Casting Terms (Click on thefigure 1 to view)
Casting Terms (figure 1 to view)1. Flask: A metal or wood frame,
without fixed top or bottom, in which the mold is formed. Depending
upon the position of the flask in the molding structure, it is
referred to by various names such as drag - lower molding flask,
cope - upper molding flask, cheek - intermediate molding flask used
in three piece molding.
2. Pattern: It is the replica of the final object to be made.
The mold cavity is made with the help of pattern.
3. Parting line: This is the dividing line between the two
molding flasks that makes up the mold.
4. Molding sand: Sand, which binds strongly without losing its
permeability to air or gases. It is a mixture of silica sand, clay,
and moisture in appropriate proportions.
5. Facing sand: The small amount of carbonaceous material
sprinkled on the inner surface of the mold cavity to give a better
surface finish to the castings.
6. Core: A separate part of the mold, made of sand and generally
baked, which is used to create openings and various shaped cavities
in the castings.
7. Pouring basin: A small funnel shaped cavity at the top of the
mold into which the molten metal is poured.
8. Sprue: The passage through which the molten metal, from the
pouring basin, reaches the mold cavity. In many cases it controls
the flow of metal into the mold.
9. Runner: The channel through which the molten metal is carried
from the sprue to the gate.
10. Gate: A channel through which the molten metal enters the
mold cavity.
11. Chaplets: Chaplets are used to support the cores inside the
mold cavity to take care of its own weight and overcome the
metallostatic force.
12. Riser: A column of molten metal placed in the mold to feed
the castings as it shrinks and solidifies. Also known as "feed
head".
13. Vent: Small opening in the mold to facilitate escape of air
and gases.
Figure 1 : Mold Section showing some casting terms
Steps in Making Sand Castings There are six basic steps in
making sand castings as given below:
Pattern making: The pattern is a physical model of the casting
used to make the mold. The mold is made by packing some readily
formed aggregate material, such as molding sand, around the
pattern. When the pattern is withdrawn, its imprint provides the
mold cavity, which is ultimately filled with metal to become the
casting. If the casting is to be hollow, as in thecase of pipe
fittings, additional patterns, referred to as cores, are used to
form these cavities.Core making: Cores are forms, usually made of
sand, which are placed into a mold cavity to form the interior
surfaces of castings. Thus the void space betwe en the core and
mold cavity surface is what eventually becomes the casting.Molding:
Molding consists of all operations necessary to prepare a mold for
receiving molten metal. Molding usually involves placing a molding
aggregate around a pattern held with a supporting frame,
withdrawing the pattern to leave the mold cavity, setting the cores
in the mold cavity and finishing and closing the mold.Melting and
Pouring: The preparation of molten metal for casting is referred to
simply as melting. Melting is usually done in a specifically
designated area of the foundry, and the molten metal is transferred
to the pouring area where the molds are filled.Cleaning: Cleaning
refers to all operations necessary to the removal of sand, scale,
and excess metal from the casting. Burned-on sand and scale are
removed to improved the surface appearance of the casting. Excess
metal, in the form of fins, wires, parting line fins, and gates, is
removed. Inspection of casting for defects and general quality is
performed.
The pattern is the principal tool during the casting process. It
is the replica of the object tobe made by the casting process, with
some modifications. The main modifications are theaddition of
pattern allowances, and the provision of core prints. If the
casting is to behollow, additional patterns called cores are used
to create these cavities in the finishedproduct. The quality of the
casting produced depends upon the material of the pattern,
itsdesign, and construction. The costs of the pattern and the
related equipment are reflectedin the cost of the casting. The use
of an expensive pattern is justified when the quantity ofcastings
required is substantial.
Functions of the Pattern
1. A pattern prepares a mold cavity for the purpose of making a
casting.
2. A pattern may contain projections known as core prints if the
casting requires a core and need to be made hollow.
3. Runner, gates, and risers used for feeding molten metal in
the mold cavity may form a part of the pattern.
4. Patterns properly made and having finished and smooth
surfaces reduce casting defects.
5. A properly constructed pattern minimizes the overall cost of
the castings.
Figure 2: A typical pattern attached with gating and risering
system
Pattern Allowances
Pattern allowance is a vital feature as it affects the
dimensional characteristics of thecasting. Thus, when the pattern
is produced, certain allowances must be given on thesizes specified
in the finished component drawing so that a casting with the
particularspecification can be made. The selection of correct
allowances greatly helps to reducemachining costs and avoid
rejections. The allowances usually considered on patterns andcore
boxes are as follows:
Shrinkage or Contraction Allowance All most all cast metals
shrink or contract volumetrically on cooling. The metal shrinkage
is of two types: i. Liquid Shrinkage: it refers to the reduction in
volume when the metal changes from liquid state to solid state at
the solidus temperature. To account for this shrinkage; riser,
which feed the liquid metal to the casting, are provided in the
mold.
ii. Solid Shrinkage: it refers to the reduction in volume caused
when metal loses emperature in solid state. To account for this,
shrinkage allowance is provided on the patterns.
