Foundary or Casting

8/13/2019 Foundary or Casting

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Casting Terms

Mold consists of two halves:

Cope= upper half of mold

Drag= bottom half

Mold halves are contained in a box, called a

flask

The two halves separate at the parting line

Mold cavity is formed by packing sand around

a pattern, which has the shape of the part


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Casting Terms contd.

When the pattern is removed, the remaining cavityhasdesired shape of cast part

The pattern is usually oversized to allow for shrinkage

of metal as it solidifies and cools

Sand for the mold is moist and contains a binder tomaintain shape

Through downsprue, metal enters a runnerleading tothe main cavity


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Casting Terms contd.

At top of downsprue, a pour ing cupis often

used to minimize splash and turbulence as the

metal flows into downsprue

Riseris a reservoir in the mold which is a

source of liquid metal to compensate for

shrinkage during solidification

The riser must be designed to freeze after the

main casting in order to satisfy its function


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Types of Patterns

(a) Single Piece pattern

(b) split or two piece pattern

(c) match-plate pattern

(d) cope and drag pattern


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Single Piece Pattern

Made from one piece and does not contain

loose pieces or joints.

Inexpensive.

Used for large size simple castings.

Pattern is accommodated either in the cope or

in the drag.


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Split Pattern

The upper and the lower parts of the split piece patterns are

accommodated in the cope and drag portions of the mold

respectively.

Parting line of the pattern forms the parting line of the mould.

Dowel pins are used for keeping the alignment between the

two parts of the pattern


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Loose Piece Pattern

Certain patterns cannot be withdrawn once

they are embedded in the molding sand. Such

patterns are usually made with one or more

loose pieces for facilitating from the molding

box and are known as loose piece patterns.

The main body of the pattern is drawn first

followed by loose piece.


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Loose piece pattern


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Match Plate Pattern

It consists of a match plate, on either side ofwhich each half of split patterns is fastened.

The match plate with the help of locator holes canbe clamped with the drag.

After the cope and drag have been rammed withthe molding sand, the match plate pattern isremoved from in between the cope and drag.

Match plate patterns are normally used inmachine molding.

By using this we can eliminate mismatch of copeand drag cavities.


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Cope & Drag pattern

Each half of the pattern along with gating

system is fixed to a separate metal/wood plate.

The two moulds of each half of the pattern are

finally assembled with the help of alignment

pins and the mould is ready for pouring.

Cope and drag patterns are used for producing

big castings which as a whole cannot be

conveniently handled by one molder alone.


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Sweep Pattern

It is used for generating large shapes which are

axi-symmetrical.

Making a sweep pattern saves a lot of time,

money and labour as compared to making a

full pattern


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Sweep Pattern


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Follow Board Pattern

A follow board is a wooden board and is used

for supporting a pattern which is very thin and

fragile and which may give way and collapse

under pressure when the sand above the

pattern is being rammed.


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Follow Board Pattern


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Skeleton Pattern

A skeleton of pattern is made with the help of

wooden strip. It is filled by packing sand.


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Skeleton Pattern


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Pattern Allowances

A pattern is larger in size as compared to the

final casting, because it carries certain

allowances due to metallurgical and

mechanical reasons for example, shrinkageallowance is the result of metallurgical

phenomenon where as machining, draft,

distortion, shake and other allowances areprovided on the patterns because of

mechanical reasons.


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Shrinkage Allowance

Liquid Shrinkage:- reduction in volume when

the metal changes from liquid state to solid

state at the solidus temperature. Riser is used

to accommodate

Solid Shrinkage:- Reduction in volume caused

when metal loses temperature in solid state.

Shrinkage allowance is provided on Pattern.


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The metal shr inkage depends upon:

1. The cast metal or alloy.

2. Pouring temp. of the metal/alloy.

3. Casted dimensions(size).

4. Casting design aspects.

5. Molding conditions(i.e., mould

materials and molding methods

employed)


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Contraction Allowance


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Machining Allowance

Machining Allowance is provided for

i. Castings get oxidized in the mold and duringheat treatment; scales etc., thus formed need to

be removed.ii. It is the intended to remove surface roughness

and other imperfections from the castings.

iii. It is required to achieve exact castingdimensions.

iv. Surface finish is required on the casting.


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Machining allowance depends on

i. Nature of metals.

ii. Size and shape of casting.

iii. The type of machining operations to beemployed for cleaning the casting.

iv. Casting conditions.

v. Molding process employed


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Machining allowance for various

metals


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An illustration of a pattern shown below having

proper draft allowance. Here, the moment the pattern

lifting commences, all of its surfaces are well away

from the sand surface. Thus the pattern can be

removed without damaging the mold cavity.


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Taper amount depends on

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.


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Draft values for patterns


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Distortion or cambered allowance:

A Casting will distort if

It is of irregular shape,

All it parts do not shrink uniformly i.e., some partsshrinks while others are restricted from during so,

It is u or v-shape, The arms possess unequal thickness,

It has long, rangy arms as those of propeller strutfor the ship,

It is a long flat casting, One portion of the casting cools at a faster rate as

compared to the other.


