patterns

1. Patterns

2. INTRODUCTION TO PATTERN MAKING
• A pattern is a mold forming tool in the hands of foundry men.
• 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.
• A pattern is required even if one object has to be cast.
• A pattern may be defined as a model or form around which
sand is packed to give rise to a cavity known as mold cavity
in which when, molten metal is poured, the result is the CAST
OBJECT.
7/28/2017 2

3. Difference between pattern and casting
• The main difference between a pattern and the casting is their dimensions.
• A pattern is slightly larger in size as compared to the casting, because a pattern,
– carries Shrinkage allowance, it may be of the order of 1 to 2 mm/ 100 mm.
– is given a Machining allowance to clean and finish the required surfaces.
– carries a Draft allowance of the order of 1 and 3 degrees for external and
internal surfaces respectively
– carries core prints.
• A pattern may not have all holes and slots which a casting will have. Such holes and
slots unnecessarily complicate a pattern and therefore can be drilled in the casting
after it has been made.
• A pattern may be in two or three pieces whereas a casting is in one piece.
• A pattern and the casting also differ as regards the material out of which they are
made.
7/28/2017 3

4. Functions of a patterns
• A pattern prepares a mold 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.
• Runner, gates and risers (used for introducing and feeding molten
metal to the mold cavity) may form a part of the pattern.
• A pattern may help in establishing locating points on the mold and
therefore on the casting with a purpose to check the casting
dimensions.
• Patterns establish the parting line and parting surfaces in the mold.
• A pattern may help position a core (in case a part of mold cavity is
made with cores), before the molding sand is rammed.
• Patterns that are properly made and having finished and smooth
surfaces, reduce casting defects.
• Properly constructed patterns minimize overall cost of the castings.
7/28/2017 4

5. Pattern materials
• The following factors assist in selecting proper pattern material:
 The number of castings to be produced. Metal patterns are preferred
when the production quantity is large.
 The desired dimensional accuracy and surface finish required for the
castings.
 Nature of molding process i.e., sand casting, permanent mold casting,
shell molding, investment casting etc.
 Method of molding i.e., hand or machine molding.
 Shape, complexity and size of the casting.
 Type of molding materials i.e., sand etc.
 The high probability of changing the casting and hence the pattern in
near future.
Selection of pattern materials
7/28/2017 5

6. Materials for making patterns
• Patterns may be constructed out of the following
materials.
(a) Wood (b) Metal
(d) Plastic (d) Plaster (POP)
(e) Wax
7/28/2017 6

7. WOOD
• The most common materials for making patterns for sand casting is the wood.
Advantages
– Inexpensive.
– Easily available in large quantities.
– Easy to machine and to shape to different configurations and forms
– Easy to join to acquire complex and large pattern shapes
– Light in weight
– Easy to obtain good surface finish
– Wooden patterns can be preserved for quite long times with the help of suitable wood
preservatives.
Limitations
– Wooden patterns are susceptible to shrinkage and swelling.
– They possess poor wear resistance.
– They are abraded easily by sand action.
– They absorb moisture, consequently get deformed and change shape and size.
– They cannot withstand rough handling.
– They are weak as compared to metal patterns.
7/28/2017 7

8. Metal
• Metal patterns are employed where large number of
castings have to be produced from the same patterns.
• Metal patterns are cast from wooden patterns.
• The different metals and alloys used for making
patterns are,
– Aluminium and Aluminium alloys
– Steel
– Cast Iron
– Brass
– White Metal
7/28/2017 8

9. Advantages of Metal Patterns
– Unlike wooden patterns, they do not absorb moisture. They retain their shape.
– They are more stronger and accurate as compared to wooden patterns.
– They possess life much longer than wooden patterns.
– They can withstand rough handling.
– They do not distort
– They possess greater resistance to abrasion. They have accurate dimensional tolerances.
They are far stable under different environments.
– It is easy to obtain smooth surface finish.
– They possess excellent wear resistance and strength to weight ratio.
Limitations of Metal Patterns
– Expensive as compared to wood patterns.
– Are not easily repaired e.g. (Aluminium patterns).
– Ferrous patterns get rusted.
– They (ferrous patterns) are heavier than wooden patterns,
– They cannot be machined so easily as wooden ones.
7/28/2017 9

