Common Situations in the Production Process
During the printing and dyeing as well as weaving processes of knitted fabrics, the following situations are inevitable:
Wet and hot environment in the dyeing and finishing process.
Despite efforts to minimize or eliminate tension during the printing and dyeing process of knitted fabrics, it's impossible to completely avoid tension. The tension causes longitudinal stretching of the fabric, leading to decreased fabric density and deformation.
Knitted fabrics are being dried away from stable conditions.
Tension also exists during yarn processing and knitting processes, leading to latent tension effects.
Manifestations of Shrinkage Issues
Due to the aforementioned situations, knitted fabrics face a series of issues that can lead to shrinkage:
Dyeing and finishing processes occur in a hot and humid state, allowing water molecules to enter the amorphous regions of fibers, and reducing intermolecular forces. Under external force, molecular segments displace, causing fiber elongation. If dried in this stretched state, new hydrogen bonds form between the molecular chains, causing the elongated portion of the fiber to not fully revert to its original state. This deformation in the hot and humid state is unstable.
Knitted fabrics consist of interlocking loops with a stable structural form. When subjected to repeated stretching, especially longitudinally, the fabric experiences plastic deformation, leading to longitudinal elongation, narrowing of width, and loop transfer, deviating from its stable state. This unstable state tends to revert to the original stable state under appropriate conditions.
Drying knitted fabrics in their elongated state may appear stable on the surface but is temporary.
Tension during yarn processing and fabric knitting processes also causes fiber shrinkage.
Main Causes of Knitted Fabric Shrinkage
In summary, the main causes of knitted fabric shrinkage are:
Reversion of fiber deformation in a hot and humid state.
Reversion of fabric deformation due to departure from stable fabric structural states.
Increase in fiber diameter due to moisture absorption, leading to yarn shrinkage.
Controlling Shrinkage Rate of Printed Knitted Fabrics
To overcome the issue of high shrinkage rates in cotton knitted fabrics, especially considering that printing on knitted fabric occurs in a relaxed width state, the following measures can be taken:
Minimize tension as much as possible in each process during printing and dyeing production. Avoiding plastic deformation of the fabric and fibers in the wet state and preventing fabric and fiber elongation are the ideal methods to prevent fabric shrinkage. Printing enterprises have adopted various effective methods of relaxed processing in knitting fabric printing and pre-treatment and post-finishing equipment selection. With the development of knitted fabric printing and related equipment, the relaxed processing of knitted fabrics is continuously improving. Shortening the printing process flow has also received attention and has been implemented in production practices. The adoption of these measures will significantly benefit in reducing fabric shrinkage.
Conduct superfeed drying, also known as relaxation drying, to reduce tension and bring the fabric closer to a fully balanced state. Superfeed drying can be combined with pre-printing stretching processes to control fabric shrinkage effectively.
Mechanical pre-shrinking involves specialized equipment. Firstly, relax the fabric by superseding
to allow room for pre-shrinkage, then enhance the fabric's plasticity with steam to relax internal stresses, followed by diffusion to cause longitudinal shrinkage and lateral expansion or longitudinal squeezing, forcing the fabric to revert from its stretched state during weaving or dyeing processing. Mechanical pre-shrinking is one of the effective measures to overcome significant shrinkage in knitted fabrics.
Other Measures to Reduce Fabric Shrinkage
Reduce tension significantly during bleaching and scouring processes.
Apply relaxed tension during wet finishing processes to prevent excessive shrinkage. Machines should be well-maintained to ensure smooth operation and to avoid wrinkles when tension is relaxed.
Strictly control the process after silk finishing, as post-process stretching can lead to increased longitudinal shrinkage. Effective width indicators for semi-finished products should be established, and quality control should be conducted at each stage to ensure the desired effective width.
For machines where tension requires manual control, strict adherence to operational procedures and maintenance to reduce operational tension can effectively reduce fabric shrinkage.
Enhance inspection of silk finishing process conditions, and where possible, use direct rollers combined with fabric spreaders.
For fabrics with high longitudinal shrinkage rates, such as khaki and houndstooth, pre-shrinkage treatment is necessary. Pre-shrinkage treatment not only reduces the shrinkage rate but also improves the hand feel and clarity of patterns.
For fabrics with significant warp shrinkage, improvements in fabric specifications should be considered to widen the effective width of the fabric, reducing longitudinal shrinkage.
Resin finishing can reduce shrinkage and improve elasticity, though care must be taken regarding the hand feel. Additionally, PU coating can also reduce shrinkage.
For T/C blended synthetic fabrics, precise control of the silk finishing process and operations can effectively control shrinkage rates.
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