1.1 TYPES OF
PATTERNS:
Patterns may be classified as
temporary and permanent patterns depending upon the material used for the
pattern. Temporary patterns of soft wood are easily made. However, they soon
wear, warp or crack and so have a short life. Hardwood patterns are used more
than any other types of patterns.
The portions that
wear may be protected by sheet metal. Permanent patterns are made of metals or
plastics that are easily cast and machined .Patterns may be classified from the
utility point of view. The following factors affect the choice of a pattern.
(1)
Number of castings to be produced.
(2)
Size and complexity of the shape and size of casting.
(3) Types of molding method to be used.
1.1.1.
LOOSE PATTERN:
A loose pattern is simply a replica of the desired
casting. it is slightly larger than the casting and it may have several
projection called core prints that the resulting casting does not have Loose
patterns get their name because they are not attached or mounted on a plate or
frame. These patterns may be made of wood of metal depending upon the volume of
production. The gates, runners and risers are added during Molding. This makes
mould slow labour intensive .Due to this, loose patterns are used when the
number of casting to be made is small, say, up to 100 loose patterns are two
types:
a)
One piece or solid pattern
b)
Split pattern
(A)
ONE PIECE OR SOLID 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. The molding process is
quite inconvenient and time consuming. So, such patterns are used for producing
a few large casting, for ex. Stuffing box of steam engine.
(B)
SPLIT OR PARTED PATTERN:
These patterns are
split along the parting plane to facilitate the extraction of the pattern out
of the mould before the pouring operation. Molding with a split pattern has
already been explained under art. Fig 1.1 shows a split pattern for casting a
bush. The two parts of the pattern are joined together with the help of dowel
pins.
Fig. 1.1 Split Pattern
1.1.2.
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 piece. A loose piece
is attached to the main body of the pattern by a pin or dovetail slide. While
molding, Sand is rammed securely around the loose piece.
Then the pins are
removed. The sand is then packed and rammed around total pattern. When the main
pattern is drawn, the loose piece remains in the mould. These are then
carefully rapped and drawn as shown in figure 1.2
Fig. 1.2 Loose Piece Pattern
DRAWBACKS:
Another technique
to make a mould with one piece solid patterns is the use the drawbacks. A
drawback is portion of the mould which can be drawn back horizontally in order
to allow removal of the patterns. It may be rammed around a rigid support
called an "arbor" to facilitate moving it, fig 1.3.
Fig. 1.3 Draw Back Of Loose Pattern
1.1.3.
GATED PATTERNS:
A gated pattern is
simply one or more loose patterns having attached gates and runners, fig 1.4.
Since the gates and runners are not to be cut by hand, gated patterns reduced
the Molding time somewhat. Because of their higher cost, these patterns are
used for producing small casting in mass production systems and on molding
machines.
Fig. 1.4 Gated Pattern
1.1.4.
MATCH PLATE PATTERN:
A match
plate pattern is a split pattern having the cope and drag portions mounted on
opposite sides of a plate called the “match plate" that conforms to the
contour of the parting surface. The gates and the runners are also mounted on
the match plate, fig 1.5. So that very little hard work is required this result
in higher productivity. This type of pattern is use for a large number of
casting. Several patterns mounted on one match plate if the size of casting is
small the patterns need not all be for the same casting.
Fig. 1.5 Match Plate Pattern
1.1.5.
COPE AND DRAG PATTERN:
A cope and drag pattern is split pattern having the cop
and drag portion each mounted on separate match plates. These patterns are used
in when in the production of large casting. A complete mould is too heavy and
widely to be handled by a single workers. The patterns are accurately located
on the plates so that when the two separately made mould halves are assembled
together the mould cavity is properly formed.
1.1.6.
SWEEP PATTERNS:
A sweep is section or board of proper contour that is
rotated about one edge. To shape mould cavities having shapes of rotational
symmetry, Fig 1.6. This type of pattern is used when a casting of large size is
to be used in a short time. A complete pattern is not necessary and would be
very expensive for very large casting where the tolerances are large. The
moulds are made manually, either in a pit or on the foundry floor. Thus, this
type of molding referred to as pit and floor molding once the mould is ready,
the sweep pattern and the post about which it is rotates, are removed before
pouring. The mould cavity, large kettles of C.I. are made by sweep pattern.
Fig. 1.6 Sweep Pattern
1.1.7.
SKELETON PATTERN:
For large casting having simple geometrical shapes,
skeleton pattern are used just like sweep patterns, these are simple wooden
frames the outline the shape of the part to be cast and are also used as guide
by the molder in the hand shaping of the mould. Type of patterns is also used
in pit or floor molding process. Again
a complete pattern is not necessary and would be quite expensive for a large
casting where the tolerance is large, and when only a small number of casting
is to be made.
Fig. 1.7 Skeleton Pattern
Fig 1.7 shown as skeleton
pattern for a casting pipe it is ripped the construction of wood. Which form
and outline of the pattern to be made. The frame work is fill them with the
loam sand and rammed. The sand must be true and smooth. For round shape the
pattern is made into halves which are joined together by means of screw or with
glue etc. This type of pattern is used for casting water pipe, turbine casting
etc.
