U2 p2 moulds & moulding materials

Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid.
ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Casting Processes
Moulding & Moulding Materials

MOLDING MATERIALS
Choice of molding materials is based on their processing properties.
 Sand: Green sand (mixture of sand, clay, water, and some organic
additives) is the most commonly used.
 Metals and alloys: Are used in multiple-use mold casting processes.
 Plaster: Plaster of Paris (or calcium sulfate or gypsum), with various
additions to improve green strength, dry strength, permeability etc.
 Ceramic: Ceramic molds are similar to plaster molds, except that
the mold can withstand the higher melting point metals.
 Graphite: Powdered graphite can be combined with cement, starch,
and water and compacted around a pattern.
 Rubber: Several types of artificial elastomers can be compounded in
liquid form and poured over a pattern to form a Semi-rigid mold.
Rubber molds are flexible and used for intricate pattern or
shapes small castings of low melting point materials.

Properties:
1. Refractoriness
2. Cohesiveness
3. Permeability
4. Collapsibility
5. Plasticity
6. Dry strength
7. Hot strength
8. Adhesiveness
Other requirements:
9. Cheap
10.Easily available,
11.Low thermal expansion
coefficient,
12.Reusable,
13.Chemically inert and
14.Non sticking to the
casting surface.
MOLDING SAND
PROPERTIES

 Refractoriness is the ability of moulding sand to withstand the
high temperatures of melt so that it does not fuse with melt.
 Presence of sand imparts this property to the moulding sand.
 Higher the pouring temperature higher will be the required
refractoriness and vice versa.
 Presence of impurities retards refractoriness.
 Refractoriness also increases with increase in size of sand.
Coarser the size higher will be the refractoriness.
 This property of sand is measured by the sinter point rather
than melting point of sand.
REFRACTORINESS

 Permeability is the ability to permit gases to escape through it.
 Permeability is the property of the moulding sand due to which
it allows the gases inside the mold (during solidification of
casting) to escape to the surroundings.
 Permeability is a function of the size of the sand particles,
amount and type of clay or bonding agent, moisture content,
compacting pressure.
 The mould must be enough porous to permit the gases to
escape and avoid defects due to entrapped gases.
 The rate in millimeter per minute at which air will pass through
the sand under a standard condition of pressure is used as
index of permeability.
PERMEABILITY

Finer the grain
low is the permeability
Addition of water
increases permeability upto a limit
Fig (a) Effect of grain size permeability
Fig (b) Water content on permeability

 Collapsibility is the ability of disintegration of the cohesive
mould as a result of metal shrinkage.
 Collapsibility is generally obtained by adding organic
material, such as cellulose, cereals etc., that burn out when
these are exposed to hot metal. The combustion reduces both
the volume and strength of the restraining sand.
 Thus collapsibility allows sand mold to collapses automatically
after solidification of the casting as well as free contraction of
the cast material.
 The lack of collapsibility, hinders contraction of the cast
material resulting in tears and cracks in the casting.
COLLAPSIBILITY

 Plasticity is also known as Flowability.
 Plasticity is the property of moulding sand due to which it
flow around and over the pattern to acquire the desired
shape during molding.
 Moulding sand should be plastic so that it flows and acquire
the desired shape under the pressure.
 Flowability of moulding sand increases as clay and water
content increase.
PLASTICITY

 Cohesiveness is defined as the ability to retain a given shape.
 Thus due to cohesiveness, rammed moulding sand particles
are bonded together once the pattern is withdrawn from mould.
 Cohesiveness is obtained by coating the sand grains with
clays that become cohesive when moistened.
 Cohesiveness of sand is ascertained by amount of bonding
materials present such as clay in presence of moisture.
 It may be Green strength, dry strength, hot strength.
COHESIVENESS

 Green means containing water while strength refers to the
compressive strength of moulding sand.
 Green strength is the property of the green sand to retain
the shape of the mould.
 Thus green strength refers to the compressive strength of
moulding sand containing moisture.
 Adequate green strength and toughness is must for
making and handling the mould of moist sand.
GREEN STRENGTH

 When the moisture in the molding sand is completely
expelled, it is called dry sand.
 Dry strength is the ability of the dry sand to retain the
shape of mould cavity as well as to withstand the
metallostatic forces.
 Lack of dry strength may enlarge the mould cavity owing
to erosion and wall movement due to metallostatic
pressure of molten metal.
DRY STRENGTH

 Hot strength is the ability of the sand to hold the shape of
mould cavity at high temperature of sand due to the
presence of molten metal in the mould.
 The pouring of melt expels all the moisture present in the
mould and heat of melt increases the temperature of
moulding sand to high values.
 At this stage deformation of sand grains occur which
may change the shape of mould cavity.

