Rheology of PIM Feedstocks SPECIAL FEATURE

Metal Powder Report Volume 72, Number 1 January/February 2017 metal-powder.net

Rheology of PIM feedstocks SPECIAL FEATURE

Christian Kukla, Ivica Duretek, Joamin Gonzalez-Gutierrez and Clemens Holzer

Introduction constant viscosity is called zero-viscosity h0 (Fig. 1). After a certain

> g

Powder injection molding (PIM) is a cost effective technique for shear rate ( ˙1), viscosity starts to decrease rapidly as a function of

producing complex and precise metal or ceramic components in shear rate, this is known as shear thinning or pseudoplastic behav-

mass production [1]. The used raw material, referred as feedstock, ior. For highly ﬁlled compounds like PIM feedstocks a yield stress

consists of metal or ceramic powder and a polymeric binder can be observed. Thus the viscosity increases dramatically when

mainly composed of thermoplastics. The thermoplastic binder decreasing the shear stress and the zero shear viscosity is hard to

composition gives plasticity to the feedstock during the molding measure and thus shear thinning is observed even at very low shear

process and holds together the powder grains before sintering. rates. Around a certain higher shear rate ˙2 a second Newtonian

> g

Most binder systems are made of multi-component systems plateau can be observed and at very high shear rates ( ˙3) the

with a range of modiﬁers which fulﬁll the above mentioned plateau can change to an increasing viscosity curve due to formation

requirements. The ﬂow behavior of the feedstock is the result of of particle agglomerates that can restrict the ﬂow of the binder

complex interactions between its constituents. The viscosity of the system. In various industrial processes the shear rate usually ranges

3 7 1

feedstock and its reproducibility batch by batch is the base for a between 10 and 10 s . The very high shear rates occur, e.g. in the

production of high quality green parts with low scrap rates. Thus molding of thin walled parts [3–5].

rheology is a key factor for the production of high quality PIM Additional importance has the temperature dependence of the

parts, the characterization of feedstocks themselves and for reli- viscosity. It can be described according to the Arrhenius equation

able results of numerical simulation of the PIM process. especially for semi-crystalline polymers, whereas the WLF (Wil-

From the rheological point of view the PIM-feedstocks are liam, Landel, Ferry) equation is used for amorphous plastics. For

highly-ﬁlled polymeric suspensions. The ﬂow behavior is further thin walled parts the temperature dependency of the viscosity is

complicated by particle–particle interactions, which cause their more important; due to the narrow ﬂow channels the shear rate is

redistribution and reorientation in the binder, and thereby inﬂu- very high which causes a high temperature rise. The temperature

ence the bulk rheological behavior [2]. Due to these interactions

PIM feedstocks, compared to thermoplastics, have their own spe-

ciﬁc rheological behavior. Furthermore effects such as yield stress,

wall slip, phase separation, and pre-shearing can have a signiﬁcant

effect on the ﬂow behavior, the accuracy of the measurement and

thus on related results of injection molding simulation.

In general, the ﬂow properties of feedstocks depend on the

temperature, binder composition, powder content and powder

characteristics (particle size distribution and shape of particles).

The viscosity curve describes the dependence of the viscosity on

the shear rate. At very low shear rates, for thermoplastics the

viscosity curve usually changes to a horizontal viscosity line. The

viscosity value at very low shear rates is more or less constant or

independent from the shear rate (Newtonian behavior). This

FIGURE 1

Viscosity of one suspension.

E-mail address: [email protected].

SPECIAL FEATURE Metal Powder Report Volume 72, Number 1 January/February 2017

TABLE 1

Different types of rheometers.

Measuring instrument Measurement Measurement Measured material data tools range

4 2 1

Rotational rheometer Plate & plate 10 –10 s - Zero viscosity, viscosity curves – complex viscosity

0 00

(rotational & oscillating mode) Cone & plate - Storage- and loss modulus, G , G

- Normal force normal stress differences

3 3 1

Rotational rheometer Cup & bob 10 –10 s - Low viscosities

1 5 1

High pressure capillary rheometer Capillary 10 –10 s - Viscosity curves – slip velocity curves – critical shear stresses

Slit dies

PCA FEATURE SPECIAL 1 7 1

Injection molding machine rheometer [7] Slit dies 10 –10 s - Pre-shearing ﬂow correction – viscosity curves – slip velocity

curves – critical shear stresses

Rotational rheometry

Principle

The most important rheological property is viscosity, which repre-

sents the resistance to ﬂow. Fluid deformation under steady shear

ﬂow can be appropriately described by considering the parallel-plate

model. This model helps to deﬁne shear stress and shear rate and it is

schematically shown in Fig. 3-left. The upper plate, with area A, is

moved by a shear force F and the velocity vmax is measured. The

lower plate remains stationary (v = 0). Between the plates there is a

gap y where the liquid sample is sheared. The rheological parameters

can be measured when two conditions are satisﬁed: (i) samples must

adhere to the plate surface, assuming the no-slip condition at the

solid boundary; and (ii) the ﬂow is a laminar ﬂow.

