IIIE SECTION A MANUFACTURING TECHNOLOGY NOTES 1. patterns material allowances
Patterns for castings:
Pattern (Click on Figure 2 to view a typical pattern)
The pattern is the principal tool during the casting process. It is the replica of the object to
be made by the casting process, with some modifications. The main modifications are the
addition of pattern allowances, and the provision of core prints. If the casting is to be
hollow, additional patterns called cores are used to create these cavities in the finished
product. The quality of the casting produced depends upon the material of the pattern, its
design, and construction. The costs of the pattern and the related equipment are reflected
in the cost of the casting. The use of an expensive pattern is justified when the quantity of
castings required is substantial.
Figure 2: A typical pattern attached with gating and risering system
Functions of the Pattern
1. A pattern prepares a mold cavity for the purpose of making a casting.
2. A pattern may contain projections known as core prints if the casting requires a
core and need to be made hollow.
3. Runner, gates, and risers used for feeding molten metal in the mold cavity may
form a part of the pattern.
4. Patterns properly made and having finished and smooth surfaces reduce casting
5. A properly constructed pattern minimizes the overall cost of the castings.
Types of patterns:
1. Solid pattern: Simplest form of pattern, without joints, partings nor any loose
pieces in construction called Solid and single piece pattern.It is used for large
castings of simple shape.
2. Split patterns : When patterns cannot be made of of single piece because of
difficulties encountered in removing them from the mould . To eliminate this
difficulty some patterns are made of two parts, so that half of the pattern will rest
in the lower part of the mould and half in the upper part. The split in the patterns
occurs at the parting line of the mould. The two parts of the patterns are aligned
with the help of dowel pins. Some times the patterns are constructed with three or
more parts and such patterns are known as Multi piece pattern.
3. Loose piece patterns. These patterns are made up of loose pieces which is
necessary to facilitate the removal of the pattern from the mould when undercuts
are produced. Loose patterens arev removed separately through the cavity formed
on the main pattwern, after the pattern has been rmoved. These loose pcs may
have to be turned or moved before taking them out. These also need to be fastened
loosely to the main patterns by wodden dowel pins .
4. Gated Patterns : in production work where several castings are required , gated
patterns are used. Such patterns are made up of metals to give them strength and
to eliminate any warping tendency.The gates or runners of the molten metal are
formed by the connecting parts between the indivisual patterns.
5. Match plate : Match plate pattern is made by fastening each half of a split pattern
to the opposite side of one plate. The plate provides the substantial mounting of
patterns and is widely used in machine moulding.The gates and runners are alos
attached to the plate.The plate is equipped with locator holes which fit into the
pins provide don the drag portion of the flask.It is used for producing small
castings in larger quantities in moulding machines
6. Follow board : It may be used with either single or multigated patterns. Patterns
required follw boards are diificult to make as split patterns. It is also used with
thin patterns requiring support . the board is routed out so that the pattern rests in
it upto the parting line, and this board then acts as a moulding board for the first
7. Sweep patterns : Pattrns of symmetrical and regular shape,usually of large size
are constructed by using this. ( Which has the shape corresponding to the desired
casting. )which sweeps thecdesired shape into the sand mould,thus eliminating
need for costly 3 dimensinal patterns.The seep pattern is arranged to rotate about a
central axis on a needle.
8. ODD shaped pattern: An odd side copes or false cope is used when the shape of
the casting does not produce a convenient flat spilt in the pattern, such as cranked
lever. The pattern is solid and an odd side (False cope) is used to assist in
providing of a contoured split between cope and drag. The odd side is often made
up of POP.
Patterns may be constructed from the following materials. Each material has its own
advantages, limitations, and field of application. Some materials used for making patterns
are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and
resins. To be suitable for use, the pattern material should be:
1. Easily worked, shaped and joined
2. Light in weight
3. Strong, hard and durable
4. Resistant to wear and abrasion
5. Resistant to corrosion, and to chemical reactions
6. Dimensionally stable and unaffected by variations in temperature and humidity
7. Available at low cost
The usual pattern materials are wood, metal, and plastics.
The most commonly used pattern material is wood, since it is readily available and of
low weight. Also, it can be easily shaped and is relatively cheap.
Normally white pine ( Type of wood) is used for making patterns because its straight
grained, light and easy to work.and has little tendency to warp. When a more durable
wood is required for fragile partrns and hard use,Mahogany is preferred. Other woods
suitable for pattern making are : Cherry, beech,poplar,bass wood and maple.the last being
especially desirable for work on the lathe.
The main disadvantage of wood is its absorption of moisture, which can cause distortion
and dimensional changes. Hence, proper seasoning and upkeep of wood is almost a pre-
requisite for large-scale use of wood as a pattern material. Kiln drying which is requires
only a short time reduces the moisture to an minimum, drives off volatile matter and is
Metal : Metal is used as pattern materials in production work because of its ability
withstand hard use.They do not change their shapen when subjected to the moisyt
conditions, and requires minium of maintainance work to keep them in operating
Aluminium is best of all metals as it is easily workable. Light in weight, and resistant to
Ploysterene: This is known as consumable pattern because the heat of the molten metal
vaporizes the pattern so that it leaves the mould in the form of gas. The plastic is light,
easily sculptured with a knife, and the pattern can be made of several parts cemented
together, Special care is required in ramming the mould, as this material is fragile. It is
best suited for cases where it is difficult or impossible to draw the pattern.