The rate of contraction with temperature is dependent on the
material. For example steelcontracts to a higher degree compared to
aluminum. To compensate the solid shrinkage, ashrink rule must be
used in laying out the measurements for the pattern. A shrink rule
forcast iron is 1/8 inch longer per foot than a standard rule. If a
gear blank of 4 inch indiameter was planned to produce out of cast
iron, the shrink rule in measuring it 4 inchwould actually measure
4 -1/24 inch, thus compensating for the shrinkage. Exercise 1
The casting shown is to be made in cast iron using a wooden
pattern. Assuming only shrinkage allowance, calculate the dimension
of the pattern. All Dimensions are in Inches.
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The shrinkage allowance for cast iron for size up to 2 feet is
o.125 inch per feet For dimension 18 inch, allowance = 18X 0.125 /
12 = 0.1875 inch 0.2 inchFor dimension 14 inch, allowance = 14X
0.125 / 12 = 0.146 inch 0.15 inchFor dimension 8 inch, allowance =
8 X 0.125 / 12 = 0.0833 inch 0. 09 inchFor dimension 6 inch,
allowance = 6 X 0.125 / 12 = 0.0625 inch 0. 07 inch
The pattern drawing with required dimension is shown in next
page.Draft or Taper Allowance
By draft is meant the taper provided by the pattern maker on all
vertical surfaces of thepattern so that it can be removed from the
sand without tearing away the sides of the sandmold and without
excessive rapping by the molder. Figure 3 (a) above shows a pattern
havingno draft allowance being removed from the pattern. In this
case, till the pattern iscompletely lifted out, its side s will
remain in contact with the walls of the mold, thustending to break
it. Figure 3 (b) is an illustration of a pattern hav ing proper
draftallowance. Here, moment the pattern lifting commences, all of
its surfaces are wellaway from sand surface. Thus pattern can be
removed without damaging moldcavity.Draft allowance varies with the
complexity of the sand job. But in general inner details of the
pattern require higher draft than outer surfaces. The amount of
draft depends upon the length of the vertical side of the pattern
to be extracted; the intricacy of the pattern; the method of
molding; and pattern material.
Figure 3 (a) Pattern Having No Draft on Vertical Edges
Figure 3 (b) Pattern Having Draft on Vertical Edges
Machining or Finish Allowance
The finish and accuracy achieved in sand casting are generally
poor and the refore whenthe casting is functionally required to be
of good surface finish or dimensionally accurate,it is generally
achieved by subsequent machining. Machining or finish allowances
aretherefore added in the pattern dimension. The amount of
machining allowance to beprovided for is affected by the method of
molding and casting used viz. hand molding ormachine molding, sand
casting or metal mold casting. The amount of machiningallowance is
also affected by the size and shape of the casting; the casting
orientation; themetal; and the degree of accuracy and finish
required. Exercise 2
The casting shown is to be made in cast iron using a wooden
pattern. Assuming only machining allowance, calculate the dimension
of the pattern. All Dimensions are in Inches
Solution 2
The machining allowance for cast iron for size, up to 12 inch is
o.12 inch and from 12
inch to 20 inch is 0.20 inch For dimension 18 inch, allowance =
0.20 inchFor dimension 14 inch, allowance = 0.20 inchFor dimension
8 inch, allowance = 0.12 inchFor dimension 6 inch, allowance = 0.12
inchThe pattern drawing with required dimension is shown in Figure
above.Distortion or Camber Allowance
Sometimes castings get distorted, during solidification, due to
their typical shape. Forexample, if the casting has the form of the
letter U, V, T, or L etc. it will tend to contractat the closed end
caus ing the vertical legs to look slightly inclined. This can be
preventedby making the legs of the U, V, T, or L shaped pattern
converge slightly (inward) so thatthe casting after distortion will
have its sides vertical.
The distortion in casting may occur due to internal stresses.
These internal stresses are caused on account of unequal cooling of
different section o f the casting and hindered contraction. Measure
taken to prevent the distortion in casting include:
i. Modification of casting design
ii. Providing sufficient machining allowance to cover the
distortion affect
iii. Providing suitable allowance on the pattern, called camber
or distortion allowance (inverse reflection) Figure 4: Distortions
in Casting
Rapping Allowance
Before the withdrawal from the sand mold, the pattern is rapped
all around the verticalfaces to enlarge the mold cavity slightly,
which facilitate its removal. Since it enlarges thefinal casting
made, it is de sirable that the original pattern dimension should
be reduced toaccount for this increase. There is no sure way of
quantifying this allowance, since it ishighly dependent on the
foundry personnel practice involved. It is a negative allowanceand
is to be applied only to those dimensions that are parallel to the
parting plane.Core and Core Prints
Castings are often required to have holes, recesses, etc. of
various sizes and shapes. Theseimpressions can be obtained by using
cores. So where coring is requ ired, provisionshould be made to
support the core inside the mold cavity. Core prints are used to
servethis purpose. The core print is an added projection on the
pattern and it forms a seat in themold on which the sand core rests
during pouring of the m old. The core print must be ofadequate size
and shape so that it can support the weight of the core during the
castingoperation. Depending upon the requirement a core can be
placed horizontal, vertical andcan be hanged inside the mold
cavity. A typical job, its pattern and the mold cavity withcore and
core print is shown in Figure 5.
