2. 19ME303 MANUFACTURING TECHNOLOGY - I
OBJECTIVE:
To impart knowledge on
Important basic concepts of manufacturing processes.
Various fabrication and forming processes.
Bulk deformation processes, sheet metal and plastics manufacturing
processes.
3. 19ME303 MANUFACTURING TECHNOLOGY - I
COURSE OUTCOMES:
At the end of the course, the students will be able to
Gain an understanding and appreciation of the breadth and depth of the casting
process.
Understand about various welding techniques
Comprehend the principles of various metal forming processes.
Explore the fascinating applications of sheet metals forming.
Appreciate metal forming and material forming processes.
4. UNIT I METAL CASTING PROCESSES
Part 1:
Sand casting – Patterns: Types, allowances, materials, design – Moulding sand:
Types, properties – Core making - Solidification & Cooling - Riser and gating design –
Methods of sand testing – CO2 process - Moulding machines – Melting furnaces.
Part 2:
Special casting processes – Shell, investment casting – Pressure die casting –
Centrifugal casting – Casting defects –Inspection methods
5. UNIT II METAL JOINING PROCESSES
Part 1:
Gas welding: Types - Equipment’s – Flame characteristics – Arc welding: Equipment - Electrodes –
Coating and specifications - Resistance welding: Spot, butt, seam and percussion welding –Gas
metal arc welding – Submerged arc welding –Electro slag welding – Tungsten Inert Gas welding.
Part 2:
Principle and application of special welding processes –Plasma arc welding – Thermit welding –
Electron beam welding, Laser Beam Welding, Friction stir welding, Ultrasonic Welding – Weld defects
– Brazing and soldering process – Methods and process capabilities – Filler materials and fluxes for
all processes
6. UNIT III METAL FORMING PROCESSES
Part 1:
Hot working and cold working of metals –Forging processes – Open, impression
and closed die forging – Characteristics- Types of Forging Machines – Typical
forging operations.
Part 2:
Rolling of metals – Types of Rolling mills - Flat strip rolling – Shape rolling– Defects
in rolled parts- Wire and Rod - Tube drawing - Extrusion – Types - Equipment.
7. UNIT IV SHEET METAL PROCESSES
Part 1:
Sheet metal Characteristics - shearing, bending and drawing– Stretch forming –
Formability of sheet metal – Bending force calculations – Test methods.
Part 2:
Working principle and applications of special forming processes - Hydro forming –
Rubber pad forming – Metal spinning –Explosive forming - Magnetic pulse forming -
Peen forming - Super plastic forming.
8. UNIT V PROCESSING OF PLASTIC COMPONENTS
Types of plastics - Characteristics of the forming and shaping processes
– Moulding of Thermoplastics –Injection moulding – Plunger and screw
machines – Compression moulding - Transfer moulding –Industrial
applications –Blow moulding – Rotational moulding – Film blowing –
Extrusion - Thermoforming - Bonding of Thermoplastics -Elastomers –
Processing Reinforced plastics.
9. TEXT BOOKS:
Serope Kalpajian, Steven R.Schmid, “Manufacturing Engineering and
Technology”, Pearson Education, Eighth Edition, 2020.
P.N. Rao, “Manufacturing Technology: Foundry, Forming and Welding -
Volume 1”, Tata McGraw-Hill Publishing Limited, 2019.
10. UNIT I - Metal Casting Processes
Manufacturing
Manufacturing in its broadest sense is the process of converting raw
materials into useful products.
It includes
i) Design of the product
ii) Selection of raw materials and
iii) The sequence of processes through which the product will be
manufactured.
11. Casting
Casting is the process of producing metal parts by pouring molten metal
into the mould cavity of the required shape and allowing the metal to
solidify. The solidified metal piece is called as “casting”.
13. Two Categories of Casting Processes
Expendable mold processes: It uses an expendable mold which must be
destroyed to remove casting
Mold materials: sand, plaster, and similar materials, plus binders
Advantage: more complex shapes possible
Disadvantage: production rates often limited by time to make mold rather than
casting itself
Permanent mold processes – It uses a permanent mold which can be
used over and over to produce many castings
Made of metal (or, less commonly, a ceramic refractory material)
Advantage: higher production rates
Disadvantage: geometries limited by need to open mold
14. Capabilities and Advantages of Casting
Can create complex part geometries that can not be made by any other
process
Can create both external and internal shapes
Some casting processes are net shape; others are near net shape
Can produce very large parts (with weight more than 100 tons), like m/c
bed
Casting can be applied to shape any metal that can melt
Some casting methods are suited to mass production
Can also be applied on polymers and ceramics
15. Disadvantages of Casting
Different disadvantages for different casting processes:
Limitations on mechanical properties
Poor dimensional accuracy and surface finish for some processes; e.g.,
sand casting
Safety hazards to workers due to hot molten metals
Environmental problems
16. Parts Made by Casting
Big parts
Engine blocks and heads for automotive vehicles,
wood burning stoves, machine frames, railway wheels,
pipes, bells, pump housings
Small parts
Dental crowns, jewelry, small statues, frying pans
All varieties of metals can be cast - ferrous
and nonferrous
17. Sand Casting
Sand Casting is simply melting the metal and pouring it into a
preformed cavity, called mold, allowing the metal to solidify and then
breaking up the mold to remove casting.
In sand casting expendable molds are used.
So for each casting operation you have to form a new mold.
Most widely used casting process.
Parts ranging in size from small to very large
Production quantities from one to millions
Sand mold is used.
21. Sand Casting Mold
Flask : Supports the mould itself – Cope on top, - drag on bottom
Pouring Basin : 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 mould
cavity
Gate : the inlet to the mould cavity
Riser : Supply additional molten metal to the casting as it shrinks during
solidification. – Blind riser, Open riser
Core : inserts made from sand – to form hollow regions
Parting line : Separate cope and drag
22. Steps in Sand Casting
Mold Preparation
Melting Process
Pouring Technique (Pour molten metal into sand mold)
Solidification Process (Allow metal to solidify)
Mold Removal (Break up the mold to remove casting)
Cleaning, Finishing, and Inspection
Heat treatment of casting is sometimes required to improve
metallurgical properties
23. Mold Preparation
A cavity of desired Shape and Size is called Mold.
The cavity in the sand mold is formed by packing sand around a
pattern, separating the mold into two halves.
The mold must also contain gating and riser system.
For internal cavity, a core must be included in mold.
A new sand mold must be made for each part
24. Melting
Melting involves melting of metals and melt treatment
Melt treatment: fluxing, degassing, alloying (grain refinement and
modification)
Fluxing: To minimize the Oxide / Nitride
Degassing: To remove the O2, H2
Alloying (grain refinement and modification): Ti in Al alloys, V & Al used in Steel, Al-Ti-B
Providing Molten Material (with suitable furnace) having
Proper Temperature (to have desired fluidity, time before solidification begins)
Desired Quantity,
Acceptable Quality, and Reasonable Cost
25. Pouring
To introduce the molten metal into the mold to produce a
high quality casting that is fully dense and free of defects
by
Allowing escape of all air or gases present in the cavity or those
generated by mold metal reaction
Minimum turbulence and time for feeding the metal
Minimum pick up of impurities from atmospheric air
26. Solidification Process
It is depends on design of gating system and temperature
gradient established after pouring of melt
Castings should be designed so as to have directional
solidification (one which proceeds form one direction to
another) to avoid casting defects
Shrinkage related defects porosity and crack under excessive restraint
conditions
Hot cracks are typical examples
27. Mould Removal
Casting is taken out of the mould without any damage
Ease of casting removal depends type of mould being used
Single use molds These are destroyed after each casting and so no
serious difficulty
Multiple use molds removal of complex shape casting impose major
mould design problem”
28. Cleaning, Finishing, and Inspection of Casting
Extra material remains attached with casting in form of metal
solidified
at gate and riser and
along the mold parting lines
Mold material adhered to the casting surface
All these Material should be Removed from the Finished
Casting
Castings are inspected for defects
29. Patterns
A pattern is a replica of the object to be cast, used to prepare the cavity
into which molten material will be poured during the casting process.
Patterns used in sand casting may be made of wood, metal, plastics or
other materials.
30. Type of Patterns
Single piece or Solid pattern
Split pattern or two piece pattern
Gated pattern
Match plate pattern
Loose piece pattern
Sweep pattern
Skeleton pattern
Segmental or part pattern
Shell pattern
31. Single piece or Solid pattern
Solid pattern is made of single piece without joints, parting lines or
loose pieces. It is the simplest form of the pattern.
Its removal from sand is easy.
32. Split pattern or two piece pattern
When solid pattern is difficult for
withdrawal from the mold cavity, then
solid pattern is splited in two parts.
Split pattern is made in two pieces
which are joined at the parting line by
means of dowel pins.
The splitting at the parting line is done
to facilitate the withdrawal of the
pattern.
34. Gated pattern
In the mass production of castings, multi cavity moulds are used. Such
moulds are formed by joining a number of patterns and gates and
providing a common runner for the molten metal.
These patterns are made of metals, and metallic pieces to form gates
and runners are attached to the pattern.
35. Match plate pattern
This pattern is made in two halves and is mounted on the opposite
sides of a wooden or metallic plate, known as match plate.
The gates and runners are also attached to the plate.
This pattern is used in machine molding.
36. Loose piece pattern
Loose piece pattern is used when pattern is difficult for
withdrawal from the mould.
Loose pieces are provided on the pattern and they are the
part of pattern.
The main pattern is removed first leaving the loose piece
portion of the pattern in the mould. Finally the loose piece is
withdrawn separately leaving the intricate mould.
37. Sweep pattern
Sweep patterns are used for forming large circular moulds of symmetric
kind by revolving a sweep attached to a spindle.
