Unit V

Powder metallurgy and

Plastic Molding
 Powder Metallurgy
• Powder metallurgy is a branch of metallurgy which deals with the production of metal and non-
metal powders and subsequently manufacturing of components by using these powders.
• Powder metallurgy (P/M) components are manufactured by mixing of metal or metal and non
metal powders, compacting with simultaneous or subsequent heating at elevated temperatures
using a controlled atmosphere to develop metal or metal like components with satisfactory strength
and density.
• Steps involved in manufacturing powder metallurgical component are

1. Powder production
2. Blending or Mixing
3. Compacting (i.e.
Pressing)
4. Sintering
5. Sizing or Impregnation
6. Testing and inspection
 Powder production methods
1. Mechanical:
A. Machining
• This method is used to produce filings, turnings, chips, etc. which are
subsequently pulverized by crushing and milling.
• Relatively coarse powders are obtained
• The powder particles are of irregular shape
B. Crushing
• The solid materials are crushed by hammers, jaw crushers, gyratory
crushers, etc.
• The powder particles of brittle materials are angular in shape and
ductile materials are flaky in shape.
• Any material can be crushed to powder form; however, the method
is very much suitable for brittle materials.

(a) Roll crusher (b) Ball mill

 Powder production methods
C. Milling
• Milling is the most important and widely used method for the production of powders of required grade and
fineness.
• Milling is done by using equipments such as ball mills, rod mills, disk mills, eddy mills etc.
• In the ball milling method, the material to be powdered is tumbled or rotated in a container with large number of
hard balls.
• The balls are made of steel, alloy steel, or white cast iron.
D. Shotting
• In this method, molten metal is poured on a vibrating screen and the liquid droplets are solidified either in air or a
neutral gas.
• The size and character of the powder depends on the temperature of molten metal, size of openings in the screen
and frequency of vibrations of the screen.
• Shape of particles is nearly spherical.
E. Graining
• Graining involves the same procedure as the shotting, the only difference being the solidification of molten metal
droplets is done in water.
• The powders obtained by shotting and graining methods are coarse and subsequently other pulverization
methods are used for further reduction of size.
 Powder production methods
F. Atomization
• Produce a liquid-metal stream by injecting molten metal through a small orifice
• Stream is broken by jets of inert gas, air, or water
• The size of the particle formed depends on:
1. Temperature of the metal
2. Metal flowrate through the orifice
3. Pressure of jet
4. Nozzle size and jet characteristics
• The process consists of main three stages
• Melting: Melting is done by induction, arc, plasma or electron-beam technique to maintain purity of melt.
• Atomization: Atomization is done by high velocity water, compressed air or inert gas.
• Solidification and cooling: The disintegrated particles are solidified in controlled atmosphere, vacuum , air or
water.
• Main two types of techniques:
• Water Atomization
• Gas Atomization
 Powder production methods
2. Physical
A. Condensation
• In this method, metal vapors are condensed to obtain metal powders.
• This method is highly suitable for volatile metals because they get easily transformed to their vapours.
• Large quantities of Zn, Mg and Cd powders are manufactured by this method.
• The powder shape is nearly spherical.
B. Thermal decomposition
• Fine metal powders of some metals like Fe, Ni, W, Mo, Co, Mg, etc. are manufactured by thermal
decomposition of their respective carbonyl vapours.
• However, the method is highly suitable for the manufacture of Fe and Ni powders.
• Fe aid Ni carbonyls are produced by passing CO over a spongy or powdered metal at some suitable
temperature (200 to 270°C) and pressure (70 to 200 atmospheres)
• These carbonyls are volatile liquids and their vapours decompose at one atmospheric pressure and
temperature of 150 to 400°C
 Powder production methods
3. Chemical
A. Reduction
• The largest volume of metallurgical powder made by the process of oxide reduction.
• Reduce metal oxides with H2/CO
• Powders are spongy and porous and they have uniformly sized spherical or angular shapes which are
ideal for compacting
 Powder production methods
B. Intergranular corrosion
• It is a fact that the grain boundaries of any metal corrode faster than the grains.
• In this method, grain boundary area of the metal under interest is corroded by a suitable electrolyte so as to
separate out the grains from the polycrystalline metal.
• The powder of stainless steel is made by this process.
C. Electro-chemical
• Metal powder deposits at the cathode from aqueous solution
• Powders are among the purest available (99.99%)
• The particle shape is dendritic
• Method is generally used to manufacture powders of Cu, Be, Fe, Zn,Ni, Cd, Ag etc
• The conditions which favour the powder formation on cathode are:
1. High current density
2. Low metal ion concentration
3. High acidity
4. Low temperature
 Blending and Mixing
• To make a homogeneous mass with uniform distribution of
particle size and composition
• Powders made by different processes have different sizes and
shapes
• Mixing powders of different metals/materials
• Add lubricants (<5%), such as graphite and stearic acid, to
improve the flow characteristics and compressibility of
mixtures
• Combining is generally carried out in Air or inert gases to
avoid oxidation
• Liquids for better mixing, elimination of dusts and reduced
explosion hazards Some common equipment geometries
used for blending powders
• Hazards: Metal powders, because of high surface area to
volume ratio are explosive, particularly Al, Mg, Ti, Zr, Th (a) Cylindrical, (b) rotating cube, (c) double
cone, (d) twin shell
 Compaction
• Press powder into the desired shape and size in dies using a hydraulic or mechanical press.
• Pressed powder is known as “green compact”
• The purpose of compacting:
1. To consolidate the powder into the desired shape .
2. To impart the desired level and type of porosity .
3. To provide adequate strength for handling.
• Stages of metal powder compaction:
 Sintering
• Sintering is carried out to increase strength and hardness of a green compact and consists of heating
the compact to some temperature under controlled conditions with or without pressure for a definite
time.
• The possible diffusion mechanisms are
 Surface diffusion
 Volume diffusion
 Grain boundary diffusion
 Evaporation and condensation
 Sintering
• It promotes solid-state bonding by diffusion.
• Diffusion is time-temperature sensitive. Needs sufficient time
• Because material heated very close to MP, metal atoms will be released in the vapour phase from the
particles.
• Vapour phase resolidifies at the interface.
 Characterization of Powders
• Powder Characteristics:
 Chemical composition
 Porosity & Microstructure.
 Shape, size and distribution.
 Flow rate
 Specific surface
 Density

