Nylon Yarn Chilling: The Core Code That Determines Quality and Performance

Nylon Yarn Chilling: The Core Code That Determines Quality and Performance

In the nylon yarn production chain, chilling is a critical link between spinning and post-processing, serving as the “invisible engineer” that determines the yarn’s final quality. This process not only directly impacts the yarn’s physical and mechanical properties and appearance, but is also closely linked to the durability, comfort, and even production efficiency of downstream textile products. For those engaged in nylon yarn foreign trade, a thorough understanding of the technical logic and process details of chilling is not only a key vehicle for delivering professional value to clients, but also crucial for maintaining product competitiveness.

1. Chilling: The Scientific Principle of Nylon Yarn “Setting”
Nylon yarn chilling is essentially a physical process in which high-temperature molten filaments extruded from the spinneret of a spinning machine are rapidly cooled to below their glass transition temperature through a specific medium, transforming them from a highly elastic state to a glassy state and fixing their molecular chain arrangement. The core tension in this process lies in “controlled cooling”: the need for rapid cooling to prevent filament adhesion and ensure forming stability, while also precisely controlling the cooling rate to reduce internal stress and lay the foundation for subsequent processing. From a molecular perspective, molten nylon molecular chains are in intense motion. If not promptly cooled after spinning, the chains will orient freely due to thermal motion, resulting in a shapeless yarn with extremely low strength. The cooling medium (usually air or water) rapidly removes heat from the yarn through heat exchange, causing the molecular chains to rapidly slow down and “freeze” themselves, forming a solid yarn with a certain degree of crystallinity and orientation. At this point, core yarn characteristics such as diameter, breaking strength, and elongation have been initially determined. Subsequent processes such as stretching and twisting can only be optimized based on this foundation and cannot fundamentally change the structural foundation established during the cooling and forming stage.

II. Core Processes and Equipment for Nylon Yarn Cooling and Forming

Currently, the mainstream nylon yarn cooling and forming processes in the industry can be categorized into two main types: “air cooling” and “water bath cooling.” These two methods complement each other based on different application scenarios and product requirements, and their equipment configurations and process parameters also differ significantly. (I) Air Cooling: The Mainstream Choice for Medium and Low Denier Yarns

Air cooling uses clean compressed air as the cooling medium. It can be categorized as cross-flow cooling or circumferential cooling, depending on the airflow direction. It is the preferred process for producing nylon yarns with a denier below 150D.

The cross-flow cooling system consists of a bellows, filter, rectifier, and fan. The cooling air is blown through the filament bundle perpendicular to its direction of travel. Its advantages lie in its simple structure, low maintenance costs, and easy adjustment of the cooling air speed and temperature, meeting the molding requirements of conventional nylon 6 and nylon 66 yarns. However, due to airflow uniformity limitations, cross-flow cooling is more suitable for spinning lines with single-row spinnerets. Wide filament bundles can easily suffer from uneven cooling.

Circumferential cooling, on the other hand, applies a 360° uniform airflow to the filament bundle through an annular air duct. The airflow undergoes multiple layers of rectification, resulting in laminar flow. This ensures a consistent cooling rate around the circumference of the filament bundle, effectively addressing the uniformity issues inherent in cross-flow cooling. This process is particularly suitable for the production of fine-denier (e.g., below 30D) and ultra-fine-denier nylon yarns, significantly reducing yarn unevenness and improving dyeing consistency. However, the circular air system requires extremely high air cleanliness, requiring multi-stage filtration, and the equipment investment and energy consumption are relatively high.

In terms of process parameter control, three key indicators for air cooling are crucial: first, cooling air temperature, which is typically controlled at 20-25°C, with temperature fluctuations of ≤±1°C to avoid temperature instability leading to variations in yarn crystallization; second, air speed, which is typically 0.3-0.8 m/s for cross-wind air and 0.5-1.2 m/s for circular air. Excessive air speeds can easily cause yarn vibration, while too low can lead to insufficient cooling efficiency; third, the temperature difference between air temperature and spinning temperature must be maintained within a reasonable range of 180-220°C, ensuring a stable cooling rate of 500-800°C/s to achieve rapid shaping while avoiding excessive internal stress. (II) Water Bath Cooling: A Special Solution for High-Denier, Industrial Yarns

Water bath cooling, also known as wet cooling, involves directly immersing the spun yarn in room-temperature or low-temperature water for cooling. It is primarily used in the production of industrial nylon yarns with a denier of 300D and above (such as tire cord and fishing net yarn).