HOT STRENGTH

 Adhesiveness is the property of the molding sand by which
it is capable to adhere to the surface of the molding flask.
 Adhesiveness renders sand particles capable of sticking to
the surface of moulding box of flask.
 Because of Adhesiveness moulding sand mass is held in
the moulding box and can be manipulated as desired
without any chance of its falling out.

ADHESIVENESS

Material used for making green sand moulds consists following:
1. Sand (70-85%): to provide refractoriness
2. Clay (10-20%): to act as binder, along with water, impart tensile
and shear strength to the molding sand
3. Water (3-6%): to activate the clay and provide plasticity
4. Organic additives (1-6%): to enhance desired sand properties
• Moulding sand composition must be carefully controlled to assure
Satisfactory and consistent results.
• Exact composition may vary slightly depending on whether casting is
Ferrous or non-ferrous.
• Good molding sand always represents a compromise between
conflicting factors such as:
Size of sand particles, Amount of bonding agent
(such as clay), Moisture content, Organic matter
COMPOSITION OF MOULDING SAND

CONSITUTENT OF MOULDING SAND
CLAY
Clay is generally used as binding agent in the molding sand to provide
the strength, because of its low cost and wider utility.
The most popular types of clay used are:
1. Kaolinite or fire clay (melting point: range of 1750 to 1787°c )
2. Bentonite (melting point: range of 1250 to 1300 0c), two types
3. Sodium bentonite or western bentonite
4. Calcium bentonite or southern bentonite
 Bentonite can absorb more water which increases its bonding power.
 Sodium bentonites produce better swelling properties (volume
increases some 10 to 20 times), high dry strength which lowers the
risk of erosion, better tolerance of variations in water content, low
green strength and high resistance to burnout which reduces clay
consumption.
 In contrast, calcium bentonites have low dry strength but higher
green strength.

• Water activates clay so that it
develops the necessary plasticity and
strength.
• Amount of water used should be
properly controlled.
• Water in molding sand is often referred
as “tempering” water.
• Water in excess -------- free water
• A part of the water absorbed by clay
helps in bonding while the remainder
up to a limit helps in improving the
plasticity.
• Excessive water decreases the
strength and formability.
• Normal percentages of water used are
from 2 to 8%.
Water
Fig. Effect of Water content on (a)
sand properties (b) green strength

• Additives are added to sand to enhance the specific properties.
• Since molding material is often reclaimed and recycled, the
temperature of the mold during pouring and solidification is also
important.
• If organic materials are added to provide collapsibility, a portion will
bum during the pour. Some of the mold material may have to be
discarded and replaced with new one.
ADDITIVES

ADDITIVES
Cereals:
- Finely ground corn flour or ground starch from corn.
- 0.25 to 2.00 percent
- Increase green or dry strength and collapsibility.
Ground Pitch:
- By-product of coke making.
- up to 2.0 percent
- Improve hot strength and casting finish on ferrous castings
Gilsonite:
- About 0.4 to 0.8 percent.
- A mineral
- Improve casting finish

ADDITIVES
Sea Coal:
- 2 to 8 percent.
- A finally ground soft coal.
- Grey and malleable iron molding sands.
- Improve the surface finish.
- Improve ease of cleaning the castings.
Wood Flour:
- 0.5 to 2.0 percent
- Enhance thermal stability.
- Control the expansion of sand by burning out at
elevated temperature
Fuel Oil:
A little fuel oil is sometimes used as a replacement of
water, which lowers total % of moisture present.

ADDITIVES
Silica Flour:
• Pulverized silica, finer than
200 mesh, is called silica
flour.
• Up to 35 percent
• Increase hot strength
Iron Oxide:
• 0.25 to 1.0 percent
• To obtain added hot
strength
Perlite:
• An aluminum silicate mineral
• 0.5 to 1.5 percent
• Better thermal stability of the
sand
• Riser insulator
Molasses, Dextrin:
• Cane or blackstrap molasses,
unrefined, and containing 60
to 70 % sugar solids,
• Used for increased dry
strength.
• Dextrins may also be used for
the same purpose

• It is also important for each sand grain to be coated uniformly
with the additive agents.
• This is achieved by putting the ingredients through a muller, a
device that kneads, rolls, and stirs the sand.
• After mixing, the sand is often discharged through an aerator,
which fluffs it so that it does not pack too hard during handling.
ADDITIVES
Figure (a) shows a continuous and (b) batch-type muller, which
use blades and wheels to produce the mixing

SAND
 Sand contains 50 to 95 % of the total material of moulding
sand.
 Generally the purest silica sand, (99.8 +%SiO2) is
considered the most refractory and thermally stable.
 Excessive amounts of iron oxide, alkali oxides and lime can
reduce the fusion point in some sands.
 The shape of sand grains may be rounded, angular, or sub-
angular depending on their geologic history.
 Compounded grains are agglomerated particles of angular
or sub-angular sands.