10. Plastic
• Advantages
– Durable
– Provides a smooth surface.
– Moisture resistant.
– A plastic pattern does not involve any appreciable change in its size or shape.
– Lightweight.
– Wear and corrosion resistant.
– Provides good surface finish.
– It possesses low solid shrinkage.
• Limitations
– Plastic patterns are fragile and thus light sections may need metal
reinforcements.
– Plastic patterns may not work well when subject to conditions of severe shock as
in machine moulding.
7/28/2017 10

11. PLASTER
• Advantages
– can be easily worked by using wood working tools.
– Intricate shapes can be cast without any difficulty.
– It has high compressive strength (up to 285 kg/cm2)
• Disadvantages
– Can be used for small castings only
• Plaster Patterns Material: Plaster patterns may be
made out of Plaster of Paris or Gypsum cement.
• Applications: Plaster is used for making (i) Small and
intricate patterns, and (ii) Core boxes.
7/28/2017 11

12. WAX
• Advantages
Wax patterns provide very good surface finish
They impart high accuracy to the castings.
After being molded, the wax pattern is not taken out of
the mold like other patterns; rather the mold is
inverted and heated; the molten wax comes out and/or
is evaporated. Thus there is no chance of the mold
cavity getting damaged while removing the pattern.
• Applications
• Wax patterns find applications in Investment
casting process.
7/28/2017 12

13. PATTERN MATERIALS:
(1) Wood: The most commonly used pattern material is wood, the main reason
being the easy availability, low weight, it can be easily shaped and cheap
 Ex: Pine, Teak
(2) Metal: Metal patterns are extensively for casting, because of their strength,
accuracy, good dimensional stability, durability and smooth surface finish
 Many materials such as Cast iron, Brass can be used as pattern materials
 Aluminium and White metal are most commonly used, these materials are
light weight, it can be easily worked, corrosion resistant
(3) Plastics: This are also used as pattern materials because of their low weight,
easier formability, good dimensional stability, smooth surfaces and durability
 Ex: Epoxy resin
(4) Plasters: It has high strength, it can be easily formed into complex shapes and
it can be used only for small pattern
 Ex: Gypsum cement
(5) Waxes: Excellent material for investment casting and it has good surface
finish
 Ex: Paraffin Wax, Bees Wax
(6) Rubbers: Mainly used for investment casting

14. PATTERN ALLOWANCES
• A pattern is always larger in size as compared to
the final casting, because it carries certain
allowances.
• The various pattern allowances are below
(a) Shrinkage or contraction allowance.
(b) Machining or Finish allowance.
(c) Draft or Taper allowance.
(d) Distortion or camber allowance.
(e) Shake or rapping allowance.
7/28/2017 14

15. Shrinkage Allowance
• Almost all cast metals shrink or contract volumetrically after solidification and
therefore to obtain a particular sized casting, the pattern 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 or alloy.
• The metal shrinkage depends upon
• The cast metal or alloy.
• Pouring temperature of the metal or alloy.
• Casting dimensions (size).
• Casting design aspects.
• Molding conditions (i.e., mold materials and molding methods employed).
• Cast iron poured at higher temperatures will shrink more than that poured at lower
temperature.
• Wood patterns used to make metallic patterns are given double allowance; one for
the shrinkage of the metal of the pattern and the other for that of metal to be cast.
7/28/2017 15

