K. GO£OMBEK et al.: RHEOLOGICAL PROPERTIES OF FEEDSTOCKS AND THE STRUCTURE ...
163–171
RHEOLOGICAL PROPERTIES OF FEEDSTOCKS AND THE
STRUCTURE OF INJECTION MOULDERS FOR SINTERING
COMPOSITE TOOL MATERIALS BASED ON MMCS
REOLO[KE LASTNOSTI ME[ANIC IN STRUKTURA
VBRIZGANIH REZKARJEV ZA SINTRANE KOMPOZITNE
ORODNE MATERIALE NA OSNOVI MMCS
Klaudiusz Go³ombek1, Grzegorz Matula1, Jaros³aw Miku³a1, Mirko Sokovi}2
1Silesian University of Technology, Institute of Engineering Materials and Biomaterials, Gliwice, Poland
2University of Ljubljana, Faculty of Mechanical Engineering, Ljubljana, Slovenia
klaudiusz.golombek@polsl.pl
Prejem rokopisa – received: 2015-10-20; sprejem za objavo – accepted for publication: 2015-12-24
doi:10.17222/mit.2015.318
New functional tool composite materials with a metallic matrix and reinforced with hard carbide phases have been developed.
The effect of the polymer binder has been determined for the used moulding a mixture of hard carbide powders based on WC,
TiC, (W, Ti)C, doped with VC, NbC and/or TaC and powders of a metallic matrix in the form of Co and/or Ni. The relevant type
of polymer binder was selected, its optimum volume fraction was determined, the rheological properties of the polymer-powder
slip were investigated along with thermal debinding conditions, i.e., atmosphere, time and temperature, as well as solvent
debinding conditions, i.e., solving time and temperature and sintering conditions were matched. The structure and mechanical
properties of the produced tool materials were examined, especially their resistance to abrasive wear, hardness and bending
strength.
Keywords: composite tool materials, reinforced metal matrix, powder injection moulding (PIM)
Razvili smo nove funkcionalne kompozitne orodne materiale s kovinsko matrico, oja~ane s trdimi karbidnimi fazami. Dolo~ili
smo u~inek polimernega veziva, uporabljenega za brizganje zmesi trdih prahov, na osnovi WC, TiC in (W,Ti) C, legirane z VC,
NbC in/ali TaC, ter prahov kovinske matrice v obliki Co in/ali Ni. Izbrali smo ustrezno vrsto polimernega veziva in dolo~ili
njegov optimalen volumski dele`. Raziskali smo tudi reolo{ke lastnosti sistema polimer-prah skupaj s pogoji termi~nega razpada
veziva, (atmosfera, ~as in temperatura), kot tudi pogoje razgradnje topil (~as in temperatura raztapljanja) usklajeno s pogoji
sintranja. Dolo~ili smo strukturo in mehanske lastnosti izdelanih orodnih materialov, zlasti odpornost proti abrazivni obrabi,
trdoto in upogibno trdnost.
Klju~ne besede: kompozitni orodni materiali, oja~ana kovinska matica, oblikovanje prahov z vbrizgavanjem
1 INTRODUCTION
Research institutes active in the field of tool materials
for many years have been endeavouring to develop a
"perfect" tool material possessing high ductility, resis-
tance to dynamic loads and high abrasive-wear resis-
tance. The manufacturing costs of engineering materials
would be markedly lower if such a tool, often coated
with protective coatings, had been developed with cost
savings associated with machining, especially manu-
facturing downtimes and the necessary replacement of a
worn tool. Even the high costs of investigations of
properties and applications of modern tool materials and
the related manufacturing costs do not constitute a
barrier for the development of this field of research.
The use of injection moulding or the extrusion of
sintered tool materials represents one of the modern
directions of the research. Powder forming and sintering
technologies offer unlimited opportunities for selecting
the chemical composition of the tool composites pro-
duced. Classical powder metallurgy based on uniaxial
pressing and sintering with potential isostatic pressing at
a high sintering temperature prevents the fabrication of
tools with complicated shapes. The injection extrusion or
forming techniques of a polymer-powder slip, under-
going rapid advancements, make it possible to produce
relatively small parts with complicated shapes and a
developed area, and also make it possible to produce
materials not requiring plastic working or machining.
