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MODULE -I
ME 220 MANUFACTURING TECHNOLOGY
Casting
• Casting is one of the oldest manufacturing process to shape
metals into useful products
• Casting involves pouring molten metal into a mould cavity,
upon solidification, the metal takes the shape of the cavity
• Casting first was used around 4000 B.C. to make ornaments,
arrowheads, and various other objects
• A Wide variety of products can be cast, and the process is
capable of producing intricate shapes in one piece, including
those with internal cavities, such as engine blocks
Outline of Metal Casting Process
Why Casting?
• Casting can produce complex shapes and can incorporate
internal cavities or hollow sections
• Very large parts can be produced in one piece
• Casting can utilize materials that are difficult or uneconomical
to process by other means
• The casting process can be economically competitive with
other manufacturing processes
Sand Casting
Sand casting consists of:
(a)Placing a pattern (having the shape of the desired casting) in
sand to make an imprint
(b)Incorporating a gating system
(c)Removing the pattern and filling the mould cavity with
molten metal
(d)Allowing the metal to cool until it solidifies
(e)Breaking away the sand mould
(f) Removing the casting
Steps in Sand Casting
Features of Mould in Sand Casting
Features of Mould in Sand Casting
• The flask supports the mould itself. Two-piece moulds consist of a cope ontop and a drag on the bottom; the seam between them is the parting line.
• When more than two pieces are used in a sand mould, the additional partsare called cheeks.
• Pouring basin or pouring cup: into which the molten metal is poured
• Sprue :through which the molten metal flows downward
• Runner: channel that carry the molten metal from the sprue to the mouldcavity.
• Gates are the inlets into the mould cavity.
• Riser : supply additional molten metal to the casting as it shrinksduring solidification
• Cores : They are placed in the mould to form hollow regions orotherwise define the interior surface of the casting
• Vents: which are placed in moulds to carry off gases produced whenthe molten metal comes into contact with the sand in the mould and thecore.
Sands
• Most sand-casting operations use silica sand (𝑆𝑖𝑂2) as themould material.
• Sand is inexpensive and is suitable as a mould materialbecause of its high-temperature characteristics and highmelting point
• There are two general types of sand: naturally bonded (banksand) and synthetic (lake sand).
• Sand having fine, round grains can be packed closely and,thus, forms a smooth mould surface and enhances mouldstrength
• Fine grains lower mould permeability (where fluids and gasespenetrate through pores)
• The mould also should have good collapsibility to allow thecasting to shrink while cooling and, thus, to avoid defects inthe casting
Types of Sand Moulds
There are three basic types of sand moulds:
1.Green-sand Mould :
• Mould is a mixture of sand, clay, and water.
• The term “green” refers to the fact sand in the mould ismoist or damp while the metal is being poured into it
• Green-sand moulding is the least expensive method ofmaking moulds
• Because of their higher strength, these moulds generallyare used for large castings
Types of Sand Moulds
2.Cold-box Mould :
• Various organic and inorganic binders are blended into the sand
to bond the grains chemically for greater strength
• Moulds are more dimensionally accurate than green-sand
moulds, but are more expensive.
3. No-bake mould :
• A synthetic liquid resin is mixed with the sand and the mixture
hardens at room temperature
• The bonding of the mould in this and in the cold-box process
takes place without heat, they are called cold-setting processes.
Patterns
• Pattern is a model of a casting, constructed in such a way that itcan be used for forming an impression(mould) in damp sand
• Patterns are used to mould the sand mixture into the shape of thecasting and may be made of wood, plastic, or metal
• The selection of a pattern material depends on the size andshape of the casting, the dimensional accuracy and the quantityof castings required, and the moulding process
• Patterns may be made of a combination of materials to reducewear in critical regions, and they usually are coated with aparting agent to facilitate the removal of the casting from themoulds.
