What is Preshrinking Finishing

1. Mechanical Preshrinking Finishing

Mechanical preshrinking is a physical finishing process to reduce fabric shrinkage after water immersion and lower the shrinkage rate. During dyeing and finishing, fabrics are subjected to warp tension, which reduces the crimp height of warp yarns and stretches the fabric. When hydrophilic fiber fabrics get wet, fibers swell and yarn diameters increase. This raises the crimp height of warp yarns and shortens fabric length, resulting in shrinkage.

The shrinkage rate refers to the percentage of fabric length reduction against its original length after shrinking. For mechanical preshrinking, fabrics are first moistened by steam or water spray, then compressed in the warp direction to increase yarn crimp height, followed by slack drying. After treatment, the shrinkage rate of cotton fabrics can be reduced to below 1%. Meanwhile, mutual extrusion and friction between fibers and yarns also improve fabric softness.

For wool fabrics, relaxation preshrinking is applied. Fabrics are soaked in warm water or steamed, then dried slowly under a relaxed state to achieve shrinkage in both warp and weft directions.

2. Functions & Methods of Preshrinking Finishing

Preshrinking finishing is designed to shrink fabrics in warp and weft directions in advance, so as to cut down the final shrinkage rate and meet quality requirements for garment manufacturing.

The rubber blanket type preshrinker is widely used. When the rubber blanket bends under force, its outer arc stretches while the inner arc contracts. When bent reversely, the stretched part contracts and the contracted part stretches. During operation, moistened fabrics attach closely to the rubber blanket and pass through the compression zone. As the blanket shifts from the stretched outer arc to the contracted inner arc, fabrics are clamped tightly without sliding and compressed along with the blanket, achieving preset shrinkage.

Generally, fabrics are moistened to a moisture content of 10%–15% and heated before feeding into the machine to soften fibers and enhance plasticity. In some cases, fabrics undergo light sizing or foam sizing and cylinder drying after singeing, to optimize preshrinking control and results.

After rubber blanket compression, partial moisture is removed by pre-drying. Fabrics are then fully dried and ironed on a slack dryer to eliminate wrinkles caused by excessive shrinkage, delivering full hand feel and soft luster.

3. Key Process Points for Preshrinking Finishing

Three parameters must be kept stable throughout production: running speed of the preshrinker, moisture regain of fabrics entering the rubber blanket unit, and steam pressure of the pressure cylinder.

Shrinkage test marks shall be made on fabrics to monitor and verify the preshrinkage rate. Timely adjustments are required once abnormalities are found to ensure stable shrinkage performance. Strict management of operators and equipment is essential to prevent defects. Common defects and solutions are listed below:

· Fish scale marks: Caused by aging rubber blankets or uneven preshrinking. Grind or replace worn rubber/blanket components.

· Warp wrinkles: Result from residual wrinkles from previous processes or excessive warp tension during feeding. Avoid wrinkles in upstream procedures and loosen the tension frame to reduce warp tension.

· Roll marks: Caused by insufficient feeding tension, loose fabric, uneven pressure on rollers or inconsistent tightness on fabric selvedges. Adjust the tension frame to increase feeding tension and balance pressure on feeding rollers.

· Weft skew: Arises from incorrect feeding position, uneven guide rollers or unbalanced roller pressure. Correct feeding position, keep guide rollers flat and balance pressure at both ends of rollers.

· Excess residual moisture: Caused by over-moistening. Reduce running speed to ensure thorough drying.

· Insufficient preshrinkage: Leads to poor dimensional stability, due to overhigh pressure cylinder position, low cylinder temperature, insufficient roller pressure or excessive tension on fabric inlet and outlet. Lower the pressure cylinder, control water spray, raise steam pressure, increase roller pressure and reduce inlet/outlet tension.

· Narrow fabric width: Caused by insufficient original width from upstream processes or excessive feeding tension. Follow standard procedures in previous working stages and properly lower feeding tension.

4. What is Shrinkage Rate

Fabrics are made of fibers. Fibers swell after absorbing water, with shortened length and enlarged diameter. The shrinkage rate is the percentage difference between fabric length before and after water immersion against its original length. Fibers with higher water absorption swell more severely, resulting in a higher shrinkage rate and poorer dimensional stability.

Yarn length differs from finished fabric length, which is defined as weaving shrinkage rate.

Weaving Shrinkage Rate (%) = (Original Yarn Length − Fabric Length) ÷ Original Yarn Length

Fiber swelling after water immersion further shortens fabric length and causes shrinkage. Weaving tension and fabric structure directly affect weaving shrinkage rate: low weaving tension produces compact fabrics with high weaving shrinkage rate and low water shrinkage rate; high weaving tension leads to loose fabrics with low weaving shrinkage rate and high water shrinkage rate.

In dyeing and finishing, preshrinking finishing is adopted to increase weft density and weaving shrinkage rate in advance, so as to effectively lower the final water shrinkage rate of fabrics.