The use of powder forming based on polymer binders, in
particular injection moulding or extrusion, has become
the subject of research in numerous research institutes
and universities.
Moreover, our own research of high-speed steels
manufactured by powder injection moulding and
pressureless forming prove that the structure and wear
resistance are similar with commercial high-speed steels,
but with less ductility. In addition, the manufacturing
technique employed, especially the debinding and
sintering process carried out in protective atmospheres,
permits the use of furnaces that are cheaper than vacuum
furnaces, which is important in process lines. The moni-
toring and maintaining of a narrow range of sintering
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Materiali in tehnologije / Materials and technology 51 (2017) 1, 163–171 163
UDK 665.7.035.6:620.175.2:620.1 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 51(1)163(2017)
temperature is undoubtedly a difficult aspect in industrial
conditions. The use of modern, polymer-binder-based
powder-forming technologies, in particular injection
moulding, for preparing metal-matrix-reinforced tool
composites creates a promising outlook for the fabrica-
tion of functional materials. WC, (W,Ti)C carbides
doped with VC, NbC and/or TaC, frequently used as
hard reinforcing phases of sintered carbides based on co-
balt, especially in this configuration, were used for fabri-
cating composite tool materials, as TiC phases, mainly
VC, inhibit the growth of a WC carbide grain in sinter-
ing. Metal, constituting a matrix of sintered carbides,
must exhibit a smaller affinity for carbon than a carbide
metal, and in the liquid state it has to wet the carbide
grains, interpenetrate the grains, filling in pores and
exhibiting the limited solubility of carbide grains. Nickel
and cobalt, often used as a matrix material, satisfy such
conditions.1-22 Some research groups propose to use
cheaper compounds for the metal matrix such as Fe, but
the sintering of WC carbides with a steel matrix causes
the dissolving of this type of carbides and the precipi-
tation of M6C carbides with lower hardness.
The paper provides an overview of the fabricating of
new Co- and/or Ni-based functional composite tool ma-
terials reinforced with hard carbide phases. The essence
of the investigations concerns the application of a
state-of-the-art polymer-powder slip forming technology
for manufacturing composite tool materials in the form
of sintered carbides ensuring high resistance to abrasive
wear, corrosion and diffusion in the tools’ working con-
ditions.
2 EXPERIMENTAL PART
The experimental mixtures of powders used for
manufacturing sintered carbides being the main com-
ponent of the feedstock, as presented in Table 1 and in
Figures 1 to 4. The mixtures produced by Baildonit S.A.
(CC1, CC2, CC3) come as a granulated product and
feature a high flow rate and are intended mainly for
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Figure 3: a) Morphology of CC2 powder, b) X-ray energy-dispersive
plot of the area in Figure 3a
Slika 3: a) Morfologija prahu CC2, b) EDS-spekter podro~ja ozna~e-
nega na Sliki 3a
Figure 1: Morphology of CC1 granules
Slika 1: Morfologija CC1 granul
Figure 2: a) Morphology of CC1 powder, b) X-ray energy-dispersive
plot of the area in Figure 2a
Slika 2: a) Morfologija prahu CC1, b) EDS-spekter podro~ja ozna-
~enega na Sliki 2a
pressing shaped sections and then for sintering. The
CC1, CC2, CC3 mixtures with a lubricant added exhibit
the high compatibility required in moulding in a closed
die. The mixture produced by Tetra Carbides-TC does
not contain a lubricant.
Table 1: Applied mixture of carbides
Tabela 1: Uporabljena me{anica karbidov
Designation Averageparticle size
Amount of components in
volume fractions, %
CC1 2–3 μm 57WC, 20TiC, 14Ta(Nb)C, 9Co
CC2 2–3 μm 87WC, 5TiC, 8Co
TC Tetra
Carbides d50 = 3.11μm
33WC, 33TiC, 25TaC, 8NbC,
Co
CC3 1–2 μm 69WC, 20 (TiC,TaC), 2VC,5Co, 4Ni
The addition of a lubricant was considered when
selecting a binder. Mixtures in the form of a granulated
product make it possible to measure the powder grain
size, hence only the average size of the powder grains
given by the manufacturers is presented in Table 1.
Grain size tests were made for the mixture of tetra
carbides powders with a Malvern Mastersizer 2000
instrument for measuring the size of the particles with
the laser-diffraction method.