Figure 5: A Typical Job, its Pattern and the Mold Cavity
Core Boxes:
1. Half box: Used to form two identical halves of a symmetrical
core. After they are shaped and baked, the core halves are pasted
togeyher.
2. Dump box: designed to form a complete core that requires no
pasting.
3. Split box: consists of two halves which are clamped
together.
4. Strickle box: used when a core with an irregular shape is
required. 5. Right & Left box: when two half cores made in the
same box cannot be pasted together to form an entire core.
6. Gang box: where large number of cores are to be made.
7. Sweep & skeleton box: looks like a sweep & skeleton
pattern. Used for large cores required in small quantities.Green
Sand Molding
Green sand is the most diversified molding method used in metal
casting operations. Theprocess utilizes a mold made of compressed
or compacted moist sand. The term "green"denotes the presence of
moisture in the molding sand. The mold material consists ofsilica
sand mixed with a suitable bonding agent (usually clay) and
moisture.
Advantages
Most metals can be cast by this method.Pattern costs and
material costs are relatively low.No Limitation with respect to
size of casting and type of metal or alloy used
Disadvantages
Surface Finish of the castings obtained by this process is not
good and machining is often
required to achieve the finished pro duct.
Sand Mold Making Procedure
The procedure for making mold of a cast iron wheel is shown in
(Figure 8 (a), (b), (c).
The first step in making mold is to place the pattern on the
molding board.
The drag is placed on the board ((Figure 8 (a) Dry facing sand
is sprinkled over the board and pattern to provide a non sticky
layer.
Molding sand is then riddled in to cover the pattern with the
fingers; then the drag is completely filled.
The sand is then firmly packed in the drag by means of han d
rammers. The ramming must be proper i.e. it must neither be too
hard or soft.
After the ramming is over, the excess sand is leveled off with a
straight bar known as a strike rod.
With the help of vent rod, vent holes are made in the drag to
the full de pth of the flask as well as to the pattern to
facilitate the removal of gases during pouring and
solidification.
The finished drag flask is now rolled over to the bottom board
exposing the pattern. Cope half of the pattern is then placed over
the drag pa ttern with the help oflocating pins. The cope flask on
the drag is located aligning again with the help ofpins ( (Figure 8
(b)).
The dry parting sand is sprinkled all over the drag and on the
pattern. A sprue pin for making the sprue passage is located at a
small distance from the pattern. Also, riser pin, if required, is
placed at an appropriate place. The operation of filling, ramming
and venting of the cope proceed in the same manner as performed in
the drag. The sprue and riser pins are removed first and a pouring
basin is scooped out at the top to pour the liquid metal. Then
pattern from the cope and drag is removed an d facing sand in the
form ofpaste is applied all over the mold cavity and runners which
would give the finished casting a good surface finish. The mold is
now assembled. The mold now is ready for pouring (Figure 8 (c)
)Figure 8 (a)
Figure 8 (b)
Figure 8 (c)
Molding Material and Properties
A large variety of m olding materials is used in foundries for
manufacturing molds andcores. They include molding sand, system
sand or backing sand, facing sand, partingsand, and core sand. The
choice of molding materials is based on their processingproperties.
The properti es that are generally required in molding materials
are:
1. Refractoriness: It is the ability of the molding material to
resist the temperature of the liquid metal to be poured so that it
does not get fused with the metal. The refractoriness of the silica
sand is highest.2. Permeability: During pouring and subsequent
solidification of a casting, a large amount of gases and steam is
generated. These gases are those that have been absorbed by the
metal during melting, air absorbed from the atmosphere and the
steam generated by the molding and core sand. If these gases are
not allowed to escape from the mold, they would be entrapped inside
the casting and cause casting defects. To overcome this problem
themolding material must be porous. Proper venting of the mold also
helps in escaping thegases that are generated inside the mold
cavity.3. Cohesiveness: This is the ability of sand particles to
stick together. Insufficient strength may lead to a collapse in the
mould or its partial destruction during conveying, turning over or
closing.3.1 Green Strength: The molding sand that contains moisture
is termed as green sand. The green sand particles must have the
ability to cling to each other to impart sufficient strength to the
mold. The green sand must have enough strength so that the
constructed mold retains itsshape.3.2 Dry Strength: When the molten
metal is poured in the mold, the sand around the mold cavity is
quickly converted into dry sand as the moisture in the sand e
vaporates due to the heat of the molten metal. At this stage the
molding sand must posses the sufficient strength to retain the
exact shape of the mold cavity and at the same time it must be able
to withstand the metallostatic pressure of the liquid material.3.3
Hot Strength: As soon as the moisture is eliminated, the sand would
reach at a high temperature when the metal in the mold is still in
liquid state. The strength of the sand that is required to hold the
shape of the cavity is called hot strength.
