STRUCTURE
FOR PURE METALS:
At the mould walls, metal cools rapidly. Produces
solidified skin or shell (thickness depends on composition,
mould temperature, mould size and shape etc)
• These are of equiaxed structure.
• Grains grow opposite to heat transfer through the
mould
• These are columnar grains
• Driving force of the heat transfer is reduced away
from the mould walls and blocking at the axis
prevents further growth
NITC
PURE METALSHave clearly defined melting/freezing point,
solidifies at a constant temperature.
Eg: Al - 6600C,
Fe - 15370C,
and W- 34100C.
NITC
Size and distribution of the overall grain structure throughout
a casting depends on rate & direction of heat flow
(Grain size influences strength, ductility, properties along
different directions etc.)
CONVECTION- TEMPERATURE GRADIENTS DUE TO
DIFFERNCES IN THE DENSITY OF MOLTEN METAL AT DIFFERENT
TEMPERATURES WITHIN THE FLUID - STRONGLY EFFECTS
THE GRAIN SIZE.
Outer chill zones do not occur in the absence of convection
NITC
FOR ALLOYS:
• Alloys solidify over a range of temperatures
• Begins when temp. drops below liquidous,
completed when it reaches solidous.
• Within this temperature range, mushy or pasty
state
(Structure as in figure)
• Inner zone can be extended throughout by adding
a catalyst.- sodium, bismuth, tellurium, Mg
(or by eliminating thermal gradient, i.e. eliminating
convection. (Expts in space to see the effect of lack of
gravity in eliminating convection)
(refresh dendritic growth- branches of tree, interlock, each
dendrite develops uniform composition, etc)
NITC
SOLIDIFICATION TIME
During solidification, thin solidified
skin begins to form at the cool mould
walls.
Thickness increases with time.
For flat mould walls
thickness  time
(time doubled, thickness by 1.414)
NITC
CHVORINOV’S RULE
solidification time (t) is a function of volume of
the casting and its surface area
t = C ( volume/ surface area )2
C is a constant [depends on mould material, metal
properties including latent heat, temperature]
A large sphere solidifies and cools at a much slower rate
than a small diameter sphere. (Eg- potatoes, one big and
other small)
Volume  cube of diameter of sphere,
surface area  square of diameter
NITC
Solidification time for various shapes:
Eg: Three pieces cast with the SAME volume, but different shapes.
(i)Sphere, (ii)Cube, (iii)Cylinder with height = diameter.
Which piece solidifies the fastest?
Solution: Solidification time = C (volume/surface area)2
Let volume = unity. As volume is same, t = C/ surface area2.
Sphere: V= 4/3 (π r3); i.e. r = (3/4 π)1/3
A= 4 π r2 = 4 π (3/4 π)1/3 = 4.84
Cube: V = a3; ie a = 1; A = 6 a2 = 6.
Cylinder: V = πr2h = 2 π r3; ie, r = (1/2 π) 1/3
A = 2 πr2 + 2πrh = 6 πr2 = 5.54.
Then, t cube = 0.028C ; t cylinder = 0.033C ; t sphere= 0.043C
Metal poured to cube shaped mould solidifies the fastest.
NITC
SHRINKAGE AND POROSITY
METALS SHRINK(CONTRACT) DURING
SOLIDIFICATION
- CAUSES DIMENSIONAL CHANGES
LEADING TO CENTRE LINE SHRINKAGE, POROSITY,
CRACKING TOO
NITC
SHRINKAGE DUE TO:
(1).CONTRACTION OF
MOLTEN METAL AS IT
COOLS PRIOR TO
SOLIDIFICATION
1
(2) CONTRACTION OF
SOLIDIFYING METAL,
LATENT HEAT OF
FUSION
2
T
3
(3) CONTRACTION OF
SOLIDIFIED METAL
DURING DROP TO
AMBIENT TEMP
Time
OUT OF THESE, LARGEST SHRINKAGE DURING COOLING
OF CASTING (ITEM 3) eg:pure metal
NITC
SOLIDIFICATION CONTRACTION FOR VARIOUS METALS
METAL Volumetric Solidification Contraction
Al
6.6
Grey cast Iron
Expansion 2.5
Carbon Steel
2.5 to 3
Copper
4.9
Magnesium
4.2
Zinc
6.5
NITC
• POROSITY DUE TO SHRINKAGE OF GASES
AND METAL TOO.