A mixture of polypropylene (PP) and paraffin (PW)
was used as a binder for producing the feedstock. The
effect of the presence of stearic acid (SA), as a sur-
factant, on the feedstock viscosity was also examined.
The feedstock was prepared with the Rheomex
CTW100p instrument by Haake shown in Figure 5
making it possible to record the torque of the vanes
during the homogenisation of components. The fraction
of the relevant components of the binder and powder is
shown in Table 2. In order to cover the surface of the
carbides with stearic acid (SA), carbide powders, stirred
strongly for 30 min so that SA is distributed evenly
across the surface of carbides, were added to the SA
dissolved in ethanol. The mixture was then heated to
60 °C to evaporate the ethanol. The so-prepared carbide
powders covered with the SA coating were next mixed
with the main binder as PP and PW. Rheological tests
were carried out in a capillary rheometer Rheoflixer by
ThermoHaake at 170 °C, 180 °C and 190 °C.
Table 2: Amount of components in all manufactured polymer-powder
slurry in volume
Tabela 2: Volumski dele` komponent v vseh proizvedenih kombina-
cijah go{~e polimer-prah
Designation Powder PP, % invol. frac.
PW, % in
vol. frac.
SA, % in
vol. frac.
CC160SA4 CC1 18 18 4
CC157SA2 CC1 20,5 20,5 2
CC154SA0 CC1 23 23 0
TC60SA4 TetraCarbides 18 18 4
CC2 CC2 18 18 4
CC3 CC3 18 18 4
In order to prepare a homogenous mixture with a low
viscosity enabling injection moulding or extrusion,
carbide powders were covered with a thin layer of stearic
acid, thus increasing their wettability when mixing with
other binder components and to decrease a ready
feedstock’s viscosity.
The coagulation speed of 10 s–1 to 10000 s–1 was
chosen during the investigations and the length and
width of the capillary is, respectively, 30 mm and 1 mm.
The melting point for the binders used was determined
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 4: a) Morphology of CC3 powder, b) X-ray energy-dispersive
plot the area in Figure 4a
Slika 4: a) Morfologija prahu CC2, b) EDS-spekter podro~ja
ozna~enega na Sliki 4a
Figure 5: Scheme of Haake Rheomex CTW100p apparatus for the
torque measurment and homogenization
Slika 5: Shema naprave Haake Rheomex CTW100p za merjenje
navora in homogenizacijo
with differential scanning calorimetry (DSC) using a
Perkin Elmer instrument, Diamond model, in order to
determine the extrusion or injection moulding tempe-
rature of the prepared polymer-powder mixtures. The
amount of heat can be recorded with the instrument and
its software, presented as a peak chart as a heat effect of
the process curve, while the area of the field underneath
the peak equals the enthalpy of such a transformation.
The AB Machinery AB-400 piston injection moulder
with a retractable mould heated to 150 °C was used for
the injection moulding. The extruded sections were pro-
duced in the Rheomex CTW100p twin-screw extruder.
The structural observations of the examined com-
posite tool materials were made with a LEICA MEF4A
light microscope with the light field technique and the
morphology of the powder grains and the structure of the
materials produced was viewed with a scanning electron
microscope ZEISS SUPRA 35 at the accelerating voltage
of 20 kV using SE and BSE detection.
The bending strength of the injection moulders was
measured with a ZWICK Z100 tensile testing machine
fitted with an adapter for three-point bending. The test
was made in line with PN ISO 3327: "Determination of
bending strength".
3 RESULTS AND DISCUSSION
The results of the investigations into the grain size
distribution of the tetra carbides powder (d10 = 1.45 μm,
d50 = 3.11 μm and d90 = 7.36 μm), and especially the
values d10 and d90, allow us to calculate the filling ratio of
the injection-moulded section SW according to the
following dependence SW = 2.56 / (log (d90/d10). The SW
value calculated is 3.64, which allows for the injection
moulding of the powder examined. Powder with a SW
coefficient of 2 is the most recommended for injection
moulding. It is not recommended to mould powder with
a SW coefficient of 7 with its powder grain size distribu-
tion characteristic being very narrow. The characteristic
of the examined powder’s grain size distribution is
relatively broad, therefore, the pores forming between
large grains may be filled by small particles. No grain
size distributions tests and SW coefficient calculations
were made for the mixtures of powders manufactured by
Baildonit, as such powders are prepared as a granulated
product and mainly intended for pressing.