Actually a sweep is a template of wood or metal and is attached to the
spindle at one edge and the other edge has a contour depending upon
the desired shape of the mould.
38. Skeleton pattern
When the castings are very large, they require very large amount of
material for making the pattern.
In such cases, a skeleton of the pattern with a strickle board is used
instead of full pattern.
The skeleton frame is filled with loam sand and the strickle board is
used to remove the excess sand.
39. Segmental or part pattern
Patterns of this type are generally used for circular castings, for
example wheel rim, gear blank etc.
Such patterns are sections of a pattern so arranged as to form a
complete mould by being moved to form each section of the mould.
The movement of segmental pattern is guided by the use of a central
pivot.
40. Shell pattern
This pattern is mainly used for making large pipe fittings.
These patterns are usually made into two pieces and joined accurately
by dowels.
This is a hollow pattern. Its outside shape is used for making the mould.
41. Pattern allowances
• A pattern is different from the casting in dimensions and
shape.
• The various allowances given in patterns to obtain the correct
size and shape in the finished casting are:
Shrinkage allowance
Finish allowance
Draft allowance
Size tolerances or shake allowance
Distortion allowance
43. Shrinkage Allowance
Almost all materials shrink when they are changed from liquid to solid
state. An allowance for this shrinkage must be made is called Shrinkage
Allowance.
It is provided to compensate for shrinkage of material
Therefore, the pattern is made oversize to allow materials and also
their shapes to shrink.
Amount of allowance depends upon type of material, its composition,
pouring temperature etc.
44. Finish or Machining allowance
In most of the cases an accurate finish cannot be obtained with casting or
moulding technique.
Therefore we have to give some allowance to machine the component in
order to finish the part.
For example, if we want to bore a hole in the casting, the hole pattern must
be made small in order to allow the material in the hole to be bored out.
Machining allowance also should be provided.
Amount of allowance depends upon size and shape of casting, type of
material, machining process to be used, degree of accuracy and surface finish
required etc.
45. Draft allowance
Taper allowance is also a positive allowance and is given on all the
vertical surfaces of pattern so that its withdrawal becomes easier.
To facilitate easy withdrawal of the pattern.
Draft is added to the exterior dimensions of a pattern.
46. Size tolerances or shake allowance
It is also known as rapping allowance.
Before withdrawing the pattern it is rapped and thereby the size of the mould
cavity increases.
Actually by rapping, the external sections move outwards increasing the size
and internal sections move inwards decreasing the size.
This movement may be insignificant in the case of small and medium size
castings, but it is significant in the case of large castings.
This allowance is kept negative and hence the pattern is made slightly
smaller in dimensions 0.5-1.0 mm.
To facilitate the enlargement of the mould that takes place during the removal
of the pattern
47. Distortion allowance
Castings of irregular shape distort during solidification because of the
metal shrinkage.
Provided on patterns whose castings tend to distort on cooling
On U, V, T, L shaped castings
48. Pattern Materials
Various factors affecting the selection of pattern material are:
Accuracy and surface finish requirements.
For getting high accuracy and good surface finish, metal and plastic patterns are used.
Type of casting process and moulding method.
For machine moulding, metal patterns are used. For loam moulding plaster of Paris is used. White metal patterns
are also used for die-casting processes.
Possibility of frequent design changes.
Economically viable materials should be used such as wood.
Number of castings to be produced.
When large number of castings are produced repeatedly, metal patterns are preferred.
Intricacy of the casting to be produced.
For making complicated shapes white metal is best suited.
49. Requirements for the pattern material
It should be :
Easily available
Easy to machine
Light in weight
Strong, hard and durable
Dimensionally stable
Repairable and reusable
Able to facilitate good surface finish after machining.
51. 1.Wood
Wood is the most popular and commonly used material for pattern making.
It is cheap, easily available in abundance, repairable and easily fabricated in various
forms using resin and glues.
It is very light and can produce highly smooth surface.
Wood can preserve its surface by application of a shellac coating for longer life of
the pattern.
But, in spite of its above qualities, it is susceptible to shrinkage and warpage and its
life is short because of the reasons that it is highly affected by moisture of the
molding sand.
Teak wood, mahogany, shisham, kail and deodar are commonly used.
52. Wood
Advantages
Wood can be easily worked.
It is light in weight.
It is easily available.
It is very cheap.
It is easy to join.
It is easy to obtain good surface
finish.
It can be easily repaired.
Disadvantages
It is susceptible to moisture.
It tends to warp.
It wears out quickly due to sand
abrasion.
It is weaker than metallic
patterns
53. 2.Metal
Metallic patterns are preferred when the number of castings required is large
enough to justify their use.
These patterns are not much affected by moisture as wooden pattern.
The wear and tear of this pattern is very less and hence posses longer life.
Moreover, metal is easier to shape the pattern with good precision, surface
finish and intricacy in shapes.
It possesses excellent strength to weight ratio.
The main disadvantages of metallic patterns are higher cost, higher weight
and tendency of rusting.
54. Cast Iron
It is cheaper, stronger, tough, and durable and can produce a smooth surface finish.
It also possesses good resistance to sand abrasion.
Advantages
It is cheap
It is easy to file and fit
It is strong
Good surface finish
Disadvantages
It is heavy
It is brittle and hence it can be easily broken
It may rust
55. Brasses and Bronzes
These are heavier and expensive than cast iron and hence are preferred for
manufacturing small castings.
They possess good strength, machinability and resistance to corrosion and wear.
Brass and bronze pattern is finding application in making match plate pattern.
Advantages
1. Better surface finish than cast iron.
2. Very thin sections can be easily casted.
Disadvantages
1. It is costly.
2. It is heavier than cast iron.
56. Aluminum Alloys
Aluminum alloy patterns are more popular and best among all the metallic patterns because of
their high lightness, good surface finish, low melting point and good strength.
They also possesses good resistance to corrosion and abrasion by sand and there by enhancing
longer life of pattern.
Advantages
1. Aluminum alloys pattern does not rust.
2. They are easy to cast.
3. They are light in weight.
4. They can be easily machined.
Disadvantages
1. They can be damaged by sharp edges.
2. They are softer than brass and cast iron.
3. Their storing and transportation needs proper care.
57. White Metal (Alloy of Antimony, Copper and Lead)
Used for making complicated and fine shapes in die-casting
production.
Advantages
1. It is best material for lining and stripping plates.
2. It has low melting point around 260°C.
3. It can be cast into narrow cavities.
Disadvantages
1. It is too soft.
2. Its storing and transportation needs proper care.
3. It wears away by sand or sharp edges.
58. 3.Plastic
Plastics are getting more popularity now a days because the patterns made
of these materials are lighter, stronger, moisture and wear resistant, non sticky
to molding sand, durable and they are not affected by the moisture of the
molding sand.
Moreover they impart very smooth surface finish on the pattern surface.
These materials are somewhat fragile, less resistant to sudden loading and
their section may need metal reinforcement.
The plastics used for this purpose are thermosetting resins. Phenolic resin
plastics are commonly used.
These are originally in liquid form and get solidified when heated to a
specified temperature.
59. 4.Plaster
This material belongs to gypsum family which can be easily cast
and worked with wooden tools and preferable for producing highly
intricate casting.
The main advantages of plaster are that it has high compressive
strength and is of high expansion setting type which compensate
for the shrinkage allowance of the casting metal.
It is also preferred for production of small size intricate castings
and making core boxes.
60. 5.Wax
Patterns made from wax are excellent for investment casting
process.
The materials used are blends of several types of waxes, and
other additives which act as polymerizing agents, stabilizers, etc.
The commonly used waxes are paraffin wax, shellac wax, bees-
wax, ceresin wax, and micro-crystalline wax.
61. Design Considerations in Casting
Design the part so that the shape is cast easily.
Select a casting process and material suitable for the size, mechanical
properties, etc.
Locate the parting line of the mold in the part.
Locate and design the gates to allow uniform feeding of the mold cavity with
molten metal.
Select an appropriate runner geometry for the system.
Locate mold features such as sprue, screens and risers, as appropriate.
Make sure proper controls and good practices are in place.
62. Design Considerations in Casting - Design of cast part
Corners, angles and section thickness: avoid using sharp corners and
angles (act as stress raisers) and may cause cracking and tearing
during solidification. Use fillets with radii ranging from 3 to 25 mm
Figure. Suggested design modifications to avoid defects in castings.
Note that sharp corners are avoided to reduce stress concentrations.
63. Design Considerations in Casting - Design of cast part
Examples of designs showing the importance of maintaining uniform cross-
sections in castings to avoid hot spots and shrinkage cavities
64. Design Considerations in Casting - Design of cast part
Sections changes in castings should be blended smoothly into each other.
Location of the largest circle that can be inscribed in a particular region is
critical so far as shrinkage cavities are concerned (a & b). Because the cooling
rate in regions with large circles is lower, they are called hot spots. These
regions can develop shrinkage cavities and porosity (c & d).
Cavities at hot spots can be eliminated by using small cores (e). It is important
to maintain (as much as possible) uniform cross sections and wall thicknesses
throughout the casting to avoid or minimize shrinkage cavities. Metal chills in
the mold can eliminate or minimize hot spots.
65. Figure- The use of metal padding (chills) to increase the rate
of cooling in thick regions in a casting to avoid shrinkage
cavities
Figure - Examples of design modifications to avoid
shrinkage cavities in castings.
Design Considerations in Casting - Design of cast part
66. Design Considerations in Casting - Design of cast part
Flat areas: large flat areas (plain surfaces) should be avoided, since they may
warp during cooling because of temperature gradients, or they develop poor
surface finish because of uneven flow of metal during pouring. To resolve this
one can break up flat surfaces with staggered ribs.
