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.
Figure 5.9 Sequence of brazing process.
6.4.1 BRAZING METAL
Brazing Metal Temperature ( °C) Application
Copper/copper alloy 850-1100 Steels, carbides, HSS, etc
Copper with phosphorus 750-850 Cooper, copper alloys, etc
Brass 850-1000 Steel, cast iron, copper, nickel, etc.
Silver alloy 600-850 Copper, copper alloys, steels, etc.
Aluminium alloy 500-600 Aluminium, aluminium alloys.
6.4.2 FLUXES
1. Fluxes are added into the braze joint to remove any of oxides present or prevent the
formation if the oxides so that the base metal and the filler remain pure during joining.
2. The fluxes generally used are combinations of borax, boric acid, chlorides, fluorides
and tetra borates (Figure 5.10).
3. For ferrous material, normally a mixture of borax and boric acid in the paste form
(ratio 3:1).
Figure 5.10 Silver brazing flux.
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EXERCISE 5.2
Brazing is widely used in industries because of its many advantages.
Describe any five (5) advantages.
5.5 SOLDERING
Figure 5.11 Soldering process.
1. Soldering is a method of joining similar or dissimilar metals by means of filler metal
whose liquidus temperature is below 450°C (Figure 5.11).
2. Though soldering offers a good joint between two plates, the strength of the joint is
limited by the strength of the filler metal used.
3. Soldering normally used for obtaining a neat leak proof joint or a high conductivity
electrical joint.
4. Not suitable for high temperature service because of low temperature of filler used.
5. The filler metal enters the soldered joint is similar to brazing by means of capillary
action.
6. Soldering joint must be cleaned to provide chemically clean surfaces to obtain a
proper bond. It is generally done by using solvent cleaning, acid pickling and even
mechanical cleaning.
7. To remove the oxides form the joint surfaces and prevent oxidation, fluxes are
generally used, rosin and sorin plus alcohol based flux.
8. For mass production, soldering is an automated process called dip soldering and
wave soldering, which has been applied extensively in the electronics assembly
industry, like an electrical printed circuit board (PCB).
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5.5.1 FILLER METALS
1. Filler metals used are normally called as solders which are essentially alloys of lead
and tin (62% tin + 38% lead).
2. Filler starts to melt at 183°C depending on the composition.
Figure 5.12 Filler metals.
5.6 ADHESIVE BONDING
Figure 5.13 Example of adhesive bonding.
1. Adhesive bonding is a joining process where a substance, in a liquid or semi liquid, is
applied to adjoining workpiece to provide a permanent bond.
2. Sticking, or adhesive bonding, of metals is becoming very popular in the automotive,
aircraft, and packaging industries because of the advantages that this technique can
offer.
3. The recent development in the chemistry of polymers, adhesives now cheap, can be
applied easily and quickly, and can produce reasonably strong joints.
4. Adhesive bonding can also be employed in producing joints of dissimilar metals or
combinations of metals and non-metals like ceramics or polymers.
5. This certainly provides greater flexibility when designing products and eliminates the
need for complicated, expensive joining processes.
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5.7 MECHANICAL FASTENING
1. Bolts, screws, and nuts are among the most commonly used threaded fasteners.
2. If the joint is to be subjected to vibration (such as in aircraft, engines, and machinery),
several specially designed nuts and lock washers are available.
3. Mechanical fasteners consists with two types : Permanent and Non Permanent (Figure
5.14).
Figure 5.14 Permanent and non-permanent fastening systems.
Example 5.3
Describe the considerations in choosing mechanical fastening as a joining method than
welding.
Solution:
Mechanical fastening may be preferred for the following reasons:
1. Ease of manufacturing.
2. Ease of assembly and transportation..
3. Ease of part replacement, maintenance, or repair.
4. Ease in creating design that require movable joint (e.g. hinges, sliding mechanism).
5. Lower overall cost of manufacturing the product.
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6.7.1 THREADED FASTENER
Bolts, screw and nut are among the most commonly used threaded fastener.
If the joint is to be subjected to vibration, e.g. engine and high-speed machinery, several
designed nuts and lock washers are available (lock pin, spring washer, etc.).
Figure 5.15 Example of some threaded fastener.
6.7.2 RIVETS
1. The most common method of permanent or semi permanent mechanical jointing.
2. Hole for the rivets are required in the component to be joined. The members to be
joined are drilled or punched (Figure 5.16).
3. Installing a rivet consists of inserted through the holes and then closed by forming a
head/end of the rivet.
4. Figure 5.16 and 5.17 show rivet and rivet gun used for riveting.
Figure 5.16 Example of rivets: (a) solid, (b) tubular, (c) split, and (d) compression.
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Figure 5.17 Rivet and rivet gun.
6.7.3 METAL STITCHING / STAPLING
1. This operation is fast and particularly suitable for joining thin metallic and non-
metallic materials (Figure 5.18).
2. Metal stitching has become a generic phrase that describes methods of repairing
cracks in cast metals without welding.
3. Example: stapling of cardboard containers (crate) for consumer product.
Figure 5.18 Stapling process.
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EXERCISE 5.3
List six (6) advantages of welding process compare to riveting.
6.7.4 SEAMING
1. Based on the simple principle of folding two thin pieces of material together.
2. The performance of seams may be improved with adhesives, coating, seals and
soldering.
3. Stage in forming a seaming joining shown in Figure 5.19.
4. Example: tops of beverage cans and containers for food (Figure 5.20).
Figure 5.19 Stage in forming a seaming joining.
Figure 5.20 Example of products that used seaming process.
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6.7.5 CRIMPING
1. Method of joining without using fasteners. It can be done with beads and dimples.
2. Example: caps on glass bottle are attached by crimping (Figure 5.21).
Figure 5.21 Example of crimping process.
