Anali PAZU - Letnik 1, leto 2011, številka 2
Time-Dependent Properties of Multimodal Polyoxymethylene
based binder for Powder Injection Molding*
asovno odvisne lastnosti multimodalnih, na polioksimetilenu
temelje ih veziv praškov za brizganje
Joamin GONZALEZ-GUTIERREZ**, Gustavo Beulke STRINGARI**, Barbara
ZUPAN I **, Galina KUBYSHKINA***, Bernd VON BERNSTORFF**** and Igor EMRI**
**Center for Experimental Mechanics, University of Ljubljana / Pot zaBrdom 104, Ljubljana, Slovenia
***Elektromaterial Lendava d.d / Kolodvorska 8, Lendava, Slovenia
****BASF Aktiengesellshaft / Carl-Bosch-Straße 38, Ludwigshafen, Germany
E-mail: ie@fs.uni-lj.si
Abstract: Powder injection molding (PIM) is one of the most versatile methods for the manufacturing of small
complex shaped components from metal, ceramic or cemented carbide powders for the use in many
applications. PIM consists of mixing the powder and a polymeric binder, injecting this mixture in a mold,
debinding and then sintering. Catalytic debinding of polyoxymethylene (POM) is attractive since it shows high
debinding rates and low risk of cracking. This work examines the possibility of using bimodal POM as the
main component of the binding agent by studying its time-dependent and thermal properties and comparing
them to monomodal POM. Furthermore, possible optimization of the binder formulation was investigated by
the addition of short molecular weight polymeric chains (wax) to bimodal POM, as to create a multimodal
material. It was observed that the magnitude of the complex viscosity for the commercial bimodal material was
more than 2 times lower than for the chemically identical monomodal POM within the investigated frequency
range. Viscosity values were observed to drop as the content of wax was increased, without compromising
thebinders mechanical properties in solid state. A new formulation of bimodal POM plus 8 wt.% of added wax
provided the most appropriate results from investigated combinations. This work has shown how the addition
of short polymeric chains in POM influences its time-dependent properties in solid and molten state, which can
be an important tool for the optimization of binders designed to be used in PIM technology.
Key words: Polyoxymethylene; Powder; Technology; Binder; Time-dependence
Povzetek: Praškasto brizganje (PIM) je ena najbolj vsestranskih metod za proizvodnjo majhnih kompleksno
oblikovanih sestavnih delov iz kovine, keramike ali cementnega karbida v prahu za uporabo v številnih
aplikacijah. PIM je sestavljen iz mešanja prahu in polimernega veziva, injiciranja te mešanice v kalup,
razvezovanja in nato sintranja. Kataliti no razvezovanje od polioksimetilena (POM) je privla no, ker kaže
visoko stopnjo razvezovanja in majhno tveganje za razpoke. To delo obravnava možnost uporabe bimodalne
POM kot glavne sestavine veziva s študijem asovne odvisnosti in toplotnih lastnosti in jih primerjati z
monomodalnim POM. Raziskana je bila možna optimizacija veziva z dodajanjem kratkih polimernih verig
(vosek) bimodalnemu POM za ustvarjanje multimodalnega materiala. Ugotovljeno je bilo, da je obseg
kompleksnih viskoznosti za komercialne bimodalne materiale ve kot 2-krat nižji kot za kemi no enake
monomodalne POM znotraj opazovanega frekven nega obmo ja. Vrednosti viskoznosti so padale, ko se je
vsebnost voska pove ala, ne da bi ogrozili mehanske lastnosti veziva v trdnem stanju. Nova oblika bimodalne
POM plus 8 ut.% dodanega voska je najbolj ustrezala, glede na raziskane kombinacije.
Klju ne besede: polioksimetilen; prašek; tehnologija; vezivo; asovna odvisnost
129
Anali PAZU - Letnik 1, leto 2011, številka 2
The process of Powder Injection Molding (PIM),
invariably, consists of four steps: I) Feedstock preparation,
II) injection molding, III) debinding and IV) sintering (7) (8).
Figure 1 illustrates the main stages of PIM.