These sand particles may differ
in the following ways:
i) Average grain size, grain
size distribution and grain
shape.
ii) Chemical composition.
iii) Refractoriness and
thermal stability.
• Mixed origin
new molding sand
addition of new silica sand
disintegrated cores
reclaimed sand

Three main classification of sand are:
1.Natural sand
2.Synthetic sand
3.Special sand.
CLASSIFICATION OF MOULDING SAND

NATURAL SAND (GREEN SAND)
 Natural Sand or Green sand are the sands taken from river
beds and are dug from pits and purely natural.
 They possess an appreciable amount of clay (acts as a binder)
and moisture.
 Natural moulding sands are also obtained by crushing and
milling soft yellow sand stones.
 During milling operation, clay aggregates breaks down and are
uniformly distributed over the sand grains.
 Due to their low cost and easy availability, these are used
for most of the ferrous and non-ferrous castings.

SYNTHETIC SAND
 Synthetic Sand is an artificial sand obtained by mixing
relatively clay free sand, binder and other materials as
required.
 It is a better moulding sand as its properties can be
easily controlled by varying the mixture content.

APPLICATIONS OF NATURAL AND
SYNTHETIC MOULDING SAND

SPECIAL SAND
 Special Sand contains the mixtures of inorganic
compounds.
 Cost of these sands are more but they offer high
temperature stability, better cast surfaces etc.
 Special sands used are zircon, olivine, chamotte,
chromite etc.

Basically there are FOUR types of sands:
1. Silica sand
2. Zircon sand
3. Olivine sand
4. Chromite sand
TYPES of SANDS

 In Silica sand silica grains (SiO2)forms the majority of the
molding sand (up to 96%), rest being the other oxides such
as alumina, sodium and magnesium oxide.
 These oxides (impurities) should be minimized to about
2% since they affect the fusion point of the silica sands.
 Shape of the grains may be round, sub-angular, angular and
very angular.
 Main source is the river sand which is used with or without
washing.

TYPES OF SANDS
SILICA SAND

Zircon sand or zirconium silicate (ZrSiO4): typical composition is
ZrO2-66.25%; SiO2-30.96%; Al2O3 -1.92%; Fe2O3-0.74% & other oxides
• It is very expensive.
• It has a fusion point of about 2400°c.
• Low coefficient of thermal expansion, high thermal
conductivity, high chilling power, and high density.
• It requires a very small amount of binder (about 3%).
• It is generally used to manufacture precision steel castings
requiring better surface finish.
TYPES OF SANDS
ZIRCON SAND

Olivine sand
• It contains minerals Fosterite (Mg2Si04) and Fayalite
(Fe2si04).
• It is a very versatile sand and the same mixture can be
used For a range of steels.
TYPES OF SANDS
OLIVINE SAND

Chromite sand
• It is crushed from the chrome ore whose typical
composition is
Cr2O3-44%; Fe203-28%; SiO2-2.5%; CaO-0.5%; and Al2O3 + MgO - 25%
• Fusion point is about 1800 °c.
• It requires a very small amount of binder (about 3%).
• It is used to manufacture heavy steel castings requiring
• Better surface finish.
TYPES OF SANDS
CHROMITE SAND

COMPARISON OF DIFFERENT MOULDING SANDS

SOME TYPES OF SAND
The moulding sands according to their use, are further classified as below
1. Green Sand
 The sand in its natural or moist state is called green
sand.
 It is a mixture of silica sand with 18 to 30 percent clay,
having total amount of water 6 to 8 percent.
 The molten metal is poured in the green sand moulds
without any prior baking (Heating).
 it is used for simple, small and medium size castings.

2.Dry Sand :
 The green sand moulds when baked or dried before
pouring the molten metal are called dry sand mould.
 The sand in this condition is called dry sand.
 Dry sand has more strength, rigidity and thermal stability
as compared to green sand,
 These moulds are used for large and heavy castings.

