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ELECTROCHEMICAL PROCESS
These methods are based on the electrolysis of molten
solutions of metals or fused salts.
The metals are electrically deposited on the cathode ofan electrolytic cell as a sponge or powder or at least in a
physical form in which it can be easily disintegrated
into a powder.
Advantages of the process:The technique has a number of advantages, e.g.
The product is usually of a high commercial purity.
A considerable range of powder qualities can be
obtained by varying bath compositions.
Frequently the product has excellent pressing and
sintering properties.
The cost of the operation may in some cases be low.
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Limitations:
Alloy powders cannot be produced.
The product of process is frequently in activecondition (presence of chemicals on powder particles)
which may cause difficulties in washing and drying it
(contaminationoxidation with atmospheric oxygen
may occur). The cost of operation may be high in some cases.
Basic principe of the process and e!"ipment "sed:
The equipment used is an electrolytic bath made of
steel, and lined from inside with rubber. Twoelectrodes are inserted in the bath.
!athode is made of lead while anode is made of the
same metal whose powder is being produced.
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Principe:
The basic principle is the electrolysis process in whichdecomposition of a molten saltaqueous solution into its
ions is obtained by the passage of electric current. The
metallic ions are deposited at the cathode which can be
removed with a brush and collected at the bottom.
The electrolytic tan"s have conical bottoms with a
valve. #uction pipes are connected to these bottoms and
powder is removed from the tan".The efficiency of the tan"process depends on the
deposition rate.
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Figure$ %lectrolytic !ell &peration for 'eposition of
owder #chematic.
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owder production at cathode is favored by$
high current density*
wea" metal concentration*
addition of acids*
low temperature* avoidance of agitation, and*
suppression of convection.
+ ery fine powder can be obtained when the current flowing is so
strong in relation to the strength of the solution that hydrogen is stronglyevolved from the cathode.
-ydrogen evolution is encouraged by$
(i) increasing cell voltage*
(ii) diminishing the sie of the cathode*
(iii) bringing the anode and cathode closer together*
(iv) increasing the temperature*
(v) wea"ening the strength of the metallic solution
(vi) adding acid
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+ /hen metal is deposited without evolution of
hydrogen, the deposit may be ductile and compact if the
current is 0ust not great enough to cause hydrogen
formation, or very hard with large crystals using strong
solutions and large quantities of electricity, or sandy and
brittle with little cohesion using very small current.
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#ESI$% CO%SI#ERATIO%S:
An outstanding characteristics of electrolytic powder
process is the large number of variables which eitherhave to be selected and fixed before plant is erected, or
which have to be controlled during operation. The most
important are*
(i) %lectrolytes(ii) %lectrodes
(iii) !urrent
(iv) Flow of electrolyte
(v) #tructural considerations
(vi) After treatment
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Eectro&tes:
The choice of the type of electrolyte will depend largely upon the
cost of the chemicals involved.
%lectrolyte should not corrode the apparatus i.e., it should be of non
corrosive nature.
!oncentration of the electrolyte should remain same with the passage
of time.
Cost: 1elatively pure salts of copper which are cheap and freely available
are uncommon, and therefore most copper powder production has
been derived from sulphatesulphuric acid baths.
#ome scientists are in favor of copper chloride bath because of bettercathode efficiency, lower cell voltage and less power consumption. 2t
is claimed that the chloride bath produces a more dendritic powder
with better pressing properties.
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2n the case of sulphate electroytes, the presence of a small amount of
chloride improves the anode current efficiency. #uch additions may,
however, cause corrosion problems in the cells and deterioration of the
"eeping qualities of the powder.
2ron powder sulphate or chloride baths. -aving selected the type of bath, the exact composition must then be
chosen and thereafter maintained with considerable care
/ith copper sulphatesulphuric acid electrolytes, it has been found that
cathode current efficiency improved as the copper content increased,reaching a maximum of 34.5 6 at 78 gm.liter, and decreased with
increasing acid, being 39.3 6 at :8 gm.liter. The apparent density of
powder produced increased to a maximum of ;.447 gm.ml. at
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The electrolyte composition does not necessarily stay
constant during electrolysis. ariations are usually caused
mainly by differences in anodic and cathodic current
efficiencies. 2n the case of copper, the concentration of metal in the
bath generally rises. #ubsidiary effects are caused by
evaporation, by dragout when the powder is removed, and
by the chemical solution of the electrodes when the current
is interrupted. 1eplace the electrolyte with fresh solution.
