Evaluation of DTY Quality and Its Impact on Weaving, Along with Improvement
1. Strength and Elongation
For fibers, higher strength is not always better. For instance, damage to polyester fabrics is often not due to inadequate fiber strength. For low-elasticity polyester filaments, a strength of 2.5 CN/dtex and an elongation of 12% to 30% can meet the requirements for weaving and use. Its toughness, defined as (strength × elongation / 2), is better when it’s higher. Fibers with high toughness not only have better intrinsic quality but also offer better abrasion and bend resistance. In practical production, different weaving methods have varying requirements for toughness. For example, the strength of DTY is required to be lower on a weft knitting machine compared to a warp knitting machine. The elongation range for the weft yarn using DTY is broader, while the elongation for the warp yarn should not be too high.
In DTY production, the strength and elongation are generally controlled by adjusting the stretching multiple of the yarn. As the stretching multiple increases, the strength of the yarn correspondingly increases, while the elongation decreases. However, if the stretching multiple is too high, both the strength and the elongation of the yarn will decline.
2. Shrinkage Properties
Shrinkage properties include shrinkage rate (KE), curl stability (KB), and boiling water shrinkage rate. Generally, only the first two items or the boiling water shrinkage rate need to be measured. The shrinkage rate and curl stability can be calculated using the following formulas:
Lg: length under heavy load
Lz: length under light load
Lb: length after removing the heavy load and then applying the light load
A high shrinkage rate gives DTY a full hand feel, aesthetically pleasing appearance, and good elasticity. Curl stability indicates the degree of gradual loss of shrinkage during weaving and usage, which is inversely proportional to the shrinkage rate. Therefore, in actual production, both indicators must be considered.
DTY with a higher shrinkage rate can cover some dyeing unevenness, so it is often controlled to be slightly higher during production. However, if DTY undergoes package dyeing in post-processing, a high shrinkage rate leads to a large boiling water shrinkage rate, making the package difficult to unwind after dyeing. If DTY yarn is twisted or dyed after being woven into fabric, it is not restricted, allowing the shrinkage rate to be set higher.
The process factors affecting shrinkage properties include stretching multiple, winding tension, and the temperatures and differences in the first and second heating chambers.
3. Dyeing Uniformity
If the physical properties of DTY, such as strength, shrinkage rate, elongation, and unevenness of yarns, are high, it may lead to color differences and transverse striping after weaving. Therefore, DTY must be controlled during production, and timely inspection of finished products is necessary to avoid downgrading.
(A) Testing Dyeing Uniformity
Currently, the method used to determine the dyeing uniformity of DTY involves weaving a 5 cm length of sock tube from each DTY package, with a standard DTY dyed under normal conditions placed in between. The sections are dyed with disperse dyes and assessed under standard lighting. According to the GB 250-1995 standard, color matching is performed using gray cards, classified into five levels with nine grades. Any color difference exceeding level four compared to the standard is deemed first-grade quality. The most common issues in production are transverse striped yarns and transparent yarns. Transverse striped yarns refer to color differences within the same sample, while transparent yarns appear shiny on the fabric surface, feeling thin to the touch and appearing more transparent under light compared to normal yarns. Possible causes include:
Insufficient deformation during processing.
Insufficient false twist.
(B) Factors Affecting DTY Dyeing Uniformity
These can be categorized into the quality of DTY's raw materials and the processing conditions:
Variations in cooling airflow speed, temperature, and relative humidity.
Defects during winding and forming.
High unevenness in POY (Partially Oriented Yarn).
Insufficient time for stripping fibers after dropping.
Mechanical factors in DTY processing: including damage or poor operation of yarn guide devices, yarn paths, and false twist devices.
Poor uniformity in the melt and spinning temperatures.
Causes of stiff yarn and inadequate twisting resulting in bad shrinkage.
Formation of dark strands measuring 30–50m at the tail ends.
Poor uniformity in POY oiling.
Fluctuations in winding tension.
Oil absorption on the surfaces of POY tubes.
Improper DTY processing operations, such as yarns running out of their normal paths or empty winding of broken yarns.
Inappropriate selection of DTY processing conditions, including fluctuations in false twist tension, excessively high D/Y ratio, or inadequate heating chamber temperatures.
Improper inspections and subjective factors.
4. Hairy Fiber
The existence of hairy fibers can affect the subsequent textile processes and the quality of the fabric. Different weaving methods have varying requirements for hairy fibers:
Less Strict Requirements for Hairy Fibers:
(1) Low-elasticity yarns used in weaving machines may undergo false twisting, sizing, or network treatments before weaving.
(2) In knitting processes, weft yarns are less affected due to shorter and slower weaving procedures.
