Spinning process
Unveiling the Spinning Process: From Melting to Yarn.
The conventional spinning process unfolds through the following sequential stages:
Dry Slicing → Melt Extrusion → Blending → Metering → Filtration → Spinning → Cool Forming → Oil Application → Winding → UDY Bobbin.
1.Melt Extrusion
The slicing relies on its own weight as it enters the screw extruder through the feeding port. Due to the rotation of the screw, the slices move forward along the groove, with the outer side of the screw sleeve equipped with heating elements. Heat is transferred to the slices through the sleeve.
Simultaneously, the friction and compression between the slices within the screw extruder generate a certain amount of heat. The slices undergo heat-induced melting and are compressed by the extruder, resulting in a specific molten pressure.
2. Blending
Utilize a static mixer to uniformly blend the molten material at the outlet, thereby enhancing the uniformity of the melt and reducing the temperature and residence time discrepancies between the pipe wall and the center during the molten material passage through the bending tube. If the melt is already uniform, the use of a static mixer may be unnecessary.
3.Metering
The molten material outputted by the screw extruder is distributed to various spinning locations by a distribution pipe and metered by metering pumps. This ensures consistent linear density and uniform filament dryness in the yarn. Simultaneously, the metering pump pressurizes the molten material to meet the requirements of high-pressure spinning.
4.Filtration
Before spinning, impurities in the molten material are filtered using filtering materials such as filter sand. Filter materials include materials like emery, sea sand, ultra-fine glass beads, and filter nets. In high-pressure spinning, the filtering layer creates higher resistance, generating frictional heat in the molten material, raising the temperature, and improving the rheological properties of the molten material.
5.Spinning
After filtration, the molten material is distributed by a distribution plate to various spinneret holes on the spinning plate. It is then ejected from the spinneret holes, forming a fine flow of molten material.
6.Cool Forming
The process in which the fine flow of molten material is cooled by a cooling medium, solidifying into a filament, is known as cooling and shaping. Simultaneously, due to the stretching action of the spinneret, the molten material flow gradually refines before solidification. The cooling and shaping process is completed within the spinning window, and the forced cooling air blown out from the upper blowing window of the spinning channel ensures uniform cooling conditions.
7.Oil Application
Due to the initial dryness of the fibers, they are prone to static electricity, and there is poor cohesion between individual filaments, resulting in loose strands with a high friction coefficient, making post-processing difficult.
Therefore, it is essential to apply oil to the strands during spinning. Although the specific oiling positions may vary with different spinning processes, the primary purpose of oiling is to facilitate strand bundling, reduce static electricity, and enhance smoothness. In conventional spinning, oiling is typically performed using an oil wheel. The strands, after cooling and shaping, pass through the channel before reaching the winding machine, where they come into contact with the oil wheel to facilitate oiling.
8.Winding
Winding is accomplished through the coordination of upper and lower thread guides, a cross thread guide, and friction rollers. The oiled strands, after changing direction and adjusting tension through the upper and lower thread guides, are wound onto the bobbin through the cross thread guide. The tube and the friction roller on the winding head make contact under a certain pressure, maintaining the same linear speed through friction transmission—this speed is the spinning speed. Winding tension affects the shaping of the bobbin and the annealing tension.
Winding tension, overfeed rate, and oiling agent are three crucial factors in winding formation.
Spinning process conditions
The spinning process conditions encompass a series of critical parameters.
These include the temperature and pressure during the extrusion of the molten material, the uniformity achieved through the mixing process, the precise metering by metering pumps, the filtration to remove impurities, and the controlled cooling and solidification during the shaping process.
Additionally, factors such as the use of a static mixer for uniformity, the application of oil to the strands for improved handling, and the winding tension during the winding process all contribute to the overall conditions of the spinning process.
Each of these elements plays a vital role in ensuring the quality and consistency of the spun yarn.
1. melting temperature (Tm)
The molten material temperature, also known as spinning temperature, is crucial for ensuring good spinnability and excellent physical and mechanical properties of the final yarn. The molten material temperature needs to be controlled appropriately to ensure complete melting of the chips and prevent drastic thermal degradation of polyester macromolecules.
Typically, within a range of characteristic viscosity between 0.64 and 0.66, the molten material temperature is preferably controlled in the range of 285 to 290 degrees Celsius. Exceeding 300 degrees Celsius can lead to rapid thermal degradation of polyester macromolecules. Within the mentioned temperature range, as the temperature increases, the flow viscosity of the molten material gradually decreases, improving the uniformity and rheological properties of the molten material, enhancing spinnability.
Lowering the pre-orientation degree (birefringence index n value) of the winding yarn, reducing irregularities in the cross-section, and decreasing spinning tension are observed at higher temperatures. After stretching, the tensile strength and elongation of the stretched yarn also tend to increase. Therefore, within the constraints of minimal viscosity reduction, a higher temperature is desirable.
