EFFECT OF NATURAL AND SYNTHETIC MOULDING
SAND ON THE MECHANICAL PROPERTIES (HARDNESS,
TENSILE STRENGHT, COMPRESIVE STRENGHT) OF A
CAST ALUMINIUM BAR.
CHAPTER ONE
INTRODUCTION
1.1 Background of the Study
Foundry is where the elaborate metal casting process takes
place. A metal foundry may specialize in making one particular
item or a variety of products using different alloys. Based on the
production requirement there are small foundries that employ
few people and large industrial foundries that employ thousand
of workers(John Campbell, complete casting handbook second
edition,2015).
Foundries
are
one
of
the
largest
contributors
to
the
manufacturing recycling movement, melting and recasting
millions of tons of scrap metal every year to create new durable
goods. Moreover, many foundries use sand in their molding
process. These foundries often use, recondition and reuse sand
which is which is another form of recycling (John Campbell 2015).
Foundry operations of the present day is an outcome of
thousands of years of experimenting with different metals and
casting methods. Very little documented evidence of casting and
foundry operations are available of the ancient time. The proof
of casting are in the form of jewelries, household items and
religious items that were unearthed during excavations or
preserved as historic relics (John Campbell, 2015).
The foundry process was first documented circa 1500 by
vannoccio brirnguccio who is known as the ‘’father of the
foundry industry’’. Rapid growth in the casting industry took
place in the industrialization age. with the invention of
machines and equipment in the industrial age, casting could be
done on a large scale. The scientific revolution in this century
increased the demand of casting products for automobiles,
machines, weapons etc. in the modern era, metal casting finds
its application from kitchen wares to aircraft components (John
Cambell,2015).
Types of foundries
Depending upon the type of end users, the wide variety of
moulding or the casting process can be basically divided into
commercial foundry and hobby foundry (John Campbell,2015).
Commercial foundry
Here the production activity is on a large scale and in
abundance to the international standards of manufacturing
practice. This type of foundry offers semi-finished or fully
finished products like railroads equipment, machine etc. major
types of casting employed in commercial foundries are sand
casting, die casting, investment casting and lost foam casting.
According to type foundry activity that the foundries are
involved in, commercial foundries can be classified into:
 Captive foundry
 Independent foundry
 Jobbing foundry
 Production foundry
 Semi production shop
Hobby foundries
These are small but fully equipped foundry units set up
inside a house or in the backyard such foundries are set up
by amateurs who loves to create casting products according
to one’s desire. The home foundries also serve other purpose
like mending the worn out mental items and for casting
special parts or components that are not available in the
markets.
1.2 Statement of the problem
Physical examination of product cast using natural molding sand and
the same product using synthetic molding sand are found to have
some physical differences in terms of good surface finish and density.
1.3 Aim and objectives
The aim of this study is to investigate the effect of natural and
synthetic moulding sand on some selected mechanical properties
(hardness,tensile
strength,
compressive
smoothness) of a cast aluminium bar.
strength
and
surface
The specific objectives of this project are;
 To cast an aluminium bar using both synthetic and natural
moulding sand.
 To determine the aforementioned mechanical properties of the
cast aluminium bar using natural and synthetic moulding sand
and comparing them.
1.4 Significant of the study
 Helps towards increasing the body of knowledge in the
implementation of the effect of foundry sand on quality of
aluminium casting.
 Helps to understand the best sand to be used between natural
moulding sand and synthetic moulding sand.
 Helps to condition molding sand to meet desired properties of
cast metal.
1.5 Scope and Delimitation
This project is limited to investigating the effect of synthetic and
natural sand on the aforementioned mechanical properties of cast
aluminium bar.
CHAPTER TWO
2.0 LITERATURE REVIEW
2.1
Aluminium
Aluminium ia soft, lightweight, fireproof and heat resistant metal.
It is a white metal produced by electrical processes from the oxide
(aluminia), which is produced from a clay mineral called bauxide.
Bauxide is hydrated aluminium oxide. The chief impurities are oxide,
silica, clay and titanium oxide. It is found in india in the states of
Bihar and modhya Pradesh (John E. Hatch,Aluminium properties and
physical metallurgy,1984).
Manufacture of aluminium
a) Primary manufacturing: involves 4 processes
I.
Extraction of the ore, bauxite
II.
Purification of bauxites to pure Al oxide
III.
Synthesis of cryolite, Na3AlF6 and Al fluoride to be used
in electrolytic reduction process
IV.
Electrolytic reduction of Al oxide to Al.
b) Secondary production: over 50% of Al used to make new
products comes from scraps of which two third is new scrap, Al
can be recycled at a low cost (using about 5% of the energy
required for primary production).
The bauxite is purified and then dissolved in fused cryolite (double
fluoride of aluminium and sodium). The aluminium is then separated
from this solution by electrolysis at about 910C (US Geological Survey
Commodity Summaries, January 2021).
Properties of aluminium
Pure aluminium has silvery colour and lusture. It is ductile and very
good conductor of heat and electricity. It has a very high resistance
to corrosion than the ordinary steel. Its specific gravity is 2.7g/cm3
and melting point is 658C, its tensile strength varies from 95 to
157MN/m2. In proportion to its weight it is quite strong. In its pure
state the metal would be weak and soft for most purposes but when
mixed with small amount of other alloys, it becomes hard and rigid.
It may be blanked, formed, drawn, turned, cast, forged and die cast.
Its good electrical conductivity is an important property and is
broadly used for overhead cables. It forms useful alloys with iron,
zinc, copper and other metals (John E. Hatch,1984).
