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Production and Processing Methods of Fleece Yarn from Polyester Filament FDY
1、Causes of Fleece Yarn in Normal Polyester Filament FDY Production
In the production process of ordinary polyester filament FDY, the generation of fleece yarn occurs when the spinning speed and cooling rate are very high. Due to stress concentration, the skin layer bears significant tension, making it susceptible to cracks that lead to fleece yarn. Therefore, selecting excellent cooling conditions to maintain uniform radial structure is crucial. A successful approach is to establish an effective cooling zone and use a honeycomb-type side blowing device to create airflow for adequate cooling of the melt filament.
In the stretching process, as processing speed (i.e., the speed of the second heat roller) increases, the output proportionally increases, production costs decrease, and dyeing uniformity improves. However, if the processing speed is too high, it results in increased breakage and fleece yarn, making it imperative to balance and determine an appropriate processing speed. Insufficient stretching, leading to low tension, can also cause significant movement of the yarn, resulting in fleece yarn and breaks. Conversely, excessively high tension negatively affects yarn formation and unwinding.
Regarding the relationship between the adhesion of oil agents and fleece yarn generation, maintaining a higher concentration of oil emulsion and oil content in the filament can reduce fleece yarn production. However, care must be taken to avoid overly high oil concentrations, which may reduce the permeability of the oil agent and degrade the quality of the filament. Methods for oiling FDY can include nozzle oiling and roller oiling. Nozzle oiling effectively reduces spinning tension but results in poor uniformity, leading to significant tension fluctuations during stretching and increased dye spots. Roller oiling provides uniform application and better dye uniformity but increases spinning tension, which raises the rates of fleece yarn and breakage and increases consumption while decreasing winding efficiency. Therefore, roller oiling can be adopted, and by adjusting the roller speed and the wrap angle of the yarn to the roller, spinning tension can be effectively lowered, reducing fleece yarn and breakage.
2、Causes of Fleece Yarn in Special-shaped Polyester Filament FDY Production
To endow fibers with excellent luster, hand feel, and pilling resistance, as well as to give fabrics a unique style and superior performance, there is often a need to produce a type of special-shaped polyester filament in industrial practice. However, fleece yarn and breakage occur frequently in the production of special-shaped polyester filament, with the design of the spinneret being a key component in manufacturing shaped fibers. For instance, when producing flat filaments using a rectangular-hole spinneret, uneven normal stress along the walls of the holes leads to uneven extrusion swelling of the melt, resulting in a high incidence of fleece yarn and breakage during spinning and stretching. By using a dumbbell-shaped hole spinneret, the unevenness of the melt extrusion swelling can be effectively minimized, and the shape degree can be significantly improved.
The production of special-shaped filaments requires higher uniformity in the drying of flakes and moisture content than conventional fibers, which theoretically necessitates strengthened drying conditions. However, high luster flakes have a noticeably lower crystallization rate compared to semi-dull flakes, making them easier to adhere together, and in severe cases, clumping may occur at the pre-crystallization feeding zone, disrupting normal production. Therefore, pre-crystallization should be conducted under milder conditions, appropriately lowering the pre-crystallization temperature and extending the time the flakes stay in pre-crystallization to achieve a certain degree of crystallization to ensure that they do not stick together during drying.
If the moisture content of dry flakes is too high, or if the viscosity of dry and wet flakes drops excessively, this can lead to an increase in fleece yarn and breakage during spinning. Spinning temperature greatly influences the processing performance of special-shaped yarns. While lowering the spinning temperature is beneficial for increasing shape degree, it may also increase the pressure bulging effect at the spinneret holes, leading to more fleece yarn and breakage during spinning. Selecting an appropriate spinning temperature, such as 293°C, is ideal, as it balances shape degree with a relatively low occurrence of fleece yarn and breakage. The cooling conditions for shaping are critical parameters affecting shape degree and the quality of post-stretch products; faster cooling results in higher shape degree. However, due to the possible formation of a core-sheath structure from high shape degrees and rapid cooling, fibers are more prone to generate fleece yarn and breakage during stretching, which may also worsen dyeing performance. Therefore, to reduce fleece yarn and breakage while considering shape degree, milder cooling conditions should be employed whenever possible.
1、Thermal Degradation of Polyester Melt
Polyester PET has excellent thermal stability but is sensitive to impurities. Pure PET begins to degrade at temperatures between 250-300°C, with significant volatile product release occurring above 350°C. The degradation process involves chain scission at the ester sites, resulting in the formation of carboxylic acids and vinyl ester end groups, which can undergo ester exchange reactions with the hydroxyethyl ester end groups in PET, releasing acetaldehyde as the primary volatile product. At higher temperatures, volatile products such as CO, CO2, CH4, C2H2, C2H4, and benzene can also be observed, making the actual reaction more complex.
The melt delivery pipeline is heated using gas-phase thermal media. The main pipeline for gas-phase thermal media distributes thermal media steam from the desuperheater to the jacket of the melt delivery pipeline, entering at the lowest point of each segment.
