2. Pattern is a model or the replica of the object to be cast. It is a larger
in size as compared to the final casting, because it carries certain
allowances due to metallurgical and mechanical reasons for example,
shrinkage allowance is the result of metallurgical phenomenon where
as machining, draft, distortion, shale, and other allowances are
provided on the patterns because of mechanical reasons.
Top center is the clay original, then the two
part plaster mold used for casting the lead at
above, and wax cast from mold, sprued for
better brass casting, not yet cast. 2008-01-12.
homepages.waymark.net/mikefirth/tapper688
1b.jpg
3. Functions of Patterns:
• A Pattern prepares a mould cavity for the
purpose of making a casting.
• A Pattern may contain projections known as
core prints (corazón) if the casting requires a
core and need to be made hollow.
• Patterns properly made and having finished
and smooth surfaces reduce casting defects.
• Properly constructed patterns minimize
overall cost of the casting.
4. The pattern material should be:
1.
2.
3.
4.
5.
6.
7.
Easily worked,shaped and joined.
Light in weight.
Strong,hard and durable.
Resistant to wear and abrasion .
Resistant to corrosion,and to chemical
reactions.
Dimensionally stable and unaffected by
variations in temperature and humidity.
Available at low cost.
11. Reason for allowances:
Solidification Shrinkage
Most metals undergo
noticeable volumetric
contraction when cooled
Three principle stages of
shrinkage:
Shrinkage of liquid as it
cools from the solidification
temperature
Solidification shrinkage as
the liquid turns into solid
Solid metal contraction as
the solidified metal cools to
room temperature
Figure Dimensional changes experienced by a
metal column as the material cools from a
superheated liquid to a room-temperature solid.
Note the significant shrinkage that occurs upon
solidification.
12. Prediction of porosity after casting
Minuto 1:12. Se observa la contracción del metal, no hay tolerancia para
compensarla, genera porosidad residual.
14. Dimensional Allowances
Typical allowances
Cast iron 0.8-1.0%
Steel 1.5-2.0%
Aluminum 1.0-1.3%
Magnesium 1.0-1.3%
Brass 1.5%
Shrinkage allowances are incorporated into the
pattern using shrink rules
Thermal contraction might not be the only factor for
determining pattern size
Surface finishing operations (machining, etc.)
should be taken into consideration
15. Solidification Shrinkage
Amount of liquid metal contraction depends on the
coefficient of thermal contraction and the amount of
superheat
As the liquid metal solidifies, the atomic structure
normally becomes more efficient and significant
amounts of shrinkage can occur
Cavities and voids can be prevented by designing the
casting to have directional solidification
Hot tears can occur when there is significant tensile
stress on the surface of the casting material
16. Types of Pattern Allowances:
THE VARIOUS PATTERN
ALLOWANCES ARE:
1.
2.
3.
4.
Shrinkage or contraction allowance.
Machining or finish allowance.
Draft of tapper allowances.
Distortion or chamber allowance.
5. Shake or rapping allowance.
17. 1.ShrinkageAllowance:
All most all cast metals shrink or contract
volumetrically on cooling.
1.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
mold.
2.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 patterns.
18. Almost all cast metals shrink or contract
volumetrically after solidification and therefore the
pattern to obtain a particular sized casting 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/alloy.
The metal shrinkage depends upon:
1. The cast metal or alloy.
2. Solidification temp.of the metal/alloy.
3. Casted dimensions(size).
4. Casting design aspects.
5. Molding conditions(i.e.,mould materials
and molding methods employed)
19. Material Dimension Shrinkageallowance
(inch/ft)
Grey Cast Iron
Up to 2 feet
2 feet to 4feet
Over 4feet
0.125
0.105
0.083
CastSteel
Upto2feet
2feetto6feet
over6feet
0.251
0.191
0.155
Aluminum
Upto4feet
4feetto6feet
over6feet
0.155
0.143
0.125
Magnesium Upto4feet
Over4feet
0.173
0.155
RATE OF CONTRACTION OF VARIOUS METALS :
20. i.
ii.
iii.
iv.
i.
ii.
iii.
2.MachiningAllowance:
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 the intended to remove surface roughness and other
imperfections from the castings.
It is required to achieve exact casting dimensions.
Surface finish is required on the casting.