FIGURE 2

The rotational rheometer with parallel plate ﬁxtures (Fig. 3-

Types of rheometers used for different ranges of the shear rate.

right) is widely used to measure the viscosity, viscoelastic proper-

ties and the normal stress functions as functions of shear rate and

rise decreases the viscosity. Above 100 s the increase in temper- temperature at a low shear rate range. The measured geometry is

ature due to shear dissipation has to be taken into account for determined by the radius R and the gap height H. In contrast to the

viscosity measurements [6]. parallel-plate model (Fig. 3-left) the moved surface performs a

Generally viscosity curves of polymers cannot be measured in rotational movement. For the analysis of the measurements, the

3 7 1

the large range of shear rates between 10 and 10 s with one maximum shear rate on the edge is used. In addition to viscosity,

type of rheometer (Fig. 2). For the shear rate range between 10 this rheometer can perform other rheological tests, such as stress

and 1 s , the rotational rheometer is used under steady-state relaxation, creep, oscillatory and ramp tests.

shearing conditions. In the transition range between 5 and Rotational rheometers with parallel plates are suitable to mea-

100 s it is used under oscillating shear conditions. In the shear sure feedstocks. However, the particle size dp or size of agglomer-

2 4 1

rate range between 10 and 10 s the high pressure capillary ates should be considered; the minimum gap height H should be at

rheometer is utilized. For even higher shear rates the injection least 5cd_ p [5].

molding machine rheometer can be used [7]. Table 1 gives a good In a Newtonian ﬂuid, the viscosity is constant and independent

overview of the use of the different types of rheometers. of shear rate and shear stress is proportional to the applied strain.

In this paper we present the two most common methods of However, most PIM-feedstocks are pseudoplastic ﬂuids and the

obtaining viscosity curves for feedstock materials: rotational rheo- shear stress correlates to the shear strain by, e.g. a power law

metry and high pressure capillary rheometry. equation.

FIGURE 3

Flow behavior between two parallels plates [4] (left); parallel-plate rheometer (right).

Metal Powder Report Volume 72, Number 1 January/February 2017 SPECIAL FEATURE

FIGURE 4

Smooth plate (a), plate with slits (b), serrated plate (c), plate with glued sand paper (d).

The no-slip condition between ﬂuid and solid boundary in plates (Fig. 4). The investigation of the effect of surface roughness

contact with the ﬂuid is one of the most classical assumptions on wall-slip formation during rheological measurements of bronze

in continuum ﬂuid mechanics. For Newtonian ﬂuids, the assump- (CuSn8) PIM-feedstock was of particular interest [13].

tion of no-slip condition leads to good agreement with experi-

SPECIAL FEATURE

mental observations. Measurement procedure

For two-phase (or multi-phase) systems, slip at the wall of the Dynamic rheological tests (i.e. frequency sweeps) must be done in

measuring geometry could originate from steric, hydrodynamic, linear viscoelastic region (LVR) to prevent overstrain of the sample

viscoelastic, chemical and gravitational forces acting on the dis- and not to destroy its elastic structure. The LVR can be measured

persed particles immediately adjacent to the walls. As a result of using a strain sweep test. In a strain sweep test, the frequency of the

these forces, a low viscosity depleted layer forms between the wall test is ﬁxed and the amplitude is incrementally increased. To

and the bulk ﬂuid, which then acts as a lubricant to produce slip determine the linear viscoelastic region, the storage modulus

effects. Factors that could lead to slip effects include large dispersed should be plotted against the amplitude. A good rule for deﬁning

(or flocculated) particles, concentrated solutions, low flow rate, the end of the linear region is to find the amplitude at which the

electrically charged particles or walls, and smooth measuring value of the storage modulus changes by 5% with respect to the

geometries. Therefore, performing measurements of PIM-feed- value at the lowest measured amplitude.

stocks on rheometers with smooth measuring geometries could Fig. 5 shows a comparison of storage modulus versus strain for

lead to wall-slip effects [8,9]. CuSn8 feedstock and silicon oil using the smooth plate-plate