4. Collapsibility: The molding sand should also have
collapsibility so that during the contraction of the solidified
casting it does not provide any resistance, which may result in
cracks in thecastings.Besides these specific properties the molding
material should be cheap, reusableand should have good thermal
conductivity.5. Flowability: Ability to behave like a fluid so
that, when rammed it will flow to all portions of a mould and pack
all-round the pattern and take up the required shape. It increases
as clay and water content increase.
6. Adhesiveness: Sand particles must be capable of adhering to
another body i.e. should cling to the sides of moulding boxes.Shell
Molding Process ( also called Croning process or C-process)It is a
process in which, the sand (100-150 mesh) mixed with a
thermosetting resin is allowed to come in contact with a heated
pattern plate (200 0C) (preferably made of grey CI), this causes a
skin (Shell) of about 3.5 mm of sand / plastic mixture to adhere to
the pattern. Then the shell is removed from the pattern. The cope
and drag shells are kept in a flask with necessary backup material
and the molten metal is poured into the mold. Advantages: This
process can produce complex parts with good surface finish 1.25 m
to 3.75 m, and dimensional tolerance of 0.5 %. A good surface
finish and good size tolerance reduce the need for machining. The
process overall is quite cost effective due to reduced machining
and cleanup costs. Floor space and sand quality are reduced.
Unskilled labour can be employed. Uses: The materials that can be
used with this process are cast irons, and aluminum and copper
alloys. It is used for mass production of steel casting of less
than 100 kg.
Limitations: The main limitations are its high costs of pattern,
resin & equipment; uneconomical for smal runs, maximum casting
size & weight are limited; shrinkage factors vary with casting
practice.
Molding Sand in Shell Molding Process
The molding sand is a mixture of fine grained quartz sand and
powdered bakelite. There are two methods of coating the sand grains
with bakelite. First method is Cold coating method and another one
is the hot method of coating. In the method of cold coating, quartz
sand is poured into the mixer and then the solution of powdered
bakelite in acetone and ethyl aldehyde are added. The typical
mixture is 92% quartz sand, 5% bakelite, 3% ethyl aldehyde. During
mixing of the ingredients, the resin envelops the sand grains and
the solvent evaporates, leaving a thin film that uniformly coats
the surface of sand grains, thereby imparting fluidity to the sand
mixtures.In the method of hot coating, the mixture is heated to 150
-180oC prior to loading the sand. In the course of sand mixing, the
soluble phenol formaldehyde resin is added. The mixer is allowed to
cool up to 80 90o C. This method gives bet ter properties to the
mixtures than cold method.
Permanent Mold Process
In the processes like green sand moulding / dry sand moulding, a
mold need to be prepared for each of the casting produced. For
large-scale production, making a mold, for every casting to be
produced, may be difficult and expensive. Therefore, a permanent
mold, called the die may be made from which a large number of
castings can be produced. , the molds are usually made of cast iron
or steel, although graphite, copper and aluminum have been used as
mold materials. For higher melting alloys such as brass and ferrous
alloys, the mould must contain large proportion of stale
carides.
It is a casting process involving pouring a molten metal by
gravity into a steel (or cast iron) mold and is similar to the sand
casting process . In distinction from sand molds, which are broken
after each casting a permanent mold may be used for pouring of at
least one thousand and up to 120,000 casting cycles with the rate
5-100 castings/hour. Manufacturing metal mold is much more
expensive than manufacturing molds for Sand casting or investment
casting process mold. Minimum number of castings for profitable use
of a permanent mold is dependent on the complexity of its
shape.
A mould material should have a high melting temperature, enough
strength not to deform in repeated use, high thermal fatigue
resistance to resist premature cracks and low adhesion.The process
in which we use a die to make the castings is called permanent mold
castingor gravity die casting, since the metal enters the mold
under gravity. Some time in die -casting we inject the molten metal
with a high pressure. When we apply pressure ininjecting the metal
it is called pressure die casting process. All cast metals can be
cast by permanent mould method. Zn, Cu, Al, Pb, Mg and Sn alloys,
steel and CIs are most often cast by this method. Used for small
& medium sized castings (upto 10 kg) non ferrous, but
impractical for large , metals/ alloys with high melting
temperatures. Permanent mold casting process
The interior surfaces of the two parts (cope and drag) of a
permanent mold are coated with a thin ceramic coating. The mold is
preheated before coating to 150-260C.
