Control Of Fabric Width Iin Textile Processing

Control of Fabric Width During Processing

The control of fabric width is one of the most important indicators during the processing stage. In practice, fabric design has the most direct influence on the finished width. Factors such as the loom-state width of the grey fabric, the type of reed, the fineness of the raw materials, the fabric weave structure, and the warp and weft density of the grey fabric all have a direct impact on the finished width. Taking polyester woven fabrics as an example, common types include conventional flat fabrics, highly twisted simulation-finished fabrics, elastic fabrics, and staple-fiber-blend wool-like fabrics. The dyeing and finishing process also significantly affects fabric width.

01 Influence of the Process Flow

Different fabric types require different dyeing and finishing processes. The typical process flow for conventional flat fabrics is as follows:

Fabric preparation → Scouring → Dyeing → After-treatment → Dewatering → Drying → Setting → Inspection

The dyeing and finishing process for highly twisted simulation fabrics is as follows:

Fabric preparation → Pre-shrinking → Pre-setting → Alkali reduction → Washing → Dyeing → After-treatment → Dewatering and drying → Setting → Inspection

The dyeing and finishing process for elastic fabrics is as follows:

Fabric preparation → Open-width scouring → Pre-shrinking → Pre-setting → Dyeing → After-treatment → Dewatering → Drying → Setting → Inspection

The dyeing and finishing process for blended wool-like fabrics is as follows:

Fabric preparation → Singeing → Pre-shrinking → Pre-setting → Dyeing → After-treatment → Dewatering → Drying → Setting → Decatizing → Inspection

Based on the above processes for different fabric types, the main steps in the typical dyeing and finishing of polyester woven fabrics include 12 procedures: fabric preparation, (open-width) scouring, pre-shrinking, pre-setting, alkali reduction, dyeing, after-treatment, dewatering, drying, setting, decatizing, and inspection.

02 Influence of the Processing Steps

1 Fabric Preparation

The main tasks of the fabric preparation step are marking, sewing heads, and weighing. Although the length of time that rolled grey fabric is left to rest after unrolling affects its weave shrinkage, this effect is weak. For warp elastic fabrics, weft elastic fabrics, and warp-weft bidirectional elastic fabrics, excessively long resting times after unrolling will increase weave shrinkage. The shrinkage in the width of weft elastic grey fabrics directly affects the finished width. Shrinkage of the warp yarns in warp elastic fabrics causes the weft yarns to become more curved, ultimately narrowing the width in the weft direction. After fabric preparation, the pressure on the fabric at the top and bottom of the fabric truck differs. Whether for elastic fabrics or highly twisted fabrics, if the grey fabric is subjected to uneven external influences during the release of internal stresses after unrolling, the width variation across the entire batch will be uneven. This ultimately results in inconsistent widths after pre-shrinking or pre-setting, adversely affecting subsequent processing. For polyester woven highly twisted fabrics, particularly elastic fabrics, the storage time after unrolling and batching should not be too long. Excessive storage time will lead to uneven fabric widths after pre-shrinking, causing significant issues in later stages.

2 Scouring

Scouring includes rope scouring and open-width scouring. Rope scouring is the pre-treatment for ordinary polyester woven fabrics, aimed at removing impurities from the fabric without affecting the quality of subsequent processes. The key process parameters in pre-treatment are the heating rate, maximum process temperature, auxiliary chemical concentration, and holding time. In a hot and wet state, polyester, as a thermoplastic fiber, always undergoes changes. The higher the temperature and the faster the heating rate, the more pronounced these changes become. To mitigate this effect, the pre-treatment temperature does not exceed the glass transition temperature of the polyester fiber. Therefore, rope scouring has little impact on the finished width of ordinary flat polyester woven fabrics.

For elastic fabrics, especially weft elastic and two-way elastic fabrics, open-width scouring is required to effectively control the shrinkage of the grey fabric width. Open-width scouring is carried out in an open-width scouring machine, which contains multiple wash tanks allowing different scouring temperatures. Starting with cold water, then warm water, the process of open-width scouring combined with continuous width expansion allows the width of weft elastic grey fabrics to shrink uniformly by more than 20% after processing. Slow and uniform shrinkage is the foundation and key to maintaining a flat fabric surface. Low-temperature open-width scouring is a simple and effective process route to achieve slow, uniform shrinkage of polyester elastic fabrics. Using an open-width scouring machine to scour wide-width plain elastic fabrics eliminates internal stresses in the grey fabric under mild hot and wet conditions, reducing the chance of fabric creasing during subsequent processing. If the width shrinkage is excessive, the speed of the open-width scouring machine can be reduced. A weft elastic (or two-way elastic) grey fabric entering the machine at 210 cm typically exits at a width of around 180 cm (including selvages). If the width after scouring exceeds 160 cm, it indicates that the machine speed is too fast.

