casting fundamentals and basics concepts.ppt
TRANSCRIPT
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2MODULE
COURSE OUTCOMES
Fundamentals of Casting process
Solidification of metals
Fluid flow of molten metal
Various casting process
Casting defects & quality
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:Casting / Foundry
Casting processes basically involve the introduction of a molten
metal into a mold cavity, where upon solidification, the metal
takes on the shape of the mold cavity.
Applications :
Cylinder blocks, liners, machine tool beds, pistons, piston rings,
mill rolls, wheels, housings, water supply pipes, bells
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Examples of Cast Parts
Crank handle formed by casting; some areas
were machined and assembled after casting
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Examples of Cast Parts
C-clamps formed by casting (left) and machining
(right)
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Examples of Cast Parts
Complex part formed by casting
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Aluminum piston for an internal
combustion engine:
(a) as-cast (b) after machining.
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: A molding flask is one which holds the sand mould intactFlask
: Lower molding flaskDrag
: Upper molding flaskCope
Pattern : It is a replica of final object to be made with somemodifications. The mold cavity is made with the help of pattern material
: It is the freshly prepared refractory material used forMolding sandmaking the mold cavity.
: It is used for making hollow cavities in the castings.Core
Core Print : A region used to support the core
: A small funnel shaped cavity at the top of the mold intoPouring basinmolten metal is poured.
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Mold
cavity
chaplet
Mold
cavity
chaplet
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Parting Line / Parting Surface : Interface that separates thecope and drag
: The channel through which the molten metal is carriedRunnerfrom the sprue to the gate.
Gate: A channel through which the molten metal enters the moldcavity
: Chaplets are used to support the cores inside the moldChapletscavity .
: It is a reservoir of the molten metal provided in the casting .Riser
: Small opening in the mold to facilitate escape of air and gases.Vent
: The passage through which the molten material from theSpruepouring basin reaches the mold cavity.
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Pattern Material
Wood, metals & alloys, plastic, plaster of Paris, plastic
and rubbers, wax, and resins.
Material select io n depends on size and shape of cast ing .
To be suitable for use, the pattern material should be:
1. Easily worked, shaped and joined2. Light in weight
3. Strong, hard and durable
4. Resistant to wear and abrasion
5. Resistant to corrosion, and to chemical reactions6. Dimensionally stable and unaffected by variations
in temperature and humidity
7. Available at low cost
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Sand Casting
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Pattern Allowances
Pattern allowance is a vital feature as it affects thedimensional characteristics of the casting
The selection of correct allowances greatly helps to
reduce machining costs and avoid rejections.
1. Shrinkage or contraction allowance
2. Draft or taper allowance
3. Machining or finish allowance
4. Distortion or camber allowance5. Rapping allowance or shake allowances
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Shrinkage or Contraction Allowance
All most all cast metals shrink or contract volumetr ical lyon cooling.
The metal shrinkage is of two types:
(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 thecasting, are provided in the mold.
(2) Solid Shrinkage: It refers to the reduction in volume during the
cooling of the cast metal to room temperature.
To account for this, shrinkage allowance is provided on the patterns.
The rate of contraction with temperature is dependent on the material.
For example steel contracts to a higher degree compared to
aluminum.
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Shrinkage
Metal Percent Contraction (-)Expansion(+)
Aluminum
Zinc
Gold
Copper
Brass
Carbon Steel
LeadGray Cast Iron
-7.1%
-6.5%
-5.5%
-4.9%
-4.5%
-2.5-4%
-3.2%+2.5%
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Draft or Taper Allowance
Taper is provided on all vertical surfaces of the pattern so that it
can be removed from the sand without tearing away the sides of the
sand mold.
Draft allowance varies with the complexity of the sand job.
Inner details of the pattern require higher draft than outer surfaces.
The amount of draft depends upon the length of the vertical side of
the pattern to be extracted; the intricacy of the pattern; the method of
molding; and pattern material.
