Yarn Inspection

Yarn inspection mainly conducts scientific testing and analysis on yarn fineness, yarn strength and elongation, evenness, twist, moisture regain and moisture content, and hairiness, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Yarn fineness: Yarn fineness is the core indicator for measuring the thickness degree of the yarn, usually expressed by the linear density (tex) or the count (English count Ne and metric count Nm). The inspection report needs to clearly indicate the adopted measurement system, and record the measured average value, unevenness (CV% value) and deviation rate, to evaluate the uniformity of the yarn and whether it meets the design requirements.

Yarn strength and elongation: This testing item includes the breaking strength, elongation at break and their unevenness. A strength testing machine is used for the tensile test under standard conditions, and the report needs to provide the breaking strength (unit: cN or N) and the elongation at break (unit: %). These data are directly related to the end-breakage rate of the yarn during the weaving process and the durability of the final fabric.

Evenness (yarn faults): The thickness variation and various faults of the yarn along the length direction are detected through a capacitive or photoelectric evenness tester (such as the USTER® TESTER). The report will provide the unevenness (CVm%), the number of frequent faults (thin places, thick places and neps) and the yarn fault grading chart, which is the key basis for evaluating the appearance quality and weavability of the yarn.

Twist: The twist refers to the number of turns per unit length of the yarn, divided into the twist (twists per metre or twists per inch) and the twist factor. During the inspection, the twist direction (S twist or Z twist) needs to be distinguished. The twist indicator in the report affects the strength, handle, lustre and subsequent processing performance of the yarn, and must match the product specification and the final use.

Moisture regain and moisture content: The percentage of the weight of the moisture contained in the yarn relative to its dry weight is called the moisture regain, and the percentage relative to the wet weight is called the moisture content. It is determined after equilibration under standard temperature and humidity conditions (such as 20±2 ℃ and 65±4% RH). This indicator affects the weight, strength, conductivity and the conditioned weight for trade settlement of the yarn.

Hairiness: The hairiness refers to the fibre ends or loops protruding from the main body of the yarn. A hairiness tester (such as the USTER® HAIRINESS TESTER) is used to measure the total hairiness length per unit length of the yarn or the hairiness index (H value) of a specific length (such as ≥3 mm). The hairiness condition affects the definition, handle and pilling of the fabric.

Testing Scope

Classification by raw material: The yarn inspection report is applicable to the yarns made of various fibre raw materials, including natural fibre yarns (such as cotton yarn, wool yarn, flax yarn and spun silk), chemical fibre yarns (such as polyester yarn, viscose yarn and nylon yarn) and various blended yarns (such as polyester-cotton yarn and wool-polyester yarn). The testing emphasis and standards of the yarns of different raw materials may be different.

Classification by yarn structure: The testing scope covers spun yarns (ring-spun yarn, rotor-spun yarn, vortex-spun yarn, etc.), filament yarns (monofilament, multifilament and textured yarn) and special yarns (such as fancy yarn, core-spun yarn and plied yarn). The report needs to test the corresponding items according to their structural characteristics, for example, the plied yarn needs to additionally test the plying twist.

Classification by processing procedure: The inspection report covers the yarn quality control in each link from spinning to before weaving, including the self-inspection of the spinning mill, the ex-factory inspection, and the incoming acceptance inspection of the downstream enterprises such as weaving, knitting and trade. It is an important document for the handover and quality judgement of each link in the supply chain.

Classification by product use: According to the final use of the yarn, the strictness and items of the inspection may be adjusted. For example, the yarns used for high-grade shirt fabrics have extremely high requirements for the evenness and hairiness; the yarns used for denim pay more attention to the strength and abrasion resistance; and the yarns used for knitted underwear emphasize the softness and nep control.

According to the trade contract requirements: As a bulk trade commodity, the inspection report of the yarn is the basis for the settlement and claim of both the buyer and the seller. The testing scope must strictly follow the standards (such as GB, ISO, ASTM and AATCC) and the specific items specified in the trade contract, letter of credit or the technical agreement agreed by both parties.

Testing Methods

Sampling and specimen preparation: Representative sampling is carried out in accordance with the relevant product standards (such as GB/T 398 and FZ/T 10007). The samples need to be fully conditioned under standard atmospheric conditions (temperature 20±2 ℃ and relative humidity 65±4%) (usually not less than 24 hours), to ensure the accuracy and comparability of the test results.