 Chemical composition
• Chemical composition and impurities in metal powders are determined by standard techniques of
chemical analysis such as gravimetric, volumetric, colourometric, etc.; or they can be determined by
spectroscopy.
• The chemical composition and impurities strongly influence pressing and sintering characteristics.
 Characterization of Powders
 Porosity & Microstructure
→ For the determination and observation of these properties, microscopy is used.
→ The powder is mounted in some suitable medium for observation under microscope.
→ Depending on its suitability, either hot mounting or cold mounting method is used.
 Particle Shape
→ The typical shape of powder are dendritic, acicular, fibrous, flaky, shperoidal, granular as
shown
 Characterization of Powders
 Particle Size
→ Powder size is classified as fine powder and coarse powder. Powder size is also determined by
microscope.
→ Usual particle size range of powders used in P/M is between 1 to 1000 microns
 Particle Size Distribution
→ Powder particle size distribution is classified as wide distribution and narrow distribution.
→ Powder particle size distribution can be measured by using one or more of the following
methods:
Sieve method
Microscopic method
Sedimentation method time
Elutriation method
 Characterization of Powders
 Flow rate of powder
• The flow rate is a very important characteristic of powders which measures the ability of a
powder to be transferred.
• An apparatus which is used to determine flow rate is called flow meter.
 Characterization of Powders
 Specific surface :

• It is defined as the total surface area of a powder per unit weight (cm 2/gm).
• It depends on size, shape, density and surface conditions of the particles.
• It is evaluated either by permeability method or adsorption method.
 Characterization of Powders
 Density:

(A) Apparent density :

• The apparent density (or packing density) of a powder is defined as the mass per unit volume of
loose or unpacked powder.