Generally, depending on the type of spinning production, the temperature of the thermal media steam in the melt delivery pipeline is between 280°C-290°C. The spinning box and its components are heated by gas-phase thermal media, with heating conditions similar to those of the melt delivery pipeline. The normal operating temperature range for the spinning box is usually 275-285°C. The melt is insulated by the thermal media from the polyester terminal polymerization kettle until the production of raw filaments. If the thermal media insulation temperature is too high, and the melt remains in the pipeline for an extended period, degradation of macromolecules occurs more severely. During the process of pressurized extrusion using a metering pump and drawing by a traction machine, defects can arise in the raw filaments, leading to breakage and the generation of fleece yarn.
2、Cooling Process of the Filament Bundle
The air-blowing device is located in the pressure air chamber directly beneath the spinning component. Its main function is to blow air into the melt filament flow to rapidly cool the molten polymer. The air-blowing device evenly distributes cooling air to each spinning position to ensure high-quality, uniform cooling of the filament bundle. If the cleanliness of the cooling air is insufficient, or if the air pressure and flow are improperly set, it can lead to filament adhesion and breakage, resulting in fleece yarn generation. Addressing cooling issues requires the air-blowing internal steel mesh to be dust-free. If contamination occurs or if the filament bundle at the spinning position experiences turbulence from the air, the air-blowing tube must be replaced. To ensure the quality of normal air blowing, it is mandated that the blowing mesh of the air-blowing tube must be replaced regularly to guarantee clean cooling air for the filament bundle, preventing fleece yarn due to air issues.
3、Filament Bundle Passage Process
After the melt is extruded from the components into a filament bundle, it will pass through oiling rollers, guiding rods, upper and lower cleaning guides, the spinning passage, larger and smaller guiding rollers, idler rollers, and traction machines. If their surfaces in contact with the filament bundle are not smooth or have defects, they will damage the filament bundle, resulting in fleece yarn. To address deficiencies in the filament passage, thorough inspections of each roller must be increased, with timely treatment and replacement of any identified issues. Regular calibration of the clearance between the pin plates is essential to reduce friction on the filament bundle and ensure the filament passage operates normally, thereby reducing fleece yarn generation.
a. If the rollers in the passage are incorrectly aligned or have surface defects or burrs, increased friction when the filament bundle comes into contact with them will lead to the generation of fleece yarn.
b. The upper and lower cleaning guides consist of two parallel pin plates with a small gap in between, through which the filament bundle passes. The gap between the pin plates can be adjusted within the range of 0.5-1.2 mm. Their primary function is to tear or jam the filament bundle when excessive fleece yarn or defects appear. If the gap adjustment is improper, increased friction when the filament bundle passes through can generate fleece yarn and may even jam the filament bundle.
The spinning components are key equipment in short fiber devices, playing a crucial role in filtering and removing impurities from the melt, homogenizing the polyester melt, distributing the melt uniformly to each microhole on the spinneret, and extruding the filament bundle from the spinneret.
a. Abnormal Pressure Rise in Components
If the pressure in the components fluctuates significantly, the linear density, tensile strength, and elongation of the raw filament can vary greatly, potentially resulting in knotted filaments, breakage, and the generation of fleece yarn.
b. Component Leaks
There are two common forms of component leakage:
The causes of leakage in components include defects in the production precision and material of the sealing gasket, which severely impacts the sealing performance of the spinning components. This results in leakage due to poor sealing after the components are installed. Precision defects in the sealing gasket within the spinneret are particularly detrimental to spinning production, as the melt can enter the central non-flow zone at the top of the spinneret under pressure, forming a dead zone where the melt cannot flow. This non-flowing melt gradually decomposes under prolonged high temperatures until it turns yellow or black. When maintenance is performed, the decomposing gases can force the decomposed melt back into the spinneret's outlet area, causing black filaments to emerge during operation, resulting in a high breakage rate.
Leakage from the inner sealing gasket of the spinneret can also cause the melt to seep out from the center bolt of the spinneret. This leakage can gradually spread to the surface of the spinneret, leading to the appearance of black slurry on the surface and dripping down, adhering to the running filament bundle, causing damage that results in fleece yarn and slurry clumps. In severe cases, normal production is not possible.
1、External leakage occurs within 24 hours of installation, where the melt seeps out from the component's inlet, often resulting in a large amount of leaking melt appearing as a white slurry dripping from the component's outer wall.
2、Delayed leakage occurs a week after installation, where the melt seeps from the center bolt of the spinneret or the junction between the spinneret and the component body. The leaked melt, after being subjected to high temperatures for an extended period, degrades and turns brown or black. The degraded melt, when extruded from the spinneret, leads to the formation of common black filaments.
c. Poor Maintenance Precision
The polyester short fiber device incorporates regular maintenance, with an interval of 48 hours between maintenance sessions. If the maintenance precision is poor, fleece yarn can appear within 48 hours, and breakage may occur erratically.
d. Countermeasures
Optimizing the sand loading plan for the components is crucial, as the quality and proportion of metal filtering sand affect the filtering performance of the components. To slow the pressure rise of the components while ensuring filter performance, it is advisable to switch to high-pressure-resistant filtering sand that does not deform under 25 MPa pressure, along with multiple optimizations of the sand loading plan to gradually reduce the pressure rise rate. Additionally, controlling component leakage requires improving the precision of sealing gaskets, with thickness deviations kept within 0.02 mm and not exceeding 0.04 mm, and selecting high-quality materials for the sealing gaskets to resolve leakage issues.
