The shrinkage problem of knitted fabrics is mainly related to factors such as fiber performance, moisture and heat plasticity, and the structural characteristics of knitted fabrics.

Situations that may easily occur during production

In the dyeing and finishing production process of knitted fabrics, the following situations are inevitably present:

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

2. In the dyeing production process of knitted fabrics, efforts have been made to create conditions for the fabrics to be processed under low or no tension. However, so far, it is impossible to completely avoid tension during the processing of knitted fabrics. The presence of tension causes the fabric to stretch longitudinally, reducing fabric density and causing deformation.

3. Knitted fabrics are dried away from a stable state;

4. The processing of knitted fabric yarns and the weaving process also have potential tension.

Specific manifestations of shrinkage problems

Due to the existence of the above situations, a series of problems that pose a risk of shrinkage to knitted fabrics arise. Specific manifestations include:

1. Since the dyeing and finishing process occurs in a humid and hot state, water molecules enter the amorphous region of the fibers, reducing the intermolecular forces. At this time, under external force, the molecular chain segments displace, causing the fibers to elongate. If dried in this elongated state, new hydrogen bonds form at the new positions between the molecular chains, and the elongated part of the fiber fails to retract to its original state. The stability of deformation under this humid and hot condition is unstable.

2. Knitted fabrics are formed by interlocking loops, which have a most stable structural form. If the knitted fabric is in a stable structural state, it remains dimensionally stable and does not shrink when wet. However, in actual production, the fabric, especially when subjected to significant repeated stretching longitudinally, causes the yarn fibers to undergo plastic deformation, leading to elongation in the fabric's length, narrowing in width, and shifting of loops, moving the fabric structure away from its stable state. This state is unstable, and under appropriate conditions, there is a tendency to return to the original stable state.

3. When knitted fabrics are dried in an elongated state, they may appear stable on the surface, but this "stability" is temporary.

4. During the processing of knitted fabric yarns and the weaving process, the influence of tension has potential tension, which can also cause the yarn fibers to retract.

Main reasons for knitted fabric shrinkage

In summary, the reasons for knitted fabric shrinkage include:

1. Recovery of fiber deformation in a humid and hot state. Cotton knitted fabrics washed in a relaxed state allow water molecules to break newly formed hydrogen bonds, relaxing internal stress, causing the elongated parts to retract, resulting in shrinkage.

2. Recovery of knitted fabric deformation. When the structure of knitted fabric is far from a stable state, during wet washing, the plasticity of the fibers is enhanced, and the recovery force is strengthened, leading to deformation recovery, elongation, and retraction. This is the main reason for the shrinkage of cotton knitted fabrics.

3. Fiber moisture absorption increases the yarn diameter, leading to yarn shrinkage and causing shrinkage. The increase in yarn diameter shortens the length of the yarn, resulting in fabric shrinkage (including length and width) and increased density.

The above describes the basic reasons for the shrinkage of knitted fabrics. As for the factors affecting shrinkage and shrinkage conditions of knitted fabrics in different situations, such as different fiber compositions, different organizational structures, different dyeing and finishing processes and equipment, whether the fabric has undergone mercerization, stretching shaping, pre-shrinking treatment, etc., further practice, exploration, summarization, improvement, and enhancement are needed.

Control of shrinkage rate in printed knitted fabrics

To overcome the problem of high shrinkage rates in cotton knitted fabrics, combined with the characteristics of printing knitted fabrics in a split and flat state, the following measures and methods can be taken:

1. Loose processing should minimize tension in each process during dyeing and finishing production. It is ideal to avoid plastic deformation of the fabric in a wet state and to prevent elongation of the fabric and fibers, which is the best method to prevent fabric shrinkage. In fact, printing companies have considered loose processing methods in the dyeing and finishing of knitted fabrics, and many effective loose processing methods have been adopted in the selection of equipment for printing, pre-treatment, and post-finishing of knitted fabrics. With the development of knitted fabric printing and printing equipment, loose processing of knitted fabrics will continue to improve. Additionally, shortening the printing process has also attracted attention and has been put into practice. The adoption of the above measures will undoubtedly help reduce plastic deformation of knitted fabrics in a wet state, avoid elongation of fabrics and fibers, and significantly reduce fabric shrinkage.

