Shrinkage in knitted fabrics is primarily influenced by fiber properties, moisture-heat plasticity, and the structural characteristics of the knit.

Common issues that may arise during the production process

During the printing and dyeing as well as weaving processes, knitted fabrics inevitably encounter the following situations:

1. The humid and hot environment of dyeing and finishing processes;

2. During the printing and dyeing process for knitted fabrics, every effort has been made to create conditions that allow processing under low or zero tension; however, to date it remains impossible to completely eliminate tension throughout the manufacturing process. The presence of tension causes longitudinal stretching of the fabric, resulting in a reduction in fabric density and deformation.

3. The knitted fabric is dried in a non-steady state;

4. Potential tension also exists in the yarn processing and knitting processes of knitted fabrics.

Specific manifestations of shrinkage issues

Due to the aforementioned conditions, a series of issues that pose a risk of shrinkage in knitted fabrics can arise. Specifically:

1. During dyeing and finishing processes, the fabric is subjected to a wet-heat environment, in which water molecules penetrate the amorphous regions of the fibers, reducing the intermolecular forces. Under external stress, the polymer chain segments then shift, causing the fibers to elongate. If the fabric is dried while in this elongated state, new hydrogen bonds form at the altered positions between the fiber chains, preventing the elongated portions from returning to their original configuration. Consequently, the dimensional stability of deformation under wet-heat conditions is inherently unstable.

2. Knitted fabrics are formed by interlooping yarn loops, resulting in a highly stable structural configuration. When a knitted fabric is in a stable structural state, it exhibits dimensional stability and does not shrink upon exposure to water. However, during actual production, particularly under repeated and substantial longitudinal stretching, the yarn fibers undergo plastic deformation, leading to longitudinal elongation, transverse narrowing, and loop shifting. Consequently, the fabric structure deviates from its stable state, rendering it inherently unstable. Under appropriate conditions, such fabrics tend to revert to their original stable state.

3. When knitted fabrics are dried under tension, their surface appearance may seem stable; however, this “stability” is only temporary.

4. During yarn processing and fabric weaving, yarns are subjected to drawing and tension, which give rise to latent tension that can also cause the yarn fibers to contract.

Main Causes of Shrinkage in Knitted Fabrics

In summary, the causes of shrinkage in knitted fabrics are:

1. Recovery of fiber deformation under humid and hot conditions. When cotton knitted fabrics are washed in a relaxed state, water molecules can break the newly formed hydrogen bonds, thereby relieving internal stresses and causing the elongated portions to contract, resulting in shrinkage.

2. Recovery of deformation in knitted fabrics. When knitted fabrics are subjected to wetting and washing—conditions that deviate from their stable state—the fibers exhibit increased plasticity, which enhances the fabric’s recovery force and promotes deformation recovery and elongation–retraction. In cotton knitted fabrics, this results in shrinkage followed by restoration to the original stable state; this is the primary cause of shrinkage in such fabrics.

3. Fiber moisture absorption causes the yarn diameter to increase, leading to yarn shrinkage and subsequent fabric shrinkage. The increase in yarn diameter also results in a reduction in yarn length, which in turn causes the fabric to contract in both length and width, thereby increasing its density.

The foregoing discussion has outlined the fundamental causes of shrinkage in knitted fabrics. As for the specific factors influencing shrinkage under various conditions and the resulting shrinkage behavior—such as knitted fabrics composed of different fiber types, those with different structural configurations, those subjected to different dyeing and finishing processes and equipment, and whether the fabric has undergone mercerization, stretching and setting, or pre-shrinking treatments—further practical experimentation, exploration, summarization, refinement, and improvement are still needed.

Control of Shrinkage in Printed Knitted Fabrics

To address the issue of high shrinkage in cotton knitted fabrics, and taking into account the characteristic of printing on knitted fabrics in the cut-and-flat state, the following measures and methods can be adopted:

1. Loosening processes should be employed to minimize tension at every stage of the printing and dyeing production process. Efforts should be made to prevent plastic deformation of fabrics in the wet state and to avoid elongation of both the fabric and its fibers; this is the most effective approach for preventing fabric shrinkage. In practice, printing enterprises have adopted loosening techniques in the finishing of knitted fabrics and other textiles, and the selection of equipment for pre-treatment and post-finishing has also taken these techniques into account, resulting in the implementation of many effective loosening methods. It is expected that, with the continued development of knitted fabric printing and the associated equipment, loosening processes for knitted fabrics will be further refined and improved. In addition, efforts to streamline the printing process have garnered attention in production and are now being put into practical application. The adoption of these measures will undoubtedly be highly beneficial in reducing plastic deformation of knitted fabrics in the wet state, preventing elongation of the fabric and fibers, and minimizing fabric shrinkage.