During the feedstock preparation the metal or ceramic
powder and an organic multicomponent binder are
combined in a variety of compounding equipment, such as
extruders and batch mixers. The mixture is then pelletized
to an appropriate shape for feeding into the molding
machine. The binder is simply a carrying medium for the
powder and once a part is molded, the binder is removed in
a subsequent step. The powder content usually ranges from
50 to 65% in volume, although there are claims of
optimized commercial formulations in which even more
than 80 % is used. If the powder content is found to be
below 50 vol.%, the sintering ability of the feedstock and
the final density of the part are significantly lowered. From
another standpoint, it is also important to keep the viscosity
of the feedstock as low as possible in order to facilitate the
injection molding process.This is the reason why a powder
content higher than 65 vol.% should be carefully
handled(9).
The injection molding process is mainly identical to
conventional plastic injection molding. Nevertheless, some
machine hardware changes are usually required to process
a specific feedstock based on its compressibility and
viscosity. In addition, injection molding machines for
processing of powdery materials are optimized with wearresistant components and screw geometry adopted to lower
the compression rate and extend the compression zone as
compared to screws used for thermoplastics(1). Control of
the molding process is vital for maintaining tight tolerances
in subsequent steps. Most design advantages of PIM
technology are captured during molding by relying on the
flexibility of incorporating complexities in the tool. A
molded part is called a “green part” and is oversized to
allow shrinkage during sintering (2).
1. Introduction
Powder injection molding (PIM) is a technology for
manufacturing complex, precision, net-shape components
from either metal or ceramic powder. The potential of PIM
lies in its ability to combine the design flexibility of plastic
injection molding and the nearly unlimited choice of
material offered by powder metallurgy, making it possible
to combine multiple parts into a single one (1). Furthermore,
PIM overcomes the dimensional andproductivity limits of
isostatic pressing and slip casting, the defects and tolerance
limitations of investment casting, the mechanical strength
of die-cast parts, and the shape limitation of traditional
powder compacts (2). Finally, the use of finer powder
particles in PIM feedstock allows for the densification of
parts during sintering, which yields high performance parts
made of material systems that are difficult to sinter using
conventional methods(3). PIM is generally best suited to
produce parts less than 6 mm in thickness and weighting
less than 100 grams(4). Therefore, industries that demand
miniaturization of complex components can benefit from
using PIM in their manufacturing process.
Due to the demand of high performance materials and
the miniaturization of complex components in various
fields, PIM market has exceeded the USD $1 billion mark
in 2007, becoming approximately six times larger than 15
years before(5). This impressive growth rate is not expected
to slow down in the next few years. As the recent report
from Global Industry Analysts announced, a combined
world metal and ceramic powder injection molding market
is forecasted to reach USD $3.7 billion by the year 2017(6).
The metal powder injection molding (MIM) is still
considered the largest segment of this market, accounting
for more than 70% of global output. Although PIM is
globally widespread, Europe and Asia-Pacific account for
the major share of MIM segment, while USA is still the
largest market forCeramic Injection Molding (CIM) (6).
Binder
Powder
Feedstock
I) Mixing of multi-component binder
and metal (or ceramic) powder
II) Injection molding of the mixture (feedstock)
Binder
Solvent
Catalyst
Heat
TEMP
Binder
IV) Sintering of brown part
III) Debinding of green part
Fig. 1. Flow chart illustrating the main stages of PIM process.
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Anali PAZU - Letnik 1, leto 2011, številka 2
Low viscosity at the molding temperature and a high
drop in viscosity at high shear rate are very important
properties of the binder for mold filling with less energy.
This is particularly important when dealing with
complicated geometries (14). When cooled, the binder
should be strong and rigid to prevent distortion of the
molded part. Finally, highthermal conductivity, low
thermal expansion coefficient, good adhesion with powder,
no orientation, and be chemical inertness with respect to
the powder are also desirable properties that should be
taken into account when selecting a binder (15).
As previously mentioned, catalytic debinding is the
fastest debinding method available. However, POM has a
higher viscosity compared to other binder systems and in
some instances this can complicate the injection molding
process. On the other hand, POM is a polymer with a high
degree of crystallinity in solid state and as such the molded
part with this binder has very good mechanical properties
and dimensional stability, which are very desirable to
ensure a final part with the correct geometry.Therefore, it
is important to find a way to decrease the viscosity of the
molten binder without deterioratingits mechanical
properties in solid state. A common method of lowering the
viscosity of polymers is to decrease their molecular weight,
but this in turn causes a drop in their solid mechanical
properties. Thus other methods need to be investigated.
