manufacturing science-1 unit-5 and unit 4

7/29/2019 Manufacturing Science-1 Unit-5 and Unit 4

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MANUFACTURING SCIENCE-1

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Centrifugal Casting:

In this process, the mold is rotated rapidly about its central axis as the metal is poured into it. Because of

the centrifugal force, a continuous pressure will be acting on the metal as it solidifies. The slag, oxides

and other inclusions being lighter get separated from the metal and segregate towards the center. Thisprocess is normally used for the making of hollow pipes, tubes, hollow bushes, etc., which are

axisymmetric with a concentric hole. Since the metal is always pushed outward because of the

centrifugal force, no core needs to be used for making the concentric hole.

The mold can be rotated about a vertical, horizontal or an inclined axis or about its horizontal and

vertical axes simultaneously. The length and outside diameter are fixed by the mold cavity dimensions

while the inside diameter is determined by the amount of molten metal poured into the mold.

Horizontal Centrifugal Casting Vertical Centrifugal casting

Features:

Following are the main features of centrifugal casting process:

Process is suitable only for products, which have rotational symmetry.

General process is economical for ring shaped objects, tabular shaped objects and hollow

cylinders, e.g. compressor cases, winding spools, furnace rollers etc.

No core is needed to form the bore as in static casting.

Temperature gradients during cooling can be controlled to some extent by controlling speed of

rotation. Centrifugal pressures can be applied to advantage in checking premature freezing and

imparting strength to the casting.

Main advantage of centrifugal casting is that the porosity free castings are obtained.

Advantages:

Formation of hollow interiors in cylinders without cores

Less material required for gate

Fine grained structure at the outer surface of the casting free of gas and shrinkage cavities and

porosity


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Disadvantages:

More segregation of alloy component during pouring under the forces of rotation

Contamination of internal surface of castings with non-metallic inclusions

Inaccurate internal diameter

Casting Terms:

1. Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed.Depending upon the position of the flask in the molding structure, it is referred to by various

names such as drag lower molding flask, cope upper molding flask, cheek intermediate

molding flask used in three piece molding.

2. Pattern: It is the replica of the final object to be made. The mold cavity is made with the help ofpattern.

3. Parting line: This is the dividing line between the two molding flasks that makes up the mold.4. Molding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a

mixture of silica sand, clay, and moisture in appropriate proportions.

5. Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of themold cavity to give a better surface finish to the castings.

6. Core: A separate part of the mold, made of sand and generally baked, which is used to createopenings and various shaped cavities in the castings.

7. Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten metalis poured.

8. Sprue: The passage through which the molten metal, from the pouring basin, reaches the moldcavity. In many cases it controls the flow of metal into the mold.

9. Runner: The channel through which the molten metal is carried from the sprue to the gate.10.Gate: A channel through which the molten metal enters the mold cavity.11.Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own

weight and overcome the metallostatic force.

12.Riser: A column of molten metal placed in the mold to feed the castings as it shrinks andsolidifies. Also known as feed head.


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13.Vent: Small opening in the mold to facilitate escape of air and gases.

Mold Section showing some casting terms

Steps in Making Sand Castings:

There are five basic steps in making sand castings:

1. Patternmaking

2. Core making

3. Molding

4. Melting and pouring

5. Cleaning

1. Pattern making:The pattern is a physical model of the casting used to make the mold. The mold is made by

packing some readily formed aggregate material, such as molding sand, around the pattern.

When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled

with metal to become the casting. If the casting is to be hollow, as in the case of pipe fittings,

additional patterns, referred to as cores, are used to form these cavities.

2. Core making:


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Cores are forms, usually made of sand, which are placed into a mold cavity to form the interior

surfaces of castings. Thus the void space between the core and mold-cavity surface is what

eventually becomes the casting.

3. Molding:Molding consists of all operations necessary to prepare a mold for receiving molten metal.

Molding usually involves placing a molding aggregate around a pattern held with a supporting

frame, withdrawing the pattern to leave the mold cavity, setting the cores in the mold cavity

and finishing and closing the mold.

4. Melting and Pouring:The preparation of molten metal for casting is referred to simply as melting. Melting is usually

done in a specifically designated area of the foundry, and the molten metal is transferred to the

pouring area where the molds are filled.

5. Cleaning:Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from

the casting. Burned-on sand and scale are removed to improved the surface appearance of the

casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed.

Inspection of the casting for defects and general quality is performed.

Die casting:

Die casting is an efficient method of creating a broad range of shapes, die castings are one of the most

mass produced components today and are found in many items in and around the home. Many toy carsuse die casting in their production, as do real vehicles. Die casting offers high accuracy in its products

with a good quality surface finish which is suitable for many products without the need for extra

polishing or machining.

Die Casting Machines:

A die casting machine performs the following functions

Holding the two die halves firmly together.

Closing the die.

Injecting molten metal into the die.

Opening the die.

Ejecting the casting out of the die.


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A die casting machine consists of four basic elements namely

(i) Frame

(ii) Source of molten metal and molten metal transfer

(iii) Dies

(iv) Metal Injection Mechanism.

These machines are classified on the basis of injection mechanisms and are of two types:

(i) Hot chamber Die casting, and

(ii) Cold chamber Die casting.

The main difference between these two types is that in hot chamber, the holding furnace for the liquid

metal is integral with the die casting machine, whereas in the cold chamber machine, the metal ismelted in a separate furnace and then poured into the die-casting machine with a laddle for each

casting cycle which is also called shot.

The Process:

Die casting first requires the creation of a steel mould (called a die) of the part to be cast, these moulds

once created are fitted to the die casting machine and injected under pressure with the desired molten

metal or alloy of choice. There are two methods of injection, these being hot chamber and cold

chamber.

Hot Chamber method:

Hot chamber casting machines use oil or gas powered piston to drive the molten metal heated within

the machine into the die. The piston pulls back allowing the molten metal to fill what is called the

goose neck once the liquid metal has filled the goose neck the piston can then force the liquid metal

into the die. The clamping force used to inject the metals can range from 400-4000 tons. This method

has fast cycle times which can be as low as seconds when producing small parts.


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Hot Chamber Die Casting

Cold Chamber Method:

Cold chamber casting machines do not heat the metal, the molten metal must be ladled into the cold

chamber manually or by an automatic ladle system, the molten metal is then forced into the die by a

hydraulic piston at high pressure.

Cold Chamber Die Casting

Advantages of Die Casting Process:

(i) Very high rates of production can be achieved.

(ii) Close dimensional tolerance of the order of 0.025 mm is possible.


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(iii) Surface finish of 0.8 micron is achievable.

(iv) Very thin sections of the order of 0.50 mm can be cast.

(v) Fine details may be produced.

(vi) Less floor space is required.

(vii) Longer die life is obtained.

(viii) Unit cost is minimum.

Disadvantages of Die Casting Process:

(i) Not economical for small runs.

(ii) Only economical for non-ferrous alloys.

(iii) Heavy castings cannot be cast. In fact, the size of the dies and the capacity of the die casting

machines available limit the maximum size.

(iv) Cost of die and die casting equipment is high.

(v) Die castings usually contain some porosity due to entrapped air.

