ADVANCE MANUFACTURING
TECHNOLOGY
SYLLABUS
UNIT I
Hot machining, Machining of Plastics, Unit heads,
Plastics cooling, electro forming, Surface Cleaning and Surface
Treatments, Surface Coatings, Paint Coating and Slushing, Adhesive Bonds,
Adhesive Bond Joints, Adhesives, Surface Coating for Tooling, Graphite Mould
Coating, Vacuum Mould Process.
Introduction,
Types of Composites materials, Agglomerated Materials, Reinforced materials,
Laminates, Surface Coated Materials, Production of Composite Structures, Fabrication
of particulate composite Structures, Fabrication of reinforced Composite,
Fabrication of Laminates, Machining, Cutting and Joining of Composites.
UNIT II
Introduction,
Polymers, Polymerization, Addition of Polymers, Plastics, Types of plastics,
Properties of Plastics, Processing of Thermoplastic Plastics,
Injection Moulding, Extrusion Process, Sheet forming processes, Processing of
Thermosetting Plastics, Compression Moulding, Transfer Moulding, Casting of
Plastics, Machining of plastics, other processing methods of plastics.
Introduction,
casting, thread chasing, Thread Rolling, Die Threading and Tapping, Thread
Milling, Thread Measurement and Inspection
Theoretical
basis of metal forming, classification of metal forming processes,
cold forming, hot working, Warm working, Effect of variables on metal
forming processes, Methods of analysis of manufacturing processes,
Open Die forging, Rolling Power Rolling, Drawing, Extrusion.
UNIT IV
UNIT IV
Introduction,
Product Application, Limitation of Die Casting, Die Casting Machines,
Molten metal Injection systems, Hot chamber machines, Cold chamber machines,
Die casting Design, Design of Die casting Dies, Types of Die
casting Dies, Die design, Die material, Die Manufacture, Die Lubrication
and Coating, Preheating of Dies, Vacuum Die Casting, Recent trends In Die
Casting Process.
Definition,
Cost accounting or costing, Elements of costing, cost structures, Estimation
of cost elements, Methods of estimating, Data requirements of cost
estimating, Steps in making cost estimate, Chief factors in cost estimating,
Numerical examples, calculation of machining times, Estimation of total unit
time.
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UNIT-I
Hot
machining, Machining of Plastics, Unit heads, Plastics cooling, electro forming,
Surface Cleaning and Surface Treatments, Surface Coatings, Paint
Coating and Slushing, Adhesive Bonds, Adhesive Bond Joints, Adhesives, Surface
Coating for Tooling, Graphite Mould Coating, Vacuum Mould Process.
Introduction,
Types of Composites materials, Agglomerated Materials, Reinforced materials,
Laminates, Surface Coated Materials, Production of Composite
Structures, Fabrication of particulate composite Structures, Fabrication of
reinforced Composite, Fabrication of Laminates, Machining, Cutting and
Joining of Composites.
1.1 HOT MACHINING
The machining of work pieces of
such materials, by conventional methods, is extremely difficult and many cases,
impossible. Very low cutting speed and feeds will have to be employed, resulting
in heavier loads on machine bearing and sides. Also, it will be quite a problem
to correctly select cutting tool materials, tool life or tool geometry.
Heat-resistant materials contain considerable amount of alloying elements, have
a tendency to weld onto the cutting
tool, loose very little of their strength, even when heated to temperatures as
high as 800oC, have a very high shear strength, combine high tensile
strength with high toughness, are susceptible to considerable work-hardening
and have low thermal conductivity. All these features lead to the development
of high cutting forces, and temperature, and to intensive cutting tool wear. In
addition, the surface finish obtained in machining is poor. Consequently, tools
for machining heat- resistant materials should be very carefully sharpened and
lapped. Tool geometry should be properly selected.
To overcome these problems,
entirely new machining methods have been developed. Some of these : ECM, EDM
and USM have already been discussed. The method of “Hot Machining” basically
consists of applying localized heat, ahead of cutting tool, to reduce the shear
strength of the work piece metal (thus improving its machinability), and to
permit the easy formation of the cutting chip. The chip is usually produced in
the form of a long smooth chip with lessened shock to the tool.
The application of correct amount
of heat, in the required place, is of maximum importance. Hence, the type of
heat and its application needs to be studied with care. Heating of the work piece
also influences tool wear. Therefore, heating in the cutting process improves
machinability; when the increase in tool life, due to the reduction of the work
done in cutting, is greater than the detrimental effect of the high temperature
on the tool, leading to increase wear. It has been established that the
temperature – interval in machining with heating of the work piece should be
taken 35 to 40oC lower than the temperature – interval for annealing
and aging.
The heating temperature depends
upon the cutting speed and the rate of feed, since the amount of heat generated
in cutting increases with the speed and feed. Thus in truing a particular grade
of stainless steel, heating temperature is:
> 500oC at cutting
speed of 19m/min
= 350oC at cutting
speed of 300m/min
= 230oC at cutting
speed of 375m/min
Advantages:
1.
The process id economical and in many case has reduced
the operating costs.
2.
Production gets increased.
3. Good surface finish can be obtained, superior to that
obtained on these materials at room temperature.
4.
Little evidences any adverse micro structural change.
1.1.1 Heating
devices: The
work piece can be heated by various methods of heating as high-frequency
induction heating or electric – arc heating devices mounted on the carriage, by
resistance heating with the application of an electric current in the cutting
zone, by flame heating, by Plasma arc heating. Sometime, the blank is preheated
in a furnace before being loaded into the machine tool.
1.2 UNIT HEADS
The idle motions are carried out
with the help of a traverse motor and its electrical brake. Depths of cut and
various intermittent motions are controlled by a series of trip stops secured
to the head. While a dead stop can be used to ensure the accuracy of cutter
depth. The longitudinal movement of the head can be actuated also through the
rotation of a plate or cylindrical cam, or when required for arduous duties, by
hydraulic power.
The “Unit Head” has opened up
avenues of multiple- operation machines for the completion of components which
would need a line of machine tools, each of which would need to be fully tooled
and manned. Also inter stage handling and storage has been eliminated.
It is possible to load one or
more components and not to remove them from the fixture on the machine until
the completion of a wide range of operations. On the completion of the
machining of these parts, the heads can be dismantled from the bases, and these
with the bases, passed into stores until required for the machining of other
components. The “Unit Heads” have made considerable headway in the production
of medium to large- scale components.
Advantages:
2.
They can be mounted and remounted in a variety of
positions on standard interchangeable bases.
3.
High production rates, along with consistent high
accuracy.
4.
Number of handling times reduced.
5.
Less floor space needed for machines and for spring.
6.
Operators more fully employed.
7.
Physical efforts of operators reduced.
8.
Good economical recovery rate.
The unit head is available in a
wide range of sizes. Power rating of driving motors ranges from about 0.2kW to
22kW, with spindle speeds from 41 to 200 rev/min, and with feeds of 0.025 to
3.50 mm/ rev.
Each unit head needs a control
panel and such panels can be housed in separate cabinets or enclosed within a
standard base. When a machine setup includes several heads, a combined control
board can be enclosed within the framework of the base.
The majority of the unit heads
are designed for boring, broaching, chamfering, counter boring, countersinking,
drilling, end milling, face milling, gage milling, reaming, and sawing,
shot-facing, tapping, thread rolling and turning. The front faces of most of
the heads are provided with means to allow the fixing of multiple-spindle
drilling heads, to permit the drilling of more than one hole simultaneously.
The versatility of drilling unit heads has been discussed under art.
1.3 PLASTIC TOOLING
Epoxide resins are more costly
than any other tool material. But they are lighter then other materials, being
1/4th weight of Zinc alloys and less than 1/4th weight of
cast iron. Also the cost saving due to the reduction in time and labour
involved in marking plastic tools outweighs the material cost. Other properties
of plastics have been discussed. Reinforcing with fiberglass can increase their
tensile strength.
Epoxide resins can be poured,
cast, laminated or moulded into intricate shapes with negligible shrinkage and
finish with a minimum amount of surface finishing. Consequently, the greatest
saving in cost is obtained with tools of complex shape, for which the cost of
machining and final finishing will be very high.
Compared with any of the tooling
metals, plastics are soft and have a much shorter life than comparable tools in
steel. It is not economical, therefore, to use plastic tools when: tool shapes
are conventional, the component material is thick and quantities and large.
Faulty handling too can damage plastic tools, more easily.
1.3.1 Applications:
Plastic tools are being used in
many industries:
1.
For drilling jigs, routing jigs and fixtures for
assembling, brazing and welding. In the majority of cases inserts are provided
to prevent endure wear.
2.
In plastic industry for the production of moulds for
both thermosetting and thermoplastic materials, for vacuum forming and for the
injection blow moulding of polythene products.
3.
In foundries for the production of patterns and core
boxes. The entry orifice of the latter is normally fitted with a hardened steel
insert to counteract the abrasive affect of the blown sand.
4.
Metal forming tools for drop hammers, hammers blocks
multipart press tools, piercing – punch plates, rubber press tools, spinning
chucks and stretch press formers. Most of these tools can be given extra
support by the inclusion in the mould of metal or fiber glass frames or
supports.
5.
Plastic tool also offer many advantages where short
runs and prototypes are required or where a set of tools is required very
quickly.
General tapers and blending radii
assist in producing a strong tool. The thickness of component metal should
rarely exceed about 1.5 mm while radii less than 4.75 mm are to be avoided.
The metal formed by plastic tools
includes:
Aluminum alloys, brass and other
copper alloys, mild steel, nimonic, stainless steel and titanium.
1.3.2 Production of plastic
tools: Plastic tools can be produced by two methods: by Casting and
building up reinforced layers of resin and glass fiber. The casting process is
used for the production of tools of large mass, such as forming dies and
punches. Selected fillers are added to the resin to reduce the cost of the mass
and provide the properties required in the tool. Inserts and supports can be
embodied in the casting to provide strength where required. Casting is least
time – consuming and the more reproducible of the two methods.
1.4 MACHINING OF PLASTICS:
Plastic can be machined, but in most cases, machining of plastics is not
required. Moulding and forming methods
can obtain acceptable surface quality and dimensional accuracy. However, there are certain plastics like PTFE
(Polytetra fluoroethylene) which are sintered products and are not mould able
by usual techniques, as they do not melt. For such “thermo stable plastics”
machining is a viable alternative to moulding.
The machining of plastics (by operation such as turning, drilling and
milling) has special features due primarily to the structure of the
material. It also depends upon the
binder upon the binder and the filler and the method of moulding the component.
For example, the machining of thermosetting plastics allows optimum cutting
variables and tool geometry to be employed because these do not soften on
heating, whereas thermoplastic resins soften under heat. The permissible
maximum temperature in the cutting zone is 160OC for thermo-setting
resins and only 60OC to 100OC for thermoplastics.
Special features of the machining of plastics are:-
1. The tendency of
certain plastics to splitting.
2. High elasticity
(40 times as much as that of steels). Therefore, they must be carefully
supported, to avoid their deflection during machining.
3. Non-homogeneous
structure of the material, with components of different hardness. This results
in poor surface finish after machining.
4. Plastics have a
strong abrading action on cutting tools.
5. Their low
thermal conductivity results in poor heat dissipation from the cutting zone and
in over-heating of the cutting edges.
6. The intense dust
formation, especially for thermosetting plastics, makes it necessary to use
special dust -removing devices.
7. The hygroscopic
of plastics excludes the use of liquid cutting fluids. Compressed air is
commonly used for cooling.
8. Reinforced
plastics are very difficult to machine.