Alloys
Aluminium really comes into its own when you combine it with other
metals to make aluminium alloys. (an alloy is a metal mixed together
with other elements to make a new material with improved
properties, it might be stronger or melt at higher temperature). A few
of metals commonly used to make aluminium alloys include boron,
copper, silicon, lithium, magnesium, tin, manganese and zinc.
Alluminium can be mixed with one or more of these elements
mentioned above depending on the job u are trying to do (John
Farndon,2001).
Composites
Aluminium can be combined with other materials in a different way
in composites (hybrid materials made from two or more material that
retain their separate identity without chemically combining, mixing
or dissolving) so for example, aluminium can act as the background
materials (matrix) in what is called a metal matrix composite (MMC)
reinforced with particles of silicon carbide, to make a string, stiff,
light weight material suitable for it wide range of aerospace,
electronic and automobile uses and (crucially) better than aluminium
alone (Chris Woodford,2021).
Mechanical properties of materials:
The mechanical properties of materials define the behavior of
materials under the action of external forces called load, there are a
measure of strength and lasting characteristics of the material in
service and are of good importance in the design of tools, machine
parts and structures. The mechanical properties of metals are
determined by the range of usefulness of the metal and establish the
service that is expected (Saif M. Mechanical Properties of Materials,2022).
The following are some of the selected mechanical properties that
will be limited to this study.
1. HARDNESS.: Hardness is the ability of a material to resist
indentation or surface abrasion. Hard materials resist
scratches or being worn out by friction with another body.
Test on hardness may be classified into:
 Scratch test
 Indentation test
- The scratch test consist of processing a loaded diamond
into the surface of the specimen and then pulling the
diamond so as to make scratch. The hardness number is
now determine on the basis of ;
I.
Load required to make a scratch of a given width or
II.
The width of the scratch made with a given load.
- The indentation test consists of pressing a body of
standard shape into the surface of the specimen in the
commonly used brinell hardness test a hardened steel ball
of a given diameter is squeezed into the surface of test
specimen under a fixed standard load and then surface
area of the indent is measured. The converse of hardness
is known as softness.
2. MALLEABILITY: Malleability is the property of a material
which permits the materials to be extended in all directions
without rapture. This property generally increases with
increase temperature. Gold, silver, copper, aluminium, lead,
zinc etc are common examples of malleable materials. It is
the property of a material which permit it to be hammered or
rolled into sheets of other sizes and shapes.
3. DUCTILITY: Ductility is a property of a material which
enables it to be drawn into a thin wire. Mild steel, copper,
aluminium are good examples of a ductile material.
4. BRITTLENESS: Brittleness implies lack of ductility. A
material is said to be brittle when it cannot be drawn out by
tension to smaller section. Brittle fracture takes place
without warning and the property is generally undesirable.
5. ELASTICITY: When a material has a load applied to it, the
load causes the material to deform.
The elasticity of a material is the power of coming back to
its original shape or position after deformation when the
stress or load is released. Steel, aluminium, copper etc
may be considered perfectly elastic within certain limits.
6. PLASTICITY: Plasticity is the reverse of elasticity. It is the
property that enables the formation of a permanent
deformation in a material. A plastic material will retain
exactly the shape it takes under load even after the load is
removed.
7. STRENGHT: The strength of a metal is its ability to
withstand various forces to which it is subjected during a test
or in service. The strength of a material enables it to resist
the fracture under load.
It is usually defined as:
 Tensile strength
 Compressive strength
 Proof stress
 Shear strength
2.2 Casting
Casting is a process that creates solid metal shapes (casting) by filling
voids in molds with liquid metal and then letting the object cool. It
could also be viewed as a manufacturing process that involves
pouring molten metal into mould to create a 3D metal part. The mold
contains a hollow cavity of a desired and the molten metal is allowed
to cool down to form a solidified part.
Primarily, casting produces ingots and shapes. An ingot is a casting
produced into a simple shape intended for further processing, such
as metal extrusion and forging. Shape castings to produce complex
geometries closer to the final part (Bernier and Adederian A.A, 2018).
Types of metal casting
Metal casting falls into two groups by the fundamental nature of the
mould design that is, permanent mould casting. It can be further
divided into groups depending on its pattern material.
a) Expandable mould
b) Permanent mould
c) Composite mould
a) Expandable mould casting
Expandable mould casting uses a temporary non-reusable mould to
produce the final casting as the foundry breaks the mould to get the
casting out. Sand, clay and plaster are common materials used to
make these moulds. To improve the quality of disposable mould,
binders known as bonding agents are typically used. Expandable
mould casting can be used to cast complex, sophisticated shapes.
Types of expandable mould casting
1. Permanent pattern
2. Sand casting
3. Plaster moulding
4. Ceramic mould
5. Shell mould
6. Expandable pattern
7. Lost foam
8. Investment casting
Sand casting
Sand casting is the most common example of expandable mould
casting which generally uses permanent pattern to create the mould.
Molten metal is poured into an expandable sand mold cavity by
gravity or force which then solidifies to form the cavity shaped part
(Groover Foundation Of Modern Manufacturing, 2010).
Investment casting
In investment casting, a wax pattern is coated with a refractory
material to make the mould which is melted away before pouring
molten metal into the cavity to solidify.
Permanent mould casting
Sometimes called non-expandable mould casting permanent mould
casting uses permanent moulds reused after each production cycle.
