Types of molding sand:
Green sand:
Natural sand with moisture
Dry sand:
Not suitable for large castings
Facing sand:
This sand is used directly next to the surface of the pattern and comes into contact with the
molten metal when the mould is poured.
As a result, it is subjected to the severest conditions and must possess, therefore, high strength
and refractoriness. This sand also provides a smoother casting surface and should be of fine texture.
It is made of silica sand and clay, and some additives without the addition of used sand.
Facing sand is always used to make dry sand moulds while system sand is frequently used for
green sand molding.
Parting sand:
This sand is used to prevent adhering of two halves of mould surfaces in each molding box
when they are separated. Thus, to ensure good parting, the mould surface (at contact of cope and
Drag) should be treated with parting sand or some other parting material.
It is also sprinkled or applied on the pattern surface (before the molding sand is put over it) to
avoid its sticking and permit its easy withdrawal from the mould. The parting sand is fine dry sand.
Backing or floor or black sand:
This is the sand which is used to back up the facing sand and to fill the whole volume of the
flask. Old, repeatedly used molding sand is mainly employed for this purpose.
Core sand:
The core sand mainly consists of silica sand and an organic binder, with very little, if any, clay
content. The presence of clay in core sand reduces its permeability and collapsibility. The core sand
may contain small percentages of other constituents also, to enhance its properties.
Loam sand:
50 % of clay and dried hard and using for large castings
Types of patterns:
The following factors affect the choice of a pattern.
(i) Number of Castings to be produced.
(ii) Size and complexity of the shape and size of casting
(iii) Type of molding and castings method to be used.
(iv) Machining operation
(v) Characteristics of castings
Different types of patterns:
1)
2)
3)
4)
5)
Single piece pattern
Split piece pattern
Loose piece pattern
Gated pattern
Match pattern
6) Sweep pattern
7) Cope and drag pattern
8) Skeleton pattern
9) Shell pattern
10) Follow board pattern
11) Segmental Pattern
12) Multi piece pattern
Figure 1: Single piece, Split, Match-plate, Cope and Drag Pattern
Single piece pattern:
This is the simplest type of pattern, exactly like the desired casting. For making a mould, the pattern
is accommodated either in cope or drag.
Used for producing a few large castings, for example, stuffing box of steam engine.
Split pattern:
These patterns are split along the parting plane (which may be flat or irregular surface) to facilitate
the extraction of the pattern out of the mould before the pouring operation. For a
more complex casting, the pattern may be split in more than two parts.
Loose piece pattern:
When a one piece solid pattern has projections or back drafts which lie above or below the parting
plane, it is impossible to with draw it from the mould. With such patterns, the projections are made
with the help of loose pieces. One drawback of loose faces is that their shifting is possible during
ramming.
Figure: Loose piece pattern
Gated pattern:
A gated pattern is simply one or more loose patterns having attached gates and runners.
Because of their higher cost, these patterns are used for producing small castings in mass
production systems and on molding machines.
Match plate pattern:
A match plate pattern is a split pattern having the cope and drags portions mounted on opposite
sides of a plate (usually metallic), called the "match plate" that conforms to the contour of the
parting surface.
The gates and runners are also mounted on the match plate, so that very little hand work is required.
This results in higher productivity. This type of pattern is used for a large number of castings.
Piston rings of I.C. engines are produced by this process.
Sweep pattern:
A sweep is a section or board (wooden) of proper contour that is rotated about one edge to shape
mould cavities having shapes of rotational symmetry. This type of pattern is used when a casting of
large size is to be produced in a short time. Large kettles of C.I. are made by sweep patterns.
Figure: Sweep pattern
Cope and drag pattern:
A cope and drag pattern is a split pattern having the cope and drag portions each mounted on
separate match plates. These patterns are used when in the production of large castings; the
complete moulds are too heavy and unwieldy to be handled by a single worker.
Skeleton pattern:
For large castings having simple geometrical shapes, skeleton patterns are used. Just like sweep
patterns, these are simple wooden frames that outline the shape of the part to be cast and are also
used as guides by the molder in the hand shaping of the mould.
This type of pattern is also used in pit or floor molding process.
Figure: Skeleton pattern
Segmental Pattern:
It is just a similar to that of a sweep pattern. The working structure of segmental pattern and sweep
pattern is almost similar. For designing require shape or structure of mold they both employ a part
of pattern. As the name suggest segmental pattern is in the form of segments and used for molding
circular or round structures. In sweep pattern there is complete rotation but in segmental pattern
there is no complete rotation. For creating mold, we can rotate partly to get required output.
Shell pattern:
Figure: Shell pattern
Follow board pattern:
A follow board is not a pattern but is a device (wooden board) used for various purposes.
Figure : Follow board pattern
Pattern allowances:
There are some allowances which are responsible for the difference in the dimensions of the
casting and the pattern. These allowances are considered when a pattern is designed for casting. In
this article we will discuss those allowances 1.
Shrinkage allowance:
After solidification of the metal from further cooling (room temp.) dimensions of the patterns
increases. So pattern size is bigger than that of the finished cast products. This is known as
shrinkage allowance.
a)
b)
c)
d)
e)
f)
It depends on:
Dimensions of casting
Design and intricacy of casting
Resistance of mol to shrinkage
Molding materials used
Method of molding used
Pouring temp of the molten metal
2.
a)
b)
c)
d)
e)
3.
4.
a)
b)
c)
d)
e)
f)
5.
Draft or taper allowance:
Pattern draft is the taper placed on the pattern surfaces that are parallel to the direction in
which the pattern is withdrawn from the mould (that is perpendicular to the parting plane),
to allow removal of the pattern without damaging the mould cavity.
It depends on:
the method of molding
the sand mixture used
the design (shape and length of the vertical side of the pattern)
economic restrictions imposed on the casting
intricacy of the pattern
Distortion allowance:
This allowance is taken into consideration when casting products of irregular shapes. When
these are cooled they are distorted due to metal shrinkage.
Finishing or machining allowance:
Machining allowance or finish allowance indicates how much larger the rough casting should
be over the finished casting to allow sufficient material to insure that machining will "clean up" the
surfaces.
This
machining allowance is
added
to
all
surfaces
that
are
to
be
machined. Machining allowance is larger for hand molding as compared to machine molding.
It depends on:
Machining operation
Characteristics of metal
Methods of castings
Size, shapes and volumes of castings
Degree of finish required in castings
configuration of the casting
Shaking or rapping allowance:
To take the pattern out of the mould cavity it is slightly rapped to detach it from the mould cavity. So
the cavity is increased a little.
TYPES of Patterns