The reason for the poor brushing effect in fabric finishing has been identified!

Purpose of Raising

Raising is carried out to improve the style and appearance of fabrics, give the surface a more wool-like appearance and better warmth retention, make the fabric bulkier and fuller, provide a softer handle, improve wearing performance, and increase the added value of the fabric.

Principle of Raising

In a wire raising machine, two types of raising rollers are mounted on a drum. One type is covered with metallic wires bent in the same direction as the fabric travel, and the other with wires bent in the opposite direction; these are commonly called the carding raising roller and the pulling raising roller. As the fabric runs through the raising machine, the rotating carding and pulling raising rollers move relative to the fabric. It is this relative motion that causes the wire tips to scrape the fabric first in one direction and then in the opposite direction, thereby pulling the fibers out to form a pile. This is the principle of fabric raising.

Common Faults in Raising and Their Solutions

Faults during raising mainly arise from three sources: unsuitable fabric or fabric processing methods, unsuitable or imperfect raising machinery, and incorrect raising operation. The faults most frequently encountered include uneven raising, torn or damaged fabric, excessive loss of weft strength, excessive fiber loss during raising, yarn chafing marks, and insufficient thickness. Among these, uneven raising has complex causes and is extremely difficult to remedy. Other faults can be avoided with sufficient care during raising. Raising defects attributable to the fabric itself originate from the fiber, yarn, loom preparation, weaving, and pretreatment (scouring, dyeing, and finishing).

1. Faults Caused by Fibers and Yarns and Their Solutions

(1) Weft strength loss is affected by fiber type. The shorter the fiber or the lower the yarn twist, the greater the loss in weft strength. Such fabrics already have low strength before raising, and a large amount of lint is shed during raising, so the raising effect is poor. The fiber length used must be at least twice the length of the pile to be pulled from the fabric, preferably with an average length exceeding 10 mm.

(2) Yarns made from longer fibers with low twist are suitable for raising even if their strength is not high. Conversely, yarns made from longer fibers with high twist are difficult to raise, but can produce a beautiful, uniform, and orderly pile. If the weft twist is too high, not only is raising difficult, but the pile appearance is also unsatisfactory.

From the above, it can be seen that if yarns of the same count but from different manufacturers or with different twists are woven into fabrics of the same construction, uneven raising is inevitable. This cannot practically be corrected even by adjusting the raising method. Uneven blending in blended yarns can also cause raising defects.

Uneven yarn tension during pirn winding leads to differences in moisture regain, causing weft-bar defects during raising; this is caused by differences in weft shrinkage after weaving. Yarns with lower shrinkage produce fine groove-like raising defects in the warp direction after raising. Uneven weft yarn thickness is a fatal defect for raising. Hard-spun yarns are more difficult to raise than soft yarns. Uneven yarn count and twist also seriously affect raising. Generally, in the same yarn, thicker portions have lower twist and thinner portions have higher twist, which further causes uneven raising.

Different dyeing conditions also affect the raising effect to varying degrees. The amount of residual dyeing auxiliaries and softeners in dyed fabrics can also cause uneven raising. For short-staple cotton fabrics dyed with direct dyes, light shades are difficult to raise and dark shades are easier. Uneven raising caused by dyeing can be improved by carrying out an appropriate softening treatment before raising.

2. Faults Caused by Fabric Construction, Specifications, and Weaving, and Their Solutions

Fabric structure and construction greatly affect the raising effect. Uneven fabric is often not easily detected at first and usually becomes apparent only after dyeing or raising. The key is that warp and weft tensions must be uniform. Alternating tight and slack selvage tension is the most difficult problem to solve; in such cases, the fabric needs further finishing. Uneven raising caused by uneven weft tension is related to pirn winding and weaving. If weft tension is low in a certain area of a piece, groove-like raising in the warp direction will appear in that area. Uneven weft density not only causes uneven raising, but the lower-density areas may also be over-raised and their strength markedly reduced. Generally, for fabrics with many weft floats, a short, dense raising method with reduced raising force and an increased number of passes is often used to improve the raising effect.

3. Faults Caused by Fabric Preparation Methods

(1) Improper moisture content. For light raising or raising where reduced fiber loss is required, the desired result cannot be achieved if the moisture content is unsuitable. Uneven moisture content causes uneven raising. Cotton fabrics are always raised dry; however, if the fabric is too dry, fiber loss during raising increases. If the moisture content is too high, warp creases easily form during raising and the pile is difficult to pull out.

(2) Improper pretreatment before raising. The fabric must be absolutely free from warp creases before raising. If warp creases are present, the fabric must be wetted out and dried before raising.

(3) Improper seam. The seam must be flat, straight, even, and secure. If the seam is wavy or the pieces are joined at an angle, a considerable length of fabric at both ends of the seam will be unevenly raised. If the seam is not secure, it may be scraped open during raising and the fabric may become entangled in the wire-covered roller, damaging the card clothing and creating a safety hazard.

4. Faults Caused by the Raising Machine and Raising Method

(1) Faults caused by drum centrifugal force. At high raising machine speeds, centrifugal force causes the center of the drum to bulge, resulting in over-raising of the central part of the fabric.

(2) Faults caused by improper raising methods. After two narrow pieces have been raised for a long time, if a wide piece is then raised, the central part of the fabric may be over-raised, forming stripe-like uneven raising. Therefore, when raising narrow fabrics, the contact position between the fabric and the card clothing must be shifted frequently.

If medium-width fabrics are run for a long time and then wide fabrics are raised, the selvages of the wide fabric may be under-raised while the center is over-raised. Because tension is higher at the two selvages and slightly lower in the center, the center becomes hairier than the selvages. Therefore, medium-width fabrics should also be shifted frequently in preparation for running wide fabrics.

If the card clothing is wound with excessive tension, it may lose elasticity, chafe the weft yarns, cause excessive fiber loss, and result in uneven raising. For some fabrics, the fabric tension between the running wire-covered rollers is already out of balance at the start of raising, affecting the operation. The relationship between fabric tension and the speeds of the carding and pulling raising rollers is critical. Once this relationship is clearly out of balance, weft tension increases, causing weft chafing or warp yarn breakage. If fabric tension is low, the wires penetrate deeply into the fabric, easily producing long pile and even weft-bar unevenness. If fabric tension is too high, the belts on the wire-covered rollers slip, which not only fails to improve raising efficiency but also easily causes torn selvages.

Editor’s Note

In short, yarns with uniform fiber and yarn quality, low twist, and high strength should be selected so that the woven fabric can achieve the expected raising effect. Increasing fabric speed does not necessarily improve raising efficiency, and increasing drum speed does not necessarily improve the raising effect. If the speed is increased too much, the contact time between the wires and the fabric is shortened, and the raising effect actually decreases.