RELATED TO DUCTILITY
AND SURFACE FINISH
(DUCTILITY V/S POROSITY CURVES FOR
DIFFERENT METALS)
- ELIMINATION BY VARIOUS MEANS
(ADEQUATE SUPPLY OF LIQUID METAL, USE
OF CHILLS, NARROWING MUSHY ZONECASTING SUBJECTED TO ISOSTATIC PRESSING
NITC
POROSITY BY GASES
LIQUID METALS HAVE HIGH SOLUBILITY FOR
GASES
DISSOLVED GASES EXPELLED FROM
SOLUTION DURING SOLIDIFICATION
(Hydrogen, Nitrogen mainly)
ACCUMULATE IN REGIONS OF EXISTING
POROSITY OR
CAUSE MICROPOROSITY IN CASTING
- TO BE CONTROLLED
NITC
Ductility
Effect of microporosity on the ductility of quenched and
tempered cast steel – Porosity affects the ‘pressure tightness’ of
cast pressure vessel
Elongation
Reduction of area
0
5
10
Porosity(%)
15
NITC
FLOW OF MOLTEN METAL IN MOULDS
Important: pouring basin, mould cavity & riser
GATING SYSTEM Design -fluid flow, heat transfer, influence
of temperature gradient,
FLUID FLOW
Without turbulence
or with minimized turbulence
HEAT FLOW INFLUENCED BY MANY FACTORS
FLUIDITY-A characteristic related to viscosity.
TEST OF FLUIDITY - USING A SPIRAL MOULD.Fluidity Index
NITC
TEST FOR
FLUIDITY
USING A SPIRAL
MOULD.
FLUIDITY INDEX IS
THE LENGTH OF
THE SOLIDIFIED
METAL IN THE
SPIRAL PASSAGE.
GREATER THE
LENGTH, GREATER
THE FLUIDITY
INDEX.
PATTERN
• Model of a casting constructed such that it
forms an impression in moulding sand
NITC
PATTERN
• 1st step- Prepare model (pattern)
Differs from the casting
Differences
Pattern Allowances.
• To compensate for metal shrinkage,
• Provide sufficient metal for machining
• Easiness in moulding
• As Shrinkage allowance, Draft allowance, Finishing
allowance, Distortion or camber allowance,
Shaking or rapping allowance
NITC
MATERIAL
1. WOOD.
2. METAL
Al, CI, Brass,
3. For special casting processes,
Polystyrene which leaves mould as gas
when heated also used.
Types- many
Simple-Identical patterns;
Complex, intricate- with number of pieces.
Single or loose piece; Split; gated; Match Plate;
Sweep; Segmental; Skeleton(frame, ribbed), skell;
Boxed Up; Odd shaped etc. Sketches--
NITC
Material
1. WOOD.
(+) Cheap, easily available, light, easiness in surfacing,
preserving (by shellac coating), workable, ease in
joining, fabrication
(-) Moisture effects, wear by sand abrasion, warp during
forming, not for rough use.
Must be properly dried/ seasoned,
free from knots, straight grained
Egs. Burma teak, pine wood, mahogany, Sal, Deodar,
Shisham, Walnut, Apple tree
NITC
2. METAL:
For durability, strength
Egs: Al alloys, Brass, Mg alloys, Steel, cast Iron for
mass production
(first, wooden pattern is made, then cast in the metal)
Type of material depends on shape, size, number of
castings required, method of moulding etc.