Applications:

The typical products made by die casting are carburetors, crank cases, magnetos, handle bar housings,

parts of scooters and motor cycles, zip fasteners, head lamp bezels, and other decorative automobile

items.

Moulding:

Moulding is the process of making a mould with the desired cavity to pour the molten metal. Desirable

Characteristics of mould are:

1. The mould must be strong enough to hold the weight of the metal.

2. The mould must resist the erosive action of the rapidly flowing molten metal during

pouring.

3. The mould must generate minimum amount of gas when filled with molten metal.

4. The mould must provide enough venting so that any gases formed can pass through the

body of the mould itself, rather than penetration the material.


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5. The mould must be refractory enough to withstand the high temperature of the molten

metal and strip away cleanly from the casting after cooling.

6. The mould must permit the casting to contract after solidification.

The basic components found in many molds are shown below

The terms for the parts of a mold are

Pouring cup - the molten metal is poured in here. It has a funnel shape to ease pouring accuracy

problems.

Runner/sprue - a sprue carries metal from the pouring cup to the runners. The runners

distribute metal to the part.

Gate - a transition from the runner to the cavity of the part

Riser - a thermal mass where excess metal will remain in a liquid state while the part cools. As

the cooling part shrinks, the molten metal in the riser will feed or fill in the shrinkage. Risers can

also be used to collect impurities that rise in molten metal.

Mold cavity - this is the final shape of the part.

Vent - a narrow escape passage for gases that would otherwise be trapped in the mold.

Parting line - a line of separation that allows the mold (made in two pieces) to be put together

to make a full cavity. Note that this line does not have to be a straight line, and is often

staggered to make the mold making easier.

Cope - the upper part of a casting mold

Drag - the lower part of a casting mold

Methods and Types of Moulding Processes:

The different moulding processes may be classified as follows:

According to the method used:

(a) Floor Moulding

(b) Bench Moulding


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(c) Pit Moulding

(d) Machine Moulding

According to the mould materials:

(a) Green Sand Moulding

(b) Dry Sand Moulding

(c) Loam Sand Moulding

(d) Core Sand Moulding

Moulding Process:

Floor Moulding:

This method of moulding is commonly used for preparing the mould of heavy and large size of jobs

which cannot be conveniently moulded through bench moulding method. In floor moulding, the floor

itself acts as a drag. It is preferred for such rough type of castings where the upper surface finish has no

importance.

Bench Moulding:

Bench moulding is done on a work bench of a height convenient to the moulder. It is best suited to

prepare the mould of small and light items which are to be casted by non-ferrous metals.

Pit Moulding:

Large size of jobs which cannot be accommodated in moulding boxes are frequently moulded in pits.

Here, the pit acts as a drag. Generally, one box, i.e. cope is sufficient to complete the mould. Runner and

riser, gates and pouring basin are cut in it.

Machine Moulding:

Machine moulding method is preferred for mass production of identical casting as most of the moulding

operations such as ramming of sand, rolling over the mould, and gate cutting etc. are performed by the

moulding machine. Therefore, this method of moulding is more efficient and economical in comparison

to hand moulding.

Types of Moulds:

Green Sand Moulding:


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Green sand consists of silica sand, 10 to 15 percent clay and 4 to 6 percent moisture content. All these

materials are thoroughly mixed and riddled. It should also be given the required condition by proper

tempering.

Dry Sand Moulding:

This process of moulding is just similar to green sand moulding except the composition of constituents in

mixture. Here, in the preparation of mixture for dry sand moulding, special binding materials such as

resin, molasses, flour, or clay are mixed to give strong bond to the sand. All parts of mould are

completely dried before casting. Dry sand moulding is widely used for large size of work such as parts of

engine, large size of fly wheel and rolls for rolling mill. This process is costlier than green sand moulding

but much superior in quality.

Loam Sand Moulding:

This process is used for extremely large size of casting which are to be made in very small numbers.Loam sand moulds are prepared with coarse grained silica sand, clay, coke, horse manure and water.

This process of moulding is performed in different way. First, a rough structure of desired shape is made

by hand by using bricks and loam sand. This structure is then finished by means of strickle and sweep.

The surfaces of structure are blackened and dried before being casted.

Core Sand Moulding:

For core sand moulding, mixture is prepared with silica sand, olivine, carbon and chamotte sands. Sand

that contains more than 5% clay may not be used as a core sand. For core making by hand, the core

sand is filled and rammed in the core box properly. The whole operation takes a short time after that

the core box is withdrawn and the core removed.

Design of Riser:

The function of a riser is to supply addition molten metal to a casting to ensure a shrinkage porosity free

casting. Shrinkage porosity occurs because of the increase in density from the liquid to solid state of

metals. To be effective a riser must solidify after the casting and contain sufficient metal to feed the

casting or portion of a casting.

Risers are reservoirs of molten material. They feed this material to sections of the mold to compensate

for shrinkage as the casting solidifies. There are different classifications for risers.

Top Risers: Risers that feed the metal casting from the top.

Side Risers: Risers that feed the metal casting from the side.

Blind Risers: Risers that are completely contained within the mold.

Open Risers: Risers that are open at the top to the outside environment.


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Casting solidification time can be predicted using Chvorinovs Rule.

Where tTS is the total solidification time of the part or riser, C is a mold constant, V is the volume of

metal, and Asurfis the total surface area of the part or riser. Chvorinovs Rule provides guidance on why

risers are typically cylindrical. The longest solidification time for a given volume is the one where the

shape of the part has the minimum surface area. From a practical standpoint, the cylinder has the least

surface area for its volume and is easiest to make. Since the riser should solidify after the casting, we

want its solidification time to be longer than the casting. If we want the riser to take 20% longer than

the riser then we can write the following expression:

The term occurs so frequently it is given a special name. It is called the casting modulus. By using the

variable M to represent the casting modulus and simplifying, the above equation can be reduced to.

This expression is used for the simplest method for desiging a riser. It is called the modulus method.

While modern computer methods make it easier to optimize the riser, an initial guess of the correct

geometry is needed.


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Design of Runner:

The runner is a horizontal distribution channel that accepts the molten metal from the sprue and

delivers it to the gates.

One runner is used for simple parts, but-two runner systems can be specified for more

complicated castings.

The runners are used to trap dross (dross is a mixture of oxide and metal and forms on the

surface of the metal) and keep it from entering the gates and the mold cavity.

Commonly, dross traps are placed at the ends of the runners, and the runner projects above the

gates to ensure that the metal in the gates is trapped below the surface.

Runner System Layout

Risers (size and location) are extremely useful in affecting-front progression across a casting and are

essential feature in the mold layout. Blind risers are good design features and maintain heat longer than

open risers. Risers are designed according to six basic rules:

1. The riser must not solidify before the casting.

2. The riser volume must be large enough to provide a sufficient amount of liquid metal to

compensate for shrinkage in the cavity.

3. Junctions between casting and feeder should not develop a hot spot where shrinkage porosity

can occur.

4. Risers must be placed so that the liquid metal can be delivered to locations where it is most

needed.


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5. There must be sufficient pressure to drive the liquid metal into locations in the mold where it is

most needed.

6. The pressure head from the riser should suppress cavity formation and encourage complete

cavity filling.