Shrinkage: pattern dimensions also should allow for shrinkage of the metal
during solidification and cooling. Allowances for shrinkage, known as
patternmaker’s shrinkage allowances, usually range from about 10 to 20
mm/m.
67. Design Considerations in Casting - Design of cast part
Draft: a small draft (taper) typically is provided in sand mold pattern to
enable removal of the pattern without damaging the mold. Drafts
generally range from 5 to 15 mm/m. Depending on the quality of the
pattern, draft angles usually range from 0.5° to 2°.
Dimensional tolerances: tolerances should be as wide as possible,
within the limits of good part performance; otherwise, the cost of the
casting increases. In commercial practices, tolerances are usually in the
range of ± 0.8 mm for small castings. For large castings, tolerances may
be as much as ± 6 mm.
68. Design Considerations in Casting - Design of cast part
Lettering and markings: it is common practice to include some form of
part identification (such lettering or corporate logos) in castings. These
features can be sunk into the casting or protrude from the surface.
Machining and finishing operations: should be taken into account. For
example, a hole to be drilled should be on a flat surface not a curved
one. Better yet, should incorporate a small dimple as a starting point.
Features to be used for clamping when machining.
69. Moulding Sand
Principal raw material used in moulding is sand
Sand is formed by the breaking up of rocks due to the action
of natural forces such as frost, wind, rain, heat and water
currents.
The principal ingredients are:
Silica sand grains
Clay
Moisture
Miscellaneous materials
70. Moulding Sand
Silica Sand grains:
Basic components of the moulding sand.
Moulding sand contains 80-90% of silica.
Obtained from quartz rocks or by decomposition of granite composed of quartz and feldspar.
Silicon oxide imparts refractoriness, chemical resistivity and permeability.
Clay:
Clay may be defined as those particles of sand (under 20 microns in diameter) that fail to settle at a
rate of 25 mm/minute, when suspended in water.
It is an important ingredient of moulding sand.
Holds the sand together.
Bonding depends on two factors – amount & quality.
Moulding sand contains 5-20% clay.
71. Moulding Sand
Moisture:
Clay imparts bonding action and strength to the moulding sand in the
presence of moisture.
2-5% of water is added to sand.
When water is added – it penetrates and forms a microfilm coating on each
particle.
Miscellaneous:
Oxides of iron, limestone, magnesia, soda and potash and other substances
are found.
Good moulding sand contains < 2% impurities.
72. Types of Moulding Sand
Moulding sands can be generally classified into:
Natural Moulding sand
Synthetic sand
Special sands
According to use, sands are classified as:
Green sand
Dry sand
Loam sand
Facing sand
Backing sand
Parting sand
Core sand
System sand
73. (i) Natural sand:
Natural sand is directly used for molding and contains 5-20% of clay as
binding material.
It needs 5-8% water for mixing before making the mold. Many natural sands
possess a wide working range of moisture and are capable of retaining
moisture content for a long time.
Its main drawback is that it is less refractory as compared to synthetic sand.
Many natural sands have weak molding properties.
These sands are reconditioned by mixing small amounts of binding materials
like bentonite to improve their properties and are known as semi-synthetic
sand.
74. (II) Synthetic Sands:
Synthetic sand consists of silica sand with or without clay, binder or moisture.
It is a formulated sand i.e. sand formed by adding different ingredients. Sand
formulations are done to get certain desired properties not possessed by
natural sand.
These sands have better casting properties like permeability and
refractoriness and are suitable for casting ferrous and non-ferrous materials.
These properties can be controlled by mixing different ingredients.
Synthetic sands are used for making heavy castings.
75. (III) Special sands
These are used when special characteristics are needed, which are not
ordinarily obtained from other sands.
Various special sands used are zircon, olivine, chamotte, chromate and
chrome-magnetite.
Zircon – Chemically zircon is zirconium silicate.
Olivine – Orthosilicate of iron and magnesium.
Chamotte – Produced by calcining high-grade fire clay at about 1100
°C and crushing it to the required grain size.
76. Green sand
Green sand is a mixture of silica sand and clay. It constitutes 18 % to
30 % clay and 6 % to 8 % water.
The water and clay present is responsible for furnishing bonds for the
green sand.
It is slightly wet when squeezed with hand. It has the ability to retain the
shape and impression given to it under pressure.
It is easily available and has low cost.
The mould which is prepared in this sand is called green sand mould.
It is commonly used for producing ferrous and non-ferrous castings.
77. Dry Sand
After making the mould in green sand, when it is dried or baked is
called dry sand.
It is suitable for making large castings.
The moulds which is prepared in dry sand is known as dry sand
moulds.
The physical composition of the dry sand is same as that of the green
sand except water.
78. Loam Sand
It is a type of moulding sand in which 50 % of clay is present.
It is mixture of sand and clay and water is present in such a quantity, to
make it a thin plastic paste.
In loam moulding patterns are not used.
It is used to produce large casting.
80. Parting Sand
Parting sand is used to prevent the sticking of green sand to the pattern
and also to allow the sand on the parting surface of the cope and drag to
separate without clinging.
It serves the same purpose as of parting dust.
It is clean clay free silica sand.
81. Facing Sand
The face of the mould is formed by facing sand.
Facing sand is used directly next to the surface of the pattern and it
comes in direct contact with the molten metal, when the molten metal is
poured into the mould.
It possesses high strength and refractoriness as it comes in contact
with the molten metal.
It is made of clay and silica sand without addition of any used sand.
82. Backing Sand
Backing sand or flour sand is used to back up facing sand.
Old and repeatedly used moulding sand is used for the backing
purpose.
It is also sometimes called black sand because of the addition of coal
dust and burning when it comes in contact with the molten metal.
83. Core Sand
The sand which is used to make core is called core sand.
It is also called as oil sand.
It is a mixture of silica sand and core oil. Core oil is mixture of linseed
oil, resin, light mineral oil and other binding materials.
For the sake of economy, pitch or flours and water may be used in
making of large cores.
84. System Sand
In mechanical sand preparation and handling units, facing sand is not
used.
The sand which is used is cleaned and reactivated by adding of water,
binder and special additives.
And the sand we get through this is called system sand.
System sand is used to fill the whole flask in the mechanical foundries
where machine moulding is employed.
The mould made with this sand has high strength, permeability and
refractoriness.
85. Properties of Moulding sand
The moulding sand should possess the following properties:
Porosity or Permeability
Flowability
Refractoriness
Chemical Resistivity
Green strength
Dry strength
Hot strength
Cohesiveness or strength
Collapsibility
Adhesiveness
86. Properties of Moulding sand
Permeability:
It is also termed as porosity of the molding sand in order to allow the escape of any air,
gases or moisture present or generated in the mold when the molten metal is poured into it.
All these gaseous generated during pouring and solidification process must escape
otherwise the casting becomes defective.
Permeability is a function of grain size, grain shape, and moisture and clay contents in the
molding sand.
Flowability or plasticity:
It is the ability of the sand to get compacted and behave like a fluid.
It will flow uniformly to all portions of pattern when rammed and distribute the ramming
pressure evenly all around in all directions.
Flowability increases as clay and water content increases.
87. Properties of Moulding sand
Refractoriness:
Refractoriness is defined as the ability of molding sand to withstand high temperatures
without breaking down or fusing thus facilitating to get sound casting.
It is a highly important characteristic of molding sands.
Molding sand with poor refractoriness may burn on to the casting surface and no smooth
casting surface can be obtained.
The degree of refractoriness depends on the SiO2 i.e. quartz content, and the shape and
grain size of the particle.
Chemical Resistivity:
The moulding sand should not chemically react with the metallic mould or moulding box.
Otherwise the casting will be distorted.
88. Properties of Moulding sand
Green strength:
The moulding sand that contains moisture is termed as green sand. It should have enough
so that the constructedmould retains its shape
Dry strength
When the moisture in the moulding sand is completely expelled, it is called dry sand. When
the molten metal is poured into mould, the sand around the mould cavity converted into dry
sand as the moisture in the sand immediately evaporates due to heat in the molten metal. At
this stage, it should retain the mould cavity and withstand the metallostatic forces.
Hot strength
After all the moisture is eliminated, the sand would reach a high temperature when the
metal in the mould is still in the liquid state. The strength of the sand is required to hold the
shape of the mould cavity is called hot strength.
89. Properties of Moulding sand
Cohesiveness or strength:
This is the ability of the sand to stick together.
When the sand is rammed, the sand particles stick with each other and do not collapse
when the moulding box is removed for pouring.
It depends on the moisture content, grain size and shape.
Collapsibility:
After the molten metal in the mold gets solidified, the sand mold must be collapsible so that
free contraction of the metal occurs and this would naturally avoid the tearing or cracking of
the contracting metal.
Adhesiveness:
It is property of molding sand to get stick or adhere with foreign material such sticking of
molding sand with inner wall of molding box.
90. Sand Additives
Additives are used to develop special properties in the mould and their effect on castings.
Facing material:
The objective of using face material is to obtain smooth surface on the casting. The various materials
used for this purpose are, different forms of carbon, charcoal, gas carbon, coke dust, black lead,
graphite.
These materials are applied by : Mixing with moulding sand, painted with a brush and applied as a
spray.
Fire clay:
It offers a good bond when mixed with burnt sand. It is hydrated aluminium silicate from the same
source as that of sand.
Clay wash:
It is mixture of fire clay and water. It is used where strong bond is required.
91. Sand Additives
Parting materials:
This prevents the moulding sand from adhering to the moulding box or
to the pattern.
Non-silica parting compund is made from powdered phosphate rock.
