L. URTEKIN et al.: FABRICATION AND SIMULATION OF FEEDSTOCK FOR TITANIUM-POWDER ...
619–625
FABRICATION AND SIMULATION OF FEEDSTOCK FOR
TITANIUM-POWDER INJECTION-MOLDING CORTICAL-BONE
SCREWS
IZDELAVA IN SIMULACIJA VHODNE SUROVINE ZA
INJEKCIJSKO BRIZGANJE VIJAKA NA OSNOVI TITANOVEGA
PRAHU ZA KORTIKALNO KOST
Levent Urtekin
1
,As ým Genç
2
, Fatih Bozkurt
3
1
Ahi Evran University, Engineering Faculty, Mechanical Engineering Department, 40100 Kýrsehir, Turkey
2
Gazi University TUSAª-Kazan Vocational School, 06500 Ankara, Turkey
3
Vocational School of Transportation, Eskisehir Technical university, 26470 Eskisehir, Turkey
Prejem rokopisa – received: 2018-04-26; sprejem za objavo – accepted for publication: 2018-11-29
doi:10.17222/mit.2018.088
Titanium-powder injection molding is a combination of plastic injection and powder metallurgy. Using this technology, near-net
titanium parts are produced. In this study, feedstock-development experiments were performed with Ti-6Al-4V powders and
binders for the production of titanium cortical-bone screws. Critical solid-loading and optimum solid-loading values were
determined to specify the most appropriate binder system and ratio by volume. The critical solid-loading rate was determined as
62 % by volume while the optimum solid-loading rate was 60 % by volume. The rheological properties of the feedstocks were
obtained with a capillary rheometer, and the thermal properties with a TGA analysis. The rheological behavior and thermal
properties of PW/PE/SA and PEG8000/PP/SA feedstocks at different mixing ratios were determined. A simulation of the flow
was made with the Moldflow program, designing a screw and a mold. For the two different feedstocks, skeletal binders PP and
PE were identified and simulations were carried out. The knowledge that the feedstock skeletal-binder properties predominate
was obtained as the data for the flow in the mold in the light. Experiments and simulations showed that the water-based
feedstock is a suitable binder system for the cortical-bone screw molding.
Keywords: feedstock, rheology, powder injection molding, Ti-6Al-4V cortical-bone screw
Injekcijsko brizganje prahov na osnovi titana (Ti) je kombinacija brizganja plastike in metalurgije prahov (PM). S to tehnologijo
je mo`no izdelati kovinske izdelke kompliciranih oblik. V {tudiji so avtorji na osnovi preizkusov razvili osnovno surovino
(polnilno maso) za injekcijsko brizganje izdelkov kompliciranih oblik iz prahu zlitine Ti-6Al-4V in ustreznega veziva
(plastifikatorja). V tem primeru je bil kot izdelek za preizkuse izbran vijak za kortikalno kost. Dolo~ili so kriti~ne in optimalne
vsebnosti posameznih komponent polnilne mase oziroma razmerje med volumskim dele`em prahu in primernega polimernega
veziva. Kriti~ni volumski dele` kovinskega prahu je bil 62 %, medtem ko je bil optimalni dele` 62 %. Reolo{ke lastnosti pol-
nilne mase so dolo~ili s kapilarnim reometrom, termi~ne lastnosti pa s termo-gravimetri~no analizo (TGA). Dolo~ili so reolo{ko
obna{anje in termi~ne lastnosti me{anic plastifikatorjev: parafin/polietilen/stearinska kislina (PW/PE/SA) in polietilen-
glikol/polipropilen/stearinska kislina (PEG8000/PP/SA) razli~nih sestav. S programskim paketom Moldflow so simulirali pretok
materiala v orodju za injekcijsko brizganje in na osnovi simulacij dimenzionirali (oblikovali) optimalno obliko vijaka in orodja.
Dve vrsti polnilnih mas so identificirali kot skeletni vezivi: propilen (PP) in polietilen (PE) in z njima izvr{ili simulacije.
Pomembno je bilo spoznanje, da ima skeletno vezivo odlo~ilno vlogo za lahek pretok polnilne mase v orodju. Preizkusi in
simulacije so pokazali, da je polnilna masa na vodni osnovi primeren vezivni sistem za injekcijsko brizganje vijakov kortikalne
kosti.