The cores are inserted and installed in the mold assembly.
The mold is closed.
The molten metal is poured into the mold.
After the casting has solidified and cooled down to the desired
temperature the mold is opened and the casting is withdrawn from
it.
The gating system is cut away from the casting.
The finish operations are carried out.
Permanent cores are commonly used for permanent mold castings,
however if a casting has cavitys shape not allowing a withdrawal of
the core it is made of chemically bonded sand or other materials
used for preparation of expendable cores. New consumable cores are
added after each pour. The process combining permanent mold and
consumable parts (cores) are called semi-permanent casting.
Advantages: Advantages of permanent mold casting process are
determined by relatively high cooling rate caused by solidification
in metallic mold:
Better mechanical properties.
Homogeneous grain structure and chemical composition.
Low shrinkage and gas porosity.
Good surface quality: 40-250 inch (1-6 m) Ra.
Low dimensional tolerances: typically about 0.04 (1 mm}.
Little scrap process.
Disadvantages:
The cost of tooling is usually higher than for sand castings
The process is generally limited to the production of small
castings of simple exterior design, although complex castings such
as aluminum engine blocks and heads are now commonplace.
Die casting
It is a process, in which the molten metal is injected into the
mold cavity at an increased pressure up to 30,000 psi (200 MPa).
The reusable steel mold used in the die casting process is called a
die. It is a highly productive method of casting parts with low
dimensions tolerance and high surface quality. The parts
manufactured by die casting method are automotive connecting rods,
pistons, cylinder beds, electronic enclosures, toys, plumbing
fittings. This process is particularly suitable for Pb, Mg, Sn, and
Zn alloys. The molten metal injection is carried out by a machine
called die casting machine. If the parts are small, several parts
may be cast at one time in what is known as multiple-cavity
die.There are two principal die casting methods: hot chamber method
and cold chamber method.
Cold chamber die casting: In the cold chamber die casting
machines hydraulically operated plunger forces a molten metal to
flow in the cold cylinder (chamber). A principal scheme of the cold
chamber die casting machine is shown in the picture
Cold chamber process:
When the pressure chamber is filled with a molten metal the
plunger starts traveling forward and builds up a pressure forcing
the metal to flow through the sprue to the die cavity.
After the metal has solidified the plunger returns to its
initial position allowing a new portion of the molten metal to fill
the pressure chamber.
The die then opens and the ejector pins removes the casting from
the die.
The casting cycle now may be repeated.
Cold chamber method is mainly used for casting Aluminum alloys,
Magnesium alloys, Copper alloys and zinc alloys (including
zinc-aluminum alloys).
Hot chamber die casting: In this, die casting machines the
pressure chamber (cylinder) and the plunger are submerged in the
molten metal in the pot (crucible).
Hot chamber machines have short casting cycle (about 1 sec.).
They are capable to cast thin wall casting with good filling the
cavity under precise temperature control of the molten metal.
Hot chamber process may be used for casting low melting metals,
which are chemically inert to the material of the plunger and other
parts of the casting machine: zinc alloys (except zinc alloys
containing more than 10% of aluminum), tin alloys and Magnesium
alloys.
Maintenance of these machines is more expensive as compared to
the cold chamber process.
Hot chamber process:
The plunger goes up allowing the melt to fill the cylinder
space. The die is closed at this stage.
The plunger goes down forcing the melt to flow through the
gooseneck into the die cavity.
After the die has been filled with the melt the plunger is held
under a pressure until the solidification is completed.
The die opens. The casting stays in the die part equipped with
ejectors.
The plunger goes up and the melt residuals return through the
gooseneck back to the pot.
The ejectors push the casting out of the die.
Advantages of die casting:
High productivity.
Good dimensional accuracy.
Good surface finish: 2-100 inch (0.5-2.5 m) Ra.
Thin wall parts may be cast.
Very economical process at high volume production.
Fine Grain structure and good mechanical properties are
achieved.
Intricate shapes may be cast.
Small size parts may be produced.
Disadvantages of die casting:
Not applicable for high melting point metals and alloys (eg.
steels) Large parts can not be cast.
High die cost.
Too long lead time.
Some gases my be entrapped in form of porosity.
Centrifugal Casting
In this process, the mold is rotated rapidly about its central
axis as the metal is pouredinto it. Because of the centrifugal
force, a continuous pressure will be acting on the metalas it
solidifies. The slag, oxides and other inclusions being lighter,
get separated from themetal and segregate towards the center. This
process is normally used for the making ofhollow pipes, tubes,
hollow bushes, etc., which are ax symmetric with a concentric
hole.Since the metal is always pushed outward because of the
centrifugal force, no core needsto be used for making the
concentric hole. The mold can be rotated about a
vertical,horizontal or an inclined axis or about its horizontal and
vertical axes simultaneously.The length and outside diameter are
fixed by the mold cavity dimensions while the insidediameter is
determined by the amount of molten metal poured into the mold.