Compared to air cooling, water has a thermal conductivity over 25 times that of air, resulting in higher cooling efficiency and instantaneous cooling of the yarn. This is particularly suitable for high-denier yarns with high melt viscosity and slow heat dissipation. Furthermore, water bath cooling allows precise control of crystallinity by adjusting the water temperature (typically 15-25°C) and the length of time the yarn is immersed in the water (generally 1-3m). Lower water temperatures and longer immersion times increase yarn crystallinity, significantly improving the breaking strength during subsequent drawing. However, water bath cooling places extremely stringent requirements on water quality. Impurities and ions in the water can directly adhere to the surface of the yarn, leading to problems such as broken ends and lint during post-processing. Therefore, water treatment systems such as ion exchange and precision filtration are essential. Furthermore, after the yarn emerges from the water, it must be subjected to vacuum suction or hot air drying to remove surface moisture, which increases equipment complexity and energy consumption.

III. Analysis of Key Factors Affecting Cooling Forming Results

Cooling forming is a multi-parameter process. In addition to the choice of process type, factors such as raw material properties, equipment precision, and environmental conditions all influence the final result. Among these, raw material moisture content, spinning speed, and cooling medium conditions are the most critical variables.

(I) Raw Material Properties: The “Innate Foundation” of Cooling Forming
The moisture content of nylon chips is the primary raw material factor affecting cool forming. Nylon is a polar polymer that readily absorbs moisture from the air. If the moisture content of the chips exceeds 0.05%, the moisture will vaporize during the spinning process due to heat, resulting in bubbles in the molten yarn. These bubbles cannot be eliminated during the cooling and forming stage, ultimately causing “slubbed” yarn defects and significantly reducing breaking strength. Therefore, nylon chips must be vacuum-dried before spinning to keep the moisture content below 0.02% to provide uniform molten yarns for cooling and forming.

In addition, the molecular weight distribution of the nylon chips also affects the cooling effect. Chips with a narrow molecular weight distribution have more stable melt viscosity, a more uniform temperature distribution of the yarn after spinning, and a consistent crystallization rate during cooling, effectively reducing the yarn evenness CV value. Conversely, chips with a wide distribution are prone to uneven cooling, resulting in fluctuating yarn properties.

(II) Spinning Speed: The “Dynamic Balance” of Cooling Efficiency

Spinning speed and cooling and forming efficiency have a significant positive correlation, but this relationship has a critical threshold. When the spinning speed increases from 1000 m/min to 3000 m/min, the residence time of the filament in the cooling zone decreases from 0.2s to 0.07s. This reduction in residence time must be compensated by increasing the cooling air velocity or lowering the cooling medium temperature to ensure the required cooling rate.

If the spinning speed exceeds a critical value (typically 4000 m/min), even at maximum cooling intensity, the filament may enter the stretching stage before fully cooling below the glass transition temperature. This can cause the filament to “neck” during stretching and a sharp drop in elongation at break. Therefore, when designing the cooling system, the cooling capacity must be matched to the target spinning speed. For high-speed spinning lines (speeds ≥ 3500 m/min), a combination of “circular air blowing + secondary cooling” is typically used to ensure adequate filament shaping.

(III) Cooling Medium: The “Core Guarantee” of Stable Quality

The condition of the cooling medium directly determines the uniformity and stability of cooling. For air cooling, in addition to temperature and air velocity, relative humidity is also crucial. When relative humidity exceeds 70%, condensation easily forms on the yarn surface, causing it to stick. Below 30%, moisture evaporation accelerates, resulting in differential cooling rates between the inside and outside of the yarn. Therefore, the relative humidity of the cooling air should be controlled within the optimal range of 40%-60%.

For water bath cooling, the conductivity and pH of the water are key control indicators. Conductivity must be ≤5μS/cm to prevent metal ion adhesion and affect yarn dyeability. The pH must be maintained in a neutral range of 6.5-7.5 to prevent acidic or alkaline water from corroding the yarn surface. Furthermore, the suspended solids content in the water must be ≤1ppm, and water quality testing must be conducted every four hours to ensure compliance.