Although permanent mould caasting produces repeated parts due to
the reuse of the same mould, it can only produce simple castings as
the mould needs to be opened to remove the castings.
Types of permanent mould casting
1. Die casting
2. Centrifugal casting
3. Gravity casting
4. Low pressure/vacuum
1. Die casting: it is a permanent mould casting method in
which liquid metal is poured into the dies at pressure
ranging from 0.7 to 700mpa, where it solidifies as a metal
cast.
2. Gravity casting: the molten metal is poured into the muold
from a crucible only under gravity in gravity casting.
3. Composite mould casting: As the name implies these uses
both expandable and reuseable casting mould to produce
casting. These typically include materials such as sand,
wood, graphite and metal.
Application and characteristics of metal casting
Nearly every engineering product we use from washing
machines to pillar drills, cars to bicycles are manufactured
using metal parts that are most likely to be made using one
of the metal casting processes. This age-old manufacturing
process has improved its precision and tolerance over time
(Beeley P. Foundry Technology, 2001).
Typically, castings are used to make car engine blocks,
crankshafts and power tool housing such as, pillar drills,
plumbing parts, turbine blades, gearbox housing, metal
statues and gears.
Advantages of metal casting
1. Metal casting can produce complex shapes
2. Significant components can be produced in one piece cast
3. Almost all materials can be cast
4. Low tooling cost
5. Features like internal cavities or hollow sections can be
easily cast
6. Materials that are difficult or expensive to manufacture
using other manufacturing processes can be cast
7. Near net shape often without or minor post-processing
8. Wide material choice
Disadvantages of metal casting
1. Relatively coarse surface finish hence wider tolerance
has to be allowed and is not suitable for mating
interface
2. Metal casting, such as shell moulding, has a limit in
terms of size and pattern
3. Pattern are time consuming and expensive to make
4. Die casting can be very expensive for smaller to
medium quantities due to high die cast
5. Part size and material choices depend on the casting
processes chosen. For instance only nonferrous metal
can be used for permanent mould castings.
Material suitability
Although almost all metals can be used the most common ones are
iron, steel, aluminium, magnesium and copper based alloys such as
bronze.
Zinc, alluminium, magnesium and brass are widely used in die
casting, whereas aluminium alloy, brass alloy, cast iron and cast
steel are popular sand casting materials (Beeley P. 2001).
Factors to be considered before choosing a suitable metal casting for
a given engineering product design;
1. Part shape and size
2. Required quality
3. Required tolerance
4. Material
Foundry sand
Foundry sand consist primarily of clean uniformly sized, high quality
silica sand or lake sand that is bonded to form molds for ferrous (iron
and steel) and nonferrous (copper, aluminium, brass) metal castings.
The general sources of receiving moulding sands are the sea shores,
rivers, lakes, deserts and granular elements of rocks (Bernier and
Adederian A.A ,2018).
Types of moulding sands
Moulding sands can be classified mainly into two types namely:
i.
Natural moulding sand
ii.
Synthetic moulding sand
Natural moulding sands contains sufficient amount of binder
material
Whereas, synthetic moulding sands are prepared artificially using
basic sand moulding constituents
 Silica sand 85-91%
 Binder 6-11%
 Water or moister content 2-3% and other additives in proper
proportion by weight with perfect mixing and mulling in
suitable equipments.
2.2.1
Constituent of a Moulding Sand
Foundry sand consists primary of clean, uniform sized, high
quality silica sand, bentonite, and additives that is bonded to
form moulds for ferrous (iron an steel) and non ferrous (copper,
aluminium, brass) metal casting.
Although these sands are
clean prior to use, after casting they may contain ferrous (iron
and steel) industries account for approximately 95% of foundry
sand used for castings. The automotive industry and its parts
suppliers are the major generators of foundry sand (Wajaga 1982)
The most common casting process used in the foundry industry
is the sand casting system. Virtually all sand cast moulds for
ferrous casting are of the green sand type.
Green sand consists of high quality silica sand, about 10%
bentonite clay (as the binder), 2% to 5% water and about 5%
sea coal (a carbonaceous mould additive to improve casting
finish). The type of metal being cast determines which additives
and what gradation of sand is used. The green sand used in
the process constitutes up-wards of 90% of the moulding
materials used.
In addition to green sand moulds, chemically bonded sand cast
system are also used.
These systems involve the use of one or more organic binders
(usually proprietary) in conjunction with catalyst and different
hardening/setting procedures. Foundry sand makes up about
97% of the mixture. Chemically bounded system are most often
used for “cores” (used to produce cavities that are not practical
to produce by normal moulding operations) and for moulds for
non-ferrous castings.
(Wajaga, 1982) Foundry is one of the oldest of all industries both
ancient and
medieval history offering examples of the
manufacturing and use of castings. Casting in Nigeria is as old
as history. The Benin and Ife bronze casting and the Adazi
Akwa Iron casting readily come to minds through these are
more of art work than engineering products yet they are first
class quality casting products which readily serve our needs in
the past. In Europe by the end of medieval period decorated
bronze and other types of casting were being used in European
churches and domestic life. Whilst Aluminium made a more
somber appearance the shapes of cannon short and grave slabs,
those are also very common in the Eastern Europe most
especially in Kiev the capital city of Ukraine of the former USSR.
In the 10th century, Biringvecro wrote a debate account of metal
founding giving an impression of working condition and the
emotion of the foundry man which would not have been out of
place even in recent time. Moulding boxes and sand were in
use between them.
