Polymer in question is Nylon 6,6. Looking to understand flow through spin pack and how changes in the design will affect fiber spinning.
Spinning of polymer into filaments involves two types of fluid flow- one is shear flow before the extrusion i.e. flow through a spinneret and the other is elongational flow (extensional flow). These flows have to be in a stable region in order for the spinning to occur without defects or breaks in the spun filament. In most polymer production operations, the production rate is often limited by the onset of flow instabilities. Therefore, understanding the causes for fluid instabilities is important for successfully carrying out spinning of a polymer fluid.
Instabilities during flow through spinneret
Two types of flow instabilities are observed when polymer fluid flows through a cylindrical path and is subsequently extruded from a spinneret. These are called melt fracture and extrudate swell (also known as die swell). Both instabilities are due to the visco-elastic properties of the fluid. Extrudate swell (die swell) occurs, when the fluid is pushed through the fine holes of a spinneret and is extruded on the other end.
The third type of instability occurs during the extensional or elongational flow of the polymer fluid in the spinning zone. This is called draw resonance.
As the polymer is pushed through a spinneret hole under pressure, some energy is utilized in the flow of the fluid but part of the energy is stored as elastic energy. This has been explained earlier. When the polymer comes out on the other end, this stored energy is released and the polymer fluid relaxes to give a swelling of the extruded fluid jet.
Extrudate swell is measured as the ratio of diameter of the swollen extrudate/diameter of the spinneret.
High extrudate swell results in improper flow or spinning of the material in the spinning line and therefore must be minimized. It may also result in fusing of filaments if the spinneret holes are placed very close to each other. In order to reduce the extrudate swell, the flow conditions must be controlled in such a way so as to control the elastic character of the polymer fluid.
Thank you. I have been studying as best I can the melt extrudant flow and the elongational flow. It seems a lot of the activity I have seen from the fibers can be linked to the extrudant flow instabilities. This helped a lot and helped clear things up!
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