Plastics can be machined with H.S.S. and cemented -carbide tools. In machining a plastic material with a filler
of glass, quartz or mica type, a satisfactory tool life can be obtained only
with carbide -tipped tools. Only diamond
tools are suitable for turning high-strength plastics of this type. The strength of cast parts of laminate
plastics is 40 to 50 percent less than that of the parts made by compression
moulding. Therefore, higher cutting
speeds and feeds can be used in their machining than for strong thermo-setting
plastics. The main trouble in turning
laminated plastics is the peeling of the surface layer.
The cutting variables are also influenced by the life of cutting tool
which is subject to abrasive wear in machining most engineering plastics. Dulling of the cutting tool leads to a poor
surface finish and to breaking out of the material at the points the cutting tools
enters and leaves the cut. This makes it necessary to use more keenly sharpened
cutting tools for plastics. The need for
sharp cutting edges follows from the high elasticity of plastics.
The selection of cutting variables is also influenced by the low heat
conductivity of the plastics, since; in machining the tool may be within a
closed volume (as in drilling) with no cooling facilities. This may lead to
charring of the machined surface.
The cutting tool angles for machining plastics are made somewhat
different than those of tools for ferrous and non-ferrous metals. The rake
angles are positive and relatively larger.
Because of the visco elastic behavior of thermoplastics, some of the
local elastic deformation is regained when the load is off. Therefore tools must be made with large
relief angles (200 to 300).
Abrasive machining of plastics has many advantages over machining with
metal cutting tools. These include the absence of splitting and crack
formation, and the better surface finish that can be obtained.
In grinding, the contact between the wheel and the surface being ground,
should be as short as possible, to avoid burns.
Organic glass is commonly ground with coated abrasive, applying an ample
amount of water as a coolant. If possible, however, grinding should be replaced
by polishing with a felt, broadcloth or flannel wheel charged with lapping
paste, the process is known as “Buffing”. The buffing wheels are of diameter
250 mm, 40 to 60 mm wide and of speed 2000 rev/min. Medium and fine lapping pastes are used as
the buffing compound for plastics. Laminate fabric base, asbestos-fiber and
glass -fiber laminate can be cut with abrasive wheels (SiC) of grain size 24 to
46 and with a 5% emulsion as the cooling fluid.
1.5 ELECTRO- FORMING
Electro-forming is a process of
precision metal parts that are usually thin in section, by electro-deposition
on to a form which is shaped exactly to the interior form of the product &
which is subsequently removed
In the
process, a slab or plate of the material of the product is immersed into
electrolyte & is connected to the positive terminal of a low voltage, high
current d.c power. So it becomes an anode. A correctly prepared mandrel or a
pattern of correct shape & size is immersed at
some distance from the anode &is connected to the negative terminal. The
mandrels are made from variety of materials both metallic & non-metallic.
If the material is non-conducting, a conductive coating must first be applied
to perform electroplating. The mandrel should poses mirror like finish. When
the circuit are closed metal ion are removed from the anode, transported from
the electrolyte towards the cathode and deposited there. After the deposition
the master is removed or destroyed. A
metal shell is confirms exactly the masters. It may take hours or day to deposit
of sufficient thickness. The thickness of electroforms ranges from0.25-25 mm.
The processes very much simple electroplating with the diff. That where is the
electroplating, the deposit stays in place (on the cathode), in electroforming,
it is stripped from the form, the electroformed products are typically from
nickel, iron, copper, and more recently from copper-tin, nickel, cobalt and
nickel,
1.5.1 Advantages:
1.
Low plant cost, cheap tooling & absence of heavy
equipment
2.
Low labor operating cost
3. The process can be designed to operate continuously
throughout day and night
4. Electro deposition can produce good dens deposit, and
compared with casting, electroforming offers high purity, freedom from porosity
with the homogeneous structure these important quality are seldom obtained to
such a degree in machine parts, stamping or forging
5. There is no restriction the internal complexity of
electro-forms and this advantage eliminates in many instances, the costly
joining processes.
6. The process has no equal for the reproduction of fine
or complex details
7. The use of inserts has widened the application of the
process. Metal inserts are attached to or are embedded in a wax or fusible
alloy master, &, when the master is melted, the inserts remain attached to
the electro-form.
8.
A high quality surface finish is both obtained on
interior &external surfaces of the electro forms. Accuracies as close as
.005 mm
9. complex thin –walled parts can be produced with
improved electrical properties
10. Shell-like
parts can be produced
11. Quickly
&economically
1.5.2. Mandrels:
The mandrels, the mould or the matter, is the most expensive item
in the process of electro-forming. Mandrels can be made of several metallic
&non-metallic materials the metallic materials are aluminum, brass, and
carbon steel. a common feature of these mandrels is their oxide formation film –which
facilitates their separation from electro form with out any surface treatment.
Depending on shape of electro
–form mandrels are of three types-permanent; semi permanent & expandable
1.5.2.1 Permanent mandrels
These mandrels are usually made of metals or
glass or rigid plastics .the surface of non-metallic mandrels is made
conductive by metalizing by electro-forming or a chemical deposition technique.
For close tolerances work such as gear &gauges, stainless steel is
recommended. Such materials can be used indefinitely, with a minimum treatment
to preserve the smooth surface. Adhesion is minimized by the application of
thin coating of a parting compound
1.5.2.2.
Semi-permanent mandrels
for straight sided
components or components which have
several cuts up to about .013 mm, semi permanents mandrels are used .these are
made of steel with fusible coating, compounded usually made from wax
&graphite .,to remove electro-form the fusible layer is melted &after
removal , mandrels is cleared & rebuilt
1.5.2.3. Expendable mandrels
For complex electro-form are made
from
(1) Plaster, which after
electro-forming are removed by breaking.
(2) Plastic resins &fusible
alloys which are melted.
(3) Aluminum & zinc which are
dissolved chemically
(4) Brass
Plastic resins are commonly used
for decorative work where tolerances are wide without undercuts. The surface
finish of mandrels made from fusible metal alloys can be removed by
electroplating a layer of copper .025 to .050 this copper layer is dissolved
from electroform after fusible alloy is melted
1.5.3 Applications
1.
Moulds & dies feature high in the list. Moulds for
the production of artificial teeth rubber 7 glass products, & high strength
thermosetting plastics are now commonplace. The moulds can be made with
undulating parting lines which have made a considerable impact upon the
production of thermoplastic toys & novelties.
2.
Radar 7 electronic industries –radar wave-guides,
probes, complicated grids screens 7 meshes can be produced easily
3.
Spline, thread & other types of form gauges
4.
Cathode for e c m & electrodes for E D M
5.
Electro-formed core boxes with inbuilt heating
elements.
6. Electro-formed precision tubing, parallel & tapered
formed to different shapes to eliminate the need for bending which the bore.
7.
Electrotypes floats bellows, venturi tubes, fountain
pen caps, reflectors, heat exchanger parts honey comb sandwich, parts for gas
appliances 7 musical instruments, radio parts , filter & dies .
1.5.4 Electro-Forming Is Particularly Used For:
1.
High cost metals
2.
Low production quantities
3.
Quantity of identical parts, for example a multi
–impression mould
4.
The possibility of using a single master for production
of a number of electro-forms
5. Where as intricate female impression is required, son
that it would be much easier to produce a male form
During the manufacture of virtually all metal; parts, filling, fine metal chips , pieces of chips , remnants of wastage or abrasive grit may get into holes or channels of parts . Also oil, dirt, grease, scale, and other foreign materials remain affected to the main part surface the purpose of surface cleaning &getting rid of all above materials is two fold – firstly they may get into holes or channels of part. Subsequently, in operation finished goods they may be carried by lubricants into the bearings, where they may lead to overheating & premature wear of the bearing & even to breakdown of the whole machine. This can be properly avoided by cleaning. Thorough cleaning of parts is essential for high quality of their performance.
The
second purpose of surface cleaning is to prevent corrosion, & to combine a
decorative appearance with protective coating. All metals will oxide &
corrode, when expose to certain environments, unless protected with an air
thrust. Before application of any protective coating, it is essential that the
surfaces of the part be prepared by proper cleaning to good adhesion. Various surface cleaning methods are:
1.6.1
CHEMICAL CLEANING METHODS
1.6.1.1 Alkaline Cleaning: In this method
dipping them in aqueous solution of alkaline silicates, caustic soda, or
similar cleaning agents cleans parts. Some type of soap is added to aid in
emulsification. Wetting may be added to the solution to help in thorough
cleaning of the parts. The method satisfactorily removes grease &oil. The
cleaning action is by emulsification of oils & grease. Special washing
machines are employed in lot &mass production. Washing m/c may be of
single, two or three chambers types.
(a) In a single chamber washing
m/c, washing chamber is equipped with a bank of pipes with nozzles. A pump
delivers the cleaning fluid, drawn up from main tank to the pipes. The nozzles
are arranged so that the part or unit is washed from all sides simultaneously
with powerful streams of fluid. The parts may be transferred in washing machine
by chain conyver. The cleaning fluid is heated by a steam coil of 60 to 80,
& therefore parts ejected from the machine dry fairly soon.
(b) A two-chamber washing machine
has two washing chambers. The parts are cleaned in first chamber & then
rinsed of washing solution in the second chamber.
(c) In three –chamber machines
third chamber is used for drying.
1.6.1.2. Solvent Cleaning:
Small parts are cleaned of oils, dirt, grease, and fats by dipping in
commercial organic solvents, such as naphtha, acetone, carbon tetra chloride.
The parts are then rinsed once or twice in a clean solution of same solvent.
The vapors of these solvents are toxic & therefore require ventilation. The
method is particularly suitable for aluminum, brass, lead, which are chemically
active & might get attacked by alkaline cleaners
1.6.1.3. Emulsion Cleaning: In this method of
cleaning, the action of organic solvent is combined with that of an emulsifying
agent. The solvent is of generally of petroleum origin & emulsifying agent,
which are soap or a mixture of soap & kerosene oil, include nonionic
polyesters, high molecular weight sodium, amine salts of alkyl aryl
sulphonetes, acid esters of polyglycerides, glycerols & polyalcohols.
Cleaning is done either by spraying or dipping the metal parts in solution
& then rinse & drying. The method is suitable for parts & aluminium,
lead or zinc while are attached by alkaline solution
1.6.1.4. Vapor
Degreasing: Vapor Degreasing is a similar process, except that the
solvent vapors are used as the cleaning agent. The solvent is heated to its
boiling point and the parts to9 be cleaned, are hung in its vapors. The vapor
condenses on the surface of the parts wash off the oil and grease.
1.6.1.5. Pickling: The process is used to
remove dust or oxide scale from surface of components. The parts are filled in
a tank filled with an acid solution, which is 10 to 12% of sulphuric acid in
water, & is at temp from 65 to 85 C. the solution acts to loosen the hard
scale from the components surface & removes it. The acid solution should
not react with metal while from the scale. For this an inhibitor agent is added
to9 the solution. Pickling process only removes oxide scale.
1.6.1.6. Ultra- Sonic Cleaning: Very dirty
small parts, especially those of intricate shape with hard-to-access inside
surfaces, are difficult to clean in ordinary washing facilitates. Such parts
are cleaned much more efficiently by ultra sonic cleaning method. The method is
effected in three stages – prelimary, ultrasonic, & rinsing of parts in a
clean washing medium (kerosene, trichloroethylene)
Ultrasonic energy is produced by
a high frequency generator, which feeds high frequency electric energy to
transducers that transforms electric energy into inaudible sound energy. The
transducers are fixed to the bottom or the sides of a stainless steel tank
designed to afford the optimum acoustic conditions. High velocities are
imparted to particles of cleaning liquid in the tank. Cavitations bubbles of
microscopic dimensions are formed on the surface of the components. The
cleaning action is done by formation of bubbles, which practically blast all contaminants
from all types of components in seconds, penetrating every crack or crevice
& removing all loose parts. The cleaning liquid includes water, water based
solutions, mild acid & caustic soda,
which are thermostatically controlled to operate at temp of about 45 C. The
effectiveness of this method is 99%.