!ontrol of temperature is also important. 2t was found
that as the temperature increases from 98 to 4; !, thecurrent efficiency increased from 44.< to 39.5 6 and the
apparent density from ;.589 gm.ml. to ;.>54 gm.ml.
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Eectrodes:
The sie, shape and disposition of electrodes may
vary widely.
The anode may be soluble or insoluble and may be placeddirectly in the electrolyte or within a porous pot.
The anode may be of pure or impure metal, or in the formof scrap supported in a bas"et. ?nless, however, special
precautions are ta"en, impure anodes may cause operatingdifficulties or at least contamination of the powder by the
formation of slimes.
2t is unusual for the area of the anode to be larger or smaller
than that of the cathodes, for the purpose of balancing the
electrode efficiency.For similar reasons, in order to improve the distribution of
powder deposit on the cathodes, it is recommended to use anodes
with rows of holes bored in them
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2n the case of cathodes, the choice may depend upon whether the deposit
is going to be stripped off or allowed to fall off in the form of a sponge
or powder, or weather it is intended to ma"e a coherent brittle deposit.
2n the former case, the choice is mainly a matter of minimiing
corrosion, especially at the liquid level, and facilitating clean stripping.
For copper copper rod, Al sheets, b sheet.
For iron @b, o, Ta, / or b sheets /hen the deposit is of a brittle nature, it may be removed either by
"noc"ing it off or flexing the sheet cathode.
#ponge deposits may be removed using brushes.
Bayers of graphite paint or oils may be employed to facilitate the
separation. !astor oil oxidied with 97 6 perchloric acid applied by
preimmersion has been used.
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2t is not unusual to ma"e the deposition upon a cathode
starting sheet which is substantially crushed along with
the deposit. For example, iron gaue has been
recommended and used. This becomes embrittled
during the electrolysis and is readily crushed.
2t has even been proposed to employ coldpressed and
unsintered or sintered cathode which easily
disintegrate.
1otating electrodes
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C"rrent:
The choice of a specific operating current density will depend
mainly upon whether a coherent brittle or powdery spongy deposit is to
be made. 2n the former case the current density will be low, in the latter
it will be high.
2n each case there may be an optimum density which gives the
highest current efficiency, but this may not necessarily be the same
density which produces the most suitable grade of powder.
#ome wor"ers have found that rising temperature increases the currentefficiency.
Apparent density of the product is unaffected by current density.
The frequency at which the current is interrupted has a most important
influence upon the particle sie of the powder, and the longer theintervals the larger the particle.
The greater the interval between current interruptions, the higher
is the apparent density.
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'o( of Eectro&te:2n practice, convection and development of gas
bubbles cause a considerable flow of electrolyte overthe cathodes, and an important practical difficulty is tomaintain this reasonably constant. 2t would appear thata certain minimum forced circulation would be helpful
in attaining this. 2n an experiment it was found that stirring theelectrolyte coarsened the powder and increased theapparent density.As stirring is advantageous from the point of view ofevening out bath variables, but to some extentdisadvantageous in increasing the density andtherefore reducing the compressibility.
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Str"ct"ra:&wing to the substantial changes in behavior of anelectrolytic powder cell when its sie is increased, it is
advisable that, when such a process is advised in thelaboratory, it should be operated as a unit cell with fullsied electrodes before an attempt is made to design thefinal plant.#tructural design factors involve ta"ing decision upon
the sie and nature of the electrodes, whether theyshould be stationary or rotary, or be sheets, tubes orrods, etc., whether the cathodes should be lifted out ofthe cell for scrapping or not, whether the scrappingshould be manual or mechanical.&ther problems concern with the corrosive nature of theelectrolyte$ such as tan" construction and linings,contacts, electrolyte handling, cooling or heating, usedanode treatment, etc.