5.7.6 SPRING AND SNAP FASTENER
1. Various spring and snap in fastener as shown in Figure 5.22.
2. Such fasteners are widely used in automotive bodies and household appliance.
3. They are economical and permit easy and rapid component assembly.
Figure 5.22 Example of spring and snap-in fastener to facilitate assembly.
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6.7.7 SHRINK AND PRESS FIT
1. Component may also be assembled by shrink fitting and press fitting.
2. Shrink fitting is based on the principle of differential thermal expansion and
contraction of two components.
3. Example : Mounting gears and cam on shaft (Figure 5.23)
Figure 5.23 Bearing installation equipment (a) Installation bearing on shaft (b)
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SUMMARY
In this chapter we studied that:
1. fusion welding is defined as melting together and coalescing materials by means of
heat;
2. oxyfuel-gas, arc, and resistance welding are among the most commonly used joining
operations. Gas welding uses burn energy; to supply the necessary heat, arc and
resistance welding use electrical energy instead;
3. in all of these processes, heat is used to bring the joint being welded to a liquid state.
Shielding gases are used to protect the molten-weld pool and the weld area against
oxidation. Filler rods may or may not be used in oxyfuel-gas and arc welding to fill the
weld area;
4. joining processes that do not rely on fusion or pressure at the interfaces include
brazing soldering and adhesive bonding. These processes instead utilize filler
material that requires some temperature rise in the joint. They can be used to join
dissimilar metals of intricate shapes and various thicknesses;
5. adhesive bonding is a joining method to produce good bond strength. Adhesives
have other favorable characteristics, such as the ability to seal, to insulate, to prevent
electrochemical corrosion between dissimilar metals, and to reduce vibration and
noise by means of internal damping in the bond;
6. mechanical fastening is one of the oldest and most common joining methods. Bolts,
screws, and nuts are common fasteners for machine components and structures
which are likely to be taken apart for maintenance, for ease of installations, or for
various other reasons;
7. rivets are semipermanent or permanent fasteners used in buildings, bridges, and
transportation equipment. A wide variety of other fasteners and fastening techniques
is available for numerous permanent or semipermanent applications.
REFERENCES
1. S. Kalpakjian, S.R. Schmid, Manufacturing Engineering and Technology 5 edition,
th
Prentice Hall, 2005.
2. Steve F.Krar Technology of Machine Tools, , Mc Graw Hill, 2005.
3. Howard B. Cary. 2005. Modern Welding Technology. 6th ed. New Jersey: Prentice-
Hall.
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CHAPTER 6: POLYMER PROCESSING
6.0 INTRODUCTION
Plastics are made from fossil fuels-petroleum, natural gas, coal, wood and some plant
materials, air, water and sometimes other common materials. They are designed in many
different formulas for different purposes. Today plastics are taken for granted. They form part of
our everyday life to such an extent that it would be impossible to imagine being without them.
This situation has come about in the last 60 years or so. Products are now designed and made
form plastic materials because they offer property combinations not available with other
materials, such as lightness, resistance to corrosion colour, transparency and ease of
processing. In spite of their limitations their exploration is generally limited by the designer’s
inequity. “Well its made of plastic” it is statement that is often heard. The common high tonnage
plastics used are polyethylene, polystyrene, polyvinyl chloride and polypropylene. Figure 6.1
shows various products made of polymers.
Figure 6.1 Various products made of polymers.
LEARNING OBJECTIVES
The objectives of this unit are to:
1. recognize the type of polymer materials;
2. describe the injection moulding process;
3. distinguish the function of injection moulding systems;
4. select the right welding process to join or cut the right product.
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6.1 POLYMER MATERIALS
1. Polymers are materials which consist of very long chain like molecules.
2. There are two major classes of plastics:
i. Thermoplastics
ii. Thermosets
3. Examples of thermo-plastic material are:
i. Cellulose acetate
ii. Nylon
iii. Polyethylene
iv. Polystyrene
v. Polypropylene
vi. Polyvinyl chloride
vii. Polycarbonate
4. Example of thermosetting material are:
i. Phenolics
ii. Melamine formaldhyde
iii. Formaldehyde epoxies
iv. Urea
v. Thermoset
vi. Polyester
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6.2 INJECTION MOLDING
1. Injection Molding is a manufacturing technique for making parts from thermoplastic
material.
2. In some circumstances, thermoses plastics can also be used with injection molding.
Molten plastic is injected at high pressure into a mold, which is the inverse of the
product's shape.
3. The mold is made by a moldmaker (or toolmaker) from metal, usually either steel or
aluminium, and precision-machined to form the features of the desired part.
4. Injection molding is very widely used for manufacturing a variety of parts, from the
smallest component to entire body panels of cars.
5. Injection Molding is the most common method of production, with some commonly made
items including bottle caps and outdoor furniture.
6. Figure 6.2 shows the schematic illustration of a typical injection molding component.
6.2.1 ADVANTAGES
1. Injection molding is a very economical method of producing large quantities of a complex
component very rapidly that is finished in one operation.
2. Sprues and runners can be reground and reused.
3. Generally mold costs are expensive and to amortise these high tooling costs require
reasonable production runs to be economic.
Example 6.1
Briefly describe the injection molding process.
Solution:
Injection molding is a process in which a polymer is heated to a highly plastic state and
forced to flow under high pressure into a mold cavity, where it solidifies. The molding is
then removed from the cavity.
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6.2.2 EQUIPMENT
Figure 6.2. Schematic illustration of a typical injection molding component.
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6.2.3 INJECTION SCREW
1. The comments for the extrusion screw operation apply to the injection screw equally.
2. The compression ratios are usually in the range 2.5:1 to 4:1 and the common L/D ratios
are between 15 and 20 to 1.
3. Most screws are capable of producing injection pressures of 30,000 psi (210 MPa).
4. The one important difference from the extruder screw is the addition of a back-flow
check valve fitted to the end of the screw.