It was determined according to tests of the torque
tested during the homogenisation of powder mixtures
with a binder that, irrespective of the powders used, their
maximum fraction should not exceed 60 %. The tested
torque of stirrers is considerably decreased by adding
stearic acid.
Figure 6 shows a torque chart for a mixture of 64 %
CC1 carbide with 13 % of paraffin and polypropylene in
volume. The mixture was produced at 170 °C. The cha-
racteristic of the curve presenting the torque according to
the feedstock mixing time is unstable, signifying an
excessive fraction of carbide powders and inhomoge-
neous distribution of a binder in the matrix, despite a
long homogenisation time. Besides, the curve does not
show a falling tendency, despite long mixing, thus such a
high content of powder cannot be used. In the case of a
mixture with only 50 % content of carbides, the torque
of the stirrers falls below 1 Nm after 20 min, proving the
low viscosity of the polymer and powder mixture. A
smooth and falling characteristic of the curve signifies
the mixture’s high homogeneity.
A 50 % content of carbides in the produced feedstock
of the moulder or extruder ensures its low viscosity, but
may cause numerous problems in the debinding of such
a high fraction of a binder or may lead to the distortion
of specimens during sintering as a result of high
shrinkage of the sinter. Hence, a well-prepared feedstock
should be characterised by a possibly high fraction of
powder and relatively low viscosity enabling its forma-
tion. It has been concluded according to data from the
literature and our own studies that the feedstock’s visco-
sity is considerably reduced by applying a small amount
of stearic acid as an active surfactant.
Table 3 shows the results of torque tests for the
stirrers homogenising a polymer and powder slip for 1 h,
containing 60 % of tetra carbides and a binder in the
form of a paraffin (PW) and polypropylene (PP). The
torque value is substantially decreased by using stearic
acid (SA) covering the surface of the carbides (Table 3).
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166 Materiali in tehnologije / Materials and technology 51 (2017) 1, 163–171
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Figure 6: Torque measurements of feedstock based on PP and PW
with 64 % amounts of CC1 mixture carbides
Slika 6: Meritve navora me{anice, ki temeljijo na PP in PW s
64 %-nim dele`em zmesi karbidov CC1
Figure 7: Influence of binder type on rheological behaviour at 170 °C
Slika 7: Vpliv vrste veziva na reolo{ko obna{anje pri 170 °C
A low torque value of mixtures containing polypro-
pylene (PP) and paraffin (PW) corresponds to a low
viscosity. The torque is only negligibly reduced by in-
creasing the content of stearic acid from 4 % to 8 %,
hence its content in further investigations did not exceed
4 %.
Table 3: Torque measurements of feedstock based on PP. PW and SA
with 60 % amounts of TC mixture carbides covered by SA
Tabela 3: Meritve navora me{anice, ki temeljijo na PP, PW in SA s
60 %-nim dele`em TC zmesi karbidov, ki jih zajema SA
Time, min
Torque, Nm
0 % SA 4 % SA 8 % SA
5 12.20 2.20 1.80
10 7.30 5.90 1.40
15 5.50 1.80 1.30
20 6.00 1.70 1.20
25 5.20 2.10 1.20
30 4.90 1.90 1.20
35 5.50 1.80 1.10
40 5.20 1.60 1.20
45 4.90 1.60 1.10
50 4.90 1.60 1.10
55 5.50 1.50 1.00
60 4.70 1.40 1.10
The outcomes of the rheological tests indicate a
smaller viscosity of a powder mixture with PP and PW
in respect of a mixture containing High-Density Poly-
ethylene (HDPE) instead of PP, and this is independent
of the homogenisation rate. Viscosity is also consider-
ably lowered by applying PW, as confirmed by the
results of the rheological tests provided in Figure 7. In
addition, paraffin allows us to use solvent debinding,
expediting the rate of thermal debinding and this short-
ens the duration of the whole cycle. Figure 8 presents
the effect of the fraction of carbides coated and uncoated
with stearic acid on the viscosity of the polymer and
powder mixture. Polypropylene and paraffin are the main
binder components, regardless the content of stearic acid.