One particular method is to modify the polymer
molecular mass distribution by changing its modality, in
other words to have more than a single peak in the
molecular mass distribution of the polymeric material.
Figure 2 shows a schematic representation of a bimodal
and monomodal molecular mass distribution.
The effects of bimodality have been previously studied
in some polymers. For example, Emri & von Bernstorff(16)
reported that bimodal Polyamide 6 (PA6) showed a
significant improvement on the relaxation modulus as
compared to conventional monomodal PA6; a difference of
almost two decades in time dependency between the
monomodal and bimodal PA6 materials. Similar results
were observed for the same materials in terms ofcreep
compliance by Kubyshkinaet al (17).
Before sintering, the organic binder must be removed
without disrupting the molded powder particles; this
process is commonly referred as debinding. Organic
polymers have to be removed completely from the “green
part”, since carbon residues can influence the sintering
process and affect the quality of the final product
negatively. Moreover, binder removal is one of the most
critical steps in the PIM process since defects can appear
due to inadequate debinding, causing problems like
bloating, blistering, surface cracking and large internal
voids. It has been shown that the rate of binder removal
plays a main role in the defect appearance due to structural
changes in capillaries inside the green part (10). For these
reasons, the debinding is the most expensive and timeconsuming stage in the PIM technology. Three main
methods can be applied depending on the composition of
the binder: thermal, solvent, and catalytic. Catalytic
debinding is by far the fastest method of removing the
binder from the molded part; it is based on the solid-tovapor catalytic degradation of polyoxymethylene (POM),
which occurs when such polymer is exposed to high
enough temperatures (110 to 150 °C) in the presence of
nitric or oxalic acid vapor(11). It is observed that the
debinding rate is proportional to the temperature at which
the process is executed. However, this operating parameter
is limited by the increasing dimensional distortion as the
melting temperature of POM is approached (2).
Sintering is the last stage of the PIM process; it is a
thermal treatment that transforms metallic or ceramic
powder into bulk material with improved mechanical
strength that in the majority of cases has residual porosity.
Sintering is performed at temperatures below the melting
temperature of the major constituent in the metal or
ceramic powder, generally within 70 to 90% of the melting
point (12). The temperature inside the sintering furnace is
high enough to start the recrystallization process of the
metal or ceramic particles, but low enough so that the
particles remain unmelted. At such temperatures, the
particles recrystallize into each other causing them to fuse
together(4).Depending on the material, debound parts are
sintered at temperatures ranging from 1200 to 1600°C
under a controlled atmosphere(13).
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li PAZU - Letnik 1, leto 2011, številka 2
stribution in polymeric
Fig. 2. Schematic representationn for monomodal and bimodal molecular mass distr
materials.
Fig. 3. Complex viscosity of mono
no- and bimodal PA6 as function of frequency.
Viscosity measurements have been recently
re
performed
by our group on monomodal and bimoodal PA6 and are
shown in Fig. 3. As can be appreciated
ed, the viscosity of
bimodal PA6 is significantly lower
lo
than its
monomodalcounterpart in all the rang
nge of frequencies
studied. Therefore, one can conclude tha
that bimodality has
brought the required properties toPA6, i.e.,
i.e lower viscosity
in the molten state and high mechanical
al properties in the
solid state. Unfortunately, PA6 does not
ot exhibit catalytic
decomposition and could not be used inn ccatalytic PIM. For
this reason, it was decided to investig
tigate if a similar
behavior could be achieved inn POM through multimodality
in its molecular mass distributi
tion.
The present work focusess oon the characterization of the
time-dependent properties of
o monomodal POM and
different formulations of bim
imodal POM-based materials.
The aim is to understand how
h
the addition of short
polymeric chains influences the
th macroscopic properties of
POM and thus design a binder
bin
more suitable to the
requirements of PIM technolog
ogy.