5. The screw of the valve is to stop any back flow of molten polymer across.
6.2.4 NOZZLE
1. A nozzle is screwed into the end of the barrel to provide a means of transferring plastic
melt from the barrel to the mold via the sprue bushing.
2. It acts as a simple "push to seal" connection.
3. The nozzle is a region where the melt can be heated both by friction and conduction
from a heater band before entering the relatively cold channels of the mold.
4. Contact with the mold causes heat transfer from the nozzle and in cases where this is
excessive it is advisable to use nozzle retract during the screw back part of the cycle.
5. There is the possibility of the melt freezing in the nozzle.
6.2.5 CLAMPING SYSTEM
1. The clamping mechanism is the second major portion of the injection molding machine.
2. The function of the clamp mechanism is to make sure the following sequence is carried
out:-
i. Bring the mold together quickly.
ii. Close the mold gently.
iii. Part it slowly.
iv. Move it rapidly to full open.
v. Strip the mold slowly.
3. While carrying out the above actions, the clamp must be capable of holding the mold
closed during the injection and cooling part of the cycle.
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4. There are two distinctive types of system:
a) hydraulic
b) toggle
5. The hydraulic system uses oil under pressure that operates on .a piston connected to
the moving platen of the machine.
6. This causes the mold to close and the clamp force can be adjusted so that molten
plastic does not leak from the mold.
EXERCISE 6.1
1. An injection-molding machine is divided into two principal components; identify them.
2. What are the two basic types of clamping units?
6.2.6 CONTROL SYSTEMS
1. The trend today is for the control systems of injection molding machines to move from
process indicators and controllers to the sophisticated microprocessor fully controlled
machine.
2. The control of some machines can go to the extent of start up control and quality control
monitoring and instructions to the machine.
3. The normal parameters monitored and controlled are the barrel temperature controls.
4. There are at least two barrel zones and generally three as well as a nozzle zone to be
heated and controlled.
5. Many systems now use proportional controllers in which the heat is supplied as a
proportion of a maximum figure depending upon the difference the controller is away
from a predetermined value.
6. This gives a smoother temperature control and should eliminate the normal over and
under shooting of the barrel temperature.
7. Still widely used is sequence control in which timers control the length of the functions
such as the injection and cooling cycle.
8. Limit switches are used to activate the injection cycle, mold open position, screw back
distance.
9. The sequence control allowed the machine to repeatedly perform the same cycle of
operations.
10. The use of a pressure transducer in the mold, such as under an ejector pin or in a
special.
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11. Cavity meant that the pressure in the mold could be monitored and used to control the
machine.
12. The means that mold filling speed and mold pressure can be controlled to give a better
quality control of the component and ensure density control of the part as well as
ensuring the component does not flash.
13. The use of microprocessors now means that once the parameters are known for the
production of a component the set up operation can be automatically adjusted when the
component is put on the machine again.
6.2.7 MOLDING CYCLE
1. Granules or pallet of plastic powder (Figure 6.3) are poured or fed into a hopper, a large
open bottomed container, which feeds the granules down to the screw.
2. A heater heats up the tube and when it reaches a high temperature a screw thread
starts turning.
3. The screw is turned by hydraulic or electric motor that turns the screw feeding the
pellets up the screw's grooves.
4. As the screw rotates, the granules are move forward along the heater section that melts
then into a liquid.
5. The heated plastic is forced under the pressure of the injection screw to take the shape
of the mold by filling inside mold cavity.
6. Holding time, reduce and maintain the pressure at the hold value while the plastic cools.
The water-cooling channels assists in cooling the mold and the heated plastic solidifies
into the part.
7. The cycle is completed when the mold opens.
8. The part is ejected with the assistance of ejector pins within the mold.
Figure 6.3 Plastic granule.
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6.2.8 THE MOLD
1. The purpose of the mold is to shape the injected liquid plastic to the required
dimensions and hold the material until it cools and can be ejected as a solid part (Figure
6.4). The simplest type of mold consists of two halves, which contain a cavity or
impression. One half of the mold is stationary and generally contains the sprue through
which the polymer melt is injected.
2. The moving half mold is bolted to the moving platen, and generally contains the ejection
mechanism, either pins or plate.
3. The ejection of the molding from the mold is by ejector pins or plate.
4. Ejector pins can be operated either mechanically or hydraulically. Hydraulic ejection is
used where the machine is not attended and a number of ejection strokes are required.
5. Ejector plates are used where there is not enough area for pins or an even ejection is
required. The plate can be actuated by rods, chains or hydraulically.
6. In most cases the ejector pin or plate is operated by a set screw hitting a stop. When the
mold opens.
7. The two halves of the mold must mate together accurately so that no leakage of plastic
can occur at the split line.
8. To increase the production rate of the process, cooling channels are included in the
mold. This allows the mold temperature to be controlled.
Figure 6.4. Picture of the mold.
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6.2.9 GATES FOR INJECTION MOLDING
1. Gate is a constriction between the feed system and the mold cavity, and serves several
purposes. Freezes rapidly and prevents material either entering or leaving the mold.
2. Provides an easy means of separating molding from the feed system and it provides
small portion of the feed system where there is a very rapid shearing of the material.
3. Incorrect gating is one of the most frequent causes of faults in injection molding.
EXERCISE 6.2
What are the functions of gates in injection molds?
7.2.10 HOT RUNNERS
1. The hot runner system has the sprue and runner remaining molted all the time.
2. This has the advantage that only the molding requires to solidify, which for thin walled
components will be much faster that the sprue.
3. There is no degating necessary, and no sprue and runner to recycle and it is often used
in conjunction with a pin gate.
4. The sprue and gate are kept molten with electric heaters and an insulating nozzle.
7.2.11 CORES
1. Designers may require a design that is so complex that it cannot be made from a two
part mold, such as a re-entrant feature.
