Hot-Melt Yarn Roll Forming

Hot-Melt Yarn Roll Forming: A Core Process Determining Product Performance

In the hot-melt yarn production chain, roll forming is the critical link between raw material melting and finished product application. This process not only directly determines the linear density uniformity, thermal shrinkage stability, and mechanical properties of the hot-melt yarn, but also profoundly impacts processing efficiency and final product quality in downstream textile and nonwoven industries. This article systematically analyzes the key points of the full hot-melt yarn roll forming process, deeply analyzing the mechanisms by which key parameters influence product performance, and provides industry practitioners with practical guidance for process optimization and selection.

I. The Core Value and Technical Framework of the Roll Forming Process
As a functional fiber with thermal bonding properties, hot-melt yarn’s core advantage lies in its ability to bond itself or to other materials through heating. The stability of this property directly depends on the precise control of the roll forming process. Roll forming essentially involves cooling and stretching the molten extruded spinning melt, then passing it through a specialized winding mechanism and heat treatment (singeing) process to form a yarn package with a stable physical structure and properties. From a technical perspective, a complete roll-to-roll system consists of four core modules: the spinning and cooling module cools and solidifies the melt stream into nascent fibers; the stretching and shaping module adjusts the fiber’s molecular orientation through mechanical stretching; the winding module achieves regular yarn packaging; and the singeing module removes surface hairiness, improving yarn smoothness and bonding stability. The coordinated operation of these four modules forms a “closed-loop system” for controlling the performance of hot-melt yarns.

In actual production, the level of roll-to-roll process directly determines the market competitiveness of the product. For example, in the apparel interlining industry, hot-melt yarns produced using precise roll-to-roll can achieve a bonding temperature fluctuation of ≤±2°C, increasing bond strength by 15%-20%. In nonwovens production, uniformly fired hot-melt yarns can maintain a longitudinal/transverse strength ratio within 1.2, significantly outperforming products produced using conventional processes. II. Analysis of Key Process Parameters in the Winding Process
(I) Spinning Cooling: The “Innate Shaping Period” of Fiber Structure
The spinning cooling stage is the foundation of winding. Its core task is to rapidly cool the melt stream extruded from the spinning beam into nascent fibers with a certain degree of crystallinity. Cooling air temperature, air velocity, and cooling distance are three key parameters that influence cooling effectiveness.
Cooling air temperature: Typically controlled at 20-25°C. Excessively high temperatures result in slow cooling, low fiber crystallinity, and increased breakage during subsequent stretching. Excessively low temperatures can cause uneven cooling between the fiber surface and interior, creating internal stress and leading to yarn relaxation after winding. Experience has shown that when cooling air temperature fluctuations exceed ±1°C, the elongation at break of nascent fibers can vary by more than 5%.
Cooling air velocity: Typically set at 0.5-1.2 m/s. Excessively high air velocity can cause fiber oscillation, affecting linear density uniformity. Excessively low air velocity results in insufficient cooling efficiency. For fine-denier hot-melt yarns (linear density ≤ 20D), gradient air velocity control is required to prevent fiber adhesion during the cooling process.
Cooling distance: This refers to the distance from the spinneret to the cooling air ring, typically 80-150mm. A cooling distance that is too short will result in insufficient cooling and uneven thermal shrinkage of the fibers; a distance that is too long may cause the fibers to sag under gravity, affecting tensile stability.