Factors of Directional Solidification:

As the molten metal cools and solidifies in the mould, shrinkage of metal will take place which creates

voids inside. Solidification of metal free of internal voids and shrinkage is called as a directional

solidification. The factors which are used to control this directional solidification are

(a) Proper design and positioning of risers,

(b) Proper design and positioning of gating systems,

(c) Use of padding,

(d) Use of metal chills, and

(e) Use of exothermic materials.

Riser:

It is a passage of sand made in the cope part of moulding box through which the molten metal rises after

the mould is filled up completely. The main functions of a riser are given below:

a. Riser acts as a reservoir and feeds the molten metal to the casting to compensate the shrinkage

during solidification.

b. It permits the escape of gas, air and steam as the mould cavity is being filled up with the molten

metal.

c. It controls the solidification time, which should be greater in it than that in the mould cavity.

d. It helps to ensure that the mould cavity has been completely filled up with molten metal.

Use of Padding:

Padding means adding of some extra metal to the original section of casting in varying thickness to

attain the required directional solidification.

Use of Chills:


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Chills in shape of extra metal are also used in achieving directional solidification. If a casting consists of

sections of uneven thickness, rate of cooling will be different as per thickness of section. The thin

sections tend to solidify earlier than the thick ones, resulting in uneven shrinkage and severe distortion.

To accelerate the cooling rates of thick sections, chills are inserted in these sections and thus, obtain the

desired directional solidification.

Use of Exothermic Materials:

The proper directional solidification of casting can be further controlled by the use of exothermic

materials. These materials produce large amount of heat when come in contact with the molten metal.

These are added to the surface of molten metal through riser side. The materials used as exothermic

materials are the oxides of iron, copper and nickel etc. mixed with suitable amount of aluminum.

Sodium Silicate Molding Process (CO2):

In this process, the refractory material is coated with a sodium silicate-based binder. For molds, the sand

mixture can be compacted manually, jolted or squeezed around the pattern in the flask. After

compaction, CO2 gas is passed through the core or mold. The CO2 chemically reacts with the sodiumsilicate to cure, or harden, the binder. This cured binder then holds the refractory in place around the

pattern. After curing, the pattern is withdrawn from the mold.

The sodium silicate process is one of the most environmentally acceptable of the chemical processes

available. The major disadvantage of the process is that the binder is very hygroscopic and readily

absorbs water, which causes a porosity in the castings.. Also, because the binder creates such a hard,

rigid mold wall, shakeout and collapsibility characteristics can slow down production. Some of the

advantages of the process are:

A hard, rigid core and mold are typical of the process, which gives the casting good dimensional

tolerances.

Good casting surface finishes are readily obtainable.

Sodium Silicate-CO2 Moulding Material:

Moulds (and cores) can also be made from a sand that receives its strength from the addition of 3 to 4%

sodium silicate, a liquid inorganic binder that is also known as water glass. The sand can be mixed with

the liquid sodium silicate in a standard muller and can be packed into flasks by any of the methods. It

remains soft and mouldable until it is exposed to a flow of CO2 gas, after which it hardens in a matter of

seconds by the reaction.

The CO2 gas is nontoxic and odorless, and no heating is required to drive the reaction. The hardened

sands, however, have poor collapsibility, making shakeout and core removal difficult. Unlike most other

sands, the heating that occurs as a result of the pour makes the mould even stronger (a phenomenon

similar to the firing of a ceramic material). In addition, care must be taken to prevent the carbon dioxide

in the air from hardening the sand before the mould-making process is complete. A modification of the

CO2 process can be used when certain portions of a mould require higher strength, better accuracy,


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thinner sections, or deeper draws than can be achieved with ordinary moulding sand. Sand mixed with

sodium silicate is packed around a metal pattern to a depth of about 1 inch, followed by regular

moulding sand as a backing material. After the mould is fully rammed, CO2 is introduced through vents

in the metal pattern. This hardens the adjacent sand, and the pattern can now be withdrawn with less

possibility of damaging the mould.

Inspections of Casting:

Inspection of castings is done to detect the internal and external defects in them. The inspection

methods can be broadly classified as:

a. Destructive testing methods.

b. Non-destructive testing methods.

Due to some drawbacks destructive testing methods is not used much. Hence non-destructive testing is

used. Various methods of these are discussed below:

Visual inspection:

Visible defects that can be detected provide a means for discovering errors in the pattern equipment or

in the molding and casting process. Visual inspection may prove inadequate only in the detection of sub

surface or internal defects.

Dimensional inspection:

Dimensional inspection is one of the important inspections for casting. When precision casting is

required, we make some samples for inspection the tolerance, shape size and also measure the profile

of the cast. This dimensional inspection of casting may be conducted by various methods:

Standard measuring instruments to check the size of the cast.

Contour gauges for the checking of profile, curves and shapes

Coordinate measuring and Marking Machine

Special fixtures

X-Ray Radiography:

In all the foundries the flaw detection test are performed in the casting where the defects are not

visible. This flaw detection test is usually performed for internal defects, surface defects etc. These testsare valuable not only in detecting but even in locating the casting defects present in the interior of the

casting. Radiography is one of the important flaw detection tests for casting. The radiation used in

radiography testing is a higher energy (shorter wavelength) version of the electromagnetic waves that

we see as visible light. The radiation can come from an X-ray generator or a radioactive source.

Magnetic particle inspection:


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This test is used to reveal the location of cracks that extend to the surface of iron or steel castings, which

are magnetic nature. The casting is first magnetized and then iron particles are sprinkled all over the

path of the magnetic field. The particles align themselves in the direction of the lines of force. A

discontinuity in the casting causes the lines of the force to bypass the discontinuity and to concentrate

around the extremities of the defect.

Fluorescent dye-penetration test:

This method is very simple and applied for all cast metals. It entails applying a thin penetration oil-base

dye to the surface of the casting and allowing it to stand for some time so that the oil passes into the

cracks by means of capillary action. The oil is then thoroughly wiped and cleaned from the surface. To

detect the defects, the casting is pained with a coat of whitewash or powdered with tale and then

viewed under ultraviolet light. The oil being fluorescent in nature, can be easily detect under this light,

and thus the defects are easily revealed.

Ultrasonic Testing:

Ultrasonic testing used for detecting internal voids in casting is based on the principle of reflection ofhigh frequency sound waves. If the surface under test contains some defect, the high frequency sound

waves when emitted through the section of the casting, will be reflected from the surface of defect and

return in a shorter period of time. The advantage this method of testing over other methods is that the

defect, even if in the interior, is not only detected and located accurately, but its dimension can also be

quickly measured without in any damaging or destroying the casting.

Fracture test:

Fracture test is done by examining a fracture surface of the casting. it is possible to observe coarse

graphite or chilled portion and also shrinkage cavity, pin hole etc. The apparent soundness of the casting

can thus be judged by seeing the fracture.

Macro-etching test (macroscopic examination):

The macroscopic inspection is widely used as a routine control test in steel production because it

affords a convenient and effective means of determining internal defects in the metal. Macroetching

may reveal one of the following conditions:

Crystalline heterogeneity, depending on solidification

Chemical heterogeneity, depending on the impurities present or localized segregation and

Mechanical heterogeneity, depending on strain introduced on the metal, if any.