3 Pre-shrinking

Pre-shrinking is carried out in rope form inside a dyeing machine, at a temperature higher than scouring but lower than dyeing. Pre-shrinking is a pre-treatment conducted under high-temperature, hot and wet conditions. Besides removing sizing agents, oils, stains, and other impurities, this process aims to eliminate internal stresses within the fabric as much as possible, allowing the fabric width to shrink moderately. Under high-temperature, hot and wet conditions, regions of high crystallinity within the fibers tend to disorient, while regions of low crystallinity tend to orient. This orientation and disorientation phenomenon within the thermoplastic fibers leads to a more uniform overall orientation of the fabric, releasing the internal stresses formed during weaving and storage. The result is increased dimensional stability, a smoother fabric surface, and more stable width. Pre-shrinking of polyester woven fabrics is analogous to mercerizing of cotton fabrics.

During pre-shrinking, the warp-direction tension comes from the machine's nozzle, resulting in relatively high tension in the warp direction and lower tension in the weft direction. During pre-shrinking, weft yarns are short, and each weft yarn passes over a number of warp yarns equal only to the warp count of that fabric. Each warp yarn passes over a number of weft yarns equal to the weft density multiplied by the length of the fabric piece. Therefore, the resistance encountered by warp yarns during shrinkage is far greater than that for weft yarns. Additionally, under the nozzle tension, warp yarn shrinkage becomes even more difficult, ultimately leading to greater weft-direction shrinkage than warp-direction shrinkage for non-elastic polyester woven fabrics after pre-shrinking. When thermoplastic fibers shrink under high-temperature, hot and wet conditions, the thickened weft yarns effectively shorten the warp yarns wrapped around them, and similarly, warp yarns shorten the weft yarns. After pre-shrinking, the interaction between yarns results in a more reasonable fabric width and length. Understanding the shrinkage behavior of grey fabrics with different twists, raw materials, and weaves during weaving and dyeing/finishing is fundamental to the rational design of the finished width.

During pre-shrinking, the greater the nozzle tension, the more pronounced the fabric width shrinkage. If the nozzle opening is too small, the fabric cannot pass through smoothly, making it susceptible to damage and uneven shrinkage. If the nozzle opening is too large and the nozzle tension is set too high, fabrics without selvage structures are prone to edge curling (selvedge curl). Once edge curling occurs, subsequent processing and finishing setting become very difficult. Although increasing the fabric entry temperature can improve edge curling, excessively high water temperatures inside the machine during pre-shrinking can also cause uneven shrinkage.

Stepwise heating with holding periods at different temperatures is the primary method for extending process time and moderating process conditions during pre-shrinking of highly twisted fabrics. Extending the time and moderating the conditions both enhance the pre-shrinking effect. Uniform shrinkage, dimensional stability, and a flat fabric surface are the main quality indicators for pre-shrinking. If the fabric width is too wide after pre-shrinking, or if the fabric surface shows splits, these issues are caused by improperly designed pre-shrinking processes. If a stepwise heating pre-shrinking process is used, consider appropriately reducing the nozzle tension and the fabric circulation speed. This increases the immersion time of the fabric in the pre-shrinking bath, allowing shrinkage in a relaxed, high-temperature, hot and wet state, which is beneficial for improving the pre-shrinking effect.

4 Pre-setting

Pre-setting is carried out on a stenter frame. For polyester woven fabrics, pre-setting is a continuation of pre-shrinking, with the same objective. The former is performed under hot and wet conditions, while the latter is performed under dry heat conditions. The results of slow pre-shrinking under hot and wet conditions require consolidation and reinforcement through pre-setting under dry heat conditions. During pre-setting, temperature, width, tension, and machine speed are the main process parameters. The pre-setting temperature is slightly higher than the final setting temperature, which benefits the dimensional stability of the fabric after pre-setting. The pre-setting width should be slightly smaller than the final finished width. During setting, the greater the warp tension, the wider the fabric width and the flatter the fabric surface. Increasing warp tension increases the chance of warp yarns being straightened, causing weft yarns to become more curved, thus tending to narrow the width. The machine speed depends not only on the length of the heating chamber but also on fabric thickness, raw material properties, weave structure, and other factors.

The pre-setting width is determined based on the pre-shrunk width, the flatness of the fabric surface before pre-setting, and the target finished setting width. The adjustment range for the width during pre-setting should not be too large, nor should the temperature fluctuations be excessive. Otherwise, after alkali reduction and dyeing, the widths of fabrics from the same batch before final setting may differ too much, making final setting impossible.