2.2.2
(i)
Types of Foundry Sand
Olivine Sand:- This is orthosilicate or iron and magnesium, its
density, conductivity and refractoriness are higher than those
of silica sand. Its fusion point is high; about 1800oC hence it is
favoured for heavy sections of alloy steel castings. Its resistance
to slag reaction makes it suitable for the casting of high
manganese steels.
(ii)
Zircon Sand:- This is the sand from zirconium silicate, which
is highly refractory. It is used extensively as a core and mould
wash for heavy and alloy steel castings. It has extremely high
specific gravity of about 4.6 hence flasks are heavy to handle.
They are also inert to chemical, and molten metal action hence
it enhances good surface finish of cast. It also has high heat
conductivity, hence it promotes quick formation of a solidified
metal later, thus casting of fine-grained structure are produced.
Also, the unit cost is high.
(iii)
Chromite and chrome-magnestic sand :- These sands have a
high degree of refractoriness, high density and high chilling
power.
Binder and temper water requirements of chromite
sands are very low. These sands are useful particularly where
the chilling tendency is to be increased to control solidification.
They are also suitable as facing materials in mould for steel
casting.
(iv)
Chamotte Sand :- This is produced by calcimining high-grade
fireclay.
The last four are synthetic sand.
Functions of Foundry Sand
In sand moulding technique, a good quality (sand) casting will
depend on the ability of the moulding material to satisfy the
following requirements:
i.
Stability:- This is the liquid metal may not cause any
deformation of the mould in high cast pieces, by washing out or
swelling. The can be enhanced by good moulding practice.
ii.
Permeability of Gases:- Pores within the sand to allow the
escape of gases. This depends on the type of moulding sand.
iii.
Smooth surface to the cast product: This is achieved by
carefully screening (sieving) the facing sand.
2.2.3
Properties of Moulding Sand
(Dr Dmitri Kopeliovich, 2012) In order for the moulding to perform
the above three factors, the moulding sand is expected to have
a combination of desirable properties depending on the type and
size of cast metal. From the viewpoint of stability (both physical
and thermal) of the mould, the relevant properties are green
strength, dry strength, and refractoriness. Properties required
of a good moulding sand include the following.
i.
Fineness:- Is the measure of the actual sizes of a sand mixture.
The fineness number assigned to sand is approximately the
sieve (screen), which would just pass the sand if its grains were
all the same size.
ii.
Plasticity:- This is the ability of a moulding mixture to deform
in a mould or core box to acquire precisely the shape of the
casting or core pattern and then retain this shape when the
patterns are removed.
The plasticity of moulding sands
depends on the composition and moisture content of the
moulding mixture, method of mixture preparation, and the size
and shape of sand grains. Sand mixture with a large content of
clay have a higher plasticity.
iii.
Collapsibility:- Is the property of moulding sand and the core
mixtures that permit them to be knocked out easily from cooled
castings. Good collapsibility is usually associated with a loss of
strength of the binder on completion of formation of a casting.
iv.
Hyroscopicity:- Is the ability of moulding mixtures to absorb
moisture from the surrounding atmosphere after drying
prepared mould and cores should not be hygroscopic.
Otherwise, especially when they stand for a certain while before
being filled with the molten metal, they will be moistened and
lose the surface strength, which will lead to the appearance of
gas blowholes and burnt – on sand defects in castings.
v.
Refractoriness:- This is the ability of the core sand to
withstand the temperature of the melting without softening or
fusion. The higher the grain size of the quartz sands the higher
the refractoriness. The higher the amount of impurity the lower
the refractoriness.
vi.
Strength: The strength of the core of moulding sand is being
considered or measured in terms of green strength, dry or baked
or cured sand and hot strength. Strength is the ability of the
sand to exhibit non-crumbleness in assembly, transportation,
and pouring. This property depends in the size of the sand
proper, moisture contents and the density.
vii.
Permeability:- The ability of the material should be able to
allow trapped air, water vapour and other gases to pass through
the walls is called permeability.
Permeability is affected by grain size, grain shape and the
moisture content of the sand mixture.
Moulding sand with
grains of identical size had the highest permeability of gases.
Increasing the content of clay in the sand mixture lowers the
permeability to gases.
In terms of grains shape, spherical grains have the best
permeability to gases.
Too much moisture is not good for the moulding sand because
it would increase the amount of water vapour. Also, the amount
of clay content of the sand will determine the optimum required
water content.
In general low day content will have lower
moisture content than sands rich in clay.
Summarily, the grain size, shape and distribution of the sand
grains (fineness), the type and quantity of the bonding
materials, the density with which it is packed, and the
percentage of the tempering water, are important factors in
regulating the degree of permeability of the moulding sand.
The permeability is expressed as the volume of air (in cu cm)
that will pass in unit time through unit area of sand rate of flow
of air through a standard specimen of sand, the pressure (in cm
of water) and the time in seconds being noted.
viii. Porosity:- This is the property of the sand allowing for the
escape of evolved gases.
This property is function of grain
shapes and sizes.
Thus, when liquid metal comes in contract with the wall of a
green sand mould, steam and other gases are generated. If the
mould wall is not porous enough to allow the gases to escape
freely, the pressure in the mould cavity increase until it blows
the metal out of the mould (or, in extreme cases until the gases
explode and destroy the mould).
Adequate porosity is achieved by control of the shape and
volume of the pores that are contained within the mass of
packed sand grains.
ix.
Flowability:- This is the ability of the moulding sand to flow
and uniformly fill the moulding flask. This is related to the
humpiness, which is the ability of the sand to form lump.