1.1.8.
SEGMENTAL PATTERN:
The segmental pattern is a functionally similar to a
sweep pattern in the sense, that both employ a part of the pattern instead of a
complete pattern, for getting required shape of the mould.
Fig. 1.8 Segmental Pattern
The segmental
pattern is in the form of segment, fig1.8. And is used for molding parts having
circular shape. To create the mould, it is rotated about the post in the same
way as in swept pattern. But it is not revolved continually about the post to
prepare the mould. When one portion of the mould is completed, the pattern is
lifted up and moved of the next portion to make the next segment of the mould.
This process is continuing until the entire mould is completed. Big gears and
wheel rims are produced with the help of this process.
1.1.9.
FOLLOW BOARD PATTERN:
A follow board is
not a pattern but is a device used for various purposes as explain below.
1)
It is used for supporting a pattern which is very thin and fragile, which may
be break or collapse under the pressure when sand is rammed above it. Such a
pattern is set on a follow board which is shaped in surface pattern, fig 1.9(a).
After ramming the sand over the pattern and inverting the flask, the follow
board is withdrawn. The other flask is mounted over the first one, field with
sand which is than rammed. During this operation, pattern remains in the drag
and gets support from the rammed sand under it.
Fig. 1.9 Follow Board Pattern
2)
It helps to establish a parting plane with ease in a pattern that has an
irregular shape, fig 1.9(b). A hole is cut in to the follow board. The pattern
is placed in the follow board and then it is used in the same manner as a match
plate. A follow board is used with pattern that does not have a flat side and
these are one piece solid types. Without the help of a following board it wills
one time consuming and costly to cut out an irregular parting surface and
skilled workman will be required for this.
3)
Gated patterns are frequently set on a follow board which is shaped to the
parting surface of the mould. This serves as an interface where the mould
separate, which enables the pattern to be removed after the mould, is prepared.
STOP OFF:
This is the portion
of the pattern which is added for its own strength only if the pattern is
fragile, fig 1.10. it forms a cavity in the mould when the mould withdraws the
pattern, which is refilled with sand before pouring. Stop-offs is wooden pieces
used to reinforce some portions of the pattern which are structurally weak,
especially from the standpoint of repeated handling. They have no connection
with the completed casting.
Fig. 1.10 Stop-off
1.2:
PATTERN MATERIALS
The
requirements of good pattern are
1)
Secure the desired shape and size of the casting.
2)
Cheap and radially repairable.
3)
Simple in design for is of manufacture.
4)
Light in mass and convenient to handle.
5)
Have a high strength and long life in order to make as many moulds as required.
6)
Retain its dimensions and rigidity during the definite service life.
7)
Its surface should be smooth and wear resistant.
8)
Able to withstand rough handling.
The common
materials used in pattern making include wood, metal, plastic and quick setting
compound. Each material has its own advantages, limitation and field f
application. Also, the required accuracy, strength and life of a pattern depend
on the quantity of casting to be produced. Based on the above factors, we can
choose the patterns material as follows:
I.
Piece and short run production wood.
II.
Large scale and mass production. Metal, being more durable than wood, though
costlier.
III.
Batch production, plastic, for example, epoxy resins and also from gypsum and
cement.
A)
WOOD:
The wood used for
pattern making should be properly dried and seasoned. It should not contain
more than 10% moisture to avoid warping and distortion during subsequent
drying. It should be straight drained and free from knots.
ADVANTAGES:
1.
Light in weight.
2.
Comparatively inexpensive.
3.
Good workability.
4.
Lends itself to gluing and joining.
5.
Holds well varnishes and paints.
6.
Can be repaired easily.
LIMITATION:
1.
Inherently non uniform in structure.
2.
Process poor wear and abrasion resistance.
3.
Cannot withstand rough handling.
4.
Absorbs and gives of moisture, so that it various in volume, warps and the
changes its mechanical properties. These drawbacks, however, Can be remedied by
drying and seasoning it and then giving coats of water proof varnishes and
paints.
The
following types of wood are commonly used for pattern making:
1.
White pine: it is the most widely used wood, because of its straight grain and
light weight and because it is soft, easy to work and unlikely to warp.
2.
Mahogany: it is harder and more durable than white pine. Can be worked easily
if straight grained. It is less likely to warp than some of other woods.
3.
Maple birch and cherry: it woods are harder and heavier than white pine. They
tend to warp in large sections, so should be used for small patterns only. They
should be carefully treated, because, they pick up moisture readily.
(B)
METAL:
A metal pattern can
be either cast from a master wooden pattern or may be machined by the usual
methods of machining. Metal pattern are usually used in machine molding.
ADVANTAGES:
1.
More durable and accurate in size than wooden pattern
2.
Have a smooth surface
3.
Do not deform in storage
4.
Are resistant to wear, abrasion, corrosion and swelling
5.
Can with stand rough handling
LIMITATION:
1.