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Shake Allowance

A Pattern is shaked or wrapped to take it out of

mould. This in turn enlarges the mould cavity.

Hence ave allowance is given to pattern.

If the draft angle is provided, shake allowance

reduces.


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Pattern Material Factors affecting selection of material:-

No. of castings to be produced.

Metal to be cast.

Dimensional accuracy & surface finish.

Shape, complexity and size of casting.Casting design parameters.

Type of molding materials.

The chance of repeat orders.

Nature of molding process.

Position of core print.


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Desirable Properties of Pattern

Material

1. Easily worked, shaped and joined.

2. Light in weight.

3. Strong, hard and durable.

4. Resistant to wear and abrasion.

5. Resistant to corrosion, and to chemicalreactions.

6. Dimensionally stable and unaffected byvariations in temperature and humidity.

7. Available at low cost.


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Material for pattern Making

Wood

Metal

Plastic

Plaster

Wax.


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Wood cont.

Limitations:

Susceptible to shrinkage and swelling.

Possess poor wear resistance.

Abraded easily by sand action.

Absorb moisture.

Cannot withstand rough handling.

Life is very short.


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Metal Patterns

These are employed where large no. of castings haveto be produced from same patterns.

Advantages:

Do not absorb moisture

More stronger

Possess much longer life

Do not wrap, retain their shape

Greater resistance to abrasionAccurate and smooth surface finish

Good machinability


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Metal Patterns cont.

Limitations:

Expensive

Require a lot of machining for accuracy

Not easily repaired

Ferrous patterns get rusted

Heavy weight , thus difficult to handle


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Plastic Pattern

Advantages:

Durable

Provides a smooth surface

Moisture resistant

Does not involve any appreciable change in size or shape Light weight

Good strength

Wear and corrosion resistance

Easy to make Abrasion resistance

Good resistance to chemical attack


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Plastic Cont.

Limitations:

Plastic patterns are Fragile

These may not work well when subject toconditions of severe shock


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Pattern Color code

The patterns are normally painted with contrastingcolors

The color code used is,

1. Red or orange on surface not to be finished andleft as cast

2. Yellow on surfaces to be machined

3. Black on core prints for unmachined openings

4. Yellow stripes or black on core prints formachined openings


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Moulding Material

Hot strength:- when sand reaches to high temp.

its strength to retain shape of cavity.

Permeability:-ability of sand to allow gases to

escape from the mould


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Properties of Moulding Material

Refractoriness:-Ability to withstand hightemperature of the molten metal and avoid fusion.

Green Strength:-Sand with moisture is green sandand its property to retain shape of mould is greenstrength.

Dry strength:- Sand without moisture is dry sand.Its ability to retain shape of cavity and withstandmetallostatic force is dry strength.


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Moulding Sand

Silica Sand (SiO2)

Clays are binding agents to provide strength

Moisture


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Types of sand

Facing Sand:-This sand is used next to pattern

to obtain cleaner and smoother casting surfaces

Mould Wash:-Carbonaceous material applied

on the inner cavity after the pattern is

withdrawn. This is done to prevent metal

penetration and prevent sand fusion.


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Core & Core Prints


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Core & core prints

Core is a Body made of refractory material

which is set into prepared mould before

closing and pouring it.

Core should be able to collapse after the metal

is solidify

Chaplets are used to support cores if the core

are very big in size. They are made with samemetal or alloys being casted.


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Type of Cores

Green Sand core:-it is formed by the pattern

itself using moulding sand.

Dry Sand Core:- They are made separately

and positioned in the mould after the pattern

is taken out.


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Backing sand:-Foundry sand which is used for

ramming after facing sand is applied to pattern.

Parting sand:-Sand which is sprinkled on the

parting surfaces of mould before they are

prepared.


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3. Permanent Mold Processesa. Gravity Die casting

b. Low and High Pressure Die Casting

4. Special Casting Processes

a. Lost Wax

b. Ceramics Shell Molding

c. Evaporative Pattern Casting

d. Vacuum Sealed Molding

e. Centrifugal Casting


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Shell Moulding process


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Procedure

Sand and additives are mixed in a muller for1min and then resin is added and heated foranother 3 min.

Only metal patterns can be used as it issubjected to 200-350 C.

Coated sand is allowed to be in contact withmetallic pattern and is heated.

Pattern is removed and clamped together andready for pouring.


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Cont.

Advantages:

a. Dimensionally accurate as well as tolerance

of 0.030.13mm can be achieved.

b. Smoother surface can be achieved.

c. Lower draft angle required.

d. Thin sections can also be made.e. Small amount of sand is used.


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Limitations:

a. Patterns are expensive, so only used for large

scale production

b. Size of casting is limited

c. Highly complicated shapes can not be

prepared

d. Sophisticated equipments are required for

handling


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Cont.

When sufficient thickness is attained, the

mould is reverted and is heated to take wax out

of it (Removal of pattern).

