Screw Extrusion for Nylon Yarn

Screw Extrusion for Nylon Yarn

In the nylon yarn production system, the screw extrusion process is the core link between raw materials and finished yarn. Its technical level directly determines key properties such as yarn strength, elasticity, and uniformity. For textile companies, material research and development institutions, and buyers, a thorough understanding of the technical details of screw extrusion not only optimizes production efficiency but also provides core support for product innovation and quality control. This article will comprehensively analyze the nylon yarn screw extrusion process from five perspectives: technical principles, key parameters, process advantages, industry applications, and common problem-solving, providing a systematic reference for industry practitioners.

I. Screw Extrusion: The “Core Engine” of Nylon Yarn Forming

Screw extrusion is the key process for converting solid nylon chips into a continuous molten stream for spinning into yarn. Essentially, it achieves precise material transformation through a continuous process of “heating – melting – conveying – pressurization.” The core equipment in this process is the screw extruder, which primarily consists of a screw, barrel, heating and cooling systems, a feeding device, and a control system. These components work together to ensure the nylon material is formed under stable process conditions.

1.1 Core Process Principle: Four-Stage Precision Control

The screw extrusion process for nylon yarn can be divided into four key stages. Process parameters in each stage must be strictly controlled; deviations in any step can result in yarn quality defects:

Feeding Stage: Dried nylon chips (moisture content must be kept below 0.05% to prevent bubbles during melting) enter the barrel through a feed hopper and are conveyed by the screw’s spiral grooves to the heating zone. During this stage, a uniform feed rate (typically 10-30 kg/h, adjusted according to yarn fineness) is crucial to prevent fluctuations in feed rate and unstable extrusion output. During the melting stage, the barrel is heated in stages (typically divided into 3-5 heating zones, with the temperature gradually increasing from 220°C at the feed end to 260-280°C at the discharge end), gradually softening and melting the nylon chips. The shearing action of the screw further enhances the melting process, ensuring a uniform melt. (The melt viscosity must be controlled between 100-500 Pa·s; a higher viscosity can lead to extrusion difficulties, while a lower viscosity can affect yarn strength.)

Dose-by-dose, the molten nylon melt is pushed toward the front of the barrel by the screw. It passes through a filter (typically 80-120 mesh to ensure melt cleanliness) to remove impurities. The filter then enters the metering pump, which delivers the melt to the spinning pack at a constant flow rate (accuracy ≤±1%), providing a stable supply of raw material for subsequent spinning. During the extrusion stage, the melt is extruded through the spinneret of the spinning assembly (the spinneret orifice diameter is designed based on the yarn fineness, generally 0.1-0.5mm), forming continuous filaments. These filaments then undergo subsequent processes such as cooling, oiling, and stretching, ultimately becoming nylon yarn.

1.2 Core Equipment: The “Golden Pair” of Screw and Barrel

The screw and barrel are core components of the extrusion process, and their structural design directly impacts extrusion efficiency and melt quality.

Screw Structure: A “gradually tapered screw” is commonly used for nylon extrusion. The depth of the screw groove gradually decreases from the feed section to the metering section, ensuring gradual compaction and melting of the material during conveying. The screw’s length-to-diameter ratio (L/D) is typically 25-30. A screw that is too long can lead to excessive material degradation, while a screw that is too short can result in incomplete melting. Furthermore, the screw surface should be nitrided (hardness ≥ HV850) to reduce melt adhesion and extend service life. Barrel Design: The barrel utilizes segmented heating and cooling control, with each heating zone equipped with an independent temperature sensor and temperature controller to ensure temperature fluctuations of ≤±2°C. The barrel’s inner wall is precision-machined (roughness Ra ≤ 0.8μm), and the clearance between the barrel and the screw is controlled to 0.1-0.3mm, ensuring a tight seal while preventing excessive wear.

II. Key Process Parameters: The “Secret Code” that Determines Nylon Yarn Quality

During the screw extrusion process, coordinated control of parameters such as temperature, screw speed, and metering pump flow rate is crucial for achieving consistent yarn quality. Nylon yarns of different specifications (such as nylon 6, nylon 66, or products with varying fineness and strength requirements) require differentiated process parameter settings. 2.1 Temperature Control: Balancing the “Critical Point” of Melting and Degradation
Nylon materials (especially nylon 6) are extremely temperature-sensitive. Excessively high temperatures can easily lead to material degradation (producing small volatile molecules that affect yarn color and strength). Excessively low temperatures can lead to incomplete melting, resulting in “unmelted particles” and increased yarn breakage rates. The extrusion temperature ranges for different nylon grades are as follows:
Nylon 6 (PA6): Barrel temperatures from the feed end to the discharge end are 220-230°C, 240-250°C, 250-260°C, and 260-270°C, respectively. The spinneret temperature is 265-275°C. Nylon 66 (PA66): Due to its higher melting point (approximately 265°C), the barrel temperature needs to be increased accordingly, to 240-250°C, 260-270°C, 270-280°C, and 280-290°C, respectively. The spinneret temperature is 285-295°C.