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Pattern
material
Height of the
given surface
(inch)
Draft angle
(External
surface)
Draft angle
(Internal
surface)
Wood
1
1 to 2
2 to 4
4 to 88 to 32
3.00
1.50
1.00
0.750.50
3.00
2.50
1.50
1.001.00
Metal and
plastic
1
1 to 2
2 to 4
4 to 8
8 to 32
1.50
1.00
0.75
0.50
0.50
3.00
2.00
1.00
1.00
0.75
Draft Allowances of Various Metals
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Taper on patterns for ease of removal from the sand mold
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Machining or Finish Allowance
Machining or finish allowances are added in the pattern dimensionto have good surface finish or dimensionally accurate
The amount of machining allowance to be provided is affected by
the method of molding and casting used viz. hand molding or machine
molding, sand casting or metal mold casting.
The amount of machining allowance is also affected by the size andshape of the casting; the casting orientation; the metal; and the degree
of accuracy and finish required.
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Metal Dimension (inch) Allowance (inch)
Cast ironUp to 1212 to 20
20 to 40
0.120.20
0.25
Cast steel
Up to 6
6 to 2020 to 40
0.12
0.250.30
Non ferrous
Up to 8
8 to 12
12 to 40
0.09
0.12
0.16
Machining Allowances of Various Metals
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Distortion or Camber Allowance
Castings 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. itwill tend to contract at the closed end causing the vertical legs to look
slightly inclined.
This can be prevented by making the legs of the U, V, T, or L shaped
pattern converge slightly (inward) so that the casting after distortionwill have its sides vertical
The distortion in casting may occur due to internal stresses.
These internal stresses are caused on account of unequal cooling of
different section of the casting and hindered contraction.
To prevent the distortion in castings include:
i. Modification of casting designii. Providing sufficient machining allowance to cover the distortion
affect
iii. Providing suitable allowance on the pattern, called camber or
distortion allowance (inverse reflection)
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Distortions in Casting
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Rapping Allowance
Before the withdrawal from the sand mold, the pattern is rapped all
around the vertical faces to enlarge the mold cavity slightly, which
facilitate its removal.
Since it enlarges the final casting made, it is desirable that the
original pattern dimension should be reduced to account for thisincrease.
There is no sure way of quantifying this allowance, since it is highly
dependent on the foundry personnel practice involved.
It is a negative allowance and is to be applied only to those
dimensions that are parallel to the parting plane.
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Fluid flow
2 principles of fluid flow are relevant togating design: Bernoullis theorem and
the law of mass continuity.
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Fluid flow
Bernoullis theorem
Based on
- principle of conservation of energy
- frictional losses in a fluid system
Conservation of energy requires that,
Constant2
2
g
v
g
ph
f
g
v
g
ph
g
v
g
ph
2
2
2
22
2
2
11
h= elevation
p= pressure at elevation
v= velocity of the liquid
= density of the fluid
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Fluid flow
Mass continuity
States that for an incompressible liquid
the rate of flow is constant.
Subscripts 1 and 2 pertain to two different
locations in the system.
2211vAvAQ
Q= volumetric rate of flow
A = cross-sectional area of the liquid stream
v= velocity of the liquid
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Fluid flowSprue profile
Relationship between height and cross-
sectional area at any point in the sprue is
given by
Velocity of the molten metal leaving the
gate is
When liquid level reached heightx, gate
velocity is
1
2
2
1
h
h
A
A
ghcv2
xhgcv 2
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Fluid flow
Flow characteristics Reynolds number, Re, is used to characterize
aspect of fluid flow.
It represents the ratio of the inertia to the viscous
forces in fluid flow and is defined as
vDRe
v= velocity of the liquid
D= diameter of the channel
= density
n = viscosity of the liquid.
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Flow Characterist ic s 0 < Re < 2000 => laminar flow
2000 < Re < 20 000 =>mixture of laminar and turbulent
flow , generally regarded as harmless in gatingsystems.
Re > 20 000 => severe turbulence
In gating systems, Re typically ranges from 2000 to20,000
Techniques for minimizing turbulence
Dross or slag can be eliminated by vacuum casting
Use of filters eliminates turbulent flow in the runnersystem
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Turbulence can be reduced by the design of a gating system
that promotes a more laminar flow of the liquid metal.
