Vortex-Spun Slub Yarn: Air-Jet Process, Test Data and Fabric Uses

Vortex-spun slub yarn uses an irregular filament as its core. Staple fibers wrap around it inside the vortex unit, creating controlled thick and thin sections. In the reported trial, an Orient nozzle ran at 0.55 MPa and 400 m/min. The team formed the slubs in the filament before vortex spinning, so the frame did not need to slow down to make the effect.

When we review a textured-yarn brief, we start with the fabric and the machine. A slub that looks clear on a cone may disappear in a dense knit. In an open construction, the same profile can look too strong. The target structure, fiber blend, dye route, and amount of visible core help define a useful sample.

How slub yarn forms on ring and rotor machines

Spinners once treated thick places as yarn faults. Over time, they learned to control those sections and use them to add texture to fabric. The study describes about 50 years of development in ring-spun slub yarn, with faster progress after 1986 and wider growth from 1995 onward.

Conventional ring spinning, siro spinning, compact spinning, and compact-siro spinning can all make slub yarn. In a common route, operators change the back-roller speed. The rollers feed more or less fiber into the drafting zone, so the yarn alternates between thicker and thinner sections. Spinners can set the base count, average count, slub length, spacing, and thickness to suit the fabric design.

Rotor spinning offers two more methods. One changes the feed and delivery speeds to adjust the drafting ratio. The other places damping elements of different sizes in the rotor groove. As fibers gather, those elements disturb the bundle and help form thick sections. Mills can choose either method or combine them, depending on the desired fabric effect.

Why air-jet vortex spinning needs a different route

Air-jet vortex spinning wraps fibers around a core with high-speed air. The process supports high output, digital control, and fewer operators. However, operators have found it difficult to change staple-fiber feed quickly enough to form controlled slubs directly in the vortex frame.

Slowing the frame can make the yarn uneven, but it reduces output and may add strain to the equipment. Another option is to spin a separate fine slub core on ring equipment, then feed that yarn into the vortex frame. The reported high-count core range of 2.95–11.81 tex can add a costly step before the final yarn is made.

The study cites 420,000 air-jet vortex spindles in China as of October 2024. This is a dated industry figure. The development described here focuses on a narrower production question: how to feed an uneven core through a high-speed vortex unit while covering both its thick and thin sections.

For a broader comparison of the spinning routes, see our vortex yarn vs. ring yarn guide.

Making the filament core for vortex-spun slub yarn

Preparing the staple-fiber cover

The outer layer starts with staple-fiber preparation. Operators select and open the fibers, remove trash, and card them into sliver. Moisture or oil can soften the stock and improve spinnability. The sliver then passes through three drawing passages, which blend and align the fibers before vortex spinning.

At the vortex frame, the distance between the front-roller nip and the semicircular opening in the spindle holder needs to match the core filament. The study links this setting to filament linear density, slub thickness ratio, and slub spacing. With a suitable distance, staple fibers can wrap the core through both thick and thin sections. A setting that works for one filament profile may not suit another.

The process combines two-stage conditioning with moisture and a penetrating, softening, antistatic oil. Fast fiber transfer, oiling at the vortex stage, and an adjusted feed ratio help improve wrapping and reduce neps or weakly twisted sections. The researchers also attribute lower friction against the vortex tube wall, less static, and cleaner equipment surfaces to the second oil treatment. They do not quantify those changes.

During the reported run, the WOS system monitored defects over 100,000 m of yarn. Operators changed the harmful-defect classification from the nine-grade setting A3 + B3 × C3 + D2 to the five-grade setting D2 + F + G. These codes belong to that setup. Mills should set defect limits against their own machine, yarn construction, and buyer specification.

Core-filament content and yarn-count planning

Table 1 relates slub-filament linear density to the minimum linear density of the finished core-spun yarn. The calculation accounts for 1.8%–2.2% filament elongation during vortex spinning.