Binders:
Flours, resin, linseed oil, cereal products, dextrin and molasses are
typical binders.
They increase air setting strength, toughness and collapsibility and
prevent sand from drying rapidly.
92. Core Making
Many cast parts have interior holes (hollow parts), or other cavities in their shape that are
not directly accessible from either piece of the mould.
Such interior holes are generated by inserts called cores.
Cores are made by baking sand with some binder so that they retain their shape when
handled.
Binders added to the sand are linseed oil, phenol, bentonite, urea and water.
To improve the properties of the sand, additives such as pitch corn flour, straw, graphite,
cow dung and sea coal are also added.
The mould is assembled by placing the core into the cavity of the drag.
The cope is placed on top of this and the mould is locked.
After the casting is done, the sand is shaken off, the core pulled away and usually broken
off.
93. Core Sand
The sand which is used to make core is called core sand.
It is also called as oil sand.
It is a mixture of silica sand and core oil. Core oil is mixture of linseed
oil, resin, light mineral oil and other binding materials.
For the sake of economy, pitch or flours and water may be used in
making of large cores.
95. Green Sand Core:
When a pattern leaves a core as a part of the mould, that body of sand used to
make the core is called green sand core as this core is formed by the pattern itself.
The green sand core is made out of the same sand as the mould and is suitable for
vertical mouldings only.
96. Horizontal core:
This core is positioned horizontally
in the mould and is commonly used
in foundries.
It is usually cylindrical in shape.
It may also have other shapes
depending on the cavity needed.
It is seated in the mould cavities
made by the core prints of the
pattern.
97. Vertical core:
This core is placed vertically in
the mould.
The upper end of the core is
forced into the cope and the
lower end into the drag.
On the cope, the core needs
more taper (15º) so that it does
not damage the mould in the
cope while the cope and drag are
assembled.
98. Balanced core:
This core is supported and balanced at one end only.
It extends horizontally in the mould.
This core needs only one core print and produces an opening at only
one side of the casting.
99. Hanging & cover core and Wing Core
Hanging and cover core:
This core hangs from the cope.
It is supported from the top and hangs
vertically in the mould.
It has no support at its bottom.
This core is also known as cover core as it
covers the mould.
Wing core:
This core is used to form the hole or recess in
the casting which is not in line with the parting
line.
Depending upon the usage, the core may also
be called drop core, tail core, chair core or
saddle core.
100. Ram-up core & Kiss core
Ram-up core:
This core is set in the mould with the pattern
before ramming.
It is used when the core detail is located in
an inaccessible position.
When the pattern is not provided with core
prints and no seat is available for the core to
rest, the core is held in position between the
cope and drag simply due to the pressure of the
cope.
Such a core is know as kiss core.
101. Core Prints
A Core must be supported in the mould cavity. Wherever possible, this
is done by providing core prints.
Core prints are extensions of the core which rest in similar extensions
of the mould cavity so that core remains supported in the mould cavity
without the core falling to the bottom of the cavity.
Core prints may be of horizontal, vertical, balanced, wing and core
types.
102.
103. Core Prints
The print design depends on the direction of the core axis and the number of openings.
Each opening corresponds to a separate print for core to support.
Major considerations in core print design are:
The print must balance the body, so that core stays in place during mould assembly.
Must withstand the buoyancy force of the metal and not get crushed.
Must not shift during mould filling.
Should minimize the deflection of the core.
Should maximize the heat transfer from the core to the mould.
Should allow the internal gases generated in the core to escape to the mould.
Asymmetrical hole should have foolproof prints to prevent incorrect assembly.
The prints of adjacent cores may be combined into one.
104. Core Boxes
Any kind of hollowness in form of holes and recesses in castings is obtained by the
use of cores.
Cores are made by means of core boxes comprising of either single or in two parts.
Core boxes are generally made of wood or metal and are of several types.
The main types of core box are
Half core box,
Dump core box,
Split core box,
Strickle core box,
Right and left hand core box
Gang core box.
105. Half core box
This is the most common type of core box. The two identical halves of a
symmetrical core prepared in the half core box.
106. Dump core box
Dump core box is similar in construction
to half core box.
The cores produced do not require
pasting, rather they are complete by
themselves.
If the core produced is in the shape of a
slab, then it is called as a slab box or a
rectangular box.
A dump core-box is used to prepare
complete core in it.
Generally cylindrical and rectangular
cores are prepared in these boxes.
107. Split core box
Split core boxes are made in
two parts.
They form the complete core
by only ramming.
The two parts of core boxes
are held in position by means
of clamps and their alignment
is maintained by means of
dowel pins and thus core is
produced
108. Gang core box
When a number of cores needed are more, a gang core box is used.
At one time many core may be made in this box
109. Right and left hand core box
Some times the cores are not symmetrical about the center line.
In such cases, right and left hand core boxes are used.
The two halves of a core made in the same core box are not identical
and they cannot be pasted together.
110. Strickle core box
This type of core box is used when a core with an irregular shape is desired.
The required shape is achieved by striking off the core sand from the top of the core box with a
wooden piece, called as strickle board.
The strickle board has the same contour as that of the required core.
111. Solidification and Cooling
Solidification mechanism is essential for preventing defects due to shrinkage.
As soon as the molten metal is poured in a sand mold, the process of solidification
starts.
During solidification, cast forms develops cohesion and acquires structural
characteristics.
The mode of solidification affects the properties of the casting acquires a
metallographic structure which is determined during solidification. The
metallographic structure consists of:
Grain size, shape and orientation
Distribution of alloying elements
Underlying crystal structure and its imperfections
112. Solidification and Cooling
Volume shrinkage/volume contraction occurs during three stages:
Liquid contraction (shrinkage): liquid contraction occurs when the metal
is in liquid state.
Solidification contraction (shrinkage): solidification contraction occurs
during the change from liquid to solid
Solid contraction (shrinkage): solid contraction occurs when the metal
is solid; solid contraction occurs after solidification; solid contraction
does not influence shrinkage defects.
113. The Solidification Process
Molten material is allowed to solidify into the final shape
Casting defects occur during solidification
Gas porosity (solved by adding the vent)
Shrinkage (solved by using the riser to add the molten metal)
Two stages of solidification
Nucleation
Growth
114. The Solidification Process
A metal in molten condition possesses high energy
As the molten metal cools, it loses energy to form crystals
Since heat loss is more rapid near the mold walls than any other place, the first metal
crystallites called ‘nuclei’ form here.
Nuclei formed as above tend to grow at the second stage of solidification.
The crystal growth occurs in a dendrite manner.
Dendrite growth takes place by the evolution of small arms on the original branches of
individual dendrites:
Slow cooling makes the dendrites to grow long whereas fast cooling causes short dendrite
growth.
Since eventually dendrites become grains, slow cooling results in large grain structure and
fast cooling in small grain structure in the solidified metal.
115. The Solidification Process
As solidification proceeds, more and more arms grow on an existing
dendrite and also more and more dendrites form until the whole melt is
crystallized.
116. Solidification of Pure Metals
Pure metals generally posses
Excellent thermal and electrical conductivity(e.g. Cu and Al).
Higher ductility, higher melting point, lower yield point and tensile strength, and
Better corrosion resistance, as compared to alloys.
As metals posses high melting points, they exhibit certain difficulties in
casting,
Difficulties during pouring
Occurrence of several metal-mold reactions
Greater tendency toward cracking
Their mode of solidification, which may produce defective castings.
Above freezing point the metal is liquid and below freezing point, it is in solid.
117. Solidification of Pure Metals
A pure metal solidifies at a constant temperature equal to its freezing point (same as melting point).
119. The solidification occurs at prescribed time duration.
Local solidification time: time between freezing start and freezing completion. In this time, the molten
metal heat of fusion is delivered into mould.
Total solidification time: time between pouring and final solidification
First liquid cooling occurs till freezing starts. Then solidification occurs for a time duration, till freezing
completes. Even after solidification is over, solid cooling occurs at a particular rate as shown in the
figure.
The grain structure in pure metals depends on the heat transfer into the mold and thermal properties
of the metal.
The mold wall acts as a chiller and hence solidification starts first in the molten metal closer to the
mold wall. A thin skin of solid metal is first formed near the mold wall.
The solidification continues inwards towards the mold center. The initial skin formed near the mold
wall has gone through fast removal of heat and hence fine, equiaxed and randomly oriented grains
are formed.
120. When the solidification continues inwardly, heat is removed through the mold wall and thin solid
skin. Here the grains grow as needles with preferred orientation. As these needles enlarge, side
branches develop, and as these branches grow, further branches form at right angles to the first
branches. This type of grain growth is referred to as dendritic growth.
121. Solidification of alloys
Alloyed metals possess:
Higher tensile strengths
Better high temperature strengths
Better corrosion resistance
Improved machinability and workability
Lower melting points
Improved castability
122. Solidification of alloys
Mushy zone formation
In alloys, solidification will not occur at a particular temperature. It happens at a temperature range.
This range depends on the alloy composition.
123. Solidification of alloys
Solidification occurs between liquidus line and solidus line. Freezing starts at liquidus
temperature and ends at solidus temperature. A skin layer is formed at the mold end and the
dendrites grow in a similar fashion normal to the mold wall.
However, because of the temperature difference between the liquidus and solidus line, the
nature of the dendritic growth is such that an advancing zone is formed in which both liquid
and solid metal exist together.
The solid portions are the dendrite structures that have formed sufficiently to hold small
regions of liquid metal in the matrix.
This solid–liquid region has a soft consistency and hence called the mushy zone.
Depending on the conditions of solidification, the mushy zone can be a narrow zone, or it
can exist throughout the casting.
Slowly the liquid islands solidify as the temperature of the casting goes down to the solidus.