Requirements of a Good Pattern
• Light in weight
• Simple in design and ease of manufacture
• Smooth and wear resistant surface
• Retain its dimensions and rigidity during the definite service life
• High strength and long life
• Ability to withstand rough handling
• Cheap and readily repairable
Types of Pattern
1. Single piece or solid pattern
2. Split pattern
3. Loose piece pattern
4. Match plate pattern
5. Gated pattern
6. Sweep pattern
7. Cope and drag pattern
8. Follow board pattern
Single Piece or Solid Pattern
Split Pattern
Loose Piece Pattern
Match Plate Pattern
Gated Pattern
Sweep pattern
Follow Board Pattern
Pattern Materials
Materials commonly used are:
1. Wood
2. Metal
3. Plastic
4. Wax
5. Quick setting compounds
Wood
• The wood used for pattern material should be properly dried and seasoned
• It should be straight grained
• It should be free from knots
• It should be free from insects and excessive sap wood
Types of wood commonly used for pattern making
• White pine
• Mahogany
• Mapple
• Teak
Metals
• Where durability and strength are required, patterns are made from metals
• A metal pattern can be either cast from master wood pattern or be machined by the methods of machining
• Metal patterns are used in machine moulding
Metals commonly used for pattern making
• Aluminium
• Brass
• Whit metal
• Cast Iron
Plastic
• Plastic patterns are highly resistant to corrosion
• Strong and dimensionally stable
• Surface of the pattern is smooth
Plastics employed for pattern making
• Phenol formaldehyde
• Polyester resin
• Ploy acrylates
• Polyethylene
• Poly vinyl chloride
Quick Setting Compounds
• Gypsum patterns are capable of producing castings with
intricate details and to very close tolerances
• Gypsum can be easily formed , has plasticity and can be easily
repaired
Pattern Allowances
• While making patterns certain dimensional allowances must be
given in the pattern so that casting obtained is of required
specification.
• The allowances usually provided in a pattern are
1. Shrinkage allowance
2. Draft or taper allowance
3. Machining allowance
4. Rapping or shaking allowance
5. Distortion allowance
Pattern Allowances (Cont…)
1. Shrinkage allowance: An allowance added to the pattern to compensate for the metal shrinkage that takes place while the metal solidifies
2. Draft or taper allowance: It is the taper provided on the verticalsurface of a pattern to facilities its removal from mould withoutexcessive rapping or breakage
3. Machining allowance :Machining or finishing allowance is the extramaterial provided on certain details of casting may be machined toexact dimensions
4. Rapping or Shaking allowance: This allowance is provided for theenlargement of the mould cavity because of excessive rapping
5. Distortion allowance : This allowance is provided on the pattern tocompensate for possible distortion of casting because of the unequalcooling rates at different sections of casting and uneven internalstresses
Cores
Cores
• For castings with internal cavities or passages, such as thosefound in an automotive engine block or a valve body, cores areutilized.
• Cores are placed in the mould cavity to form the interiorsurfaces of the casting and are removed from the finished partduring shakeout and further processing
• Cores must possess strength, permeability, the ability towithstand heat, and collapsibility
• Cores are anchored by core prints
• To keep the core from shifting, metal supports (chaplets) maybe used to anchor the core in place
Sand-moulding Machines
When large number of castings is to be produced, hand moulding
consumes more time, labour and also accuracy and uniformity in
moulding varies.
To overcome this difficulty, machines are used for moulding.
Based on the methods of ramming, moulding machines are classified
as follows:
1.Jolt moulding machine
2.Squeeze moulding machine
3.Jolt-squeeze moulding machine
4.Sand slinger
Jolt Moulding Machines
Jolt Moulding Machines
• A jolt machine consists of a flat table mounted on a piston-cylinderarrangement and can be raised or lowered by means of compressedair.
• In operation, the mould box with the pattern and sand is placed onthe table. The table is raised to a short distance and then droppeddown under the influence of gravity against a solid bed plate. Theaction of raising and dropping (lowering) is called 'Jolting'.
• Jolting causes the sand particles to get packed tightly above andaround the pattern. The number of 'jolts' may vary depending on thesize and hardness of the mould required. Usually, less than 20 joltsare sufficient for a good moulding.