Strict Control Requirements for Hairy Fibers:
(1) Low-elasticity yarns used in water-jet weaving machines.
(2) Warp yarns, where the production speed is high, must pass through needle eyes, making the presence of hairy fibers prone to breakage.
The generation of hairy fibers is related to both DTY processing conditions and the quality of POY (Partially Oriented Yarn). Causes include:
Spinning temperatures that are too high or low, low pressure in spinning components, breaches in filter materials, improper cooling and molding conditions, inappropriate selection of POY oil agents, and abrasion of yarns by guiding devices during winding and forming.
POY with potential issues may not create breakage or hairy fibers during spinning and winding, but during processing to DTY, factors like heat and stretching can reveal weak points in the yarn, leading to breakage or hairy fibers.
Even high-quality POY can produce hairy fibers if proper conditions are not maintained during processing into DTY. The main causes of hairy fibers in the conversion from POY to DTY include:
(1) Incorrect selection of D/Y ratio and stretching multiples.
(2) Excessively high heating chamber temperatures.
(3) Over-abrasion of yarns by guiding devices.
5. Stiff Yarn and Tight Spots
Stiff yarn refers to segments of DTY that are tight and not fluffy. Tight spots indicate less severe stiff yarn, where the continuous length is very short, almost point-like. Stiff yarn and tight spots can be identified through visual inspection and appearance. Stiff yarn can also be recognized on dyed sock tubes, appearing darker than normal yarns in strip form. Tight spots may appear as dark dots on sock tubes and are generally harder to discern.
The primary causes of stiff yarn and tight spots include:
High unevenness in POY yarn thickness and elongation.
Poor wettability of POY spinning oil agents.
Uneven distribution on the surface of the yarn.
Incompatibility between POY spinning speed and DTY processing speed.
Fluctuations in the entry and exit speed of the yarn in the stretching area.
Adhesion among individual filaments in the twisting area.
Inappropriate false twisting devices.
Incorrect selection of D/Y ratio and stretching multiples.
Insufficient tension in the yarns in the second heating chamber.
Polyester filament is widely used in the fields of clothing and decoration due to its unique style. With continuous improvements in spinning technology and the pursuit of comfort and diversity in clothing, the quality requirements for polyester filament have also increased. However, due to limitations in the control and testing methods during the DTY production process, some quality issues only become apparent during weaving.
Tracking the after-sales service situation of a certain company over the past three years, the most frequently reported quality issues by customers include color differences, stiff yarns, hairy fibers, unwinding issues, network problems, and others. The proportions of these issues are illustrated in Figure 1: Quality Issue Proportions.
Analysis and Discussion
1. Color Differences or Stripes
(A) Solutions
To address the three dyeing-related issues—color absorption, curling shrinkage rate, and linear density:
Full Process Management Starting from POY Raw Materials:
Ensure uniformity in linear density during production. Promptly address issues with leaking components and metering pumps, and strictly manage fiber distribution.
Strengthen management to prevent mixing batches and misaligned yarns, ensuring stability in melt quality, component cycle, and cooling conditions.
Stability in Production Process Parameters:
Maintain stability in pressure, pressure differentials, temperature, and speed, and establish strict fluctuation diversion standards.
Spindle Position Management:
Reduce variations in spindle positions.
Replacement of Expired DTY Equipment Parts:
Regularly replace components like false twisting discs, rubber rings, and rollers to keep all spindle processing conditions consistent.
Consistency in Yarn Paths During DTY Processing:
Ensure that the yarn path remains consistent throughout the processing.
Adopting Stricter Color Matching Standards:
Current practices show that using gray card standards may no longer meet users' increasingly stringent dyeing requirements, often leading to minor color differences. Many manufacturers are now adopting a level 4.5 standard, with three color subdivisions (deep, medium, light) referencing woven samples to further improve dye uniformity.
Tailoring DTY Processing to Fabric Characteristics:
For weft-knitted fabrics, increasing DTY's curling shrinkage rate enhances elasticity and fullness, helping to mask minor striping after dyeing and finishing.
Among these conditions, the consistency of DTY processing conditions is where issues are most likely to arise. Due to the complexity of components and spindle positions in DTY production, there is a higher likelihood of operational errors. Experimental comparisons indicate that the filament path between the first and second rollers is critical during the elongation process. The POY yarn completes stretching and false twisting between these two components, making this segment the core of the elongation. Any inconsistencies in heating, stretching, twisting, or cooling can lead to significant differences in shrinkability and fluffiness, resulting in greater color variations and substantial losses. Therefore, operators must conduct a thorough inspection of this yarn path after yarn formation.