IV. Innovations in Cooling Forming Technology and Industry Application Cases

As downstream textile industries continue to raise their standards for nylon yarn quality, cooling forming technology is also undergoing continuous innovation, evolving from “single cooling” to “precise control” and “energy-saving and high-efficiency.” The following two cases fully demonstrate the value added brought about by technological innovation.

Case 1: Application of an Intelligent Circular Air Blowing System in Ultra-fine Denier Nylon Yarn

To meet demand for ultra-fine denier (15D/36F) nylon yarn for high-end women’s apparel fabrics, a foreign trade company introduced a circular air cooling system equipped with an intelligent sensing system. This system uses infrared temperature sensors to monitor the surface temperature of each yarn in real time. Combined with an AI algorithm, it automatically adjusts the wind speed and temperature of the circular air duct to maintain a circumferential temperature difference within ±0.5°C. After implementing this technology, the yarn’s evenness (CV) value decreased from 2.8% to 1.5%, and the dyeing color difference grade improved from level 4 to level 4.5. The product successfully entered the European high-end fabric market, with a unit price increase of 30% compared to conventional products. Furthermore, the intelligent adjustment system reduced cooling air energy consumption by 18%, achieving a dual improvement in quality and efficiency.

Case 2: A Breakthrough in Industrial Nylon Cord Using Gradient Water Bath Cooling
A tire nylon cord manufacturer developed a gradient water bath cooling process to address the problem of uneven crystallization and insufficient tensile strength caused by traditional water bath cooling. This process divides the cooling water tank into three sections, with the water temperature gradually decreasing from 25°C at the inlet to 10°C at the outlet. This allows for gradual crystallization of the yarn and reduces internal stress accumulation. Testing has shown that the breaking strength of cord produced using this process has increased from 8.5 cN/dtex to 9.8 cN/dtex, and the thermal shrinkage has decreased from 3.2% to 1.8%, fully meeting the performance requirements of high-end tire carcass materials. Exports of this product have increased by 45% year-on-year, successfully replacing imported products.

V. Cooling and Forming Process Optimization Directions and Practical Recommendations

For nylon yarn manufacturers, optimizing the cooling and forming process should focus on the three core goals of improving uniformity, reducing energy consumption, and adapting to product needs. Based on actual production practices, the following four areas can be addressed:

Equipment Upgrade and Modification: For cross-blow cooling lines, low-cost modifications such as adding rectifier grids and optimizing the airbox structure can improve airflow uniformity by over 20%. For high-denier yarn production lines, it is recommended to gradually replace them with gradient water bath cooling systems. This performance improvement, especially when producing industrial products such as cord and conveyor belt yarn, can directly translate into market competitiveness.​
Refined Parameter Control: A linked database connecting “raw materials, processes, and performance” is established to preset optimal ranges for key parameters such as cooling medium temperature, air speed, and dwell time for different deniers and types of nylon yarn. For example, when producing nylon 6 ultrafine yarn, the annular air temperature can be set to 22±0.5°C and the air speed to 0.8±0.1m/s; when producing nylon 66 industrial yarn, the water bath temperature is controlled at 18±1°C and the immersion length is 2.5m.

Improving Cooling Medium Quality: The air cooling system requires monthly replacement of the primary filter and quarterly replacement of the secondary filter to ensure Class 1000 air cleanliness. The water bath cooling system utilizes a dual-stage water treatment process combining reverse osmosis and EDI, equipped with an online water quality monitor to provide real-time alarms and automatic adjustments for conductivity and pH values. Visual Process Monitoring: High-definition cameras and temperature sensor arrays are installed in the cooling area, providing real-time visualization of the yarn’s operating status and cooling temperature distribution through the production management system. If temperature fluctuations exceed ±1°C or yarn jitter exceeds 0.5mm, the system automatically triggers an alarm and adjusts parameters, keeping the defective rate below 0.1%.

Conclusion
Chilling and forming may appear to be an “intermediate step” in nylon yarn production, but in reality, it is a “watershed” that determines the quality grade of the product. From freezing and setting the molecular chain to precisely controlling yarn properties, from optimizing and upgrading traditional processes to the innovative application of intelligent technologies, each breakthrough in chilling and forming technology is driving nylon yarn development towards higher strength, better uniformity, and greater compatibility with downstream demands.