Pattern allowance is a vital feature as it affects the dimensional characteristics of the
It is important to arrive at a pattern which gives proper size and shape The allowances
cannot be arrived precisely and mathematically as the size and shape of the castings
depends on product design, mould design, shrinkage and contraction charecterstics of the
metal being cast .The process of trial and error on the pattern is best suited to fix the
..Thus, when the pattern is produced, certain allowances must be given on the sizes
specified in the finished component drawing so that a casting with the particular
specification can be made. The selection of correct allowances greatly helps to reduce
machining costs and avoid rejections. The allowances usually considered on patterns and
core boxes are as follows:
1. Shrinkage or contraction allowance
2. Draft or taper allowance
3. Machining or finish allowance
4. Distortion or camber allowance
5. Rapping allowance
Shrinkage or Contraction Allowance ( click on Table 1 to view various rate of contraction of various
All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage
is of two types:
i. 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
ii. 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 rate of contraction with temperature is dependent on the material. For example steel
contracts to a higher degree compared to aluminum. To compensate the solid shrinkage, a
shrink rule must be used in laying out the measurements for the pattern. The shrink rule
for CI is 10mm per mtrs longer than the standard rule.
In actual practice , shrinkage allowances varies with the casting design,type of metal ,
pouring temp and resistance of the mould to the normal condition of the casting caused
by projecting lugs, cores etc.
When metal patterns are to be cast from original patterns,double shrinkage must be
The various rate of contraction of various materials are given in Table 1.
Table 1 : Rate of Contraction of Various Metals
Material Dimension Shrinkage allowance
Grey Cast Iron
0.6 to 1.2 mtrs
over 1.2 mtrs
Cast Steel 0.6 mtrs
0.6 to 1.8 mtrs
over 1.8 mtrs
Aluminum 1.2 mtrs
1.2 to 1.8 mtrs
over 1.8 mtrs
Magnesium Up to 1.2 mtrs
Over 1.2 mtrs
The casting shown is to be made in cast iron using a wooden pattern. Assuming only
shrinkage allowance, calculate the dimension of the pattern. All Dimensions are in Inches
The shrinkage allowance for cast iron for size up to 2 feet is o.125 inch per feet (as per
For dimension 18 inch, allowance = 18 X 0.125 / 12 = 0.1875 inch » 0.2 inch
For dimension 14 inch, allowance = 14 X 0.125 / 12 = 0.146 inch » 0.15 inch
For dimension 8 inch, allowance = 8 X 0.125 / 12 = 0.0833 inch » 0. 09 inch
For dimension 6 inch, allowance = 6 X 0.125 / 12 = 0.0625 inch » 0. 07 inch
The pattern drawing with required dimension is shown below: surfaces and facilitate
Draft allowances : The allowance given on the pattern in the form of a slight taper in the
direction parallel to which it is being withdrawn, to avoid the tearing of the edges while
removing a pattern from the mould . This provides a slight clearance for the pattern as it
is lifted up. This taper and clearance is known as draft..The amount of the draft depends
on the shape and size of the casting,method of production,and whether moulded by hand
or machine. Machine moulding requires minimum draft.
The amount of draft on the exterior surfaces are 10-20 mm/mtrs.On interior holes which
are fairly small,the draft should be 30mm/mtr.In green sand moulding the interior surface
requires more draft than exterior surfaces.
Finish allowances: The finish dimensions of a component are to be observed in the
drawing and extra material to be provided in such places so that there will be extra metakl
to be removed to maintain the final size.The extra amount to be added is depends on
sizeand shape of the casting,but for small and ave size castings it is usually 3
mm.Whereever possible surfaces to be machined should be cast in the drag side of the
mould. However where finish surfaces are cast in the cope, an extra allowance has to be
Distortion allowances : This allowances applies only to those castings, of irregular
shapes like U shape and those having large flat areas, which are distorted in the process
of cooling as a result of metal shrinkage. Such an allowance depends on the judgment and
experience of the pattern maker, who understands the shrinkage pattern of the metal.
Shake allowance: When a pattern is rapped in the mould before it is withdrawn, the
cavity in the mould is slightly increased. In an average sized casting this increase in size
can be ignored. In large castings or in one that must fit without maching,shake
allowances must be considered by making the pattern slightly smaller.
Pattern construction details
Fillets : A fillet is a concave connecting surface for the rounding out of a corner at two
intersecting planes.In all castings shapr corners should be avoided. Rounded corners and
fillets assist materials in moulding, since there is less tendency for the sand to break out
when the pattern is withdrawn.The metal flows into the mould more easily and there is
less danger of sand washing into the mould. The appearance of the casting is improved,
and are generally stronger having fewer internal shrinkages
A casting in a mould cools on the out side first.As the coolings progresses to the centre,
the grains of the metal arrange themselves normal to the surface in a dendritic structure.
In patterns having a sharp corner there is a tendency for the metal at the corners to open
up because of shrinkage.
Fillets are made up of wood, leather or wax.
Section Thickness :As for as service and design factors permit,all sections should be
uniform as possible. When light sections are adjacent to heavy sections, the transition
should be as gradual as possible, since abrupt changes in thickness always results in
starin, which are likely to cause cracks.
Sanding and Shellacing the patterns : For getting good finish, patterns are made
smooth and shellac is used for this purpose.