Slub-filament linear density (tex)	Yarn linear density (tex) at maximum core-filament content
	≤32%	20%	16%
2.22	≥7.40	≥11.10	≥14.80
3.33	≥11.10	≥16.65	≥22.20
4.44	≥14.80	≥22.20	≥29.60
5.56	≥18.50	≥27.80	≥37.10
7.78	≥25.90	≥38.90	≥51.90
10.00	≥33.30	≥50.00	≥66.67
11.11	≥37.03	≥55.55	≥74.10
13.33	≥44.40	≥66.65	≥88.87
14.44	≥48.10	≥72.20	≥96.27
16.67	≥55.57	≥83.35	≥111.10
18.89	≥62.96	≥94.45	≥125.93
22.22	≥74.07	≥111.10	≥148.13

The first and third column labels need confirmation before mills use this table to set a core ratio. For example, 2.22 tex divided by 7.40 tex equals 30%, while 2.22 tex divided by 14.80 tex equals 15%; the headings read ≤32% and 16%. The trial yarn below represents a separate design case: 111.1 dtex equals 11.11 tex, or about 49% of a 22.7 tex yarn before the elongation correction. Confirm the calculation basis with the research team.

Reported trial settings and yarn results

The trial yarn had a linear density of 22.7 tex. The sheath used viscose staple fiber at 1.33 dtex, and the core used FDY slub filament at 111.1 dtex. Table 2 lists the machine settings from this run.

Parameter	Setting	Parameter	Setting
Nozzle model	Orient	Nozzle pressure (MPa)	0.55
Spinning speed (m/min)	400	Total draft ratio	396
Back-zone draft ratio	3.0	Feed ratio	0.99
Winding ratio	1.01	BR starting ratio (%)	190
Friction-roller pulling force (mN)	120	Roller gauge (mm)	43 × 45
Yarn-chain setting	1.4	Top-roller hardness	73

Table 3 gives the reported yarn measurements. The 100% pass rate applies to the slub-specification assessment in this trial.

Item	Linear-density deviation (%)	Linear-density CV (%)	Single-yarn tenacity (cN/tex)	Breaking-strength CV (%)	Fiber-content deviation (%)	Slub-specification pass rate (%)
Slub yarn	1.0	0.5	17.6	11.3	+1.4 (viscose); −1.4 (filament)	100

The researchers do not list the sample size or full strength-test conditions. Use these measurements as trial results, not bulk acceptance limits. Before comparing suppliers or approving a production lot, agree on the sampling plan, conditioning, test method, and buyer tolerances.

What controlled core exposure changes

The process can leave selected parts of the filament visible. The research team uses this controlled incomplete covering to create a distinct fabric surface. A buyer brief should define how much core may show, along with slub length, spacing, and thickness. An approved fabric sample gives the mill a clearer reference than a cone alone.

Core exposure also affects hand feel and color. The core and sheath may respond differently to dye, while a larger exposed area makes the filament effect easier to see. Too much exposure can change surface uniformity, however. We compare the effect in the intended fabric structure before settling a specification.

Fabric uses for this yarn

Woven fabric

Woven options include twill, slub cloth, linen-look fabric, and denim-style constructions. Single dyeing can create a surface similar to sand-washed denim. The researchers also describe their fabric as more breathable and stronger than pure cotton denim; however, their published test figures do not show a side-by-side comparison. Compare fabrics of similar construction, weight, and finish before using those advantages as product claims.

Home and decorative textiles

Wide-width rapier looms can use the yarn in curtains and wall-covering base cloth. Mills can weave a pale solid fabric, then add color through single dyeing, double dyeing, or printing. For upholstery or curtains, the sample should also match the expected cleaning method and dimensional requirements.

What is slub yarn used for?

Slub yarn adds a deliberate thick-and-thin texture to fabric. This core-spun construction suits knitted tops, rib and jacquard fabrics, woven twill, linen-look cloth, denim-style fabric, curtains, and decorative base cloth. The right slub size depends on the construction: a fine effect may suit a dense knit, while a larger effect can read better across a wider woven surface.