Stearic acid improves the wettability of metallic and
ceramic powders by covering their oxided (polar) surface
that adsorbs the hydrophilic part of the chain as a result
of the existing electrostatic forces between the powder
and the wetting agent. The non-polar part of the chain
should be mixed without limitations with other polymers
present in the binder.
Apart from the decreasing viscosity, stearic acid acts
as a lubricant in contact between the powder and the die
surface or the surface of another particle. It also prevents
powders from migrating during high-speed homogeni-
sation. A process of migrating the powder inside the
capillary or the destruction of the binder structure occurs
most probably during the high-speed homogenisation of
a mixture not containing stearic acid, presented in
Figure 8. This is manifested by a strongly falling
viscosity together with an increased homogenisation
speed. The viscosity of mixtures containing stearic acid
is not so much dependent on the homogenisation speed,
hence the growing speed of homogenisation does not
have such a strong effect on the structure of a homo-
genous mixture.
Considering the feedstock viscosity, the maximum
applicable fraction of carbides uncoated with stearic acid
is 50 %. If stearic acid is used for a mixture containing
50 % of carbides, the viscosity is greatly reduced and a
higher volume fraction of carbides can be obtained. The
maximum volume fraction of powders applicable in a
mixture for injection moulding could be determined by
investigating the technological properties of the polymer
and the powder mixtures containing binder-carbides.
A test in a capillary rheometer could not have been
made due to the excessive viscosity of the mixture con-
taining 68 % of powder. Three polymer-powder mixtures
are shown in the diagram. Two of them contain addition-
ally SA, apart from the main binder components. It can
be concluded by analysing the research outcomes that
the content of stearic acid, similar to the tetra carbides, is
strongly reducing the viscosity of the examined
polymer-powder mixtures containing CC1 carbides. The
mixture with the lowest content of powder and without
SA possesses the highest viscosity. For the homogeni-
sation speed of 5000 s–1 and 10000 s–1, the viscosity of
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Figure 9: Influence of the type of carbides on rheological behaviour
of the feedstock at 170 °C
Slika 9: Vpliv vrste karbidov na reolo{ko obna{anje me{anice pri 170 °C
Figure 8: Influence of SA on rheological behaviour of binder and
carbides CC1 type mixtures at 170 °C
Slika 8: Vpliv SA na reolo{ko obna{anje veziva in zmesi karbidov
CC1 pri 170 °C
the CC154SA0 mixture is accordingly equal and lower
than the viscosity of the CC160SA4 mixture. The
CC157SA2 mixture has the lowest viscosity, irrespective
the homogenisation speed. Figure 9 shows the results of
the viscosity tests according to the type of powder used.
Regardless of the powder used, the content of the binder
was 40 %, including 4 % of SA.
A mixture with tetra carbides powders applied has
the highest viscosity. This is most likely connected with
the fact that Baildonit’s powder mixtures intended for
pressing contain about 2 % of volume fraction of lubri-
cant, most often paraffin, additionally increasing the
wettability and reducing the viscosity. Mixtures designed
for the industrial manufacturing of sintered carbides
possess a high homogeneity and lubricant are tightly
covering the surface of carbides. It is thus easier to
prepare a polymer powder slip for injection moulding
and the slip itself has better properties. Regardless of the
mixture type, the viscosity is lower than 1000 Pa·s,
hence each of the investigated mixtures is suitable for
injection moulding. Figure 10 compares the results of
the viscosity tests for a mixture with tetra carbides
applied with pure polypropylene. The viscosity of poly-
propylene, often used for injection moulding, is higher
than the homogenisation speed by 100 s–1 to 5000 s–1,
which confirms the earlier conclusion that a mixture with
tetra carbides is suitable for injection or extrusion
moulding.
Figure 11 presents the diagrams of dependency bet-
ween stress and homogenisation speed for CC1S60SA4
and CC260SA4 mixtures. The stress is growing as the
homogenisation speed grows. Higher stresses are present
in a CC1S60SA4 material due to its higher viscosity.
The binder’s melting point and debinding tempe-
rature start tests were made to determine an injection
temperature. An injection temperature of 170 °C was
determined according to such measurements. The results
of the binder’s melting point and crystallisation tempe-
rature tests, determined with Differential Scanning Calo-
rimetry (DSC), are shown in Table 4. The melting point
of polypropylene is 163 °C; however, when mixed with
paraffin in the same fraction, the melting point falls to
137 °C.