Klju~ne besede: osnovna surovina, reologija, injekcijsko brizganje prahu, Ti-6Al-4V, vijak za kortikalno kost
1 INTRODUCTION
Titanium and its alloys have outstanding properties in
terms of low density, high specific strength, excellent
corrosion resistance, high-temperature resistance and
non-magnetic properties. Due to these excellent proper-
ties, titanium and its alloys are used in many industrial
areas. It is especially preferred in the fields of military,
defense, automotive, medical and sports equipment.
1–3
Regardless of these excellent features, its production is
limited due to poor process ability and high production
costs. Powder production technology provides a good
solution for eliminating this production problem of
titanium and its alloys. However, it is difficult to produce
three-dimensional (complex-shaped) parts using the
conventional P/M method. Less complex-shaped parts
are produced using the conventional P/M.
2–3
Another
attractive method is the powder-injection-molding (PIM)
method. The method that can produce clear parts, using
microproduction technology is formed by combining
plastic injection and powder metallurgy. The production
of titanium and its alloys with PIM results in complex
shapes, uniform microstructures and high perform-
ance.
4–5
The PIM production consists of four stages. The
four-step process includes the stages of mixing, injection
molding, debinding and sintering. The mixing step is
Materiali in tehnologije / Materials and technology 53 (2019) 5, 619–625 619
UDK 620.17:621.762:669.295 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(5)619(2019)
*Corresponding author e-mail:
levent.urtekin@ahievran.edu.tr

carried out by mixing some of the organic binders iden-
tified as binders at low temperatures. The new mix, ob-
tained by mixing powders and binders, is called feed-
stock. This feedstock fills the mold cavity during
molding. We denote the debinding stage as the removal
of the binders. It includes two stages: solvent debinding
and thermal debinding. In the final step, debinded parts
are sintered. Sintering ensures that the samples become
fully dense. Feedstock features are very important in the
PIM process. The feedstock defines the final properties
in the final part. Rheological and thermal properties of a
feedstock are two important criteria for powder injection
molding and debinding.
6–10
Ti-alloys were used as an alternative to stainless
steel. This is because of the high corrosion resistance and
biocompatibility of Ti-alloys. The mechanical properties
of Ti alloys are close to those of the bone. Therefore, it
has a wide range of uses.
11–13
If the machining and
cutting tool is not suitable, the implant screw is at risk
and the optimum performance cannot be achieved. The
difficulties of machining Ti-alloy and achieving a series
production are overcome with metal-injection molding.
This method is useful in the production of complex-
geometry parts that do not require secondary processing,
and small parts.
14–15
The injection-molding step consists
of different stages such as heating and melting the raw
material, injecting the molten raw material into the mold
cavity, bringing the molten raw material into the desired
shape, packaging, chilling/solidification and emptying
the molded product.
16
Some defects may occur during
the molding of the feedstock. These defects are related to
the injection pressure of the feedstock, the pressing pres-
sure and the temperature application.
17
Some commercial
software packages, such as Moldflow, Poly-Flow, Solid-
works Flow Simulation and Moldex3D, can be pur-
chased and used to avoid these defects. The Moldflow
software is used to estimate the injection parameters of
the feedstock during injection molding.
In this study, the MIM method was chosen because of
the difficulties in achieving a mass production of both
Ti-implant manufacturing and Ti machining. In this
context, experimental and simulation studies were
carried out for Ti-alloys to determine the appropriate
binders. The feedstock was optimized by mixing tita-
nium alloys and binders. The critical powder-loading rate
was determined using a capillary rheometer. The rheolo-
gical and thermal properties of the feedstocks obtained
during the mixing process were investigated. In addition,
an implant-screw design was used to examine the flow
characteristics for a four-hole mold. For the two different
binding systems, the rheology data was supported by
simulating an in-mold usage of the PP and PE binders as
the basis numerals in the square bracket.
2 EXPERIMENTAL PART
2.1 Materials
In the study, a spherical Ti–6Al–4V alloy powder was
used as the main material. Its density was 4.47 g/cm
3
and
the average powder size was 13.4 microns. In Figure 1,a
powder-size analysis is shown.
In powder-injection molding, typically, powders
smaller than 10 microns in size are used for ceramics,
and spherical powders smaller than 20 microns in size
are used for metals.
1,3,4,9
The chemical composition of
Ti-6Al-4V is given in Table 1.