3. Loam Sand :
 Loam sand contains much more clay as compared to
ordinary moulding sand.
 The clay content is of the order of 50%.
 Sweep or skeleton patterns may be used for loam
moulding.
 It is used for loam moulding of large grey iron castings.

4. Facing Sand :
 This sand is used directly next to the surface of the
pattern and comes in contact with the molten metal
when the mould is poured.
 It is fresh sand i.e., without the addition of used sand.
 It must possess high strength and refractoriness.
 The layer of facing sand in a mould usually ranges from
20 to 30 mm.

5. Backing Sand :
 It is the sand which backs up the facing sand.
 It is the floor sand which is repeated used.
 Backing sand has black colour due to the addition of
coal dust and burning on coming in contact with molten
metal.
 Before use, the backing sand should, however, be
cleaned off the foreign matter like fin nails etc.

6. System Sand:
 System sand is one which is used in a mechanical sand
preparation and handling system(mechanized foundries).
 In mechanized foundries, no facing sand is used, rather the
complete otherwise used sand is cleaned and reactivated
by the addition of waters, binders and special additives.
 Since the whole mould is made of this system sand, the
strength, permeability and refractoriness of this sand
must be higher than those of backing sand.

7. Parting Sand
 This sand is clay free sand and consists of dried silica sand, sea
sand or burnt sand.
 Parting sand prevent green sand from sticking to the pattern and
also to allow the sand on the parting surface of the cope and drag
to separate without clinging.
8. Core Sand
 The sand which is used for the preparation of the cores is
called core sand. It is also called oil sand.
 It is the silica sand mixed with linseed oil or any other oil as
binder.

VARIABLES AFFECTING MOLDING SAND PROPERTIES
Following are the main variable:
1. Sand grain shape and size
2. Clay and water
3. Method of preparing sand mold

SAND GRAIN SHAPE AND SIZE
 Shape and size of the sand grains greatly affects the various
molding sand properties.
 Sand grain size could be coarse or fine (higher the grain
fineness number (GFN) lower the grain size).
 Coarse grains have more void space between the grains
consequently provide good permeability and better
resistance to high temperature melting and expansion,
 Finer grains have lower permeability, however they provide
better surface finish to the casting produced.
 Higher the grain size, lower would be the refractoriness.

1. SAND GRAIN SHAPE AND SIZE
 Purity of sand grains improves the refractoriness.
 Distribution of the grain size also plays an important role.
 Uniform-size grains give good permeability, while a
wide distribution of sizes enhances surface finish.
 Grain shape could be compounded, angular, sub-
angular and round.
 Round grains give good permeability and minimize the
amount of clay required because of their lower surface
area.
 Angular grains give better green strength because of
the mechanical interlocking of the grains.

EFFECT OF MOISTURE, GRAIN SIZE
AND SHAPE ON MOULD QUALITY

2.Clay and Water
 An optimum amount of water is to be used for a given
clay content to obtain maximum green compression
strength.
 During the sand preparation clay is uniformly coated
around the sand grains.
 Water then reacts with the clay and forms a linkage of
silica-water-clay-water-silica (or clay) throughout the
moulding sand.
 Any additional amount of water increases the plasticity
and dry strength but reduces the green compression
strength.

EFFECT OF CLAY AND MOISTURE CONTENT
ON GREEN COMPRESSIVE STRENGTH

3.Method of Preparing Sand Mold
 Degree of ramming increases the bulk density or
mold hardness of the sand and is related to the other
properties.
 Increased ramming increases the strength.
 Permeability of green sand decreases with degree of
ramming.

TYPES OF SAND GRAINS
Rounded Grains:
 Have least contact with one another in rammed structure
 Sand is highly permeable to gases
 Low strength
 Can not pack up to optimum extent
 Requires minimum amount of binder
Sub angular Grains:
 Have comparatively lower permeability and greater strength than
rounded grains.
Angular grains
 Have defined edges and flat surfaces
 Higher strength and low permeability than Sub angular Grains
 Requires more amount of binder
Compound grains
 These grains are cemented together such that they fail to separate when
screened.
 These may consists rounded, angular, sub angular or a combination of three

Moulds
• Mould or Mould cavity contains molten metal and is essentially
a negative of the final product.
• Mould is obtained by pattern in moulding material (sand).
• Mould material should posses refractory characteristics and
withstand the pouring temperature.

Characteristics
1. Should have the desired shape and size.
2. Must be produced with due allowances for shrinkage of the
solidifying material.
3. Any geometrical feature desired in the finished casting must exist
in the cavity. Consequently, the mold material must be able to
reproduce the desired detail.
4. should have a refractory character so that it will not contaminate
the molten material.
5. The mold must be made from a material that can withstand
repeated use.