Now, the pattern is preheated and molten metal

is poured in it


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Advantages

a. Since the pattern is withdrawn by melting , so

shapes which are difficult to produce by any

other method are possible to fabricate.

b. Very fine details can be incorporated.

c. Dimensional accuracy and good surface

finish can be achieved.

d. Casting is ready for use as very little or no

machining is required.


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Cont.

Controlled mechanical properties can be

achieved.

Limitations

a. Size is limited.

b. Expensive due to larger manual labourinvolved.

Di C ti


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Die Casting

Die casting involves injecting molten metal at

high pressure into a metallic die


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Cold Chamber die casting:-

Furnace is not integralwith casting


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Die Casting


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Casting defects

Gas defects

Shrinkage cavities

Molding material defects

Pouring metal defects

Metallurgical defects


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Gas Defects

These defects are mainly caused by lower gas

passing tendency of the mould.

Blow holes and open blows:-

Spherical, flattened or

elongated cavities caused by moisture left in

the core.

On the surface they are called open blows and

inside they are called blow holes


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Shrinkage cavity

These are caused by liquid shrinkage occurring

during the solidification of the casting. To

compensate for this, proper feeding of liquid

metal is required.


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Moulding material defects

These defects are caused by characteristics of

molding material

Cuts & Washes:-

Appears as rough spots and area of excess metal

due to erosion of moulding sand by the

flowing molten metal.

Caused by poor strength of moulding sand or

molten metal flowing at high velocity.


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R t


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Runout:-

A runout is caused when the molten metal leaks

out of the mould. Caused by faulty mouldmaking or faulty flask.

Rat tails and buckles:-

caused by compression failure of the skin ofmould cavity. Due to excessive heat, sandexpands resulting in a small line on castingsurface. Buckles are rat tails that are severe.

Reasons may be poor expansion properties ofsand or high pouring temperature.

Swell:


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Swell:-

Under the metallostatic force mould wall my

move back causing a swell in the dimension.This may result in increased feedingrequirement.

Drop:-

The dropping of loose moulding sand or lumpsfrom the cope surface into the mould cavity.

Caused by improper ramming of the copeflask.



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Pouring metal defect

Misrun & Coldshut:-

Misrun is caused when the metal is unable to fill

the mould cavity completely.

Coldshut is caused when two metal streams

while meeting in the mould cavity do not fuse

together properly.

These are caused due to lower fluidity of molten

metal or section thickness is too small.


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Metallurgical defects

Hot tears:-

Since metal has low strength at higher

temperature, any unwanted cooling stress may

cause rupture of the casting.

Hot spots:-

These are caused by chilling of the casting.


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CASTING SYSTEM DESIGN

Aim of an effective gating system are as follows:-

Mould should be completely filled in smallestpossible time.

Metal should flow smoothly in cavity withoutturbulence.

Unwanted material should not be allow toenter mould cavity.

Entry of metal should be properly controlledso that atmospheric air can be released.


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Cont..

A proper thermal gradient should exist in

casting so that it can be cooled without any

shrinkage cavity or distortion.

Metal flow should not cause gating or moulderosion.

Adequate supply of molten metal is ensured.

Gating design should be economical, simple

and easy to remove after solidification.


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Pouring Basin Main function of PB is to reduce the

momentum of flowing liquid.

Molten metal if poured directly in cavity cancause erosion. Molten metal is poured in

pouring basin and act as a reservoir.

It also separates dirt and slag which float on

the surface, with the help of a skim core.

Sprue


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Sprue

Sprue is the channel through which molten

metal is brought into parting line.

While moving downward, metal velocity

increases, hence requires smaller cross section

area.

A1V1=A2V2

So a straight sprue will result in air inclusion in

mould. Hence a tapered sprue is always used.

R


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Runner

Located in horizontal plane, runner connects

the sprue to ingates.

For ferrous metal, runner is in the cope and the

in-gate is in drag in order to trap slag anddross.

Runner should always be full in order to trap

slag.

G I


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Gates or In-gates

Opening through which molten metal enters the

mould cavity.

Top gate:-

molten metal enters the cavity from the top.

Advantages:

a. Mould cavity is filled very quickly

b. Favorable temperature gradient exist

Disad antages:


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Disadvantages:-

a. As metal directly falls into the mould through

height, it is likely to cause mould erosion.

b. It may cause turbulence in mould cavity.

c. Not suitable for nonferrous alloys.

Application:-

Suitable for ferrous alloys and simple castingshapes

Bottom gate


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Bottom gate

Metal enters the mould cavity from bottom.Advantages:-

a. It would not cause mould erosion

b. Very useful for deep moulds.

Limitations:

a. Cause unfavorable temperature gradient.

b. Side risers are to be used with these.

P ti t


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Parting gate

Most widely used gates, metal enters the cavity

at the parting plane.

For cope it is bottom gate and for drag it is top

gate.

Advantages:-

a. Easiest and most economical to prepare.

b. Incorporate advantages of both top and

bottom gate.


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Limitations:

Not useful if drag part is too deep.

St t


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Step gate

Molten metal enters the cavity through number

of vertical gates. Such gates are used for heavy

and large casting.

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