Challenges spinners face during core-spun yarns production

Spinners face dual challenge in commercial core-spun yarn production: managing complex spinning geometry and maintaining consistent quality while dealing with the inherent differences in the core and sheath materials. The primary obstacles are quality defects that disrupt downstream processing and necessitate reduced production speeds.

Quality issues

The main defects encountered in high-volume production are related to the failure of the staple fibers to completely and permanently cover the core filament:

1. Grin-through (incomplete core coverage): This defect is a result of a failure of aesthetics and coverage, where the core filament (like spandex or polyester) is visible through the staple fiber sheath.

Improper positioning: Inaccurate alignment of the core filament relative to the roving, often forming a large 'V' under the front drafting roller nip often compromises the desired look of the yarn and fabric (e.g., exposed elastane), often making the product commercially unacceptable.

Insufficient tension: Core filament pre-tension is too low, causing it to wander off the center axis. This often leads to inconsistent yarn properties and poor coverage.

2. Strip-back (Barberpole effect): Also known as ‘Barberpole,’ this is a critical functional defect where the sheath fibers slip along the core filament, leaving a bare section of the core with a clump of fiber at one end.

Low cohesion/twist: Insufficient twist is applied, failing to build the necessary cohesion to bind the sheath fibers firmly to the smooth filament core. This results in severe end-breaks during subsequent processes like warping and weaving/knitting, drastically lowering machine efficiency and increasing costs.

Abrasion: The yarn is excessively rubbed by machine parts during further processing leading to sheath fibers being easily pulled away from the core filament.

3. Core/sheath voids: These are sections of yarn with either a missing core or a missing sheath.

Core voids: (Short or Long sections with no core):  The core filament is broken due to over-drafting, nicked guide surfaces, or mishandling of the core supply tube.

Sheath voids: (Sections with exposed core): There is a break in the staple fiber roving during drafting, or the roving is over-drafted.

How to ease these challenges

To deal with the above challenges, spinners can employ specific mechanical, process, and raw material strategies to ensure quality and optimize production efficiency:

Process and machine adjustments

Precise core alignment: For grin-through and voids ensure the core filament is positively and selectively positioned with respect to the roving as it enters the front roller nip, maintaining perfect alignment throughout spinning.

Optimal pre-tension: For the grin-through process apply a specific level of pre-tension (e.g., 100 g for some nylon cores) to the filament before it enters the drafting zone. This keeps the core centered and aids in axial orientation for better strength.

Optimize twist level: While higher twist reduces production speed, a carefully selected, relatively high twist level is necessary to ensure the staple fibers are sufficiently wrapped to build cohesion and prevent slippage.

Use heavier traveler: For increasing the speed of spinning, use a heavier traveler than normally required for an equivalent count of conventional yarn. This increases tension, helping to prevent contraction of the core and bulking the cover yarn uniformly.

Manufacturing techniques

• Doubling (Plying): For applications like sewing thread, twisting two or more single core-spun yarns together improves the binding of the sheath fibers, significantly minimizing the strip-back problem (Source 1.2).
• Heat setting (For elastic yarns): For yarns containing spandex/elastane, heat setting (steaming) must be performed after spinning. This stabilizes the yarn, ‘sets’ the twist, and equalizes the elongation properties of the different components, preventing snarling and improving dimensional stability.
• Spinning system selection: Using the Siro spinning system has been shown to produce yarns with better overall quality, strength, and elongation properties compared to conventional ring spinning for core-spun yarns (Source 1.1).

Quality control and maintenance

• Raw material quality: Only use uniform, high-quality roving and carefully handle continuous filament tubes to prevent damage to the extremely fine core yarns.
• Regular maintenance: Strictly maintain and replace worn-out parts like cots, aprons, rings, travelers, and drafting rollers. Poor mechanical condition is a direct cause of defects and yarn breaks.
• Draft control: Keep the main spinning draft below a certain limit (e.g., below 30) for elastic core-spun yarns to maintain quality.