However, the molten material temperature should not be too high, as excessive temperatures can exacerbate the degradation of polyester macromolecules. This can result in decreased screw pressure or fluctuations in component pressures, causing fluctuations in the solidification point of the fibers, increased irregularities in yarn dryness, and elevated dyeing irregularities. Additionally, issues such as filament discontinuity, excessive fluff during winding, and increased breakage may occur.
Conversely, a too-low temperature can lead to poor spinnability due to increased shear stress in the spinning holes caused by high viscosity, resulting in material rupture. Yarn produced at temperatures below 280 degrees Celsius, termed weak yarn, exhibits low strength and elongation, prone to fluff and breakage during stretching, making processing difficult. Temperature fluctuations during actual production should be controlled within a range of ±1°C to minimize fiber dyeing variations.
The suitability of molten material temperature selection is evaluated not only based on the operation of spinning and stretching and the quality indicators of the final yarn but also by assessing the viscosity drop of the non-oiled yarn. A Δn value less than 0.04, with minimal fluctuations, indicates proper molten material temperature selection. Control of the molten material temperature can be achieved through temperature regulation in the screw extruder and spinning box. Additionally, factors such as frictional heat generation should be considered. The screw can be divided into feeding, compression, and metering sections. In practical use, to facilitate temperature control, the screw can be divided into several heating control zones.
2. Screw extrusion pressure
Pressure at the outlet of the screw extruder, measured and controlled by a pressure sensor. Screw extrusion pressure is crucial for overcoming the resistance of the melt within pipelines and equipment like mixers, ensuring a certain melt pressure at the inlet of the metering pump. According to literature, accurate metering output from the pump requires a pre-pressure of 2 MPa; otherwise, there may be insufficient or fluctuating pump supply, resulting in finer yarn or uneven filament.
Taking the example of the VC406A spinning machine, when spinning filament with a specification of 167 dtex at a spinning speed of 1000 m/min, the pipeline resistance is 2.6 MPa. To ensure normal production, there needs to be at least 4.6 MPa of screw extrusion pressure on the spinning machine.
In practical production, controlling a pressure of 6.5-7.5 MPa is recommended. Although higher screw extrusion pressure is beneficial for spinning, excessively high pressure requires faster screw speeds, leading to increased backflow of melt in the extruder, higher energy consumption, and the risk of accidents if it exceeds the equipment’s pressure resistance range.
3. Pump delivery
The term “pump delivery” refers to the mass of melt transported by the metering pump per unit of time. The magnitude of pump delivery directly affects the thickness of the spun yarn. Pump delivery can be determined through calculation and adjusted according to actual conditions. The calculation formula is:
Where:
– Q is the pump delivery (g/min)
– D is the finished yarn linear density (dtex)
– R is the stretch ratio
– v is the spinning speed (m/min)
– K is the fiber shrinkage coefficient (typically taken as 1.05–1.10)
In actual production, pump delivery is not directly controlled but achieved by controlling the pump’s speed. The pump speed ( N ) can be determined by the formula:
N = \frac{Q}{\gamma \eta C}
Where:
– N is the metering pump speed (r/min)
– Q is the pump delivery (g/min)
– gamma is the melt density (g/cm³)
– eta is the efficiency of the metering pump (generally around 98%)
– C is the metering pump capacity (cm³/r)
The allowable speed for metering pumps is generally in the range of 15–40 r/min, with the preferable range being 20–30 r/min. If the calculated speed is outside the applicable range, it can be adjusted by changing the specifications of the metering pump.
3. Component pressure
The term “component pressure” is used to overcome the resistance encountered by the melt as it passes through the filtering layer and spinneret holes, and it is closely related to the uniformity of fiber quality.
In high-pressure spinning, maintaining component pressure within the range of 9.8–24.5MPa results in better quality of winding yarn. As the component is used over time, impurities gradually accumulate in the filtering layer, increasing the resistance and causing a continuous rise in component pressure. When considering component pressure, the process mainly focuses on initial pressure and the rate of pressure increase. Initial pressure refers to the pressure stabilizing 30 minutes after the start of new component spinning, also known as the starting pressure. It is related to the composition of the filtering layer, pump delivery, melt temperature, and viscosity, typically ranging from 9.8 to 14.7MPa. The rate of pressure increase is the degree to which component pressure rises per unit time (hours or days) during normal component use. The daily rate of pressure increase should be less than 6%. A fast rate of pressure increase shortens the lifespan of the component. The component should be replaced when the pressure reaches a maximum of 30MPa. Continuing to use it may damage the metering pump, cause deformation of the spinneret plate, or result in material leakage.
4. Temperature, humidity, and airspeed (air volume)
The parameters for the cooling air in the production of filament yarn generally include temperature, humidity, and airspeed (air volume), along with the distribution of airspeed on the side-blowing window.
The cooling air temperature is typically maintained between 20 and 30 degrees Celsius. If the spinning speed increases, the air temperature should be appropriately lowered to expedite cooling. Currently, a common temperature setting is around 28°C.