Figure 9 (Vertical Centrifugal Casting), Figure 10 ( Horizontal
Centrifugal Casting)
True Centrifugal Casting: Molten metal is poured into a rotating
mould to produce tubular parts such as pipes, tubes and rings.
Semi- Centrifugal Casting: Speed is not as high as in true
casting. In this method, centrifugal force is used to produce solid
castings rather than tubular parts. Density of the metal in the
final casting is greater in the outer sections than at the centre
of rotation. The process is used on parts in which center of
casting is machined away, such as wheels and pulleys.
Centrifuged: Several identical or nearly similar mouds are
located radially about a vertically arranged central riser or sprue
which feeds metal into cavities through a number of radially gates.
The entire mould is rotated with central sprue which acts as axis
of rotation. Thus, it is not purely centrifugal process. Suitable
for small, intricate parts where feeding problems are
encountered.
Advantages
Formation of hollow inte riors in cylinders without cores
Less material required for gate
Fine grained structure at the outer surface of the casting free
of gas and shrinkage cavities and porosity Disadvantages
More segregation of alloy component during pouring under the
forces of rotation
Contamination of internal surface of castings with non -metallic
inclusions
Inaccurate internal diameter.As the mold material steels, Cast
irons, Graphite or sand may be used. The rotation speed of
centrifugal mold is commonly about 1000 RPM (may vary from 250 RPM
to 3600 RPM).
A centrifugal casting machine is schematically presented in the
above picture:
Centrifugal casting is carried out as follows: The mold wall is
coated by a refractory ceramic coating (applying ceramic slurry,
spinning, drying and baking).
Starting rotation of the mold at a predetermined speed.
Pouring a molten metal directly into the mold (no gating system
is employed).
The mold is stopped after the casting has solidified.
Extraction of the casting from the mold.
Centrifugal casting technology is widely used for manufacturing
of iron pipes, bushings, wheels, pulleys bi-metal steel-bronze
bearings and other parts possessing axial symmetry.
Squeeze casting: Squeeze casting is a method combining casting
and forging technologies. In contrast to other casting techniques
(sand casting, die casting), in which a molten metal is poured
(injected) into the mold cavity after the two parts of the mold are
assembled, squeeze casting mold is closed after a portion of molten
metal has been poured into the preheated bottom die. The upper die
lowers towards the bottom die causing the melt to fill the mold
cavity. The squeezing pressure is applied until full solidification
of the casting. A scheme of the process is shown in the picture
given below. Squeeze castings are characterized by:
low shrinkage and gas porosity;
enhanced mechanical properties because of fine grain structure
caused by rapid solidification ;
good surface quality.
Squeeze casting is commonly used for processing aluminum and
magnesium alloys. This process is also used for fabrication of
reinforced metal matrix composites where molten aluminum
infiltrates a fiber reinforcing structure.
Continuous casting
It is a casting method, in which the steps of pouring,
solidification and withdrawal (extraction) of the casting from an
open end mold are carried out continuously. Cross-sectional
dimensions of a continuous casting are constant along the casting
length and they are determined only by the dimensions of the mold
cavity.
The length of a continuous casting is limited by the life time
of the mold.
Continuous casting technology is used for both ferrous and
non-ferrous alloys.
Traditional continuous casting processes use stationary (or
oscillating) molds, in which the solidified bar moves relative to
the mold surface. Friction caused by the movement results in
formation of micro-cracks and other defects in the surface regions
of the casting. Lubricating oil supplied to the mold surface and
self-lubricating graphite molds decrease the friction / sticking
and reduce the defective surface zone. This defective zone is
commonly machined (milled) prior to Rolling.
The alternative continuous casting methods use moving endless
molds (rolls, belts, wheels) characterized by zero relative
movement between the mold and casting surfaces. Strips and slabs
fabricated by Continuous casting in traveling mold have low defect
surface. The castings may be further processed (rolled) without
surface machining. Depending on the mold position (vertical or
horizontal) continuous casting machines may be vertical or
horizontal.
Vertical continuous casting Steels are commonly cast in these
machines. Molten metal is continuously supplied from the ladle to
the intermediate ladle (tundish) from which it is continuously
poured into the mold at a controllable rate keeping the melt level
at a constant position.
The water-cooled copper mold (primary cooling zone) extracts the
heat of the metal causing its solidification. The mold oscillates
in order to prevent sticking with the casting.
When the casting goes out from the mold it is cooled in the
secondary cooling zone by water (or water with air) sprayed on the
casting surface.
Most of vertical continuous casting machines are equipped with
strand guide units bending the casting and changing its
configuration from vertical to horizontal. The casting is
continuously extracted from the mold by the withdrawal unit
followed by a cut-off unit.