In addition, the cooling system must be adjusted in real time based on temperature feedback. When the barrel temperature exceeds the set value, the cooling fan automatically activates to prevent local overheating.

2.2 Screw Speed: The “Knob” for Controlling Extrusion Output and Shear Strength

Screw speed directly affects the shear strength and extrusion output of the melt:

Low speed (20-40 r/min): Suitable for fine-denier nylon yarn (e.g., below 50D). This has lower shear strength and can reduce material degradation, but the extrusion output is low, requiring a high-precision metering pump to ensure stable flow. High speed (50-80 rpm): Suitable for coarse denier yarns (e.g., 200D and above). This allows for higher extrusion throughput and improves production efficiency, but requires enhanced cooling and temperature control to prevent melt degradation due to excessive shear heating.

Generally, screw speed fluctuations should be controlled within ±1 rpm to ensure consistent extrusion output.

2.3 Metering Pump Flow Rate: A “Stabilizer” for Yarn Uniformity

The metering pump’s flow rate accuracy directly determines the yarn’s fineness uniformity (CV value). For high-quality nylon yarns (such as high-stretch yarn for textiles and high-tenacity yarn for industrial applications), the metering pump’s flow rate accuracy must be within ±0.5%. In actual production, the flow rate must be calculated based on the yarn fineness and the number of spinning heads:

For example, when producing 100D/36F nylon 6 yarn (single yarn fineness ≈ 2.78D), at a spinning speed of 1200 m/min, the flow rate per metering pump is approximately 5.3 mL/h, according to the formula: “Metering pump flow rate (mL/h) = Yarn fineness (D) × Spinning speed (m/min) × Number of spinning heads / (9000 × Melt density)” (Nylon 6 melt density is approximately 1.08 g/cm³).

III. Core Advantages of Screw Extrusion: Why is it the preferred choice for nylon yarn production?

Compared to traditional kettle melt spinning, screw extrusion offers significant advantages in nylon yarn production, making it particularly suitable for large-scale, high-quality yarn production.​
3.1 Continuous Production: Significantly Improved Efficiency and Stability

The screw extrusion process enables continuous operation from raw material feeding to melt extrusion, eliminating the frequent downtime and refilling required by batch-based processes. This can improve production efficiency by 30%-50%. Furthermore, continuous production reduces batch-to-batch process fluctuations, enabling yarn quality stability (e.g., fineness CV value and strength deviation) to be controlled within ±3%, far superior to the ±5% of batch-based processes.

3.2 Strong Material Adaptability: Meeting Diverse Yarn Demands

The screw extrusion process is compatible with various nylon raw materials (e.g., nylon 6, nylon 66, and modified nylon), and can functionalize the yarn by adding functional masterbatches (e.g., antimicrobial, UV-resistant, and color-treated). For example, adding 2%-5% of antimicrobial masterbatch during the extrusion process can produce antimicrobial nylon yarn suitable for medical and sportswear applications. Adding carbon black masterbatch can create UV-resistant yarn for outdoor textiles.
3.3 Energy Consumption and Cost Advantages: Reducing Overall Production Costs
The screw extruder’s heating system utilizes segmented temperature control, resulting in high energy efficiency (approximately 70%-80%). Compared to the kettle process (approximately 50%-60%), this reduces energy consumption per unit product by 20%-30%. Furthermore, the screw extrusion process is highly automated (capable of fully automated PLC control), reducing manual operations and labor costs by 15%-20%. It also reduces scrap rates due to human error (scrap rates can be kept below 1%).

IV. Industry Applications: Screw Extrusion Empowers Nylon Yarn in Multiple Applications
Nylon yarn produced through screw extrusion, with its excellent strength, elasticity, and wear resistance, is widely used in a variety of fields, including textiles, industry, and healthcare. The performance requirements of different applications for yarns are driving technological advancements in the extrusion process.
4.1 Textile Industry: Highly Elastic and Delicate “Fabric Core”
In the textile industry, nylon yarns (such as high-stretch and low-stretch nylon yarns) produced through screw extrusion are core raw materials for high-end fabrics:

Sportswear: By optimizing extrusion temperature (e.g., nylon 6 extrusion temperature controlled at 255-265°C) and stretching processes, high-stretch yarns with elongation at break ≥300% and rebound ≥90% are produced. These sportswear exhibits excellent elastic recovery, tailored to the needs of the human body during exercise.

Underwear Fabrics: Extrusion using fine-denier spinnerets (0.1-0.2mm diameter) produces fine-denier nylon yarns below 50D. These fabrics are delicate, breathable, and skin-friendly. The addition of moisturizing masterbatch also enhances the fabric’s skin-protecting properties.​
4.2 Industrial Sector: High-Strength, Wear-Resistant “Structural Support”
Industrial nylon yarns (such as tire cord and cable yarns) require extremely high strength and wear resistance. The screw extrusion process is optimized to meet these requirements through the following technologies:

Increasing melt viscosity: The extrusion temperature of nylon 66 is controlled at 275-285°C to ensure a stable melt viscosity of 300-400 Pa·s. Combined with high stretching (stretching ratio of 4-6x), high-strength yarn with a breaking strength of ≥8 cN/dtex is produced. This yarn is used in tire cord, improving the tire’s load-bearing capacity and durability.

