Covered yarn = Core-spun yarn?
On the Example of Spandex Covered Yarn and Spandex Core-Spun Yarn:
Spandex Covered Yarn (SCY): Utilizing spandex filament as the core, this elastic yarn is formed by wrapping synthetic filaments or non-elastic short fibers around the spandex filament in a highly regular spiral fashion. In a tense (stretched) state, there may be spandex exposure.
Spandex Core-Spun Yarn: With spandex filament as the core, non-elastic short fibers are wrapped around it. Generally, there is minimal spandex exposure when stretched. Both types share a structural foundation of spandex filament as the “core,” surrounded by other fibers. However, referring to them interchangeably as “covered yarn” oversimplifies the distinction due to differences in outer materials and covering processes.
From a structural perspective:
- Core-Spun Yarn is also known as Covered Yarn/Composite Yarn (✔)
From the angle of covering processes:
- Core-Spun Yarn ≠ Covered Yarn (✔)
Initially developed with cotton fiber as the “shell” and polyester short fiber as the “core,” core-spun yarn has evolved into three main types:
- Short fiber covering short fiber.
- Short fiber covering filament.
- Filament covering filament.
Machine-Covered or Air-Covered?
I. Based on the number of coverings: Single Cover & Double Cover
- Single Cover (SC): A single layer of filament wrapped around the spandex core in a highly regular spiral. This yarn, commonly used for socks and weft-knitted elastic fabrics, may exhibit spandex exposure in tense conditions.
- Double Cover (DC): Two layers of filament spirally wrapped around the spandex core, typically in opposite directions. This creates a highly covered structure, minimizing spandex exposure even in tense states. Known for good elastic balance, it often doesn’t require additional sizing treatment before subsequent processes. Primarily used for legwear, elastic bands, compression stockings, and similar fabrics.
II. Based on the covering equipment: Spandex Cover & Air Cover
- Spandex-Covered Yarn (SCY): Filaments are continuously rotated and wound around the steadily stretched spandex core on a covering machine, resulting in a yarn with “twist” (Twists Per Meter – TPM). The fabric is characterized by smoothness, stiffness, and abrasion resistance.
- Air-Covered Yarn (ACY): Both the outer filament and the spandex core are simultaneously drawn through a specific nozzle, subject to high-pressure compressed air, forming a rhythmic network of yarn with periodic points. Its main characteristic is a soft and smooth feel.
Air-Covered and Machine-Covered yarns have their advantages and disadvantages in downstream weaving. While Air-Covered yarns may require sizing during warp sizing on jet looms to prevent fuzziness and breakage, they have significantly higher production capacity and lower cost compared to Machine-Covered yarns.
III. Important Process Parameters for Covered Yarn
The primary physical indicators for evaluating the quality of spandex-covered yarn include five aspects: twist (for machine-covered yarn), breaking elongation, evenness of strands, creep performance, and tensile strength.
During spinning, a crucial process parameter affecting these quality indicators is the pre-draw ratio of spandex (Spandex Stretch). An inadequate pre-draw ratio limits the elasticity of the covered yarn, while excessive pre-draw may lead to yarn breakage, reducing product quality and increasing wastage, ultimately lowering production efficiency. The spandex content also influences the elasticity of the covered yarn, with higher spandex content resulting in better elasticity.
The tensile strength of air-covered yarn increases with the increase of spandex pre-draw ratio. Due to the inherent resilience of spandex, the outer fibers of the yarn curl when the yarn is relaxed, and with higher pre-draw, this curling effect increases. This enhances the stretchability of the core yarn, leading to increased breaking elongation and obtaining greater elasticity.
However, excessive pre-draw may bring the spandex filament’s deformation close to its critical value, causing a decrease in the tensile strength of the covered yarn. Still, it can benefit the evenness of the yarn strands. Therefore, the choice of pre-draw ratio should consider the stretch characteristics of different specifications of spandex filaments. Conventional covered yarns achieve the best anti-creep performance when spandex is pre-drawn 3.5 times.
In specific applications, adjustments should be made according to the style requirements of downstream fabrics.
Twist is essential, as higher twist enhances the cohesion between outer fibers and the spandex core, resulting in higher yarn strength. However, excessive twist can lead to a stiff fabric with poor drape. Therefore, the twist of machine-covered yarn generally adjusts according to the yarn count/Denier (unit for yarn thickness) and the style requirements of the fabric.
- Twist: The number of turns that the outer fiber filament makes around the spandex core per unit length. One turn is one twist. For example, if 1 meter is wound 1000 times, it is called 1000 twists.
- Breaking Elongation: The elongation ratio when the yarn is stretched to break, expressed as a percentage.
- Evenness of Strands: Refers to the uniformity of thickness along the length of the yarn.
- Creep Performance: The phenomenon of deformation over time under constant tensile force.
In conclusion, the article explores the distinctions between Spandex Covered Yarn (SCY) and Spandex Core-Spun Yarn, shedding light on their structural differences, covering processes, and various applications. It emphasizes that while both can be seen as having a spandex core enveloped by other fibers, a careful examination reveals nuanced disparities in the materials used for covering and the covering techniques.
The discussion extends to the evolution of core-spun yarn, highlighting its initial development with cotton and polyester fibers and its current classification into three main types. The article then delves into the intricacies of single and double covering, emphasizing their respective uses and characteristics.
Furthermore, the piece navigates the realms of machine-covered yarn (SCY) and air-covered yarn (ACY), unraveling their unique features and applications in downstream weaving. The advantages and disadvantages of each, particularly the considerations for warp sizing in air-covered yarn, are thoughtfully presented.
The article concludes by elucidating critical process parameters for evaluating the quality of covered yarn, touching upon aspects such as twist, breaking elongation, evenness of strands, creep performance, and tensile strength. It underscores the importance of the pre-draw ratio of spandex in determining yarn quality and the necessity of considering different spandex specifications for optimal performance.
Overall, this comprehensive exploration provides valuable insights for those in the textile industry, offering a nuanced understanding of spandex-covered yarns and their applications. It serves as a guide for decision-making in selecting the appropriate yarn type based on specific fabric requirements and production considerations.