New method for measuring the friction between hot viscous glass and metals

The present study proposes a new experimental approach to quantify the slid ing of hot viscous glass on a substrate material, in conditions being repre sentative of the loading of glass gobs into forming moulds. Using a gob-loa ding sensor, the method consists in calculating the energy dissipated by th e glass gob when crossing a feeding funnel, with a direct measurement of th e velocity loss of the gob following its dynamic contact with the funnel. E xperimental data combined with a mechanical modelling has been performed wi th two funnel geometries: a cylindrical funnel and a conical funnel. The st rain dissipation energy induced by the deformation of the Viscous gob insid e the funnel has been first calculated. It is shown that the cylindrical fu nnel allows strain modes consuming four times less energy than the conical funnel, in agreement with experimental observations A. global analysis of t he motion allowing the calculation of the total energy lost by the gob duri ng the contact with the funnel is then proposed, taking into account the pr evious dissipation energy calculation. A motion equation is thus deduced, g iving the velocity of the gob as a function of time, glass/gob friction coe fficient and other experimental parameters. The dissipated kinetic energy i s strongly influenced by the dissipated strain energy, thus influencing the value of the calculated friction coefficients. The gob-loading sensor is a powerful tool increasing knowledge in the forming of viscous materials, to gether with a comparison and discrimination of various materials and surfac e states, which may be used in the glass forming process with the aim to av oid the use of lubricant and to make the process more competitive.