casting design unit 3
TRANSCRIPT
Design for casting process
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Overview
Introduction
Pattern design
Design of pattern allowances
Moulding sand properties – testing
Gating types
Design – aspiration effect – effect of friction and velocity distribution
Cooling & Solidification
Mechanism of solidification – rate of solidification
Riser
Riser design – placement
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Design is the critical first step in the development of cost effective, high quality
castings.
The important advantage of casting design is able to produce highly complex
functional shapes quickly and easily.
Design process includes mold construction, mold filling & material solidification.
Fluid life is the ability of the molten alloy to fill the mold cavity, flow through thin
narrow channels to form thin walls and sections, and conform to fine surface detail.
In addition to temperature of the molten metal, fluid life also depends on chemical,
metallurgical, and surface tension factors.
Introduction
Introduction
Shrinkage
Liquid shrinkage
Liquid to solid shrinkage or solidification shrinkage
Solid shrinkage
Solidification shrinkage
Directional, eutectic, and equiaxed
Pouring Temperature
Fluid Flow
Heat Transfer Considerations
Introduction
Shrinkage allowances
Since the metal shrinks on solidification 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.
Draft allowances
The pattern needs to incorporate suitable allowances for draft, which
means that its sides are tapered so that when it is pulled from the sand, it will
tend not to drag sand out of place along with it. This is also known as taper
which is normally between 1 and 3 degrees.
Pattern design
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.
Corners and fillets
The intersection of surfaces in casting must be smooth and form no
sharp angles.
Fillets facilitate the removal of the pattern from the mould, prevent the
formation of cracks and shrink holes in the casting.
Pattern design
Sprues, gates, risers, cores, and chills
The patternmaker or foundry engineer decides where the sprues, gating
systems, and risers are placed with respect to the pattern.
Where a hole is desired in a casting, a core may be used which defines a
volume or location in a casting where metal will not flow into.
Sometimes chills may be located on a pattern surface, which are then
formed into the sand mould. Chills are heat sinks which enable localized
rapid cooling.
The rapid cooling may be desired to refine the grain structure or
determine the freezing sequence of the molten metal which is poured into
the mould.
Pattern design
Rapping or shake allowance
To take the pattern out of the mould cavity it is slightly rapped to detach
it from the mold cavity. Due to this, the cavity in the mould increases
slightly. So, the pattern is made slightly smaller.
Distortion allowance.
This allowance is considered only for castings of regular shape which
are distorted in the process of cooling because of metal shrinkage
Pattern design
Permeability
Strength or cohesiveness
Refractoriness
Plasticity or flowability
Collapsibility
Adhesiveness
Co – efficient of expansion
Properties of moulding sand
Moisture content test
By the loss of weight after evaporation
By moisture teller
By moisture teller based on chemical reaction
Clay content test
Permeability test
Fineness test
Strength test
Hardness test
Sand Testing
The term gating system refers to all passage ways through which the molten
metal passes to enter the mold cavity
Since the way in which liquid metal enters the mold has a decided influence
upon the quality and soundness of a casting, the different passages for the
molten metal are carefully designed and produced.
A gating system should avoid sudden or right angle changes in direction.
Sudden change in direction causes mold erosion, trubulence and gas pick-
up.
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Principle of gating system
Fill the mold cavity completely before freezing
Introduce the liquid metal into the mold cavity with low velocity and little
turbulence, so that mold erosion, metal oxidation and gas pick-up is
prevented
Help to promote temperature gradients favorable for proper solidification
Regulate the rate at which liquid metal enters into the mold
Be practicable and economical to make end
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Function of gating system
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Components of gating system
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Components of gating system
Pouring cup
A pouring cup makes it easier for the ladle or crucible operator to direct
the flow of metal from crucible to sprue
A pouring cup is a funnel shaped cup which forms the top portion of the
sprue
Pouring basin
A pouring basin may be made out of core sand, metal or it may be cut
or molded in the cope of sand mold
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Components of gating system
Sprue
A sprue feeds metal to the runner which in turn reaches the casting
through the gates
Gates
A gate is channel which connects runner with mold cavity through which
molten metal flows to fill the mold cavity
A gate should feed liquid metal to the casting at a rate consistent with
rate of solidification
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Components of gating system
Parting line gate
Top gate
Wedge gate
Pencil gate
Bottom gate
Step gate
Multiple gate
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Types of gate
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Types of gate
Parting line gate
Side gate
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Types of gate
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Types of gate
Gating technique must be designed to take account of the weight and shape
of the individual casting, the fluidity of the metal and its relative susceptibility to
oxidation.