2. Thus to reduce the part may entail a mold with cores.
3. To enable the component to be withdrawn from the mold, the cores will have to be
operated before or during the mold opening cycle.
4. This can be done by hydraulics or actuated by the machine’s hydraulics, or by guide pins
set at an angle to move the core as the mold opens.
5. The cores slide in guides that are locked into position by angled heel plates as the mold
locks up.
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7.3 POLYMER EXTRUSION
1. Extrusion molding is a plastic shaping process in which a continuous workpiece is
produced by forcing molten thermoplastic material through a shaped die orifice (Figure
6.5).
2. As the hot plastic workpiece is carried along a conveyor, it is cooled and cut to the
desired length.
l
Figure 6.5. Schematic illustration of a typical extruder.
6.3.1 PROCESS CHARACTERISTIC
1. It is a continuous, high volume process.
2. Accurately controls material thickness.
3. Products are cut to desired lengths.
4. Has low tooling cost.
5. Can produce intricate profiles.
7.3.2 PROCESS SCHEMATIC
1. Thermoplastic materials are fed from a hopper into the heated barrel of an extruder
(Figure 6.6).
2. A rotating helical screw inside the barrel pushes the plastic through the barrel toward
the die located at the end of the machine as the plastic progresses along the barrel.
3. A heating jacket carefully controls the temperature of the plastic.
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4. Molten plastic is then forced through the die opening and around the mandrel to
produce a hollow workpiece.
Figure 6.6 Thermoplastic materials are heated in a barrel of an extruder.
.3.2 WORK PIECE GEOMETRY
1. Granular compounds and pellets are used in extrusion molding.
2. Scrap parts may be chopped and mixed with virgin materials.
3. Generally, only thermoplastic materials are extruded.
4. Typical profile extrusions are pipe, film or sheet, rain gutter components, and windows
components (Figure 6.7).
Figure 6.7. Typical profile extrusion.
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.3.3 PROCESS SEQUENCE
1. Blow molding a shape is a common industrial process.
2. The example shown below is of the production of a plastic bottle (Figure 6.8).
3. The plastics normally used in this process are; polythene, PVC and polypropylene.
Figure 6.8 Extrusion process to produce plastic bottle
1. The process is similar to injection molding and extrusion.
2. The plastic is fed in granular form into a 'hopper' that stores it.
3. A large thread is turned by a motor, which feeds the granules through a heated section.
4. In this heated section the granules melt and become a liquid and the liquid is fed into a
mold. Air is forced into the mold, which forces the plastic to the sides, giving the shape
of the bottle.
5. The mold is then cooled and is removed.
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EXERCISE 6.3
Using the 'Equipment and Processes', answer the following injection and blow moulding
questions:
1. Complete the diagrams by adding missing words and parts.
A. __________ of plastic powder (polystyrene, nylon, _____________ and polythene)
are poured or fed into a hopper which stores it until it is needed.
B. The ________ is turned on. This warms up and melts the granular plastic. A
________ turns a screw thread which pushes the granules along the heater section
causing it to change to a________.
C. The liquid plastic is forced into a mould by ___________.
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D. __________ air is ‘blown’ into the mould. This forces the liquid plastic against the
sides of the ________. In this example it forms the shape of a plastic __________.
E. The plastic is allowed to _____ and the mould is ‘_____’ and the plastic bottle
removed. The entire process is _________ hundreds or thousands of times.
6.4 COMPRESSION MOLDING
Figure 6.9 (a) Compression mould (b) Sequence of compression molding process.
Compression molding started when pressure is applied by a hydraulic press to the two halves
of the mold which puts the material and the mold under a compressive load (Figure 6.9).
1. Compression molding started when pressure is applied by a hydraulic press to the two
halves of the mold which puts the material and the mold under a compressive load
(Figure 6.9).
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2. The method of operation of a compression molding process is as follows:
Into the bottom half of a die is placed a measured amount of material.
There has to be a slight excess of material required to make the product, to
ensure complete filling of the mold.
This gives rise to a certain amount of flash or excess material that squeezes out
of the die closing line.
The two half molds are then closed and the heated molds cause the plastic resin
to polymerise.
Once the crosslinking ("curing") is completed the component is solid and can be
ejected whilst still hot.
The mold opens and the component is ejected and removed from the mold.
The mold is then cleaned to remove the flash and spilt plastic resin by
compressed air, ensuring a clean mold ready for the next plastic resin charge.
Normally the patterns are heated by electric heaters, although steam has been
used.
6.5 TRANSFER MOLDING
(a)
(b)
Figure 6.10 (a)Transfer mould (b) Sequence of transfer moulding process.
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1. Transfer molding is a development of compression molding.
2. Transfer molding consists of melting the plastic resin in a separate chamber and then
injecting it into a closed mold.
3. It is in principle similar to the ram injection molding of thermoplastic materials (Figure
6.10). There are a number of advantages of transfer molding. These are:
a) Shorter molding cycle times - because when the material is going through the
runners and gates there can be large enough flow restrictions to add a sensible amount
of heat to the liquid resin, hence saving time in the chamber.
b) Tighter Dimensional Control - because the material is injected into a closed mold,
there is little or no flash and dimensions across the parting lines are held. There is less
flash and thus finishing costs are lower.
c) Insert molding - the plastic material enters the mold as liquid, compared to
compression molding, thus the pressure on delicate metal inserts is less and it is less
likely for pins or thin mold sections to bend.
d) Encapsulation Transfer molding is the only feasible method of encapsulating delicate
electronic components where maximum density is required with low molding pressures.
e) 'Family' Molds. Transfer molding is well suited for producing parts of different sizes
which must be assembled together. One shot gives several parts all connected together
by the runners and cured at the same time. In compression molding the parts would
likely be molded separately and cured for different times, which may result in slight
colour differences.
