Casting Design Unit 3

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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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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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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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


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Equation of Continuity

Volume rate of flow

Bernoulli’s Theorem

Linear velocity of flow

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Fluid flow

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F K

Choke area

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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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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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