16. Machining Allowance
• A casting is given an allowance for machining, because
– Castings get oxidized in the mold and during heat treatment; scales etc., thus formed need
to be removed.
– It is intended to remove surface roughness and other imperfections from the castings.
– It is required to achieve exact casting dimensions.
• How much extra metal or how much Machining allowance should be provided,
depends upon:
• Nature of metal i.e., ferrous or non-ferrous. Ferrous metals get scaled whereas non-ferrous ones do not.
• Size and shape of the casting. Longer castings tend to warp and need more material (i.e., allowance) to be added
to ensure that after machining the casting will be alright.
• The type of machining operation (i.e., grinding, turning, milling, boring etc.) to be employed for cleaning the'
castings. Grinding removes much lesser metal as compared to turning.
• Casting conditions i.e., whether casting conditions result in a rough casting or a semi-finished one. Casting conditions
include the characteristics of mold-materials etc.
• Molding process employed. Die casting produces parts which need little machining (allowance) whereas sand-
casting, require more machining allowance.
• Number of cuts to be taken. Machining allowance is directly proportional to the number of cuts required for finishing the casting.
• The degree of surface finish desired on the cast part.
7/28/2017 16

17. Draft or Taper Allowance
• It is given to all surfaces perpendicular to the parting line.
• Draft allowance is given so that the pattern can be easily
removed from the molding material tightly packed around it
without damaging the mold cavity.
• The amount of Taper depends upon
– Shape and size (length) of the pattern in the depth direction in contact with the
mold cavity.
– Molding method.
– Mold materials.
• Draft allowance is imparted on internal as well as external
surfaces; of course it is more on internal surfaces.
7/28/2017 17

18. Fig. shows two patterns — one with taper allowance and the
other without it. It can be visualized that it is easy to draw the
pattern having taper allowance, out of the mold without
damaging mold walls or edges.
Taper on external surfaces = 10 to 25 mm/metre.
Taper on internal surfaces = 40 to 65 mm/metre,
7/28/2017 18

19. Distortion Allowance
• A Casting will distort or warp if:
– it is of irregular shape,
– all its parts do not shrink uniformly i.e., some parts shrink while others are
restricted from doing so,
– it is U or V-shaped,
– it has long, rangy arms as those of the propeller strut for the ship,
– it is a long flat casting,
– the arms possess unequal thickness,
– one portion of the casting cools at a faster rate as compared to the other, etc.
• Distortion can be practically eliminated by providing an allowance and
constructing the pattern initially distorted i.e., outsize in the opposite
direction so that the casting after cooling neutralizes the initial distortion
given on the pattern and acquires the correct shape.
• The amount of distortion allowance may vary from 2 to 20 mm depending
upon the size, shape and material of the casting.
7/28/2017 19

20. Shake Allowance
• A pattern is shaken by striking the same with a wooden piece
from side to side. This is done so that the pattern is loosened
a little in the mold cavity and can be easily removed.
• In turn, therefore, shaking enlarges the mold cavity which
results in a bigger sized casting.
• Shake allowance is normally provided only to large castings
because it is negligible in case of small castings and is thus
ignored.
• The magnitude of shake allowance can be reduced by
increasing the taper.
7/28/2017 20

21. TYPES OF PATTERNS
7/28/2017 21

22. • For selecting a particular kind of pattern for making a casting,
one may consider the following points:
– Quantity of castings to be produced.
– The size and the complexity of the shape of the casting to be
produced.
– Type of molding method to be used (i.e., hand or machine
molding).
– Problems associated with the molding operation such as
withdrawing the pattern from the mold etc.
– Other difficulties resulting from poor casting design or pattern
design.
7/28/2017 22

23. • The different types of patterns commonly used
are:
– One piece(Solid) pattern
– Split pattern
– Loose piece pattern
– Match plate pattern
– Cope and Drag pattern
– Sweep pattern
– Gated pattern
– Skeleton pattern
– Follow board pattern.
7/28/2017 23