Sulphur Print test:

Sulphur may exist in iron or steel in one of two forms; either as iron sulphide or manganese sulphide.

The distribution of sulphur inclusions can easily examined by this test.

Microscopic Examination:


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Microscopic examination can enable the study of the microstructure of the metal alloy, elucidating its

composition, the type and nature of any treatment given to it, and its mechanical properties. In the case

of cast metals, particularly steels, cast iron, malleable iron, and SG iron, microstructure examination is

essential for assessing metallurgical structure and composition. Composition analysis can also be done

using microscopic inspection. Distribution of phase can be observed by metallographic sample

preparation of cast product. Grain size and distribution, grain boundary area can be observed by thisprocedure. Distribution of nonmetallic inclusion can also be found from this process of inspection.

Chill Test:

Chill test offers a convenient means for an approximate evaluation of the graphitizing tendency of the

iron produced and forms an important and quick shop floor test for ascertaining whether this iron will

be of the class desired. In chill test, accelerated cooling rate is introduced to induce the formation of a

chilled specimen of appropriate dimension. It is then broken by striking with a hammer in such a manner

that the fracture is straight and midway of its length. The depth of chill obtained on the test piece is

affected by the carbon and silicon present and it can therefore be related to the carbon equivalent,

whose value in turn determines the grade of iron.

Defects in Casting Processes:

Figure schematically shows various defects that are experienced during casting, in particular, sand

casting processes. A brief explanation of some of the significant defects and their possible remedial

measures are indicated in the text to follow.

Shrinkage:

These are caused by the liquid shrinkage occurring during the solidification of the casting. An improperriser and gating system may give this type of defect which has a shape of a funnel.

Porosity:

Porosity is a phenomenon that occurs in materials, especially castings, as they change state from liquid

to solid during the manufacturing process. Casting porosity has the form of surface and core

imperfections which either effects the surface finish or as a leak path for gases and liquids. The poring

temperature should be maintained properly to reduce porosity. Adequate fluxing of metal and

controlling the amount of gas-producing materials in the molding and core making sand mixes can help

in minimizing this defect.

Hot tear:

Hot tears are internal or external ragged discontinuities or crack on the casting surface, caused by rapid

contraction occurring immediately after the metal solidified. They may be produced when the casting ispoorly designed and abrupt sectional changes take place; no proper fillets and corner radii are provided,

and chills are inappropriately placed. Hot tear may be caused when the mold and core have poor

collapsibility or when the mold is too hard causing the casting to undergo severe strain during cooling.

Incorrect pouring temperature and improper placement of gates and risers can also create hot tears.

Method to prevent hot tears may entail improving the casting design, achieving directional solidification

and even rate of cooling all over, selecting proper mold and poured materials to suit the cast metal, and

controlling the mold hardness in relation to other ingredients of sand.


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Scar:

It is usually found on the flat casting surface. It is a shallow blow.

Blowhole:

Blowholes are smooth round holes that are clearly perceptible on the surface of the casting. To prevent

blowholes, moisture content in sand must be well adjusted, sand of proper grain size should be used,

ramming should not be too hard and venting should be adequate.

Blister:

This is a scar covered by the thin layers of the metal.

Dross:

The lighter impurities are appearing on the top of the cast surface is called the dross. It can be taken

care of at the pouring stage by using items such as a strainer and a skim bob.

Dirt:

Sometimes sand particles dropping out of the cope get embedded on the top surface of a casting. When

removed, these leave small angular holes is known as dirts.

Wash:

It is a low projection on the drag surface of a casting commencing near the gate. It is caused by the

erosion of sand due to high velocity liquid metal.

Buckle:

It refers to a long fairly shallow broad depression at the surface of a casting of a high temperature metal.

Due to very high temperature of the molten metal, expansion of the thin layered of the sand at the mold

face takes place. As this expansion is obstructed by the flux, the mold tends to bulge out forming a V

shape.

Rat tail:

It is a long shallow angular depression found in a thin casting. The cause is similar to buckle.

Shift:

A shift results in a mismatch of the sections of a casting usually as a parting line. Misalignment is

common cause of shift. This defect can be prevented by ensuring proper alignment of the pattern for die

parts, molding boxes, and checking of pattern flux locating pins before use.

Warped casting:


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Warping is an undesirable deformation in a casting which occurs during or after solidification. Large and

flat sections are particularly prone to wrap edge. Wrap edge may also be due to insufficient gating

system that may not allow rapid pouring of metal or due to low green strength of the sand mold or

inadequate / inappropriate draft allowance in the pattern / mold cavity.

Metal Penetration and Rough Surfaces:

This defect appears as an uneven and rough external surface of the casting. It may be caused when the

sand has too high permeability, large grain size, and low strength. Soft ramming may also cause metal

penetration.

Fin:

A thin projection of metal, not intended as a part of casting, is called a fin. Fins occur at the parting of

the mold or core sections. Molds and cores in correctly assembled will cause the fin. High metal

pressures due to too large down sprue, insufficient weighing of the molds or improper clamping of flasks

may again produce the fin defect.

Cold Shut and Mis-Run:

A cold shut is a defect in which a discontinuity is formed due to the imperfect fusion of two streams of

metal in the mold cavity. The reasons for cold shut or mis-run may be too thin sections and wall

thickness, improper gating system, damaged patterns, slow and intermittent pouring , poor fluidity of

metal caused by low pouring temperature, improper alloy composition, etc.


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Powder Metallurgy:

Introduction:

Powder metallurgy is used for manufacturing products or articles from powdered metals by placing

these powders in molds and are compacting the same using heavy compressive force.

Typical examples of such article or products are grinding wheels, filament wire, magnets, welding rods,

tungsten carbide cutting tools, self-lubricating bearings electrical contacts and turbines blades having

high temperature strength.

The manufacture of parts by powder metallurgy process involves the manufacture of powders, blending,

compacting, profiteering, sintering and a number of secondary operations such as sizing, coining,

machining, impregnation, infiltration, plating, and heat treatment. The compressed articles are then

heated to temperatures much below their melting points to bind the particles together and improve

their strength and other properties. Few non-metallic materials can also be added to the metallic

powders to provide adequate bond or impart some the needed properties. The products made through

this process are very costly on account of the high cost of metal powders as well as of the dies used. The

powders of almost all metals and a large quantity of alloys, and nonmetals may be used.

The application of powder metallurgy process is economically feasible only for high mass production.

Parts made by powder metallurgy process exhibit properties, which cannot be produced by


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conventional methods. Simple shaped parts can be made to size with high precision without waste, and

completely or almost ready for installation.

Powder Metallurgy manufacturing process:

The powder metallurgy process consists of the following basic steps:

1. Formation of metallic powders.

2. Mixing or blending of the metallic powders in required proportions.

3. Compressing and compacting the powders into desired shapes and sizes in form of articles.

4. Sintering the compacted articles in a controlled furnace atmosphere.

5. Subjecting the sintered articles to secondary processing if needed so.

Applications of powder metallurgy:

The powder metallurgy process has provided a practical solution to the problem of producing refractory

metals, which have now become the basis of making heat-resistant materials and cutting tools of

extreme hardness. Another very important and useful item of the products made from powdered metals

is porous self-lubricating bearing. In short, modern technology is inconceivable without powder

metallurgy products, the various fields of application of which expand every year. Some of the powder

metal products are given as under.