Rinses are required in all cases
to ensure through removal of cleaning agent before coating. After being washed,
machines parts are dried carefully. This is done by compressed air.
1.6.1.6.1 ADVANTAGES
1.
The process can be manned easily by trained labour.
2.
The process reduces the time element.
3.
The process produces cleaner surfaces & eliminates
many manual operations & the quality hazards.
4.
The cleaning of intricate assemblies after final assembly
can reduce testing time.
1.6.1.6.2 APPLICATIONS
The process can be used in all
types of engineering factories, cafes, dairies, hospitals, & hotels &
by manufacturing jewellery. Components, which have been cleaned by the process,
include – ceramics, cutlery, electronic equipment, machine tool equipment,
watch parts.
Other components, which need to
be scrupulously cleaned, include air craft, ball race assemblies, engine
components, fuel gauges, gas turbines, gears, glass components, hydraulic
devices, jet engine parts, refrigerator parts, satellite components, &
parts for semi conductors, teliprinters parts.
1.6.1.7 Surface Polishing: Mechanical polishing of pressed or extruded metal products & many such articles is by using a wide range of wire brushes or mops, in conjunction with specially blended greases & oils the two non –mechanical techniques are chemical polishing & electrolyte polishing. Chemical polishing has made greater advantages due to increased use of aluminum for a variety of applications. Electrolyte polishing has made less progress, because it is more expensive to install.
(a) Chemical Polishing
In the process, the metallic
objects are immersed in bathtubs of selected acids. During process certain
amount of metal; mainly peaks are dissolved, producing a bright surface with
out the formation of etched pattern. For chemical polishing of aluminum alloys
the most successful of the solution used contain phosphoric, nitric acid,
sulphuric acids. Production cycle consist of following steps
(1) Immerse
for 1 to 3 min at 100 C.
(2) Remove & rinse in hit
water to remove the viscous film formation.
(3) Rinse in a mixture containing
equal amount of water & I, 42 sp. Gr. Nitric acid at Room Temperature
(4) If anodizing required, rinse
in cold water.
(5) If lacquering is called for,
rinse in cold water
The resultant surface finish is
of order of .45 to .50 with a high reflection factor of 88%.
1.6.1.7.1 ADVANTAGES
1.
The process is comparatively cheap, with low operating
cost
2.
The equipment has along life.
3.
It is very suitable for delicate, thin –walled,
embossed.
4.
Both the inside & outside surfaces are cleaned
easily
5.
The process can be combined with barrel – polishing to
reduce time & cost.
6.
The process can be included in aluminum anodizing cycle
7.
Improved reflectivity usually is obtained.
(b) Electrolyte Polishing
When a metallic component is
immersed in electro polishing electrolyte, the current line leads from surface
peaks, tangentially, causing a higher current density on the peaks than on
valleys. Thus greater metal dissolution takes place on peaks to take smoother
surface than on valleys. This viscous film protects the micro valleys from the
action of current but permits minute peaks to dissolve. The rate of metal
removal is 3 to 10 microns.
A wide range of metals &
alloys can be electro typically polished, but the main industrial uses of the
process are for polishing of alloys & stainless steel, copper alloys,
nickel, & aluminum alloys.
1.7 CLEANING
AND FINISHING OF FORGINGS
1) Removal of oxide scales: A thin layer of scale which is caused
by contact of heated steel with air is formed on surface of steel forgings.
Amount of scale depends upon the forging temperature & length & time of
operation. The simple way to remove the scale is by employing steam or
compressed to blow away the scale.
2) Cleaning by pickling: Process is used to remove hard scale from
surface of forging. It consists of immersing the forgings in a tank filled with
an acid solution, which is 12 to 15 % concentrate of sulphuric acid in water.
The acid solution should not react with clean metal while removing the scale.
For this an inhibitor is added to solution.
3) Tumbling process: Process is used to remove the scale & for
general cleaning of the forgings. The forging along with abrasive materials
such as coarse sand or small particle is placed in barrel. The tilted barrel is
rotated at low speed s. the forging & abrasive roll over themselves. This
action loses the scale from surface of forging & removes it along with
affecting a general cleaning of forging.
4) Blast cleaning: Process
consists of directing a jet of sand, grit or metallic shots against the
forging. The blast force is obtained from compressed air or centrifugal force
through suitably designed apparatus. This process removes the scale & a
smooth surface finish is imparted to the forging.
1.7.1 CLEANING
AND FINISHING OF CASTING
After casting is solidified &
cooled down sufficiently in an expandable mould, the first is freezing the
casting from mould. The operation is called as “SHAKE OUT OPERATION” since a
great deal of heat & dust are involved in this operation, the operation is
usually mechanized. Shake out is usually done by means of vibratory knockouts,
jolting grids & vibrators. The mould is intensively jolted & broken up.
After shaking the casting out of mould, it is conveyed to the fettling shop for
cleaning and finishing. The process consists of following operations – core
removal, cleaning of surfaces, and removal of gates, risers & fins.
1) CORE REMOVAL OR CORE KNOCKOUT: This operation is also done
manually. Hampering & vibrations will loosen & breaks up cores.
Stationary 7 portable vibrators are employed for this purpose. To knockout from
heavy engines, it is disadvantages to use air drills. Removal of cores by hydro
blasting is more sanitary process keeping in view dust problems. The operation
consists of breaking up & washing out the cores with a jet of water
delivered at a pressure of 25 to 100 mm.
2) CLEANING OF SURFACES: The process involves the removal of all
adhering sand &oxide layer & produces a uniformly smooth surface.
Mechanical methods are employed for this purpose, since cleaning by hand with
wire brush is tedious & costly.
I) Tumbling: This method
is used for cleaning & light casting. The Castings are loaded into a
tumbler or barrel along with white iron picks. Rotation of barrel causes
casting & jack stars to tumble. Jack stars abrade the surface of casting
&also abrade surface of one another. This operation removes adhered sand
& oxide scale from the surface of the casting. The rotational speed of
barrel is 30 rev/min.
Ii) Sand Blasting and
Shot-Blasting: This method is widely used to clean surfaces of light,
medium & heavy castings. In these machines, dry sands or shots is blown by
stream of compressed air against the surfaces of casting. The impact of
abrasive particles traveling at a high speed, on the surface removes adhering
sand & oxide layer. Velocity of abrasive particles leaving the nozzle of
m/c ranges from 35 to 75 m/s & pressure 0.7 MPA
iii) Airless Shot Blasting:
In this shots are hurled on the surface of casting by a fast rotating paddle.
For harder castings, the shots are made up of white iron, steel, whereas for
softer non ferrous castings, these are made up of copper, bronze, glass or mild
iron,. The wheel rotates at 1800 to 2500 rev /min. The velocity of shots
striking the surface is about 60 to 72 m /s.
Iv)Hydro Blasting: This
is most effective surface cleaning method. Here two operations are accomplished
simultaneously –core knockout & surface cleaning. Casting are placed on a
rotary & stationary table & high velocity jets containing about 15% sand & 85 % water under a pressure of
10 to 20 mpa . The jet velocity can be up to 100 m/s.
v) Removal of Gates and Risers:
Gates, runners, risers, and spure can be removed before or after cleaning
operations. In brittle materials, these are simply broken off from the
castings. in more ductile materials , following the following methods are used
to remove them –power hacksaws , band saws , disk type cutting edges , abrasive
cut of edges , flame cutting with an oxyacetylene cutting torch & arc
cutting for heat resistant steels which are not amiable to gas cutting.
vi) Powder Cutting: Process
by which risers & gates can be easily removed from castings made of
oxidation resistant alloys. Preheated iron powder is introduced in oxygen
stream. This burning iron then attacks the metal riser by a process of fluxing
& oxidation.
vii) Some minor defects defected
may sometimes be repaired by welding without affecting the function of finished
castings.
viii) Finished castings are
subjected to various heat treatments to modify mechanical properties or reduce
residual stress.
1.8 SURFACE
COATING:
The various surface coating on
machine parts are used for protective, decorative, wear resistant and
processing purpose. The different types of surface coatings used for this
purpose are: metallic coatings, phosphate coatings, oxide coatings and plastic
coatings etc.
1. Electro – deposited coatings:
This process of coating also known as “electroplating”, comprises
preparation of the surface to the plated, plating itself, and degreasing of the
surface. The part to be electro-plated is made the cathode and the metal to be
deposited is made the anode and both are placed in a tank containing an
electrolyte. The process is carried out at a voltage of 10 V (D.C.) and current
density of upto 10A/dm². When the circuit is closed, metallic ions from the
anode migrate to the cathode and get deposited there. Some characteristic of
electrodeposited coatings as given below:
a) Copper plating: Used for masking of steel parts from
carbonization a case of hardening heat treatment, plating for improved running-
in of plated surfaces as an under layer for multi-layer coatings. Coating
thickness: 5 to 25µm.
b) Chrome plating: Wear – resistant protective and decorative
coating. It result in improved retention of lubricant and lower co-efficient of
friction. Coating thickness: 30 – 40µm.
c) Cadmium plating: Coating for protection against corrosion of
steel in moist atmosphere (marine corrosion) and for improved running in of
mating surfaces. Plating thickness: 15 µm.
d) Nickel plating: Undercoat of chrome, corrosion protection for
steel, wear qualities and for decoration. Plating thickness: upto 25µm.
e) Lead plating: Resistance to chemical corrosion.
f) Zinc plating: Low- cost protection of steel and iron against
atmosphere corrosion and fro decoration. Plating thickness: upto 15µm.
g) Silver plating: Electrical contacts. Good anti galling and
seizing qualities at high temperature. Plating thickness: 2.5 to 12.5µm.
h) Tin plating: Coating for protection against weak acidic media,
non-toxic protection in food, for subsequent soldering and for masking in
nitriding. Plating thickness: 3 to 12µm.
i) Gold plating: Infrared reflectors, electrical contacts,
jewellery.
j) Borating: High hardness coating.
k) Lead-indium plating: Electro-deposit of lead on a silver plated
surface followed by indium plating forms a satisfactory bearing surface.
l) Phosphating: Anti-corrosive coating. Plating thickness: 0.5 -
1µm.
m) Brass plating: Brass plating is frequently used as a base for
bonding rubber and rubber like materials to the metal. It improves appearance,
provides soldering surface, which is abrasive resistant. Since brass tarnishes
(it is satin yellow to bronze initially and when turns to black to green on
exposure), it must be covered with lacquer, when used for decorative purposes.
Brass plating is used on steel, zinc aluminium, and copper plate.
1 8.1 Electro-less plating: The method of plating differs from the
conventional method of plating that is electro-plating, in that no external
source of electricity is used in the process. The plating is obtained with the
help of chemical reaction. For example, for nickel plating, a metallic salt of
nickel, nickel chloride is reduced with the reducing agent, such as sodium
hypophosphate. Nickel metal so obtained is deposited on the work piece. Nickels
are two most commonly used metal, for this process.
1.8.1.1 Advantages:
1. This
process can be used for plating
non-conducting materials such as
plastic and ceramics.
2. This
process does not produce hydrogen embitterment.
3. Cavities,
recesses and inner surface of tubes can be plated successfully.
4. The
coating has excellent wear and corrosion as compared to electro-plating.
1.8.1.2 Disadvantages
The process is costlier as
compared to electro-plating. Some times; some portions of the base of a work
piece are not to be electro-plated for decorative purpose or for the sake of
economy. This is known as “blacking out”. For this the complete base is
prepared for electro-plating. The base is heated to 100ºc. Paraffin wax is
applied to the portion to be black out and then the work piece is completely
cooled. After that the electro-plating is carried out.
