Patterns
and Types of Patterns
1.0 Brief
introduction
In casting, a pattern is a replica of the object to
be cast, used to prepare the cavity into which molten material will be poured during
the casting process.
Patterns used in sand casting may be made of wood,
metal, plastics or other materials. Patterns are made to exacting standards of
construction, so that they can last for a reasonable length of time, according
to the quality grade of the pattern being built, and so that they will
repeatably provide a dimensionally acceptable casting.
The making of patterns, called patternmaking
(sometimes styled pattern-making or pattern making), is a skilled trade that is
related to the trades of tool and die making and moldmaking, but also often
incorporates elements of fine woodworking. Patternmakers (sometimes styled
pattern-makers or pattern makers) learn their skills through apprenticeships
and trade schools over many years of experience. Although an engineer may help
to design the pattern, it is usually a patternmaker who executes the design.
·
Introduction of Pattern
Patterns are required to make moulds. The mould is
made by packing suitable moulding material, such as moulding sand around the
pattern. When the pattern is withdrawn, the imprint provides the mould cavity,
which is ultimately filled with molten metal to become casting.
A pattern may be defined as 'a full size model of the
desired casting which when packed or embedded in a suitable moulding material,
produces a cavity called mould'. This mould, when filled with molten metal
forms the desired casting after solidification of the molten metal.
The pattern very closely conforms to the shape and
size of the desired casting, except that it carries pattern allowances to
compensate metal shrinkage, provide sufficient metal for machined surfaces, and
facilitate moulding. The process of making a pattern is known as pattern
making.
Patterns used in sand casting may be made of wood, metal,
plastics or other materials. Patterns are made to exacting standards of
construction, so that they can last for a reasonable length of time, according
to the quality grade of the pattern being built, and so that they will
repeatably provide a dimensionally acceptable casting.
·
Pattern Material
The materials used in the pattern should be cheap in
cost and easily available in the market. The material should have a good surface finish. The material should have withstood high temperatures
and does not change its shape at high temperatures.
Generally, we use 5 different types of
material to make the patter and those are:
- a. Wood
- b. Metals
- c. Plaster
of Paris
- d. Plastics
- e. Wax
Wood:
As we all know woods are easily available, and the
price is quite low so it is satisfied us some basic criteria which I mentioned
above.
Some advantages using wood in pattern.
- a. Wood
is light in weight
- b. Easily
Available in the market
- c. You
can make any shape using wood
- d. Woods
gives good surface finish
However wood is attracted to moisture and sometimes it
can change shape on high temperature or after dry out from moisture, this is an
important con of using wood as a pattern.
Not only this reason woods are very week in strength,
and it wears out quickly due to its low resistance to sand abrasion.
For these above reasons, it is not used for very big
product casting.
Generally, pines deodar, walnut, teak’s are used for
making a pattern.
Metals:
In metals, cast iron, brass, aluminum are generally
used in patterns. It gives smooth surface finish, this is the only reason that
metals are used in large production casting workshops.
These are some advantages of using Metal Pattern:
- a. Smooth
surface finish can be obtained by metal patterns.
- b. Deformation
is less.
- c. Closer
dimensional tolerance
Although there are some disadvantages of using this
type of pattern like it is a little bit costlier, heavy, sometimes rusting
effect occurred on the surfaces of the metals.
Plaster of Paris:
It is generally used if you need to set up the pattern
quickly. The main advantage of this pattern is it can easily cast into
intricate shapes.
However, it is not for repetitive usages as it is
fragile.
Plastics:
Different types of plastics are nowadays used in
pattern because of their lighter weight, strength, and dimensionally stable and
also for cheap in cost.
Thermoplastics and polystyrene are commonly used for
making patterns, and Thermosetting plastics such as phenolics and epoxies are
also used in a pattern.
There are few advantages of using pattern and those
are:
- a. Light
in weight.
- b. Cheap
in price.
- c. It
posses good compressive strength.
- d. No
tension of rusting or moisture absorbing.
However, they are a little weak in strength and not
good abrasion-resistant.
·
Colour Coding of Pattern
Patterns are normally painted with different colors in
such a way that mold maker would be able to understand the how to treat that
particular surface. There is no universally accepted color code for a pattern.
However, the common color codes used in pattern are
- 1. Clear/
No color: denotes the parting face of the split pattern
- 2. Yellow:
the seats for loose core print painted yellow.
- 3. Yellow
strip on Red: indicates the seats for loose pieces.
- 4. Red
strips on yellow background: for supports/ stop-offs
- 5. Black:
the surface must be left as in the casting. This surface does not need a
finishing operation.
- 6. Red:
the surface which painted red required to be machined.
·
Pattern Making Allowances
Although the pattern is used to produce a casting of
the desired dimension, it is not dimensionally identical with the casting. For
metallurgical and mechanical reasons, a number of allowances must be provided
on the pattern to get dimensionally correct casting.