• The disadvantage of this type is that, the density and hardness of therammed sand at the top of the mould box is less when compared toits bottom portions.
Squeeze Moulding Machine
Squeeze Moulding Machine
• In squeeze machine, the mould box with pattern and sand in it is
placed on a fixed table as shown in figure
• A flat plate or a rubber diaphragm is brought in contact with the upper
surface of the loose sand and pressure is applied by a pneumatically
operated piston.
• The squeezing action of the plate causes the sand particles to get
packed tightly above and around the pattern.
• Squeezing is continued until the mould attains the desired density.
• In some machines, the squeeze plate may be stationary with the mould
box moving upward.
• The disadvantage of squeeze machine is that, the density and hardness
of the rammed sand at the bottom of the mould box is less when
compared to its top portions.
Jolt Squeeze Moulding Machine
Jolt Squeeze Moulding Machine
• Jolt squeeze machine combines the operating principles of 'jolt' and 'squeeze'machines resulting in uniform ramming of the sand in all portions of the moulds
• The machine makes use of a match plate pattern placed between the cope and thedrag box.
• The whole assembly is placed on the table with the drag box on it.
• The table is actuated by two pistons in air cylinders, one inside the other. Onepiston called 'Jolt piston' raises and drops the table repeatedly for a predeterminednumber of times, while the other piston called 'squeeze piston' pushes the tableupward to squeeze the sand in the flask against the squeeze plate. In operation, sandis filled in the drag box and jolted repeatedly by operating the jolt piston.
• After jolting, the complete mould assembly is rolled over by hand.
• The cope is now filled with sand and by operating the squeeze piston, the mouldassembly is raised against the squeeze plate. By the end of this operation, the sandin the mould box is uniformly packed.
• The match plate is now vibrated and removed. The mould is finished and madeready for pouring.
Sand Slinger
Sand Slinger
• A sand slinger is an automatic machine equipped with a unit that
throws sand rapidly and with great force into the mould box. Figure
shows a sand slinger. Sand slinger consists of a rigid base, sand bin,
bucket elevator, belt conveyor, ramming head (sand impeller) and a
swinging arm.
• In operation, the pre-mixed sand mixture from the sand bin is picked
by the bucket elevator and is dropped on to the belt conveyor.
• The conveyor carries the sand to the ramming head, inside which there
is a rotating impeller having cup shaped blades rotating at high speeds
(around 1800 rpm).
Shell Mould Casting
Shell Mould Casting
• Shell-mould casting can produce many types of castings with closedimensional tolerance and good surface finish a low cost
a)A mounted pattern made of a ferrous metal or aluminium is
heated to 175 ̊ C -370 ̊ C
b) Coated with a parting agent such as silicone
c) Clamped to a box or chamber
• The box contain fine sand, mixed with 2.5% to 4% thermosetting resinbinder, such as phenol-formaldehyde that coats the sand particles
• The box is either rotated upside down or the sand is blown over thepattern , allowing it to coat the pattern
• The assembly is then placed in an oven for a short period of time tocomplete the curing of the resin
• The shell hardens around the pattern and is removed from the patternusing built-in –ejector pin
Ceramic Mould Casting
Ceramic Mould Casting
• Also called cope and drag investment casting
• Uses refractory mould materials suitable for high temperatureapplications
• The slurry is a mixture of fine grained Zircon (ZrSi𝑂4 ),aluminium oxide and fused silica, which are mixed withbonding agent and poured over the pattern, which has beenplaced in a flask
• The pattern may be made of wood or metal
• After setting , moulds are removed, dried, burned off to removevolatile matter and baked
Investment Casting
Investment Casting
• Also called lost wax process
• The pattern made of wax or plastic
• The pattern is made by injecting molten wax or plastic into a
metal die in the shape of the pattern
• The pattern is then dipped into a slurry of refractory material
such as fine silica and binder(such as water, ethyl silicate and
acids)
• After tis initial coating has dried, the pattern is coated
repeatedly to increase its thickness
• The term investment derives from the fact that the pattern is
invested with the refractory material
Investment Casting (Cont…)
• The one-piece mould is dried in air and heated up to a
temperature of 90̊ C to 175̊ C
• It is held in an inverted position for about 12 hours to melt out
wax
• The mould is then fired to 650̊ C to 1050̊ C for about 4 hours
depends on metal to be cast
• After the metal has been poured and has solidified, the mould is
then broken up and the casting is removed
• Number of patterns can be joined to make one mould, called a
tree
Vacuum Casting
Vacuum Casting
• A mixture of fine sand and urethane is moulded over metal dies
and cured with amine vapour
• The mould is then held with a robot arm and immersed partially
into molten metal held in an induction furnace
• The vacuum reduces the air pressure inside the mould to about
two-thirds of atmospheric pressure, thus drawing the molten
metal into the mould cavities through a gate in the bottom of the
mould.