124. As solidification continues and the dendrites grow, an imbalance in
composition between the solidified metal and the remaining molten
metal will develop. This composition imbalance will finally result in the
segregation of the elements.
Figure - Characteristic grain structure in an alloy casting, showing
segregation of alloying components in center of casting
125. Solidification Time
Solidification takes time
Total solidification time TST = time required for casting to solidify after
pouring
TST depends on size and shape of casting by relationship known as
Chvorinov's Rule
where TST = total solidification time; V = volume of the casting; A = surface
area of casting; n = exponent usually taken to have a value = 2; and Cm is
mold constant
126.
127.
128.
129.
130.
131.
132.
133. Type of Gates
Figure. Three types of gating system. (a) Top gating, (b) bottom gating and (c) parting line gating.
134.
135.
136.
137.
138.
139.
140.
141.
142.
143.
144.
145.
146.
147.
148.
149.
150.
151. SLAG TRAP SYSTEM:-
Runner extension:-
This is a blind alley ahead the gates.
The clean metal will go into the mould after filling up the runner extension in
which the slags and dross will be remained.
This should be twice the runner width.
Whirl gate:-
It utilizes the principle of centrifugal action to throw the dense metal to the
periphery and retain the lighter slag at the centre.
The entry area should be 1.5 times the exit area.
152.
153.
154.
155.
156. Sand testing
Molding sand and core sand depend upon shape, size, composition and distribution of
sand grains, amount of clay, moisture and additives.
The increase in demand for good surface finish and higher accuracy in castings
necessitates certainty in the quality of mold and core sands.
Sand testing often allows the use of less expensive local sands.
It also ensures reliable sand mixing and enables a utilization of the inherent properties
of molding sand.
Sand testing on delivery will immediately detect any variation from the standard quality,
and adjustment of the sand mixture to specific requirements so that the casting defects
can be minimized.
Thus sand testing is one of the dominating factors in foundry and pays for itself.
157. Sand testing methods
The following tests are performed to judge the molding and
casting characteristics of foundry sands:
Grain fineness test
Moisture content test
Clay-content test
Permeability test
Compression strength test
Mould and core hardness test
158. Grain Fineness Test
For carry out grain fineness test a sample of dry silica sand
weighing 50 gms free from clay is placed on a top most sieve
bearing U.S. series equivalent number 6.
A set of eleven sieves having U.S. Bureau of standard
meshes 6, 12, 20, 30, 40, 50, 70, 100, 140, 200 and 270 are
mounted on a mechanical shaker.
159. Grain Fineness Test
The series are placed in order of fineness from top to bottom. The free silica
sand sample is shaked in a mechanical shaker for about 15 minutes.
After this weight of sand retained in each sieve is obtained and the retained
sand in each sieve is multiplied by 2 which gives % of weight retained by each
sieve.
The same is further multiplied by a multiplying factor and total product is
obtained.
It is then divided by total % sand retained by different sieves which will give
G.F.N.
GFN = Total Product / Total % of sand retained on each sieve
160.
161. Moisture Content Test
The moisture content of the molding sand mixture may determined by drying a weighed
amount of 20 to 50 grams of molding sand to a constant temperature up to 100°C in a
oven for about one hour.
It is then cooled to a room temperature and then reweighing the molding sand.
The moisture content in molding sand is thus evaporated.
The loss in weight of molding sand due to loss of moisture, gives the amount of
moisture which can be expressed as a percentage of the original sand sample.
As the conventional method is time consuming, direct reading instruments are often
used to quickly assess the moisture content such as :
Moisture teller and moisture meter. Percentage of moisture content =
(W1-W2)/(W1) %
Where, W1-Weight of the sand before drying,
W2-Weight of the sand after drying.
162. Moisture Content Test
Moisture Teller:
(i) The instrument blows hot air through the moist sand in a pan, the bottom of which is made of 500-mesh metal screen.
The sand sample is spread over the pan in a thin layer, and hot air is blown for a period of approximately 3 minutes through a 50
gm sample.
The moisture is effectively removed and a precision balance determines the loss in weight of the sample.
(ii) Another ‘moisture teller’ utilizes calcium carbide to measure the moisture content.
A measured amount of calcium carbide in a container along with a separate cap consisting of measuring quantity of moulding
sand is kept in the moisture teller.
The apparatus is then shaken vigorously such that following reaction takes place:
CaC2 + 2H2O C2H2 + Ca(OH)2
The amount of C2H2 produced is proportional to the content of the moisture.
The moisture content is directly measured from a calibrated scale on the instrument.
163. Moisture Content Test
Moisture meter:
This instrument enables the user to determine the moisture content of a
sand almost instantly.
The two arms of the instrument are inserted in the given sample of
moulding sand held in a container, and a small electric current, supplied
from a dry battery is passed through the moist sand.
The wetter the sand, the more easily the current flows, and the
deflection of an indicator gives a measure of the moisture content.
164. Clay content Test
Dry thoroughly a small quantity of prepared moulding sand.
Separate 50 grams of dry moulding sand and transfer the same to a wash bottle. Add in it 475 cc of distilled
water and 25 cc of a 3% NaOH solution.
Using a rapid sand stirrer, agitate the whole mixture for about 10 minutes.
Fill the wash bottle with water up to the mark indicated on the same.
After the sand etc. has settled for about 10 minutes, siphon out the water, from the wash bottle.
Clay is dissolved in water and gets removed along with the same.
To the sand thus left in the wash bottle, add more water, stir the solution again and let the sand settle down.
Repeat above step till the water over the settled sand is clean. This assures that the whole of the clay has
been removed from the sand.
Dry the settled down sand.
The clay content can be determined from the difference in weights of the initial and final sand samples.
165. Permeability Test
Permeability is measured by the quantity of air that passes through a
standard specimen of sand under the given pressure (p) at prescribed time (t).
In this test a standard rammed 5.08 x 5.08 cm2 size test-piece is used.
The equipment consists of a water tank on which an inverted bell or air holder
is floating.
The specimen tube is connected to a manometer and air holder by tube.
Mercury is used at the bottom of the specimen to provide an airtight seal.
When the pressure in the manometer reaches 10 gm/cm2 it is closed.
Permeability is defined as the volume of air (v) 2000 cc air that will pass under
pressure (p) of 10 gm/cm2 through 5.08 cm2 area (a) specimen.
P = vh/pat
Where, P = permeability
v = volume of air passing through the
specimen in c.c.
h = height of specimen in cm
p = pressure of air in gm/cm2
a = cross-sectional area of the specimen in
cm2
t = time in minutes.
166. Compression strength Test
The compression strength of the molding sand is determined by placing standard specimen at
specified location and the load is applied on the standard sand specimen to compress it by
uniform increasing load using rotating the hand wheel of compression strength testing setup.
As soon as the sand specimen fractures for break, the compression strength is measured by the
manometer.
Mould and Core Hardness Test:
Hardness of the mould or core can be tested with the help of an indentation hardness tester.
The instrument resembles with dial indicator, carries a spring loaded spherical indenter which
penetrates into mould surface.
The depth of penetration with respect to the flat reference surface of the tester is indicated on the
dial of the instrument in terms of hardness unit (calibrated).
179. Classification of moulding machines according to the
method of removing the pattern from the mould:
Straight-draw moulding machine
Stripping-plate moulding machine
Turn-over moulding machine
180. Straight-draw moulding machine:
In this machine, the pattern is fixed
on the pattern plate on the table, and
the flask or moulding box is placed
over it and filed with sand.
It is then roughly rammed round the
edges of the box.
The squeeze head is next swung
over in position and it squeezes the
mould.
The flask is then lifted from the
pattern by stripping pins.
181. Stripping-plate moulding machine:
The stripping plate is arranged between
the flask and pattern plate.
The stripping plate has a recess whose
contours equal those of the pattern.
When the mould is ready the pattern is
withdrawn from the mould downwards
through the stripping plate, which
supports the mould when the pattern is
removed.
1. Pattern
2. Moulding box
3. Pattern plate
4. Stripping plate
182. Turn-over moulding machine
This is used for large size, high moulds,
having parts which might easily break away.
The flask rests on the pattern plate during
the moulding operation.
Then the flask together with the work table
is rotated 180º and pins lift the table
together with the pattern out of the mould.
192. Melting Furnaces
Melting furnaces used in the foundry industry are of many diverse
configurations.
The selection of the melting unit is one of the most important decisions,
foundries must make with due consideration to several important
factors including:
The temperature required to melt the alloy.
The melting rate and quantity of molten metal required.
The economy of installation and operation.
Environmental and waste disposal requirements.
193. Blast Furnace
Coke Blast furnace is a type of furnace for smelting metal ore usually iron ore.
The combustion material and ore are supplied from the top while air flow is
supplied from the bottom of the chamber, so that the chemical reaction takes
place throughout the ore, not only at the surface.
This type of furnace is typically used for smelting iron ore to produce pig iron,
the raw material for wrought iron and cast iron.
The blast furnace relies on the fact that the unwanted silicon and other
impurities are lighter than the molten iron that is its main product.
The furnace is built in the form of a tall, chimney-like structure lined with
refractory bricks.
194. Blast Furnace
Process:
The iron ore with suitable amount of coke and flux is charged in the blast
furnace.
These materials are lifted through a hoisting mechanism to the top of the
furnace and charged through the double ball arrangement into the throat.
The hot air blast enters the furnace through tuyers and rises upwards.
Through the downwards moving charge as the charge is melted, the molten
metal is collected at the bottom and the slag floats over its top surface.
The normal capacity of this furnace range from 800 to 1200 tonnes of pig iron
per 24 hour.