Types of Moulds
Basically moulds are two types:
1. Expendable moulds-
– are made of sand and is used for single casting which
break upon solidification.
2. Permanent moulds-
– are made of metal or graphite (costly) and used repeatedly
for large number of castings which do not break upon
solidification.
Fig. Expendable moulds

Continued…..
Open mould closed mould

Moulding
• Moulding is the process of making sound mould of sand
by means of pattern.
Types of moulding:
1. Hand moulding-
are used for odd castings generally less than 50 no.
and ramming is done by hands which takes more time.
2. Machine moulding-
are used for simple castings to be produced in large
numbers. Ramming is done by machine so require
less time.

3. Bench moulding
moulding is done on a bench of convenient height to the
moulder and is used for small castings.
4. Floor moulding
moulding is done on the foundry floor and is used for
all medium and large castings.
5. Pit moulding
moulding is done in a pit which act as drag and is used
for very large castings
Continued…..

Machine Moulding
• Moulds are produced by machines for mass production
of castings.
• A moulding machine performs following functions:
– Filling of sand
– Ramming of sand
– Lifting of pattern from mould
– Rolling mould section
Following are the main moulding machine:
 Jolt M/c
 Squeezing m/c
 Sand slinger
 Diaphragm moulding m/c
 Stripper plate m/c
M/c moulding requires mounted patterns and is faster and
more uniform than bench moulding

Expandable-Pattern Casting Process
Figure 11.11 Schematic illustration of the expandable-pattern casting process, also
known as lost-foam or evaporative casting.

Methods of Moulding
SAND MOULDING
 Hand ramming is the preferred method of mold making when only a
few castings are to be made from any given design, and some small
foundries still make their molds by hand moulding.
 In most cases sand molds are made by specially designed molding
machines.
Various moulding methods differ in following aspects:
 Type of flask required,
 Method of packing the sand within the flask,
 Whether mechanical assistance is provided to turn or handle
the mold
 In all cases the moulding machines greatly reduce the labor and Skill
required, and give to castings with better dimensional accuracy and
consistency molding usually begins with a pattern and a flask.
 Sand is generally packed in the flask by following basic techniques.

MACHINE MOULDING
JOLTING:
 Sand is placed on top of the pattern.
 Pattern, flask, and sand are then lifted and dropped
several times.
 Kinetic energy of the sand produces optimum packing
around the pattern.
 Jolting machines can be used on the first half of a
match-plate pattern or on both halves of a cope-and-
drag operation.
SQUEEZING:
 Uses either an Air-operated squeeze head, or a Flexible
diaphragm to compact the sand.
 Squeezing provides firm packing near the squeeze
head, but the density diminishes farther into the mould.
 High-pressure machines with a flexible diaphragm
(commonly called Taccone machines) can produce a
more uniform density around all parts of an irregular
pattern.

 Combination of jolting and squeezing: is often used to produce a more uniform
density throughout the mold.
 A match-plate pattern is positioned between the cope and drag sections of a flask,
and the assembly is placed upside down on the molding machine.
 A parting compound is sprinkled on the pattern, and the top section of the flask is
filled with sand.
 Entire assembly is then jolted a specified number of times to pack the sand
around the pattern.
 A squeeze head is then swung into place, and pressure is applied to complete the
upper portion of the mold.
 Flask can be inverted and operations repeated on the cope half, or the cope and
drag can be made on separate machines using cope-and-drag patterns.
 Except for small molds, molding machines usually provide mechanical assistance
for inverting the heavy molds.
 Gating system and runners can be hand cut or can be made part of pattern.
 After completing the mold, tapered flask may be removed to prevent possible
damage to the flask during the pour.
 A slip jacket, an inexpensive metal band, may be positioned around the mold to
hold the sand in place.
 Heavy metal weights are often placed on top of the molds to prevent the sections
from separating as the hydrostatic pressure of the melt presses the cope upward.

TYPES OF MOULDS
GREEN-SAND, DRY-SAND, AND SKIN-DRIED MOLDS
 Problems of green-sand moulds can be reduced by heating the mould to a
temperature of 300°f or higher, and baking until most if moisture is driven off.
 This strengthens the mould and reduces the amount of gases generated
when the hot melt enters the cavity.
 These dry-sand molds are not very popular because of the long times required
for drying, the added cost of that operation, and the availability of practical
alternatives.
 An attractive compromise is to produce a skin-dried mold, drying only the sand
that is adjacent to the mold cavity. Torches are often used to perform the
drying, and the water is usually removed to a depth of about one-half inch.
 Molds used for the casting of steel are almost always skin-dried, because the
pouring temperatures are significantly higher than those for cast iron.
 These molds may also be given a high-silica wash prior to drying to increase
the refractoriness of the surface, or the more stable zircon sand can be used
as a facing sand.
 Additional binders, such as molasses, linseed oil, or com flour, may be
added to the facing sand to provide additional strength to the skin-dried
segment.

