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Measuring the Friction between Polymer Pellets and Screw Surfaces, the bulk density
and the melting behavior simulating the conditions in a Single Screw Extruders
FH-Prof. Dipl.-Ing. Dr.mont. Gernot Zitzenbacher, e-mail: g.zitzenbacher@fh-wels.at, DI(FH) Kneidinger Christian, BSc MSc
Introduction
Single screw plasticating units (Fig. 1) are wide-
ly used in polymer processing equipment. They
can be subdivided into functional zones which
are the hopper, the solids conveying zone, the
delay zone, the melting zone and the metering
zone. In some cases additionally mixing and
degassing zones are used.
Figure 1: Schematic drawing of a single screw
plasticating unit
The properties of the solid polymer determine
the output, the pressure build up behavior and
the first point of melting. Besides the bulk den-
sity and the coefficient of pressure anisotropy
the frictional behavior of the bulk plastics is
very important. The friction between the solid
polymer and the screw surface as well as the
inner barrel surface can be described by the
external coefficient of friction. In the case of
grooved barrels additionally friction between the
solid polymer in the grooves and the solid poly-
mer in the screw channel occurs which can be
described by the internal coefficient of friction
[1].
Experimental
With the equipment (Fig. 2) utilized in this work
the bulk density, the external coefficient of fric-
tion as well as the melting behavior of polymeric
bulk materials as a function of the applied pres-
sure, the temperature and the frictional velocity
can be determined [2]. Furthermore the melting
behavior can be characterized by two different
measurement systems. The first is a position
measurement system which allows the exact
determination of the volume of the polymeric
sample. The second system is an optical trian-
gulation system which measures the volume of
the melt which is transported out of the sample
chamber. Table 1 shows the characteristics of
the test equipment.
Table 1: Characteristics of the test equipment
Characteristic
Possible values
Pressure
0.2 – 20 MPa
Velocity
0.05 – 1.5 m/s
Temperature
RT – 300 °C
Figure 2: Schematic drawing of the test equip-
ment utilized in this work, showing the mechani-
cal parts: 1) Pneumatic cylinder, 2) Load cell
normal force, 3) Piston, 4) Sample chamber, 5)
Heated rotating shaft [2]
Calculations
The bulk density ρbulk can be calculated by using
the mass of the bulk material mbulk which is filled
into the sample chamber and its volume Vbulk.
The latter is calculated from the piston position
yp and the projected area Aproj of the sample.
These measurements have to be done at tem-
peratures below the melting point of the poly-
meric material TM.
projpbulk
bulk
bulk
bulk AyV
V
m ,
(1)
The bulk density ρbulk and the density of the
solid polymeric material ρsolid itself can further-
more be used to calculate the free volume frac-
tion of the bulk ξFREE_bulk.
solid
bulk
bulkFREE
1
_
(2)
The difference between the overall mass of the
polymeric material in the sample chamber m(t)
and the mass of bulk material in the sample
chamber mbulk(t) is the melted polymeric materi-
al which penetrates into the bulk material
mmelt_IN(t).
)()()(
_tmtmtm bulkINmelt
(3)
The average penetration depth ypenetr(t) can be
calculated as follows:
bulkFREEmelt
INmelt
penetr
tm
ty
_
_)(
)(
(4)
Materials
Two different geometries of PP-H, spheroidal
and cylindrical pellets are compared.
Figure 3:a) spheroidal pellets with a Diameter of
about 5 mm and b) cylindrical pellets with a
length of about 6.3 mm and a Diameter of about
1.9 to 2.3 mm
Results
Coefficient of friction
As fig. 4 shows the external coefficient of friction
of cylinders is higher than that of the spheroidal
bulk material.
Figure 4: External coefficient of friction of sphe-
roidal and cylindrical PP-H pellets as function of
pressure [3]
Bulk Density
At low pressure and temperature the bulk densi-
ty of spheroidal pellets is higher than that of
cylindrical. This relation inverts at high pressure
and temperature.
Figure 5: Bulk density depending on pressure
and temperature
Penetration of melt into the bulk
As fig 6 shows the melt penetrates deeper into
the spheroidal bulk compared to the cylindrical
bulk[2].
Figure 6: Penetration depth of melt into the bulk
1
2
3
4
5
6
[1] C. Kneidinger, G. Zitzenbacher, and H. Hager, “Friction of Polymer Pellets at Screw Surfaces,” in Proceedings of Americas
Regional Meeting, Polymer Processing Society (PPS), 2008, pp. 1–2.
[2] C. Kneidinger, G. Zitzenbacher, M. Längauer, J. Miethlinger, and G. Steinbichler, “Evaluation of the melting behavior and the melt
penetration into the solid bed using a model experiment,” in Polymer Processing Society Conference 2015, Graz PPS, 2015.
[3] M. Längauer, K. Liu, C. Kneidinger, G. Schaffler, B. Purgleitner (Edlinger), and G. Zitzenbacher, “Experimental analysis of the
influence of pellet shape on single screw extrusion,” J. Appl. Polym. Sci., vol. 132, no. 13, 2015.
a)
b)