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After)treatment:
An electrolyte powder is generally in a reactive
condition, and is also wet with reactive electrolyte, thereare considerable problems in washing and drying it and
bringing it to a dry powder which is not only low in
oxide but reasonably stable on storage.
For example, with electrolytic iron powder, it was found
necessary to wash the cathode deposit with water, : 6
-:#&5, water, dilute citric acid, water, dilute ammonia,
and finally with distilled water before filtering, and thenmoistening with acetone before drying. %ven then it is
recommended that the powder should be annealed in
hydrogen to reduce the oxide content.
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T&rre* with copper powder, recommends annealing in a reducing
atmosphere. -e found, however, that treating the powder in a crac"ed
ammonia atmosphere often led to rapid subsequent deterioration onstorage. -e recommended treating the powder with suitable water
repellent chemicals and indicated that stearic acid dissolved in ammonia
was suitable for a commercial process.
any manufacturers avoid washing and drying difficulties byannealing the powder in a reducing atmosphere.
/hen a brittle electrodeposit is the first product, annealing may be
absolutely necessary in order to produce a powder having reasonable
pressing qualities, and is customary among iron powder producers.
&wing to the reactive nature of many electrolytic metal powders,
difficulties are frequently observed in preventing them from oxidiing or
corroding on storage. 2t is customary, at least with copper powder, to add
corrosion inhibitors to the powder.
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METAL PO+#ER TESTI%$
Characteri,ation of Meta Po(ders:• Cuality monitoring
• The following steps are involved in prior to processing into compact shape$
a) owder characteriation and testing b) owder handling and mixing
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a- Po(der Characteri,ation and Testing:
9. owder sampling
:. !hemical Testing
i) &xygen content of the powder
ii) Acid insoluble content of powders
7. articlerelated vs massrelated properties
5. article sie and particle sie distribution
i) #ieving
ii) icroscopic siing
iii) #edimentation methods
iv) !oulter !ounter and particle analysis by light obscuration
v) Baser light scattering
i l h d
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8. article shape and structure
4. #pecific surface area
>. !haracteristics determining the processing behavior of metal
powder$ i) Flow rate and apparent density
ii) !ompactibility
iii) 'imensional changes of powders due to
sintering
.- Po(der mi/ing and handing
9. #pecial precautions in handling and storing metal powders
:. owder ixing
i) ixing and demixing
ii) ixing apparatus
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PARTICLE SHAPE:• Figure shows various possible particle shapes of powders.• #pherical powders show excellent flow properties but give
poor green strength as compared to irregular powders.• /ater atomiation ranging from near spherical tohighly irregular.
• Das atomiation spherical powder particles.
• The reactivity of the metal or alloy essentially determinesthe particle shape. 2f the alloywater reaction produces astrongly adherent film then irregular particles are formed.#pherical shapes are produced when the oxide formed arehighly fusible at the melting point of the alloy as they have
no strength to overcome the forces of surface tension.• -igh melting metalsalloys have tendency to form
spherical particles because of long freeing times.• ery short freeing times for low melting metalsalloys
tend to form highly irregular particles.
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T . h P ti Sh d th M th d f P d P d ti
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Ta.e sho(s Partice Shape and the Method of Po(der Prod"ction
O%E #IME%SIO%AL
cic"ar Irreg"ar Rod)i0e
hemical decomposition !hemical decomposition
echanical comminution T+O #IME%SIO%AL
entritic 'a0e
lectrolytic echanical comminution
THREE #IME%SIO%AL
pherica Ro"nded
tomiation Atomiation
arbonyl Fe !hemical decomposition
recipitation from a liquid
reg"ar Poro"s
tomiation 1eduction of oxides
hemical decomposition
ng"ar
echanical disintegration !arbonyl @i
P d P ti
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Po(der Properties:
•rocessing conditions and final sintered properties are
determined to a very large extent by the characteristics of
the powder, such as* chemical composition
particle sie and sie distribution
particle shape
structure
surface condition
S i f P d
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Samping of Po(ders:• #tandard methods
A#T !ommittee E3
2F #tandard !ommittee (etal owder 2ndustries
Federation)
A#T #tandards E:98
2F #tandard 9
• A representative sample of the whole lot
• #amples from the entire cross section of the stream of
powder.• 1epresentative sample from a shipment consisting of
several drums.