Maintaining a certain humidity in the cooling air helps prevent static electricity generation as the yarn moves through the duct, reducing yarn jitter and bouncing. It also stabilizes indoor temperatures, facilitates heat transfer, enhances yarn cooling, and influences the crystallinity, elongation, and regain of the yarn. The relative humidity is generally maintained between 65% and 80%, typically controlled around 70%.
The airspeed (air volume) significantly affects the preorientation (birefringence) and stretch ratio of the winding yarn. With increasing airspeed, the birefringence of the winding yarn decreases, and the cold-drawing ratio increases. This is because higher airspeed improves cooling efficiency, shifts the solidification point toward the spinneret plate, shortens the deformation zone, and weakens the preorientation effect on the melt before solidification.
Additionally, higher airspeed can improve color uniformity and yarn density uniformity, reduce interference from outdoor airflow. However, excessive airspeed can cause yarn shaking, turbulence, and increased cooling on the spinneret plate, leading to an uneven quality index, especially in cooling airspeed for yarns with different linear densities.
Moreover, it is crucial to maintain a stable airspeed, as fluctuations can increase the unevenness of yarn density, which is a significant factor in uneven dyeing and uneven tensile properties. Airspeed distribution curves usually take on three forms: straight lines, arcs, and S-shapes, with straight lines and arcs being more common. Some production setups include a cooling zone in the spinning window, with the lower opening of the cooling zone insulated with asbestos boards to maintain the temperature of the spinneret plate. During normal production, it is essential to properly place the heat-insulating board.
6. Winding speed
Winding speed is a crucial factor influencing the preorientation of wound yarn. As winding speed increases, preorientation rises, resulting in lower post-drawing ratios and higher spindle production, though not in direct proportion.
Under feasible conditions, it is advisable to maximize winding speed. This not only enhances production efficiency but also contributes to improved yarn quality. According to available data, the optimal winding speed for conventional spinning is typically in the range of 900 to 1200 meters per minute. The ratio of winding speed to melt extrusion speed is referred to as the spinning head drawing ratio. As the spinning head drawing ratio increases, the post-drawing ratio decreases. The spinning head drawing ratio can be calculated using the formula:
\[ R’ = \frac{v}{\gamma \cdot d \cdot n \cdot Q} \]
Where:
– \( R’ \) is the spinning head drawing ratio.
– \( v \) is the winding speed (cm/min).
– \( \gamma \) is the melt density (g/cm³).
– \( d \) is the spinneret hole diameter (cm).
– \( n \) is the number of spinneret holes.
– \( Q \) is the pump flow rate (g/min).
7. Number of reciprocations of the traverse wire guide
The reciprocation frequency of the traverse guide determines the size of the package winding angle, which also influences the winding tension and is crucial for the quality of winding formation. In production, winding angles in the range of 6° to 7° are commonly used. The reciprocation frequency of the traverse guide can be calculated using the formula:
\[ N = \frac{360 \cdot v}{H \cdot \alpha} \]
Where:
– \( N \) is the reciprocation frequency (cycles/min).
– \( \alpha \) is the winding angle (°).
– \( H \) is the traverse stroke (m).
– \( v \) is the winding speed (m/min).
To prevent poor winding formation due to overlapping, the reciprocation frequency of the traverse guide should undergo periodic changes. The range of these changes is termed the amplitude, and the time of the changes is termed the period. Amplitude generally falls within ±15 to 25 cycles/min, and the period typically ranges from 15 to 25 seconds. As the winding speed increases, the amplitude and period should be appropriately reduced.
8. Tanker rotation and oil concentration
The amount of oil applied to the winding yarn directly determines the oil content of the final textured yarn. The higher the concentration of the oil agent, the faster the rotation speed of the oil roller, and the higher the amount of oil applied to the winding yarn. The amount of oil applied depends on the ultimate use of the yarn, such as 0.6% to 0.7% for weaving yarn, 0.7% to 0.9% for knitting yarn, and 0.5% to 0.6% for covered yarn. The rotation speed of the oil roller is typically between 10 to 20 r/min, and the oil agent concentration ranges from 10% to 16%.
To achieve uniform oiling, the rotation speed of the oil roller and the concentration of the oil agent must be coordinated. An increase in the oil agent concentration with a decrease in the oil roller rotation speed results in better splashing and spreading but poorer adhesion. Conversely, a decrease in the oil agent concentration with an increase in the oil roller rotation speed leads to poor splashing and spreading but better adhesion.
In conclusion, the intricacies of the spinning process, from melting to winding, underscore the precision and coordination required in the production of high-quality yarns. Each step, from controlling the melt temperature to adjusting the winding speed, plays a crucial role in determining the characteristics of the final product. As we navigate the complexities of fiber production, the careful calibration of parameters such as extrusion pressure, cooling airflow, and oiling amounts ensures the creation of yarns that not only meet industry standards but also excel in quality and performance. With a commitment to continuous improvement and adherence to best practices, our endeavor remains rooted in delivering yarns that meet the diverse needs of global clients, setting new standards of excellence in the textile industry.