Enhancing wear resistance: 5%-10% polytetrafluoroethylene (PTFE) masterbatch is added during the extrusion process. Through thorough mixing by the screw, the PTFE is evenly dispersed in the melt, resulting in a yarn with over 50% increased wear resistance, making it suitable for industrial cables, conveyor belts, and other applications.
4.3 Medical Field: Clean and Safe “Medical Materials”
Medical nylon yarns (such as surgical sutures and medical dressings) have stringent requirements for cleanliness and biocompatibility. The screw extrusion process ensures quality through the following measures:
Raw Material Control: Medical-grade nylon chips (compliant with FDA and CE standards) are used and vacuum-dried (120-140°C for 4-6 hours) before extrusion to ensure a moisture content of ≤0.03% to prevent microbial growth.
Clean Production: The extruder is equipped with a 120-150 mesh double-layer filter and utilizes a sterile spinning environment (Class 10000 cleanliness level). The resulting yarn is free of impurities and odor, suitable for direct use in medical sutures, and meets 100% biocompatibility standards. V. Common Problems and Solutions: Ensuring Stable Screw Extrusion Process Operation
Nylon yarn screw extrusion production is prone to problems such as melt degradation, yarn breakage, and uneven fineness. Promptly identifying the causes and implementing targeted measures are key to ensuring production continuity and product quality.
5.1 Melt Degradation: The Culprit for Yarn Yellowing and Strength Loss
Symptom: The extruded melt turns yellow, the resulting yarn’s breaking strength decreases (by more than 10% below the standard), and bubbles appear on the surface.
Cause Analysis:
Excessive barrel temperature (exceeding the nylon melting point by more than 30°C) leads to thermal degradation of the material;
Excessive screw speed, resulting in excessive shear heat, causes localized degradation;
Excessive melt residence time in the barrel (e.g., failure to drain the melt promptly during production interruptions) leads to oxidative degradation. Solution:
Lower the barrel temperature (especially the metering section and spinneret temperature), for example, for nylon 6, from 270°C to 260°C, and monitor viscosity changes in real time using a melt viscometer.
Appropriately reduce the screw speed (e.g., from 60 rpm to 45 rpm), while optimizing the screw groove depth to reduce shear heat generation.
When production is interrupted, promptly initiate the “emptying process” to drain the remaining melt in the barrel to avoid prolonged dwelling.
5.2 Yarn Breakage: An Obstacle to Low Production Efficiency
Symptom: Frequent yarn breakage during spinning (breakage rate ≥ 5 times/hour), resulting in production interruptions and increased scrap rates.
Cause Analysis:
Unmelted particles in the melt (due to insufficient heating or insufficient screw shear force) lead to yarn breakage at weak points.
Spinneret orifice blockage (due to a damaged filter, allowing impurities to enter the spinneret), affecting melt extrusion uniformity.
Excessive cooling rate, resulting in brittle fracture of the yarn due to sudden cooling. Solution:
Increase the barrel heating temperature (e.g., for nylon 66, from 280°C to 285°C) and replace the screw with a higher aspect ratio (e.g., from L/D=25 to L/D=30) to enhance melting efficiency.
Check the integrity of the filter screens, replace damaged ones, and upgrade to a double-layer filter (e.g., 80 mesh + 120 mesh) to improve filtration accuracy.
Reduce the cooling air velocity (e.g., from 0.8 m/s to 0.5 m/s) and increase the cooling distance (e.g., from 1.5 m to 2.0 m) to prevent sudden cooling of the yarn.

5.3 Uneven Fineness: A Key Issue for Unqualified Yarn Quality

Symptom: The yarn fineness CV value exceeds 5% (standard value ≤ 3%), indicating significant variations in yarn thickness within the same batch, impacting subsequent weaving quality. Cause Analysis:
Unstable metering pump flow (e.g., worn metering pump gears, reduced accuracy);
Feeding speed fluctuations (e.g., clogged feed hopper, resulting in uneven raw material supply);
Screw speed fluctuations (e.g., motor speed controller failure).
Solution:
Regularly (every three months) calibrate the metering pump and replace worn gears to ensure flow accuracy error ≤ ±0.5%;
Inspect the feed hopper outlet, clear any obstructions, and install a vibrating feeder to ensure stable feeding speed (fluctuation ≤ ±2%);
Repair the motor speed controller and replace any faulty components to ensure screw speed fluctuation ≤ ±1 r/min. Monitor speed changes in real time using a PLC system.

VI. Summary: Screw Extrusion Process – The Core Driving Force for Nylon Yarn Industry Upgrade
With the textile industry’s growing demand for high-quality, functionalized nylon yarn, screw extrusion technology is evolving toward high-precision, intelligent, and green features. In the future, by introducing AI temperature control system (to achieve temperature error ≤ ± 0.5 ° C), using new screw materials (such as tungsten carbide coated screw, the service life is extended2 times), optimized energy recovery technology (waste heat utilization rate increased to over 90%), and screw extrusion technology will further improve production efficiency and product quality, providing core support for the upgrading of the nylon yarn industry.