(II) Stretch Setting: The “Acquired Strengthening Phase” of Mechanical Properties
Stretch setting is a key step in improving the strength, modulus, and other mechanical properties of the hot-melt yarn by axially orienting the molecular chains in the nascent fibers through mechanical force. The draw ratio, draw temperature, and draw speed are the core control parameters in this step.
Stretch ratio: Depending on the type of hot-melt yarn, the draw ratio typically ranges from 2x to 5x. A draw ratio that is too low results in insufficient molecular orientation and low yarn strength; a draw ratio that is too high can easily lead to microcracks within the fibers and reduce elongation at break. For example, the stretch ratio for hot-melt yarn used in zipper tapes is generally controlled at 3.2-3.5 times, while simultaneously meeting the requirements of a strength ≥3.5 cN/dtex and an elongation at break of 15%-20%.
Stretching temperature: A staged heating method is typically used, with the preheating stage controlled at 10-20°C above the glass transition temperature, the stretching stage 5-10°C higher than the preheating stage, and the setting stage temperature close to but 5-10°C below the fiber’s melting point. Temperature control accuracy must reach ±0.5°C; otherwise, problems such as yarn strength fluctuation and unstable thermal shrinkage may occur.
Stretching speed: Directly related to production efficiency, it is generally between 200-500 m/min. The stretching speed must be coordinated with the cooling and winding speeds. Excessively fast speeds can lead to uneven stretching and the “slubby” yarn phenomenon; excessively slow speeds reduce production efficiency and increase costs. (3) Winding and Forming: The “Form-Determining Stage” of Package Quality

The core objective of the winding and forming process is to evenly wind the stretched and shaped yarn onto the paper tube at a specific winding angle and tension, creating a package with a stable structure and smooth unwinding. Winding tension, winding angle, and winding speed are key parameters in this process.

Winding tension: Typically controlled between 0.5-2 cN/dtex. Excessive tension can lead to excessive yarn stretching, affecting final performance; too little tension can result in a loose package structure, prone to problems such as edge collapse and yarn overlap. During the winding process, a tension feedback control system is required to keep tension fluctuations within ±5% to ensure uniform package density.

Winding angle: This refers to the angle at which the yarn is wound on the package, generally between 3-8°. A winding angle that is too large can result in “ears” at the ends of the package; a winding angle that is too small can cause yarn to overlap at the same location, hindering unwinding. For large packages (weight ≥ 5kg), variable winding angle technology is required to achieve uniform package formation.

Winding speed: This must be synchronized with the drawing speed, generally between 300-600 m/min. In the later stages of winding, as the package diameter increases, the winding speed must be reduced using variable frequency control to maintain a stable yarn linear speed and avoid package deformation due to excessive centrifugal force.

(IV) Singeing: The “Fine Finishing Stage” of Surface Quality

Singeing is a process that removes surface hairiness from hot-melt yarns using high-temperature flames or plasma. This significantly improves the yarn’s smoothness, pilling resistance, and bonding properties. Singeing temperature, singeing time, and cooling rate are key control parameters in this process.

Singeing temperature: Depending on the melting point of the hot-melt yarn, the singeing temperature is generally controlled 10-30°C above the melting point. Too low a temperature will result in incomplete hair removal; too high a temperature can easily cause surface melt adhesion, hindering subsequent processing. For example, the singeing temperature for PA hot-melt yarn is typically 230-250°C, while that for PET hot-melt yarn is 260-280°C.
Singeing time: Generally, it should be controlled within 0.1-0.5 seconds. A longer time can lead to a loss of yarn strength, while a shorter time can result in ineffective hairiness removal. Precise control of the singeing time can be achieved by adjusting the relative speed between the singeing device and the yarn.
Cooling rate: After singeing, the yarn must be immediately cooled to room temperature, typically at a cooling rate of 50-100°C/s. Rapid cooling prevents changes in the yarn’s surface crystallinity and maintains stable performance. Common cooling methods include cold air cooling and cold water bath cooling. Cold air cooling is more suitable for fine-denier hot-melt yarns, preventing yarn adhesion after contact with water.

