Extruding fibres: simulation and knowledge
13-10-2020 | Posted by Joaquín Martí
Mankind has used and continues to use fibres for many purposes, some of them natural fibres and, in today’s world, many of them man-made. Man-made fibres of various kinds allow us to achieve special mechanical properties, to communicate across the oceans, or to make textiles for a variety of applications.
Think of the strengths allowed by carbon fibre, the multiple applications of fibreglass, the reinforcement provided by metallic fibres, the composite materials pervading our cars and planes or, last but not least, the presence of optical fibre in our communication systems. Fibres are everywhere in our life.
The speed, bandwidth and low attenuation of fibre-optic communications make them ideal for many uses. Also, they have a reasonable cost, are immune to electromagnetic interference, are hard to eavesdrop and do not suffer from cross-talk between parallel fibres or from ambient noise.
But imagine having to produce silica fibres, free of defects and slightly thicker than a human hair, at a rate of 50 m/s. Apart from glasses, mainly silica, optical fibres can be made with polymers, typically PMMA or polystyrene; these are thicker, on the order of 1 mm, and tend to be used over shorter distances, with the advantages of their low cost and mechanical robustness.
Optical fibres are not only used for communications. For example, the sensitivity of their properties to various environmental conditions allows them to serve as sensors for variables such as strain, temperature or pressure, thus providing a means of remote monitoring.
Many of the man-made fibres are created by extrusion of a previously molten material, such as a polymer. The simulation of fibre extrusion is not an easy task. One must model the flow of a non-Newtonian fluid, taking into account the thermal coupling and the ulterior solidification process.
The material goes through a die, often with a circular cross-section, but sometimes with star-shaped, trilobal or other geometries; occasionally, the final filament arises after joining several streams that emerge through separate orifices. And, typical of optical fibre, it is not just one material being extruded, but two are being co-extruded to achieve a core and a sheath, which is how the fibres manage to guide the light.
Principia has been commissioned a number of times to study those processes for various applications. As in so many other fields, simulation is unquestionably the tool that allows making those studies. But, in all honesty, simulation only allows attempting them: succeeding in the task requires adding a considerable amount of knowledge and experience.