• Thieve sampling
• ThievesG are devices to ta"e samples from different
CHEMICAL TESTS
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CHEMICAL TESTS
a- H&drogen Loss Test:
) A#T standard % 983, 2F standard :
for the socalled hydrogen loss of !u, / and Fe powder
•) A sample of powder is heated in a stream of hydrogen
for a given length of time and at a given temperature.
•) Boss of weight an approximate measure of theoxygen content of the powder.
•) -ydrogen loss values may be lower than the actual
oxygen content &xides not reduced by hydrogen
under the test conditions such as #i&:, Al:&7, !a&, etc
•) The hydrogen loss value may be higher than the actual
oxygen content in the presence of elements forming
volatile compounds with hydrogen, i.e. # or !.
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• #ome metals volatile at the test temperature, i.e., Hn, !d and
b.
• To avoid measuring the content of !, #, or volatile metals in
the metal powder, a modified hydrogen loss test is used.• The amount of water vapor produced by heating in a stream
of dry hydrogen is determined by titration.
• Total amount of oxygen in a metal powder including oxygenin refractory oxides, fuse a sample in a small singleuse
graphite crucible under a flowing inert atmosphere at a
temperature of :;;; o! or higher.
• The oxygen is released as !& and measured by infraredabsorption or alternatively converted to !&: and measured
by a thermal conductivity difference.
A id I . C t t f C d ' P d
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Acid Inso".e Content of C" and 'e Po(der:
• #amples of Fe powder are dissolved in -!l and those of
!u in -@&7 under specified conditions.
• The insoluble matter is filtered out, ignited in a furnace and
weighed.
• #ilica, alumina, clays and other refractory materials
• 2n Fe powder, the acid insoluble may also include insoluble
carbides.
Partice Si e and Partice Si e #istri. tion
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Partice Si,e and Partice Si,e #istri."tion
Methods:
(a) #ieving
(b) icroscopic #iing(c) ethods based on #to"esI Baw
i) the 1oller Air Analyer
ii) the icromerograph
iii) Bight and Jray (#edigraph) Turbidimetry(d) !oulter !ounter and article Analysis by Bight&bscuration
(e) Baser Bight #cattering* the icrotrac article Analyer
(f) Hetasier @anoparticle sie analyer
Si i
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Sieving:
A#T sieves sie,
and ?.#. standardsieve designation,Km
Tyler sieves sie,
and Tyler sieveseries designation,Km
95 (:;; mesh)
58 (@o 7:8) 55 (7:8 mesh)
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Figure: Schematic of sieveseries stacked in order of size
Figure: Stacked sieves on ashaker with rotaryand tapping action
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Figure: Sieved size of an irregularly shapedparticle
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Methods Based on Sto0es1 La(:
• #edimentation and %lutriation
• #to"esI Baw gives the settling velocity L of spherical particles with a diameter x and a density M in a fluidmedium with density MF and viscosity N
L O g (M MF ) 9< N + x:
/here g is the gravitational constant.
++ articles which are not spherical will also settle. Their
#to"esianG sie is defined by the diameter of a sphere ofthe material which has the same settling velocity as theirregular powder particle.
• !onvection currents in the suspending fluid must be
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• !onvection currents in the suspending fluid must be
avoided.
• The relative rate of motion between the fluid and the
powder particles must be slow enough to guaranteelaminar flow, which means the 1ynolds number should be
less than ;.:
x L MF
N
/here x is the particle sie, L is the settling velocity, MF
the density of the fluid and N its viscosity.
• The particles in the suspension must be perfectly
dispersed and the suspension must be dilute enough to
guarantee independent motion, which means maximum
concentration of about 9 6 by volume of particles in the
suspending medium.
