Pattern making

Pattern: A Pattern is a model or the replica of the object to be cast.
 Except for the various allowances a pattern exactly resembles the
casting to be made.
 Patterns may be in two or three pieces, where as casting are in a
single piece.
 A pattern is required even if one object has to be cast.
 The quality of casting and the final product will be effected to a
great extent by the planning of pattern.
 A Pattern prepares a mould cavity for the purpose of making a
casting.
 A Pattern may contain projections known as core prints if the casting
requires a core and need to be made hollow.
 Risers, runners and gates may form a part of the pattern.
 Patterns properly made and having finished and smooth surfaces
reduce casting defects.
 Properly constructed patterns minimize overall cost of the casting.
Functions of Patterns:

Selection of Pattern Materials:
The following factors assist in selecting proper pattern material:
No. of castings to be produced.
Metal to be cast.
Dimensional accuracy & surface finish.
Shape, complexity and size of casting.
Casting design parameters.
Type of molding materials.
The chance of repeat orders.
Nature of molding process.
Position of core print.

Materials for making patterns:
a. Wood
b. Metal
c. Plastic
d. Plaster
e. Wax.

1. Wood Patterns:
These are used where the no. of castings to be produced is small and
pattern size is large.
Advantages:
Inexpensive
Easily available in large quantities
Easy to fabricate
Light in weight
They can be repaired easily
Easy to obtain good surface finish

Limitations:
 Susceptible to shrinkage and swelling
 Possess poor wear resistance
 Abraded easily by sand action
 Absorb moisture, consequently get wrapped
 Cannot withstand rough handling
 Life is very short
Commonly used woods for making patterns:
a. Teak
b. Pine
c. Mahogony
d. Deodar etc..

2. Metal Patterns:
These are employed where large no. of castings have to be produced
from same patterns.
Advantages:
 Do not absorb moisture
 More stronger
 Possess much longer life
 Do not wrap, retain their shape
 Greater resistance to abrasion
 Accurate and smooth surface finish
 Good machinability

Limitations:
 Expensive
 Require a lot of machining for accuracy
 Not easily repaired
 Ferrous patterns get rusted
 Heavy weight , thus difficult to handle
Commonly used metals for making patterns:
i. Cast iron
ii. Aluminium and its alloys
iii. Steel
iv. White metal
v. Brass etc..

3. Plastic Patterns:Advantages:
 Durable
 Provides a smooth surface
 Moisture resistant
 Does not involve any appreciable change in size or shape
 Light weight
 Good strength
 Wear and corrosion resistance
 Easy to make
 Abrasion resistance
 Good resistance to chemical attack
Limitations:
 Plastic patterns are Fragile
 These are may not work well when subject to conditions
of severe shock as in machine molding (jolting).

4. Plaster Patterns:
Advantages:
 It can be easily worked by using wood working tools.
 Intricate shapes can be cast without any difficulty.
 It has high compressive strength.
 Plastermaybe madeout of Plasterof parisor
Gypsumcement.
 Plastermixtureis pouredintoa mouldmadeby
a sweeppatternor a woodenmasterpattern,in
ordertoobtaina Plasterpattern.

5. Wax patterns:
Advantages:
 Provide very good surface finish.
 Impart high accuracy to castings.
 After being molded, the wax pattern is not taken out of the
mould like other patterns;
 rather the mould is inverted and heated; the molten wax
comes out and/or is evaporated.
 Thus there is no chance of the mould cavity getting damaged
while removing the pattern.
 Wax patterns find applications in
Investment casting process.

Types of patterns depend upon the following factors:
i. The shape and size of casting
ii. No. of castings required
iii. Method of moulding employed
iv. Anticipated difficulty of moulding operation

Types of Patterns:
1. Single piece pattern.
2. Split piece pattern.
3. Loose piece pattern.
4. Match plate pattern.
5. Sweep pattern.
6. Gated pattern.
7. Skeleton pattern
8. Follow board pattern.
9. Cope and Drag pattern.

1. Single piece (solid) pattern:
 Made from one piece and does not contain loose pieces
or joints.
 Inexpensive.
 Used for large size simple castings.
 Pattern is accommodated either in the cope or in the
drag.
Examples:
1. Bodies of regular shapes.
2. stuffling box of steam engine.