1. Porous products such as bearings and filters.

2. Tungsten carbide, gauges, wire drawing dies, wire-guides, stamping and blanking tools, stones,

hammers, rock drilling bits, etc.

3. Various machine parts are produced from tungsten powder. Highly heat and wear resistant

cutting tools from tungsten carbide powders with titanium carbide, powders are used for and

die manufacturing.

4. Refractory parts such as components made out of tungsten, tantalum and molybdenum are

used in electric bulbs, radio valves, oscillator valves, X-ray tubes in the form of filament,

cathode, anode, control grids, electric contact points etc.

5. Products of complex shapes that require considerable machining when made by otherprocesses namely toothed components such as gears.

6. Components used in automotive part assembly such as electrical contacts, crankshaft drive or

camshaft sprocket, piston rings and rocker shaft brackets, door, mechanisms, connecting rods

and brake linings, clutch facings, welding rods, etc.


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7. Products where the combined properties of two metals or metals and non-metals are desired

such as non-porous bearings, electric motor brushes, etc.

8. Porous metal bearings made which are later impregnated with lubricants. Copper and graphite

powders are used for manufacturing automobile parts and brushes.

9. The combinations of metals and ceramics, which are bonded by similar process as metal

powders, are called cermets. They combine in them useful properties of high refractoriness of

ceramics and toughness of metals. They are produced in two forms namely oxides based and

carbide based.

Limitations of powder metallurgy:

1. Powder metallurgy process is not economical for small-scale production.

2. The cost of tool and die of powder metallurgical set-up is relatively high.

3. The size of products as compared to casting is limited because of the requirement of large

presses and expensive tools which would be required for compacting.

4. Metal powders are expensive and in some cases difficult to store without some deterioration.

5. Intricate or complex shapes produced by casting cannot be made by powder metallurgy because

metallic powders lack the ability to flow to the extent of molten metals.

6. Articles made by powder metallurgy in most cases do not have as good physical properties as

wrought or cast parts.

7. It may be difficult sometimes to obtain particular alloy powders

8. Parts pressed from the top tend to be less dense at the bottom.

9. A completely deep structure cannot be produced through this process.

10. The process is not found economical for small-scale production.

11. It is not easy to convert brass, bronze and a numbers of steels into powdered form.

Advantages & disadvantages of powder metallurgy:

Advantages:

1. The processes of powder metallurgy are quite and clean.


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2. Articles of any intricate or complicated shape can be manufactured.

3. The dimensional accuracy and surface finish obtainable are much better for many applications

and hence machining can be eliminated.

4. Unlike casting, press forming machining, no material is being wasted as scrap and the processmakes utilizes full raw material

5. Hard to process materials such as diamond can be converted into usable components and tools

through this process.

6. High production rates can be easily achieved.

7. The phase diagram constraints, which do not allow an alloy formation between mutually

insoluble constituents in liquid state, such as in case of copper and lead are removed in this

process and mixtures of such metal powders can be easily processed and shaped through this

process.

8. This process facilitates production of many such parts, which cannot be produced through other

methods, such as sintered carbides and self-lubricating bearings.

9. The process enables an effective control over several properties such as purity, density,

porosity, particle size, etc., in the parts produced through this process.

10.The components produced by this process are highly pure and bears longer life.

11. It enables production of parts from such alloys, which possess poor cast ability.

12. It is possible to ensure uniformity of composition, since exact proportions of constituent metal

powders can be used.

13.The preparation and processing of powdered iron and nonferrous parts made in this way exhibit

good properties, which cannot be produced in any other way.

14.Simple shaped parts can be made to size with 100 micron accuracy without waste

15.Porous parts can be produced that could not be made in any other way.

16.Parts with wide variations in compositions and materials can be produced.

17.Structure and properties can be controlled more closely than in other fabricating processes.

18. Highly qualified or skilled labor is not required. in powder metallurgy process


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19.Super-hard cutting tool bits, which are impossible to produce by other manufacturing processes,

can be easily manufactured using this process.

20.Components shapes obtained possess excellent reproducibility.

21.Control of grain size, relatively much uniform structure and defect such voids and blowholes instructure can be eliminated.

Disadvantages:

1. Metal powders deteriorate quickly when stored improperly

2. Fixed and setup costs are high

3. Part size is limited by the press, and compression of the powder used.

4. Sharp corners and varying thickness can be hard to produce

5. Non-moldable features are impossible to produce

Powder Metallurgy manufacturing process:

The powder metallurgy process consists of the following basic steps:

1. Formation of metallic powders.

2. Mixing or blending of the metallic powders in required proportions.

3. Compressing and compacting the powders into desired shapes and sizes in form of articles.

4. Sintering the compacted articles in a controlled furnace atmosphere.

5. Subjecting the sintered articles to secondary processing if needed so.

Production of Metal Powders:

Metallic powders possessing different properties can be produced easily. The most commonly

used powders are copper-base and iron-base materials. But titanium, chromium, nickel, andstainless steel metal powders are also used. In the majority of powders, the size of the particle

varies from several microns to 0.5 mm. The most common particle size of powders falls into a

range of 10 to 40 microns. The chemical and physical properties of metals depend upon the size

and shape of the powder particles. There are various methods of manufacturing powders. The

commonly used powder making processes are given as under.

1. Atomization

2. Chemical reduction


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3. Electrolytic process

4. Crushing

5. Milling

6. Condensation of metal vapors

7. Hydride and carbonyl processes.

The above mentioned metallic powder making techniques are discussed briefly as under.

1. Atomization:In this process, the molten metal is forced through an orifice and as it emerges, a high pressure

stream of gas or liquid impinges on it causing it to atomize into fine particles. The inert gas is

then employed in order to improve the purity of the powder. It is used mostly for low melting

point metals such as tin, zinc, lead, aluminium, cadmium etc., because of the corrosive action of

the metal on the orifice (or nozzle) at high temperatures. Alloy powders are also produced by

this method.

2. Chemical Reduction Process:In this process, the compounds of metals such as iron oxides are reduced with CO or H2 attemperatures below the melting point of the metal in an atmosphere controlled furnace. The

reduced product is then crushed and ground. Iron powder is produced in this way

Fe3O4 + 4C = 3Fe + 4CO

Fe3O4 + 4CO = 3Fe + 4CO2

Copper powder is also produced by the same procedure by heating copper oxide in a stream of

hydrogen.

Cu2 + H2 = 2Cu + H2O

Powders of W, Mo, Ni and CO can easily be produced or manufactured by reduction process

because it is convenient, economical and flexible technique and perhaps the largest volume of

metallurgy powders is made by the process of oxide reduction.

3. Electrolytic Process:Electrolysis process is quite similar to electroplating and is principally employed for the

production of extremely pure, powders of copper and iron. For making copper powder, copper

plates are placed as anodes in a tank of electrolyte, whereas, aluminium plates are placed in to

the electrolyte to act as cathodes. High amperage produces a powdery deposit of anode metal

on the cathodes. After a definite time period, the cathode plates are taken out from the tank,

rinsed to remove electrolyte and are then dried. The copper deposited on the cathode plates is

then scraped off and pulverized to produce copper powder of the desired grain size. The

electrolytic powder is quite resistant to oxidation.