2.2.4
Bonding Clays
(Pribyl, 1976) the large family of clays used in bonding moulding
sand are called bonding clays. They are composed of one or
more of the materials kaolinite, available in Nigeria. The clays
were formed from weathering of various types of volcanic ash,
examples of kaolinite deposits in Nigeria are the Ukpor, Oshiele,
Kankara, Giro, Ozubulu etc.
Generally, clay imparts the necessary bonding strength to the
moulding sand so that after ramming the mould does not lose
its shape. However, as the quantity of the clay is increased, the
permeability of the mould is reduced.
Clay is defined as those particles of sand under 20 microns in
diameter, that fail to settle at a rate of 25mm per minute when
suspended in water.
The three types of clay commonly used are kaolinite (fireclay),
illite and bentonite. Bentonite being used most often, because
it possesses higher binding capacity.
When clay is heated, the process in it cause gradual or sudden
change of the binding properties.
The strength of moulding
mixtures containing clay increase on heating to 1000c or slightly
above.
This is due to an increase of cohesion between clay
particles as hygroscopic moisture is removed.
In the
temperature interval 350 – 6500c, clays lose all crystal water,
and hence, binding properties.
Bentonite, the most common type of bounding clays is used
with high silica sands as a green sand additive to increase the
bonding action.
It belongs to the motmorllonite group of
mineral and processes typical characteristic properties, which
impact
exceptionally
favourable
bonding
properties.
enhances strength without requiring drying.
It
Unlike other
bonding materials, bentonite can be recirculated in closed
systems and the bond is generated simply by the addition of
water. It is simple to prepare a sand mix containing bentonite
as no swabbing or dusting is required, patters are easily
stripped and moulds quickly produced. Moreover, bentonite
resists erosion of moulds and its volumetric contraction helps
in compensating the expansion of silica grains.
Bentonies are of two types: sodium bentonite and calcium
bentonite, depending on the kind of substitution metal present,
Sodium bentonite products high swelling properties, high dry
strength, very high hot strength, high liquid limit, low plasticity,
and low green strength.
Calcium bentonite produces low swelling properties, low, dry
and hot strength. Low liquid limit, high plasticity, and high
green strength.
2.3 Determination of Clay Content in Sand
The clay content in sand can be calculated by means of an
apparatus called the clay content tester. The sample under test
is first dried and cooled and then a particular amount of it say
100 grams, is taken into a receptacle or jar. Finally, 47c.c of
water and 25c.c of standard solution of Na014 are added to the
sample before the jar is securely covered and sealed.
The
apparatus has a provision for tightly holding one or two more
such jars in a frame and rotating the whole frame at about 60
rpm so that the solution gets shaking vigorously.
After about an hour’s rotation, the jar is removed and unsealed,
and the sand adhering to the cover and sides is washed into the
container. The jar is then filled with water to a predetermined
mark, usually 150mm above the bottom, the contents allowed
to settle for 10 minutes, and water siphoned off to the depth of
125mm. Water is again added up to the original mark and is
once again siphoned of after allowing the sand to settle 10
minutes. The process is then repeated a few more time allowing
only 5 minutes period for setting through 12mm. The material
that fails to settle down 125mm in 5 minutes i.e. 15mm per
minute is separated and removed from the container.
This
material is nothing but clay. The remaining sand in the jar is
filtered carefully, dried for half an hour and weighted.
The
difference between this weighted and the original weight gives
the weight of the day.
Binders
Moulding and core mixtures with days used as the binder have
a number of drawbacks. In order to obtain a high strength of a
mixture, one has to add large quantities of clay and water,
which worsens substantially the gas permeability of the mixture
and increases the probability of burnt on sand defects on
casting.
Such mixtures usually have poor yield ability and
collapsibility.
For improving the properties of moulding mixtures, clays are
often replaced by special binders. These are called on to ensure
the following properties of moulding and core mixtures:
a)
High total and surface strength after drying.
b)
High flow ability, good gas permeability, and low gas evolution
capacity.
c)
High yield ability and collapsibility
d)
Avoiding of sticking to patterns and core boxes.
These requirements are satisfied on addition of relatively small
quantities of binders (0.5 – 6%)
(Jain, 1990) the primary purpose of binders is to influence the
bonding properties of sand.
Dextrin, starch, molasses, and
sulphite Iye are common organic types of green binders.
Dextrin
increases
air
setting
strength,
toughness,
collapsibility and prevents sand from drying rapid.
and
During
pouring, it gasifies, producing extra space between trains and
allowing their expansion without distortion.
Starch is
sometimes used for better expansion and scabbing resistance
properties.
Molasses is a dark, brown, viscous liquid obtained as a by
produce during sugar refining. It enhances the bench life of
sands and imparts high dry strength and collapsibility to
moulds and cores. Due to its high viscosity and wet ability, it
increases green strength sulphite Iye is a by-product of the
cellulose (pulp) industry. It is also used for imparting better dry
strength as well as hot strength and collapsibility of moulds.
Another variety of organic binder is the oil binder. Linsed oil is
the most popular binder for cores. The necessary strength is
developed when the core prepared from sand and linsed oil is
heated to 2000 – 2400c. the cores go made have good
collapsibility and can be stored for a long time.
The most important in organic binder used in a number of
processed for moulding and core making these clays sodium
silicate. (O2 process, and cement moulding make use of this
materials as binder).
All the above classified as binder belong to the additives family.
Other types of additives are:
1.
Fibrous materials:- These are used to prevent scabbing and
improve collapsibility and the resistance to expansion defects.
