Polyester fine fibers have advantages such as a soft hand feel and a full, rich fabric, along with excellent drapability and stiffness, making the garments made from them very popular. The production of fine fibers can use conventional spinning, high-speed spinning, FDY, and stretching and warping equipment. Due to the low denier of the filaments and the low strength they can withstand, it is easy to generate fuzz and breakage during production and use. The hole diameter of the spinneret used for spinning should be smaller, and the rheological properties of the melt need to be improved, which raises the requirements for raw materials and spinning processes.
1、Raw Material Chips
Due to the low strength that fine fibers can withstand, the requirements for the polyester chips used in spinning are high. Firstly, the impurity content in the chips must be low; if the impurity content is high, it is easy to produce fuzz and breakage during spinning. Secondly, all indicators of the chips should be uniform and stable; otherwise, production and product quality will be inconsistent. Additionally, the thermal stability of the chips must be good, as spinning fine fibers requires better rheological properties of the melt, which means higher spinning temperatures. Chips with good thermal stability experience less thermal degradation.
2、Drying of Chips
When spinning fine fibers, the spinning temperature is high, leading to significant degradation. To minimize degradation, it is essential to strictly control the moisture content of the chips. The moisture content should be below 25 ppm. Additionally, it is required that the drying quality is uniform, with minimal dry chip powder, and that the viscosity during the drying process is reduced; otherwise, it is easy to produce floating fibers, fuzz, and broken fibers.
3、Spinning Temperature
Higher spinning temperatures can improve the rheological properties of the melt as it passes through the spinneret holes, prolonging the cooling time of the melt and maintaining a higher plate temperature. Generally, the temperature is controlled between 290 and 300°C. The smaller the denier of the filament, the higher the temperature used should be. After increasing the spinning temperature, the reduction in viscosity for the oil-free fiber will become larger. If the reduction in viscosity is too great, it becomes easier for fine fibers to experience fuzz and breakage. Typically, during conventional spinning, the viscosity reduction should be less than 0.03, while during high-speed spinning, it should be less than 0.015. Since the total denier for spinning fine fibers is lower and the screw output is small, to solve the issue of excessive viscosity reduction, a lower temperature for the screw and a higher temperature for the barrel can be employed. Generally, the screw temperature is set at 284 to 286°C, while the barrel temperature is set at 295 to 298°C. This approach ensures a well-flowing melt and enables smooth passage through high-speed stretching.
4、Spinning Components
(1) Component Pressure
When spinning fine fiber POY, a higher component pressure is required to improve filtering effectiveness and shear stress, which increases the temperature of the melted material and enhances its rheological properties, thereby improving spinnability. However, excessively high initial component pressure can cause the pressure to rise too quickly, shortening the service life. Generally, the component pressure used is between 12 and 18 MPa.
(2) Component Filtering Material
The filtering material for the components has changed from sea sand to metal sand. The unique amorphous structure of metal sand provides a filtering capacity that far exceeds that of sea sand, making it much more effective at filtering impurities from the melt and providing better temperature increase effects compared to sea sand. For example, when producing 166 dtex/192F specifications of POY, the optimal ratio of metal sand is coarse grain, medium grain, and fine grain at a ratio of 1:2:1, which results in good filtering effectiveness and smooth spinning of the POY.
5、Cooling Conditions
Both excessively high and low side-blown air speeds can result in increased unevenness of the yarn and affect the stretching performance due to periodic shifts in the solidification point. Therefore, good blowing conditions should feature an appropriate airspeed and smooth airflow. In high-speed spinning, the influence of inconsistent airspeed during cooling is relatively minor; changes in airspeed have less obvious effects on FDY performance compared to conventional spinning. Therefore, increasing the speed of GR1 within an appropriate range can improve the uniformity of the yarn and dyeing. To reduce the orientation and crystallinity of fine fibers, cooling conditions should be mild. High orientation and crystallinity make the stretching process of fine fibers more difficult. Thus, settings like maintaining an insulated area, increasing air temperature, or lowering airspeed should be implemented for gentle cooling. The airspeed for conventional spinning is generally 0.1–0.2 m/s, while for high-speed spinning, it is 0.25–0.35 m/s, with a relative humidity of 75% ± 5%.
6、Bundling Position
The spinning tension has a significant impact on winding formation. Spinning tension is influenced by factors such as rheological resistance, inertial force, and air friction. For fine fibers, with their larger specific surface area, the air friction is higher, so it is necessary to raise the bundling point position or shorten the channel to reduce air friction. In high-speed spinning, because of the high spinning speed, the tension along the spinning path is greater, making the position of the bundling point even more critical; otherwise, spinning and winding may not be possible. Some equipment has raised the oiling bundling position from 1.4 m to 0.7 m, which yields better results. For fibers with special cross-sections, the position of the bundling point should also be higher because these fibers have an even greater specific surface area; the initial fiber generates heat quickly, resulting in a rapid cooling rate and a significant upward shift in the solidification point. Typically, the bundling position from the spinneret to the oiling nozzle is between 0.7 and 1.0 m. Shortening the bundling distance can reduce the tension in the yarn bundle, and increasing the bundling position, along with the birefringence and crystallinity of the initial fiber, also helps minimize the fluctuations of fine fibers.
7、Oiling
Fine fibers have a larger specific surface area, so the amount of oil applied is higher than that for ordinary fibers, generally ranging from 0.7% to 1%. The oil used should have good permeability and smoothness. A dual-nozzle system is typically used for oiling.
8、Spinneret Hole Diameter
Reasonably selecting the hole diameter of the spinneret and scientifically designing the spinneret are key to producing high-quality fine fibers. The size of the holes must match the shear rate of the melt flowing through the micropores while keeping the drawing multiplier in a smaller range. The shear rate for conventional spinning can be (0.7 to 1.0) × 10,000 s⁻¹, while for high-speed spinning, it can be (1.8 to 2.2) × 10,000 s⁻¹.
9、Stretching Multiplier and Temperature
As fine fibers are used as simulated silk, they are generally processed into stretched yarns to highlight their simulated effects. To accommodate the high orientation and crystallinity of fine fibers during winding, the stretching multiplier must be reduced; the smaller the denier of the filament, the more significant the reduction. However, the strength of the finished yarn does not decrease due to a lower stretching multiplier, and the elongation does not increase. Under normal stretching temperatures, fine fibers exhibit high stretching stress, and the smaller the denier, the more likely fuzz and breakage will occur, with a larger hot water shrinkage. Experiments suggest that increasing the stretching temperature by 5 to 8°C is beneficial. If the stretching temperature is too high, the finished yarn may develop color stripes. Additionally, a lower stretching speed should be chosen to avoid generating excessive fuzz and breakage.
10、Network Processing
Fine fibers have a high coefficient of friction, making unwinding difficult. For example, with yarn specifications of 83 dtex/72F, applying strong twisting directly on the twisting machine can lead to significant fuzz due to high unwinding tension. The higher the unwinding speed, the more severe the fuzz situation becomes. Therefore, fine fibers must undergo network processing. The smaller the denier of the fine fibers, the lower their bending stiffness, making network processing easier; satisfactory network density (20 to 30 per meter) can be achieved at lower air pressures.