196. Blast Furnace
Different chemical reaction take place in different parts of the blast furnace. According to the temp in those parts, the highest
temp in the furnace is at the bottom and the lowest at the top.
According to these temp range, the blast furnace can be divided into following zones.
1. Preheating Zone (From the top to gas outlet level): Temp range in this zone is 200°C to 350°C which provided only a preheating
effect on the charge and helps in evaporating the moisture content from it.
2. Reduction Zone (from the gas outlet to nearly the max cross-section level): Temp range between 350°C to 1200°C. This is further
sub-divided into two zones.
(a) Upper Reduction Zone: (350°C to 700°C)
Here, iron oxide is reduced to metallic iron by reaction with the ascending carbon monoxide. So, this zone is also called iron oxide
reduction zone.
Fe2O3 + 3CO = 2Fe + 3CO2
In this zone limestone (flux) also starts dissociating as follows:
CaCO3 = CaO + CO2
197. Blast Furnace
(b) Lower Reduction Zone (700°C to 1200°C) Here charge becomes hotter as it descends.
The decomposition of CaCO3 started earlier is completed at about 850°C. The CO2 formed due to this decomposition react with the carbon of coke to
reduce to CO.
CO2 + C = 2CO
Reduction of iron oxide is completed here Fe2O3 + 3C = 2Fe + 3CO
The calcium oxide formed by the decomposition of limestone combines all the impurities like silica and aluminium with it to form the slag.
The higher temp of about 1200°C also causes the reduction of other oxides in the ore like P2O5, MnO2 and SiO2 etc, into respective free element P,
Mn and Si. They are absorbed by the metal formed(Fe) as above.
As a result of all these, the melting point of iron is lowered and it starts melting at about 1200°C instead of 1530°C (The melting point of pure iron)
(3) Fusion Zone(1200°C to 1600°C): Evidently this parts carries highest temperature and in this region the melting of charge is finally completed.
The iron get superheated here. The slag and molten metal are tapped separately from the furnace.
The molten metal is poured into the moulds. Where it solidified to form what is known as pig iron.
198. Cupola Furnace
For many years, the cupola was the primary method of melting
used in iron foundries.
The cupola furnace has several unique characteristics which are
responsible for its widespread use as a melting unit for cast iron.
Cupola furnace is employed for melting scrap metal or pig iron
for production of various cast irons.
The main considerations in selection of cupolas are melting
capacity, diameter of shell without lining or with lining, spark
arrester.
199. Cupola Furnace
Shape
A typical cupola melting furnace consists of a water-cooled vertical cylinder which is lined with
refractory material.
Construction
The construction of a conventional cupola consists of a vertical steel shell which is lined with a
refractory brick.
The charge is introduced into the furnace body by means of an opening approximately half way
up the vertical shaft.
The charge consists of alternate layers of the metal to be melted, coke fuel and limestone flux.
The fuel is burnt in air which is introduced through tuyers positioned above the hearth. The hot
gases generated in the lower part of the shaft ascend and preheat the descending charge.
200. Various Zones of Cupola Furnace
Various numbers of chemical reactions take place in different zones of cupola. The construction and different zones of cupola are :
1. Well
The space between the bottom of the tuyers and the sand bed inside the cylindrical shell of the cupola is called as well of the cupola.
As the melting occurs, the molten metal is get collected in this portion before tapping out.
2. Combustion zone
The combustion zone of Cupola is also called as oxidizing zone. It is located between the upper of the tuyers and a theoretical level above it.
The total height of this zone is normally from 15 cm to 30 cm.
A temperature of about 1540°C to 1870°C is achieved in this zone. Few reactions takes place in this zone these are represented as:
C + O2 → CO2 + Heat
Si + O2 → SiO2 + Heat
2Mn + O2 → 2MnO + Heat
201. 3. Reducing zone
Reducing zone of Cupola is also known as the protective zone which is located between the upper level of the combustion zone and the upper level
of the coke bed.
The temperature falls from combustion zone temperature to about 1200°C at the top of this zone.
The important chemical reaction takes place in this zone which is given as under.
CO2 + C (coke) → 2CO + Heat
4. Melting zone
The lower layer of metal charge above the lower layer of coke bed is termed as melting zone of Cupola.
The metal charge starts melting in this zone and trickles down through coke bed and gets collected in the well.
3Fe + 2CO → Fe3C + CO2
5. Preheating zone
Preheating zone starts from the upper end of the melting zone and continues up to the bottom level of the charging door.
This zone contains a number of alternate layers of coke bed, flux and metal charge. The main objective of this zone is to preheat the charges from
room temperature to about 1090°C before entering the metal charge to the melting zone.
202. Cupola Furnace
There are four stages in the cupola melting process:
Preparing the cupola
Firing the cupola
Charging the cupola
Before the blower is started, the furnace is uniformly pre-heated and the metal and coke charges,
lying in alternate layers, are sufficiently heated up.
The cover plates are positioned suitably and the blower is started.
The height of coke charge in the cupola in each layer varies generally from 10 to 15 cms.
The requirement of flux to the metal charge depends upon the quality of the charged metal and
scarp, the composition of the coke and the amount of ash content present in the coke.
Tapping Metal
203. Working of Cupola Furnace
The charge, consisting of metal, alloying ingredients, limestone, and coal coke for fuel and carbonization (8-16% of the metal charge), is fed in
alternating layers through an opening in the cylinder.
Air enters the bottom through tuyers extending a short distance into the interior of the cylinder. The air inflow often contains enhanced oxygen levels.
Coke is consumed. The hot exhaust gases rise up through the charge, preheating it. This increases the energy efficiency of the furnace. The charge
drops and is melted.
Although air is fed into the furnace, the environment is a reducing one. Burning of coke under reducing conditions raises the carbon content of the
metal charge to the casting specifications.
As the material is consumed, additional charges can be added to the furnace.
A continuous flow of iron emerges from the bottom of the furnace.
Depending on the size of the furnace, the flow rate can be as high as 100 tones per hour. As the metal melts it is refined to some extent, which
removes contaminants. This makes this process more suitable than electric furnaces for dirty charges.
A hole higher than the tap allows slag to be drawn off.
The exhaust gases emerge from the top of the cupola. Emission control technology is used to treat the emissions to meet environmental standards.
Hinged doors at the bottom allow the furnace to be emptied when not in use.
204. Cupola Furnace
Advantages:
Continuous in operation
High melt rates
Ease of operation
Relatively low operating costs
Less floor space requirements comparing with those furnaces with same capacity.
Limitations:
Since molten iron and coke are in contact with each other, certain elements like si, Mn are lost and others like sulphur are picked up. This changes
the final analysis of molten metal.
Close temperature control is difficult to maintain.
Poor cleanliness
Preparation required during every start.
Applications:
Used for melting iron and ferro alloys.
205. Open Hearth Furnace
In this furnace excess carbon and other impurities are burnt
out of pig iron to produce steel.
Normal fuels and furnaces were insufficient to manufacture
steel with its high melting point, the open hearth furnace was
developed.
206. Open Hearth Furnace
In this furnace, waste heat recovery is enough to save 70-80% of the fuel.
This furnace operates at a high temperature by using regenerative pre-heating
of fuel and air for combustion.
In regenerative pre-heating, the exhaust gases from the furnace are pumped
into a chamber containing bricks, where heat is transferred from the gases to
bricks.
The flow of the furnace is reversed so that fuel and air pass through the
chamber and are heated by the bricks.
Through this method, an open-hearth furnace can reach temperatures high
enough to melt steel.
207. Crucible Furnace
Crucible furnaces are small capacity typically used for small melting
applications.
Crucible furnace is suitable for the batch type foundries where the metal
requirement is intermittent.
The metal is placed in a crucible which is made of clay and graphite.
The charge is heated via conduction of heat through the walls of the
crucible.
The energy is applied indirectly to the metal. The heating of crucible is
done by coke, oil or gas.
208. Crucible Furnace
Coke-Fired Furnace
Primarily used for non-ferrous metals
Furnace is of a cylindrical shape
Also known as pit furnace
Preparation involves: first to make a deep bed of coke in the
furnace
Burn the coke till it attains the state of maximum combustion
Insert the crucible in the coke bed
Remove the crucible when the melt reaches to desired
temperature
Oil-Fired Furnace.
Primarily used for non-ferrous metals
Furnace is of a cylindrical shape
Advantages include: no wastage of fuel
Less contamination of the metal
Absorption of water vapor is least as the metal melts inside
the closed metallic furnace.
Crucible furnaces are classified according to the method of
removing the metal from the crucible :
Tilting furnace
Lift-out furnace
Pit furnace
209. Electric Arc Furnaces
This furnace can be described as a furnace heating charged materials by the way of an electric arc.
Capacity
These furnaces exist in all the sizes -right, from the smallest one having a capacity of around 1 ton to the largest one
having a capacity of 400 tons (approx.)
Furnace temperature
The electric arc furnace can have temperatures risen up to 1800 Celsius.
Construction
An electric arc furnace used for steelmaking consists of a refractory-lined vessel, usually water-cooled in larger sizes,
covered with a retractable roof, and through which one or more graphite electrodes enter the furnace. The furnace is primarily
split into three sections:
The shell, which consists of the sidewalls and lower steel 'bowl';
The hearth, which consists of the refractory that lines the lower bowl;
The roof, which may be refractory-lined or water-cooled, and can be shaped as a section of a sphere. The roof also
supports the refractory delta in its center, through which one or more graphite electrodes enter.
210. Direct Electric Arc Furnace
A typical alternating current furnace has three electrodes.
They may be lined with acid or basic refractories.
Electrodes are round in section, and typically in segments with threaded couplings, so that as the
electrodes wear, new segments can be added.