Electro-plated parts are usually dull and
posses little or no metallic luster. To provide finish, shine and luster to the
electro-plated parts, mops and “compos” finish them. The mop should be moved
slowly over the surface to avoid removal of any portion of electro-plated
layer, the final finish/colors be obtained by mopping with chalk. Compos, which
contain abrasive, should not be used with soft metals.
1.8.2. Phosphate coating: A
coating for steel as a preparation for painting, adhesive bonding or rust
proofing. The process involves the chemical development of a film, which
contains ferrous insoluble phosphate of manganese and ferrum, or ferrum and
zinc, by treatment with a dilute acid phosphate solution. Depending on the
phosphate structure and the method of surface preparation, the phosphate film
may be from 2 to 15 µm thick. A rapid phosphating process is known as
“bonderizing”. Phosphate coating can also be used for non-ferrous and light
metals.
1.8.3. Oxide coating: Oxide coating of steel parts is done for
decoration, rust proofing and to obtain low friction surface. The coating is obtained
by thermal, chemical and electro-chemical methods.
The thermal method involves
heating the part in air, steam or molten nitre. An oxide film 1µm thick is
formed on the part surface: the film color varies with the process temperature.
Heating in the air serves to form thin oxide films on electrical components.
The chemical methods include
alkaline and acidic oxidation. In the first method steel parts are treated with
a hot concentrated solution of caustic alkali containing oxidants. In the
second method, solution contains ortho-phosphoric acid and oxidants. The acidic
oxidation is much quicker as compared to alkaline oxidation and provides a
stronger oxide film with improved corrosion resistance. The oxide films on
steel parts are (0.8 - 3µm) and porous, and therefore do not reliably protect
the parts from corrosion. Their corrosion – resistance can be increased by
subsequent varnishing.
The chemical methods are used to
oxidant parts made of aluminium, manganese copper, zinc and their alloy. The
field of the process application is the manufacture of instruments, tools and
consumer goods.
Electro-chemical oxidation of the
parts is made of ferrous and non-ferrous metals and alloy is carried out in
solutions of caustic alkali. The parts being processed form an anode. The
process runs at lower temperature and requires less chemical agents than the
chemical alkaline oxidation. Prior to treatments of parts, these are cleaned of
corrosion spots and degreased, and after oxidation they are rinsed in water.
Decorative oxidation takes from 30 to 40 min; corrosion – resistant films
require upto 1.5 – 2 hours for their formation.
1.8.4. The coating of nickel – cobalt, zinc- cadmium and tin – lead
is obtained by methods, which are called “thermo electro- plating” or “thermo-diffusion”.
The latter consists in that individual metals are successively deposited on the
parts and in the course of subsequent heating these diffuse and form plating of
some alloy. Nickel – cobalt plating increases hardness, zinc-cadmium plating upgrades
corrosion resistance and tin-lead plating reduces porosity and improves
appearance.
1.8.5. Plastic coating: Plastic is used as decorative,
anti-corrosive, and anti-friction coatings. These are applied in liquid and
powder form. The primary materials used are thermo-plastics such as:
polyethylene, polypropylene, polyamide, polyvinyl butyral, polyurethane,
fluroplastic and caprolactum etc. these are used in form of fine powders which
on heating. Change to plastic state. The coast thickness ranges from 0.15 to
0.35 mm. before coating the parts are heated to 180- 300ºc depending on the
plastic to be used. The treatment itself lasts from 2 to 5 s. plastic coating
makes it possible to use carbon steels or non-ferrous metals.
1.8.6. Metallization: Metallization means spraying molten metal
with the aid to compressed air. The metal particle moving at a speed of 100 to
150m/s strike the surface of a part being coated and adhere to it, thus forming
a layer of strong, finely porous metal coating. The layer has a fairly high
compressive strength, even though it is brittle. The coating thickness varies
from a few hundredths of an mm to 3-4 mm. parts after being coated can be
turned and ground. The method is used to obtain decorative, protective,
antifriction and heat-resistant coatings, to restore worn out parts and correct
defects of castings. The metal being sprayed is melted OA flame (gas
metallization) or by electric arc (electro-metallization). The initial material
is metal wire. Sometimes uses are made of equipment operating on metable
powders. The surface to be coated is cleaned of oil and oxides. Sand blasting
and rough turning is employed for better adhesion of the metal being sprayed to
the surface.
1.8.7. Anodizing: Anodizing or anodic coating is a process of
providing corrosion resistant and decorative films on metals, particularly
aluminium. The process is the reverse of electro-plating, in that the part to
be coated is made anode, instead of cathode, as in electro-plating. When the
circuit is closed, a layer of aluminium oxide is formed on the anode
(aluminium) by the reaction of aluminium with the electrolyte. The layer of
aluminium oxide on the surface is highly protective.
There are two process used for
anodizing.
1.8.8) Chromic acid process: In this process, 3% solution of
chromic acid is made as the electrolyte at a temperature of about 38ºC. This
process is applicable only to those aluminium alloys containg not more than 5%
copper or a total alloy content of more than 7½%. The process produces a light
yellow colour.
1.8.9) Sulphuric acid process: The electrolyte is 15 to 25%
solution of sulphuric acid. The process is applicable to aluminium alloys
containg more than 5% copper or a total alloy content of more than 7½%. The
process produces a light yellow colour. This process shall not be applied to
parts having joints or recesses in which solution may be retained. Normal
anodized coatings are 0.0050 to 0.0075 mm thick.
1.8.10) Hot- dip coatings: Many metal parts are used for making
food containers due to the non-toxity of tin. To remove the excess tin, the
sheets are passed through rollers after these come out of the bath.
(i) Zinc coating: Giving a coating of zinc is called “Galvanizing”.
One method of doing so is by “electro-plating”. In the hot dip method, the
parts or steel sheets are fluxed by immersing them into a solution of zinc
chloride and hydrochloric acid. After that they are dipped into a molten zinc
bath. Again, to remove excess zinc, the sheets are passed through rollers after
they leave the bath. Galvanized steel sheets (and more recently, also one sided
galvanized sheet) find increased use in automotive and appliance industry in
addition to their use for roofing.
(ii) Lead coating: Load- coated sheet provides anticorrosion properties
in some media. Where tin coated and zinc coated sheets can not resist
corrosion. However, lead coated sheets cannot be used for food applications,
because lead is toxic. An alloy of 15% to 20% tin and the remaining lead can
also be used for this coating. Lead coating method is also called “terne”
coating.
(iii) Aluminum-coating: Aluminum – coated sheets can resist
corrosion by hot gases. Due to this, these are suitable for heat exchanges,
automotive exhaust systems and grill parts etc.
1.8.11. Conversion coatings: These are the coating produced when a film is deposited on the base
material as a result of chemical or electro-chemical reaction. Many metals
particularly steel, aluminium and zinc can be conversion coated. The coatings
can be phosphate coatings, chromate coatings oxalate coatings. After degreasing
and cleaning in alkali, the part is soaked in suitable acid bath, for example,
for chromate coatings, in chromic acid bath.
Conversion coatings are obtained
for corrosion protection, prepainting and decorative finish. Another important
application of this coating is as a lubricant carrier in cold forming
operations, such as wire drawing.
Oxides that form naturally on the
surface of metals are a form of conversion coating. Oxide coatings discussed
above and also “anodic coatings” fall under the category of “conversion
coatings”.
Bicycle wheel rim plating plant process chart
The rims are loaded on the fixture. The fixture can carry
upto twenty rims at a time and after the loading is done, the robot is switched
on which takes control of the whole process. The rims undergo various processes
before being unloaded for use in body assembly shop. The various process of
electroplating of the rims is as in the table below:
Table 1.1 Various process of electroplating
S. No.
|
Process sequence
|
Chemical concentration
|
Temperature
(Deg. Celsius)
|
Density
(Deg. Be)
|
Checking
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
|
Kerosene oil cleaning
Abrasive cleaning
Water rinse
Soak cleaning
Water rinse
Electro cleaning
Water rinse
Acid dipping
Water rinse (1)
Electro cleaning
Water rinse (ii)
Acid dip
Water rinse (ii)
Acid dip
Water rinse (ii)
Semi-bright Ni plating(ii)
Tri-Nickel plating
Bright Nickel plating
Drag out
Water rinse (iii)
Chrome plating
Drag out
Drag out
Water rinse (iv)
Unloading of the rim
|
M.T.kerosene oil 100%
Surclean-504-70-80cc/lit.
NAON-80-100-80 ltr
Running water
Steelex 80-10gm/ltr
Running water
Ginbond- 808
80-100gm/ltr
Running water
HCI 30-40%
Running water
Ginbond- 808
80-100gm/ltr
Running water
Sulphuric acid 10-15%
Running water
Nickel sulphate
250-300gm/ltr
Nickel sulphate
250-300gm/ltr
Nickel sulphate
250-300gm/ltr
D.M.water tank
Running water
Chromic acid
275-325gm/ltr
D.M.water tank
D.M.water tank
Running water
|
Room
60-80
Room
60-80
Room
60-80
Room
Room
Room
60-80
Room
Room
Room
45-55
45-55
45-55
Room
Room
40-50
Room
Room
Room
|
-
8-10
-
8-10
-
8-10
-
8-10
-
8-10
-
8-10
-
18-25
18-25
18-25
-
-
24-30
-
-
-
|
Once daily
Once daily
-
Once daily
-
Once daily
-
Once daily
-
Once daily
-
Once daily
-
Once daily
Once daily
Once daily
Once daily
-
Once daily
Once daily
Once daily
-
|
Nickel plating: is done as a protective
coating as it has excellent adhesive properties which form good base fro chrome
plating.
Chrome plating
is corrosion resistant and also enhances the looks of the component.
1.9 PAINT COATING & SLUSHING:
Metals parts are painted to
protect their surfaces against corrosive action of surrounded medium & also
to improve their appearance. The process of coating with paints & varnishes
is carried out on 3 stages:
1.
Preparation of the surface to be coated.
2.
Painting.
3.
Drying with finishing.
Preparation for painting consists
in cleaning & degreasing surfaces. The surface so prepared is then primed
for better adhesion of the subsequently deposited coating. Use is made of oil
varnish, oleo vituminous, water-soluble & nitro soluble primers. The prime
surface is then trited with a filter, whose layer should be as thin as
possible. Oil varnish filters & quick drawing proxylin filter are commonly
used. The clearance surfaces are painted & the preparation for painting
consists in cleaning and degreasing the surfaces, priming, luting, and
smoothing down the luted surfaces with emery clothe.
1.9.1 METHOD OF PAINTING:
Following are the methods of
industrial painting:
a) Brush painting: the brush painting is used in piece &
small lot production. It is done by hand & is slow cumbersome method, where
quick drawing paints are used. The method requires minimum of eq. But max. Of
labour. The point losses are upto 5%.
b) Spray painting: The
method consists in applying fluid paint in the atomized form. This method is
the most common & productive, but requires premises equipped with exhaust
devices & spraying eq. There are various ways of painting:
i) Mechanical spraying:
in this method paint is delivered to spray gun by pump.
ii) Air spraying: the paint
I sprayed by jet of compressed air, which carries the paint mist to the surface
being painted. The method is capable of coating 30-80 mt sq. Of surface/hour
but losses is high 40-50%.
iii) Airless spraying: in this method paint heated to 70-90C IS
FORCED through a nozzle at pressure of 2-4 n/mm sq. the production rate can be
50-200mtsq. Of surface/hour & paint losses are amt. to 25-50%.
iv) Electrostatic spraying: in this method a negative charge
delivers paint, which gets on to the surface of charge metal part being
painted. The charging is provided by high voltage const. current source.
Setting up metal screens behind the parts can also use the method for
non-metallic parts. The paint losses are less than 5%. The method makes it
possible to improve working condition to provide for fairly high productivity.
c) Dip coating: this
coating is used in automatic production. The method consists in dipping the
parts suspended from chain conveyor, in bathtub. The method is used in large
lot and mass prod. For painting parts of simple shape. The paint losses are 5%.