The casting process begins from inserting a dummy (primary) bar
into the mold. Then a molten metal is poured into mold where it
solidifies and grips the end of the dummy bar.
The dummy bar is disconnected from the casting after passing the
withdrawal unit.
Horizontal continuous casting
This is generally used for casting non-ferrous alloys.
Horizontal continuous casting in stationary mold with graphite
water-cooled molds, Twin-roll caster and Twin-belt caster are most
popular methods of this type. Due to the water cooling (primary and
secondary) solidification rate provided by continuous casting is
higher than in other casting methods therefore continuous castings
have more uniform and finer grain structure and enhanced mechanical
properties
Investment Casting Process (also called lost wax
pattern/precision cating process)The root of the investment casting
process, the cire perdue or "lost wax" method datesback to at least
the fourth millennium B.C. The artists and sculptors of ancient
Egypt andMesopotamia used the rudiments of the investment casting
process to create intricatelydetailed jewelry, pectorals and idols.
The investment casting process al os called lost waxprocess begins
with the production of wax replicas or patterns of the desired
shape of thecastings. A pattern is needed for every casting to be
produced. The patterns are preparedby injecting wax or polystyrene
in a metal dies. A numbe r of patterns are attached to acentral wax
sprue to form a assembly. The mold is prepared by surrounding the
patternwith refractory slurry that can set at room temperature. The
mold is then heated so thatpattern melts and flows out, leaving a
clean cavi ty behind. The mould is further hardenedby heating and
the molten metal is poured while it is still hot. When the casting
issolidified, the mold is broken and the casting taken out.The
basic steps of the investment casting process are (Figure 11 see
below ) :
1. Production of heat-disposable wax, plastic, or polystyrene
patterns2. Assembly of these patterns onto a gating system3.
"Investing," or covering the pattern assembly with refractory
slurry4. Melting the pattern assembly to remove the pattern
material5. Firing the mold to remove the last traces of the pattern
material
6. Pouring
7. Knockout, cutoff and finishing
Uses: for casting turbine blades, parts of motor cars, sewing
machines, typewriters etc.
Advantages
Machining can be largely reduced or eliminated since tolerances
close to +0.1 to -0.1 mm and surface finish of around 1-5microne
are possible.
Extremely thin sections (to the extent 0.75 mm)
Fine grained structure at the outer surface of the casting free
of gas and shrinkage
cavities and porosity . Sound & defect free casting may be
obtained.
.Suitable for mass production of small sized
castings.Disadvantages
Unsuitable for casting more than 5 kg weight
Precision control ir required in all stages.
Expensive in all respect. Melting Practices
Melting is an equally important parameter for obtaining a
quality castings. Anumber of furnaces can be used for melting the
metal, to be used, to make ametal casting. The choice of furnace
depends on the type of metal to be melted.Some of the furnaces used
in metal casting are as following:.
Crucible furnaces
Cupola
Induction furnace
Reverberatory furnace
Cupola
Objective of the cupola is to produce iron of desired
compositions, temperature and properties at the required rate in
the most economical manner. Advantages of cupola over other types
of furnaces are its simplicity of operation, continuity of
production and increased output coupled with high degree of
efficiency.
Cupola furnaces are tall, cylindrical furnaces used to melt iron
and ferrous alloys in foundry operations. Alternating layers of
metal and ferrous alloys, coke, and limestone are fed into the
furnace from the top. A schematic diagram of a cupola is shown in
Figure14 . This diagram of a cupola illustrates the furnace's
cylindrical shaft lined with refractory and the alternating layers
of coke and metal scrap. The molten metal flows out of a spout at
the bottom of the cupola.
Description of Cupola
The cupola consists of a vertical cylindrical steel sheet, 6 to
12 mm thick, and lined inside with acid refractory bricks which
consist of SiO2 and Al2O3.
The lining is generally thicker in the lower portion of the
cupola as the temperature are higher than in upper portion. There
is a charging door through which coke, pig iron, steel scrap and
flux is charged.
The shell is mounted either on brick work foundation or on steel
coloumns.
In a steel coloumn arrangement, used on most modern cupolas,
cupolas are provided with a drop bottom door through which debris,
consisting of coke, slag etc. can be discharged at the end of the
melt.
In drop bottom cupolas, the working bottom is built up with
moulding sand which covers the drop doors.
Through the tap hole molten metal is poured into the ladle and
it is situated at the lowest point at the front of cupola.
A slag hole is provided to remove the slag from the melt which
is opposite to the tap hole, and somewhat above the tapping
hole.
The blast is blown through the tuyeres. These tuyeres are
arranged in one or more row around the periphery of cupola.
Hot gases which ascends from the bottom (combustion zone)
preheats the iron in the preheating zone.
At the top conical cap called the spark arrest is provided to
prevent the spark emerging to outside.