Expansive as compared to wood
2.
Not easily repaired
3.
Heavier than wooden patterns
4.
Ferrous pattern can get rusted.
C)
PLASTICS:
The
use of plastics for pattern material results in following advantages:
1.
Facilitates the production process.
2.
Makes it more economical in cost and labor.
3.
Plastics patterns are highly resistant to corrosion, lighter and stronger than
wood patterns.
4.
Molding sand sticks less to plastics than to wood.
5.
No moisture absorption.
6.
Smooth surface of patterns.
7.
Strong and dimensionally stable.
Various plastics
make good materials for the production of patterns. These are the compositions
based on epoxy, Phenol formaldehyde and polyester resin, polyvinylchloride and
others. In most wide used are cold curing plastics based on epoxy resins.
PLASTIC PATTERNS ARE MADE BY ONE OF THE FOLLOWING METHOD:
I.
By injecting a plastic material in to a die.
II.
Utilizing laminated construction by building up successive layers of resin and
glass fiber.
1.3.
PATTERN ALLOWANCES:
The difference in the dimensions of the casting and the
patterns is due to the various allowances considered while designing a pattern
for a casting. These allowances are discussed below:
1.3.1.
SHRINKAGE ALLOWANCES:
Since metal shrinks on solidifications and contracts
further on cooling to room temperature, linear dimensions of patterns are
increased in respect of those of the finished casting to be obtained. This is
called the “shrinkage allowance”. It is given as mm/m. Typical values of
shrinkage allowance for various metals are given below:
C.I.,
malleable iron = 10 mm/m
Brass, cu, al = 15 mm/m
Steel = 20 mm/m
Zinc, lead = 25 mm/m
While laying out a
pattern, the dimensions are taken from pattern makers rule, called “shrink
scale”, Which is longer than a standard scale by the shrinkage value for the
appropriate metal.
13.2.
MACHINING ALLOWANCES:
Machining allowance or finish allowance indicates how
much longer the rough casting should be over the finished casting to allow
sufficient material to insure that machining will “clean up” the surface. This machining allowance is added to
all surfaces that are to be machined.
The amount about of
finish allowance depends on the material of the casting, its size volume of
production methods of molding, configuration of the casting, the position the
wall surface occupies in the in the mould and during pouring. Machining
allowance is larger for hand molding as compare to machine molding. For
internal surfaces such as bores, the allowance is about 0.8 mm greater and is
negative.
Table 1.1 typical machining allowance for sand casting
Material cast
|
0 to 30
|
Overall length of external surface, cm
|
||
30 to 60
|
60 to 105
|
105 to 150
|
||
Al alloys
|
1.6
|
3.2
|
3.0
|
4.8
|
Brass, bronze
|
1.6
|
3.2
|
3.0
|
4.8
|
C.I.
|
2.4
|
3.2
|
4.8
|
6.4
|
C.S.
|
3.2
|
4.8
|
6.0
|
9.6
|
1.3.3. PATTERN
DRAFT OR TAPER:-
Pattern draft, also termed “draw” , is the taper placed on the pattern
surfaces that are parallel to the direction in which the pattern is withdrawn
from the mould to allow removal of the pattern without damaging the mould
cavity, fig a. The draft depends upon the method of the molding, the sand
mixture used the design and economics restrictions imposed on the casting. The
common draft is 1o to 3o after applying the draft, the
largest cross section of the pattern will be at the parting line for external
surfaces and reverse will be for internal surface fig 1.11.
Fig.
1.11 Pattern Taper
1.3.4. CORNER
AND FILLETS:-
The intersection of surfaces in
casting must be smooth and form no sharp angles. For this the external and
internal corners of patterns are suitably rounded. They are called rounded
corners and fillets respectively. Fillets facilitate the removal of the pattern
from the mould, prevent the formation of cracks and shrink holes in the
casting. The radius of fillet given as=1/5 to 1/3.
1.3.5. RAPPING
OR SHAKE ALLOWANCE:-
To take the pattern out of the mould
cavity it is slightly rapped to detach it from the mould cavity Due to this,
the cavity in the mould increasing slightly. So, the pattern made slightly
smaller.
1.3.6. DISTORTION
OR CAMBER ALLOWANCE:-
This allowance is considered only
for casting of irregular shape which is distorted in the process of the cooling
because of metal shrinkage. Sometimes casting get distorted during
solidification, due to their typical shape.
For example, if the casting has the
form of the letter U,V,T or L etc. It will tend to contract
At the closed end causing the
vertical legs to look slightly inclined this can be prevent by making the leg
of the U,V,T or L shaped pattern converge slightly (inward) so that the casting
after distortion will have its sides vertical fig.1.12
The distortion in casting may occur
due to internal stresses. These internal stresses are caused on the account of
unequal cooling of different section of the casting and hindered Contraction
measure taken to prevent the distortion casting include
I. Modification of
casting design
II. Providing
sufficient machining allowance to cover the distortion affect
III. Providing
suitable allowance on the pattern, called camber or distortion.
Fig.
1.12 Distortions In Casting
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