The problem of slip can be eliminated by using serrated, rough- system. It can be seen that for feedstock the linear region ends

ened or textured rheometer tools. Some examples of surface at smaller strains (0.1%) as compared with silicon oil (10%). This

textures are shown in Fig. 4. Other geometries include the one is related to the more complex structure of feedstocks due to the

by Nickerson [10], where he used cleated surfaces on parallel disk presence of solid particles in a liquid polymer.

tools to measure the rheological properties of diverse slip-prone Slip effects can be shown in photographs by marking a reference

ﬂuids. The cleat geometry suppresses slip without application of line across the plates and sample and observing how it moves as

signiﬁcant normal force as it is often required in surface-modiﬁed the rheological measurements are performed (Fig. 6). If there is no

tools. However, in order to obtain the true viscosity behavior a slip between the plate and the sample, the line on the sample

correction factor needs to be applied since the cleats act as pores should move along with the line on the rotating plate.

and the ﬂuid has to go through them. As it can be seen in Fig. 6a and b, smooth plates present slip since

In studies by Hausnerova et al. [11] and Mizera´kova´ [12], the the line on the rotating plate moves away from the sample.

difﬁculties of rheological measurements of PIM-feedstocks due to Fig. 6c and d shows that slits on the rotating plate increase the

formation of wall slip have been reported. They used an online contact between sample and plate; however fracture of the sample

rheometer equipped with slit dies varying in surface roughness in is observed that causes other type of instabilities [13] but no slip

order to obtain valid rheological data. reduction.

A special investigation was performed on how slip can be Fig. 6e and f shows that serrated plates effectively prevent slip

eliminated and/or controlled by the use of special proﬁled surface since the line on the sample moves along with the rotating plate.

FIGURE 5

Strain sweep of CuSn8 feedstock (left) and unﬁlled silicon oil (right).

SPECIAL FEATURE Metal Powder Report Volume 72, Number 1 January/February 2017 PCA FEATURE SPECIAL

FIGURE 6

Rate sweep; (a) smooth plates at the beginning of the measurements, (b) smooth plates at 0.03161 s , w = 168, (c) plates with slits at the beginning of the

1 1

measurements, (d) plate with the slits at 0.01212 s , w = 108, (e), serrated plates at the beginning of the measurements, (f) serrated plates at 0.01333 s ,

w = 88, (g) plates with glued sand paper at the beginning of the measurements, (h) plates with glued sand paper at 0.01958 s , w = 128; w = angle of rotation.

The contact between the measuring plate and the sample is so 1.6 and 2 N and the other one between 14 and 19 N. As it can be

much better that the entire sample is being deformed as the plate seen in Fig. 7, increasing the normal force tenfold roughly

rotates, thus the line on the sample is not completely vertical but increases the viscosity by ten times at all the shear rates mea-

still continuous. sured. This increase in viscosity occurs because the particles in

Fig. 6g and h shows the plates with sand paper, which do not the feedstock are brought closer together as the feedstock is

improve the adhesion between sample and plate signiﬁcantly compressed, which leads to more particle–particle interactions

since the line on the sample remains vertical and the mark on that can reduce the ability of the binder to ﬂow, thus resulting

the rotating plate is displaced. in larger viscosities.

Therefore by looking at the photographs in Fig. 6, it can be

concluded that slits and sand paper are not effective ways of High pressure capillary rheometer

reducing slip effects, but serrated plates are effective. Principle and measuring procedure

According to the literature [10], it is suggested that when For high shear rates a high pressure capillary rheometer (HPCR) is

performing rheological measurements with texturized geome- used. Capillary rheometers measure the viscosity of a ﬂuid by

tries a normal force should be applied to compress the sample. determining the pressure required to cause it to ﬂow through a

However, if a large normal force is applied, the viscosity values small cylindrical tube or rectangular slit. Most common HPCRs

could be increased. Viscosity measurements on PIM feedstock allow routine analysis of the ﬂow and viscosity curves at shear rates

1 1 5 1

were performed with two levels of normal force: one between from approximately10 s to 10 s .

Fig. 8 shows a slit die rheometer where the melt pressure is

measured by using ﬂush mounted pressure transducers in the slit

die along the slit length.

In order to obtain a viscosity curve as a function of shear rate,

the piston speed is varied in a step wise manner. As soon as the

measured pressure changes within the speciﬁed tolerance, the ﬂow

piston speed is recorded and the piston speed is increased to the

next value. An example of a pressure proﬁle corresponding to

different shear rates is shown in Fig. 9.