2. Split piece pattern:
 Patterns of intricate shaped castings cannot be made in
one piece because of the inherent difficulties
associated with the molding operations (e.g.
withdrawing pattern from mould).
 The upper and the lower parts of the split piece
patterns are accommodated in the cope and drag
portions of the mold respectively.
 Parting line of the pattern forms the parting line of the
mould.
 Dowel pins are used for keeping the alignment
between the two parts of the pattern.
 Examples:
1. Hallow cylinder
2. Taps and water
stop cocks etc.,

3.Loose piece pattern:
 Certain patterns cannot be withdrawn once they are
embedded in the molding sand. Such patterns are
usually made with one or more loose pieces for
facilitating from the molding box and are known as
loose piece patterns.
 Loose parts or pieces remain attached with the main
body of the pattern, with the help of dowel pins.
 The main body of the pattern is drawn first from the
molding box and thereafter as soon as the loose parts
are removed, the result is the mold cavity.

4. Match plate pattern:
 It consists of a match plate, on either side of which
each half of split patterns is fastened.
 A no. of different sized and shaped patterns may be
mounted on one match plate.
 The match plate with the help of locator holes can be
clamped with the drag.
 After the cope and drag have been rammed with the
molding sand, the match plate pattern is removed
from in between the cope and drag.
 Match plate patterns are normally used in machine
molding.
 By using this we can eliminate mismatch of cope and
drag cavities.

5. Sweep pattern:
 A sweep pattern is just a form made on a wooden
board which sweeps the shape of the casting into
the sand all around the circumference. The sweep
pattern rotates about the post.
 Once the mold is ready, Sweep pattern and the
post can be removed.
 Sweep pattern avoids the necessity of making a
full, large circular and costly three-dimensional
pattern.
 Making a sweep pattern saves a lot of time and
labour as compared to making a full pattern.
 A sweep pattern is preferred for producing large
casting of circular sections and symmetrical
shapes.

6. Gated pattern:
 The sections connecting different patterns serve as
runner and gates.
 This facilitates filling of the mould with molten
metal in a better manner and at the same time
eliminates the time and labour otherwise consumed
in cutting runners and gates.
 A gated pattern can manufacture many casting at
one time and thus it is used in mass production
systems.
 Gated patterns are employed for producing small
castings.

castings
castings
 Gating system

7. Skeleton pattern:
 A skeleton pattern is the skeleton of a desired shape
which may be S-bend pipe or a chute or something
else. The skeleton frame is mounted on a metal base
 The skeleton is made from wooden strips, and is thus
a wooden work.
 The skeleton pattern is filled with sand and is
rammed.
 A strickle (board) assists in giving the desired shape to
the sand and removes extra sand.
 Skeleton patterns are employed for producing a few
large castings.
 A skeleton pattern is very economical, because it
involves less material costs.

8. Follow board pattern:
 A follow board is a wooden board and is used for
supporting a pattern which is very thin and fragile
and which may give way and collapse under pressure
when the sand above the pattern is being rammed.
 With the follow board support under the weak
pattern, the drag is rammed, and then the fallow
board is with drawn, The rammed drag is inverted,
cope is mounted on it and rammed.
 During this operation pattern remains over the
inverted drag and get support from the rammed sand
of the drag under it.
 Follow boards are also used for casting master
patterns for many applications.

9. Cope and Drag patterns:
 A cope and drag pattern is another form of split pattern.
 Each half of the pattern is fixed to a separate metal/wood
plate.
 Each half of the pattern(along the plate) is molded
separately in a separate molding box by an independent
molder or moulders.
 The two moulds of each half of the pattern are finally
assembled and the mould is ready for pouring.
 Cope and drag patterns are used for producing big castings
which as a whole cannot be conveniently handled by one
moulder alone.

(a)Split pattern
(b) Follow-board
(c) Match Plate
(d) Loose-piece
(e) Sweep
(f) Skeleton pattern

Pattern Allowances:
A pattern is larger in size as compared to the final
casting, because it carries certain allowances due to
metallurgical and mechanical reasons for example,
shrinkage allowance is the result of metallurgical
phenomenon where as machining, draft, distortion,
shake and other allowances are provided on the
patterns because of mechanical reasons.

Types of Pattern Allowances:
The various pattern allowances are:
1. shrinkage or contraction
allowance.
2. Machining or finish allowance.
3. Draft of tapper allowances.
4. Distortion or chamber allowance.
5. Shake or rapping allowance.

1.Shrinkage Allowance:
All most all cast metals shrink or contract
volumetrically on cooling.
The metal shrinkage is of two types:
1. Liquid Shrinkage:
it refers to the reduction in volume when the metal
changes from liquid state to solid state at the solidus
temperature. To account for this shrinkage; riser, which
feed the liquid metal to the casting, are provided in the
mold.
2. Solid Shrinkage:
it refers to the reduction in volume caused when
metal loses temperature in solid state. To account for
this, shrinkage allowance is provided on the patterns.