Although techniques vary widely according to these conditions, the basic
objectives must be achieved at minimum cost in moulding and fettling time
and in metal consumption.
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Design of gating system
Design of gating system
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Equation of Continuity
Volume rate of flow
Bernoulli’s Theorem
Linear velocity of flow
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Design of gating system
Fluid flow
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Design of gating system
F K
Choke area
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Design of gating system
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Sprue Design
Riser
Riser or feeder heads are a part of the feeding system.
These are reservoirs of molten metal that feed the metal in the casting
proper as it solidifies
Types of risers
Top riser
Side riser
Open riser
Blind riser
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Types of riser
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Riser location
The riser should be located at such a position to facilitate feeding metal to
compensate shrinkage, to promote directional solidification, easy feeding and
to minimize the riser effect and end effect.
The type of risers used may vary from metal to metal, in case of light metal,
foundries like Aluminum and magnesium which have low specific gravity
extensive use of top riser are made which provides maximum benefit of
metallostatic pressure. Close spacing of riser can minimize difficulties with
micro shrinkage.
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Minimum volume criterion
Modulus criterion Minimum volume criterion
According to this criterion if a riser has to feed a casting in order to compensate shrinkage, the volume of molten metal available for feeding from the riser should be at least equal to the amount of shrinkage in the casting.
That is,
Volume of the metal feed by the feeder= volume of shrinkage in the casting
Volume of the metal feed by the feeder=e*Vf
Where
Vf= Volume of feeder
e=Efficiency of Feeder
Efficiency= (volume of metal fed by riser to casting) X (volume of metal in the riser)
Volume of shrinkage in the casting=α*(Vf+Vs)
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Riser shape and size
Therefore by minimum volume criterion:-
(e*Vf)=α*((Vf+Vs)
Therefore volume of feeder
= (α*Vf)*(e-α)
The height of the riser can be determined by considering the height to diameter ratio.
Height to diameter ratio should be in the ratio of 1:5
Cylindrical risers are preferred because of its better efficiency to feed the casting.
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Riser shape and size
Modulus Criterion
Modulus Criterion:-
According to this criterion riser should stay in the molten condition for enough time must
be able to feed the casting until it completely solidifies.
A casting losses heat to the surroundings by conduction, convection and radiation.
Chrinov has shown that solidification time of casting is proportional to the square of ratio
of volume to surface of the casting.
The constant of proportionality is called the mould constant that depends on the pouring
temperature, casting and mould thermal characteristics.
Ts= (V2*K)/ (SA) 2
Where,
Ts= Solidification time, V=volume of the geometry, SA=Surface area of the geometry
K= Modulus
To compare the solidification times of two casting, their moduli can be compared
(Tcasting1)/ (Tcasting2) = (Mcasting1)/ (Mcasting2).
34(V/SA) riser > (V/SA) casting.
Mechanism of solidification
When the mould cavity is filled with molten metal, the metal adjacent to the
walls of the mould cools and solidifies first.
This results in a shell of solid metal, with center of the section remaining
liquid, while there is a zone between the liquid interior and solid exterior
wherein the metal is semisolid state.
The solidification proceed inwards the center of the section. This
solidification is called Lateral or Progressive solidification.
Longitudinal solidification occurs at right angles to the lateral solidification at
the center line is called directional solidification.
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Mechanism of solidification
The longitudinal solidification must progress from the thinnest faster cooling
sections to the heavier hotter section
The temperature gradient, in addition to being properly directed, must be
sufficiently steep so that the liquid metal can pass through the wedge shaped
channel to compensate for shrinkage as it occurs at the center line. It implies
that the progressive solidification is controlled in such a way that no portion of
the casting is isolated from the liquid metal feeding channels, during the
complete solidification cycle.
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Mechanism of solidification