5 Alkali Reduction

The purpose of alkali reduction is to make highly twisted polyester filaments finer. The reduced yarns become less rigid, and the space for yarn movement increases, resulting in improved fabric hand feel. After alkali reduction, the fabric becomes softer, and the width increases slightly. The amount of width increase is most closely related to the raw material and weave structure, typically around 1 cm. This width increase is more pronounced after high-temperature alkali reduction in the dyeing machine. The washing step after reduction but before dyeing does not affect the fabric width.

6 Dyeing

Similar to pre-shrinking, the dyeing process is also carried out under high-temperature, hot and wet conditions. The difference is that the dyeing temperature is higher than the pre-shrinking temperature, making it a second hot and wet setting treatment for the fabric. After the first hot and wet setting (pre-shrinking) and the first dry heat setting (pre-setting), the fabric width is essentially stable. Although the dyeing temperature is about 10°C higher than the pre-shrinking temperature, the thermoplastic behavior of the polyester fibers after high-temperature dry heat setting results in a slight increase in fabric width during dyeing, typically about 1 cm wider than the pre-set width.

Two main reasons account for the width increase after alkali reduction and dyeing. First, the fabric continuously circulates within the machine, and this circulation acts as a mechanical softening process. As the yarns soften, they slide more easily against each other compared to before pre-setting, causing a slight width increase. Second, the hot and wet conditions release the new stresses imposed on the fabric during the pre-shrinking and pre-setting steps, also contributing to a slight width increase.

7 After-treatment

Processes such as reduction cleaning and fixation after dyeing for conventional fabrics, highly twisted fabrics, and staple wool-like fabrics do not affect the fabric width. For elastic fabrics, especially weft elastic fabrics made from blended raw materials, when cellulose fibers are dyed with sulfur dyes and then oxidized with oxidizing agents for color development, or when reactive dyes are used on cellulose fibers followed by fixation with sodium carbonate, the process conditions must not be too severe, and the process time should not be too long. Otherwise, excessive damage to the spandex elasticity will cause an excessive increase in fabric width, ultimately resulting in a finished width that is too wide and loss of weft elasticity.

8 Dewatering, Opening, and Drying

Dewatering and opening after pre-shrinking and dyeing have little direct impact on the fabric width. Opening here refers to using equipment or manual means to flatten the rope-form fabric without entanglement, facilitating drying or setting. Although the fabric's moisture content after dewatering affects drying efficiency, excessive dewatering of some fabric types can create new crease marks on the fabric surface. In most cases, to improve processing efficiency, the aforementioned conventional polyester woven fabrics can be directly sent for pre-setting after opening, omitting the drying step, provided the stenter's heating chamber is long enough to ensure adequate pre-setting. The effect of dewatering, opening, and drying after dyeing on fabric width:

Although relaxed drying equipment and tensioned drying equipment have some influence on the fabric width after drying, this influence occurs after dyeing and is far less significant than the effect of final setting on fabric width. For some special-effect fabrics, such as bark crepe or pearl linen, relaxed drying is necessary after dewatering and opening to preserve the fabric's surface aesthetic. Most polyester woven fabrics can be directly set after opening.

9 Final Setting (Finished Setting)

For conventional flat polyester woven fabrics, final setting has the greatest impact on dimensional stability. Whether for conventional fabrics, highly twisted fabrics, wool-like staple fabrics, or elastic fabrics, the finished setting width must be based on the fabric's width before setting. Although increasing warp tension during setting can help narrow the finished width, it is impossible to significantly narrow a fabric that is too wide after dyeing solely through final setting. Similarly, it is impossible to significantly widen a fabric that is too narrow after dyeing through final setting, even by greatly reducing the warp tension during final setting.

Tension adjustment during final setting is even more critical than warp tension adjustment during pre-setting. Adjusting the warp tension during final setting by manipulating the tension bars, cloth regulators, spreading rollers, and overfeed device at the front of the stenter can affect not only the fabric weight and hand feel but also the final set width. After shrinking on a rubber blanket shrinking machine, the width of T/C fabrics increases moderately. After decatizing, the width of wool-like fabrics also increases slightly. For denim-like weft elastic fabrics with viscose staple warp yarns, the width increases after washing. Pin-plate stenter frames are more effective at maintaining warp tension (including both positive stretching tension and negative compressive tension) during final setting compared to clip-type stenter frames.

The final width of a fabric depends not on the finished setting width alone, but on the fabric's inherent characteristics and the entire dyeing and finishing process. Through rational design and meticulous processing, the fabric's final width will fully reflect its intrinsic properties, and the dimensional stability of the width will be achieved.