2. Overfeeding drying means achieving relaxed drying, reducing tension, and bringing the fabric closer to a state of full balance. Before printing knitted fabrics, overfeeding drying can be combined with the pre-stretching shaping process to appropriately control overfeeding and dry the fabric to achieve the goal of reducing the fabric shrinkage rate.

3. Mechanical pre-shrinking is performed using specialized mechanical pre-shrinking equipment. First, overfeed the fabric to keep it in a relaxed state longitudinally, allowing for pre-shrinking. Then, steam is applied to the fabric to enhance its plasticity in a relaxed state, relaxing the internal stress of the fabric. Through diffusion, the fabric shrinks longitudinally and expands laterally, or is compressed longitudinally, forcing the longitudinally elongated parts of the fabric to retract during weaving or dyeing and finishing processes, resulting in a relaxed structure. This relaxed state is then dried in a loose manner to achieve the effect and purpose of pre-shrinking. Mechanical pre-shrinking is one of the effective measures to overcome the high shrinkage rate of knitted fabrics.

Other measures to reduce fabric shrinkage rates

1. Reduce tension during the bleaching process;

2. In the dyeing and finishing process, any wet fabric drying should apply loose tension to reduce elongation and prevent excessive narrowing of the width. If using a machine that opens and dries the fabric, a small fabric container should be added after opening to avoid excessive tension between opening and water squeezing;

3. During the mercerization process, control the expansion tension to maintain the effective width of the original cotton fabric and carefully manipulate the weft tension.

4. The guide rods, rollers, and drying cylinders of the flat washing machines in each process should be maintained to ensure they are flat and smooth, avoiding wrinkles when the tension is relaxed. The guide rollers in the wrinkle water tank should operate smoothly, and the line speed of the front and rear rollers and drying cylinders of the same machine should not differ too much. The fabric should not drag on the ground under the transverse tension control, and there should be a tension adjustment device or segmented transmission adjustment device during the segmentation process on the long table to control the tension.

5. The products after mercerization should be strictly controlled. The tension during the subsequent processing may increase due to elongation after mercerization. The transverse shrinkage rate can be set as effective width indicators for semi-finished products in each process during actual production operations. The fabric removal at each machine should be measured and assessed, and the subsequent processes should accept the previous processes to ensure the semi-finished products have the required effective width.

6. For some machines, the tension needs to be manually controlled. Strict process operations should be strengthened, and mechanical cleaning should be enhanced to reduce operating tension and effectively lower the shrinkage rate of the fabric.

7. Strengthen the inspection of the processing conditions for mercerization. Dyeing factories with conditions can use direct rollers combined with fabric clips for mercerization.

8. For varieties with a large transverse shrinkage rate, such as khaki and woolen fabric, pre-shrinking treatment should be carried out to reduce the shrinkage rate. In these varieties, relying solely on mechanical relaxation of tension during tight processing can lead to a shrinkage rate as high as about 4%. This shrinkage rate should adopt pre-shrinking methods to further reduce it. After pre-shrinking treatment, not only is the shrinkage rate reduced, but a better hand feel and clear texture with a soft sheen can also be obtained.

9. For varieties with a large weft shrinkage rate, some are due to the unreasonable effective width of the raw cotton fabric. Even with reasonable processing conditions, there can still be a large weft shrinkage rate, and the shrinkage rate is even greater under poor processing conditions. Further improvement measures must be taken, and the effective width of the fabric must meet the requirements. Additionally, improvements should be made to the specifications of the raw cotton fabric to widen the effective width without increasing costs or with minimal cost increase, ensuring that the weft shrinkage rate falls within the required range.

10. Resin finishing can reduce shrinkage rates and improve elasticity (care should be taken with the hand feel). Additionally, PU coatings can also reduce shrinkage rates.

11. For T/C blended synthetic fiber fabrics, it is important to control the mercerization process and operations to better manage the shrinkage rate.