Measurement of the coefficient of internal friction

For the solid conveying in single screw extruders and the pressure build-up in the feed zone the shearing forces are of decisive importance. These forces can be quantified by a simple Coulomb equation provided that the coefficients of friction are known. Besides the external coefficient of friction there is also an internal one which characterises the friction among the particles. The knowledge of the coefficient of internal friction is especially important for calculating the pressure build-up in a grooved bush extruder. In this case the friction force on the barrel surface is a combination of internal and external friction. Since there is no smooth shear transition between the shearing area in the grooves and the smooth barrel surface an additional edge effect has to be considered. The planned measuring apparatus allows to determine the coefficients of internal and external friction as a function of pressure, temperature and velocity. A number of preinvestigations showed that the calculation of the pressure buildup and the development of the barrel surface temperature which determine the preconditions for the following melting process are of decisive importance. An apparatus with this measuring features is not available yet. The correct description of the nonisotropic pressure distribution in bulk materials which was investigated in a former research project will further improve the results of pressure build-up and temperature development in the feed zone of a single screw extruder.

A new apparatus for measuring the internal and external coefficient of friction was developed and built in the framework of this project. The knowledge of these material data is of great importance for the calculation of the pressure build up and the temperature development in the solids conveying zone of plasticising extruders. The friction apparatus works on the principles of a vertically arranged solids conveying extruder. The conveying resistance is applied with a tapered axially moveable annular piston. The backpressure is kept constant at the outlet of the apparatus with a hydraulic system. The cylinder bushes are interchangeable: Saw tooth like grooved bushes are used for measuring the internal coefficient of friction and smooth bushes are necessary for measuring the external coefficient of friction. For the temperature steady state conditions can be realised even at high pressures since the polymer bulk material flows continuously through the apparatus. A specially developed measuring technique allows to determine the influence of the pressure distribution, the torque, the temperature and the sliding velocity so that the coefficients of friction can be measured in dependence of pressure, temperature and velocity. The internal and the external coefficient of friction decrease with increasing pressures. Increasing velocity and temperature also reduce the coefficients of friction. The ratio of the internal to the external coeffcient of friction is not constant; it decreases with increasing pressures. The pellet shape has a significant influence on the frictional behaviour. As expexted rougher barrel surfaces increase the external coefficient of friction and with that the throughput rate. The external coefficient of friction of hygroscopic polymers reduces with increasing material humidity. The plastics industry is one of the branches with the greatest growth rates. The demand of shorter development periods and higher throughput rates makes the use of simulation software necessary. A systematical investigation of the processes in production lines and the development of several measuring devices for the determination of the required material data is a basic requirement. Basic research was done in this project to describe the processes in the solids conveying zone of single screw plasticising units and to measure the necessary material data for the calculation. The acquired knowledge is the basis for solving application oriented problems together with industrial partners.