Table 4: Melting point and crystallization temperature of binder
components and their mixture
Tabela 4: Tali{~e in temperatura kristalizacije komponent veziva in
njihove zmesi
Material Melting point(°C)
Crystallization
temperature (°C)
HDPE 130 61
PP 163 108
PW 58.3 47.5
SA 73 62
HDPE/PW t1 = 57.8; t2 = 127 t1 = 43; t2 = 72
PP/PW t1 = 56.6; t2 = 137 t1 = 45; t2 = 90
Thermogravimetric analysis (TGA) for the polymers
used, ready binders and ready polymer-powder mixtures
were carried out to determine the start temperature of the
thermal debinding and to select its cycle preceding
sintering. The test results are provided in Table 5. The
volume fraction of paraffin (PW) versus the main binder
of 50 % was assumed. If the PW fraction is increased,
the strength properties of the section moulded are
deteriorated, while the increased fraction of PP or HDPE
precludes the use of solvent debinding.
Table 5: Temperature of the start and finish of thermal debinding
Tabela 5: Temperatura za~etka in kon~anja toplotnega odstranjevanja
veziva
Polymer of mixture
Temperature of
beginning of
thermal debinding
(°C)
Temperature of
complete thermal
debinding (°C)
HDPE 378 503
PP 320 480
PW 198 278
SA 204 286
HDPE/PW 234 497
PP/PW 215 446
An injection temperature cannot be higher than its
value, especially considering the beginning of thermal
debinding temperature, which is 217 °C. The thermal
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Figure 11: Flow curves of CC160SA4 and CC2 feedstock at 170 °C
Slika 11: Krivulje te~enja za CC160SA4 in CC2 polnilo pri 170 °C
Figure 10: Comparison of feedstock viscosity including TC carbides
with viscosity of polypropylene
Slika 10: Primerjava viskoznosti polnil za PIM, ki vklju~uje karbide
TC z viskoznostjo polipropilena
debinding of paraffin, used as one of the binder compo-
nents, may otherwise occur. The rate of mass loss
changes at 287 °C, signifying the end of the paraffin
debinding. The binder is subject to complete thermal
debinding at 444 °C. The temperature of thermal de-
binding can be selected on the basis of thermogravi-
metric test results. Direct sintering is necessary due to
the low properties of the sections after complete thermal
debinding. High-temperature heaters often cannot be
used for debinding due to degradation products deposited
onto the heat chamber’s surface. Debinding in a separate
device should then be applied. The related necessity to
transport the specimens after debinding into a high-tem-
perature heater device forces us to use incomplete
debinding, ensuring minimum mechanical properties,
enabling the transport of the specimens. The maximum
thermal debinding temperature should be about 420 °C
when analysing the TGA curve.
A heating rate should be lowered at the temperature
of 217 °C where the paraffin debinding starts, as sec-
tions’ breaking may occur due to the growing pressure of
gaseous products of debinding in pores.
The thermogravimetric tests of the polymer-powder
CC260SA mixture were also performed. A thermal
debinding cycle shown in Figure 12 was selected based
on the results of the tests. The rate of heating was chosen
experimentally.
The heating rate in the thermogravimetric tests was
5 °C/min. Material defects such as cracks may occur
during fast heating within the range of the binder’s
thermal debinding temperature. This is caused by a
growth in the pressure of gaseous products formed as a
result of the thermal debinding. The heating rate was
lowered to 2 °C/min. for this reason, and an isothermal
interval was additionally used at 200 °C, i.e., the paraffin
debinding start temperature. Another isothermal interval
depends on several factors, such as the size of the heat
chamber, the flow rate of the shielding gases or where a
material must be transported into a chamber of another
device for sintering. Where the transport of specimens is
necessary, debinding should end at a temperature lower
than this, resulting from a thermogravimetric curve. A
thermal debinding cycle can be shortened by dissolving
one of binder components, and no isothermal interval at
200 °C, corresponding to paraffin debinding, is needed
for solvent debinding. A heating rate of feedstock can be,
therefore, increased to the debinding temperature of PP
or HDPE. The binder components such as PP or HDPE
do not undergo solvent debinding, and their role is to
maintain the formed specimens’ shape to the maximum
temperature possible.
