Nylon Hot Melt Fiber Production Process

Nylon Hot Melt Fiber Production Process

Nylon hot melt fiber has become an indispensable key material in various fields, including textiles, filtration, and medical applications, thanks to its excellent bonding properties, chemical resistance, and mechanical strength. However, the production process of seemingly slender nylon hot melt fiber embodies the collaborative innovation of polymer materials science, mechanical engineering, and precision control technology. This article will provide an in-depth analysis of the complete nylon hot melt fiber production process, from raw material screening to finished product testing, revealing the technical key points and quality control logic of each step.

1. Raw Material Selection: The Core Foundation for Product Performance

The performance of nylon hot melt fiber depends primarily on the quality of the raw materials and the design of the formulation. Unlike ordinary nylon fibers, hot melt fiber must melt at a specific temperature to produce a bonding effect. Therefore, the raw material selection must meet the dual requirements of “melting controllability” and “base material stability.”

1. Base Resin Selection Criteria

Mainstream nylon hot-melt yarn raw materials are primarily nylon 6 (PA6) and nylon 66 (PA66). These two materials exhibit distinct properties due to their different molecular structures:

Nylon 6: With a melting point of approximately 215-225°C, it offers good fluidity, moderate bonding strength after melting, and relatively low cost. It is suitable for bonding medium- and low-temperature applications, such as clothing interlinings and non-woven fabrics.

Nylon 66: With a melting point of approximately 255-265°C, it offers superior heat resistance and improved washability and dry-cleaning resistance after bonding. It is commonly used in applications requiring high stability, such as automotive interiors and filter materials.

In actual production, resin chips with a corresponding viscosity should be selected based on customer requirements for “melting temperature,” “bonding strength,” and “aging resistance.” Typically, chips with a relative viscosity of 2.4-3.0 are used to ensure fluidity after melting while preventing yarn breakage and lint during the spinning process.

2. Scientifically Formulated Functional Additives

To optimize the performance of hot-melt filaments, precisely proportioned additives must be added to the resin. Common combinations include:

Plasticizers (such as dioctyl adipate): Lower the melting point, adjust the melting rate, and ensure more uniform bonding;

Antioxidants (such as 1010): Delay oxidative degradation during high-temperature processing, improving the weatherability of the finished product;

Nucleating agents (such as talc): Refine the crystal particles and improve the mechanical strength and melt consistency of the hot-melt filaments.

The total additive dosage is typically controlled at 1%-3%, and they must be thoroughly dispersed using a high-speed mixer to avoid spinning defects caused by localized concentration variations.
2. Melt Spinning: The Critical Transformation from Chips to Yarns

Melt spinning is the core step in nylon hot-melt filament formation. Essentially, it converts solid resin into continuous yarns with a specific cross-section and fineness. The entire process requires a carefully controlled and coordinated combination of temperature, pressure, and speed.

1. Screw Extrusion: Raw Material Melting and Conveying

Resin chips first enter the screw extruder, where they are gradually melted through a three-stage temperature gradient:

Feeding section (200-220°C): Chips are preheated and softened to prevent clumping and blockage;

Compression section (230-250°C): Chips are extruded to increase density, gradually melting to form a melt;

Metering section (250-260°C): The melt is fully homogenized, and the output rate is precisely controlled by the screw speed (typically 10-30 kg/h).

The screw’s length-to-diameter (L/D) ratio is generally selected between 28 and 32:1 to ensure uniform melt mixing. The barrel must also be equipped with a precise temperature control system to keep temperature fluctuations within ±1°C to prevent local overheating that could lead to resin degradation.

2. Spinning Pack: Melt Filtration and Spinning

The homogenized melt is transported to the spinning pack via a pipeline. This process has two core functions:

Filtration and Purification: The melt passes through 80-mesh, 120-mesh, and 200-mesh metal filters to remove impurities and gel particles and prevent spinneret clogging;

Distribution and Spinning: The melt is evenly distributed to the spinneret via a distribution plate, where it is extruded through the spinneret holes to form strands. The number of holes in the spinneret is adjusted according to the final fiber fineness (e.g., 48 holes, 64 holes). The hole diameter is typically 0.2-0.4mm, and the hole shape is mostly circular (customized holes can be used for special applications).

The pressure in the spinning pack must be stable at 8-12MPa. Excessive pressure fluctuations can lead to uneven strand thickness. Therefore, an online pressure monitor is required to provide real-time feedback and adjust the screw speed.

3. Cooling and Forming: Curing and Setting the Yarn

The hot yarn (approximately 250°C) extruded from the spinneret must be immediately cooled and solidified. This is typically done using side-blown cooling:

The cooling air temperature is controlled at 20-25°C, with a relative humidity of 60%-70%.

The wind speed is 0.3-0.5 m/s and must be uniform and stable to prevent yarn fluttering, causing twisting or breakage.

The cooling distance (from the spinneret to the wind window) is generally 30-50 cm to ensure that the yarn is fully solidified but not overcooled, preparing for subsequent drawing.
3. Drawing and Setting: Imparting Mechanical Properties and Hot-melt Stability to the Yarn

Newly cooled nascent yarn (also known as “undrawn yarn”) has low crystallinity and poor strength. Drawing and setting is necessary to adjust the molecular arrangement, improve mechanical properties, and stabilize hot-melt properties.