Sharp corners and abrupt changes in sections within the
casting can be a leading cause of turbulence. Their affect
can be mitigated by the employment of radii.
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Fluidity of molten metal
Fluidity of Molten Metal : The capability of molten metal to fill mold
cavities is called f lu id i ty .
The following influence fluidity
Characteristics of molten metal Viscosity (How runnyis it when hot)
Surface tension (Development offilm ) Inclusions
Solidification pattern of the alloy
Casting parameters
Mold design (Risers, runners, gates, etc.) Mold material and its surface characteristics
Degree of superheat
Rate of pouring
Heat transfer
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Heat Transfer
Important consideration in casting
Heat flow in the system
Complex
Depends of flow characteristics
Solidification Time A function of the volume of a casting and its surface
area
Solidification time = C volume 2
surface area
Effects on solidification time Mold Geometry
Skin thickness
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Heat transfer
Temperature distribution in the mold-
liquid metal interface is shown below.
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Sol id i f icat ion o f Metals
Involves liquid metal turning back in to solid metal
The process is different for Pure metals and alloys
Can be divided into two steps:
Formation of stable nuclei
Growth of crystals
Pure Metals
Have a clearly defined melting point
Temperature remains constant during freezing
Solidifies from the walls of the mold toward the center
of the part
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Grain Structu re for Pure Metals
Two types of grains are formed for a pure metal
Fine equiaxed grains Columnar
Rapid cooling at the walls produces fine equiaxed grains
Columnar grains grow opposite of the heat transferthroughout the mold following the chill zone
Equiaxed Grains
If crystals can grow approximately equally in all directionsequiaxed grains will grow.
Large amounts of under cooling is needed near the wall ofthe mold.
Ill t ti f C t St t
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Illustration of Cast Structures
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Al loys
Solidification in alloys begins when the temperature drops below
the liquidus TL and is complete when it reaches the solidus, TS.
All
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Alloys
Within the TL and TS Temperature range, the alloy is like a slushy withcolumnardendrites
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Effects o f Coo l ing Rates
Slow cool rates results in cou rse grain struc tures(102 K/s)
Faster cooling rates produce f iner grain s tructures(104 K/s)
For even faster cooling rates, the structures are amorphous
(106
108
K/s)
Grain size influences strength of a material
Smaller grains have higher ductility and strength
Smaller grains help prevent ho t tearingand/or cracks in thecasting
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Casting Process Classifications
Expendable Mold / Reusable Pattern.1
Expendable Mold / Expendable Pattern.2
Permanent Mold / No Pattern.3
Sand CastingShell moldingCeramic-Mold Casting
Investment Casting
Evaporative-Foam Casting
Permanent Mold Casting
Die Casting
Centrifugal Casting
( Wax , Plastic ,Polystyrene
Foam )
( Wood ,Plastic ,metal )
Sh ll ldi
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Shell molding
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Sh ll ldi
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a mounted pattern, made of a ferrous metal or
aluminum, is heated to 175-370 0 C, coated with aparting agent such as silicone, and clamped to a box
or chamber containing a fine sand coated with a 2.5 -
4.0% thermosetting resin binder
the sand mixture is blown over the heated pattern,coating it evenly
the assembly is placed in an oven to complete the
curing of the resin
the shell is formed by removing the pattern two half shells are made and are clamped together in
preparation for pouring
Shell molding
S
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Shell molding
Advantages
Better surface finish
Better dimensional tolerances.
Reduced Machining.
Less foundry space required.
Semi skilled operators can handle the process.
The process can be mechanized.
Sh ll ldi
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Disadvantages
The raw materials are relatively expensive.
The process generates noxious fumes which must be
removed.
The size and weight range of castings is limited. (Size
limits of 30 g to 12 kg )
Shell molding
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Shell Molding
Applications
-Crankshaft fabrication
-Steel casting parts, fittings
-Molded tubing fabrication
-Hydraulic control housing fabrication
-Automotive castings (cylinder head andribbed cylinder fabrication).
