L3 Casting - U.pptx

Casting Processes
UNIT 1 LECTURE 2 : Casting –
Pattern & pattern making
 Pattern allowances
 Pattern materials & Types
BY
Mr. Arun Kumar Battu
The material provided here is prepared from the inspiration received from my professor Dr. Nageshwarrao. (DRDO scientist )
and also I have gone through various educational sites to collect material just to benefit the students. Regards, Mr. A K Battu (
M.Tech (IPED) NIT SURAT) DYPCOE.AK. Will look forward to improve further

Casting Terms

Pattern
• A pattern is a model or replica of the object to be
cast.
Purpose:
• To prepare a mould cavity of appropriate shape
and size for the purpose of making a casting
• NOTE: casting refers to the component to be
produced

Pattern
• The pattern exactly resembles the casting but the size of the
pattern is different from the size of the casting ( component or
object that is casted)
• Pattern size = casting size ± allowances
• Note: A pattern is slightly larger than the casting/ object
casting Pattern

Why pattern needs allowances
• To compensate metal shrinkage
• For machining so as to clean and finish the
required surfaces.
• To provide necessary draft for easy removal
from the sand mass.

PATTERN ALLOWANCES
• Shrinkage or Contraction allowance
• Machining or Finishing allowance
• Draft or Taper allowance
• Shake or Rapping allowance
• Distortion or Camber allowance
NOTE: The selection of correct allowance greatly
helps to reduce costs and avoid rejections

Shrinkage allowance
Almost all the metals shrink during cooling and expand during
heating
Shrink means reduction in volume
The molten metal is poured in to the casting cavity at a
temperature called as pouring temperature.
P.T= M.P + 150 to 200 ⁰C
P.T>M.P
• Note: Pouring temperature is always greater than Melting
point temperature to avoid solidification of molten metal
during passing through gating system.

Shrinkage allowance
• For Eg: Aluminum M.P = 660 ⁰C
P.T= 660+(150 to 200) =800 ⁰C
• similarly, Iron M.P= 1500
P.T= 1500+(150 to 200)=1700 ⁰C

Shrinkage allowance
(IRON)
P.T(L) (1700 ⁰C) (Liquid)
case1 Sensible Heat
F.T/M.P(L) ( 1539 ⁰C) (liquid)
case2 Latent Heat
F.T/M.P(S) ( 1539 ⁰C) (solid)
case3 Sensible Heat
R.T (s) ( 30 ⁰C )
Sensible Heat: Heat energy liberated or absorbed by the substance before the phase
change takes place
Latent Heat: Heat energy liberated or absorbed by the substance during phase change

Shrinkage allowance
During cooling process
The material is shrinking in all the three cases but
the shrinkage taking place in first & second case is
called liquid shrinkage
The Shrinkage taking place in the third case is called
solid shrinkage.
The liquid shrinkage in casting process is
compensated by providing riser.
The solid shrinkage is compensated by providing
allowance on the pattern.
The material provided here is prepared from the inspiration that I received from my professor Dr. Nageshwarrao. (DRDO
scientist ) and also I have gone through various educational sites to collect material just to benefit the students.
Regards, Mr. A K Battu DYPCOE.AK.

Shrinkage allowance
• Largest shrinkage in the liquid state is
aluminum
• Largest shrinkage in the solid state is
admiralty metal
• Largest total shrinkage (i.e., liquid + solid
) is steel

Shrinkage allowance
The shrinkage allowance on pattern is purely
dependent on linear coefficient of the metal.
Shrinkage allowance=Lα(Tf –Tr)
L- dimension of component,
α – coefficient of linear expansion,
Tf Freezing temperature,
Tr -Room temperature
Specification:
% of Dimensions
2in50 or 2:50

Shrinkage allowance
Coefficient of thermal expansion of brass is
largest
i.e., 23 microns/ ⁰C /m
Invar metal- α= Zero

Machining Allowance
Why we provide machining allowance?
The finish and accuracy achieved in sand casting
are generally poor
Therefore when the casting is functionally
required to be of good surface finish or
dimensionally accurate, it is generally
achieved by subsequent machining.
Machining or finish allowances are therefore
added in the pattern dimension.