The arc forms between the charged material and the electrode, the charge is heated both by current
passing through the charge & by the radiant energy evolved by the arc.
The electrodes of direct arc furnace are automatically raised and lowered by a positioning system.
The regulating system maintains approximately constant current and power input during the melting of
the charge.
The furnace is built on a tilting platform so that the liquid steel can be poured into another vessel
for transport. The operation of tilting the furnace to pour molten steel is called "tapping".
211. Indirect Electric Arc Furnace
Indirect arc furnaces generally consists of a horizontal, barrel-shaped steel shell lined
with refractories.
Melting is effected by the arcing between two horizontally opposed carbon electrodes.
Heating is via radiation from the arc to the charge.
The barrel-shaped shell is designed to rotated and reverse through aprrox 180º.
These are suitable for melting a wide range of alloys but are particularly popular for the
production of copper base alloys.
The units operate on a single-phase power supply and hence, the size is usually limited
to relatively small units.
212. Induction furnace
Induction heating is a heating method. The heating by the
induction method occurs when an electrically conductive
material is placed in a varying magnetic field.
Induction heating is a rapid form of heating in which a current is
induced directly into the part being heated.
Induction heating is a non-contact form of heating.
The induction heating power supply sends alternating current
through the induction coil, which generates a magnetic field.
213. Induction furnace
Induction furnaces work on the principle of a transformer.
An alternative electromagnetic field induces eddy currents in the metal which converts the
electric energy to heat without any physical contact between the induction coil and the work
piece.
The furnace contains a crucible surrounded by a water cooled copper coil. The coil is called
primary coil to which a high frequency current is supplied.
By induction secondary currents, called eddy currents are produced in the crucible. High
temperature can be obtained by this method.
Induction furnaces are of two types: cored furnace and coreless furnace.
Cored furnaces are used almost exclusively as holding furnaces. In cored furnace the
electromagnetic field heats the metal between two coils.
Coreless furnaces heat the metal via an external primary coil.
214. Shell moulding
This process is known as croning
process after the name of its inventor.
In this process, resin-bonded sand is
used.
The mould is formed from a mixture of
fine sand and thermosetting resin
binder such as phenol, urea and furan
placed against heated metal pattern
preferably grey cast iron.
217. Shell moulding
When the mixture is heated the resin cures causing the sand to adhere to each other, forming a
strong shell which confirms to the shape of the pattern and constitutes half of the mould.
The other half is also made in this way.
The temperature of the metal pattern is about 200-300 °C, which is the melting point of resin.
Then a silicon-parting agent is sprayed on the surface, the resin and sand mixture is deposited on the
pattern by blowing or dumping.
The resin starts melting and after a few seconds it forms a uniform resin-soaked layer of sand with a
thickness of about 4-12 mm.
The thickness of the layer depends upon the heat.
Then the shell is removed and heated again in an oven for 3-5 min at 420 °C depending upon the
type of resin used.
In this way stable shell moulds are prepared.
218. Application:
Most of industrial products like
gearbox housing, connecting rod,
small size boats, truck hoods,
cylindrical head, Camshaft, valve
body etc. are made by shell moulding.
219.
220.
221.
222.
223.
224.
225. Mercast process
Mercasting / mercast process is an improvement in precision investment casting.
In this manufacturing process, the pattern is made of mercury is used to make
mould.
The mercury pattern is formed in a special aluminium mould. At normal room
temperature, the mercury is filled into the master mould.
Then the whole unit is cooled to a temperature below -38°C, eventually the mercury
gets solidify.
The frozen mercury pattern is then removed from the master mould and dipped into
series of special slurries to form a harder ceramic shell around them.
The temperature the mercury then allow to rise, the liquefied mercury then retrieved
from mould.
The obtained mould is then used for investment casting.
227. Investment casting
Application:
Typically materials that can be cast with this process are Aluminum
alloys, Bronzes, Stainless steels, Stellite etc.
Glass mold accessory castings, Valves and fittings, Gears, Levers and
Splines are some of the popular usages.
228. Die casting
A sand mould is usable for production of only one casting. It cannot be used twice.
Die is essentially a metal mould and can be used again and again.
A die is usally made in two portions. One portion is fixed and the other is movable.
Together, they contain the mould cavity in all its details. After clamping or locking the
two halves of the dies together molten metal is introduced into the dies.
If the molten metal is fed by gravity into the dies, the process is known as gravity die
casting process. On the otherhand, if the metal is forced into the dies under pressure
(e.g., a piston in a cylinder pushes the material through cylinder nozzle), the process
is called “pressure die casting”.
The material of which the dies are made, should have a melting point much higher
than the melting point of casting material.
229. Pressure die casting
Unlike permanent mold or gravity die casting, molten metal is forced into
metallic mold or die under pressure in pressure die casting.
The pressure is generally created by compressed air or hydraulically means.
The pressure varies from 70 to 5000 kg/cm2 and is maintained while the
casting solidifies.
The application of high pressure is associated with the high velocity with
which the liquid metal is injected into the die to provide a unique capacity for
the production of intricate components at a relatively low cost.
This process is called simply die casting. The die casting machine should be
properly designed to hold and operate a die under pressure smoothly.
230. High pressure die casting
High pressure die casting (HPDC) is a process where molten metal is
injected under very high pressure into premium steel molds (dies) in
order to manufacture high precision die cast products.
The die is designed to cast engineered shapes and complex features
with great accuracy and consistent replication.
There are two types of HPDC : hot chamber die casting and cold
chamber die casting.
Although there are several similarities between the two types, they exist
separately for different purposes.
231. Hot Chamber Die Casting
Hot chamber die casting is a type of die casting that uses alloys with
low melting temperatures (i.e. Zinc, tin and lead).
Using alloys with high melting temperatures would result in damage to
the gooseneck, nozzle and other components.
In a hot chamber die casting machine, the fixed die half is called a
cover die, which is mounted to a stationary platen (large plate to which
each die half is mounted) and aligns with the nozzle of the gooseneck.
The movable die half is the ejector die and is mounted to a movable
platen, which slides along tie bars.
232. Hot Chamber Die Casting
The metal is contained in an open holding pot, which is placed in the furnace and melted to the
needed temperature.
When the plunger is in the “up” position, the molten metal flows into the shot chamber. As the
plunger moves down, it forces the molten metal through a gooseneck and into the die.
The machine pushes the moving platen towards the cover die and holds it closed with great
pressure until the molten metal is injected.
The plunger remains in the “down” position to hold the pressure while the casting “cools off.”
After solidification, the plunger is retracted and the cast part is either ejected, manually removed
from the machine or pushed off the cover die.
This ejection system, which includes an ejector die and ejector pins, allows the casting to be
pushed out while releasing the die halves.
233. Cold Chamber Die Casting
Cold chamber die casting is a type of die casting that is used for alloys with high
melting temperatures (i.e. Aluminum, brass, bronze and some Magnesium alloys).
As a contrast from hot chamber die casting (pumping molten metal into the machine),
molten metal is ladled from the furnace into the shot chamber through a pouring hole.
While the general function of the cold chamber machine is similar to hot chamber, cold
chamber works with a horizontal orientation and does not have a gooseneck.
The plunger forces metal through the shot chamber into the die.
The plunger holds the pressure and retracts after solidification.
234. Centrifugal casting
In centrifugal casting process, molten metal is poured into a revolving mold and allowed to
solidify molten metal by pressure of centrifugal force.
It is employed for mass production of circular casting, as the castings produced by this process
are free from impurities.
Due to centrifugal force, the castings produced will be of high density type and of good strength.
The castings produced promote directional solidification as the colder metal (less temperature
molten metal) is thrown to outside of casting and molten metal near the axis or rotation.
The cylindrical parts and pipes for handling gases are most adoptable to this process.
(1) True centrifugal casting
(2) Semi-centrifugal casting and
(3) Centrifuged casting
237. Semi-Centrifugal Casting
It is similar to true centrifugal casting but only with a difference that a central core is
used to form the inner surface.
This casting process is generally used for articles which are more complicated than
those possible in true centrifugal casting, but are axi-symmetric in nature.
A particular shape of the casting is produced by mold and core and not by centrifugal
force.
The centrifugal force aids proper feeding and helps in producing the castings free from
porosity.
Symmetrical objects namely wheel having arms like flywheel, gears and back wheels are
produced by this process.
238. Centrifuge Casting
This casting process is generally used for producing non-
symmetrical small castings having intricate details.
A number of such small jobs are joined together by means of a
common radial runner with a central sprue on a table which is
possible in a vertical direction of mold rotation.
The spinning action is used to fill the moulds.
The products of centrifuging are of high density.
They are free of most impurities and air inclusions.
239. Ceramic-mold Casting
The ceramic-mold casting process also called cope-and-drag
investment casting, it uses refractory mold materials suitable for high-
temperature applications.
Typical parts made are impellers, cutters for machining operations, dies
for metalworking, and molds for making plastic and rubber
Components.
Parts weighing as much as 700 kg have been cast by this process.
The slurry is a mixture of fine-grained zircon (ZrSiO4), aluminum oxide,
and fused silica, which are mixed with bonding agents and poured over
the pattern, which has been placed in a flask.
240. Ceramic-mold Casting
The pattern may be made of wood or metal. After setting, the molds (ceramic
facings) are removed, dried, ignited to burn off volatile matter, and baked.
The molds are clamped firmly and used as all-ceramic molds.
The facings then are assembled into a complete mold, ready to be poured.
The high-temperature resistance of the refractory molding materials allows
these molds to be used for casting ferrous and other high-temperature alloys,
stainless steels, and tool steels.
Although the process is somewhat expensive, the castings have good
dimensional accuracy and surface finish over a wide range of sizes and
intricate shapes.