2. Stiff Yarn
(A) Solutions
To tackle the issue of stiff yarn, it is essential first to ensure the quality of POY:
Reduce unevenness in POY linear density and oil absorption issues by promptly monitoring component status and cooling airflows, preventing fluctuations in POY that could lead to uneven tension and inadequate false twisting during subsequent processing, causing stiff yarns.
In the DTY processing, ensure the equipment is in good condition, particularly that the first and second rollers do not slip or misalign, which can lead to inconsistent yarn stretching.
The processing speed must be moderated; excessively high speeds can cause yarn vibrations, leading to unstable friction on the false twisting devices, resulting in intermittent stiff yarns.
Ensure uniform heating of the yarn in the first heating chamber and avoid issues with yarn not entering or exiting properly, as these can lead to inconsistent heating and varying twisting effects.
Strengthen online production monitoring to eliminate various instability factors, maintain good equipment conditions, and establish periodic parts replacement or maintenance schedules, possibly installing online tension systems where feasible.
3. Hairy Fibers
(A) Solutions
If there is a high occurrence of hairy fibers or fixed positions of DTY with hairy fibers, it is crucial to focus on the POY processing conditions. Sometimes, simply replacing components can eliminate the batch of hairy fibers.
If the issue is isolated to a single spindle, check the ceramic components of the DTY, particularly the single end surfaces of some yarn cakes, as this is often related to the quality of the winding ceramic parts.
4. Unwinding
(A) Solutions
The primary cause of excessive residual torque is improper stability in the second heating chamber. A high temperature difference between the first and second heating chambers prevents sufficient release of friction stress during the false twisting process, leading to inadequate yarn shaping. By appropriately increasing the temperature in the second heating chamber, residual torque can be effectively reduced.
Table 2 compares the unwinding breakage of a particular type of yarn under different temperature conditions in the second heating chamber (unwinding speed: 1500 m/min). As seen in Table 2, an appropriate setting of the second heating chamber temperature can improve unwinding breakage conditions. However, it's important to consider that too small a temperature difference may lead to issues like thick streaks or color differences during dyeing. Through experimentation, a temperature difference of 20–30°C is generally suitable for yarns with a linear density of 167 dtex.
Increasing the roll density requires appropriate forming parameters for the DTY cakes; otherwise, merely increasing winding tension to improve roll density will hit a critical limit and may cause issues such as yarn entanglement and shoulder bulging. Particularly for porous fine denier types, the higher filament count increases the fluffiness of the composite yarn. Compared to coarser denier yarns, the same weight occupies more stacking space, leading to lower roll density and a higher likelihood of breakage during weaving. Only by selecting suitable parameters—such as anti-bulge, anti-overlap, end face cone angle, and crossover angle—can the surface density of the yarn cake be made relatively uniform, resulting in high roll density without entanglement.
Furthermore, a high occurrence of hairy fibers and inadequate oil absorption can adversely affect unwinding breakage, especially in porous fine denier types. Due to the larger contact area between the yarn and the guiding device, increased friction can generate static electricity, leading to tangled hairy fibers. Poor adhesion between filaments may cause twisting and abrupt changes in unwinding tension, resulting in breakage. Thus, efforts should be made during production to minimize hairy fibers and select appropriate oil absorption rates based on different yarn types.
5. Poor Network
(A) Solutions
To improve overall network quality, the following strategies can be employed:
Increase network air pressure.
Select appropriate nozzle types.
Add 2BIS rollers to improve yarn tension stability.
Adjust the number of yarn holes to reduce single filament linear density.
These measures can effectively enhance the overall fixture strength and improve overall quality while reducing the likelihood of single-spindle network issues. Table 3 shows the relationship between the network strength of a company’s 167 dtex/47f heavy network products and customer complaints per thousand tons. It is evident that once the overall strength of the product reaches a certain requirement, the tendency for isolated or overall strength issues significantly decreases.
Single spindle network issues primarily arise from nozzle blockages or equipment defects. Conducting visual inspections to check for equipment defects is the only method to reduce single spindle network issues, including issues such as rust or looseness in the first and second rollers leading to tension differences, blocked nozzle holes, or unclosed nozzles.
In conclusion, to reduce quality defects in polyester DTY during the weaving process, comprehensive quality management must be implemented throughout production. In addition to selecting suitable production processes, it is vital to ensure the quality of POY raw materials, maintain stability in all process parameters, strengthen quality management of spindle positions before and after processing, reduce differences between equipment spindle positions, enhance product consistency, and strictly enforce inspection standards. Understanding customer needs and the characteristics of weaving machines is essential to producing satisfactory products for customers.