(B) Tap density:

• The tap density is the apparent density of the powder after it has been mechanically shaked or tapped
until the level of the powder remains constant.

• Apparent density is measured by using a standard flowmeter funnel or volumeter, and tap density by
Ro tap machine.
 Manufacturing of Typical P/M
Components
 Oil Impregnated Porous Bearings
(Self Lubricating Bearings)
 Cemented Carbides
 Cermets
 Diamond Impregnated Tools
 Refractory Metals
 Electrical Contact Materials
 Oil Impregnated Porous Bearings (Self Lubricating Bearings)
• Controlled porosity of powder metal parts has led to the production of ‘Oil Impregnated Porous
Bearings’ (Self-lubricating bearings).
• Self-lubricating bearings are made of bronze, brass, iron or aluminium alloy powders with or
without graphite.
• However, bronze bearings are widely used and are made from Cu and Sn (90 : 10) with addition of
graphite. Graphite increases porosity and also improves pressing characteristics.
• These bearings must have the following characteristics for their efficient working:
1. Sufficient porosity (30 to 50%) to retain the maximum possible amount of oil
2. Inter-connected porosity in the largest proportion and should be uniformly distributed
throughout the material
3. Sufficient strength to sustain the loads
4. Good dimensional accuracy
• When the porosity is more, the strength of the bearing is less.
• When the porosity is less, the strength of the bearing is more.
 Oil Impregnated Porous Bearings (Self Lubricating Bearings)
• The working of the bearing is as below :
• As the speed of shaft increases, the
temperature of bearing rises due to frictional
heat.
• This results in decrease of viscosity and
increase in volume of the oil.
• Due to this, the oil is pulled out from the
pores and gets rapidly circulated along with
the rotating shaft.
• With decrease of shaft speed, pressure
decreases and temperature also decreases;
and due to this, the oil goes back to pores by
capillary action.
• There is no wastage of oil and working of the
bearing is smooth and silent.
 Oil Impregnated Porous Bearings (Self Lubricating Bearings)
• The steps in the production of a porous bronze bearing are as below:
(1) Mixing
• Metal powders of Cu and Sn with small amount of fine natural graphite are blended or mixed to obtain
the desired alloy composition (90 Cu: 10 Sn).
(2) Cold compaction
• These powders are cold compacted at pressures between 20 to 50 kg/mm2 to form green compacts of
desired shape and size.
(3) Sintering
• These compacts are sintered in a reducing atmosphere at a temperature of about 800°C. A typical
sintering cycle consists of holding the compact at 400-450°C for the removal of part of the graphite
and diffusion of molten Sn into the copper, followed, by further heating to 800°C for periods as short
as 5 minutes.
• At this temperature, a tin-rich liquid phase is formed which is absorbed by the copper.
 Oil Impregnated Porous Bearings (Self Lubricating
Bearings)
(4) Repressing (i.e. Sizing) or Machining
• Distortions occurring during sintering can be eliminated by repressing (i.e. sizing) or machining.
• If the pore size is large, sizing can be done and if it is small, machining should be done.
• For small pore sizes, sizing should not be done because it may result in closure of pores.
(5) Impregnation
• The repressed or machined components are impregnated with cold or hot oil using pressure,
vacuum or a combination of these.
Applications:
 These bearings find applications in places which are inaccessible or difficultly accessible. These
are the places which are impossible or difficult for regular lubrication.
 They are also used in certain applications where it is desirable that the oil should not come in
contact and contaminate the products (e.g. in food and textile industries)
 Self-lubricating bearings are used extensively in the automotive industry and in washing
machines, refrigerators, electric clocks, and many other types of equipment.
 Cemented Carbide Tools
 These are important products of P/M and find wide applications as cutting tools, wire drawing and
deep drawing dies, drills and stone working tools.
 They are manufactured from carbides of refractory metals such as W, Mo, Ti, Ta or Nb.
 These carbides are extremely hard (hardness more than 3000 VPN) and retain their hardness upto a
very high temperature. However they are extremely brittle and hence are likely to fail with slight
shock loading.
 To increase their shock resisting ability, metals such as Co, Ni, Cr or alloys of Co —Cr or Co — Ni —
Cr are used upto 20 % and the processing is done by P/M. The hard carbide powders are bonded or
cemented together by these metals or alloys.
 For most of the common applications, carbides of W and Mo are used and the binder is Co.
 The steps in the manufacture of cemented carbides are as below:
(i) Powder manufacture :
 Carbide powders of the refractory metals are produced either from their respective oxides or metals.
 Metal oxides can be reduced to metals by carbon or hydrogen and subsequently the metals can be
converted to carbides by direct reaction with the carbon, or the metal oxides can be directly converted
to carbides in a single step by reaction with carbon.
 Co powder is obtained by the reduction of the oxide or oxalate by H 2 at temperatures of 600 to 700°C
 Cemented Carbide Tools
(ii) Milling:
 Carbide powders are mixed in the required proportion along with the powder of metallic binder by a wet
mixing method.
 Lubricants such as paraffin wax dissolved in petrol, camphor in ether or light hydrocarbons, and glycerine in
alcohol are mixed to these powders just prior to compaction which facilitates pressing and avoids defects and
cracks in the compacts.