III. The Influence of Process Parameters on Hot-melt Yarn Properties
Slight changes in the coil-to-coil process parameters can alter the final properties of the hot-melt yarn by affecting its molecular structure, crystal morphology, and surface state. A thorough understanding of these influencing mechanisms is a prerequisite for process optimization.
(I) Impact on Thermal Shrinkage
Thermal shrinkage is one of the core performance indicators of hot-melt yarns and directly affects the dimensional stability of downstream products. The stretching temperature and setting temperature during coiling and sintering are key factors influencing thermal shrinkage. Increasing the stretching temperature enhances the mobility of the molecular chains and improves their orientation, reducing the thermal shrinkage of the yarn after setting. However, if the setting temperature is too low, the internal stress of the molecular chains cannot be fully released, significantly increasing the thermal shrinkage. Practical data shows that increasing the setting temperature of PET hot-melt yarn from 180°C to 200°C reduces its thermal shrinkage at 100°C for 30 minutes from 8% to below 3%.
In addition, winding tension also affects thermal shrinkage. Excessive winding tension causes excessive elastic potential energy to be stored within the yarn, making it more susceptible to shrinkage when heated, leading to increased thermal shrinkage. Therefore, for hot-melt yarn products requiring low shrinkage, the winding tension should be minimized while ensuring package stability. (II) Impact on Mechanical Properties
The mechanical properties of hot-melt yarn, such as strength and elongation at break, are primarily determined by the draw ratio and the drawing temperature. As the draw ratio increases, the molecular chain orientation improves, the yarn strength gradually increases, and the elongation at break gradually decreases. When the draw ratio exceeds a critical value, the molecular chains break, resulting in a sharp drop in strength. For example, when the draw ratio of PA hot-melt yarn increases from 2.5 to 3.5 times, its strength increases from 2.8 cN/dtex to 4.2 cN/dtex, while the elongation at break decreases from 35% to 18%.
The effect of drawing temperature on mechanical properties is primarily reflected in the orientation efficiency of the molecular chains. Within the appropriate drawing temperature range, increasing temperature reduces intermolecular forces, making molecular chains more oriented, thereby achieving higher strength at the same draw ratio. However, when the drawing temperature is too high, the thermal motion of the molecular chains intensifies, and the oriented structure tends to relax, resulting in a decrease in strength. (III) Impact on Adhesive Properties
The adhesive properties of hot-melt yarns depend on their melt temperature, melt viscosity, and surface condition, all of which are closely related to the coil-to-coil forming process. Singeing removes surface hairiness, increasing the contact area between the yarn and the bonded material, thereby improving bond strength. Furthermore, the slight melting of the yarn surface during the singeing process creates a smooth surface, reducing air entrapment during bonding and improving the sealability of the bond.
In addition, the stretching and setting process affects the crystallinity of the hot-melt yarn, which in turn changes its melt viscosity. High crystallinity results in high melt viscosity and poor fluidity, making it difficult to fully wet the bonded material during bonding. Low crystallinity results in low melt viscosity and increased adhesive bleed-through. By adjusting the stretching temperature and setting time, the crystallinity of the hot-melt yarn can be controlled within the optimal range of 25%-35%, achieving optimal adhesive properties.