4. Crushing Process:The crushing process requires equipments such as stamps, crushers or gyratory crushes. Various

ferrous and non-ferrous alloys can be heat-treated in order to obtain a sufficiently brittle

material which can be easily crushed into powder form.

5. Milling Process:The milling process is commonly used for production of metallic powder. It is carried out by

using equipments such as ball mill, impact mill, eddy mill, disk mill, vortex mill, etc. Milling and

grinding process can easily be employed for brittle, tougher, malleable, ductile and harder


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metals to pulverize them. A ball mill is a horizontal barrel shaped container holding a quantity of

balls, which, being free to tumble about as the container rotates, crush and abrade any powder

particles that are introduced into the container. Generally, a large mass to be powdered, first of

all, goes through heavy crushing machines, then through crushing rolls and finally through a ball

mill to produce successively finer grades of powder.

6. Condensation of Metal Powders:This process can be applied in case of metals, such as Zn, Cd and Mg, which can be boiled and

the vapors are condensed in a powder form. Generally a rod of metal say Zn is fed into a high

temperature flame and vaporized droplets of metal are then allowed to condense on to a cool

surface of a material to which they will not adhere. This method is not highly suitable for large

scale production of powder.

7. Hydride and Carbonyl Processes:High hardness oriented metals such as tantalum, niobium and zirconium are made to combine

with hydrogen form hydrides that are stable at room temperature, but to begin to dissociate

into hydrogen and the pure metal when heated to about 350C. Similarly nickel and iron can be

made to combine with CO to form volatile carbonyls. The carbonyl vapor is then decomposed in

a cooled chamber so that almost spherical particles of very pure metals are deposited.

Characteristic of Metal Powders:

The performance of powder metallurgical parts is totally dependent upon the characteristics of

metal powders. Most important characteristics of metal powders are powder particle size, size

distribution, particle shape, purity, chemical composition, flow characteristics and particle

microstructure. Some of the important properties are discussed as under.

Powder particle size and size distribution:

Particle size of metal powder is expressed by the diameter for spherical shaped particles and by

the average diameter for non-spherical particle as determined by sieving method or microscopicexamination. Metal powders used in powder metallurgy usually vary in size from 20 to 200

microns. Particle size influences density/porosity of the compact, mold strength, permeability,

flow and mixing characteristics, dimensional stability, etc. Particle size distribution is specified in

terms of a sieve analysis i.e. the amount of powder passing through 20 or 40 mesh sieves.

Particle shape:

There are various shapes of metal powders namely spherical, sub-rounded, rounded, angular,

sub-angular, flakes etc. Particles shape influences the packing and flow characteristics of the

powders.

Chemical composition:Chemical composition of metallic powder implies the type and percentage of alloying elements

and impurities. It usually determines the particle hardness and compressibility. The chemical

composition of a powder can be determined by chemical analysis methods.

Particle microstructure:

Particle microstructure reveals various phases, inclusions and internal porosity.


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Apparent density:

Apparent density is defined as the weight, of a loosely heated quantity of powder necessary to

fill a given die cavity completely.

Flow characteristics:

Flow-ability of metal powders is most important in cases where moulds have to be filled quickly.Metal powders with good flow characteristics fill a mould cavity uniformly.

Mixing or Blending of Metallic Powders:

After the formation of metallic powders, proper mixing or blending of powders is the first step in

the forming of powder metal parts. The mixing is being carried out either wet or dry using an

efficient mixer to produce a homogeneous mixture.

Compacting of Powder:

Compacting is the technique of converting loose powder in to compact accurately defined shape

and size. This is carried out at room temperature in a die on press machine. The press used for

compacting may be either mechanically or hydraulically operated. The die consists of a cavity of

the shape of the desired part. Metal powder is poured in the die cavity and pressure is applied

using punches, which usually work from the top and bottom of the die as shown in figure. Dies

are usually made of high grade steel, but sometimes carbide dies are used for long production

runs. In compacting process, the pressure applied should be uniform and applied simultaneously

from above and below. The pressure applied should be high enough to produce cold welding of

the powder. Cold welding imparts a green strength, which holds the parts together and allows

them to be handled. The metal parts obtained after compacting are not strong and dense. To

improve these properties, the parts should be sintered.

Sintering:

Sintering is the process of heating of compacted products in a furnace to below the melting

point of at least one of the major constituents under a controlled atmosphere. The sinteringtemperature and time vary with the following factors-

I. Type of metal powder

II. Compressive load used, and

III. Strength requirements of the finished parts.


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Powder metallurgy die setup

In the sintering furnace, the metal parts are gradually heated and soaked at the required temperature.

During this gradually heating process, powders bond themselves into coherent bodies. Sintering results

in strengthening of fragile green compacts produced by the pressing operation. It also increases

electrical conductivity, density and ductility of the powder metal parts.

Secondary Operations:

Some powder metal parts may be used in the sintered condition while in some other cases additional

secondary operations have to be performed to get the desired surface finish, close tolerance etc.. The

secondary operations may be of following types-

i. Annealing.

ii. Repressing for greater density or closer dimensional control.

iii. Machining.

iv. Polishing.

v. Rolling, forging or drawing.

vi. Surface treatments to protect against corrosion.

vii. In some cases infiltration is needed to provide increased strength, hardness, density obtainable

by straight sintering.

viii. The procedures for plating powdered metal parts are quite different from those used for

wrought or cast metal parts. In powdered metal parts, porosity must be eliminated before the

part is plated. After the porosity has been eliminated regular plating procedures can be used.


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Plastic welding defects and causes:

Defect: Poor weld penetration or poor bonding

Cause: Improper or superficial preparation of the welding area. Speed of welding was too fast or the

temperature was too low. The weld was made with dissimilar materials.

Defect: Uneven weld bead width

Cause: Welding rod was stretched. Irregular pressure on the welding rod

Defect: Carbonized welding

Cause: Welding speed too slow Welding temperature too high

Defect: Warping

Cause: Overheated repair area Parts fixed under tension Poor site preparation

Defect: No wash or bead is visible

Cause: Welding speed too fast Temperatures too low

Defect: Low and deformed weld

Cause: Too much pressure has been applied to the welding rod. Rotary burr was not correctly positioned duringpreparation.

WHY ARE PLASTICS BEING USED MORE FREQUENTLY TODAY?

Some of the more important reasons plastics are used more today are: Low weight with good

mechanical qualities, good electrical insulating ability, excellent resistance to chemicals and weather and

low thermal conductivity. Plastics offer varied processing possibilities. Plastics can be easily cut, filed,

formed and connected. Many of the techniques which are used for wood and metals can also be applied

to plastics.


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A good chemist can make a plastic for any application. In today's world, not only is plastic being used

more, but also new plastics are being made every day. The welding characteristics of thermoplastics

today vary greatly, so plastic welding tools must be flexible yet very precise to weld these new

thermoplastics properly.

HOW DO YOU RECOGNIZE THE DIFFERENT PLASTICS?