2. Overfeed drying, also known as relaxed drying, aims to reduce tensile stress and bring the fabric closer to a state of overall equilibrium. Prior to printing on knitted fabrics, overfeed drying can be integrated with the pre-print stretching and setting process: by appropriately controlling the degree of overfeed and drying the fabric at the right time, the fabric’s shrinkage rate can be effectively reduced.

3. Mechanical pre-shrinking is carried out using specialized mechanical pre-shrinking equipment. First, the fabric is fed with overfeed to place it in a relaxed longitudinal state, creating room for pre-shrinking. Next, steam is applied to moisten the fabric, enhancing its plasticity in this relaxed condition and relieving internal stresses. Subsequently, diffusion causes the fabric to contract longitudinally while expanding laterally or being compressed longitudinally, thereby forcibly reducing the longitudinal elongation that occurs during weaving or dyeing and finishing processes. This results in a more relaxed fabric structure, which is then dried in a loose state to achieve the desired pre-shrinking effect and objective. Mechanical pre-shrinking is one of the effective measures for mitigating the high shrinkage rate of knitted fabrics.

Other measures to reduce fabric shrinkage

1. Reduce tension during the scouring and bleaching process;

2. During dyeing and finishing processes, whenever wet fabrics are dried, the tension should be kept as low as possible to minimize elongation and prevent excessive narrowing of the fabric width. If using a continuous drying machine with width-opening functionality, a small fabric-receiving box should be installed after width opening, followed by threading the fabric onto the fabric rack, to avoid excessive tension between the width-opening and water-rolling stages.

3. During the mercerization process, it is essential to maintain the effective width of the undyed cotton fabric by carefully controlling the widening tension and the weft tension.

4. The guide rollers, nipping rollers, and drying cylinders in each processing stage of the flat washing machine shall be regularly maintained to ensure they remain smooth and even, thereby preventing wrinkling when tension is relaxed. The guide rollers in the water trough must rotate freely, and the linear speeds of the nipping rollers and drying cylinders on the same machine should not differ excessively. In terms of machine-direction tension, the fabric should be kept from dragging on the floor. During the sectionalization process on long machines, tension-adjusting lifting-and-lowering frames or sectional drive adjustment devices shall be provided to control the magnitude of tension.

5. Products after mercerization must be handled with strict control; during subsequent processing, the tension should be carefully managed, as mercerization causes additional elongation and results in longitudinal shrinkage. In actual production, effective width targets for semi-finished products can be set at each processing stage, and fabric take-up at each machine should be measured and inspected accordingly. Furthermore, downstream processes must inspect and accept the preceding stages to ensure that the semi-finished products maintain their specified effective width.

6. For certain machinery, tension must be manually adjusted; strict adherence to process procedures is required, along with enhanced process control and improved machine cleaning, to reduce operating tension and effectively lower the fabric shrinkage rate.

7. Strengthen inspection of mercerization process conditions; dyeing mills with the necessary resources may employ a combination of straight rollers and cloth clamps for mercerization.

8. For fabrics with significant warp shrinkage—such as khaki and gabardine—it is essential to perform pre-shrinking treatment to reduce shrinkage. In tight-processing operations, relying solely on mechanical relaxation of tension can still result in a shrinkage rate of around 4%. To further minimize this shrinkage, it is advisable to adopt pre-shrinking methods. After pre-shrinking, not only is the shrinkage reduced, but the fabric also exhibits improved hand feel, clearer patterns, and a soft, lustrous finish.

9. For varieties with significant weft shrinkage, some exhibit substantial weft shrinkage even under optimal processing conditions due to an unreasonable effective width of the undyed cotton fabric; when processing conditions are suboptimal, the shrinkage is even greater. Therefore, further improvement measures must be implemented, and for mercerized varieties, it is essential to ensure that the effective fabric width upon delivery meets the specified requirements. In addition, the specifications for undyed cotton fabric should be refined: without increasing costs or with only a minimal increase, the effective width of the undyed cotton fabric should be widened to ensure that the weft shrinkage is reduced to within the required tolerance range.

10. Post-finishing with resins can reduce shrinkage and enhance elasticity (with careful attention to hand feel); additionally, PU coating can also lower shrinkage.

11. For T/C blended synthetic-fiber fabrics, meticulous control of the mercerization process and operating procedures can effectively manage shrinkage.