For the twisting of filament thread (typically sewing thread or embroidery thread made from high strength polyester or nylon 66 filaments), the uniformity of strength and elongation are two additional core technical indicators. Together with twist uniformity, they form the fundamental quality characteristics of filament twisted threads.
Part 1: Detailed Explanation of Strength and Elongation Uniformity Indicators
Polyester Filament Sewing Thread Strength & Elongation Test
I. Strength Uniformity Indicators
1. Average Strength and Coefficient of Variation (CV%)
● Average Strength: The arithmetic mean of the breaking strength of all tested samples, representing the overall strength level of the thread. farerthread.com
● Strength Irregularity / Strength CV%: This is the most critical evenness indicator. It uses statistical methods (typically calculating the percentage of the standard deviation to the mean, i.e., CV%) to express the dispersion of strength values of individual test samples relative to the average.
● A lower CV% value indicates that the strength of each thread segment is more consistent, signifying better evenness.
● Minimum Strength: The lowest value among all tested samples. This value directly determines the "weakest link" of the sewing thread during high-speed sewing and poses a more immediate threat than the average strength.
2. Breaking Tenacity and Tenacity Irregularity
● Breaking Tenacity: The strength normalized per unit fineness (e.g., denier, tex), facilitating comparison between threads of different specifications.
● Tenacity Irregularity: Similarly, the CV value of the breaking tenacity is calculated. This eliminates the influence of fineness differences, better reflecting the uniformity of the fiber material itself and the manufacturing process.
3. Concentration of the Strength-Elongation Curve Cluster
● On an electronic single yarn strength tester, testing dozens of samples yields a cluster of "stress-strain" curves.
● For thread with excellent evenness, these curves almost overlap into a single thick line, indicating highly consistent behavior for each thread segment throughout the entire process from initial loading, deformation, to rupture.
● For thread with poor evenness, the curves are scattered, varying in height, and may even differ in shape (e.g., some steep, some gentle).
II. Elongation Uniformity Indicators
1. Breaking Elongation and Elongation Irregularity
● Average Breaking Elongation: The mean value of the elongation percentage at which the samples break.
● Elongation Irregularity: The CV value of the breaking elongation. For sewing threads, controlling elongation evenness is equally important as controlling strength evenness because stitch formation relies on the elastic deformation and recovery of the thread.
2. Elongation at Specified Load and Its Evenness
● Definition: The elongation percentage produced when the thread is subjected to a standardized, fixed load that is less than the breaking strength. This simulates the state of the thread being moderately tensioned during the sewing process.
● Importance: The CV value of the elongation at specified load directly determines the stability of stitch tension. A high CV value means each thread segment is tensioned to a different degree, leading to inconsistent tightness in the stitches.
3. Evenness of Elastic Recovery Rate
● Refers to the percentage by which the thread can return to its original length after being stretched and then having the force released. Its evenness affects the smoothness and wrinkle resistance of the seam after it is formed.
Part 2: The Extreme Importance of Strength-Elongation Uniformity for High Strength Sewing Threads
High Tenacity Polyester Sewing Thread Strength & Elongation Test
For high strength filament sewing threads, their application scenarios (such as airbags, parachutes, outdoor equipment, car seats, leather goods) often impose stringent requirements on reliability. In these contexts, the importance of evenness is magnified to the utmost degree.
1. The Fundamental Guarantee for Realizing the Potential of "High Strength"
● Paradox: A thread with an average breaking strength of 10 kilograms, if it has poor evenness, might effectively only perform as well as a thread with good evenness and 8 kilograms strength. This is because failure always initiates at the weakest point.
● Significance: Evenness ensures that every inch of the thread can bear the load nearly equally, allowing the "high-strength" attribute to be truly manifested in the overall seam and avoiding overall failure due to a local weak spot.
2. The Cornerstone for Achieving Extreme High-Speed Sewing and Automated Production
● At ultra-high speeds of 5000 to 10000 stitches per minute, the sewing thread endures extremely complex, high-frequency impact forces, bending forces, and frictional forces. farerthread.com
● Poor Strength Evenness: Leads to random breakage at weak points during dynamic load peaks, causing disastrous frequent thread breaks that can paralyze automated production lines.
● Poor Elongation Evenness: Causes instability in hooking and thread take-up timing, directly resulting in defects like skipped stitches and poor stitch formation, making stable production impossible.
3. The Key Factor Determining the Dynamic Fatigue Life and Safety of Seams
● In many high-stress applications (e.g., safety belts, slings, athletic shoes), sewing threads endure long-term, repeated fatigue loads.
● Thread with Poor uniformity: During the fatigue process, stress concentration occurs first at the weak spots, accelerating wear and breakage, causing the seam's fatigue life to fall far short of design expectations.
● For Safety Products: Uniformity is a safeguard for safety redundancy. It means that under extreme conditions, the seam fails in a overall, gradual manner, rather than a sudden, unpredictable rupture.
4. Ensuring the Forming Quality of Complex Stitches and Special Seam Types
● In sewing multi-layer heavy materials or creating special decorative stitch types (e.g., zigzag stitches, embroidery), the sewing thread undergoes more complex paths and deformations.
● Only threads with highly uniform strength and elongation properties can ensure consistent loop sizes and identical stitch tension for every stitch, thereby forming complex seams that are aesthetically pleasing, strong, and functionally stable.
5. The Ultimate Reflection of a Manufacturer's Technical Prowess and Brand Value
● Producing "high-strength" thread is relatively easy, but producing thread that is both "high-strength and highly uniform" requires extreme control over the entire industrial chain:
Raw Materials: Must use high-quality industrial filaments with low batch-to-batch variation and high consistency.
Equipment: Requires precise twisting machines, stable tension control systems, and a constant temperature and humidity environment.
Process: Precise control over twisting and heat setting processes.
● Therefore, the level of strength-elongation uniformity is the core technical barrier distinguishing ordinary sewing thread manufacturers from top-tier industrial sewing thread manufacturers.
Summary
For high strength sewing threads, strength-elongation uniformity is not an added "advantage" but the essential "foundation" for their viability.
● Strength Evenness ensures "Reliable Non-Breaking" — making the high-strength characteristic a reality and avoiding unexpected breaks.
● Elongation Evenness ensures "Stable Formation" — making every stitch perfect and consistent.
Therefore, in the twisting of high strength filament sewing thread and embroidery thread, focusing on the strength-elongation uniformity of thread is a long-term and critically important task. The focus for twisting manufacturers cannot solely be on rapidly completing large-volume deliveries amidst order rotations; providing customers with reliable, consistently stable, and high-quality twisted thread products is an even greater core competency.
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