If the thermal debinding of a binder is carried out at a
temperature corresponding to the final temperature of PP
or HDPE debinding determined on the thermogravi-
metric curve, this is linked to the complete degradation
of a binder binding the powder particles, hence, such a
heat cycle can take place only in a high-temperature
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Figure 13: View of the fracture surface of injected CC2 materials:
a), b)
Slika 13: Pogled na povr{ini preloma injekcijsko brizganih CC2
materialov: a), b)
Figure 12: Thermal debinding cycle of CC1-60SA designed on the
basis of TGA analysis
Slika 12: Cikel toplotnega odstranjevanja veziva pri CC160SA,
zasnovan na podlagi TGA analize
furnace permitting direct sintering after finished debind-
ing.
Otherwise, it is very difficult and dangerous for the
materials manufactured to transport specimens from
low-temperature heating devices, designed mainly for
thermal debinding, to the chambers of high-temperature
devices, due to their low mechanical strength.
Injection-moulded materials exhibit the highest bend-
ing strength due to a higher moulding pressure and fewer
pores. Figure 13 shows the structure of the CC2S60SA4
material’s fracture. No gas bubbles were identified in the
material that may form during injection moulding and
reduce the bending strength. Growing bending strength
in the extruded specimens is linked to the presence of
stearic acid covering the powder surface and enhancing
the strength of the powder, i.e., binder bondage. The
bending strength of the extruded specimens depends on
the extrusion conditions. The fractures of the specimens
extruded at 140 °C and 170 °C are given in Figures 14a
and 14b, respectively.
The results of the bending-strength tests from the
injection moulded and extruded specimens are shown in
Figure 15. The injection-moulded materials provide the
highest resistance to bending due to the higher pressure
and the formation of fewer pores.
4 CONCLUSION
The methods of injection moulding and extrusion of
powders and sintering make it possible to fabricate small
complex parts made of composite tool materials based
on a cobalt or nickel-cobalt matrix, reinforced with hard
carbide phases.
It was found based on the investigations conducted
that the mixtures of carbide powders through injection
extrusion or moulding can be produced by applying a
binder in the form of paraffin or polyethylene. All the
polymer-powder mixtures presented can be used for the
injection moulding of powders or extrusion thereof and
this is evidenced by the outcomes of the rheological tests
presented. The fraction of powder in relation to powder
in a slip can be increased by applying a surfactant such
as stearic acid. Stearic acid is clearly reducing the visco-
sity of the investigated polymer-powder mixtures, hence
its use is substantiated. The binder content in injection
moulded or extruded materials should be as small as
possible, enabling slip moulding only. Excessive binder
content poses difficulties in degradation and causes a
larger shrinkage and potential distortion in sintering. The
results of the bending strength tests mainly depend on
the conditions of moulding that should be selected so
that the structure of the moulded specimens is uniform
and does not exhibit any discontinuities. It is predicted
that further investigations of such materials consisting of
the selection of appropriate debinding and sintering con-
ditions will make it possible to produce ready complex
tool materials in the form of sintered carbides characte-
rised by appropriate custom-made properties.
The application of the state-of-the-art powder mould-
ing methods for producing composite tool materials
reinforced with hard carbide phases is substantiated. The
application of injection moulding or extrusion processes,
opposite to casting processes and classical powder metal-
lurgy, allows us to manufacture composite materials with
complicated shapes and a developed geometry, including
tool materials with a wide range of content of reinforcing
K. GO£OMBEK et al.: RHEOLOGICAL PROPERTIES OF FEEDSTOCKS AND THE STRUCTURE ...
170 Materiali in tehnologije / Materials and technology 51 (2017) 1, 163–171
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 15: Bending strength of injected and extruded materials
Slika 15: Upogibna trdnost injekcijsko brizganih in ekstrudiranih
materialov
Figure 14: a) View of the fracture surface of CC160SA4 materials
extruded at 140 °C, b) view of the fracture surface of extruded
CC157SA2 materials at 170 °C
Slika14: Pogled na prelomni ploskvi ekstrudiranih materialov:
a) CC160SA4 pri 140 °C, b) CC157SA2 pri 170 °C
particles, without having to use additional procedures
and sorting typical for casting materials.
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