IV. Common Problems and Solutions in the Coil-to-Coil Forming Process
In the actual production of hot-melt yarn coil-to-coil forming, a series of quality issues often arise due to improper process parameter control or poor equipment condition, affecting product performance. The following are four common problems in the industry and their corresponding solutions. (1) Package Edge Collapse
Problem: Depression or deformation occurs at both ends of the package, causing yarn tension fluctuations during unwinding, impacting downstream processing.
Causes: Uneven winding tension, especially excessive tension at the initial stage of winding; improper winding angle setting, resulting in low winding density at both ends; poor paper tube quality and insufficient strength.
Solutions: 1. Implement a closed-loop tension control system to adjust winding tension in real time, ensuring tension fluctuations of ≤±3% throughout the winding process; 2. Optimize winding angle parameters and implement a progressive tension increase technique at both ends to increase the density at both ends of the package; 3. Select high-strength paper tubes with a radial compressive strength of ≥1500N.
(2) Excessive Yarn Hairiness
Problem: Dense yarn surface hairiness leads to uneven contact during bonding, reduced bond strength, and increased pilling.
Causes: Excessively low singeing temperature or insufficient singeing time; mismatch between drawing speed and cooling speed, resulting in microfibers on the fiber surface; worn spinning assembly and uneven spinneret holes. Solution: 1. Increase the singeing temperature by 5-10°C, extend the singeing time by 0.1-0.2 seconds, and enhance post-singe cooling. 2. Adjust the ratio between the drawing speed and the cooling air speed so that the cooling speed is slightly higher than the drawing speed. 3. Regularly replace the spinning assembly to ensure the spinneret finish Ra ≤ 0.8μm.
(III) Unstable Thermal Shrinkage
Problem: The thermal shrinkage of yarn within the same batch or roll varies by more than 2%, resulting in inconsistent dimensions in downstream products.
Cause: Excessive fluctuations in the setting temperature; uneven stretching ratio; and drastic changes in the storage environment temperature after winding.
Solution: 1. Upgrade the setting heating device and adopt a PID temperature control system to control the setting temperature fluctuation to within ±0.3°C. 2. Check the parallelism and surface roughness of the stretch rollers to ensure uniform stretching ratio. 3. Establish a constant temperature and humidity storage environment, maintaining a temperature of 20-25°C and a relative humidity of 50%-60%. (IV) Large Yarn Linear Density Deviation
Problem Symptoms: Yarn linear density deviation exceeds ±3%, not meeting customer requirements and affecting knitting or weaving uniformity.
Causes: Unstable spinning melt pressure; fluctuating cooling air temperature and speed; and unsynchronized drawing and winding speeds.
Solutions: 1. Optimize spinning manifold temperature control and implement a melt pressure feedback control system to limit melt pressure fluctuations to ≤±0.5 MPa; 2. Replace the cooling air ring with a uniform air flow design to ensure uniform cooling air temperature and speed; 3. Implement a servo synchronous control system to limit the synchronization error between drawing and winding speeds to ≤0.1%.

V. Development Trends and Selection Recommendations for Roll-to-Roll Forming Processes
(I) Industry Development Trends
With downstream applications continuously increasing their requirements for hot-melt yarn performance, the roll-to-roll forming process is developing towards intelligent, efficient, and green processes.
Intelligent: Introducing Industrial Internet technologies, installing sensors to collect real-time process parameters such as temperature, tension, and speed, and utilizing AI algorithms for data analysis and process optimization, enabling “unmanned” production. For example, after adopting an intelligent roll-to-roll forming system, one company increased its product qualification rate from 85% to 98%, and its production efficiency increased by 30%.

Efficiency: Developing high-speed winding technology has increased winding speeds to 800-1000 m/min. Simultaneously, adopting a large package design has increased roll weight from 5 kg to over 10 kg, reducing roll changes and improving production continuity.

Green: Developing low-temperature plasma singeing technology to replace traditional flame singeing reduces energy consumption by over 40% and eliminates waste gas emissions. Using a new cooling medium reduces water consumption, achieving an environmentally friendly process.

(II) Enterprise Selection Recommendations
For hot-melt yarn manufacturers, selecting appropriate roll-to-roll forming equipment and process solutions requires comprehensive consideration of three key factors: product positioning, production scale, and downstream demand. Product Positioning: For high-end hot-melt yarn production (such as for medical and automotive applications), select roll-to-roll forming equipment with high-precision temperature control, closed-loop tension control, and intelligent detection capabilities to ensure stable product performance. For mid- to low-end products (such as interlinings for general clothing), choose cost-effective equipment and focus on controlling core parameters.

Production Scale: Large-scale manufacturers should choose continuous, highly automated production lines equipped with multiple winders and centralized control systems to achieve large-scale production. Small and medium-sized enterprises can choose modular equipment to flexibly adjust production capacity based on order requirements.

Downstream Demand: Optimize process parameters based on the needs of different downstream sectors. For example, hot-melt yarns for nonwovens require focused control of thermal shrinkage and bond strength, using high setting temperatures and a thorough singeing process. Hot-melt yarns for knitwear require emphasis on elasticity and softness, and should be processed in a controlled manner.Reduce the stretching ratio and increase the elongation at break.