Determination of the coefficients of friction
Our laboratory has just installed a system for measuring the coefficient of friction to meet the international standard ISO 8295.
Read more...Melted polymers when stressed to high rate gradients present a set of specific phenomena such as swelling at the die exit, the Weissenberg effect (fluid climbing along a rotor) and the tubeless siphon effect (related to high extensional viscosity).
In this article, we will describe the phenomenon of die swell (also called extrudate swell). This is due to the preservation of the deformations memory during the passage of the contraction. In this zone, macromolecules are oriented in the direction of the flow. This memory is more marked when the transit time through the die is short:
At the die outlet, the pressure on the material falls, macromolecules relax in the direction transverse to the flow.
It is necessary to study this phenomenon because, when not controlled, it may cause instability of the strand (untimely breakdowns disturbing the continuous extrusion) and modification of the surface texture ("shark skin" aspect) in particular.
It is this relaxation that we will describe in this article.
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