241. Casting Inspection
All casting have discontinuities. Discontinuities are interruptions in the
normal crystalline lattice structure of the metal.
However, discontinuities are not always defects.
They are defects only when the weld or cast becomes unsuitable for the
service for which it was intended.
The role of inspection is to locate and determine the size of the
discontinuities,
There are two types of tests used to inspect the weld or casts:
Destructive tests
Non-destructive tests
242. Destructive tests
Destructive tests are used to determine the physical properties and to predict the
service life of the casting.
To perform this type of test, it is necessary to destroy the casting.
Various destructive tests are:
Tensile test
Compression test
Bend test
Nick-break test
Impact test
Hardness test
243. Tensile Test
This test is employed to express mechanical properties and to gain the
most useful fundamental information regarding the behavior of material.
The type of testing machine most commonly used is called the universal
testing machine.
A mechanical, electrical or optical device called extensometer measures
the amount of elongation in the test piece caused by the load accurately.
For maximum load requirements, a simple measuring device employed
is a pivoted steelyard.
244. Compression Test
This test is also performed in the universal testing machine
by using special fittings and shackles.
In this test, the uniaxial load applied is compressive.
The test specimen is compressed to obtain 80% reduction in
size.
This test is mostly used for brittle materials, which are
unsuitable for tensile test.
245. Bend Test
Bend test can be accomplished with two types of tests, the free bend
test and guided bend test.
In free bend test, bending is continued far beyond the limit of elastic
properties of metals and into plastic zone.
Guided bend test shows surface imperfections near and in the casting.
246. Nick-break test
It shows interior inclusions such as gas pockets, slag
inclusions and degree of porosity in the cast part.
The test is a simple test in which force may be applied by a
press or a sharp blow with a hammer.
The specimen used in this test usually has a width of 1.5
times the thickness.
247. Impact load
Impact testing determines the relative toughness of a
material.
Toughness may be defined as the resistance of a metal to
fracture after plastic deformation has begun.
The plastic deformation is initiated and finished by the swing
of a weighted pendulum.
The energy that is required to fracture the test piece is
proportional to the toughness of the material.
248. Hardness Testing
Hardness is the ability of a material to resist penetration,
abrasion or wear and tear.
The Rockwell and the Brinell hardness testers are two means
to determine the resistance of metal to penetration.
249. Non-Destructive Test
Non-destructive testing is a means to define and locate flaws within a material or a product
without destroying the product.
Various non-destructive testing used are:
Ultrasonic testing
Pulse-echo system
Through-transmission system
Ultrasonic resonance system
Radiographic testing
X-Ray analysis
Gamma ray analysis
Liquid penetrant testing
Magnetic particle testing
250. Ultrasonic testing
It is one of the methods of locating and determining the size of discontinuities (flaws) by means of sound waves.
Ultrasonic testing uses high frequency sound waves in the range of 20 – 25 kHz.
An electronic device called transducer emitter is used to change electrical energy to sound energy or sound waves, by utilizing
the piezoelectric principle.
Transducers are placed on the surface of the article is to be tested.
Couplant is used to make contact between the transducer and the surface of the material.
Oil, glycerin, grease or water is commonly used as a couplant.
Couplant removes air between transducers and the specimen. It provides a medium for the transfer of sound vibrations.
Ultrasonic wave is sent into the material for a very short period of time (1 to 3 millionth of a second).
Once the wave is sent through the material, it is reflected from the surface boundaries or flaws where density changes occur.
Reflected wave is received by the collector in the transducer-head.
251. Pulse Echo system
The pulse echo system generates an ultrasonic wave that passes
through the material from the transducer.
The system has evenly timed pulses of ultrasonic sound waves.
The pulses reflect from the discontinuances in their path or from any
boundary of the material.
As the wave passes through the top surface, flaw and bottom surface
there will be pip A, B and C respectively on the CRT.
The thickness is indicated by C.
It is the widely used system.
252. Through-transmission system
In this system there are two transducers. One is for sending and
other for receiving.
Like pulse echo system short pulses and at times even
continuous waves are used.
Soundness or quality of the material is being tested in terms of
the energy lost by the wave, which limits the use of this system.
The emitter sends the pulses, the receiver aligns with the emitter,
conveying the information in the form of sound waves and then
it is converted into electrical energy to the CRT.
253. Ultrasonic Resonance system
Similar to pulse echo system, except that the waves used are
always continuous, longitudinal waves.
The wave frequency is varied until the standing waves are
setup in the system, causing the item to resonate or to
vibrate at greater amplitude.
The difference in the vibration resonance is then sensed by
the transducer and this information is displayed on the CRT
screen
254. Radiographic Testing
This is based on the electromagnetic radiation of the source used.
In this process, a ray is emitted form a controllable source and this ray
penetrates a test specimen to film.
The errors and flaws in the test specimen are superimposed upon the
film, providing a visual record of the analysis for future reference.
There are two types of radiographic testing:
X-Ray analysis
Gamma ray analysis
255. X-Ray analysis
X-Ray radiographic testing is most effective in identifying small defects.
Only simple parts can be X-rayed.
Expensive, because of the cost of equipment and the extreme safety precautions that
must be taken to ensure operator safety.
X-Rays are produced when a high-energy electron collides with the nucleus of an atom.
The equipment consists of a cathode and an anode.
The cathode is a filament similar to incandescent light. The anode is the target.
The free electron due to high heat of filament free from the cathode and the anode
produces X-rays of continuous nature.
The shape of the anode target determines the pattern of the emitted X-rays.
256. X-Ray analysis
The intensity of the X-rays produced is directly proportional to the number of electrons freed at
the filament.
X-Ray analysis can be used for revealing errors in fabrication and assembly.
Gas bubbles, fractures and other cast imperfections can be identified.
The film used is made up of a thin transparent plastic sheet, coated with gelatin on both sides
and are sprayed over with tiny grains of silver bromide.
After the X-ray film is exposed, silver bromide chemically gets converted into metallic silver.
The exposed film is developed by placing it into the solution.
The solution removes the unused silver bromide from the film.
The film is dried and the inner side of the object can be viewed.
257. Gamma Ray Analysis
It is similar to X-ray analysis.
X-ray is effective only in testing material having uniform thickness of 50 mm
and below.
For material having varying thickness, gamma ray is suitable.
Another advantage is more number of pieces can be tested at a time.
The only difference is the source.
The source used here consists of radiographic isotopes like Cesium 137,
Cobalt 60, Iridium 192, Samarium 153 and other radiographic elements which
are independent of any electrical power source.
Extreme safety precautions should be taken.
258. Liquid Penetrant Testing
Liquid penetrant inspection (LPI) or Dye Penetrant Inspection is one of the most widely used nondestructive evaluation (NDE)
methods.
Its popularity can be attributed to two main factors, which are its relative ease of use and its flexibility.
LPI can be used to inspect almost any material provided that its surface is not extremely rough or porous.
Materials that are commonly inspected using LPI include metals (aluminum, copper, steel, titanium, etc.), glass, many ceramic
materials, rubber, and plastics.
Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye
from the flaw.
The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action.
There are basically two ways that a penetrant inspection process makes flaws more easily seen.
LPI produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself.
LPI produces a flaw indication with a high level of contrast between the indication and the background.
260. Liquid Penetrant Testing
Basic processing steps of LPI:
Surface Preparation: One of the most critical steps of a liquid penetrant inspection is the surface
preparation. The surface must be free of oil, grease, water, or other contaminants that may
prevent penetrant from entering flaws. The sample may also require etching if mechanical
operations such as machining, sanding, or grit blasting have been performed.
Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant
material is applied by spraying, brushing, or immersing the parts in a penetrant bath.
Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much
penetrant as possible to be drawn from or to seep into a defect. The times vary depending on the
application, penetrant materials used, the material, the form of the material being inspected, and
the type of defect being inspected. Generally, there is no harm in using a longer penetrant dwell
time as long as the penetrant is not allowed to dry.
261. Basic processing steps of LPI
Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess
penetrant must be removed from the surface of the sample while removing as little penetrant as possible
from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct
rinsing with water, or first treated with an emulsifier and then rinsing with water.
Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in
flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied
by dusting (dry powdered), dipping, or spraying (wet developers).
Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient
to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a
minimum of 10 minutes and significantly longer times may be necessary for tight cracks.
Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws
which may be present.
Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer
from the parts that were found to be acceptable.
262.
263. Magnetic Particle Testing
A nondestructive testing method used for defect detection. Fast and relatively easy to apply and part surface
preparation is not as critical as for some other NDT methods. – MPI one of the most widely utilized
nondestructive testing methods.
MPI uses magnetic fields and small magnetic particles, such as iron filings to detect flaws in components.
The only requirement from an inspectability standpoint is that the component being inspected must be made
of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys.
Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be
affective.
The method is used to inspect a variety of product forms such as castings, forgings, and weldments. Many
different industries use magnetic particle inspection for determining a component's fitness-for-use.
Some examples of industries that use magnetic particle inspection are the structural steel, automotive,
petrochemical, power generation, and aerospace industries. Underwater inspection is another area where
magnetic particle inspection may be used to test such things as offshore structures and underwater
pipelines.
264. Magnetic Particle Testing
Basic Principles:
Consider a bar magnet. It has a magnetic field in and around
the magnet. Any place that a magnetic line of force exits or
enters the magnet is called a pole.
A pole where a magnetic line of force exits the magnet is
called a north pole and a pole where a line of force enters the
magnet is called a south pole.
Editor's Notes
2. Decrease in mechanical properties because of microstructure because of uncontrolled cooling, decreased alloying composition, etc
A dental crown is a type of dental restoration which completely caps or encircles a tooth