• It begins to solidify within a very short time.
• After the mould is filled, it is withdrawn from the molten metal
Slush Casting
Slush Casting
• Solidified skin develops in a casting and becomes thicker withtime
• Hollow castings with thin walls can be made by permanent-mould casting using this principle: a process called slushcasting.
• This process is suitable for small production runs and generallyis used for making ornamental and decorative objects (such aslamp bases and stems) and toys from low-melting-point metalssuch as zinc, tin, and lead alloys.
• The molten metal is poured into the metal mould.
• After the desired thickness of solidified skin is obtained, themould is inverted (or slung) and the remaining liquid metal ispoured out.
Pressure Casting
Pressure Casting
Pressure Casting
• In pressure casting (also called pressure pouring or low
pressure casting), the molten metal is forced upward by gas
pressure into a graphite or metal mould
• The pressure is maintained until the metal has solidified
completely in the mould
• The molten metal also may be forced upward by a vacuum,
which also removes dissolved gases and produces a casting
with lower porosity
• Pressure casting generally is used for high-quality castings,
such as steel railroad-car wheels
Hot-Chamber Die Casting
Hot-Chamber Die Casting
• A piston a certain volume of metal into the die cavity through a
gooseneck and nozzle.
• Pressures range up to 35 MPa, with an average of about 15 MPa
• The metal is held under pressure until it solidifies in the die.
• To improve die life and to aid in rapid metal cooling (thereby
reducing cycle time) dies usually are cooled by circulating water
or oil through various passageways in the die block
• Low-melting-point alloys (such as zinc, magnesium, tin, and
lead) commonly are cast using this process
Cold-Chamber Die Casting
Cold-Chamber Die Casting
• Molten metal is poured into the injection cylinder (shot
chamber).
• The chamber is not heated-hence the term cold chamber.
• The metal is forced into the die cavity at pressures usually
ranging from 20 to 70 MPa, although they may be as high as
150 MPa
Centrifugal Casting
Centrifugal Casting
• The centrifugal-casting process utilizes inertial forces (causedby rotation) to distribute the molten metal into the mouldcavities
• In true centrifugal casting, hollow cylindrical parts (such aspipes, gun barrels, bushings, engine-cylinder liners, bearingrings with or without flanges, and street lampposts) areproduced
• Molten metal is poured into a rotating mould
• The axis of rotation is usually horizontal, but can be vertical forshort work pieces.
• Moulds are made of steel, iron, or graphite and may be coatedwith a refractory lining to increase mould life
Gating System
Gating System
• The molten metal is poured through a pouring basin or cup; it
then flows through the gating system (consisting of sprue,
runners, and gates) into the mould cavity.
• A good gating design ensures distribution of the metal in the
mould cavity at proper rate without excessive temperature loss,
turbulence and entrapping gases and slag
• The design of gating system depends on both the metal and
mould composition
Gate Ratio (Gating Ratio)
It is defined as the ratio of sprue area to total runner area to total
gate area i.e., Sprue area : Runner area : Gate area
Types of Gating System1. Pressurised (Choked) Gating System
In this system, the ingate serve as the choke
This system maintains a back pressure and causes the entire
gating system to be pressurised
In this system molten metal enters mould cavity uniformly
Typical gate ratio in this system 4: 3: 2
This system is adopted for metals like iron, steel, brass, etc.