24. One Piece (solid) Pattern
7/28/2017 24

25. One Piece (solid) Pattern
• It is the simplest type of pattern.
• As the name suggests the pattern is
made from one piece and does not
contain loose pieces or joints.
• It is inexpensive.
• It is used for making a few large size simple castings
• One piece pattern is usually made up of wood or
metal depending upon the quantity of castings to be
produced.
• For making the mold, one piece pattern is
accommodated either in the cope or in the drag.
7/28/2017 25

26. Split Pattern
7/28/2017 26

27. Split 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 the pattern from the mold etc.) , such
patterns are, then, made as split or two piece patterns.
• The upper and the lower parts of the split pattern are
accommodated in the cope and drag portions of the mold
respectively.
• Dowel pins are used for keeping the alignment between the two
parts of the pattern.
• The parting (surface or) line of the pattern forms the parting
(surface or) line of the mold.
• Patterns for still more intricate castings are made in more than two
pieces for facilitating their molding and withdrawing.
• A pattern having three pieces will require a three piece flask for the
molding purposes7/28/2017 27

28. Loose Piece Pattern
7/28/2017 28

29. 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
• Pieces for facilitating their removal 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 the loose parts are
removed, the result is the mold cavity.
• Loose piece patterns involve more labour and consume
more time in the molding operation.
7/28/2017 29

30. Match Plate Pattern
7/28/2017 30

31. Match Plate Pattern
• A match plate pattern consists of a match plate, on
either side of which each half of (a number of) split
patterns is fastened.
• A number 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.
• The match plate has runner and gates also attached with
it.
• After the cope and drag have been rammed with the
molding sand, the match plate pattern is removed from
in between the two (i.e., cope and drag.)
7/28/2017 31

32. Match Plate Pattern
• Cope and drag are then assembled and this completes
the mold.
• Patterns, match plate, runner and gates — all may be
made up of aluminium, because it is light and
relatively inexpensive.
• Match plate patterns are normally used in machine
molding.
• Match plate patterns are preferred for producing small
castings on mass scale.
• They produce accurate castings and at faster rates.
• Piston rings of I.C. engines are produced with the help
of match plate patterns.7/28/2017 32

33. Sweep Pattern
7/28/2017 33

34. 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 sweep pattern saves a lot of time and labour as
compared to making a full pattern.
• A sweep preferred for producing large castings of circular
sections and symmetrical shapes.
• The manufacture of large kettles of cast iron requires a sweep
pattern.
7/28/2017 34

35. Gated Pattern
castings
Gating system
7/28/2017 35

36. Gated Pattern
• Gated patterns are usually made of metal which
increases their strength and reduces the tendency to
warp.
• The sections connecting different patterns serve as
runner and gates. This facilitates filling of the mold
with molten metal in a better manner at the same
time eliminates the time and labour otherwise
consumed in cutting runners and gates.
• A gated pattern can manufacture many castings at
one time and thus it is used in mass production
systems.
• Gated patterns are employed for producing small
castings.7/28/2017 36

37. Skeleton Patten
7/28/2017 37

38. Skeleton Patten
• A skeleton pattern is the skeleton of a desired shape. The skeleton
frame is mounted on a metal base.
• The skeleton is made from wooden strips and is thus a wooden
framework.
• 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.
• If the object is symmetrical like a pipe, the two halves (of the pipe)
can be molded by using the same pattern and then the two molds
can be assembled before pouring the molten metal.
• Skeleton patterns are employed for producing a few large castings.
• A skeleton pattern is very economical as compared to a solid
pattern, because it involves less material costs.
• Castings for turbine castings, water pipes, channels, etc., are
made with the help of skeleton patterns.7/28/2017 38

39. Follow Board Pattern
7/28/2017 39

40. 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 collapse under the 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 follow board is
withdrawn.
• The rammed drag is inverted, cope is mounted on it and
rammed. During this operation pattern remains over the
inverted drag and gets support from the rammed sand of
the drag under it.
• Ultimately, the pattern is removed and the cope and
drag are assembled.
7/28/2017 40