1.9.2 ADHISIVE BONDS
Adhesive joints can be made by
applying adhesive in thin layer b/w the connected parts. They are fastening
metallic as well as non-metallic (textile laminate, foam plastic) materials.
1.9.2.1 ADVANTAGE: -
(1) Reliable
connection of parts made of very thin sheet materials.
(2) Dissimilar
metal can be joining.
(3) The process
is used to reduce production cost
(4) It ism used
to lessen the mass of parts.
(5) Highly
skilled labor is not required to bonding.
(6) The process
provides for tight and corrosion free joints.
(7) Smooth
bonded surface.
(8) Exterior
surface remain smooth.
(9) Only low
temp are involved so absence of stress or their lower concentration.
(10) Heat
sensitive material can be joined easily without any damage.
(11) Complex assemblies can be at low
cost.
(12) Adhesive contribute towards the
shock absorption and vibration.
(13) It can tolerate the thermal stress
of different expansion and contraction.
(14) Because the adhesive bonds the entire joint area, good
load distribution and fatigue resistance are obtained.
(15) The joints are sufficiently strong
in shear and with stand with dynamic and variable load.
(16) Compared to welded soldered and riveted joints,
adhesive bonded parts have uniform spread stresses and do not tend to warp.
(17) The process is very fast.
1.9.2.2 DISADVANTAGE: -
2.
Low resistance to tear off.
3.
Reduce strength of some adhesive in the course of
timing (ageing).
4.
Tendency to creep, if subjected to long-standing and
heavy load.
5.
The need of extended polymerization time.
1.9.2.3 APPLICATION: -
Adhesive have particular develop
in air –craft industry: the appearance of honeycomb structure is due to this
method. The method is used in uncritical structure such ask control surface in
aircraft body. In machine tools, adhesive are employed to bond carriage-guide
–ways to beds, and in automobile industries to fasten the friction lining to
clutch- disk brake –bands. Adhesive bonds are also used in appliance and c
consumer goods fields and also sealing, vibration damping and insulating etc.
1.9.3 ADHESIVE BONDED JOINTS:
-
Adhesive-bonding of is affected
on following types of surfaces:-
(1)
In cylindrical type of surfaces, for example, placing
bushings into the holes in the housing –types and parts discs onto the shafts,
coupling pipes together, fitting plugs and fastening lining to brake blocks.
(2)
On flat surface, for example, lap-type joining of sheet
parts with one or two straps and so on.
Typical adhesive bonded joints
are shown in figure: -
Fig. 1.2 Main type of adhesive bonded joint
The strength of joints is
dependent on the amount of clearance it which is normally kept at 0.05
to1.5mm.with increased clearance the strength of joints decreases, as the
length of overlapping is increases the force need to break down teethe joints
increases asymptotically approaching a certain limit. Surface rough ewes of the
parts bonded should be held to within 6.3 to 1micro meter. Increase in curing
time has a favorable effect on the strength of adhesive-bonded joints. With cold curing the strength grows
continuously over a long period of time, the strength of bonded with old curing
adhesive increases if the polymer process is accompanied by heating. Heating also
greatly reduces the curing time.
1.9.3.1 Making an Adhesive
Bonded joints
An adhesive-bonding process
comprises the following steps:
(1) Preparation of part surfaces.
(2) Preparation and application
of the adhesive.
(3) Assembly of parts under a pressure
of determined by the grade of adhesive.
(4) Heating of assembly product.
Fig. 1.3 Hand
Pneumatic Injector
The surface to be bonded must be
cleaned and degreased. Cleaning is done with wiping wastes, brushes or in sand
blaster. The substance used for degreasing is: acetone, trichloroethylene,
carbon tetra chloride and other organic solvents. Aluminum alloys parts are
prepared by pickling, where necessary the surface to be joined is machined to
obtain a surface finish to provide a better holding if the adhesive.
Adhesive are prepared in special
polychloroethylene or metallic containers; the will be chrome plated or varnish
with silicone. Hot curing adhesive can be stored in container for a long time.
Cold curing adhesive are prepared just before the uses there pot life is30 to
40 min.
The method of application on an
adhesive is depending upon its viscosity. Liquid adhesive, tat can be applied
with brushes and sprays are uses most commonly. Some grades of adhesive are
convenient to apply with spatulas, roller or injectors.
The adhesive can be spread in a
thin layer (0.1 to 0.2mm) with a bristle brush or a spatula. To prevent
frothing the adhesive must be applied moving the brush in one direction.
In hand pneumatic injectors,
compressed air is supplied through an inlet connection. The air extrudes the
piston by means of piston through nozzle having a dia. of 1mm.
After the application of adhesive
the parts are assembled in special fixture and clamped by means of lever
mechanism, spring or pneumatic clamping devices. Clamping force must ensure a
unit pressure of 0.05to 1MN/m2.
Lastly heat is heat is effected
in cabinets equipped with electric or gas heater. The heating temp. and the
curing temp depend upon the composition of adhesive for intake a heating temp.
of 150 to 160 c and a curing time is1.5 h is needed for a cold curing
adhesive base on epoxy resin. For a hot
curing adhesive based on epoxy resin, a curing time of 3 to 4h at 150 to 160 or
1.5 to 2h at180 to 190c is recommended.
Adhesive should be chandelled very care fully
as their constituents are toxic. The work therefore should be done with gloves
on, under proper exhaust ventilation.
1.9.4 ADHISIVES: -
There is large variety of adhesive available for
bonding with metal and metal with non-metallic materials. They can be
classified into the following main group:
(1) Adhesive Based On Epoxy Resins: -
The
available epoxy resin based adhesives are both cold and hot curing. These are
used to cold and hot joining of metal ceramics plastic wood and other material.
In cold curing adhesive a curing
element such polythene polyamide
98 to10 parts by mass) or
hexamethyldiamine (20 parts by mass) is added to 100parts by mass of resin.
Maleic anhydride (40 parts by mass) is added as a curing agent to the resin in
making hot curing adhesive. There various epoxy resin are used as given below
with the curing given within the brackets: -
Epoxy (room temp. cure, 16to32c)
Epoxy (elevated temp. cure, 93 to
177c)
Epoxy nylon (121to 177c)
Epoxy phenoilic (121 to177c)
(2) Phenol-Resin Based Resins: -
Various
compounds modify these. Curing take place 150cwith the joined component held
against each other. Phenol polyvinyl acetate is available readymade without
subsequent introduction of curing agent. These adhesive can sustain temp. Up to
100 c. phenolic rubber and phenolic resin based adhesive s modified by organic
solvent polymer or silicon compounds feature high temp. resistance Phenol
formaldehyde issued to bond foam plastic textile laminate. The common adhesive in
this group are:
Neoprene-phenolic
(135-177c)
Niotrile_phenolc (135-177c)
Butryl-phenolc (135-177c)
(3) Polyurethane Adhesive: -
These adhesive have resistance to
temp. up to 100 to 120 c and same strength as polyvinyl acetate adhesive.
(4) Special Grade Adhesive: -
These
are used to high temp. Resistance and poses high shearing strength.
1.9.4.1
Bonding Plastic Parts: -
The above-discussed adhesive and
special purpose adhesive is used to bond plastic parts. For many thermoplastic
these solvent serve as a adhesive, or example dichloro ethane for organic
glass, benzol for polystyrene, acetone for viniplast etc. the scope of
automation for adhesive-bonding process is the application of adhesive to the
mating surfaces, assembly and accurate location of the parts bonded, and
subsequent curing. Adhesive can be applied with roller and feed with injector
into the clearance b/w the mating parts; dipping the mating parts into it is
also practicable.
1.10 SURFACE COATING FOR TOOLING: -
In almost every type of production
tooling the most desirable feature to have is a very hard surface on allow
strength but tough body. Toughness is needed to survive mechanical shocks that
are impact loading in interrupted cuts. Shocks occur in even continuous chip
formation process, when the counters the localized hard spot. The example of
such tooling include metal cutting tools rock drills, cutting blades, forging
die, screw for extrusion of plastic and food products and saw mills and so on.
Other application include parts of earth moving machinery, valves and valves
seat for diesel engines and many such parts involving high heat application and
in general application requiring wear resistance. The various techniques
employed for this purpose are discussed below: -
1.10.1 Hard Facing: -
It is the
technique of depositing a layer of hard metal on component to increase the
hardness, strength of base metal. The techniques is widely used in bearings,
cam shaft, valves and valves seats, hot extrusion dies, closed dies especially
for abrasive powder, earth handling, and mining equipment of many type such as
rock drills stone crushers. Hammer mills shear blades and much type of cutting
and trimming dies.
The composing
of surfacing metal differs from that of the base metal. Hard facing materials
such include satellite and other cutting and wear resistance alloys. Tip or rod
from 5 to 10 mm thick, cat from satellite alloys, or used for hard facing of
tool y welding technique. The cutting tool material with very hard phases has
such as high alloying element concentration that they cannot be manufacture
into welding rods. The ingredients are incorporated in the flux coating or
packed inside tubular rods, and the alloys are formed in welding process
itself.
Hard facing is
by means of gas, arc or shielded arc welding techniques. Gas and shield arc
welding are more uniform composition of deposited layer. Surfacing by ordinary
a arc welding is cheaper and faster, but there is greater danger of dilution of
metal with the base metal. Deposition of tungsten carbide by an electric arc is
called spark hardening. When
thick layer are deposit one speaks of weld overlays. However the
thickness of deposit should not exceed than 2mm because the susceptibly to
cracking increase with thicker layers.
Hard facing techniques
and conditions should ensure a strong bond of the deposit with the base metal,
restrict their mixing and avoid the formation of cracks and other defects in
the deposit layer. Parts to be hard faced are first pre heated to 350 to500c;
the hard faced parts are to be cooled slowly.
Hard facing
should increase the service life of certain part by 3 to 4 times and worn parts
to be repeatedly restored.
1.10 .2 Nitriding Case
Hardening: -
It is surface
hardening processing which the surface of steel is saturated with nitrogen. In
consist of heating the part to a temp. Of 480c to 650c inside a chamber through
which a stream of NH3 is passed ammonia gets dissociated:
2NH3 = 2N +
3H2.
The nitride
parts very high surface hardness (730 to 1100 BHN). Nitriding increase the wear
resistance in air water and water vapor.
Nitriding is
usually to medium carbon and alloy steels containing Al, Cr, Mo, and other
elements capable of forming nitrides. Prior to nitriding parts should be
hardened tempered and undergo to the complete sequence of machining process
including grinding. Only finish grinding and lapping is done after nitriding.
The nitride case is usually0.2 to 0.4 mm thick.
Nitriding done
at low temp. as compared to hardening and carburizing, so it requires more
time. But since no quenching is necessary as the high hardness is obtain
directly after the operation. The feature enables the hardening effects to be
avoided.
a) Hard Chrome Plating: Hard chrome plating is done by electrolytic
electroplating technique. It is most common process for wear resistance.
b) Flame Plating: - Flame plating is a
process develops to prolong the life of certain type of tool and there wear
applications. By this process, a carefully controlled coating of tungsten
carbide, chromium carbide (Cr3C2) or aluminum oxide is applied to a wide range
of base metals the more common material, which have been successfully,
flame-plated include: aluminium, brass, bronze, cast iron, ceramics, copper,
glass/ H.S.S., magnesium, molybdenum, nickel steel, and titanium and their
alloys.
The uses a
specially designed gun into which is admitted metered amount of oxygen and
acetylene. A change of fine particles of the selected plating mixtures is
injected into the mixture of oxygen and acetylene. Immediately a valve opens to
admit he a stream of nitrogen to protect the valve during the subsequent
detonation. The mixture is now ignited and the explosion is take place, which
plasticizes the particles and hurls in them from gun with a velocity of 750
m/s. The particle gets embedded into the surface and a microscopic welding take
place, which produces a highly tenacious bond.
