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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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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.
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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
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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.
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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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