Machining Allowance Depends on.
• The amount of machining allowance to be
provided for is affected by the
 method of molding and casting used viz.
hand molding or machine molding, sand
casting or metal mold casting.
size and shape of the casting; the casting
orientation; the metal; and the degree of
accuracy and finish required.

Draft Allowance
• Making the vertical surfaces of the pattern
into inclined surfaces is called draft allowance.
Why we provide draft allowance?
Easy removal of the pattern from the mould.
Vertical surface
Inclined surface

Draft Allowance
With out using draft: During the removal of
pattern from the mould the surface of the
pattern is in contact with mould surface, any
vibrations or shaking of pattern may cause
damage to the mould walls.

Draft Allowance
With the provision of draft: As soon as the
pattern is lifted by small amount there is a
clearances forming b/w the pattern & the
mould so that no damage will take place to
the mould even the pattern is vibrated or
shaked.
1 to 3⁰
For external
draft

Draft Allowance
1 to 5⁰
1 to 3⁰
Internal & External Draft
NOTE: If the pattern is made using wax ,mercury ,polystyrene as pattern
material then there is no need to provide draft allowance
Polystyrene is used as a pattern material for producing large sized
castings like machine tool beds

Distortion Allowance
The change in the shape of the casted
component is called distortion
The tendency of distortion is not common in all
castings.

The casting will distort or wrap if:
• It is of irregular shape
• It is of U or V- shape.
• The arms having unequal thickness.
• One portion of casting cools at faster rate than
the other.

Before Casting
After Casting
Required shape of casting
Due to non uniform HT
Remedy : To eliminate this defect, an opposite distortion is provided on the pattern,
so that the effect is balanced and correct shape of the casting is produced

The amount of distortion depends on the L/T
ratio
L- length of leg
T- Thickness of leg
Note: higher the L/T ratio larger the distortion

Rapping or Shake allowance
• Due to the adhesiveness property of molding
sand , the sand gets sticks to the surface of
the pattern
• when the pattern is removed the sand sticked
on to it comes along the pattern damaging the
mould walls
• To separate the sticked molding sand, the
pattern is shaked before removal

Rapping or Shake allowance
• Due to shaking of the pattern the size of the
cavity increases
• So to reduce the cavity size the pattern size is
further reduced
• This reduction in the dimensions of pattern is
called shake allowance

Characteristics of Pattern Materials
• It should not absorb the moisture
• Light weight or low density
• Better surface finish can be produced
• Easiness in fabrication
• Easily available and cheaper

Pattern Materials
The common materials of which the pattern are
made of
• Wood
• Metal
• Plastics
• Plaster
• Wax

Pattern Materials
The above pattern materials are selected depending on
 The number of casting to be produced
 Degree of accuracy in dimensions and the quality of
surface finish required on the casting
 Method of molding to be used i.e., hand or machine.

Pattern Materials
Types of casting method to be used
i.e., sand casting, investment casting
Shape, size and complexity of the casting

Wood
It is most commonly used pattern material for
sand casting
Advantages:
Cheap & easily Available
Easily shaped and machined
Good surface finish can be easily obtained
Light in weight & manipulation is easy

Wood
Disadvantages/limitations:
Wood has a property of moisture absorption,
hence warped & changes shape and size
Wood can be used as a pattern material for few
no of castings but not for continuous casting
They cannot withstand rough handling
Wood patterns are weak as compared to metal
patterns

Wood
Commonly used woods are
Mahogany
White pine
Maple
Teak
Shisham
cherry

Metals
• Metal patterns are used where large number of
castings have to be produced from same pattern.
• Advantages:
• They do not absorb moisture
• They are stronger and accurate, hence more life
as compared to wooden pattern.
• They have greater resistance to wear
• They can withstand rough handling

Metals
• Aluminum & white metals are commonly used
Because Very low density
• Al-2.89g/cc
• White metals-3.1g/cc
• Expensive and heavier than wood
Note: the metal patterns are produced by
wooden patterns first , It is required to
produce double shrinkage on the wood
patterns.