Each particle
in the coating is elongated and flattened into thin disc. The coating has a
dense fine and grain laminar structure with negligible porosity and absence of
void and oxide layer.
The layer of
the plated materials about 0.006mm, this layer can build up by repeating the
explosion, to thickness ranging from0.05 to0.75mm, according to the requirement
of subsequent operations. The resultant layer well dense, hard and well bonded.
Because of the
hard dense structure of coatings, flame plating has provided industry with a
valuable tool for the solving of many abrasion, erosion and wear problem. For
example bushes for many applications, core pin for powder metallurgy, dies,
gauges, journals, mandrels, and seals for high duty pumps, have all being given
much longer lives.
The process
has influenced considerably certain type of cutting process, especially in the
glass leather, soap, and textile industries has proved to be great advantage
for component involving high heat application such as “hot-end” of gas turbine.
The coating
shows an excellent resistance to galling and corrosion. Flame-plated coating
can be ground and lapped if necessary. Resultant surface be with in the reason
of 0.025micro meter. Another advantage is that the components can be enabling
to mast the coatings to be placed preciously where required.
The mixture of
tungsten carbide coating can be consisting of cobalt ranging from 7% to17% and
rest of tungsten carbide.
Aluminum oxide
plating mixture is almost of Al2O3 (above 99%). Chromium carbide plating
consist of about 75%yo 85% of Cr3C2 and balance of (Ni-Cr).
1.10.3 Chemical Vapor
Decomposition: -
Chemical vapor
decomposition uses volatile metal compounds, which are carried as a vapour in a
glass stream and deposit as metal upon any surface that is hot enough to
produce the desired reaction.
Vapor phase
decomposition can be done by two methods:
i) In decomposition
method a decomposition halide is vaporized metered and transported by means of
inert carrier gas to the heated component, where it decomposes at the surface
of yield pure metal.
ii) In the second method, a
reduction process, hydrogen is used as the carrier gas through a purifier and
dry hydrogen chemically reduces the halide to pure metal on the part surface as
shown in fig. HFC, HFN. Multiple coating of Al2O3 can be
given on top of Al2O3.coating thickness is in micrometer
For depositing a layer of TiC on
carbide tool inserts a mixture of hydrogen methane and titanium trichloride gas
is form in the mixing chamber. The mixture of these gases can flow through next
chamber enrich carbide inserts are heated up to about 1000c by induction
heating or by resistance heating. The following reaction takes near the surface
of the parts:
TiCl4 +CH4 = TiC +4HCl.
TiC so produce get adhere to the
surface of the substance that is WC. The main advantage of CVD process is its
ability to produce:
a) High density coatings because
the coatings are built by atom by atom.
b) High purity materials.
c) High strength materials.
d) And complex shapes.
An emerging coating technology
used particularly for multiphase coatings, is medium temp. CVD (MTCVD). It is
being developed to machine ductile iron and stainless steels and to provide
higher resistance to crack propagation than conventional CVD.
Fig. 1.4 Reduction Method of
CVD
1.10.4 Physical Vapor Decomposition: -
In the basic
form of PVD method applying sufficient heat with help of one of many techniques
evaporates metal or an oxide. The atom or molecule so produced move in all
direction. When they come into atomic or molecular attraction of the component
that is the substrate they condense onto it to form a uniform coating.
In a variation
of method, a cathode target is bombarded by accelerated ion. This concept
dislodges or driving of single atom or small cluster into surrounding gas for
deposition on a near by substrate surface. To increase coating adhesion and
improve film structure, the substrate surface is heated to temp. From about
200c to500c.
PVD process
particularly suited to tin coating of H.S.S. tools, because it being a
relatively low temp. Processes, the tempering temp. Point of H.S.S. is not
reached. So after the PVD process the heat treatment is not needed.
1.10.5 Diffusion
Coatings: -
The surface
hardness of low carbon steel (with carbon lees than 2%) can be increased by
making hardneble by diffusing carbon or nitrogen into the surface. On heating or quenching the carbon-nitrogen
enriched surface is very hard but core remains tough. The surface can also be
hardened by ‘ion nitriding’ method where the steel surface is bombarded by low
energy nitrogen ions produced in plasma.
1.10.6 Ion
Plating: -
In this method
high-energy ion are penetrated into the surface. For cutting tools, nitrogen
ions are almost commonly user. There is virtually no change in dimension in the
last two processes.
1.11 GRAPHITE MOULD CASTING:
Graphite is
high refractory substance. Graphite moulds are used for casting metals, such as
titanium, that tends to react with many common mould materials. Graphite is
used for Moulding in much the same manner as plaster. For this graphite is
available in an investment type of mixture, which is obtained by combining
powdered graphite with cement, starch & water. This slurry is compacted
around a precision mc. metal pattern. The pattern is removed & mould is
fired at 1000 C, producing a solid graphite mould, which is then poured. After
solidification of the metal the moulds is broken for the removal of the
casting. Graphite moulds have an advantage over plaster moulds as they may be
reused. Graphite moulds can also with stand with heat of grey, ductile or
malleable metals. However the size of moulds of graphite is limited is the
order of 50 x 45 x 25cm. casting upto a mass of about 23 kg can be produced.
1.12 VACCUM MOULDING PROCESS:
Sand
can be lodged in place if air is removed from the sand mass. This principle is
employed in vacuum moulding process in which no binder is used.
1. A thin plastic sheet is draped over pattern positioned
over the mould board. Vacuum is drawn on the pattern. This makes plastic to be
tightly drawn over the pattern surface.
2. A vacuum flask is placed over the pattern & is
filted with clean unbounded sand. Pouring basin & the sprue are formed and
another plastic sheet is placed over the sand.
3. Vacuum is drawn on the sand. This makes sand very
hard.
4. Vacuum is now released on the pattern & it is
withdrawn.
5. Similarly the second half of the mould is made.
Fig. 1.5
Vacuum Moulding Process
The two moulds halves are
assembled & molten metal is poured. The plastic sheet will burn up. When
the casting is solidified, vacuum on the flask is released. The sand collapse
and the casting are taken out.
The process is also known as
V-process & has followed.
1.12.1 Advantages
1.
Saving on the binder cost, as no binder is used in
process.
2.
No defect released to moisture & binder fumes.
3.
Any sand can be used
4.
Easy shake out. However the process is quit
slowly.
1. 13 INTRODUCTION TO COMPOSITE
MATERIALS:
Composite materials can be defined as the structures
made up of two or more distinct starting materials. The starting materials can
be organic, metals or ceramics. The components of composite materials do not
occur naturally as an alloy, but are separately manufactured before these are
combined together mechanically. Due to this, they maintain their identities,
even after a composite material is fully formed. However the starting materials
combine to rectify a weakness in one material by strength in another material.
Hence composite material exhibits properties distinctly different from those of
individual materials used, to make composite. Thus composite material or
structure possesses a unique combination of properties such as stiffness,
strength, hardness, weight, conductivity, corrosion resistance & high temp.
Performance etc. that is not possible by individual materials. Thus the search
for materials with special properties to suit some specific stringent
conditions of use has given rise to development of materials called “COMPOSITE
MATERIALS”.
1.13.1 TYPES OF COMPOSITE MATERIALS:
Composite materials may roughly be classified as:
1) Agglomerated materials/ Particulate composites.
2) Reinforced materials.
3) Laminates.
4) Surface coated materials.
The particulate composite and reinforced composites
are constituted by just two phases, the matrix phase. The aim is to improve the
strength properties of matrix material. The matrix material should be ductile
with its modulus of elasticity much lower then that of dispersed phase. Also
the bonding forces between the two phases must be very strong.
In fact the particulate composite also fall in the category
of reinforced composites. Depending upon the nature of reinforced materials
(shape and size), the reinforced composites can be classified as
1. Particle reinforced composites or particulate
reinforced composites.
2. Fiber reinforced composite.
In particulate reinforced
composites, dispersed phase is in the form of exi-axed particles, whereas in
fibre-reinforced composite, it is in the form of fibers.
1.13.1.1 AGGLOMERATED
MATERIALS:
Agglomerated
materials consist of discrete particles of one material, surrounded by matrix
of another material. The material is bounded together in an integrated mass to
classic eg. Of such composite material are: concrete formed by mixing gravel,
sand, cement & water & agglomeration of asphalt & stone particles,
that is used for paving the high surfaces. Other eg. Of particulate composite
material includes:
1) Grinding and cutting wheels, in which
abrasive particles (Al2O3, Sic, CBN or carbon) are held together by a vitreous
or a resin bond.
2) Cemented carbide, in which particles of
ceramic materials such as WC, TaC, TiC & of cobalt & nickel, are
bounded together via Powder metallurgy process to produce cutting tool
materials. Many powdered metal parts & solid sintering produces various
magnetic & dielectric ceramic materials, which requires diffusion.
3) Electrical contact point from powder of
tungsten & silver or copper is process via powder metallurgy method.
4) Electrical Brushes for motored & heavy
duty & frictional materials for brake & clutches by combining metallic
& non-metal. Materials.
5)
Cu infilterated iron & silver, tungsten.
6) Heavy metal (w+6%ni+4%cu)
7) Electrical resistance welding electrodes from
mixture of cu & tung.
8) Shell moulding sand, using a resin binder,
which is polymerized by hot pattern.
9) Metal polymer structers (metal bearing in
filtered with nylon or PTFE).
10) Particleboard, in which wood chips are held
together by suitable glue.
11) Elastomers & plastics are also reinforced
with suitable particle material. The eg. Is addition of 15-30 of carbon black
in the vulcanized rubber for automobiles types?
12)
Dispersion strengthened materials: in these materials
hard, brittle and fine particles are dispersed in softer or more ductile
matrix.
Because of their unique geometry,
the properties of particulate composite can be isotropic. This property is very
important in many engineering applications
1.13.1.2 REINFORCED MATERIAL:
Reinforced materials from the
biggest and most important group of composite materials. The purpose of
reinforcing is always to improve the strength properties. Reinforcement may
involve the use of a dispersed phase (discussed in the last article) or strong
fiber, thread or rod.
Fiber-reinforced materials: in a larger number of applications, the
material should have high strength, along with toughness and resistance to
fatigue failure. Fiber reinforced materials, offer the solution. Stronger or
higher modulus filler, in the form of thin fibers of one material, is strongly
bonded to the matrix of another. The matrix material provides ductility and
toughness and supports and blinds the fibres together and transmits the load.
The toughness of the composite material increases, because extra energy will be
needed to break or pull out a fibre. Also, when any crack appears on the
surface of a fibre, only that fibre will fail and the crack will not propagate
catastrophically as in bulk material. Failure is often gradual, and repairs may
be possible.
Due to the above mentioned desirable
properties of the matrix materials, the commonly used matrix materials are;
Metals and polymers, such as, Al Cu, Ni etc. and commercial polymers strong
fibers in the relatively weak matrix. Like this, it is possible to produce
parts where strength control is developing in different directions. if the part is loaded parallel to the fibers will be
much greater than in the matrix . Even if the fibre breaks, the softness of
matrix hinders the propagations of crack .The fibre direction are tailored to
the direction of loading.
Reinforced Fibers: A good reinforcing fibre should have: high elastic
modulus, high strength, low density, reasonable ductility and should be easily
wetted by the matrix. Metallic fibres such as patented steel; stainless steel,
tungsten and molybdenum wires are used in a metal matrix such as aluminum and
titanium. Carbon fibers and whiskers are also used in a metal ultra –high
strength composite. Fibres need not be limited to metals. Glass, ceramic and
polymer fibers are used to produce variety of composite having wide range of
properties .The high modulus of ceramic fibers make them attractive for the
reinforced of the metal. The ductile matrix materials can be aluminum
magnesium, nickel or titanium and the reinforcing fiber may be of boron ,
graphite , aluminum or SiC.