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METAL PO+#ER
PROCESSI%$ TECH%I23ES
Compaction of Meta Po(ders:'ifferent ways of consolidation of metal powders*
4A- +ith appication of press"re (hich inc"des
i) ?niaxial pressing (single action or double action pressing)
ii) 2sostatic pressing
iii) 1oc"ing die compaction
iv) owder rolling
v) owder extrusion
vi) owder swaging
vii) owder forgingviii) owder 2n0ection olding
4B- +itho"t app&ing press"re s"ch as5
i) #lip mixing or slip casting
ii) ibrational compaction
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• Consoidation genera& occ"rs in three stages
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Consoidation genera& occ"rs in three stages
(a) rearrangement of particles.
(b) particles contacting by plastic deformation.
(c) mechanical loc"ing and cold welding of particles dueto surface shear strains.
2t is, therefore, some time easier to cold compact irregular
particles than spherical powder particles.
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Main varia.es in press"re compaction are:
(a) ethod of compaction
(b) !ompaction pressure, time and temperature
(c) 1ate of compaction
(d) !ompacting atmosphere
(e) Bubricants and other additives of the mix, and
(f) 'ie design()The main o.6ectives o.tained d"ring pressing are:
(9) To achieve the required part shape.
(:) To obtain the required green density.
(7) To secure sufficient green strength to permit safe handling of
the part.
(5) To provide particletoparticle contact which is necessary for
sintering.
The .asic t&pes of compacting presses are:
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The .asic t&pes of compacting presses are:
(i) echanical (single punch or rotary type) presses.
(ii) -ydraulic presses.
(iii) -ybridtype presses (mechanical presses may ma"e use ofauxiliary pneumatic or hydraulic devices).
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The powder metal must first fill the die orifice.
Filling may be done by hand or automatically from the
presshopper.
A constant volume or constant weight may be used.
ibration filling is introduced to create denser pac"ing to
avoid bridging and high porosity defects.
ressing may be done automatically.The pressure may be applied along more than one axis using
various punch and die sets designed to minimie defects.
%0ection after pressing may be carried out automatically or by hand.
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Minim"m re!"irements for an& po(der meta press:
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Minim"m re!"irements for an& po(der meta press:
(i) Adequate total pressure capability in the direction of
pressing and sufficient part e0ection capability.
(ii) !ontrolled length and speed of compression and e0ectionstro"es.
(iii) Ad0ustable die fill arrangements.
(iv) #ynchronied timing of press stro"es.(v) aterial feed and part removal systems.
$enera cassification of po(der meta"rg& parts:
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$enera cassification of po(der meta"rg& parts:
7- Cass I parts with a diameter (or thic"ness) up to
48 mm and one level parts of any contour that can
be pressed with a force from one direction.
8- Cass II parts are single level components of any
thic"ness and any contour that must be pressed
from two directions.
9- Cass III parts are two level components of any
thic"ness and contour that must be pressed from
two directions.- Cass I; parts are multilevel components of any
thic"ness and contour that must be pressed from
two direction.
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Compacting Presses and Attachments:
The presses systems used are*
(a) #ingle action press system consisting of$i) a die to form the outer contour of the part*
ii) an upper punch to form the top surface of the part*
iii) a lower punch to form the bottom surface of the part*
iv) if required, core rods to form any through holes
(for class 2 parts).
(b) 'ouble action opposed ram system consists of
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(b) 'ouble action opposed ram system consists of
a die, upper punch, lower punch and core rods (for class 2and class 22 parts).
(c) 'ouble action floating die system consists of moving upper punch, stationary lower punch, moving dietable and core rods (for class 2 = 2 parts).
Further during compaction tooling materials, clearances andtolerances require expertise and special attention is paid to(i) die design* (ii) die materials* (iii) punch* (iv) carbideinserts* (v) tolerances, clearances and finishes.
'orming and sintering in one step
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'orming and sintering in one step
• The simplest forming route for a metal powder is loose
powder sintering which is used for forming spherical
powder.
• 2n this process the metal powder is filled loosely or sha"en
into the mould by vibration and subsequently sintered in the
mould.• The mould can be made of steel or graphite and can be used
repeatedly.
• owders which are difficult to compact are often formed by
simultaneous application of pressure and temperature(pressure sintering).
