Roughly, synthetic type spinning is executed as follows: The polymer, in chips or pallet form, is heated until it melts. Subsequently, the more or less fluid plastic (depending on the exact technique) is then pressed through a spinneret, a process called extrusion, creating the long and thin filaments that form the yarn.
The spinneret in this process is crucial in terms of number of filaments as well as thickness of the yarn. A yarn with 36 filaments for example has been extruded by a spinneret with 36 "holes". The thickness of a yarn, called the count, is also influenced by the "holes" as their sizes / openings affect the yarn. Furthermore, the pressure at extruding also is essential for the count.
Depending on the plastic one would like to produce yarn from, one has to pay attention to different things and settings. This is due to the fact that plastics have different characteristics like melting temperature or cristalinity which highly influence the ability of extrusion and require different settings in order to get a solid yarn. So it is that Polyamide 6 and Polyamide 66 yarns , despite being closely related from a chemical view, fundamentally differ in terms of production values like temperature, pressure and other variables.
From a mechanical view however, there are six different systems when it comes to synthetic type spinning along the entire process:
- gravity spinning
Spinning technique, also called vertical spinning, where there is no extruder as such. The polymer is moving by the simple force of gravity, meaning the polymer is delivered to the spinning head without any mechanical help. Machines for gravitiy spinning are usually several storeys high.
- extrusion spinning
Also called melt spinning. Spinning technique where solid polymer is fed into an extruder screw that is in a spinning machine (except for direct spinning, where the polymer gets produced from its raw material in the machine). The polymer is heated, fed to a spinning pump and then goes into the spinneret. Though this technique is very similar to the gravitiy spinning technique, it has the advantage over it in its smaller size, meaning machines do not need to be seven to nine storeys high.
- core-sheath spinning
A combination of spinning (usually two) different polymers, whereas one functions as a core, the other as the sheath. The two polymers (in more or less liquid state) are brought together at the spinneret where the inner, core polymer is extruded and almost simultaneously coated with the sheath polymer. The result is usually a bicomponent yarn.
- gel spinning
Spinning technique where the polymer is only partially liquid (in a gel state), which keeps the polymer chain somewhat bound together in order to produce stronger inter-chain forces. The end result usually is a yarn with a high tenacity value. This process is also known as dry-wet spinning.
- single end yarn drawing
Not a spinning technique itself, rather a process that happens after the yarn is spun. When a yarn comes out from the spinning machine, it is usually in a state where the molecules are low in orientation (LOY) due to the amorphness. In order to improve cristalinity and to give the molecules a uniform direction, the yarn gets stretched / drawn almost immediately after it is extruded. This drawing happens on a separate machine. The end result is a FDY, meaning a fully drawn yarn, by a two-step process.
- warp drawing
Like single end yarn drawing, warp drawing happens after the extrusion process and is thus not a spinning technique per se. However, part of the drawing process has already been done once the yarn leaves the spinning machine. The yarn is therefore in POY state (partly orientated yarn). Further drawing is then done in the subsequental steps that can be texturing for example.