Sequence of Operations in Making a Ceramic Mold
Figure 11.10 Sequence of operations in making a
ceramic mold. Source: Metals Handbook, Vol. 5, 8th ed.

©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
CORES
Full-scale model of interior surfaces of part
• It is inserted into the mold cavity prior to pouring
• The molten metal flows and solidifies between the mold cavity
and the core to form the casting's external and internal surfaces
• May require supports to hold it in position in the mold cavity
during pouring, called chaplets
Figure 11.4 (a) Core held in place in the mold cavity by chaplets, (b)
possible chaplet design, (c) casting with internal cavity.

CORE PARTS
A core consists of two portions:
a) The body of the core and
b) one or more extensions called prints
 The body of the core is surrounded by molten metal
during casting process.
 Body of core has all the features which are required in final
internal surface (e.g. hole) of the castings.
 The prints are necessary to support the core in the
mould.
 They also conduct the heat (and gases produced by a
sand core) to the mould.

CORE, CORE PRINT & CORE BOX
 CORE: a sand shape that is
inserted into the mold to
produce the internal features of
a casting, such as holes or
passages for water cooling
 CORE PRINT: region added to the
pattern, core, or mold which is
used to locate and support the
core within mold
 CORE BOX: the mold or die
used to produce casting cores

ESSENTIAL CHARACTERISTICS OF CORE (SAND)
A good core must possess followings:
 High permeability to allow an easy escape to gases formed.
 High refractoriness to withstand high temperature of molten
metal
 Smooth surface.
 High collapsibility i.e. it should be able to disintegrate quickly
after the solidification of the metal is complete.
 Sufficient strength to support itself.

FUNCTIONS (PURPOSES) OF CORES
Cores are required for following :
 The cores are used to form the internal cavities.
 Cores are used to form a part of a green sand mould.
 Cores are used to strengthen the moulds.
 Cores are used as a part of the gating system.

DESIRED CHARACTERISTICS FOR A CORE
 Cores are the materials used for making cavities and internal features
which cannot be produced by the pattern alone.
 Cores are generally made of the sand and are even used in
permanent molds.
 In general, cores are surrounded on all sides by the melt and therefore
subjected to much more severe thermal and mechanical conditions
core sand should be of higher strength than the molding sand.
Following are the desired characteristics for a core
1. Green strength: a core made of green sand should be strong
enough to retain the shape till it goes for baking.
2. Dry strength: core should have adequate dry strength so that
when the core is placed in the mold, it is able to resist the cast material
pressure acting on it.
3. Permeability: the gases evolving from the melt and from the mold
may have to go through the core to escape out of the mold. Hence
cores are required to have adequate permeability.

4. Refractoriness: in most of the cases, core is surrounded all around by the
melt, it is desirable that the core material should have higher refractoriness.
5. Collapsibility: as the casting cools, it shrinks, and so the core should have
good collapsibility (ability to decrease in size). Lack of collapsibility may
provide resistance against shrinkage and can cause the casting defect of hot
tears.
6. Smoothness: surface of the core should be smooth so as to provide a
good finish to the casting surfaces in contact with the cores.
7. Friability (ability to crumble): after the casting is completely cooled, the core
should be removed from the casting before it is processed further. Hence the
friability is also an important consideration.
8. Low gas emission: because the cores are subjected to very high
temperature, the evolution of gases from the inside are very high at that
temperature. These gases are otherwise likely to produce gas inclusion
defects. So the cores should be made such that the evolution of gases is
minimum.
DESIRED CHARACTERISTICS FOR A CORE

CORE SANDS
CORE SAND CONSTITUENTS:
Core sand should contain the sand grains, binders and other additives to provide
specific properties.
Sand:
 Silica sand which is completely devoid of clay is generally used for making core
sands.
 Coarse silica (because of its higher refractoriness) is used in steel foundries
 Finer sands are used for cast irons and non-ferrous alloys.
Binders:
 Core sands need to be stronger than the molding sand.
 Clay used as binder in molding sands is not enough and so organic binders are
used.
 Generally used binders are, linseed oil, core oil, resins, dextrin, molasses, etc.
 Core oil is a mixture of linseed, soya, fish and petroleum oils and coal tar.
 These binders are burnt away by the heat of the melt and thus make the core
collapsible during the cooling of the casting.
 Amount of binder required depends to a great extent on the fineness of the
sand grains.
 Amount of clay left in the sand increases the consumption of the binder.