1. Multi-Stage Stretching: Molecular Chain Orientation and Strengthening
The stretching process is typically divided into two stages: pre-stretching and main stretching, achieved by differential speed between the front and rear rollers:

Pre-stretching: Performed at room temperature, with a stretching ratio of 1.2-1.5x. Its primary function is to eliminate internal stress in the yarn and prevent subsequent breakage during stretching.

Main stretching: Performed on heated rollers (80-100°C), with a stretching ratio of 3-5x. This aligns the nylon molecular chains along the yarn axis, significantly improving strength and modulus.

The stretching ratio must be precisely controlled based on the final product requirements: too low a stretching ratio will result in insufficient yarn strength; too high a stretching ratio can easily lead to brittle fracture and uneven shrinkage during hot melt. Typically, nylon hot melt yarns used for bonding require a breaking strength of 3.5-4.5 cN/dtex, with an elongation at break controlled at 20%-30%.

2. Heat Setting: Stabilizing and Optimizing Performance

After stretching, the yarn undergoes heat setting. The goals are:

Eliminate internal stress generated during the stretching process and reduce shrinkage during subsequent processing (typically, shrinkage is controlled within 3%);

Adjust the degree of crystallinity (controlled within 30%-40%) to ensure melt consistency during hot melting;

Improve the yarn’s surface smoothness and reduce friction damage during subsequent weaving or processing.

Heat setting is typically performed on setting rollers or setting boxes at a temperature of 120-140°C for 1-3 seconds. A setting tension of 0.5-1 cN/dtex is applied to prevent the yarn from loosening.
4. Winding and Post-Processing: Forming and Optimizing the Finished Product

The stretched and set yarn is then wound into a finished package. In some cases, post-processing may be required to meet specific application requirements.

1. Winding: Controlling Package Quality

During the winding process, the following parameters must be carefully controlled:

Winding speed: This should match the drawing speed, typically 800-1200 m/min. Excessive speeds can cause the yarn to heat up, affecting heat-melting performance.

Winding tension: This should be maintained at a constant 0.2-0.3 cN/dtex to ensure a tight and uniform package and prevent interlayer slippage.

Packaging specifications: These can be customized based on customer needs. Common roll weights are 5 kg and 10 kg, and the roll diameter should not exceed 300 mm to facilitate subsequent use.

The winding machine should be equipped with automatic wire breakage detection and a tension control system. If a wire breakage or abnormal tension occurs, the machine will immediately shut down and issue an alarm to minimize scrap.

2. Post-Processing: Customized Functional Upgrades
Depending on the application scenario, nylon hot-melt yarn may require the following post-processing:
Fiber Splitting: Splitting multifilament yarn into monofilaments or finer multifilament yarns, suitable for precision filtration materials;
Oil Coating: Applying antistatic oil or lubricant to improve the yarn’s antistatic properties and weavability;
Dyeing Pretreatment: Pre-expanding the hot-melt yarn to improve dyeing uniformity (care must be taken to avoid affecting the melting temperature).

V. Quality Inspection: Full-Process Performance Control

The production process of nylon hot-melt yarn undergoes rigorous quality inspections to ensure that each batch meets standard requirements. Key inspection items include:

1. Physical Property Testing
Fiber Density Deviation: Measured using an electronic denier, the deviation must be controlled within ±2%;
Breaking Strength and Elongation: Tested using a strength testing machine to ensure compliance with design standards;
Heat Shrinkage: After immersion in 100°C hot water for 30 minutes, the shrinkage must be ≤3%.

2. Hot Melt Performance Testing

Melting Temperature: Determined using a Differential Scanning Calorimeter (DSC). The melting point deviation must be ≤5°C.

Adhesion Strength: After laminating the hot melt filament to a substrate (e.g., cotton or non-woven fabric), the peel strength is tested. Typically, it must be ≥5N/2.5cm.

Melt Flow: Tested using a Melt Flow Rate (MFR) meter to ensure uniform wetting of the substrate after melting.

3. Appearance Inspection

Use visual inspection or automated appearance inspection equipment to check for defects such as lint, broken threads, oil stains, and foreign matter.

Inspect the package for flatness and tightness to avoid issues such as edge collapse and cobwebs.

VI. Process Difficulties and Technological Breakthroughs

The production of nylon hot-melt yarn is not an easy task, requiring overcoming multiple technical challenges. Major breakthroughs in the industry in recent years include:
Precise Melt Temperature Control: By improving the screw structure and temperature control system, melt temperature fluctuations are controlled to ±0.5°C, solving the problem of varying bond strength caused by temperature unevenness;
Fine Denier Production Technology: The development of ultra-fine spinnerets (apertures below 0.15mm) and a low-tension drawing process enables the stable production of fine denier nylon hot-melt yarn below 20 dtex, meeting the needs of high-end medical dressings;
Functional Modification: Through nanocomposite modification technology, highly heat-resistant (melting point ≥280°C) and flame-retardant nylon hot-melt yarn have been developed, expanding its application in high-temperature applications such as automotive and aviation.

Conclusion: Process Innovation Drives Application Upgrades
The production process of nylon hot-melt yarn is a systematic project of “precision control + continuous optimization.” From raw material formulation to finished product testing, subtle adjustments at every step can affect the final performance. As the requirements for material performance in the textile, automotive, and medical industries continue to increase, the production process of nylon hot-melt yarn will also develop in the direction of “finer denier, more functional, and more stable”.