The commonly used materials are wood flour, peat, straw, chaff
(dried grass), horsehair or cowhair, sawdust, shoddy, manure,
and asbestos.
2.
Reducing Agents:- These are used mainly for improving
surface finish of the castings. Examples are coal dust, pitch,
and fuel oil.
When coal dust is added it reduces metal
penetration.
It decreases the incidence to scabbing due to
increase in hot plasticity to sand. However, it tends to reduce
permeability if too much.
i.
Coal dust:- This is added to green sand mixtures to enable the
mould to strip cleanly.
A coal giving good results showed:
moisture 1.39% volatile matter, 3.81% fixed carbon 48.35% and
Ash 14.45%. Alternatively, plumbago can be employed.
ii.
Wood flour and sawdust:- These and similar materials are
added to sands to increase the permeability. In drying, they
retain moisture until the mold or core has set firmly, and then
become dry themselves, when they shrink considerably, and
may be partly burnt away, leaving additional spaces for the
escape of gas.
iii.
Coal – tar pitch:- Coal dust is often replaced in sand mixtures
by finely ground coal-tar pitch, which contains some 80%
volatile carbon aqueous matter. The amount of pitch required
in green sand mixture should thus be somewhat less than half
of the amount of coal dust.
Other additives include dextrin, gelatinized starch. The ratio of
binder to sand in these mixtures is rather high, being 1:8 or
more parts of sand.
Synthetic moulding mixture are produced in foundry shops
from initial materials with adequate qualities. Usually in their
composition we have silica sand, clay or alternatively bentonite,
water (moisture) and other binders. They have high mechanical
qualities, good gas porosity refractoriness, cohesion, chemical
resistively and then have better qualities of castings.
Synthetic moulding sands are composed of washed, sharp
grained silica to which 3 to 5% clay is added. In addition there
are special sands which are ideal for achieving special
characteristics.
Examples of these are zircon, olivine, chromite and chromemagnesite. Practically, the material generally used for mould
making is green sands so called because it used in a damp state.
However, to be suitable for moulding, the sand must possessed
some properties like, adhesiveness, permeability, collapsibility
etc.
Silica sands are specified according to their averages size and
shape.
The finer the grains, the more intimate will be the
contact and lower the permeability. However fine grains tend to
fortify the mould and lesson its tendency to get distorted. The
shapes of the grains may vary from round to angular.
2.4 Effect of the Constituents of Moulding Sand
(McGraw–Hill, 1984) Silica sand is the most abundant solid
mineral found in nature. It is primarily Si02 (99%). The balance
is made up magnesium oxide, aluminium oxide, ferric oxide and
calcium oxide, as well other trace elements silica sand exhibits
a change in volume with increasing temperature.
This
expansion can cause serious casting defect and dimensurial
problems if the green sand mix is not correctly formulated.
i.
Effect of additives:- Green sand additive other than bonding
clays are used to persistent defects, reduce scrap or improve
sand handling characteristics. Use of any additive should be
made with the knowledge of how the additives affect the total
green sand system. Since many additives are naturally
occurring minerals, each has typical properties within a range
normal variance.
ii.
Effect of moisture:- Water is added to a clay bonded moulding
sand to plasticize the clay. Each type of clay required for high
density moulding sand is generally less than that for
conventional moulding sand, and is determined by the sand,
bonding clay, and supplementary additives.
Excess moisture lead to steam and gas formation within the
mould cavity, causing oversize castings due to high deformation
as well expansion related casting defects. Rapid increase in hot
and dry strength with increasing moisture content also leads to
shake out problems and cracked castings.
iii.
Effect of clay:- The reason for using clay in moulding sand is
to bind the sand grains together. Western bentonite is the most
commonly used foundry clay. If offers about 90% of the green
strength potential of southern bentonite and significantly more
green strength than fire-clay at equivalent usage rates. Western
bentonite expands in thickness, by absorbing water into its
structure.
It is this adsorption that makes western bentonito more difficult
to mull to its optimum than either of the other two clays.
Quality Sand Control
(Pidgeon, 1963) Sand control influence of green sand mould
composition, it is of little concern to the designer of casting but
it is a matter of great concern to the foundry worker who is
expected to deliver casting of good consistent quality. Standard
test are used to control quality of sand used in the foundry
shop. The standard test includes.
i.
Grain size test
ii.
Moisture content
iii.
Clay content
iv.
Mould hardness
v.
Permeability test
vi.
Strength of mould
Though there are a lot of tests that are supposed to be carried
out on moulding sands, however there are other various types
of tests that could be carried out on moulding sands that
depend on its composition of example the hot strength test,
(collapsibility) is unsuitable for organically bonded core sands
(Osungbure, 1996) since complete decomposition of the binder
produces rapid collapse on handling.
2.5 Practical and Realistic Defects in Permanent Mould and
Foundries Technological Assessment
A number of commercial accepted melt treatment techniques
are being used by aluminium foundries to remove defect from
the molten aluminium alloy prior to casting.
These various
methods of sands preparation technique, degassing and
filtration in the furnaces and in the gating system (Gallo, 2001),
(Martins and Sigworth, 1989), (Schmahietal, 1995) and, (Neff,
1995) most foundries understand that these techniques have
an impact on the melt cleanliness of the molten aluminium alloy
prior to pouring. However, the effectiveness to evaluate their
removal not only relies on the melt cleanliness measurement
technique being used, but also on the casting quality
acceptance criteria and the final casting scrap rate.