 Organic binders develop strength by means of polymerization and
Cross-linking.
 To effect this, the cores after preparation need to be Baked.
 A proper combination of baking time is to be chosen so as to Optimize
the core properties (as shown in fig).
 General composition of a core sand mixture could be core oil (1%) and
water (2.5 to 6%).
CORE SANDS

CLASSIFICATION OF CORES
• The selection of the correct type of core depends on production
quantity, production rate, required precision, required surface finish, and
the type of metal being used.
Core can be classified as follows:
1. Based on material used for making cores
a) Sand cores b) Metal cores
2. Based on nature of use
a) Dispensable (in sand casting) b) Permanent (in die casting)
3. Based on shapes and positions of the cores in prepared moulds
a) Horizontal core
b) Vertical core
c) Balanced core
d) Hanging or cover core
e) Drop core or stop off core
f) Ram up core
g) Kiss core.

METAL AND SAND CORES
• Metal Cores are used in
permanent mould casting.
• Metal cores should be
parallel to the mould
parting line, or can be
removed before the casting
is removed from the mould,
and shaped so that is readily
freed from the casting.
• Metal cores are typically
made from cast iron or steel.
• Sand cores are made from
materials similar to those
used for chemically bonded
sand moulds.
• These cores are formed in
core boxes - similar to
pattern boxes used to make
moulds.
• Sand core are chemically
bonded sand of complex
shapes, and used in all
mould types.
Based on the material used for making cores are of two types:
Metal cores and sand cores.

TYPES OF CORES
Cores are generally made of sand & are even used in permanent molds.
BASED ON THE TYPE OF SAND USED:
1. Green sand core: these are obtained by the pattern itself during
molding.
 This is used only for those type of cavities which permit the
withdrawal of the pattern.
 Though this is the most economical way of preparing core, the green
sand being low in strength cannot be used for fairly deep holes.
 A large amount of draft is to be provided so that the pattern can be
withdrawn.
2. Dry sand cores: are those which are made by means of special
core sands in a separate core box, baked and then placed in the
mold before pouring.
Green Sand Core

TYPES OF CORES
3. Horizontal core: the most common type.
 Usually in a cylindrical form laid horizontally in the mold.
 Ends of core rest in the seats provided by the core prints on the pattern.
 Horizontal core may be made in one piece using a split core box, or in
two halves using a half core box.
4. Vertical core:
 The core is placed along a vertical axis in the mould.
 The ends of the core at top and the bottom fit into the seats provided in
the cope and drag halves of the mold.
 Both horizontal and vertical cores are used more frequently than
other cores in the foundry work. For this reason they are called stock
cores and are kept ready in various diameters and lengths.
Horizontal core Vertical core

TYPES OF CORES
5. Balanced core:
 Balanced core is suitable when the casting has an opening
only on one side and only one core print is available on the
pattern.
 Core print in such cases should be sufficiently large to support
the weight of the Core, which extends into the mold cavity, and
it should be able to withstand the force of buoyancy of the
melt surrounding it.
 To support the core in mold cavity, chaplets are often inserted.
Balanced core

TYPES OF CORES
6. Cover Core:
 Cover core is used when the entire pattern is rammed
in the drag and the core is required to be suspended
from the top of the mold.
 Unlike the balanced core, which extends horizontally in
the mold cavity, the cover core stretches vertically
downwards.
Cover core

7. Hanging Core:
 If the core hangs from the cope and does not have
any support at the bottom in the drag, it is referred to as
a hanging core.
 In this case, it may be necessary to fasten the core
with a wire or rod, which extends through the cope to
a fastening on the top side of the cope.
TYPES OF CORES
Hanging core

8. Wing core" or stop-off:
 Wing core may be used when a hole or recess is to be obtained
in the casting either above or below the parting line.
 Wing core is necessitated when it is not possible to place the
pattern in the mold such that the recess can be cored directly
or with the other types of cores.
 Since a part of the core placed in the seat becomes a stop-off
and forms a surface of casting, it is also referred as stop-off core.
 It is also known as tail core, chair core, and saddle core
according to its shape and position in the mold
TYPES OF CORES
Wing core