Types of Gating System (Cont..)
2. Unpressurised Gating System
In this system sprue base serves as choke
The typical gating ratio in this system are 1:2:2, 1:2:4, 1:3:3,
1:4:4
Such a system is adopted for light oxidisable metals like
aluminium, magnesium, etc.
Gates
• Also called ingates, are the opening through which the molten
metal enters the mould cavity
• Depending on the application, various types of gates are used in
the casting design
1.Vertical Gate
2.Bottom Gate
3.Horizontal Gate
Gating Design-Vertical Gating
𝒗𝒈 = 𝟐𝒈𝒉𝒕
Where, 𝑣𝑔=Velocity of metal at gate
g = Acceleration due to gravity
ℎ𝑡=ℎ𝑐 + ℎ𝑠
ℎ𝑐= Pouring basin (cup) height
ℎ𝑠= Sprue height
𝒕𝒇 =𝑽
𝑨𝒈𝒗𝒈
Where, 𝑡𝑓 =Time taken to fill up the mould
𝐴𝑔=Cross sectional area of the
gate
V= Volume of the mould
Gating Design-Bottom Gating
𝒗𝒈 = 𝟐𝒈(𝒉𝒕 − 𝒉)
Where, 𝑣𝑔=Velocity of metal at gate
ℎ𝑡=ℎ𝑐 + ℎ𝑠
h= static head
ℎ𝑡-h= effective head
𝒕𝒇 =𝑨𝒎
𝑨𝒈
𝟏
𝟐𝒈𝟐( 𝒉𝒕 − (𝒉𝒕 − 𝒉) )
Where, 𝑡𝑓 =Time taken to fill up the mould
𝐴𝑔and 𝐴𝑚=Cross sectional area
of the gate and mould
Fluidity of Molten Metal
• The capability of molten metal to fill mould cavities iscalled fluidity
• It consists of two basic factors:
(1) characteristics of the molten metal and
(2) casting parameters
Characteristics of the Molten Metal
• Viscosity : As viscosity and its sensitivity to temperature (viscosity index)increase, fluidity decreases.
• Surface Tension: A high surface tension of the liquid metal reducesfluidity.
• Inclusions : Because they are insoluble, inclusions can have a significantadverse effect on fluidity
• Solidification Pattern of the Alloy : The manner in which solidification takes place can influence fluidity
Fluidity of Molten Metal (Cont…)
Casting Parameters
• Mould Design : The design and dimensions of the sprue, runners, andrisers all influence fluidity
• Mould Material and its Surface Characteristics : The higher thethermal conductivity of the mould and the rougher its surfaces, thelower the fluidity of the molten metal.
• Degree of Superheat : Superheat (defined as the increment oftemperature of an alloy above its melting point) improves fluidity bydelaying solidification
• Rate of Pouring : The slower the rate of pouring molten metal into themould, the lower the fluidity because of the higher rate of coolingwhen poured slowly
• Heat Transfer. This factor directly affects the viscosity of the liquidmetal
Solidification Time
• The solidification time is a function of the volume of a casting and its
surface area (Chvorinov’s rule):
𝑺𝒐𝒍𝒊𝒅𝒊𝒇𝒊𝒄𝒂𝒕𝒊𝒐𝒏 𝒕𝒊𝒎𝒆 = 𝑪𝑽𝒐𝒍𝒖𝒎𝒆
𝑺𝒖𝒓𝒇𝒂𝒄𝒆 𝑨𝒕𝒓𝒆𝒂
𝒏
• where C is a constant that reflects
(a) the mould material
(b) the metal properties (including latent heat), and
(c) the temperature
• The parameter n has a value between 1.5 and 2,
• But usually is taken as 2
Caine’s Method
• Method for determining riser size based on experimentallydetermined hyperbolic relationship between relative freezingtimes and volumes of casting and riser
• According to Caine if the casting solidifies infinitely rapid, theriser volume should be equal to solidification shrinkage and ifthe riser and casting solidify at the same rate, the riser should beinfinitely large
• Relative freezing time or freezing ratio (𝑅𝑓) is defined as
𝑅𝑓 =
𝐴𝑉
𝑐𝑎𝑠𝑡𝑖𝑛𝑔
𝐴𝑉
𝑟𝑖𝑠𝑒𝑟
Caine’s Method (Cont…)
• Volume ratio (𝑅𝑣) is given by
𝑅𝑣 =𝑉𝑟𝑖𝑠𝑒𝑟𝑉𝑐𝑎𝑠𝑡𝑖𝑛𝑔
Caine’s formula
𝑹𝒇 =α
𝑹𝒗−𝒃+ 𝒄
Where, α= Freezing characteristics constant for the metal
b= Contraction ratio from liquid to solid
c= Relative freezing rate of riser and casting
Defects in Castings
Defects in Castings (Cont…)
Defects in Castings (Cont…)
1. Metallic projections, consisting of fins, flash, or projections suchas swells and rough surfaces
2. Cavities, consisting of rounded or rough internal or exposedcavities including blowholes, pinholes, and shrinkage cavities