Table 1: EDX analysis
Element w/% % atomic
Al 5.67 9.67
Ti 89.61 86.07
V 4.72 4.26
Its powder morphology, determined with scanning
electron microscopy, is given in Figure 2.
The binders consist of water-based polyethylene
glycol (PEG8000, the main binder), paraffin wax (PW,
the main binder), polypropylene (PP, a skeleton binder),
polyethylene (PE, a skeleton binder) and stearic acid
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620 Materiali in tehnologije / Materials and technology 53 (2019) 5, 619–625
Figure 2: Scanning-electron-microscopy analysis
Table 2: Physical properties of the binders
Property PW PEG8000 PP PE SA
Density (g/cm
3
) 0.92 1.204 0.85 0.91 0.94
Melting
temperature (°C)
51 62 189 170 68–70
Figure 1: Powder-size analysis

(SA, a lubricant). Table 2 gives the physical properties
of the binder systems.
2.2 Method
The authors carried out the study for two purposes.
The first was to provide for a serial production of the
implant. The other was to determine whether the MIM
method was appropriate. The difference between the
preferred binder combinations of PW/PE/SA and
PEG8000/PP/SA with respect to a cortical-bone screw
implant was investigated in the literature.
Solid loading has a critical effect on a feedstock.
Critical solid loading is the most efficient form of the
powder-binder ratio. Binders form a thin film layer on a
powder, providing for its flow during injection at low
temperatures. Therefore, obtaining an optimum powder-
binder mixture is a sensitive process for PIM because the
flowing material has a high viscosity when the amount of
the powder is high, and it exhibits excessively high fluid
characteristics when the amount of the powder is low.
1,3
In this study, the critical powder loading was deter-
mined using a capillary rheometer. First, the binders
were mixed with each other, and then they were mixed
with the Ti-6Al-4V alloy powder at 180 °C to obtain new
granules. The operation was carried out under a vacuum
for the obtained granules to be homogeneous and pro-
tected from the air. The critical solid loading was deter-
mined by passing the granules at different mixing ratios
through the capillary. The optimum solid loading was the
value below the 2 % critical loading by volume. Another
important point here is that the stress density of the
powder gives a preliminary idea about the optimum
powder loading.
The most important task of the mixing method is to
homogenize the powder-binder mixture. It was mixed
with a twin-screw extruder at a temperature of 180 °C at
30 min
–1
to pelletize. An indication of the homogeneity
of the feedstock is a decrease in the viscosity as the shear
rate increases. The rheological behavior of the feedstock
affects the injection moldability in a broad sense. The
feedstock viscosity is effective during the filling of the
mold cavity. At a low viscosity, jetting occurs in the
mold, whereas at a high viscosity, mold filling does not
occur. Therefore, the powder-binder ratio and viscosity
of a feedstock that is developed are of vital importance.
The capillary rheometer was used for determining the
feedstock viscosity and shear rate, and they were calcul-
ated at temperatures of 180–220 °C for the two different
binder systems.
The debinding behavior is determined with the bind-
ing properties, and this step is done as thermal debinding
and solvent debinding. Depending on the binder property
used, water-soluble or any solution-soluble binders can
be used. In this study, the most critical point was the
debinding of the main binder using solvent debinding. It
is generally recommended that a skeletal binder should
be debinded with thermal debinding. Thermal debinding
was performed by obtaining the information about the
thermal behavior of the feedstock using a TGA analysis.
In the TGA analysis, the behavior of the feedstock was
determined by heating it by 5 °C/min, to 600 °C and in
an argon atmosphere.
Cortical-bone screw implants are now produced by
machining. In the study proposed, we aimed to produce
them with the MIM method. The most important factor
here is the difficulty of a serial production and Ti ma-
chining. A standard screw that is easily applied to every
person is preferred in the study. A cortical-bone screw,
which is an orthopedic implant, is designed with reverse
engineering and Figure 3 shows its design. The mold
design was done according to the screw geometry and a
Moldflow simulation analysis was performed for the
skeletal binders that are essential for the feedstock. From
many studies reported in the literature, it is known that a
feedstock behaves in accordance with the skeletal
binding properties of the feedstock. It was aimed to be
supported by PW/PE/SA– PEG8000/PP/SA rheology
data prior to molding it with a simulation analysis.