(iii) Cold pressing and sintering :

This mixture is compacted at a pressure of 35 to 45 kg/mm2 and the compacts are heated to about 400°C for a
sufficient period to remove the lubricant by volatilisation.
Sintering of these compacts is carried out in two stages. The preliminary sintering is done using H2 atmosphere
at a temperature between 900 to 1150°C.
This is done to impart sufficient strength to the compacts.
At this stage, they can be machined or cut to a shape and size to obtain exact dimensions of the component after
final sintering.
Final sintering is done in the temperature range of 1350 to 1550°C for about two hours using H2 atmosphere or
vacuum.
The liquid phase formed at this temperature binds the particles and hence the name is cemented carbides.
During this stage of sintering, large amount of shrinkage occurs in the component and hence to obtain the final
dimensions within tolerance limit, the component must have oversized dimensions before sintering.
 Cemented Carbide Tools
(iv) Machining :
 It should be done to the extremely close tolerances and is done in two steps.
 First it is ground rough using silicon carbide grinding wheels and finally with metal bonded
diamond wheels.
 Electrospark or ultrasonic machining has also been used for threading, boring or engraving of
these cemented carbide components.
 Advantages of powder metallurgy
1. Metal plus metal components can be manufactured by P/M. There is almost no need of referring
to their equilibrium or phase diagrams. Components of any desired composition can be
manufactured.
2. Metal plus non-metal components can be manufactured.
3. Controlled porosity can be obtained in the components. This is essential for certain applications
like liquid and gas filters, self-lubricating bearings and insulating bricks.
4. It is possible to produce components with properties similar to the parent metals. Whereas, if the
components are manufactured by melting, the alloy may have different properties from their
parent metals.
5. Production of refractory metals like W, Mo, Ti, Th, etc. This is possible without melting e.g.
manufacture of ductile tungsten in wire form for incandescent lamp filaments.
6. Components from metals which are completely insoluble in the liquid state can be manufactured
with uniform distribution of one metal into the other. However, if they are manufactured by
melting and casting, the distribution of one phase into the other is non uniform.
7. Composite and dispersion hardened materials can be manufactured. e.g. cermets and thoria
dispersed tungsten filaments.
 Advantages of powder metallurgy
8. There is a little chance for contamination of metal powders during processing by P/M The
purity of the component remains the same as the original purity of metal powders.
9. P/M parts may be welded, brazed, machined, heat treated, plated or impregnated with
lubricants or other materials.
10. Close control over the dimensions of the finished component can easily be obtained.
11. No machining or minimum machining is required and hence the scrap is minimum. This gives
yield of over 99.0%.
12. Fast production of simple shaped components is possible due to lesser number of steps involved
in P/M .
13. Manufacture of cemented carbide cutting tools is only possible by P/M. The melting points of
the carbides which are used for the manufacture of these cutting tools are extremely high and
hence melting is not possible.
 Limitation of Powder Metallurgy
1. Fire Hazards : Most of the powders used in P/M are fine and fine powders of some of the metals
like Mg, Al, Zr, Ti, etc. are likely to explode and cause fire hazards when they come in contact
with air and hence, they should be preserved carefully.
2. Oxidation: Other metal powders are also likely to get oxidised slowly in air and hence, they must
be stored properly to avoid their deterioration.
3. High Capital Cost: It is not suitable to manufacture small number of components because of high
initial investment on tooling and equipment.
4. Large sized components: Large sized components can not be manufactured because of the
limited capacity of presses available for compaction.
5. Complex shaped parts: Complex shaped parts can not be manufactured with ease by P/M.
6. Corrosion resistance :P/M parts have poor corrosion resistance because they are porous. Due to
this porosity, large internal surface area gets exposed to corrosive environment.
7. Porosity: Due to the presence of porosity, mechanical properties such as ductility, U.T.S. and
toughness are poor as compared to components manufactured by conventional methods. The
surface finish is also poor.
 Applications of PM