'orming and Sintering in Separate Process Steps
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'orming and Sintering in Separate Process Steps
A/ia pressing
•The powder is charged by volume filling into a closed tool
consisting (in principle) of three parts* a die and an upper anda lower punch, and subsequently sub0ected to a pressure using
mechanical or a hydraulic press.
•The cycle begins with the tool in the filling position (9) in
which the upper and the lower punches are retracted so that a
defined filling space arises. The tool is filled with powder
from the filling shoe, the quantity of powder being determined
by the volume of the die cavity at this stage (:).
• The height of the filling space is determined by the apparent
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g g p y pp
density (filling volume) of the powder and by the required
final density of the component.
• 2t usually is equal to :.: to 7 times the height of thecomponent.
• After finishing the filling operation the powder is
compacted by the counter movement of the lower and the
upper punches until the tool has reached the pressing position (7 P 5).
• After completion of the pressing step the upper punch is
withdrawn and the compact is e0ected by moving the lower punch upwards (8). The e0ected component is pushed away
from the die by the filling shoe (4), and the tool is again
ready for a new pressing cycle.
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Compaction
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Compaction
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3nia/ia Pressing
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The e6ection s&stem
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6 &
• 'uring compaction according to the e0ection system with
two punches moving in opposite direction (figure E) the die
remains fixed and the two punches carry out thecompaction. The compact exhibits a symmetrical density
distribution with a neutral one in the middle. The counter
movements of the punches are effected by cams and toggle
0oints in the case of a mechanical press and by separatelycontrolled upper and lower cylinders in the case of a
hydraulic press.
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ISOSTATIC PRESSI%$
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ISOSTATIC PRESSI%$
• The metal powder is compacted uniformly in all directions
so that the compact becomes an accurate scale down of themould. uniform density a homogeneousmicrostructure
• For this purpose the powder is sealed in a flexible envelopeand the assembly (mouldpowder) is immersed in a fluidwhich is pressuried.
• There are virtually no residual stresses in the compactedmaterial, because there is no die wall friction.
• Figure9 shows the use of formers and use of containers withholes for support purposes.
Fig. 9$ %xamples of use of (a) container with holes and
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g p ( )
(b) (c) formers for producing shaped components by
isostatic pressing.
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Two types of processes:
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i) Cold Isostatic Pressing
ii) Hot Isostatic Pressing
< Cod Isostaic Process ))))) t(o methods are "sed $(a) free mould or Q(et .agQ process (Figure :) is suited for
(i) batch scale production*
(ii) the mold is filled and sealed outside the pressure vessel.
(iii) After the mold is introduced in to the pressure vessel, it iscompletely immersed in the pressure medium, usually watercontaining lubricating and corrosionpreventive additives*
(iv) complex parts*
(v) research and prototype wor"*(vi) several moulds in one runeven with*
(vii) differing shape, i.e. parts of different sies and shapes thatrequire the same process parameters can be pressed in the
same cycle.
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a) owder fill by weight or volume.
b) Filling of the mould from the top.
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c) Top lid put on the mould container.
d) ould is placed inside the pressure vessel.
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e) Top lid put on the pressure vessel after necessaryevacuation and filling with the pressuriing medium.
f) Top lid removed after required isostatic compaction for
removal of the mould.
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(b) fixed mould or Qdr& .agQ process (Figure 7) which is
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characteried by
(i) envelope is permanently fixed into the pressure vessel,
(ii) After the elastomeric mold is filled with powder, pressureis applied by introducing pressuried oil between the
fixed mold and the vessel wall,
(iii) only one compact at a time is used,
(iv) more simple shapes are made and
(v) more suited for mass production and faster production
rates.
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Fig.7$ #chematic of equipment for drybag isostatic pressing.
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Press"re $enerators
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• ressure is generated in the pressure medium through the
use of airdriven and hydraulically driven pumps and
pressure intensifiers. • The pressure medium typically is oil for dry bag processes*
water containing additives (watersoluble oil or rust
inhibitors) is used for wet bag processes. A filtering system
should be included with all systems to protect the pressuregenerating equipment from particulate contamination.
•#epress"ri,ation S&stems. 'epressuriation can be
accomplished with a single metering valve.