowances

Given slight taper on the vertical surfaces of the pattern

parallel to the direction of withdrawal from the mould

3. Draft or Taper allowance

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For easy withdrawal of pattern from the mould


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Pattern Allowances

Negative allowance to be provided in the pattern

When pattern is withdrawn from the mould by striking over

it from side to side size of the cavit increases

4. Rapping / Shake allowance

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To compensate this rapping allowance is provided


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Pattern Allowances

Castings having irregular shape, will not have uniform

contraction during their cooling

Results in distortion of the castin s

5. Distortion allowance

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Remedy:

An opposite distortion is provided in the pattern


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Pattern Allowances

Mould wall movement takes place because of excessive

heat and pressure applied by the molten metal on the surface

layer of the sand

Affects the size of the casting

6. Mould wall movement allowance

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Remedy:

Provide proper allowance in the pattern

Controlling the density & temperature of molten metal

Controlling the composition of the molding sand


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Scooping

It is the process of making a cavity in the sand mould

without using a pattern.

Instead it makes use of casting hand tools like shovel,

rammer etc.

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Defects in casting

Sand castings are subjected to certaindefects which in a well designed casting

are controllable by proper foundrytechnique, but are not whollypreventable.

Dept. of Mech & Mfg. Engg.

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D f t i ti


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Defects in casting Blowholes

Smooth round holes appearing in the

form group of small holes below the surface

of casting

Due to entrapped bubble of gasesPinholes:These are nothing but tiny blow holes found in largenumbersoccur either at or just below the casting surface.

Scabs

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Projection on the casting that occur when aportion of the mould lifts and metal flows

Drop

Occurs when upper surface of the mold cracks

and pieces of sand falls in to the molten metal

Shrinkage cavity- Insufficient molten metal

Void created in the casting mainly due to

uncontrolled solidification of the metal


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Internal air pocket

It appears as small holes inside the casting, due to rapid pouringof molten metal in to the mold

Shifts

An external defect caused due to core misplacement or

mismatching of top and bottom parts of the casting Fin

A thin projection of the metal

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Caused due to incorrect assembling of molds or cores Swell

Enlargement of metal cavity by metal pressure

This is caused by improper ramming

Warpage

Undesirable deformation in a casting that occurs duringsolidification


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Shrinkage cavity-Insufficient molten metal

Warpage

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General Casting DefectsBack

Scabs


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BlowholesDro

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s

Swell


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Casting Advantages Complex Part Geometries

Net Shape Processes Available

Suitable for Very Large Parts Suitable for Any Metal

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epen s on t e pec c rocess

Some Methods Suited to MassProduction.


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Casting Disadvantages

Vary By Method

Limitation on Metal Properties

Porosity Poor Dimensional Accuracy

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Safety and Environmental Concerns

High Initial Cost (Patterns)


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***Explain with sketches, the method of preparing a green

sand mould. The drag box is first placed inverted over the moulding platform as shown inFig A. The bottom half of the pattern is kept inverted in the centre of the box such that its

parting surface lies on the top face of the platform. Now the drag box is filled with themoulding sand and is rammed firmly round the pattern.

The drag is then inverted along with the pattern and placed upright as shown in Fig. B. Theother half of the pattern is then placed over the bottom half of the pattern in the drag.Fine dry sand, known as the parting sand is sprinkled over the exposed mould surfaceof the drag. After the runner or sprue pin and the riser pin are placed in the appropriateplaces, the cope box is filled with the moulding sand and suitably rammed. The pouringbasin is cut by hand around the runner or sprue pin. Suitable number of vent holes are

made by piercing the vent rod: The riser and sprue pins are withdrawn. The cope islifted off carefully and placed upside down on a prepared sand platform. The patterns

are lifted out from the cope and drag by gently rapping. Now the core is placed in itsposition. The cope is lifted and placed over the drag.

[Sketch = 2 marks Expln = 2 marks]


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