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Anali PAZU - Letnik 1, leto 2011, številka 2
min: minimum
p: polar
ω: angular frequency
Nomenclature
D: diameter
l: length
I: moment of inertia
J: creep compliance
M: shearing torque
T: Temperature
t: time
η*: complex viscosity
: rotational angle
2. Materials and Methods
2.1 Materials
Monomodal (MO-0) and bimodal (BI-0) POM were
supplied by BASF (Germany). Their notation used in this
study and corresponding thermal properties are specified in
Table 1. As it can be seen in Table 1, thermal properties of
the two types of POM have no significant difference.
Subscripts
max: maximum
Table 1. Notation and thermal properties of monomodal and bimodal POM(18).
Notation
Supplier
Molecular Mass
Distribution
Melting
Temperature (°C)
Crystallization
Temperature (°C)
MO-0
BASF
Monomodal
165.8± 0.7
134.0± 1.1
BI-0
BASF
Bimodal
164.3± 3.2
134.9± 2.8
components in a twin-screw extruder at the maximum
concentration (16 wt.%). This master batch was
subsequently diluted with previously extruded bimodal
POM until the remaining concentrations were achieved. All
the materials were extruded seven times in total to ensure a
homogeneous distribution of the short chains among the
long ones. Neat bimodal POM was also extruded (BI-00E)
and characterized in order to investigate the effects of
extrusion (Fig. 4).
In the second step of our investigation the bimodal
POM was further modified by adding different
concentrations of a polymer with low molecular weight to
obtain material with multimodal molecular weight
distribution. Bimodal POM was modified by adding
different concentrations (1, 2, 4, 8 and 16 wt.%) of a
compatible polymer with low molecular weight (referred
as ‘wax’ due to confidential terms). The blends of bimodal
POM and wax were prepared by mixing the two
Fig. 4. Flow chart for the blending of bimodal POM with wax at different concentrations.
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The list of multimodal materials (bimo
modal + wax), their
notation and thermal properties are show
own in Table 2. As
with the unmodified monomodal and bi
bimodal POM, the
thermal properties of the POM
M-wax blends did not differ
significantly among each other.
er.
Table 2. Notation and thermal
the
properties of extruded bimodal POM and its waax blends(18).
Notation
Wax Conte
ntent
(wt.%)
Molecular Mass
Distribution
Melting
Temperature (°C)
Cry
rystallization
Temp
mperature (°C)
BI-00E
0
Bimodal
164.6 ± 2.4
13
135.5
± 1.2
BI-01E
1
Multimodal
166.6 ± 3.2
13
135.4
± 0.5
BI-02E
2
Multimodal
166.9 ± 1.4
13
136.1
± 0.7
BI-04E
4
Multimodal
164.3 ± 1.3
13
135.7
± 1.0
BI-08E
8
Multimodal
165.3 ± 1.9
13
135.6
± 0.9
BI-16E
16
Multimodal
163.6 ± 0.9
13
135.6
± 1.2
°C. The heater was moved at a relatively slow rate of 2
mm/min. After melting and pr
pressing on the glass cylinder,
the polymeric materials were le
left to naturally cool down to
room temperature. Schemati
atics of the procedure are
(21
provided elsewhere (16) (17) (18) (21)
.
After each specimen was prepared,
p
they were annealed
at a temperature of 120 °C
° to erase their thermomechanical history. After aannealing, torsional creep
measurements were performed
ed at the temperature of 110
°C in the time scale of 3 h, under a constant shearing
torque (M). The rotational angl
ngle caused by this torque was
recorded as a function of tim
ime (ϕ(t)) and then the creep
compliance (J(t)) of each spe
pecimen was calculated using
the following equation:
2.2 Viscosity measurements
Viscosity measurements of all materi
erials in the molten
state were performed by means off small amplitude
oscillatory shear tests according to ASTM
M D-4440(19) using
a Haake MARS II rotational rheometer (Thermo
(T
Scientific,
Karlsruhe, Germany). Measurements w
were performed at
190 °C, which is a commonly usedd temperature for
injection molding of POM. The geometr
try used was coneplate with a 20 mm in diameter. Two frequencies
fr
sweep
were performed in each measuremen
ent, the first one
increasing from 0.1 s-1 (0.628 rad/s) to 100 s-1 (628.32
rad/s) and the second one decreasing from
om 100 s-1 to 0.1 s1
, in order to monitor any possible flow
low instability. The
applied stress was set at 200 Pa; such valu
alue was previously
determined to be within the linear viscoela
elastic region of the
materials at the specified temperature. The
Th reported results
are the average of six repetitions for each
ach of the materials
studied.