3 RESULTS
3.1 Determination of the optimal solid loading
Prior to the capillary-rheometer experiments, the
binders were mixed with each other in a dry medium for
45 min at a ratio of 65:30:5. The mixture was then mixed
for 45 min with Ti–6Al–4V using a three-dimensional
stirrer and passed through a twin-screw extruder (180 °C,
30 min
–1
) to obtain homogeneous granules. The obtained
homogenous granules were passed through a capillary
rheometer to determine the optimum solid loading. The
variation in the viscosity values by time at different solid
loads taken from the capillary rheometer are given in
Figures 4a and 4b.
The solid-loading rate for the PEG8000/PP/SA feed-
stock was between 50–62 %. No flow occurred at the
62-% solid loading. This shows that the critical loading
for this binder was determined to be 62 %. The optimum
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Materiali in tehnologije / Materials and technology 53 (2019) 5, 619–625 621
Figure 3: Cortical-bone screw-implant design

loading for the same binder was 60 % by volume. In
Figure 4b, the solid-loading rate for the PW/PE/SA
feedstock was between 47–59 %. No flow occurred at
the 59-% solid loading. This shows that the critical load-
ing for this binder was determined to be 59 %. In other
words, the rate at which the flow does not occur is the
rate at which the critical load occurs. The optimum load-
ing for the same binder was 57 % by volume.
The melt-flow index indicates the amount of the
feedstock flowing through the extruder in 10 min. In
Figures 5a and 5b, the melt-flow index and the change
in the solid loading are given. Powder-binder separations
and inhomogeneous mixtures were observed at solid-
loading values of 50 % and below by volume. The
PEG8000/PP/SA feedstock appears to be superior to the
PW/PP/SA feedstock in terms of melt-flow index values.
3.2 Rheological behavior of the Ti-6Al-4V feedstock
The viscosity and shear rate were determined using
the capillary rheometer. The viscosity-temperature
change and viscosity-shear-rate changes for different
feedstocks at 180–220 °C are given in Figures 6 and 7.
At these temperatures, it was seen that as the viscosity
increased, the shear rate decreased and the flow type
became pseudo-plastic. In the literature;
1,3,4
it is expected
that a successful feedstock has a viscosity lower than
1000 Pa.s and a shear rate of 100–10000 s
–1
.
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622 Materiali in tehnologije / Materials and technology 53 (2019) 5, 619–625
Figure 6: Viscosity versus shear rate for: a) PEG8000/PP/SA, b)
PW/PE/SA feedstocks
Figure 5: Melt-flow index versus vol.% solid loading for: a)
PEG8000/PP/SA, b) PW/PE/SA feedstocks
Figure 4: Capillary-rheometer results for: a) PEG8000/PP/SA, b)
PW/PE/SA feedstock solid loadings

As shown in Figure 6a, it was calculated that the
viscosity of the PEG8000/PP/SA feedstock varied from
1001 Pa s to 72 Pa s and the shear rate from 92 s
–1
to
1100 s
–1
. As shown in Figure 6b, it was calculated that
the viscosity of the PW/PE/SA feedstock varied from
1100 Pa s to 80 Pa s and the shear rate from 55 s
–1
to
1200 s
–1
. The viscosity-temperature change for the feed-
stocks at 180–220 °C is shown in Figure 7.
3.3. Thermal analyses of the feedstocks
The thermal behaviors of the feedstocks were ob-
tained using a TGA analysis. The TGA analysis provides
an important optimization for thermal debinding. In
Figure 8, the weight loss of the Ti-6Al-4V feedstocks
was determined at a certain heating rate. Figure 8a
shows the thermal behavior of the PEG8000/PP/SA
feedstock and Figure 8b shows the thermal behavior of
the PW/PE/SA feedstock. The binders for both feed-
stocks were completely debinded at 550 °C. However,
this process was carried out under a special protective
atmosphere.
The shear stress/shear rate relation is defined as
follows:
  −=
y
m
k() (1)
where denotes the shear stress, k denotes the constant,
 denotes the shear rate, and m denotes the flow-beha-
vior index.
3,10
For all the feedstocks, the flow-behavior
index was less than 1, ranging from 0.507 to 0.601.