Operation of Cupola
The cupola is charged with wood at the bottom. On the top of the
wood a bed of coke is built. Alternating layers of metal and
ferrous alloys, coke, and limestone are fed into the furnace from
the top. The pur pose of adding flux is to eliminate the impurities
and to protect the metal from oxidation and also to reduce melting
point of slag and to increase its fluidity for easy disposal. Air
blast is opened for the complete combustion of coke. When
sufficient metal has been melted that slag hole is first opened to
remove the slag. Tap hole is then o pened to collect the metal in
the ladle.
A constant volume of air for combustion is obtained from a
motorised blower. The air os carried from blower through a pipe
called windpipe (air blast inlet), first to a circular jacket
around the shell called windbox and then into the furnace through a
number of openings called tuyeres which are provided at a height of
between 450 to 500 mm above the working bottom or bed of the
cupola. These tuyeres are generally 4, 6 or 8 in numbers depending
on the size of cupola and may be fitted in one or more number of
rows. FIGURE 14
Zones in a Cupola:
1. Crucible Zone:
between top of the sand bed and bottom of tuyeres.
Molten metal is accumulated here.
Also caled the well or hearth.
2. Combustion or oxidizing zone:
situated normally 150 to 300 mm above the top of tuyeres.
All oxygen in the air blast is consumed here and actual
combustion takes place here.
Lot of heat is liberated and this is supplied to other zones.
Heat is also evolved due to oxidation of silicon and manganese.
Temp being 15500C to 18500C
Molten drops of cast iron pour onto the hearth.
The chemical reactions are:
C + O2 -( CO2 + Heat
Si + O -( SiO2 + Heat
2 Mn + O2 -( 2 MnO + Heat
3. Reducing zone:
extends from the top of combustion zone to the top of the coke
bed.
Reduction of CO2 to CO occur and temp drops to about 12000C at
the coke bed.
Due to the reducing atmosphere, the charge is protected from any
oxiding influence.
The reaction taking place is:
CO2 + C (Coke) -( 2CO Heat
4. Melting Zone:
starts from the first layer of metal charge above the coke bed
and extend up to a height of 900 mm.
highest temp (16000C) is developed in this zone for complete
combustion of the coke and iron is thus melted here.
A considerable carbon pickup by molten metal also occurs in this
zone:
3Fe + 2 CO -( Fe3C + CO25. Preheating Zone or charging zone:
starts from above melting zone and extends up to the bottom of
the charging door.
Contains cupola charge as alternate layers of coke, flux and
metal
They are preheated at a temp of 11000C before coming to melting
zone.
6. Stack Zone:
extends from the above the preheating zone to the top of the
cupola.
Carries the gases generated within the furnace to the
atomsphere.
Capacity of cupola:
Output is defined as the tonnes of molten metal obtained per
hour of the heat.
Capacities vary from 1 to 15 tonnes ( or even more) of melted
iron per hour.
It has been observed that 14 cm3 of cupola plan area burns about
1 kg of coke/hour.
Diameter of cupola varies from 1 to 2 m with a height of from 3
to 5 times of dia.
Cupola Operation:
1. Preparation of cupola Clean out slag & refuse on the
lining and around tuyeres
Bad spots or broken bricks are repaired.
Preparation of sand bottom is begun.
Bottom doors are raisd & bottom sand is introduced through
the charging doors and is rammed well.
Surface of sand bottom is sloped from all directions.
Slag hole & tap hole are formed.
Cupola should be thoroughly dried before firing.
2. Firing of cupola Wood is ignited on sand bottom. It should be
done 2.5-3 hrs before molten metal is required.
A bed of coke is built on the top of wood.
Coke is added to a level slightly above tuyeres and air blast is
turned on a ta lower than blowing rate to ignite coke.
As soon as red spots begin to show over the top of fuel bed,
additional coke is introduced (above the upper row of tuyeres)
3. Charging the cupola As soon as coke bed is built up to the
correct height and ignited uniformly throughout, alternate layers
of pig iron, coke and flux are charged until cupola is filled.
Suitable scrap is also added along with pig iron to control the
chemical composition of iron produced.
4. Soaking of iron Charge should soak heat for about 45 minutes
after the cupola is fully charged upto the charging door.
Charge gets slowly heated.
5. Air blast When full blast is turned on. Before turning on
blast, tuyeres openings and tapping hole are kept closed.
After the blast is on for a few minutes (10 mins), molten metal
starts accumulating in the hearth.
At the end of melt the charging is stopped but the blast is kept
on until all the metal has melted.
6. Tapping and slagging First tapping can be made 40-50 minutes
after full blast turned on.
When slag accumulates in the well, the slag hole is opened and
the slag is run off.
Molten metal is collected in ladles and is carried to the moulds
for pouring.
7. Closing the cupola Blast is shut off and prop under the
bottom door is knocked down so that bottom plates swing open.