Forms of reinforcing fibres: The fibers used for reinforcing
materials are available in different forms:
(a)
Filaments:
these are very long and continuous single fibres.
(b)
Yarns: this
is twisted bundles of filaments.
(c)
Roving:
These are untwisted bundles of gathered filaments.
(d)
Tows: These
are bundles of thousands of filaments.
(e)
Woven fabrics:
These are made from filaments, yarn or roving which have been woven at 90
degree to each other.
(f)
Mats: Fibre
form is said to be mat form when the continuous fibre is deposited in a swirl
pattern or chopped fibre is deposited in a random pattern.
(g)
Combination mat:
Here, one ply of woven roving is bonded to a ply of chopped strand mat.
(h)
Surface mats:
These are very thin, monofilament fibre mats for better surface appearance.
(i)
Chopped fibre or
roving: These are 3 to 50 mm in length.
(j)
Milled fibres:
These are of brittle materials, usually 0.5 to 3 mm in length.
(k) Whiskers: whiskers are single crystal,
in the form of fine filaments, a few microns in diameter and short in length.
These single crystal whiskers are the strongest known fibers. Their high
strength is due to the high degree of perfection and the absence of dislocation
in the structure. Their strength is many times greater than that of the normal
metals. For ex The strength of an iron whisker is found to be 13450MN\m2,
compared to about 294MPa for a piece of pure iron,. Besides metal whiskers,
long non metallic, whiskers and of graphite are being produced. They are
introduced in to resin or metallic matrix for the purpose of high strength and
high stiffness at high temperatures.
The
properties of reinforced materials will depend on:
- The properties of matrix materials.
- The properties of the fibre materials.
- The proportions of the reinforcement in the composite materials. It is never less than 20% and may go up to 80% in oriented structures.
- The orientation of the fibre, relative to the load application and relative to one another.
- The degree of bonding between the fibers and the matrix material.
- The length to diameter ratio of the fibers.There has to be some minimum fibre length, known as, critical length, lc, to get the desired strength and stiffness of the composite materials. It is given as:
L c = σ
f .d / Ø
Where, σf =Tensile strength of fibre
materials
d =diameter of fibre
Ø
=shear yield strength of the fibre matrix bond
Fig. 1.6
Reinforcing Fibers
For example, for carbon and glass
fibers, the critical length is of the order of 1mm, which may be 20 to150 times
the diameter of the fibre.
The fibre reinforcement can be
done in three ways:
1. Continuous and aligned, Fig a
2. Discontinuous and aligned, Fig
b
3. Continuous and randomly
oriented, Fig c
If the fibre length is
considerably greater than Lc e.g., 15 times or more, it is called a “continuous
fibre”, otherwise it is called “short” or “discontinuous fibre as noted above,
the properties of a composite having aligned fibre reinforcements, are highly
anisotropic, that is, they depend upon the direction in which these are
measured. Their maximum strength is along the direction of alignment. They are
very weak in the transverse direction. The arrangement is best suited for
application involving multi- direction applied stresses, for e.g., bi-axel
stresses in pressure vessel or tube. The same results can be achieved by using
bi- axially oriented or cross – ply fibers. It is apparent that the strength of
the discontinuous and aligned arrangement will be less than of the continuous
and aligned arrangement.
Applications: As discussed in the beginning, composite structures
combine the desirable properties of two or more materials. This has greatly
expanded the scope of application of all engineering materials. This has
greatly expanded the scope of application of all engineering materials. We can
produce components with exceptional strength –to –weight and stiffness
–to-weight ratio (many composite are stronger than steel, lighter than steel
and stiffer than titanium). Also, they have low conductivity, good heat
resistance, good fatigue life, adequate wear resistance and are free from
corrosion.
Reinforced concrete is a classic
example of reinforced materials. Steel rods used in the concrete to reinforce
the material take all tensile loads since concrete weak in tension but strong
in compression.
1. Glass- fibre reinforced Plastics: Here,
we have glass fibres in a matrix of unsaturated polyester. To get better
qualities to use at high temperature, high temperature polyamide resin is used
with pure SiO2 fibres. A special type of glass fibre can be used
with cement bond to form flexible type of concrete. Glass fibre reinforced
plastics are used to make: boat hulls, car bodies, truck, cabins and aircraft
fittings. The other matrix materials can be: vinyl ester and phenolic.
2. C-C composites: These composites have
graphite fibres in a carbon matrix. This material is being used to make: Nose
cone and leading edge of missiles and space shuttles, racing car disks brakes,
aerospace turbine and jet engine components, rocket nozzles and surgical
implants.
3. Graphite fibre- reinforced epoxy :( Organic or Resin matrix composites): This
material is being used to make many parts of a fighter plane: Wing span,
outrigger flaring. Overwing flaring, engine access doors, nose cone, forward
fuselage. Lid fence and strakes-flap. Flap slot door, aileron seals, Horizontal
stabilizer (Full span) and rubber. The other fibre-matrix combination can be:
Aramid fibre-Phenolic resin matrix, Boron fibre-Bismaleimide resin matrix.
4. Automative uses: Body panels, drive
shafts, spring and bumpers, Cab shell and bodies, oil pans, fan shrouds,
instrument panels and engine covers.
5. Sports equipment: Golf club shafts,
base ball parts, fishing rods, tennis rackets, bicycle frames, skis and pole
vaults.
6. Rubber
used for making automobiles tyres is now reinforced with fibres of nylon, rayon
steel or Kevlar, to provide added strength and durability. Kevlar is an organic
aramid fibre with very high tensile strength and modulas of elasticity. Its
density is about half of that of aluminum and it has negative thermal
expansion. It is flame retardant to radio signals. This makes it very
attractive for military and aerospace applications. It is also being used for
making bullet proof jackets. The trade name “Kevlar” is given by Du Pont.
7. Metal
matrix composite (MMC): As already noted, these composites are obtained by
impregnating high-strength fibres (of stainless steel, boron, tungusten,
molybdenum, graphite, AL2O3, SiC and Si3N4
etc.) with molten metal ( aluminuim , titanium, Ni and cobalt etc). These
composites offer higher strength and stiffness especially at elevated
temperatures and lower co-efficient of thermal expansion as compared to metals.
And as compared to Organic-matrix composites, these composites offer grater
heat resistance and improved thermal and electric conductivity. Hence metal
matrix composites are used where operation temperature is high or extreme
strength is desired. These will find applications in a variety segments like
automobiles and machinery.
Aluminum oxide reinforced
aluminum is used for making automotive connecting rods. Aluminum reinforced
with SiC whiskers is used to make air craft wing panels. Fibre reinforced super
alloys are used for making turbine blades. Graphite fibres in aluminum matrix
are used for Satellite, missile, helicopter structures.Graphite fibres in
magnesium matrix is used for space and satellite structures. Graphite fibres in
lead matrix are used for Strong –battery plates. A graphite fibre in copper
matrix is used for bearings and electrical contacts. Other e.g. of MMC is:
(a)
Boron fibre in aluminum: Compressor blades and
structural supports.
(b)
““ “ magnesium : Antenna structures.
(c)
“””” Titanium: Jet-engine fan blades.
(d)
Alumina ““
Lead: Strong-battery plates.
(e)
““ “ Magnesium: Helicopter transmission
structures.
(f)
SiC “‘ Super
alloy (Cobalt based): High temperature engine components.
(g)
Tungsten and Molybundum fibres in Super alloy matrix:
High temperature engine components.
8. Ceramic-matrix composites: (CMC): AS
already noted, ceramics are strong, stiff, can resist high temperatures, but
generally lack toughness. Ceramic matrix materials are: AL2O3,
SiC and Si3N4, and mullite (a compound of Al, Si, and O2).
They can retain their strength upto 1700 degree C, and also resist corrosive
environments.
Typical product applications of
ceramic matrix composite are: in jet
and automotive engines, deep-sea mining equipment, pressure vessels, structural
components’ cutting tools, and dies for extrusion and drawing operations.
Composite in development stage:
1.
Advance bismaleinmide resin matrix series for high
temperature service.
2.
Polyether ether ketone thermoplastic matrix series for
higher temperature service.
3.
Hybrid reinforcements and Knitted/stacked ply fabrics
and three-dimensional woven fabric reinforcements.
4.
Selective stitching of collated ply kits.
1.14
LAMINATES:
Laminates or laminar composites
are those structures which have alternate layers of materials bonded together
in some manner some common examples of laminar composites are given below:
1.
Plywood: it
is most common material under this category. Here, thin layer of wood veneer
are bonded with adhesives. The successive layers have different orientations of
the grain or fibre; Structural parts capable of carrying a load are made of
multi-plywood board from 25 to 30 mm thick.
2.
Bimetallic
strips used in thermostats & other heat sensing application.
3.
Safety glass
4. Sandwich
material: Here, low density core is placed between thin, high strength
High-density
surfaces, for example, corrugated cardboard. Cores of polymer foam or honeycomb
structures can be used. Wood substitutes based on red mud polymer have been
developed to be used for door shutters, windows, partitions and false ceilings.
5.
Roll cladding
(bonding) and explosive cladding (welding) of one metal upon another: The
main aim of clad material is to improve corrosion resistance while retaining
low cost, high strength and /or lightweight. Mild steel –stainless steel
combination, copper stainless steel combination are examples of metal-to-metal
laminates. Another example is “Alclad”, which is formed by cladding duralumin
with thin sheets of pure aluminium. The material is high strength composite in
which aluminium cladding provides galvanic protection for the more corrosive
duralumin. The above claddings are done by “hot roll bonding” method.
6.
Laminated
Plastic Sheet: This structure is usually made from sheets of paper or cloth and suitable resin. The resin used
includes: phenolics, polyster,silicones and epoxide. The paper and cloth provides bulk of strength, while the resin
acts as a semi rigid binder. Laminated plastic sheet can be machined, drilled,
punched and pressed to shaped. It is used in the production of gears, bearings,
electrical components, and small cabinets. Laminate fabric base gears have the
advantages over metal gears of being silent in operation and stable against the
attack of various. Aggressive media. In many cases, laminated fabric base gears
have completely replaced nonferrous gears. They are employed to transmit
rotation from electric motor in high-speed machine tools; they are mounted on
the camshafts of internal-combustion engines etc. In chemical industry,
laminate fabric base gears are used in various apparatus & instruments
where they resist corrosive attack much more efficiently then gears of bronze
brass or leather. In addition to gears, certain other transporting devices:
roller, rings etc. are also made of laminated fabric base. Laminated sheets
/plates are available in sizes of: 900*900 mm, 900 *1800 mm, and 1200*2400 mm.
The minimum thickness of sheet is 0.8 mm & it varies as follows: -
Thickness
range (mm) 0.8-1.6 1.6-4.8 6.4-9.6 12.8- 19.2 25.6- 38.4
Step(mm) 0.4 0.8 1.6 3.2 6.4
- Tufnol: this is a laminated material consisting of layers of woven textiles impregnated with a thermosetting resin. The polymer imparts rigidity, while the woven textile provides great tensile strength. Paper or asbestos may also be used as alternative reinforcements. The material (with woven textile) can be used for making seat covers &carpets.
- Laminated carbides: In laminated carbides, laminates consisting of a hard thin surface layer TiCand the form of throw away tips, are bonded by epoxy resin to the rake face of a tip body of WC. This increases the crater wear of WC cutting tool.
- Laminated wood : this sheets of wood (veneer ) , impregnated with special resins & compressed hot , form what is called ‘laminated wood ‘, which find extensive application in textile machinery & electric engineering , as well as substitute for nonferrous metal in bearing of hydraulic machinery &mechanisms operating in abrasive media . Parts of wood are machined in ordinary machine tools &wood working machinery.