4. There are disadvantages with transfer molding :
a) Mold costs -generally mold costs for transfer molding is more than for a similar
compression molding because of their greater complexity.
b) Material costs -material costs of transfer molding is higher than for compression
molding due to the material left in the well, sprue, runners and gates which cannot be
reused. For small components this scrap material can be a very sizable factor.
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6.6 BLOW MOULDING
Figure 6.11 Blow moulding.
1. Although plastic sheet and film may be produced using a split or slot die the, most
common method is the film blowing process.
2. The advantages of blown film are a faster rate of production and a stronger and tougher
product.
3. The space requirements for blown film are considerably less than cast film type line.
4. The blown film process consists of extruding the molten plastic through a circular die to
form a thin walled tube (Figure 6.11).
5. The tube is nearly always extruded vertically up.
6. As it leaves the die, internal air pressure, introduced through the die mandrel, expands
the tube, this is in effect an air mandrel.
7. The die for blown film is similar to a pipe or tube die, i.e. it is circular and may be either
centre or side fed.
8. In some cases, the die can be rotated about the mandrel during extrusion or even the
mandrel rotates.
9. This is done to minimize the effects of irregularities in die and mandrel manufacture, to
even out thick or thin sections of the extrudate.
10. Other types of die may have mandrel adjustable axially to allow for slight variations in
extrusion thickness.
11. Immediately above the die an internal air ring is used to chill the tubing.
12. At the top of the equipment is a collapsing frame and nip rolls that cause the tube to be
rolled flat.
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13. The nip rolls, one rubber coated and the other polished steel, (stainless steel or chrome
plated) are pressed tight enough together not to allow air to escape from the inside of
the blown bubble.
14. The nip rolls are driven at a faster speed than the extrusion, producing biaxial strain on
the blown film.
15. This causes the film produced to be stronger in all directions.
16. The flat tube is then brought down over the nip rolls to be wound onto a drum.
17. Blow molding is a very economical process used to produce hollow plastic products
(Figure 6.12).
18. A number of different methods of extrusion blow molding have been developed.
19. Factor such as the size of the part, the number to be made and the type of part will
influence the decision about which technique will be employed.
Figure 6.12 Close-up of the production of bottles
6.6.1 PROCESS CHARACTERISTIC
1. Inflates a softened parison tube to the con tour of a mold cavity Uses thermoplastics
2. Forms thin-walled hollow products.
3. Parting lines are present.
4. Wall thickness can be increased by increasing the parison tube wall thickness flash is
present but is minimal.
7.6.2 PROCESS SCHEMATIC
1. This setup is typical of many blow-moulding processes (Figure 6.13).
2. The parison tube is positioned in the mold cavity, and air pressure is then applied to the
parison, which forces the plastic to form to the mold cavity.
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3. Parts are usually quite uniform in thickness and are formed within a relatively short cycle
time.
Figure 6.13 Blow molding processes.
6.6 THERMOFORMING
Figure 6.14 Thermoforming centre.
1. Thermoforming is a process for forming thermoplastic sheets or films over a mold by
means of the application of heat and pressure.
2. Figure 6.14 shows the equipment for thermoforming process.
3. In this process, a sheet is (a) clamped and heated to the sag point (above the glass-
transition temperature, of the polymer), usually by radiant heating, and (b) forced
against the mold surfaces through the application of a vacuum or air pressure.
4. Fig 6.15 shows the various thermoforming processes for a thermoplastic sheet. These
processes commonly are used in making advertising signs, cookie and candy trays,
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panels for shower stalls, and packaging.
Figure 6.15 Various thermoforming processes for a thermoplastic sheet.
7.7.1 PROCESS CAPABILITIES
1. Typical parts made by thermoforming are packaging, trays for cookies and candy,
advertising signs, refrigerator liners, appliance housings, and panels for shower stalls.
2. Molds for thermoforming usually are made of aluminum because high strength is not
required, hence tooling is relatively inexpensive.
3. Quality considerations include tearing of the sheet during forming, nonuniform wall
thickness, improperly filled molds, poor part definition, and lack of surface details. Figure
6.16 shows the production line for thermoforming process.
Figure 6.16 Production line for thermoforming process.
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Example 6.2
Draw a flowchart, representing each stage of injection moulding and blow moulding.
Solution:
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SUMMARY
In this chapter we studied that:
1. polymers can be shaped by a variety of processes, including extrusion, molding,
casting, and thermoforming, as well as by some of the processes used in metalworking;
2. injection molding is a process in which a polymer is heated to a highly plastic state and
forced to flow under high pressure into a mold cavity;
3. in plastic extrusion, a polymer melt is compressed using screw to flow through a die
orifice and thus the continuous length of the plastic assumes a cross-sectional shape
that is approximately the same as that of the orifice;
4. compression molding started when pressure is applied by a hydraulic press to the two
halves of the mold which puts the material and the mold under a compressive load;
5. same as compression moulding, transfer molding consists of melting the plastic resin in
a separate chamber and then injecting it into a closed mold;
6. the blown moulding process consists of extruding the molten plastic through a circular
die to form a thin walled tube before internal air pressure expands the tube.
REFERENCES
1. S. Kalpakjian, S.R. Schmid, Manufacturing Engineering and Technology 5th edition,
Prentice Hall, 2005.
2. Steve F.Krar Technology of Machine Tools, Mc Graw Hill.
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7.0 NON CONVENTIONAL PROCESSES
CHAPTER 7 : NON CONVENTIONAL PROCESSES
7.1 INTRODUCTION
This chapter will discuss the non-mechanical method used in the material removal
processes. The non-mechanical method is applied when the strength and hardness of the
workpiece material is very high, the workpiece material is too brittle, the shape of the part is
complex and special surface finish and dimensional tolerance cannot be obtained by onther
manufacturing process. Example of advanced machining processes are electrical-discharge
machining, laser beam machining, ultrasonic machining, electrochemical machining and
water jet machining.
