Fibres which are made by chemical processing are called man-made fibres. There are two types of man-made fibres
Regenerated fibres (natural polymer fibres)
These fibres are manufactured from natural raw materials like wood. The examples of these fibres are Viscose Rayon, Cupro (Cuprammonium, Saphonifies cellulose ester etc.
These fibres are manufactured from petrochemicals. Examples of these fibres include Nylons, Polyesters, Polypropylene, Polypropylene etc
Manmade Fibre Spinning
To produce the manmade fibres the polymer in solution or molten form is passed through a spinneret (a bathroom shower like screen with very small holes). The spinneret converts the solution in very thin streams which are then solidified to from fibres.
There are four types of manmade fibres spinning, wet spinning, dry spinning, melt spinning and gel spinning.
Wet spinning: It is used for polymers that need to be dissolved in a solvent to be spun. The spinneret remains submerged in a chemical bath that leads the fibres to precipitate, and then solidify, as it emerges out of the spinneret holes. The name of the process i.e. wet spinning has got its name from this “wet” bath only. Acrylic fibre, rayon fibre, modacrylic fibres, and spandex fibres, all are manufactured through wet spinning.
Dry Spinning: is used for polymers that have to be dissolved in a solvent. However, solidification results from evaporation of the solvent. After dissolving the polymer in a volatile solvent, the solution is pumped through a spinneret. As the fibres emerge from the spinneret, air or inert gas is used to evaporate the solvent which results in solidification of the fibres that is collected on a take-up wheel. Acrylic fibre, modacrylic fibre, PBI and spandex fibre are produced by this method.
Melt Spinning: is used for the polymeric fibres or the polymers that can be melted. The polymer is melted and then pumped through a spinneret. The cooled and solidified molten fibres get collected on a take-up wheel. Nylon fibre, polyethylene fibres, polyester fibre, saran fibre, etc. are also manufactured through melt spinning.
Gel spinning: It is also known as dry-wet spinning because the filaments first pass through air and then are cooled further in a liquid bath. Gel spinning is used to make very strong and other fibres having special characteristics. Ultra high strength polyethylene is produced by this method.
There are many machine set up to produce manmade fibres depending on the fibres. Here, two simplest machines used for the extrusion of synthetic fibres will be explained.
Ram extruder is the simplest form of the extruder. It consists of a heated barrel with and a ram which applies the pressure. The polymer chips are placed in the barrel to be melted. The ram is then run to push the polymer down through the spinneret. The polymer emerging from the spinneret is passed through the air gap and consequently through water bath to cool the molten polymer into fibres.
Screw extruder is one of the widely used machines for the production of synthetic fibres by melt spinning method. Unlike ram extruders screw extruders are able to produce fibres continuously. In screw extruders the rotation of the screw pushes the chips forward into the barrel towards the melting region as shown in the figure below the polymer melt is then goes into the metering region which controls the feeding of polymer melt to the spinneret.
Spinneret converts the polymer melt into thin fibre like melt streams which are quenched in the water bath to form the solidified fibres.