Surface coated materials: the surface coating is applied to the
materials for various purposes: - protection of the material against corrosion;
for decorative, wear resistant &processing purpose. They may also be used
to :(i) improve visibility through luminescence & better reflectivity (ii)
provide electrical insulation , & (iii) improve the appearance. Surface
coating are usually classified as: metallic coatings, inorganic chemical
coating & organic chemical coating
1. Metallic coating: metallic coating of copper, chromium
nickel, zinc, lead & tin etc. are
applied by hot dipping , electro- plating or spraying techniques to protect the
base metal from corrosion & for other purpose .
2. Inorganic chemical coating: This surface coating may be
divided into: Phosphate coating, oxide coating & vitreous coating. Oxide
& phosphate coating are done to make iron or steel surface free from rust
& this is done by chemical action. These coating also provide protection
against corrosion. Vitreous coating are commonly applied to steel in the form of
a powder or frit & are then used to the steel surface by heat. These
coating are relatively brittle, but offer absolute protection against
corrosion. Enamel is an example of ceramic coating on metal & glaze on
tiles is an example of glassy ceramic on crystalline ceramic base. The glazing
as a protective coating on porcelain & stoneware ceramic is performed for
the purpose of protection from moisture absorption in ceramic materials.
Coating of TiC , TiN , Al2o3 or HFN on WC base are examples of ceramics on
ceramic & coatings of TiC & TiN on HSS base are examples of ceramics on
steel. These coatings increase the life of cutting tools.
3. Organic Coatings:
It includes paint, varnishes, enamels & lacquers. They serve to protect the base
metal & to improve its appearance.
Polymer coating on paper are used
for making milk cartons. Polymer coated textiles are used for making seat
covers & carpets. Polymer Coatings on metals act as wire insulation.
Polymer coated metals are used for making beverage cans.
1.15
PRODUCTION OF COMPOSITE STRUCTURES
Fabrication of particulate
composites: As discussed in above art. A majority of the particulate
composites are made via the powder metallurgy route. So, for details readers
should refer to chapter 10. However, a few particulate composites are made by
dispersing the particles in the matrix materials through introduction into
slurry or into a liquid melt (agglomeration of asphalt and stone particles).
Fabrication of Fibre
reinforced Composites: Many processes have been developed to fabricate
fibre-reinforced composite structures. Their aim is to combine the fibre and
the matrix into a unified form. The various fabrication techniques depend on:
the size and the form of the fibres and their orientation in the matrix
material; the shape, size and form of the product. The common fabrication
processes are: Open-Mould process, Filament winding, Pultrusion and
Matched-die-Moulding, and Laminating. Before these processes are discussed, the
following terms should be understood:
·
Prepergs: Prepergs means “Preimpregnated
with resin”. It is ready to mould material in the sheet form. Impregnated
rovings and mats make these with resin matrix under the condition in which the
resin undergoes only a partial cure. These are stored for subsequent use. These
are supplied to the fabricator, who lays up the finished shape in stacks, which
is subjected to heat and pressure. This completes the curing of the resin into
a continues solid matrix. “Lay-up” is positioning of the reinforcement
material, sometimes resin-impregnated, in the mould.
·
BMCs: are
“Bulk Moulding Compounds”. These are thermosetting resins mixed with
chopped reinforcements or filters and made into a viscous compound for compressing
moulding.
·
SMCs: are “Sheet Moulding Compounds”.
These comprise chopped fibres and resin in the sheet form approx. 2.5 mm thick.
These are3 processed further to fabricate large sheet like parts. They can
replace sheet metal, where lightweight, corrosion resistance and integral
colour are attractive features.
·
Thick Moulding Compounds: Thick Moulding
Compounds (TMC) combines the lower cost of BMC and higher strength of SMC.
These are usually injection moulded using chopped fibres of various lengths.
Used for electrical components due to their high electrical strength.
1) Open – Mould Process ~ In this process, only one mould (Die) is
employed to fabricate the reinforced part. The mould may be made of: wood,
plaster or reinforced plastic material. The various techniques in this category
are:-
a) Hand lay-up technique: In this
method, the successive layers of reinforcement mat or web (which may or may not
be impregnated with resin) are positioned on a mould by hand. Resin in used to
impregnate or coat the reinforcement. Curing the resin to permanently fix the
shape then follows it. Curing may be at room temperature or heating may speed
it up. The technique in which resin-saturated reinforcements are placed in the
mould is called “Wet lay-up”.
b) Bag Moulding: This is a technique of moulding reinforced
plastics composites by using a flexing cover (bag) over a rigid mould. The
composite material is positioned in the mould and covered with the plastic film
(bag). Pressure is then applied by a : Vacuum, auto-clave, press or by inflating the bag . An auto-clave is a
closed pressure vessel for inducing a resin cure or other operation under heat
and pressure.
i)
Vacuum-bag moulding: In this technique for moulding
reinforced plastics, a sheet of flexible, transparent material is placed over
the lay-up on the mould. After sealing the edges the entrapped air between the
sheet and the lay-up is mechanically worked out and removed by the vacuum.
Finally, the part is cured.
Fig. 1.7
Vacuum Bag Moulding
ii) Pressure-bag Moulding: It is a process for moulding reinforced
plastics in which a tailored, flexible bag is placed over the contact lay-up on
the mould, sealed and clamped in placed. Compressed air forces the bag against
the part to apply pressure while the part cures.
iii) Spray-up: In this technique, a spray gun supplies resin in two
converging streams into which chopped roving fiber is forced with the help of a
chopper. The composite material stream is then deposited against the walls of
the mould cavity. It is a low-cost method of fabricating medium strength
composite structures.
All the above open-mould
techniques are extensively used for fabricating parts such as: boats, tanks,
swimming pools, ducts and truck bodies.
2) Matched-die moulding: Matched metal dies are used for moulding
composite structure when: production quantities are large, tolerances are close
and surface quality has to be the best. The dies are heated to complete the
curing of the product during the moulding process.
i) Compression Moulding is
essentially employed for moulding BMCs.
ii) Resin- Transfer Moulding or Resin Injection Moulding: In this
technique (RTM or RIM), two piece matched cavity dies are used with one or
multiple injection points and breather holes. The reinforcing material, which
is either chopped or continuous strand material is cut to shape and draped in
the die-cavity. The die-halves are clamped together and a polyester resin is
pumped through an injection port in the die. The pressure used in the die is
low, which allows use of low cost tooling. The method is used for moulding
small non-load bearing parts.
In a variant of the above
technique, instead of the injection of only resin into the die-cavity, the
reinforcement (flake glass) is mixed with the resin in a mixing head and the mixture
is injected into the closed heated two-piece die. Flake glass is preferred to
avoid directionality of reinforcement. This method is known as “Reaction
Injection Moulding” and is being increasingly used for BMCs.
iii) SMCs cut to size, are
fabricated into parts by methods similar to metal pressing. However, curing of
the part takes place outside the press.
Fig. 1.8
Compression Moulding
3) Pultrusion: This is the process of extrusion of
resin-impregnated roving ( a bundle of fibres ) to manufacture rods, tubes and
structural shapes (Channels, I-beams and Z- Sections etc.) o0f a constant
cross-section. After passing through the resin-dip tank, the roving is dawn
through a heated die (where curing takes place) and cured to form the desired
cross-section, as it continuously runs through the machine. After the Puller
rolls, a saw cutter cuts the extruded section to the required lengths.
In “ Pulmoulding”, the process
begins with pultruding; then the part is placed in a compression mould.
Product applications are: - Golf
club shafts, because of their high damping capacity, and structural members for
vehicle and aerospace applications.
Fig. 1.9
Pultrusion
4) Filament Winding: In this process, resin impregnated strands are
applied over a rotating mandrel, to produce high strength, reinforced
cylindrical shapes. Fibers or tapes are drawn through a resin bath and wound
onto a rotating mandrel. The process is relatively slow, but the fiber
direction can be controlled and the diameter can be varied along the length of
the piece. In a variation, the Fiber bundle (made up of several thousand carbon
fibers) is first coated with the matrix material, to make a prepreg tape (endless
strip with width equal to several cms, by a meter). With both the fiber and
tape winding processes, the finished part is cured in an autoclave and later
removed from the mandrel. In axial winding, the filaments are parallel to the
axis and in circumferential winding; these are essentially perpendicular to the
axis of rotation.
Cylindrical, spherical and other
shapes are made by filament winding, for example, pressure bottles, missile
canisters, industrial storage tanks and automobile drive shafts C- fibers with
epoxy- basin resin composite is used for fabricating strength- critical
aerospace structures.
Fig. 1.10
Filament Winding Process
5) Laminating: In this
process, composite parts are produced by combining layers of resin-impregnated
material in a press under heat and pressure. The parts include, standard for
comparatively flat pieces. Two principal steps in the manufacture of laminated
fiber-reinforced composite materials are:-
(a) Lay-up, which consists of
arranging fibers in layers.
(b) Curing
We start with a preperg material
(partially cured composite with the fibers aligned parallel to each other). A
pattern of product’s shape is cut out the preperg material is then stacked in
layers into the desired laminate geometry. Curing the stacked pile under heat
and pressure in an autoclave makes a final product, or by tool press moulding,
winding the impregnated fibre on a mandrel of suitable diameter produces tubes.
The assembly is then cured in a moulding press and then the mandrel is removed.
1.16 FABRICATION OF MMC:
Basically, three approaches are
followed for fabricating MMC
1. Liquid phase approach: In this technique, the matrix material is
the molten phase and the reinforcement is in the solid state. Either one of the
conventional casting process can be used to fabricate MMC or “Pressure
infiltration casting method “can be used. In this method, a perform is made
(usually a sheet or wire) of reinforcing fibres and the liquid metal matrix is
forced into it with the help of a pressurized gas.
2. Solid phase technique: Here the Powder Metallurgy route is used
to fabricate MMC. The best example is of manufacturing WC tool material where
cobalt is used as the matrix material.
3. Two phase Processing: Here the metal matrix contains both the solid
and liquid phases. The reinforcing fibres are mixed with the matrix. The
mixture is then atomized when it leaves the nozzles and is sprayed and
deposited over the surface of a mould cavity to fabricate MMC.
PROCESSING OF CMC
The most common method of fabricating
CMC is of “Slurry infiltration”. A perform of reinforcing fibres is prepared
which is then hot pressed. Slurry containing matrix powder, a carrier liquid
and an organic binder is prepared. The perform is then impregnated with the
slurry to fabricate CMC.
1.17 MACHINING
CUTTING AND JOINING OF COMPOSITES
Conventional processes and tools
are generally not suited for machining, cutting and joining of composites.
Therefore, special methods are employed to the final processing operations for
the composites.
1. Machining: Machining of composite materials should ensure that
there is no splintering, cracking, fraying or delaminating of cured composite
edges. Standard machine tools can be used with appropriate modifications.
Cutting tools for composites include: drills, reamers, countersinks, cut-of
wheels and router bits. Common cutting tool materials are: HSS and WC. However,
poly-crystalline diamond insert tool performs satisfactory and is cost
effective. Tools must be kept sharp, to provide quality cuts and avoid
de-lamination. Tool and its geometry should be carefully selected. Cutting
speeds and feeds will depend on the type of composite material, its thickness
and the cutting method.
2. Cutting: The conventional methods for cutting uncured composites,
such as preperg ply include: manual cutting with Carbide disk cutter, scissors
and power shears. For cutting uncured composites, the main techniques are:
reciprocating knife cutting, high pressure water jet cutting, ultrasonic knife
cutting and laser cutting.
3. Joining: The common joints provided for composites structures
are: Bolted joints and Adhesive bonded joints.
Prepared
By- Er. CP SINGH
(Asst.
Prof)
(Mech
Engg Dept)
E-Max
Group of Institutions, Ambala