Figure 7.1 Non conventional process.
LEARNING OBJECTIVES
The objectives of this unit are to :
1. define the role of advanced machining, such as EDM, Laser and Water Jet machining in
the manufacturing area;
2. explain basic knowledge of advanced machining, such as EDM, Laser and Water Jet
machining;
3. recognize the typical equipments of non-conventional processes.
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7.2 ELECTRICAL-DISCHARGE MACHINING
Figure 7.2 Electric discharge machining (EDM): (a) overall setup, and (b) schematic
diagram of arc formation in EDM process (Source : Groover, 2002)
1. The Electrical-Discharge Machining (EDM) process can be used on any material
that is an electrical conductor.
2. This phenomenon has been known since the early day of electricity.
3. However it was not until the 1940s that a machining process based on this
principle was developed.
4. The EDM process has become one of the widely used in industry, especially in
the mold and die manufacturing.
7.2.1 PRINCIPLE OF OPERATION
1. The EDM process involves a controlled erosion of electrically conductive
materials.
2. The process is possible by intiation of rapid and repetitive spark discharges
between the tool and workpiece separated by a small gap of about 0.01 to 0.50
mm.
3. This spark gap is either flooded or immersed in a dielectric fluid.
4. The basic EDM consists of an electrode or tool and the workpiece connected to a
DC power supply and placed in a dielectric fluid (Figure 7.1).
5. Initially the gap between the tool and the workpiece, which consists of electrically
non-conductive dielectric fluid, is not conductive.
6. When the potential difference between the tool and the workpieceis sufficiently
high, the dielectric breaks down and a transient spark discharges through the
fluid, removing a very small amount of metal from the workpiece surface.
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7. The flowing dielectric then flushed away the small particles.
8. The amount of metal removed per spark depends upon the electrical energy
expended per spark and the period over which it is expended.
9. Two important process parameters in EDM are discharge current and frequency
discharges.
10. As either of these parameters is increased, metal removal rate and surface
roughness increases.
11. Since the process does not involve mechanical energy, the hardness, strength,
and toughness of the workpiece material do not necessarily influence the removal
rate.
12. The melting point and the latent heat of melting are important physical properties
that determine the volume of metal removed per discharge.
13. The material removal rate (MRR) can be related to melting point approximately
by the following empirical formula (Kalpakjian et. al. 2006):
MRR = 4 x 10 ITw -1.23 (8.1)
4
where MRR is in mm3/min, I is the current in amperes, and Tw is the melting point
of the workpiece in °C.
Power
Supply
Figure 7.3 Electric discharge machining (EDM): (a) overall setup, and (b) schematic
diagram of arc formation in EDM process (Source : Groover, 2002)
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i. DIELECTRIC FLUIDS
The main functions of the dielectric fluid are to:
act as an insulator until the potential differences is sufficiently high;
act as a flushing medium and carry away the debris in the gap;
act as a cooling medium;
1. The most popular dielectric fluids are hydrocarbon fluids, silicone-based oils and
de-ionised water.
2. Kerosene and water with Glysol are generally used. The requirements of a
dielectric fluid are:
i) It should have sufficient and stable dielectric strength to serve as an
insulation between tool and work till the breakdown voltage is reached.
ii) It should de-ionise rapidly after the spark discharge has taken place.
iii) It should have low viscosity and a good wetting capacity to provide an
effective cooling mechanism and remove the chip particles from the
machining gap.
iv) It should be chemically neutral so as not to attack the electrode, the
workpiece, the table or the tank.
v) Its flash point should be high so that there are no fire hazards.
ii. ELECTRODES
1. In the EDM process the shape of the electrode is impressed on the workpiece in
its complimentary form and as such the shape and accuracy of the electrode
plays a major role in the final accuracy of the workpiece machined.
2. The electrode material should have the following characteristics to serve as a
good tool:
i) It should be a good electrical and heat conductor.
ii) It should exhibit low electrode wear rates.
iii) It should be easily machinable to any shape at a reasonable cost.
3. Electrodes are made of graphite, copper, copper graphite, zinc alloy, brass,
copper tungsten, silver tungsten, and other materials.
4. The selection of electrode material depends on the type of power supply circuit
available, the type of the workpiece, and whether roughing or finishing is to be
done. Graphite is preferred for many applications because of its melting
characteristics.
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Example 7.1:
A typical alloy whose melting point = 900 °C is to be machined using EDM process.
If the discharge current used is 20 amps, what is the approximate value for the
material removal rate in the process?
Solution:
Using equation (7.1), an approximate material removal rate is
4
MRR = 4 x 10 ITw -1.23
MRR = (4 x 10 )(20)(900 -1.23 )
4
3
MRR = 185.94 mm /min
The typical parameters used in an EDM process are:
i) Spark gap 0.0125 – 0.125 mm
ii) Current 0.1 – 500 A
iii) Voltage (DC) 40 – 300 V
iv) Pulse duration 2 – 2000 µs
v) Dielectric pressure < 0.2 Mpa
vi) Surface finish 3 – 10 µm Rough
0.8 – 3 µm Finish
EXERCISE 7.1
A typical alloy whose melting point = 500 °C is to be machined using EDM process.
If the MRR is 574.709 mm /min, what is the approximate value for the discharged
3
current used in the process?
Describe some advantages of electrical-discharge machining.
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7.3 WIRE ELECTRICAL DISCHARGE MACHINING (WIRE EDM)
1. Electrical discharge wire cutting or also known as wire EDM is a process of
producing complex two and three dimensional shapes using a simple wire
eroding the material from an electrically conducting material.
2. This process is used to cut plates as thick as 300 mm and to make punches,
tools and dies from hard metals.