Fineness testing method: For the linear density (tex), the skein method is usually adopted, i.e., measuring the dry weight of the yarn of a specified length (such as 100 metres) for calculation. The count is converted by measuring the length of the yarn of a certain weight. The electronic evenness tester can also indirectly provide the fineness data.

Strength and elongation testing method: Mainly following the standard GB/T 3916 (identical to ISO 2062) "Textiles — Yarn from packages — Determination of single-end breaking force and elongation at break". A constant-rate-of-extension (CRE) strength testing machine is adopted, the appropriate clamping length, tensile speed and pretension are set, and a single yarn is stretched until breakage.

Evenness and yarn fault testing method: The capacitive test method is generally adopted, and the standard is GB/T 3292.1 (identical to ISO 16549) "Textiles — Method for yarn evenness testing — Part 1: Capacitance method". The instrument tests a certain length of yarn at a specific speed, reflects the mass variation through the capacitance change, and automatically grades and counts the yarn faults.

Twist testing method: The common methods include the direct counting method and the untwist-retwist method. The direct counting method (GB/T 2543.1) is applicable to most yarns, manually or automatically counting the number of turns within a certain length. The untwist-retwist method (GB/T 2543.2) indirectly calculates through untwisting and retwisting to the original length, and is applicable to the yarns with low twist or easy elongation.

Moisture regain testing method: The standard method is the oven method (GB/T 9995). The specimen is placed in an oven at 105-110 ℃ and dried to constant weight, and the moisture regain is calculated through the weight difference before and after drying. The resistance-type or infrared moisture meter can be used for rapid determination, but in case of a dispute over the results, the oven method prevails.

Testing Standards

Domestic Standards:

1. GB/T 398, Cotton grey yarns

2. GB/T 3916, Textiles — Yarn from packages — Determination of single-end breaking force and elongation at break using constant rate of extension (CRE) tester

3. GB/T 3292.1, Textiles — Method for yarn evenness testing — Part 1: Capacitance method

4. GB/T 2543.1, Textiles — Determination of twist in yarns — Part 1: Direct counting method

5. GB/T 2543.2, Textiles — Determination of twist in yarns — Part 2: Untwist-retwist method

6. GB/T 9995, Determination of moisture content and moisture regain of textile materials — Oven-drying method

International Standards:

1. ISO 2062, Textiles — Yarns from packages — Determination of single-end breaking force and elongation at break using constant rate of extension (CRE) tester

2. ISO 16549, Textiles — Method for yarn evenness testing — Capacitance method

3. ISO 2060, Textiles — Yarn from packages — Determination of linear density (mass per unit length) by the skein method

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Electronic single-yarn strength tester: Used for the testing of the breaking strength and elongation at break of the yarn. Modern equipment such as the USTER® UTENSO or the similar domestic instruments adopts high-precision sensors and digital control systems, and can automatically complete the testing, calculate the average value and CV value, and output a complete statistical report and curve diagram.

Yarn evenness tester: Represented by the Swiss USTER® TESTER series instruments, it is the benchmark equipment of the industry. It can test the thickness variation (CVm%) of the yarn at high speed and continuously, precisely detect and grade the various frequent yarn faults, and provide the spectrogram and the variation-length curve for analysing the causes of the unevenness.

Yarn twist tester: Divided into the manual and fully automatic types. The fully automatic twist tester (such as the Zweigle and the domestic YG155 type) can automatically complete the clamping, pretension application, untwisting/twisting, length measurement and result calculation, greatly improving the test efficiency and accuracy and reducing the human errors.

Yarn hairiness tester: Through the photoelectric detection principle, the number and degree of the lateral hairiness blocking the light when the yarn passes through the detection area are measured, thereby calculating the hairiness index (H value) and the hairiness standard deviation Sh. The advanced equipment can also provide the distribution spectrum of the hairiness length.

Standard light source cabinet and yarn blackboard machine: Used for the visual assessment of the appearance quality of the yarn. The yarn blackboard machine (such as the YG381 type) evenly winds the yarn on a special blackboard. Under a standard light source (D65 daylight), experienced inspectors rate the evenness grade, nep and impurity grades by comparison with the standard photos or by experience.

Constant temperature and humidity laboratory: All the physical property tests of the fibres and yarns must be carried out in a constant temperature and humidity laboratory. The environmental control system needs to ensure the stability of the temperature (20±2 ℃) and relative humidity (65±4%), which is the most basic and critical condition for ensuring the comparability of the test data between different batches and different laboratories.