Friction of polymer pellets at screw surfaces

C. Kneidinger, G. Zitzenbacher, H. Hager

University of Applied Sciences Upper Austria, Austria

Abstract

The external coefficients of friction of the polymer pellets at the inner barrel surface, as well as at the screw surface are decisive factors for solids conveying in single screw plasticizing units. Besides other parameters, like the screw geometry and the processing conditions, they affect the pressure build up behaviour and the temperature development in the solid plug. Screw surface treatments, such as plasma-nitriding, CrN, TiN and TiAlN coatings, additionally affect the frictional conditions between the screw surface and the polymer pellets. A new apparatus for analyzing the frictional behavior of polymer pellets at the screw surface is presented in this paper. In this testing method the pellets are pressed onto a heated, rotating shaft at a defined pressure. The shaft and the pellets can be changed easily, which allows various plastics and shafts with different surfaces and coatings to be used. Furthermore, the temperature of the heated shaft, the frictional speed and the pressure applied to the pellets and the shaft can be widely varied to simulate real processing conditions. First results of the coefficient of friction of Polypropylene at polished steel at different velocities and temperatures are presented.

Introduction

Single screw plasticizing units are widely used in polymer processing equipments. 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. The properties of the solid polymer determine the output, the pressure build up behaviour and the first point of melting. Besides the bulk density and the coefficient of pressure anisotropy the frictional behaviour 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 polymer in the screw channel occurs which can be described by the internal coefficient of friction.

Figure 1: The friction apparatus

Experimental

A new apparatus for investigating the frictional behaviour of bulk plastics on metal surfaces used in plastics processing is presented in this paper. The external friction between the solid polymer and the metal surface can be measured in dependence of pressure, velocity and temperature. Different metal surfaces and coatings can be taken into account. Additionally a melting experiment comparable to the melting mechanism in a single screw can be carried out. In this testing equipment (Figure 1) the bulk polymer is pressed onto a heated, rotating shaft at defined pressures up to 280 bar to measure the coefficient of friction of bulk polymers at different surfaces. Furthermore, the surface temperature of the shaft and the frictional velocity can be widely varied to simulate real processing conditions. The velocity can be variegated from 0.1 to 1.3 m/s and temperatures up to 400°C are possible. The pressing device (Figure 2), powered by a pneumatic cylinder, is mounted with two big ball bearings at the same bearing axis as the test shaft. Now, the frictional force between the bulk polymer and the rotating shaft causes a torque at the complete pressing device. This torque is measured precisely with a load cell (dyed orange). Due to this construction, measurement errors caused by friction in the bearings and thermal strains and forces caused by the pressing device can be avoided. This enables the accurate measurement of the frictional force. The normal force, which is applied to the pellets, is measured by a second load cell which is dyed light blue. The coefficient of friction is calculated from the

frictional force and the measured normal force. The plunger’s movement is measured with a position encoder which is integrated in the pneumatic cylinder. The shaft and the bulk polymer can be changed easily to enable the possibility of testing various plastics and shafts with different surfaces and coatings. Figure 2 shows the complete pressing device at the left side and a detailed view at the right. The sample chamber for the pellets is dyed blue, the heated shaft red, the plunger dark green and the drawing-off device with the brass scraper brown.

Figure 2: The pressing device.

The coefficients of friction of Polypropylene, Type DM55 from Borealis were measured for bulk and for a geometrically adapted solid block at the polished, hardened Boehler N690 steel shaft with a roughness of 0.14 µm (Ra). The frictional velocities were variegated from 0.1 to 0.5 m/s the pressures from 4 to 20 bar

Results and Discussion

In the first measurements the coefficient of friction turned out to be heavily dependant of the testing conditions. As Figure 3 shows, the measured values for bulk are nearly twice the values of a solid block. They tend upwards with rising pressure at lower pressures and decrease at low velocities and high pressures, unlike the values off the block which are almost the same at pressures from 4 to 20 bar. All values are increasing with rising velocities. As Figure 4 shows the values are increasing with the temperature up to 110°C and decreasing at higher Temperatures. After a running-in process the values of bulk approach to the values of a solid block and they are increasing when wear particles accumulate in the friction surface.

Figure 3: Coefficient of friction of PP DM55 block

and bulk at room temperature

Figure 4: Coefficient of friction of PP DM55, block

and bulk at a pressure of 20 bar, a velocity of 0,5 m/s and different Temperatures

Conclusion

The measured coefficients of friction of blocks are comparable to results in further investigations [1, 2, 3]. The values of the bulk are highly dependent on the measuring conditions.

Acknowledgements

Financial support for this work was provided by the Local Government of Upper Austria within the framework of the founding program “Kunststoffstandort Oberösterreich”

References

[1] S. K Biswas, KaIyani Vijayan: Friction and wear of PTFE - a review. Wear, 158 (1992) P. 193-211

[2] J. W. M. Mens, A. W. J. de Gee: Friction and wear behaviour of 18 polymers in contact with steel in environments of air and wate. Wear, 149 (1991) P. 255-268

[3] C I. Chang:  Frictional Behaviour of Solid Polymer on a Metal Surface at Processing Conditions.  Polymer Engineering and Science, Vol. 17, P. 9- 20 (1977-1)