Shrinkage (Contraction) Allowances:
When the molten metal is poured in the mould it cools and solidifies. The metal
contracts in size when it cools and solidifies. To compensate this shrinkage
the pattern must be made larger than the finished casting. Therefore, shrinkage
allowance is the amount by which the pattern must be made larger than the
desired casting.
To compensate this shrinkage, a shrink rule is used in
laying out the measurement for the pattern. Different metals have different
shrinkages and therefore a different shrink rule for each type of metal.
Machining Allowances:
A casting may require machining all over or on certain specified portion. Therefore,
additional metal must be provided on these surfaces so that there will be some
metal contact to machine. The corresponding surface of the pattern must be made
larger.
The amount of this allowance depends upon the metal of
the casting, method of machining to be used, method of casting used, size and
shape of the casting and the degree of surface finished required. Ferrous
metals need more machining allowance than non-ferrous metals. Machining allowance
may vary from 1.5 mm to 16 mm, but 3 mm allowance is quite common for small and
medium size castings.
Distortion Allowances:
This allowance is applicable only to the irregular shaped castings which are distorted
in the process of cooling. It is a result of uneven metal shrinkage. For
example, a casting in the form of a letter U will be distorted with the legs
diverging, instead of remaining parallel. To compensate this, an opposite
distortion is provided in the pattern so that the effect is neutralised and
correct casting is obtained. For making the U shaped casting, the legs of the
pattern are made convergent but as the casting cools after its removal from the
mould, the legs straighten up and remain parallel.
Rapping Allowances:
When the pattern is to be withdrawn from the mould, it is first rapped or shaken,
by striking over it from side to side, so that its surfaces may be free from
the adjoining walls of the mould. As a result of this, the cavity in the mould
is slightly increased. To compensate this, a negative allowance is to be
provided by making the pattern slightly smaller. In small and medium size
casting, rapping or shake allowance is generally neglected.
·
Applications
- 1. A
pattern is essential for the production of any metal or alloy casting.
- 2. A
pattern is used to produce the cavity in the moulding sand for pouring the
molten-metal.
- 3. A
properly manufactured pattern reduces the overall cost of casting.
- 4. A
properly manufactured pattern reduces the casting defects.
- 5. A
properly manufactured pattern provides good surface finish of the casting.
- 6. A
pattern may have projection known as core prints which helps in positioning of
core.
- 7. A
pattern establish the parting line and parting surfaces in the world.
·
Types of pattern
The pattern can be classified in different types as
per their design, some of those are mentioned below:
- a. Single
Piece Pattern
- b. Two-Piece
or Split Pattern
- c. Multipiece
Pattern
- d. Match
Plate Pattern
- e. Gated
Pattern
- f. Sweep
Pattern
- g. Loose
Piece Pattern
- h. Skeleton
Pattern
- i. Shell
Pattern
- j. Segmental
Pattern
Single Piece Pattern:
As the name denotes a single piece that means it has
only 1 section, and inside this, the shape of the pattern is made.
Split Pattern:
It has two sections and this two-section is connected
by pin named dwell pin to attach the two-portion. When we joined the two-block
the joining surface is called parting surface. Generally, it is used to make the hollow cylindrical
casting.
Multipiece Pattern:
A multipiece pattern is made by more than two pieces,
the upper one is called the cope, middle on is called cheek, and the bottom one
is named the drag portion.
Match Plate Pattern:
In match Plate Pattern, the cope and drag section of
the split pattern are mounted on the opposite of metal or wood plate.
This type of pattern is divided into two sections
along the parting line formed by the metal or wood plate.
Gated Pattern:
A gated pattern consists of:
In this type of pattern, the molten metal pours through
the runner and then it passes through the several gates and then finally to the
pattern.
Sweep Pattern:
Sweep Pattern is designed for the half shape of the
casting products, it is consisted of a mold cavity, rotating spindle, and sweep
pattern.
Its edges are contour corresponding to the outer shape
and size of the casting.
Loose Piece Pattern:
This type of pattern is used when a pattern has
projecting parts that lie below or above the main parting line in a mold. That
means when some parts of the product have a little outer or inner edge than to
casting those part we use loose piece patterns.
Skeleton Pattern:
It is generally used for casting very huge parts.
This type of pattern is not totally solid, it looks
like a rib cage. Where we make only the outer or inner surface structure.
Shell Pattern:
This type of pattern is generally made of metals and
fitted on a plate. It is like a split pattern, one half of the pattern is
placed in the upper part of the plate and another one is fitted on the bottom
of the plate.
It is used when we need to cast a large circular ring
shape product.
The final product is completed on a few repeated steps
using this pattern, 1st we mold it in one place then we rotated the pattern to
the next portion and again the molding is done
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