Plastics
• All the properties required as a pattern
material are possessed by plastics
• Plastics are considered best pattern materials
Limitation:
They may not work well when subjected to
conditions of severe shock
Example: Epoxy resin, polystyrene, PVC, Nylon

Wax
• Wax can be used as a pattern material in
investment casting for producing intricate &
complex shapes of the castings of small size
• Limitation:
• Max weight of the pattern produced by wax is
5kg
• Results in Rapt Tail Defect when used with
green sand

Mercury
• Mercury is not commercially used Pattern
material and used only for special applications
where only small size jobs to be produced
with better surface finish directly without
machining

Types of Patterns
• Single piece or solid pattern
• Two piece or split pattern
• Loose piece pattern
• Gated pattern
• Match plate pattern
• Sweep pattern
• Segmental pattern
• Skeleton pattern
• Cope and drag pattern
• Follow board pattern

Single piece or solid pattern:
Most inexpensive of all types of patterns
Used in cases where the job is very simple and does not create any
withdrawal problems.
Depending upon the quality of the casting to be produced, it is
usually made up of wood or metal.
This type of pattern is expected to be entirely in the drag and one
of the surface is expected to be flat which is used as the parting
plane

Two Piece / split Pattern
• Used when patterns of complicated shape
castings cannot be made in one-piece because
of difficulties associated with the moulding
operation.
• Upper part is accommodated in cope and lower part is
accommodated in drag
Pattern split in to two half's Pattern split in to two half's
Dowel Pins

For keeping the alignment between the two parts of the pattern, dowel pins are used
Dowels
Slot

• Patters of more complicated castings are
made in more than two pieces for their easy
removal and they have three piece flasks for
Moulding purpose.
Dowels
Cheek
Drag
Cope
Pattern

Loose piece Pattern
Patterns embedded in the moulding sand cannot be withdrawn
Such patterns are made with one or more loose pieces for their
easy removal
The two extensional parts are held
in position by the use of pins or
wires.
After being rammed up, the part
A is removed, leaving the parts B
and C still in their positions in the
sand.
These may now be carefully
removed toward the center of the
opening and lifted out
B C
A

Gated Pattern
• When Large number of identical
castings are to be produced
• Two are more Individual patterns
are connected together
• The sections connecting various
patterns serve as a runner and gates
• This facilitates filling of mould with
molten metal in better manner and
reduces the required time and
labour work
• Applicable for mass production
• Made of metals to increase strength
and reduce the tendency to wrap
Runner
Gates
Patterns

Match Plate Pattern
Large – quantity production of
small castings requires match-plate
patterns
The cope and drag portions of the
pattern are mounted on opposite
sides of a wood or metal plate.
Match plates are also integrally
cast in which cast pattern and plate
are cast as one piece.
The match plate has runner and
gates attached to it

Match Plate Pattern
used when moulding process is done using molding
machine to obtain maximum speed of molding
Advantages of the patterns are:
(a) Costly but good production rate
(b) Increase the dimensional accuracy
• Piston rings of an I.C. engine are made by using these
patterns

Sweep Pattern:
• Sweep pattern is just a
form made on a wooden
board which sweeps the
casting shape into the
sand all around the
circumference.
Post
Sweep
Green Sand
The sweep is rotated around the spindle/post, to
form cavity
Once the mould is ready, sweep pattern and post
can be removed.

Sweep Pattern
• It saves lot of time and labour work as
compared to making full pattern.
• Sweep patterns are used for making large
castings of circular sections and symmetrical
shape.

Segmental Pattern
• The working principle of
segmental pattern is similar
to sweep pattern
• The main difference is that, a
sweep is given a continuous
revolving motion to generate
required shape, whereas a
segmental pattern is a
portion of the solid pattern
itself and the mould is
prepared in parts by it.
Pivot

Skeleton Pattern
• When the casting size is very
large , but easy to shape and
few are to be made, then it is
not economical to make a large
solid pattern of that size.
• Example : Lathe Bed
Skeleton pattern
Loam sand
In such cases, pattern consisting of a wooden
frame and strips is made which is called as
skeleton pattern
Then it is filled with loam sand and rammed.

Cope and Drag Pattern
• It is another form of split
pattern
• The pattern is split about
a suitable surface or line.
• Each half of the pattern
is fixed to separate plate
and besides the pattern
it has provision for
runner and gates.
These patterns are used for producing
large casting.

Follow board Pattern
sand
A Follow board is a wooden board
and is used for supporting a pattern
which is very thin and fragile
Follow board support under weak
pattern
Follow board patterns are used for
casting master patterns for many
applications

NO YES Neutral
Understood

Thank You

L3 Casting - U.pptx

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