9. Ram-Up Core:
 Sometimes, the core is set with the pattern in the mold before
the mold is rammed. Such a core is called ram-up core it is
favored when the core detail is located in an inaccessible
position.
 It may be used for both interior and exterior portions of a
casting.
TYPES OF CORES
Ram Up core

10. Kiss Cores:
 When the pattern is not provided with core prints and no
seat is available for resting the core, the core is held in
position between the cope and drag simply by the
pressure of the cope.
 Kiss cores are useful when a number of holes are required
in the casting
 Dimensional accuracy with regard to the relative location
of the holes is not important.
TYPES OF CORES
Kiss Core

CORE MAKING
 Cores for sand casting are manufactured by packing
specially prepared sand in Core boxes.
 Core-making processes include sand preparation, core
shooting, coating/treatment and placement in mould.
 The cavity in a core box is a negative replica of the
corresponding part feature.
 The core box is made in two segments (with a parting) to
enable removal of the core.
 Complex cores are prepared by assembling or gluing two
or more cores of simpler shapes.
 The core-related activities consume significant resources.
 Thus the number and volume of cores must be minimized
to the extent possible, to reduce tooling cost and
manufacturing time.

CORE BOXES
 Core boxes are used for making cores. A core box is a
wooden or metallic type of pattern and are made either
single or in two parts.
 They may be classified according to the method of making
the core or shape of core.
The common types of core boxes are described below:
1. Half Core Box
• Half core box is used when a symmetrical core is prepared
in two identical halves which are later on pasted or
cemented together to form a complete core.
Half Core Box

CORE BOXES
2. Split Core Box
 It is made in two parts like a split pattern.
 Both the parts are joined together by means of dowel pins
to form the complete hollow cavity for making the core as
shown in fig.
Split Core Box

CORE BOXES
3. Dump Core Box
 For making the slab or rectangular shape of core,
dump core box is used.
 In construction, it is similar to half core box. The box is
made with side opening.
Dump Core Box

CORE BOXES
4. Loose Piece Core Box
 It is used for the preparation of core with the provisions
of boxes or hubs.
 This is used when the two halves of a core of which the
halves are not identical in shape and size is to be
prepared in the same core-box as shown in fig.
Loose Piece Core Box

CORE BOXES
5. Strickle Type Core Box
 Used for making unsymmetrical or irregular shapes of cores.
 A strickle core box is used when the core is required to have an
irregular shape which cannot be easily rammed by other
method.
 The desired irregular shape is achieved by striking off the core
from the top of the core box with a piece of wood called
strickle board.
 Strickle board is having same contour as that of the core.
Strickle Core Box

CORE PRINTS
 Core prints are provided so that the cores are securely and
correctly positioned in the mold cavity.
 Design of core prints takes care of the weight of the core
before pouring and the upward metallostatic pressure of the
melt after pouring.
 Core prints should also ensure that the core is not shifted
during the entry of the melt into the mold cavity.
 Main force acting on the core when melt is poured into the
mold cavity is due to buoyancy which is the difference in the
weight of the liquid metal to that of the core material of the
same volume as that of the exposed core.

DESIGN OF CORE PRINTS
 Core prints should be able to take care of weight of core before pouring
and the upward metallostatic pressure of the molten metal after pouring.
 The core print should ensure that core is not shifted during the entry of
metal into mould cavity
 The main force acting on the core when metal is poured into mould cavity is
due to buoyancy.
 Buoyant force is the difference in the weight of the liquid metal to that of the
core material of the same volume as that of the exposed core.
Mathematically
For horizontal core P = V(ρ-d)
P = Buoyant force, N
V = Volume of the core in the mould cavity, cm3 (Volume = 0.25 π D2 H)
ρ = Weight density of the liquid metal, N/cm3
d = weight density of core material= 1.65x 10-2 N/cm3
For vertical core, Buoyant force P= [0.25 π (D1
2 - D2 ) H ρ– Vd]
Where V= total volume of the core in the mould
A core should be able to support a load of 35 N/cm2 of surface area to keep core in
position . A core must satisfy following condition A= surface area
If above condition is not satisfied than provide additional support by using chaplets.

The Russian practice of dimensioning the core print is to make the
pressure acting on the core bearing area( i.e. the core print surface
area) to be less than 50- 75 % of the moulding sand compression
strength Hence

CORE PRINT DIMENSIONS
• Core print dimensions are tabulated below with
reference to fig on next slide
Table 1: Core Print Dimensions

Effectofmoisture,specimenweight,
permeabilityandgreenstrengthon
processparameters

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