3. It also can ut intricate components for the electronics industry.
4. The cutting action in wire EDM is achieved by thermal energy from electric
discharges between the electrode wire and the workpiece.
5. The wire moves past the workpiece at fast rates up to 3 m/min.
6. The spark is struck between the moving electrode wire and the workpiece,
thereby removing the material.
7. A constant gap or kerf is maintained during the cut.
8. A schematic diagram of a wire EDM operation is shown in Figure 7.2.
9. The electrode wire is of a diameter 0.05 – 0.3 mm copper or brass, which is
wound between the two spools as shown in Figure 7.2.
10. The dielectric most commonly used is deionised water which is applied as a
localised stream rather than submerging the whole workpiece.
Fresh wire spool
Used wire spool
Figure 7.4 Schematic illustration of the wire EDM process (Source : Kalpakjian and
Schmid, 2006)
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EXERCISE 7.2
A wire EDM operation is used to cut out die components from 25 mm thick
tool steel plates. However, initial cut give the poor surface finish on the cut
edge. What changes in discharge current and frequency of discharges should
be made to improve the finish?
7.4 LASER BEAM MACHINING
1. Laser beam machining (LBM) uses the light energy from a laser to remove
material by vaporization and ablation.
2. A schematic diagram of LBM is illustrated in Figure 7.5
3. The types of lasers used in LBM are carbon dioxide gas lasers and solid state
lasers. In laser beam machining process, the energy of the coherent light beam
is concentreted not only optically but also in terms of time.
4. The highly focused, high-density energy source melts and evaporates portions of
the workpiece in a controlled manner.
5. The melted material evacuate the surface at a very high velocity.
6. LBM can cut a through and blind holes depending on the requirement.
7. Important physical parameters in LBM are the reflectivity and thermal conductivity
of the workpiece surface and its specific heat and latent heats of melting and
evaporation.
8. More efficient process can be achieved if the parameters mentioned above is
reduced.
9. The cutting depth can be calculated by this formula;
t = CP/vd 8 . 2
where t is the depth, C is a constant for the process, P is the power input, v is the
cutting speed, and d is the laser-spot diameter.
10. LBM is used to perform various types of drilling, slitting, sloting, scribing, and
marking operations. LBM is not considered a mass production process, and it is
generally used on thin stock.
11. Example of workpiece in LBM are ceramic, glass, plastics, rubber, wood and
cloth.
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Figure 7.5 Schematic illustration of the laser beam machining process. (source :
Groover, 2002)
EXERCISE 7.3
Describe two (2) advantages to use of Laser Beam Machining as compared to
conventional machining.
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7.5 WATER JET MACHINING
1. Water jet machining (WJM) uses a fine , high-pressure, high-velocity stream of
water directed at the work surface to cause cutting of the workpiece.
2. A variety of materials can be cut using WJM.
3. Example of those materials are plastics, fabrics, rubber, wood products, paper,
leather and composite materials.
4. A water jet machining operation is shown in Figure 7.6.
5. The typical nozzle opening diameter is 0.05 to 1 mm.
6. The water pressure level of 400 MPa is generally used in machining operation.
7. The water jet velocity reaches 900 m/s.
8. The nozzle unit consists of a holder made of stainless steel and a jewel nozzle
made of sapphire, ruby, or diamond.
9. Cutting fluids used in WJM are polymer solutions.
10. Important process parameters include standoff distance, nozzle opening
parameter, water pressure, and cutting feed rate.
11. A smaller standoff distance is desirable in order to minimize dispersionof the fluid
stream.
12. A typical standoff distance is 3.2 mm.
13. Among the advantages of WJM process are:
No heat is produced.
No environmental pollution.
No deflection of the workpiece.
The burr produced is minimal.
14. A limitation of WJM is that the process is not suitable for cutting brittle materials
because of their tendency to crack during cutting.
7.5.1 ABRASIVE WATER JET MACHINING
1. Abrasive particles are added to the jet stream when cutting metallic, nonmetallic
and advanced composite materials workpiece.
2. This process is known as abrasive water jet machining (AWJM).
3. Typical abrasive particles used are aluminum oxide, silicon dioxide, and garnet.
4. The typical grit size of the particles are between 60 to 120.
5. The nozzle orifice diameers are in the range 0.25 – 0.63 mm which is slightly
larger than in WJM.
6. Standoff distance in AWJM is about ¼ and ½ of those in WJM.
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7. Water pressures are about the same as in WJM.
Standoff distance
Figure 7.6 Schematic illustration of the water jet machining operation.
(Source : Kalpakjian and Schnid, 2006)
Example 7.2:
Identify three (3) operations that can be performed by abrasive jet machining.
Solution:
i. Cutting small holes.
ii. Deburring or removing small flash from parts.
iii. Trimming and beveling.
iv. Removing oxides or other surfaces films.
v. Cleaning components with irregular surfaces.
EXERCISE 7.4
Explain clearly the working principles of abrasive jet machining.
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7.6 ULTRASONIC MACHINING
1. Ultrasonic machining, also known as ultrasonic impact grinding, is the use of
ultrasonically-induced vibrations delivered to a tool.
2. When combined with an abrasive slurry, ultrasonic machining allows the tool to
create accurate cavities of virtually any shape in hard, brittle materials.
3. This machining process is nonthermal, nonchemical, and nonelectrical. It does not
change the metallurgical, chemical or physical properties of the workpiece.
4. The cutting vibration uses in the ultrasonic machining process begins with converting
a high-frequency electrical signal into an oscillatory mechanical motion.
5. This motion is acoustically transmitted through a metal tool holder and cutting tool
assembly.
6. This linear oscillation is typically at a rate of 20,000 times per second, and, when
used with an abrasive slurry flowing around the cutting tool, microscopic grinding
occurs.