In general the viscosity of feedstocks follows a curve as shown in

Figs. 10 and 11. Both ﬁgures are showing the ﬂow curve (left) and

the calculated viscosity (right). The used feedstocks are:

Catamold 316LA by BASF and

Carbonyl Fe with thermoplastic binder system

For a correct calculation of the viscosity corrections are neces-

sary. The Bagley correction considers the pressure drop at the

entrance of the capillary (only for round dies). To take care of

FIGURE 7

the non-Newtonian behavior of the feedstock the Weissenberg–

Effect of normal force on viscosity measurements performed with serrated

plates. Rabinowitsch correction is incorporated.

Metal Powder Report Volume 72, Number 1 January/February 2017 SPECIAL FEATURE

FIGURE 8

High pressure capillary rheometer for PIM with slit die. SPECIAL FEATURE

FIGURE 9

Pressure proﬁles in the slit die at different shear rates.

FIGURE 10

Flow curve and viscosity curve for Catamold 316LA; h – height of the slit die [14].

FIGURE 11

Flow curve and viscosity curve for carbonyl Fe feedstock with standard thermoplastic binder system; h – height of slit die [14].

SPECIAL FEATURE Metal Powder Report Volume 72, Number 1 January/February 2017

Feedstocks can show special ﬂow behavior like slip ﬂow or yield References

stress. For the characterization of slip ﬂow a special procedure is [1] H.D. Rainer von Both, Druckﬁltration und Eigenschaften von Siliciumcarbid-

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used including the measurement with different diameters of

Karlsruhe, Karlsruhe, 2000.

capillaries or respectively different heights of slits [14]. For the

[2] E. Windhab, Rheologie und Rheometrie von Emulsionen und Scha¨umen im

determination of the yield stress several procedures can be imple-

Lebensmittelbereich – Unterlagen des Rheologieseminars 1998, Institut fu¨r

mented as shown in [15]. Mechanische Verfahrenstechnik und Mechanik, Universita¨t Karlsruhe, 1998.

[3] M.R. Kamal, A.J. Mutel, J. Polym. Eng. (1985) 5.

[4] G. Schramm, A Practical Approach to Rheology and Rheometry, Gebrueder

Summary HAAKE, GmbH, Karlsruhe, 2000.

[5] T. Osswald, N. Rudolph, Polymer Rheology, Carl Hanser, Mu¨nchen, 2015.

The viscosity of feedstocks for powder injection molding is

PCA FEATURE SPECIAL

[6] C. Kukla, I. Duretek, S. Schuschnigg, J. Gonzalez-Gutierrez, A. Gooneie, C. Holzer,

crucial for a proper quality of the ﬁnal products. Feedstocks

Ibereo 15, Coimbra, 7–9 September, 2015.

show a complex rheological behavior which can be character- [7] W. Friesenbichler, I. Duretek, J. Rajganesh, S.R. Kumar, Polimery 56 (2011) 58–62.

ized by mainly two methods – rotational and high pressure [8] S. Mallik, N.N. Ekere, R. Durairaj, A.E. Marks, A. Seman, Mater. Des. 30 (2009) 4502–

4506.

capillary rheometers. In this paper these methods have been

[9] H.A. Barnes, J. Non-Newton. Fluid Mech. 56 (1995) 221–251.

described. Since feedstocks are more complex than other

[10] C.S. Nickerson, California Institute of Technology, Pasadena, (Doctoral thesis), 2005.

materials, care must be taken to ensure that no-slip condition [11] B. Hausnerova, D. Sanetrnik, G. Paravanova, 7th International Conference on

Times of Polymers (TOP) and Composites, 22–26 June, Ischia, AIP Conference

at the solid boundary exists, that the ﬂow is laminar,

Proceedings, vol. 1599 (1), (2014), pp. 518–521.

and corrections are made to take into account their non-New-

[12] S. Mizera´kova´, Online rheological characterization of highly ﬁlled polymer melts

tonian behavior. Only then reliable viscosity data can be

employed in powder injection moulding with the special regard to wall-slippage,

expected. (Master Thesis), Thomas Bata University in Zlin, 2013.

[13] C. Kukla, I. Duretek, J. Gonzalez-Gutierrez, C. Holzer, EURO PM2015, Reims, 4–7

October, 2015.

Acknowledgment [14] C. Kukla, W. Friesenbichler, I. Duretek, M. Thornagel, EURO PM2008, Mannheim,

29 September–1 October, 2008.

We thank the Polymer Competence Center Leoben, PCCL for

[15] I. Duretek, C. Kukla, G.R. Langecker, C. Holzer, EURO PM2013, Gothenburg, 15

providing the rotational rheometer for measurements.

September–18 September, 2013.