In order to work with plastics, it is necessary to determine what the material is. Different recognition

methods are used for determining what the plastic is. The most common one is the flame test. (See

Leister Leaflet 21A for details). Also on the market is a chemical test. Some plastics are clearly marked

with the standard symbols listed below:

PVC-P polyvinylchloride soft

PVC-U Polyvinylchloride hard

PE-HD Polyethylene high density

PE-LD Polyethylene low density

ECB Ethylene-Copolymer-Bitumen

EVA Ethylene-Vinylacetate-Copolymer

EPDM Ethylene-Propylene-Terpolymer-Rubber

ABS Acrylnitrile-Butadiene Styrene-Copolymer

PA Polyamide

PC Polycarbonate

PMMA Polymethylmethacrylate

PP Polypropylene

PS Polystyrene

PTFE Polytetrafluorethylene

PUR Polyurethane

So that you can recognize which plastic you are dealing with, the following simple test is recommended:1. A short test weld with the available welding rod. If the welding rod is fixed firmly, the problem is

solved.

2. Take a small sliver or shaving of the material to be welded and light it with a match, observe the flame

and smell the smoke.

On PVC: blackish smoke and acrid smell.

On Polyethylene: no smoke, the material drips like a candle and also smells of wax

On Polypropylene: no smoke, the material drips like a candle and smells of burnt oil

On Polyamide: no smoke, pulls to form thread, smells of burnt horn.

On Polycarbonate: yellowish sooty smoke. Sweetish smell.

On ABS: blackish smoke, soot flakes, sweetish smell.

Applications of Plastics:

Plastics offer a variety of benefits to make them the perfect product to be used on a wide variety ofapplications. Some of these advantages are:

Light Weight

Strong


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Translucent or Transparent

Flexibility

Price competitive

Types of Plastics:

Plastics can be divided into two major categories:

1. Thermoset or thermosetting plastics:

Once cooled and hardened, these plastics retain their shapes and cannot return to their

original form. They are hard and durable. Thermosets can be used for auto parts,

aircraft parts and tires.

Examples include polyurethanes, polyesters, epoxy resins and phenolic resins.

2. Thermoplastics. Less rigid than thermosets, thermoplastics can soften upon heating and

return to their original form. They are easily molded and extruded into films, fibers and

packaging.

Examples include polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC).

Thermoset or Thermosetting Plastics:

1. Polyurethane Plastics:

Polyurethane plastics belong to the group that can be thermosetting. Polyurethane is the only plastic

which can be made in both rigid and flexible foams. The flexible polyurethane foam is used in

mattresses, carpets, furniture etc. The rigid polyurethane foam is used in chair shells, mirror frames and

many more. Due to the property of high elasticity, some polyurethane plastics are used in decorative

and protective coatings. The high elasticity makes these polyurethane plastics resistant to a chemical

attack.

2. Epoxy:

Epoxies are used in numerous ways. In combination with glass fibers, it is capable of producing

composites that are of high strength and that are heat resistant. This composite is typically used for

filament wound rocket motor casings in missiles, in aircraft components, and in tanks, pipes, tooling jigs,

pressure vessels, and fixtures. Epoxies are also found in gymnasium floors, industrial equipment,

sealants, and protective coatings in appliances.

Phenolic:

Phenolic plastics are thermosetting resins used in potting compounds, casting resins, and laminating

resins. They can also be used for electrical purposes and are a popular binder for holding together plies

of wood for plywood.

Thermoplastics:

1. Vinyl Plastics:

Vinyl plastics belong to the thermoplastic group. Vinyl plastics are the sub-polymers of vinyl derivatives.

These are used in laminated safety glasses, flexible tubing, molded products etc.

2. Polyacrylics Plastics:


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Polyacrylics belong to the group of thermoplastics. Polyacrylics are transparent and decorative.

Polyacrylics plastics can be shaped in any form like the windshields for airplane.

3. Polyvinyl Chloride

Polyvinyl Chloride, commonly referred to as PVC or vinyl, was first invented in Germany around 1910. It

didn't become a useful product in the United States, however, until the late 1920s. It becameparticularly useful during World War II when it was used as a substitute for rubber, which was in short

supply. Polyvinyl Chloride is resistant to abrasion and is both weather and chemical resistant. Today, it is

commonly found in upholstery, wall coverings, flooring, siding, pipe, and even apparel. In fact, vinyl is

perhaps the best known of all plastics.

4. Polyethylene Terephthalate (PETE):

PETE is one the most recycled plastic. It finds usage in various bottles like that of soda and cooking oil,

etc.

5. High Density Polyethylene (HDPE):

HDPE is generally used in detergent bottles and in milk jugs.

6. Polyvinyl Chloride (PVC):

PVC is commonly used in plastic pipes, furniture, water bottles, liquid detergent jars etc.

7. Low Density Polyethylene (LDPE) :- LDPE finds its usage in dry cleaning bags, food storage containers

etc.

8. Polypropylene (PP) :- PP is commonly used in bottle caps and drinking straws.

9. Polystyrene (PS) :- PS is used in cups, plastic tableware etc.

Characteristics of Plastics:1. Mechanical properties:

Mechanical properties refer to displacement or breakage of plastic due to some mechanical change such

as applying some load.

Mechanical properties are dependent on the temperature, force (load), and the duration of time the

load is applied. It may also be affected by ultra-violet radiation when used outside.

2. Thermal properties:

Thermal properties include heat resistance or combustibility.

Thermoplastic has a larger coefficient of thermal expansion or combustibility and a smaller thermal

conductivity or specific heat than other material such as metals.

3. Chemical properties:

Chemical resistance, environmental stress crack resistance , or resistance to environmental change are

referred as chemical properties.

When a plastic contacts chemicals, there is some kind of change. After having a plastic in contacted with

chemicals under no stress for about a week, changes in appearance, weight and size of the plastic are

examined. Such changes are referred to as chemical properties.


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4. Electric properties:

Electric properties are also referred to as electromagnetic properties. Electric properties include

insulation, conductivity and electro-static charges.

Due to their good insulation property, plastics are often used in electric fields. However, plastics do have

a defect; they are easily electrified.

5. Physical properties:

Specific gravity, index of refraction and moisture absorption are called physical properties.

The specific gravity of the plastic is small, and it varies depending on the character of high polymer , or

thermal and mechanical treatment of the plastic.

Materials for Processing Plastics:

Most Plastic resins have to be combined, compounded, or otherwise chemically treated with processing

materials before they are ready for processing.

One of the following additions is usually employed

1. Plasticizers

2. Fillers

3. Catalyst

4. Initiators

5. Dyes and Pigments

Plasticizers:

i) Organic Solvents, resins, and even water are used as plasticizers.

ii) These substances act as internal lubricants improving flow of and giving toughness and flexibility to

the material

iii) Plasticizers are also used to prevent crystallization by keeping the chains separated from one

another.

Fillers:

i) Typical fillers which include wood floor, asbestos fibre, glass fibre, cloth fibre, mica, slate powders,

may be added in high proportion to many plastics essentially to improve strength, dimensional stability,

and heat resistance.

Catalyst:

These are usually added to promote faster and more complete polymerization.

As such they are also called accelerators and hardeners.

Initiators:

It is used to initiate the reaction, i.e., to allow polymerization to beginThey stabilize the ends or reaction sites of the molecular chains.

H2O2 is a common initiator.