7. The machined area becomes counterpart of the cutting tool used.
8. Using this technology, we can cut almost limitless assortment of types and shapes of
cuts to meet any design requirements.
( a ) ( b )
Figure 7.7 (a) Equipment (b) Schematic diagram.
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9. Principle of ultrasonic machining:
Tool is hammering the abrasive particles.
The impact of the abrasive particles which move freely.
Cavitation erosion of abrasive slurry.
Chemical erosion.
Figure 7.8 Principle of ultrasonic machining.
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10. Advantage of ultrasonic machining:
Machining any materials regardless of their conductivity.
USM apply to machining semi-conductor such as silicon, germanium etc.
Especially, USM is suitable to precise machining brittle material.
USM does not produce electric, thermal, chemical abnormal surface because it is not
by electric, thermal, chemical action but by mechanical mechanism.
11. Disadvantage of ultrasonic machining:
UM has low material removal rate.
Tool wears fast in UM.
Machining area and depth is restraint in UM.
EXERCISE 7.5
Explain clearly the working principles of ultrasonic machining. Use sketches to
support your answer.
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SUMMARY
In this chapter we have studied the advanced machining processes available. These
processes are alternative processes that can be utilised for specified application. The
processes that being discussed in detail are:
1. Electrical discharge machining.
2. Wire electrical discharge machining.
3. Laser beam machining.
4. Water jet machining.
5. Abrasive water jet machining.
6. Ultrasonic machining.
REFERENCES
1. Manufacturing Engineering Technology, 5th Edition, Serope Kalpakjian and Steven
Schmid, Prentice Hall 2006.
2. Manufacturing Technology, P N Rao, McGraw Hill 2000.
3. Fundamentals of Modern Manufacturing, 2nd Edition, Mikell P. Groover 2002.
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8.0 POWDER PROCESSING
CHAPTER 8 : POWDER PROCESSING
8.1 INTRODUCTION
Powder processes products are today used in a wide range of industries, from automotive
and aerospace applications to power tools and household appliances. Controllable
characteristics of products prepared using various powder technologies include mechanical,
magnetic, and other unconventional properties of such materials as porous solids,
aggregates, and intermetallic compounds. Competitive characteristics of manufacturing
processing (e.g., tool wear, complexity, or vendor options) also may be closely regulated.
Example of typical product made using powder processes include connecting rods, piston
rings, gears, cams, bushings, bearings, surgical implants, magnets and metal filters (Figure
8.1).
Figure 8.1 Example of typical product made using powder processes.
LEARNING OBJECTIVES
The objectives of this unit are to :
1. describe methods of producing metal powders;
2. indentify several type of powder metallurgy part and component widely used in
market;
3. define common ceramic materials in industry;
4. identify the application of ceramic product;
5. identify manufacturing processes of ceramic materials.
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8.2 PRODUCTION OF METAL POWDERS
1. Powder processes is a forming and fabrication technique consisting of three major
processing stages.
2. First, the primary material is physically powdered, divided into many small individual
particles.
3. Next, the powder is injected into a mold or passed through a die to produce a weakly
cohesive structure (via cold welding) very near the dimensions of the object
ultimately to be manufactured.
4. Finally, the end part is formed by applying pressure, high temperature, long setting
times (during which self-welding occurs), or any combination thereof.
5. The powder metallurgy process consists of the following operations in sequence,
which involving the powder production, blending, compaction, sintering and finishing
operations.
6. Figure 8.2 shows the outline of processes and operations involved in making powder
metallurgy parts.
Pressing
Isostatic pressing
Atomization Rolling Atmosphere
Extrusion
Reduction
Electrolytic deposition
Carbonyls
Cold Sintering
compaction
Metal Blending Secondary
powders and finishing
Hot
compaction
Additive Coining
Lubricant
Isostatic pressing Forging
Machining
Heat treating
Impregnation
Figure 8.2 Outline of processes and operations involved in making powder metallurgy
parts.
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8.2.1 METHODS OF POWDER PRODUCTION
1. There are several methods of producing metal powders, and most of them can be
produced by more than one method.
2. The choice depends on the requirements of the end product.
3. The microstructure, bulk and surface properties, chemical purity, porosity, shape and
size distribution of the particles depend on the particular process used.
4. These characteristics are important because they significantly affect the flow and
permeability during compaction and in subsequent sintering operations.
5. Particle sizes produced range from0.1 to1000 μm.
6. Example methods of producing metal powders are:
atomization;
reduction;
electrolytic deposition;
carbonyls;
comminution;
mechanical alloying;
miscellaneous methods;
nanopowders;
microencapsulated powders.
Figure 8.3 Metal powders.
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9.2.2 BLENDING METAL POWDERS
1. Blending refers to when the powders of the same chemical composition but possibly
different particle size are intermingled.
2. Different particle are blended to reduce porosity.
3. Mixing refers to powders of different chemistries being combined.
4. An advantage of PM technology is the opportunity to mix various metals into alloys
that would be difficult or impossible to produce by other process.
5. The process accomplished by mechanical means such as, rotation in drum, rotation
in a double-cone container, agitation in a screw mixer and stirring in a blade mixer
(Figure 8.4).
6. To achieve best result in mixing, the container will be filled between 20% - 40% full.
7. The containers are usually designed with internal baffles or other ways of preventing
free-fall during blending of different size of powder, because variations in settling
rates between sizes result in segregation.
8. Other ingredients are added during blending or mixing process:
a. Lubricant, zinc and aluminum is added in small amounts as to reduce friction
between particles and at the die wall during compaction.
b. Binder, which are required in some cases to achieve adequate strength in the
pressed but unsintered part.
c. Deflocculants, which inhibit agglomeration of powder for better flow
characteristics during subsequent processing.
Figure 8.4 Mixing and blending processes of metal powders. (a) crushing (b) mixing (c)
hammering.
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