NITC
TYPES OF PATTERNS
NIT CALICUT
1. SINGLE PIECE PATTERN.
NITC
NIT CALICUT
2. SPLIT PATTERN (TWO PIECE )
NITC
NIT CALICUT
2. a, THREE PIECE SPLIT PATTERN
NITC
NIT CALICUT
3. LOOSE PIECE PATTERN
NITC
4. COPE AND DRAG PATTERN
•
•
•
NIT CALICUT
•
COPE AND DRAG PARTS OF THE PATTERN
MOUNTED ON SEPARATE PLATES.
COPE HALF AND DRAG HALF MADE BY
WORKING ON DIFFERENT MOULDING
MACHINES.
THIS REDUCES THE SEPARATE COPE AND DRAG
PLATE PREPARATION.
GENERALLY FOR HIGH SPEED MECHANISED
MOULDING.
NITC
5. MATCH PLATE PATTERN –
Pattern generally of metal and plate making
NIT CALICUT
parting line metal/wood.
NITC
6. FOLLOW BOARD PATTERN.
For thin sections.
NIT CALICUT
THIN PATTERN
NITC
7. GATED PATTERN
-
NIT CALICUT
Gating system is a part of the pattern.
NITC
8. SWEEP PATTERN
–
For large size castings in small numbers. Template of
NIT CALICUT
wood attached to a sweep used.
NITC
9. SEGMENTAL PATTERN
–
NIT CALICUT
For rings, wheel rims, large size gears.
NITC
10. SKELETON PATTERN.Stickle board used to scrape the excess sand.
Eg. Oil pipes, water pipes, pipe bends, boxes, valve bodies etc.
NIT CALICUT
Stickle board
NITC
NIT CALICUT
11. SHELL PATTERN
NITC
12. BUILT UP PATTERN
–
Also called lagged up patterns- For barrels, pipes,
NIT CALICUT
columns etc
NITC
13. LEFT AND RIGHT PATTERN
–
NIT CALICUT
For parts to be made in pairs.
Eg: legs of sewing machine, wood working lathe,
garden benches, J hangers for shafts, brackets for
luggage racks etc.
NITC
•
•
•
•
•
NIT CALICUT
Type of pattern depends on:
Shape and size of casting,
number of castings required,
method of moulding employed,
easiness or difficulties of the moulding
operations,
• other factors peculiar to the casting.
NITC
Pattern,
Finish Allowance,
and Wall Thickness
Min
Wall
mm
Metal
Pattern
Oversize Factor
(each direction)
Finish Allowance
(smaller number
for larger sizes)
Aluminum
1.08 - 1.12
0.5 to 1.0 %
4.75
Copper alloys
1.05 - 1.06
0.5 to 1.0 %
2.3
Gray Cast Iron
1.10
0.4 to 1.6 %
3.0
Nickel alloys
1.05
0.5 to 1.0 %
N/A
Steel
1.05 - 1.10
0.5 to 2 %
5
Magnesium
alloys
1.07 - 1.10
0.5 to 1.0 %
4.0
Malleable
Irons
1.06 - 1.19
0.6 to 1.6 %
3.0
CHARACTERISTICS OF
PATTERN MATERIALS
CHARACTERISTIC
RATING
WOOD
MACHINABILITY
WEAR RESISTANCE
STRENGTH
WEIGHT
REPAIRABILITY
RESISTANCE TO:
•
•
AL
STEEL
PLASTIC
CAST IRON
E
P
E
E
E
G
G
G
G
P
F
E
E
P
G
G
F
G
G
F
G
E
G
P
G
CORROSION (by water) E
SWELLING
P
E
E
P
E
E
E
P
E
E- Excellent; G- Good; F-fair, P- Poor
NITC
Functions of pattern
•
•
•
•
•
•
Moulding the Gating system;
Establishing a parting Line,
Making Cores,
Minimising casting Defects,
Providing Economy in moulding
Others, as needed