3. Discontinuities, such as cracks, cold or hot tearing, and cold shuts.
• If the solidifying metal is constrained from shrinking freely,cracking and tearing may occur
• Cold shut is an interface in a casting that lacks complete fusionbecause of the meeting of two streams of liquid metal fromdifferent gates.
4. Defective surface, such as surface folds, laps, scars, adhering sandlayers, and oxide scale.
Defects in Castings (Cont…)
5. Incomplete casting, such as misruns (due to premature
solidification), insufficient volume of the metal poured, and runout
(due to loss of metal)
6. Incorrect dimensions or shape, due to factors such as improper
shrinkage allowance, pattern-mounting error, irregular contraction,
deformed pattern, or warped casting
7. Inclusions, Which form during melting, solidification, and
moulding; these are generally non-metallic
Design Considerations for Castings
There are two types of design issues in casting:
(a) Geometric features, tolerances, etc., that should be incorporated
into the part
(b) Mould features that are needed to produce the desired casting
Steps involved in design of casting
1. Design the part so that the shape is cast easily
2. Select a casting process and a material suitable for the part, size, required production volume, mechanical properties, and so on.
3. Locate the parting line of the mould in the part.
4. Locate and design the gates to allow uniform feeding of the mould cavity with molten metal.
5. Select an appropriate runner geometry for the system.
6. Locate mould features, such as sprue, screens, and risers, as appropriate.
7. Make sure proper controls and good practices are in place.
Design Considerations for Castings
(Cont…)
• Corners, angles, and section thickness. Sharp corners, angles, and fillets should be avoided as much as possible, because they act as stress raisers and may cause cracking and tearing of the metal (as well as of the dies) during solidification
• Shrinkage. To avoid cracking of the casting during cooling, thereshould be allowances for shrinkage during solidification
• Draft. A small draft (taper) typically is provided in sand-mouldpatterns to enable removal of the pattern without damaging the mould
• Dimensional tolerances. Dimensional tolerances depend on theparticular casting process, size of the casting, and type of pattern used.
• Lettering and markings. It is common practice to include some formof part identification (such as lettering or corporate logos) in castings
• Finishing operations. In designing a casting, it is important toconsider the subsequent machining and finishing operations that maybe required.
Design Considerations for Castings
(Cont…)
• Selecting the Casting Process. Casting processes cannot be selectedseparately from economic considerations
• Locating the Parting Line. A part should be oriented in a mould so thatthe large portion of the casting is relatively low and the height of thecasting is minimized.
• Locating and Designing Gates. Gates are the connections between therunners and the part to be cast
• Multiple gates often are preferable and are necessary for large parts
• Gates should feed into thick sections of castings
• A fillet should be used where a gate meets a casting
• The gate closest to the sprue should be placed sufficiently far awayfrom the sprue so that the gate can be easily removed
• Runner Design. The runner is a horizontal distribution channel thataccepts molten metal from the sprue and delivers it to the gates.