In the literature, the flow-behavior indices of the
feedstocks prepared with HDPE alumina, Ti alloy and
PEG alumina are in ranges of 0.522–0.529, 0.50–0.56
and 0.41–0.66, respectively, and are in agreement with
the experimental data obtained within the scope of the
study. In this context, all of the feedstocks tested were
pseudo-plastic and suitable for injection molding in
terms of flow-behavior indices and were compatible with
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Materiali in tehnologije / Materials and technology 53 (2019) 5, 619–625 623
Figure 8: TGA analysis of feedstocks
Figure 7: Viscosity versus temperature for: a) PEG8000/PP/SA and b)
PW/PE/SA feedstocks
Figure 9: Weight loss versus temperature of the PEG8000/PP/SA and
PW/PE/SA feedstocks

the data from the literature.
10
In Figure 9, the debinding
of the binders from the feedstocks is given based on the
temperature. At a temperature of 550 °C, the binders
were debinded and the pre-sintering was completed.
Another factor for the feedstock rheology is the
activation energy. It is important that the feedstock in
injection molding has a low viscosity and low activation
energy. For the feedstocks, the following equation is
used
log  = Ea/R (1/T) (2)
to find the activation energy from the logarithmic visco-
sity and 1/T variations.
Here, Ea is the flow activation energy, R is the ideal
gas constant (R = 8.31 J/molK) and  is the viscosity. In
injection molding, it is desirable to have low activa-
tion-energy situations. A low flow activation energy
means that the energy required for the flow is low, i.e.,
the viscosity is low.
3
When Figure 10 is examined, it is
seen that the PEG8000/PP/SA feedstock is better in
terms of activation energy.
3.4. Cortical-bone screw-implant design and simula-
tion
The bone screw still currently in use was designed
with reverse engineering due to the difficulties of the
processing of the Ti-6Al-4V cortical screw bone and its
serial use, and a four-hole mold was manufactured. The
parameters related to molding were given in another
study.
4
Mold-filling simulations were performed using
the Autodesk Moldflow 2014 software. The rheological
and thermal properties of the feedstock were measured
and used for mold-filling simulation studies of the corti-
L. URTEKIN et al.: FABRICATION AND SIMULATION OF FEEDSTOCK FOR TITANIUM-POWDER ...
624 Materiali in tehnologije / Materials and technology 53 (2019) 5, 619–625
Figure 12: Filling-time analysis for PE and PP
Figure 10: Activation energy for feedstocks
Figure 11: Injection-pressure analysis for PE and PP

cal-bone screw geometry. Simulation studies determined
appropriate processing conditions for injection molding
cortical-bone screws. The results obtained with the simu-
lation studies were investigated based on the feedstock
characteristics determined by the mold-filling behavior
and rheology. Figure 11 shows the injection pressure
(PE and PP) and Figure 12 shows the filling time (PE
and PP). The simulation study was carried out using two
different feedstocks. The first one included PE
(PW/PE/SA) and the other included PP (PEG8000/
PP/SA). Previous work indicated that the feedstock has
skeletal binding properties. In this context, using a
simulation study, the injection pressure for PE was
62.18 MPa, the mold filling time was 0.21 s, the flow
rate was 24.21 cm
3
s
–1
and the injection temperature was
238 °C. In the analyses made for PP, the injection
pressure was 35.25 MPa, the mold filling time was
0.10 s, the flow rate was 12.11 cm
3
s
–1
and the injection
temperature was 232 °C. As in the study of rheology, it is
seen that the PP skeletal-binding feedstock is more
suitable for the simulation study.
4 CONCLUSIONS
In conclusion, the rheological and thermal properties
of Ti-6Al-4V feedstocks were optimized in this study.
The debinding temperature and environment of the bind-
ing systems of the prepared feedstocks were determined.
The wax-polymer system and the polyethylene glycol-
polymer system were compared. Especially for the
molding of complex-shaped geometries, it was deter-
mined that it is appropriate to mold the powder-binder
ratio at volumetric mixing ratios that are lower at a cer-
tain level than the determined ratio. The PEG8000/
PP/SA feedstock (with a volume of 60 %) was found to
be superior in terms of rheological and thermal proper-
ties to the PW/PE/SA feedstock (with a volume of
57 %). Higher solid loadings can be tried for PEG-based
feedstocks using finer powders. The Ti-6Al-4V
cortical-bone screw molding with a volumetric 60-%
PEG8000/PP/SA feedstock was successfully accom-
plished. As a result of the Moldflow simulation, experi-
ments using the PP skeleton binder for the PEG8000/
PP/SA feedstock were found to be more suitable for the
injection pressure, flow rate and filling-time parameters.
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