HOW MUCH EXTRA METAL OR HOW MUCH
MACHINING ALLOWANCE SHOULD BE
PROVIDED, DEPENDS ON THE FACTORS LISTED BELOW:
Nature of metals.
Size and shape of casting.
The type of machining operations to be employed for
23. 3.Draft or TaperAllowance:
It is given to all surfaces perpendicular to parting
line.
Draft allowance is given so that the pattern can
be easily removed from the molding material
tightly packed around it with out damaging the
mould cavity.
The amount of taper depends upon:
i. Shape and size of pattern in the depth
direction in contact with the mould cavity.
ii. Moulding methods.
iii. Mould materials.
iv. Draft allowance is imparted on internal as well
as external surfaces;of course it is more on
internal surfaces.
30. 4. Distortion or cambered allowance:
A CASTING WILL DISTORT OR WRAP IF :
i. It is of irregular shape,
ii. All it parts do not shrink uniformly i.e.,some
parts shrinks while others are restricted from
during so,
iii. It is u or v-shape,
iv. The arms possess unequal thickness,
v. It has long,rangy arms as those of propeller strut
for the ship,
vi. It is a long flat casting,
vii. One portion of the casting cools at a faster rate
31.
32. 5.Shake allowance:
A patter is shaken or rapped by striking the same
with a wooden piece from side to side.This is
done so that the pattern a little is loosened in the
mold cavity and can be easily removed.
In turn,therefore,rapping enlarges the mould
cavity which results in a bigger sized casting.
Hence,a ²ve allowance is provided on the pattern
i.e.,the pattern dimensions are kept smaller in
order to compensate the enlargement of mould
cavity due to rapping.
The magnitude of shake allowance can be
reduced by increasing the tapper.
33. Pattern Layout:
Steps involved:
Get the working drawing of the part for
which the pattern is to be made.
Make two views of the part drawing on a
sheet,using a shrink rule. A shrink rule is
modified form of an ordinary scale which has
already taken care of shrinkage allowance for
a particular metal to be cast.
Add machining allowances as per the
requirements.
Depending upon the method of molding,
provide the draft allowance.
34. Pattern Construction:
Study the pattern layout carefully and establish,
a. Location of parting surface.
b. No.of parts in which the pattern will be made.
Using the various hand tools and pattern making
machines fabricate the different parts of the pattern.
Inspect the pattern as regards the alignment of
different portions of the pattern and its dimensional
accuracy.
Fill wax in all the fillets in order to remove sharp
corners.
Give a shellac coatings(3 coats) to pattern.
impart suitable colors to the pattern for
identification purposes and for other informations.
35. Design Considerations in Castings
Location and orientation of the parting line is important
to castings
Parting line can affect:
Number of cores
Method of supporting cores
Use of effective and economical gating
Weight of the final casting
Final dimensional accuracy
Ease of molding
36. Design Considerations
Various allowances incorporated into a
casting pattern.
Two-part mold showing the parting line and the
incorporation of a draft allowance on vertical
surfaces.
37. Design Considerations
Figure 11-16 (Left) Elimination of a core by
changing the location or orientation of the
parting plane.
Figure 11-17 (Right) Elimination of a dry-
sand core by a change in part design.
38. Design Considerations
It is often desirable to minimize the use of cores
Controlling the solidification process is important
to producing quality castings
Thicker or heavier sections will cool more slowly, so
chills should be used
If section thicknesses must change, gradual is better
If they are not gradual, stress concentration points can be
created
Fillets or radii can be used to minimize stress concentration
points
Risers can also be used
39. Parting Line and Drafts
Figure 11-18 (Top left) Design where the location of the parting plane is specified by the
draft. (Top right) Part with draft unspecified. (Bottom) Various options to produce the top-
right part, including a no-draft design.
40. Section Thicknesses
(Above) Typical guidelines for section change transitions in castings.
Figure a) The “hot spot” at section r2 is cause by intersecting sections. B) An interior fillet and exterior
radius lead to more uniform thickness and more uniform cooling.
41. Design Modifications
Hot spots are areas of the material that cool more
slowly than other locations
Function of part geometry
Localized shrinkage may occur
Hot spots often result from intersecting sections of various thickness.
42. Design Modifications
Parts that have ribs may experience cracking due to
contraction
Ribs may be staggered to prevent cracking
An excess of material may appear around the parting
line
The parting line may be moved to improve appearance
Thin-walled castings should be designed with extra
caution to prevent cracking