Dyes and Pigments:

These are added in many cases, to color the material to different shades


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Plastic Moulding Processes:

i) Compression Moulding

ii)Transfer Moulding

iii)Injection Moulding

iv)Jet Moulding

v)Extrusion

Compression molding:

Compression molding is a method of molding in which the molding material, generally preheated, is first

placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is

applied to force the material into contact with all mold areas, while heat and pressure are maintained

until the molding material has cured.

Common plastics used in compression molding processes include.

Polyester

Polyimide (PI)

Polyamide-imide (PAI)

Polyphenylene Sulfide (PPS)

Polyetheretherketone (PEEK)

Fiber reinforced plastics

Principle of working:

a) The compression molding starts, with an allotted amount of plastic or gelatin placed over or inserted

into a mold.

b) Afterward the material is heated to a pliable state in and by the mold.

c) Shortly thereafter the hydraulic press compresses the pliable plastic against the mold, resulting in a

perfectly molded piece, retaining the shape of the inside surface of the mold.d) After the hydraulic press releases, an ejector pin in the bottom of the mold quickly ejects the finish

piece out of the mold and then the process is finished.

e) Also depending on the type of plunger used in the press there will or won't be excess material on the

mold.

Factors affecting Compression Moulding:

Amount of material


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Heating time and technique

Force applied to the mold

Cooling time and technique

Advantages:

Low initial setup costs Fast setup time

Capable of large size parts beyond the capacity of extrusion techniques

Allows intricate parts

Good surface finish (in general)

Wastes relatively little material

Can apply to composite thermoplastics with unidirectional tapes, woven fabrics, randomly

orientated fiber mat or chopped strand

Compression molding produces fewer knit lines and less fiber-length degradation than injection

molding.

Disadvantages: Production speed is not up to injection molding standards

Limited largely to flat or moderately curved parts with no undercuts

Less-than-ideal product consistency

Transfer molding:

Transfer molding is similar to compression molding in that a carefully calculated, pre-measured

amount of uncured molding compound is used for the molding process.

The difference is, instead of loading the polymer into an open mold, the plastic material is pre-

heated and loaded into a holding champers called the pot.

The material is then forced/transferred into the pre-heated mold cavity by a hydraulic plungerthrough a channel called sprue. The mold remains closed until the material inside is cured.


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Process in Transfer molding:

1. The pre-heated, uncured molding compound is placed in the transfer pot.

2. A hydraulically powered plunger pushes the molding compound through the sprue(s) into the pre-

heated mold cavity. The mold remains closed until the material inside is cured (thermosets) or cooled

(thermoplastics).

3. The mold is split to free the product, with the help of the ejector pins.

4. The flash and sprue material is trimmed off.

Plastic used in this Process:

Epoxy Polyester (Unsaturated)

Phenol-formaldehyde Plastic (PF, Phenolic)

Silicone rubber (SI)

Advantages:

Product consistency better than compression molding, allowing tighter tolerance and more

intricate parts


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Production speed higher than compression molding

Fast setup time and lower setup costs than injection molding

Lower maintenance costs than injection molding

Ideal for plastic parts with metal inserts

Disadvantages:

Wastes more material than compression molding (scraps of thermosets are not re-useable).

Production speed lower than injection molding

Injection molding:

Injection molding is a manufacturing process for producing parts from both thermoplastic and

thermosetting plastic materials.

Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to

the configuration of the mold cavity.

Conventional Single Stage Plunger Type


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Single Stage Reciprocating Screw Type

Two stage plunger or Screw plasticiser type

Process of Injection Moulding:

The process starts with feeding plastic pellets in the hopper above the heating cylinder of the

machine.

The resin falls into and is pushed along the heated tube by reciprocating screw until a sufficient

volume of melted plastic available This may take from 10 Sec to 6 min.

The entire screw is then plunged forward to force the plastic into the mould.

Each shot may produce one or several parts, depending on the die used.

The ram is held under pressure for a few seconds so that the moulded part can solidify.

It then retracts slightly, and the mould open

Knockout pins eject the moulded piece.


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Applications:

milk cartons,

Packaging bottle caps

automotive dashboards

pocket combs

and most other plastic products available today.

Extrusion Moulding:

Extrusion is one of the most widely used manufacturing processes across many industries.

Essentially, it is not much different from squeezing tooth paste out of the tube.

Anything that is long with a consistent cross section is probably made by extrusion.

Common examples are spaghetti, candy canes, chewing gums, drinking straws, plumbing pipes,door insulation seals, optical fibers, and steel or aluminum I-beams.


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Process:

The plastic extrusion molding process usually begins with a thermoplastic in the form of pellets

or granules.

They are usually stored in a hopper (a funnel-shaped receptacle) before they are delivered to a

heated barrel.

The molten plastic is then forced through a shaped orifice, usually a custom steel die with shape

of the cross section of the intended part, forming a tube-like or rod-like continuous work piece.

Cooling of the work piece should be as even as possible.

Plastics used in this process:


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Acrylonitrile Butadiene Styrene (ABS)

Acrylic

Polycarbonate (PC)

Polyethylene (PE)

Polypropylene (PP)

Polyester Polystyrene (PS)

Polyvinylchloride (PVC)

Advantages:

Low initial setup costs

Fast setup time

Low production costs

Disadvantages:

Moderate production speed

Average precision Limited to parts with a uniform cross section

Thermoforming Process:

A plastic thermoforming process usually begins with a sheet of thermoplastic material formed

by the extrusion process using a slotted die.

Thin-gage materials (less than 1/16 inch thick) usually come in rolls; and heavy-gage materials

(up to 1/2 inch thick) normally come in sheets.

The sheet of plastic material is first heated to become a flexible membrane.

This soft, rubber-like membrane is placed on the mold and stretched to cover the entire

surface.

Vacuum, external air pressure, and mechanical forces are used to rid the air bubbles andimprove the surface quality.

The plastic part remains in the mold until it solidifies. Excess material is trimmed after the part

is removed from the mold.


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Process in Thermoforming:

1. The plastic sheets used in thermoforming is usually made by extrusion. The one-sided mold is usually

made by aluminum.

2. This sheet of plastic material is first heated to become a flexible membrane. It is soft but still not

liquid or gooey.

3. The soft, rubber-like membrane is placed on the mold and stretched to fit. Vacuum, external airpressure, and mechanical forces are used to rid the air bubbles.

4. The plastic part is removed from the mold after it cools and hardens.

5. Trimming, drilling, and other finishing processes may be needed to obtain the final product

Aluminum is the most common thermoforming mold material due to its very high coefficient of thermal

conductivity that allows speedy and consistent cooling cycle.

Plastics used in this process:

Acrylonitrile Butadiene Styrene (ABS)

Acrylic

Polycarbonate (PC)

Polyethylene (PE)

Polypropylene (PP)

Polystyrene (PS)

Polyvinylchloride (PVC)

Advantages:

Low initial setup costs

Fast setup time

Low production costs

Less thermal stresses than injection molding and compression molding

More details and better cosmetics than rotational-molded products

Disadvantages:

Geometries limited to thin shells or shallow shapes

One side of the product can be precisely controlled by the mold dimensions while the other side

cannot.

manufacturing science-1 unit-5 and unit 4

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