(1)
whereIp is the polar moment
ntum for the circular crosssection geometry of the cylind
indrical specimen and l is the
length of the specimen. The ap
applied torque was selected to
be within the linear viscoel
elastic region for all POM
materials at the specified tempperature. Finally, the reported
results are the average over two
wo repetitions carried for each
of the materials studied.
2.3 Creep compliance measurements
Creep measurements in the solid state
ate were performed
following the methodology developed aat the Center for
Experimental Mechanics at the Universit
sity of Ljubljana(20).
Cylindrical specimens with diameter D = 5.80 ± 0.05 mm
and length l = 40.0 ± 1.0 mm were
we
prepared by
gravimetrical casting from all of the m
materials listed in
Table 1 and 2. The casting procedure sta
tarts by filling up a
glass tube with polymer pellets. The mat
aterial is melted by
a movable heater positioned around thee glass tube and a
pressure of about 1 MPa is applied to the molten polymer
using a piston. The temperature of the hea
eater was set at 220
3. Results and Discussion
3.1 Viscosity
The viscosity of all mat
aterials was investigated in
frequency domain. Based on these measurements, it is
clear that all the materials
ls show a nearly constant
magnitude of the complex viscosity
vi
at low frequencies,
134
Anali
li PAZU - Letnik 1, leto 2011, številka 2
followed by a drop at higher freque
uencies, which is
commonly referred as a shear-thinning bbehavior. Figure 5
shows the shear thinning behavior for
or monomodal and
bimodal POM.
The viscosity results can be summa
marized using two
parameters:
and
. Th
The first parameter
represents the magnitude of th
the complex viscosity at the
lowest angular frequency at which experiments were
performed (0.628 rad/s), while
ile the second parameter is the
magnitude of the complexx viscosity at the highest
frequency reachable by the eexperimental set up (628.3
rad/s). The viscosity results are
re shown in Fig. 6.
Fig. 5. Shear thinning behavior of monomodal and bimodal POM at 190 °C.
Fig. 6. Complex viscosity at the minimum
m
and maximum frequency studied for all mat
aterials investigated at
190 °C.
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It is clear that the addition of short
rt polymeric chains
resulted in materials with better flowability. The
commercial bimodal POM (BI-0), whi
hich has a higher
content of short polymeric chains, sho
hows a significant
decrease in viscosity in both low and high
hi frequencies in
comparison to monomodal POM (MO-00). At the lowest
frequency bimodal POM has 4 times lowe
wer viscosity and at
the highest frequency the viscosity is tw
twice smaller. The
reduction of viscosity is a consequencee of the introduced
short polymeric chains, which can act ass internal
i
lubricants
reducing the friction between the longg chains and thus,
increasing the flowability of the m
molten polymer,
particularly when subjected to hig
high shear rates.
Theextrusion protocol used to preparee P
POM/wax blends
led also to a reduction on viscosity; itt was
w observed that
BI-00E had about 17% lower viscosity tha
than BI-0 due to the
breaking of longer chains during extrusion
e
cycles.
Regarding the new formulations propose
sed in this work, it
is possible to notice that the flowabilityy of
o POM is clearly
improved by increasing the concentratio
tion of wax added.
For instance, by adding 16 wt.% of wax (BI-16E),
is reduced by 34% when com
ompared to BI-00E
and by 59% when compared to BI-0. Ass already discussed
in this work, improving flow propertie
rties is of extreme
importance to mold filling off complex
c
geometries as those
usually found in PIM.