Surface and sub-surface gas defects have always been common
and troublesome defects in aluminium and other castings
poured in green sand moulds.
Within the past 30 years,
however, innovations in synthetic binder technology have
resulted in movement away form green sand moulding and
towards total to bake moulding and core making processes and
the accompanying new types of casting defects.
Generally speaking there are three major sources that may
contribute to defect in Aluminium casting these are:
i.
High initial gas content of the melt, originating from either the
charge ingredients, melt practice or atmospheric humidity.
ii.
Mould – Metal reaction between evolved mould and core gases
at the solidifying casting surface.
iii.
The percentage of mixture consistent of the moulding materials.
In addition, any combination of these, there sources may have
cumulative effect on promoting porosity formation. However,
the gases normally held responsible for sub-surface porosity
defects are nitrogen and hydrogen.
There is a definite distinction between porosity defects are
chemical in nature and result when liquid metal becomes
supersaturated with dissolved gases during melting or pouring.
The appearance of the sub-surface porosity defects resulting
from the proceeding sources may take numerous shape but
usually form as either small, spherical holes (sometime
elongated or pear-shaped) called pin holes, or larger, irregularly
rounded holes or irregularly shaped fissure types defects.
Permanent mould and sand aluminium foundries may have
inclusion scrap defect after radiographic or fluorescent
penetrate
inspection,
requirements.
if
they
have
such
inspection
as
However, the majority of the inclusion scrap
defect that a foundry experience typically occur after casting.
However, the majority of the inclusion scrap defect that a
foundries experience typically occur after the machining
operation such casting will be rejected because of poor
machineability (i.e. hard spots) and because of failing to meet
stringent cosmetic requirements on machined surfaces (i.e.
cylinder heads, pistons, structural castings, bearing housing
etc). cosmetic requirement may cause a casting to be scrapped
if inclusions were bigger than 400 microns 0.0157 or 0.4mm
such size is considered to be about the smallest size defect that
could be seen by the naked eye, in a surface by quality personal.
Thus most inclusion are only discovered when the customer
complains. Unfortunately, trouble-shooting after the customer
is still on of the main ways of establishing melt quality control
(control of aluminium casting quality by vacuum solidification
tests, AFS transactions.
CHAPTER THREE
MATERIALS AND METHODS
3.1 Materials
The following materials are essentially required to be put in
place for the full execution and actualization of the undertaken
project:
Molding sand (Natural and Synthetic sand)
Wood (for pattern making).
Aluminium scrapts.
Suitable (Required testing machines).
Molding box.
3.2
Methods
The various methods necessary for achieving this particular
project are discussed below:
3.1 Preparation of Pattern
3.1.1
Introduction
Much preparation is necessary before the pattern maker can
actually start producing the pattern. The preparatory work
includes decisions about

The type and form of materials to be use.

The type of pattern to suit the method of moulding to be
adopted.

Considerations as regards the value of allowances to be used.

The method of grating and feeding to be followed
3.1.2
Materials Used for the Pattern
Being a jobbing work, hard wood will be use as materials for the
production of the pattern for the aluminium bar.
3.2 Construction of Moulding Box
The moulding box to be used is constructed using some logs of
hard wood. The box should be wider, so as to accommodate the
pattern and for easy withdrawal, during moulding operation.
3.3 Mould Making
Method to be adopted for moulding is floor moulding using one
box. Through this method, certain amount of sieved sand about
half of wheel – barrow will be pack directly on the floor,then a
straight –edge or spirit level is use to press down the sand. After
which a replica of the aluminium bar is introduced by stamping
it on the floor. Dressing the edges of the mould where the
pattern is been placed is done for easy removal. The moulding
box will be placed, with the application of parting powder. Why
do we apply parting powder? The reason why we apply parting
powder is to avoid our pattern not to stick with the wall of the
mould during removal of pattern. We then put runner and riser,
the purpose of putting runner is to serve as passage where
molten metal will flow through the cavity, while riser serves
indicate if molten metal has filled the cavity. We then pack the
sieved sand to fill the moulding box and then rammed to top
level. While the spirit level is been used to scrape off the excess
sand from top of the mould. A hole is now pinch round the top
of the mould in order to improve the permeability of moulding
sand, we then put mark at the edges of the moulding box, the
cope will then be lifted off and set aside whilst a certain amount
of handwork will be carried out by wetting the edges of the
patterns with soaked foam, we then rapped the pattern in order
to achieve clearance and for easy removal of pattern from mould
cavity.
3.3.1
Post Moulding Operation
This involves operations to be carried out after moulding is been
completed. The post moulding operations to be carried out is as
follows:
1.
Venting:
After
successfully
completing
the
moulding
operations, venting of the mould is carried out. This is
necessary for escape of gases, when molten metal is poured into
mould. It is thus helpful to improve the permeability of the sand,
and prevent defects due to gaseous entrapment inside the
mould.
2
Pattern Removal: This will be done after venting of mould is
carried out. Care must be taken during removal of the pattern.
The method of removal depends on the type of pattern being
used. In this case, we will be using single pieced pattern,
however, care must be taken so that the mould does not
collapse or turn during removal.
3.
Mould Dressing: The moulds will be dress after pattern
removal, as the removal of the pattern could cause some edge
tearing of the moulds. Ingots will be cut-out using hand trowel
and carefully dressing the edge of the moulds.
4.
Mould Blowing: Blowing of the mould will be carried out in
order to get rid of excess sand that might have dropped during
removal of the pattern. Crucible furnaces are frequently used
for this work. They may be either the stationery or the felting
type. Coke is commonly used as fuel for the stationary.