 Almost all cast metals shrink or contract volumetrically
after solidification and therefore the pattern to obtain a
particular sized casting is made oversize by an amount
equal to that of shrinkage or contraction.
 Different metals shrink at different rates because
shrinkage is the property of the cast metal/alloy.
 The metal shrinkage depends upon:
1. The cast metal or alloy.
2. Pouring temp. of the metal/alloy.
3. Casted dimensions(size).
4. Casting design aspects.
5. Molding conditions(i.e., mould
materials and molding methods
employed)

The contraction of metals/alloys is always volumetric, but the
contraction allowances are always expressed in linear
measures.

2. Machining Allowance:
A Casting is given an allowance for
machining, because:
i. Castings get oxidized in the mold and during heat
treatment; scales etc., thus formed need to be
removed.
ii. It is the intended to remove surface roughness and
other imperfections from the castings.
iii. It is required to achieve exact casting dimensions.
iv. Surface finish is required on the casting.

How much extra metal or how much
machining allowance should be provided,
depends on the factors listed below:
i. Nature of metals.
ii. Size and shape of casting.
iii. The type of machining operations to be
employed for cleaning the casting.
iv. Casting conditions.
v. Molding process employed

Machining Allowances of Various Metals:

3. Draft or Taper Allowance:
 It is given to all surfaces perpendicular to parting line.
 Draft allowance is given so that the pattern can be easily
removed from the molding material tightly packed around
it with out damaging the mould cavity.
 The amount of taper depends upon:
i. Shape and size of pattern in the depth
direction in contact with the mould cavity.
ii. Moulding methods.
iii. Mould materials.
iv. Draft allowance is imparted on internal as
well as external surfaces; of course it is more
on internal surfaces.

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 draft allowance 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 pattern
having proper draft allowance. Here, the
moment the pattern lifting commences, all
of its surfaces are well away from the sand
surface. Thus the pattern can be removed
without damaging the mold cavity.

4. Distortion or cambered
allowance:
A casting will distort or wrap if :
i. It is of irregular shape,
ii. All it parts do not shrink uniformly i.e., some
parts shrinks while others are restricted from
during so,
iii. It is u or v-shape,
iv. The arms possess unequal thickness,
v. It has long, rangy arms as those of propeller
strut for the ship,
vi. It is a long flat casting,
vii. One portion of the casting cools at a faster rate
as compared to the other.

5. Shake allowance:
 A patter is shaken or rapped by striking the same with a
wooden piece from side to side. This is done so that the
pattern a little is loosened in the mold cavity and can be
easily removed.
 In turn, therefore, rapping enlarges the mould cavity which
results in a bigger sized casting.
 Hence, a –ve allowance is provided on the pattern i.e., the
pattern dimensions are kept smaller in order to compensate
the enlargement of mould cavity due to rapping.
 The magnitude of shake allowance can be reduced by
increasing the tapper.

Pattern Layout:
Steps involved:
Get the working drawing of the part for which the
pattern is to be made.
Make two views of the part drawing on a sheet, using a
shrink rule. A shrink rule is modified form of an
ordinary scale which has already taken care of
shrinkage allowance for a particular metal to be cast.
Add machining allowances as per the requirements.
Depending upon the method of molding, provide the
draft allowance.

Pattern Construction:
Study the pattern layout carefully and establish,
a. Location of parting surface.
b. No. of parts in which the pattern will be
made.
 Using the various hand tools and pattern making
machines fabricate the different parts of the pattern.
 Inspect the pattern as regards the alignment of different
portions of the pattern and its dimensional accuracy.
 Fill wax in all the fillets in order to remove sharp
corners.
 Give a shellac coatings(3 coats) to pattern.
 impart suitable colors to the pattern for identification
purposes and for other informations.

Pattern Colors:
Patterns are imparted certain colors and
shades in order to:
i. Identify quickly the main body of pattern and
different parts of the pattern.
ii. Indicate the type of the metal to be cast.
iii. Identify core prints, loose pieces, etc.,
iv. Visualize the surfaces to be machined, etc.

the patterns are normally painted with contrasting
colors such that the mould maker would be able to
understand the functions clearly.
The color code used is,
1. Red or orange on surface not to be finished and left
as cast
2. Yellow on surfaces to be machined
3. Black on core prints for unmachined openings
4. Yellow stripes on black on core prints for
machinsed openings
5. Green on seats of and for loose pieces and loose
core prints
6. Diagonal black strips with clear varnish on to
strengthen the weak patterns or to shorten a
casting.

Reference:
1. A text book of Production Technology Vol. I / O.P.
Khanna / Dhanpat Rai Publications
2. A text book of Production Technology (Manufacturing
Processes) / P. C. Sharma / S. Chand & Company Ltd
3. Manufacturing Technology (Second Edition) / P N Rao
/ Tata McGraw-Hill Publishing Company Ltd

Pattern making

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Pattern making