In their initial state, the fibre-forming polymers are solids and therefore must be first converted into a fluid state for extrusion. This is usually achieved by melting, if the polymers are thermoplastic synthetics (i.e., they soften and melt when heated), or by dissolving them in a suitable solvent if they are non-thermoplastic cellulosics. If they cannot be dissolved or melted directly, they must be chemically treated to form soluble or thermoplastic derivatives. Recent technologies have been developed for some specialty fibres made of polymers that do not melt, dissolve, or form appropriate derivatives. For these materials, the small fluid molecules are mixed and reacted to form the otherwise intractable polymers during the extrusion process.

The Spinneret

                  The spinnerets used in the production of most manufactured fibres are similar, in principle, to a bathroom showerhead. A spinneret may have from one to several hundred holes. The tiny openings are very sensitive to impurities and corrosion. The liquid feeding them must be carefully filtered (not an easy task with very viscous materials) and, in some cases, the spinneret must be made from very expensive, corrosion-resistant metals. Maintenance is also critical, and spinnerets must be removed and cleaned on a regular basis to prevent clogging.
                  
                   As the filaments emerge from the holes in the spinneret, the liquid polymer is converted first to a rubbery state and then solidified. This process of extrusion and solidification of endless filaments is called spinning, not to be confused with the textile operation of the same name, where short pieces of staple fibre are twisted into yarn. There are four methods of spinning filaments of manufactured fibres: wet, dry, melt, and gel spinning.

Wet Spinning

                  Wet spinning is the oldest process. It is used for fibre-forming substances that have been dissolved in a solvent. The spinnerets are submerged in a chemical bath and as the filaments emerge they precipitate from solution and solidify.
Because the solution is extruded directly into the precipitating liquid, this process for making fibres is called wet spinning. Acrylic, rayon, aramid, modacrylic and spandex can be produced by this process.

Dry Spinning

                  Dry spinning is also used for fibre-forming substances in solution. However, instead of precipitating the polymer by dilution or chemical reaction, solidification is achieved by evaporating the solvent in a stream of air or inert gas.

                   The filaments do not come in contact with a precipitating liquid, eliminating the need for drying and easing solvent recovery. This process may be used for the production of acetate, triacetate, acrylic, modacrylic, PBI, spandex, and vinyon.

Melt Spinning

                  In melt spinning, the fibre-forming substance is melted for extrusion through the spinneret and then directly solidified by cooling. Nylon, olefin, polyester, saran and sulfar are produced in this manner.

                   Melt spun fibres can be extruded from the spinneret in different cross-sectional shapes (round, trilobal, pentagonal, octagonal, and others). Trilobal-shaped fibres reflect more light and give an attractive sparkle to textiles.
                  
                   Pentagonal-shaped and hollow fibres, when used in carpet, show less soil and dirt. Octagonal-shaped fibres offer glitter-free effects. Hollow fibres trap air, creating insulation and provide loft characteristics equal to, or better than, down.

Gel Spinning

                  Gel spinning is a special process used to obtain high strength or other special fibre properties. The polymer is not in a true liquid state during extrusion. Not completely separated, as they would be in a true solution, the polymer chains are bound together at various points in liquid crystal form. This produces strong inter-chain forces in the resulting filaments that can significantly increase the tensile strength of the fibres. In addition, the liquid crystals are aligned along the fibre axis by the shear forces during extrusion. The filaments emerge with an unusually high degree of orientation relative to each other, further enhancing strength. The process can also be described as dry-wet spinning, since the filaments first pass through air and then are cooled further in a liquid bath. Some high-strength polyethylene and aramid fibres are produced by gel spinning.

Stretching & Orientation

While extruded fibres are solidifying, or in some cases even after they have hardened, the filaments may be drawn to impart strength. Drawing pulls the molecular chains together and orients them along the fibre axis, creating a considerably stronger yarn.

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