Purpose of pre-shrinking
The purpose of pre-shrinking the fabric is to pre-shrink the fabric in the warp and weft directions. A certain amount of shrinkage is performed to reduce the shrinkage rate of the final product and meet the quality requirements of garment processing.
During the dyeing and finishing process, the fabric is subjected to tension in the warp direction, and the buckling wave height in the warp direction is reduced, resulting in elongation. When hydrophilic fiber fabrics are soaked in water, the fibers swell and the diameters of the warp and weft yarns increase, thereby increasing the buckling wave height of the warp yarns and shortening the length of the fabric, causing shrinkage. The percentage of the length shortened to the original length is called shrinkage. The finishing process of using physical methods to reduce the shrinkage of fabrics after soaking in water to reduce shrinkage is also called mechanical pre-shrinking finishing. Mechanical preshrinking is to wet the fabric by spraying steam or spray first, and then apply mechanical extrusion in the warp direction to increase the buckling wave height, and then dry it through loose drying. The shrinkage rate of pre-shrunk cotton can be reduced to less than 1%, and due to the mutual extrusion and rubbing between fibers and yarns, the softness of the fabric’s hand will also be improved.
Wool fabrics can be relaxed and preshrunk. The fabrics are padded with warm water or sprayed with steam. , slowly drying in a relaxed state, causing the fabric to shrink in both the warp and weft directions.
Mechanical pre-shrinking treatment
Mechanical shrinking uses a compressible rubber blanket to carry the fabric for shrinking.
When the rubber blanket is curled under force, the outer arc elongates and the inner arc contracts. If it is bent in the opposite direction, the originally elongated side becomes contracted, and The side that contracts stretches.
During the pre-shrinking process, the fabric with a certain humidity runs close to the rubber blanket, passing through the compression zone, from the outer arc of the rubber blanket to the inner arc, that is, from the tensile part to the inner arc. In the shrinkage part, because the fabric is clamped by the rubber blanket and the pressure roller and is not allowed to slide, it is compressed as the rubber blanket recovers and shrinks, resulting in a certain shrinkage.
Usually, the fabric is properly moistened (10% to 15%) and heated before the machine, so that the fiber becomes softer and has greater plasticity. It is also used in Before wetting (after singeing finishing), it should be penetrated with light pulp or foam sizing and dried in a drying cylinder to strengthen pre-shrinkage control and improve pre-shrinkage effect.
When the fabric is pre-shrunk by the rubber blanket, it is first pre-dried to remove part of the moisture. After leaving the machine, it is dried and ironed by a tension-free blanket dryer to eliminate compression shrinkage. Wrinkles caused by excessive shrinkage for a plumper feel and soft shine.
Operation points
Pre-shrunk Three things must be kept stable during processing: the speed of the preshrinking machine remains stable, the moisture regain of the fabric entering the rubber blanket unit remains stable, and the steam pressure of the pressure-bearing drying cylinder in the rubber blanket unit remains stable.
During the processing, shrinkage monitoring marks must be made on the fabric to check and verify the pre-shrinkage rate of the fabric. If problems are found, adjustments must be made in a timely manner to ensure a stable shrinkage rate.
During the operation of preshrinking equipment, personnel operation management and equipment management must be strengthened to prevent defects from occurring. At the same time, it is necessary to master the solutions to defects to reduce the defect rate.
Problems and Solutions
Problems that are prone to occur during pre-shrinking finishing: fish scale spots, warp wrinkles, wrinkles, weft skew, wet fabric drop, insufficient shrinkage, narrow door width, etc.
1. Fish scale spots are caused by the partial aging of rubber blankets and uneven fabric pre-shrinkage. Grind or replace the defective and aging rubber or felt blankets.
2. Warp wrinkles are caused by the fact that the fabric before shrinking has already wrinkled or the warp tension is too high when the fabric enters the shrinking machine. In the pre-processing process, attention should be paid to preventing the occurrence of warp wrinkles. That is, during pre-shrinking, the stretch cloth frame can be relaxed to reduce the warp tension to solve the problem of warp wrinkles.
3. The crepe feeding tension is too small, the cloth surface is too loose, the pressure on the left and right of the cloth roller is unequal, the edges of the gray fabric are too loose or the edges on both sides are unevenly tight. Can cause wrinkles. The adjustable stretch frame can increase the cloth feeding tension, and the cloth feeding roller can be adjusted to maintain uniform pressure on both sides.
4. The causes of weft skew include incorrect cloth feeding position, uneven cloth guide roller, and uneven pressure on both sides of the cloth feeding roller. During processing, the cloth feeding position should be adjusted to ensure that the cloth guide roller is straight, and the pressure should be adjusted to make the pressure at both ends of the cloth feeding roller uniform.
5. The wet cloth is caused by excessive moisture in the fabric during processing. Therefore, during the pre-shrinking process, the spray volume and operating speed must be controlled, and the vehicle speed can be reduced so that the drop cloth can be fully dried.
6. Insufficient shrinkage results in poor pre-shrinking effect and poor dimensional stability. This is caused by reasons such as the position of the pressure roller being too high, the temperature of the pressure roller being too low, the pressure of the cloth feeding roller being insufficient, and the tension of the cloth entering and exiting being too high. The position of the pressure roller can be appropriately lowered, the spray moisture supply can be controlled, the steam pressure of the pressure roller can be increased, the pressure of the cloth feeding roller can be increased, and the cloth tension in and out can be reduced.
7. Narrow door width. The door width before preshrinking is insufficient due to the previous processing, or the fabric feeding tension is too high during preshrinking, causing the door width to become narrower after preshrinking. In the previous processing steps, the process must be strictly followed to ensure that the width of the incoming fabric meets the requirements; at the same time, during the pre-shrinking process, attention should be paid to appropriately reducing the fabric feeding tension.
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