3.4 Melting/Pouring

Charging of aluminium materials into the crucible furnace

Putting on furnace i.e. firing the furnace

When metal is in molten state, it has to be freed of impurities,
so aluminium need degassing

Preheat the longs to prevent chilling the crucible.

Remove or bale out the crucible pot from the furnace and pour
molten metal directly to a well prepared mould.
3.5 Solidification of the Casting
Many things need to be put into consideration when we are
talking of solidification of casting.
It governs the grow structure and the defects that are
introduced during the solidification stage. Both of these affects
the properties of the casting. Solidification involves two steps
that is nucleation and growth.
3.5.1
Nucleation
It refers to the process in which tiny solid particles, called nuclei
are formed when liquid materials cols below the liquidous
temperature. In practice, nuclei are offered but are dissolved
back into liquid material unless considerable super cooling
occurs. In the absence of growth of these nuclei, solid grain
structure is not generated in the mould.
3.5.2
i.
There are two Kind of Nucleation
Homogenous Nucleation: It occurs without help of foreign
particles.
ii.
Heterogeneous Nucleation: It occurs with the help of foreign
particles, such as the materials and the impurities in a casting.
3.5.3
Heterogeneous Nucleation
Since in heterogeneous nucleation the solid phase crystallizes
on a foreign particle the initial nucleus size is large and so the
degree of super cooling needed for solidification is smaller than
in homogeneous nucleation. As in a sand casting there is an
abundance of foreign particles (sand).
On the mould surface, heterogeneous nucleation is the
dominating mechanism in the early stage of solidification.
On the other hand solidification in central part of the casting
would be expected to occur by homogeneous nucleation
because of the scarcity of foreign particles in this region unless
the materials itself contains impurities.
3.5.4
Grain Structure
Based on the above understanding, one can understand the grain
structure developed in a sand casting.
For the purpose of the discussion, consider a rectangular
aluminium pattern in which molten metal is to be poured.
The materials will start cooling when it reaches its liquidous
temperature, some nuclei appear but are soon dissolved back
into the liquid. With further cooling bigger nuclei are produced
and
since
sand particles are present on the surface,
heterogeneous nucleation will contribute to a few layers of
equated grains. (sand being a poor conductor of heat).
During solidification materials shrinks and the flow of liquid is
needed to compensate for this shrinkage. The accessibility of
liquid to such locations is the easiest in case of the columnar
grains and worst for the equiaxed grains.
The dendritic fall between the columnar and equated grain
structures in terms of the difficulty of feeding.
3.6
Centerline Feeding Resistance (CFR)
The case of feeding a casting us expressed quantitatively by CFR
defined as CFR = (time during which crystal are forming at the
centerline x 100/ (total solidification time)
During the formation of crystals at the centerline, a mushy state
comprised of the liquid and solid crystals exists in the center
region. This region needs feedings because of the solidification
shrinkage. The feeding in the mushy zone is difficult.
Thus the highest the value of CFR, the more difficult is the
feeding.
Riser Design
The function of the riser is to feed liquid metal into the casting
locations where the shrinkage of liquid during cooling is likely
to produce voids, known as shrinkage voids or shrinkage
cavities.
The requirement for these functions to occur effectively are as
follows:
1.
The riser size is large enough for feeding
2.
The riser is the last part in the mould to solidify
3.
Is placed at a proper location so that the solidification material
does not impede the flow of molten metal form it to the desired
locations.
Gating System Design
The function of the gating system is to facilitate filling of the
mould cavity at the proper rate without excessive temperature
loss, objectionable turbulence, entrapped gases, slag and dross
(oxides). The recommended time for filling is 0.5mm. 1 to 2 min,
being the maximum.
The use of the following equations has been made in analyzing
the some gating system elements.
Continuity equation: q = av
Beroulli’s equation
Where q = flow rate
a
=
area of flow
v
=
flow velocity
h
=
height from a dutum
p
=
pressure
w
=
weight per unit volume
Using the above equations, the following results may be
obtained.
1.
For a top gating system v = 2gh
2.
For a bottom gating system
Filling time, tf = (a am / ag.2g) (hf – ht – hm)
In order to avoid aspiration of gases during mould filling, a
sprue in the form of a tapered cylinder (as opposed to the
straight cylinder) in needed.
Fluidity
It is a materials property, which signifies the relative ability of
the molten material to fill a mould cavity, this definition, as
opposed to viscocity takes into account the effect of decreasing
temperature, which produces crystallization in the metal
stream and thus impedes its flow.
Method of fluidity
i.
Spiral tube method
ii.
Suction tube method
The solubility of gas is governed by the sieverts laws which
states that the solubility of gas s = kpg
Where k is a material constant and pg is the partial pressure of
gas I the atmosphere over the melt, in view of this the following
measures are employed to control gases in metals.
The gas may come from various sources such as solubility,
aspiration and reaction of hot metal with mould. The remedy is
controlled melting or other measures (as discussed earlier) for
reduced solubility of gases, reduced moisture content and
organic materials in the moulding sand, and increase
permeability of the mould.
Defect due to dirt: This defect arises because of loose dirt leaf in
the mould during cleaning.
As sand is light: It arises to the top of the mould surface when
the molten material is poured in.
3.7 Machining/Cutting of the cast into test samples: Finally, the
product is cut into sizes suitable for performing test on the machine
whilst observation and deductions are made based on the result.