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MoldExpendable
Uses a polystyrene foam pattern which evaporates
with molten metal to form a cavity for the casting.
Polystyrene foam pattern includes sprue, risers,gating system and internal cores (if needed)
Polystyrene inexpensive and easily processedinto patterns
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Ad t f d d l t
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Advantages of expanded po lystyrene process:
1. Pattern need not be removed from the mold
2. Simplifies and speeds mold-making, because two
mold halves are not required as in a conventional
green-sand mold
Disadvantages:
1. A new pattern is needed for every casting
2. Economic justification of the process is highlydependent on cost of producing patterns
E ti P tt C ti f E i Bl k
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(a) Metal is poured into mold for lost-foam casting of a 60-hp. 3-
cylinder marine engine; (b) finished engine block.
(b)(a)
Evaporative Pattern Casting of an Engine Block
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I t t C ti
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Investment Casting
Investment Cast ing Character ist ics
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Investment Cast ing Character ist ics
Advantages:
Complex shapes possible
Thin wall sections possible
High production rates
High dimensional accuracy
Disadvantages:
Limited weight range
Expensive Machinery & Dies
Expensive Unit Cost, Labor Intensive
Mold is not reusable
Typical parts produced by investment cast ing.
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yp ca pa s p oduced by es e cas g
Products such as rocket components, and jet
engine turbine blades
Die casting
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Die casting typically makes use of non-ferrous alloys.
The four most common alloys that are die cast are
Aluminum alloys, Copper alloys, Magnesium alloys,Zinc alloys
The molten metal is injected into die cavity under highpressure
Pressure maintained during solidification
Die casting
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Hot chamber die
casting
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Cold chamber die casting
Advantages of die casting
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Advantages of die casting
Excellent dimensional accuracy
Smooth cast surfaces
Thinner walls can be cast
Inserts can be cast-in (such as threaded inserts,
heating elements, and high strength bearing
surfaces).
Reduces or eliminates secondary m/c ing operations.
Rapid production rates.
Disadvantages of die casting
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The main disadvantage - very high capital cost.
To make die casting an economic process a large
production volume is needed.
Die casting is limited to high fluidity metals (Zinc,Aluminum, Magnesium, Copper, Lead and Tin) (Not
applicable for high melting point metals and alloys
(eg. steels)
Casting weights must be between 30 grams and 10 kg
Limited die life
Disadvantages of die casting
Centrifugal Casting
http://en.wikipedia.org/wiki/Capital_costhttp://www.substech.com/dokuwiki/doku.php?id=steels_and_cast_irons&DokuWiki=b1069e39936ec6f4469f6b221001434bhttp://www.substech.com/dokuwiki/doku.php?id=steels_and_cast_irons&DokuWiki=b1069e39936ec6f4469f6b221001434bhttp://en.wikipedia.org/wiki/Capital_cost -
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Centrifugal Casting
In this process, the mold is rotated rapidly about its
central axis as the metal is poured into it.
Because of the centrifugal force, a continuous pressure
will be acting on the metal as it solidifies.
The slag, oxides and other inclusions being lighter, getseparated from the metal and segregate towards the
center.
This process is normally used for the making of hollowpipes, tubes, hollow bushes, etc., which are ax- symmetric
with a concentric hole.
Centrifugal Casting
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The mold can be rotated about a vertical, horizontal or an
inclined axis or about its horizontal and vertical axes
simultaneously.
The length and outside diameter are fixed by the mold
cavity dimensions while the inside diameter is
determined by the amount of molten metal poured.
Since the metal is always pushed outward because of the
centrifugal force, no core needs to be used for making the
concentric hole.
Centrifugal Casting
Defects Cavities
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Defects
Metallic projections
Fins
Flash
Massive projections Swells
Rough surfaces
Cavities
Internal or external
Blow holes
Pin holes
Shrinkage cavities
Discontinuities
Cracks
Cold or hot tearing
Cold shunts
Casting Defects
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Casting Defects