3.2 Creep compliance
In Fig. 7 the creep compli
pliance determined at 110° C
after 10000 s (∼3h) for eachh oof the POM-based materials
investigated is shown. Creep co
compliance values at selected
time and temperature enable a direct correlation to process
conditions, since both paramet
eters are very similar to those
commonly applied during thee catalytic debinding process
of POM. Therefore, a low valu
lue of creep compliance under
these conditions is impor
ortant to ensure minimal
deformation of the molded par
art resulting in a better quality
of the sintered product. Bimoodal (BI-0) and monomodal
POM (MO-0) showed no sig
significant difference in their
creep compliance at the m
measuring conditions. This
indicates that the addition of short molecules does not
significantly affect the solid mechanical
me
properties of POM.
It can be said that the extrusio
sion protocol also showed no
significant change in the creep
ep performance of POM, since
BI-00E shows only slightly lo
lower creep compliance than
neat bimodal POM in the condi
ditions here defined.
Fig. 7. Creep compliance measured
red at 110 °C, after 104 seconds, for all materials investig
stigated.
From another standpoint, the addition
on of up to 2 wt.%
of wax yielded no significant change inn creep
c
compliance,
it appears as if BI-01E and BI-02E
2E have a slight
improvement in the resistance to deforma
mation as compared
to BI-00E, but the change cannot be consi
nsidered significant.
On the other hand, materials BI-04E and
a
BI-08E have
shown creep compliances slightly higher
er than BI-01E and
BI-02E, but at the samelevel as the extrud
uded bimodal POM.
136
Finally, the highest creep com
ompliance, and consequently,
the lowest resistance to defor
ormation among all materials
investigated was observed forr BI-16E.
B
The possible explanationn for the observed behavior
could be that the smaller mole
lecules arrange themselves in
the spaces between the largerr oones when in the molten state.
As the material solidifies a mo
more closely packed structure
is obtained, such tighter stru
ructure prevents the material
Anali PAZU - Letnik 1, leto 2011, številka 2
from creeping. This hypothesis is supported by the
polarized micrographs presented by Emri and von
Bernstorff(16)and Kubyshkina et al(17)for PA6 and by
Stringari et al(18) for POM-wax mixtures, which shows a
finer structure for the materials with the higher content of
shorter chains. However, when the content of shorter
molecules exceeds a certain concentration, they may
actually facilitate the movement of larger chains and as a
result the materials start to creep more.
Acknowledgements
This research work is co-funded by the European
Commission Erasmus Mundus Executive Agency,
Slovenian Research Agency- ARRS, and the Slovene
Human Resources and Scholarship Fund – Ad futura.
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4. Conclusions
This work has shown that macroscopic properties of
POM are sensitive to the addition of short polymeric
chains. For instance, the commercial bimodal POM (BI-0)
which has a larger content of short polymeric chains
presents time-dependent properties in solid state similar to
those shown by standard monomodal POM (MO-0) with
larger molecular chains. However, BI-0 exhibits a much
better flowability, which is extremely important for filling
the cavities of complex geometries as those usually
encountered during the powder injection molding process.
The extrusion protocol has shown no negative effects in
both, binder’s flowability and its resistance to deformation.
These results suggest that not only the ratio between long
and short chains affects the time-dependent properties of
POM, but also the distribution and interactions between
them.
Also, for the blends of bimodal POM and wax, it was
observed that the addition of short polymeric chains to
bimodal POM showed a further decrease in molten
viscosity. At low wax content (up to 4 wt.%), the solid
creep properties of the blends remained almostunchanged .
However, the addition of higher concentrations, for
instance 16 wt.%, already shows a relatively larger
increase in creep compliance. The investigated mixture of
bimodal POM and 8 wt.% wax had viscosity values (at
190 °C) up to 19% lower than the commercial bimodal
material submitted to the same extrusion protocol. At the
same time the creep compliance of the same mixture is
only 9% higher at the conditions here studied (at 110 °C,
after 104 s), which is still lower than the 14% increase
when the content of wax reached 16 wt.%. So it can be said
that a compromise was reached at 8 wt% wax content,
lower viscosity and not so high increase in creep
compliance.
In general, it can be concluded that the addition of short
polymeric chains may be an interesting tool for the
optimization of binders designed to be used in powder
injection molding technology, and that measuring the timedependent properties of binder in solid and molten state
can be an important tool for determination of the optimal
formulation of a PIM binder.
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