Automotive Applications High Temperature Applications

Aerospace Applications
Atomic Energy Applications
Other Applications
Plastic Moulding
 Plastics or polymer
• Definition: A group of engineered materials characterized by large molecules that are built up by
the joining of smaller molecules.
• They are natural or synthetics resins.
• Properties of plastics:
1. Light weight
2. Good resistance to corrosion
3. Ease of fabrication into complex shapes
4. Low electrical and thermal conductivity
5. Good surface finish
6. Good optical properties
7. Good resistance to shock and vibration.
Classification of Polymers
 Manufacturing With Plastics
• The various methods used for plastic processing are
1. Compression Moulding
2. Transfer Moulding
3. Extrusion Moulding
4. Injection Moulding
5. Blow Moulding
6. Thermoforming
7. Calendering
8. Polymer Foam Processing and Forming
 Manufacturing With Plastics
• Extrusion moulding.
• Widely used for continuous production of film, sheet, tube, and other profiles.It is also used in
conjunction with blow moulding.
• Thermoplastic pellets & powders are fed through a hopper into the barrel chamber of a screw extruder.
A rotating screw propels the material through a preheating section, where it is heated, homogenized,
and compressed, and then forces it through a heated die and onto a conveyor belt.
• As the plastic passes onto the belt, it is cooled by jets of air or sprays of water which harden it
sufficiently to preserve its newly imparted shape.
• It continues to cool as it passes along the belt and is then either cut into lengths or coiled.
• Low tool cost, numerous complex profile shapes possible, very rapid production rates, can apply
coatings or jacketing to core materials (Such as wire).

• Usually limited to sections of uniform cross section.

 Manufacturing With Plastics

(a) Side view cross‑section of an extrusion die for solid regular

shapes, such as round stock; (b) front view of die, with
profile of extrudate.
Extruder Screw
• Progress of polymer melt through barrel leads
• Divided into sections to serve several functions: ultimately to the die zone
• Before reaching die, the melt passes through a
– Feed section - feedstock is moved from hopper and screen pack - series of wire meshes supported by
a stiff plate containing small axial holes
preheated
• Functions of screen pack:
– Compression section - polymer is transformed into – Filter out contaminants and hard lumps
fluid, air mixed with pellets is extracted from melt,
– Build pressure in metering section
and material is compressed
– Straighten flow of polymer melt and
– Metering section - melt is homogenized and remove its "memory" of circular motion
sufficient pressure developed to pump it through from screw
die opening
 Manufacturing With Plastics
• Injection Moulding
• The polymer is melted and then forced into a mould.
• Thermoplastic pellets melted and melt injected under high pressure (70 MPa) into a mold. Molten plastic
takes the shape of the mold, cools, solidifies, shrinks and is ejected.
• Molds usually made in two parts (internal and external part).
• Use of injection molding machine mainly used for thermoplastics (gears, cams, pistons, rollers, valves,
fan blades, rotors, washing machine agitators, knobs, handles, camera cases, battery cases, sports
helmets etc…)
• Limitations:

• High initial tool and die costs; not

economically practical for small runs.
 Plastics Processing: Injection Molding

(1) mold is closed (2) melt is injected into cavity.

(3) screw is retracted. (4) mold opens and part is ejected.

Cycle of operation for injection molding
 Manufacturing With Plastics
• Blow Molding
Molding process in which air pressure is used to inflate soft plastic into a mold cavity
• Important for making one‑piece hollow plastic parts with thin walls, such as bottles
• Because these items are used for consumer beverages in mass markets, production is typically organized
for very high quantities
• Accomplished in two steps:
1. Fabrication of a starting tube, called a parison
2. Inflation of the tube to desired final shape
• Forming the parison is accomplished by either
– Extrusion or
– Injection molding
 Extrusion Blow Molding

Figure : Extrusion blow molding:

(1) extrusion of parison;
(2) parison is pinched at the top and sealed at the bottom around a metal
blow pin as the two halves of the mold come together;
(3) the tube is inflated so that it takes the shape of the mold cavity; and
(4) mold is opened to remove the solidified part.
• Extrusion Blow Molding

• Extrusion Blow Molding involves manufacture of parison

by conventional extrusion method using a die similar to
that used for extrusion pipes.
• Extrusion Blow Molding is commonly used for mass
production of plastic bottles.

• The production cycle consists of the following steps:

1.The parison is extruded vertically in downward direction

between two mold halves.
2. When the parison reaches the required length the two
mold halves close
• resulting in pinching the top of parison end and
sealing the blow pin in the bottom of the parison end.
3.Parison is inflated by air blown through the blow pin,
taking a shape conforming that of the mold cavity. The
parison is then cut on the top.
4.The mold cools down, its halves open, and the final part is
removed.
1 3

2 4
5
 Injection Blow Molding

1. In Injection Blow Molding method a parison is produced

by injecting a polymer into a hot injection mold around a
blow tube or core rod.

2.Then the blow tube together with the parison is removed

from the injection mold and transferred to a blow mold.
Following operations are similar to those in the extrusion
blowing molding.

3. Injection Blow Molding is more accurate and controllable

process as compared to the Extrusion Blow Molding.
Figure : Injection blow molding: (1) parison is It allows producing more complicated products from a
injected molded around a blowing rod; (2) wider range of polymer materials.
injection mold is opened and parison is
transferred to a blow mold; (3) soft polymer is 4. However production rate of Injection Blow Molding
inflated to conform to the blow mold; and (4) method is lower than that of Extrusion Blow Molding.
blow mold is opened and blown product is
removed.
 Manufacturing With Plastics
• Thermoforming
• In this process, a thermoplastic sheet can be formed into a three- dimensional shape by the application
of heat and differential pressures.
• First, the plastic sheet is clamped to a frame and uniformly heated to make it soft and flowable.
• Then a differential pressure (either vacuum or pressure or both) is applied to make the sheet conform
to the shape of a mould or die positioned below the frame.
• It is possible to use most of the thermoplastic materials. The starting material is a plastic sheet of
uniform thickness.
• It is a relatively simple process and is used for making such parts as covers, displays, blister
packaging, trays, drinking cups and food packaging.
• Limitations:

• Limited to parts of simple configuration, high scrap, and limited number of materials from which to
choose.
Manufacturing With Plastics