Raising and Shearing Finishing of Polyester Fabrics

Raising and Shearing Finishing of Polyester Fabrics
The processing of velvet-like polyester fabrics such as suede-like, corduroy, and velvet mostly involves raising with a raising machine. This process uniformly lifts fiber ends from the yarns, covering the fabric surface with a layer of fluff, thereby achieving a soft and full handfeel, enhancing warmth retention, and improving aesthetic appeal.
1. Raising Principle
Raising utilizes the relative speed difference between steel needles or burr hooks and the moving fabric to evenly pull out a layer of fluff on the fabric surface. This makes the fabric loose, thick, soft, and more warm-retentive, while also concealing its weave texture and softening its patterns.
2. Raising Equipment
Common raising machines can be divided into two categories: wire raising machines and burr raising machines. Wire raising machines have a strong raising effect, high velvet-producing capacity, and high efficiency, but they cause relatively greater damage to the fabric’s strength. They are suitable for raising polyester woven fabrics and blended fabrics.
(1) Wire Raising Machines
Based on the rotation direction of the card clothing rollers and the orientation of the needle tips, wire raising machines are classified into single-action type and double-action type.
For the single-action wire raising machine, all card clothing rollers have their needle tips oriented in the same direction. The rollers rotate in the opposite direction to the main raising cylinder but in the same direction as the fabric movement. Adjusting the rotational speed of the card clothing rollers can achieve varying degrees of raising effect, but the raising action is intense, and the lifted fluff tends to lie flat.
The double-action wire raising machine is equipped with two groups of card clothing rollers with equal quantity and opposite needle tip orientations (forward and reverse). The forward-needle rollers (with tips pointing in the same direction as fabric movement) and reverse-needle rollers (with tips pointing opposite to fabric movement) are alternately mounted on the main raising cylinder. The speeds of these two types of rollers can be adjusted separately according to fabric type, fabric running speed, and the desired raising effect. For this reason, double-action wire raising machines are more widely used at present.
(2) Burr Raising Machines
There are two ways to install burrs on the raising machine: fixed burr type (burrs fixed on a frame) and rotary burr type (burrs sleeved on small shafts to allow rotation). Burr raising machines are suitable for wet raising and water raising processes. Their raising action is relatively gentle, resulting in less strength loss of the fabric and better handfeel and luster of the finished product. However, their raising efficiency is low, so they are rarely used for raising polyester woven fabrics.
3. Raising Methods
According to the fabric moisture content during the raising process, raising methods are divided into dry raising, wet raising, and water raising.
Dry Raising
Dry raising refers to the raising process carried out when the fabric is in a dry state. In a dry condition, fibers have high rigidity and low extensibility. Therefore, the card clothing exerts a large force on the fabric during raising, making fibers prone to breakage and producing more fallen fluff. The resulting fluff is short, dense, and the fabric surface is rough with upright fluff. Pure polyester fabrics, polyester-cotton blended fabrics, and polyester-viscose blended fabrics can all adopt dry raising.
Dry raising of polyester fabrics can be conducted in two ways:
Direct raising after the grey fabric is pre-set. The fluff produced by this method has slightly poor bulkiness, but it features low production cost and short production cycle.
Raising after pre-treatment of the grey fabric (removing spinning oil and other impurities), followed by dehydration, application of raising oil, drying and heat setting. The fluff produced by this method has good bulkiness, but it has higher production cost and longer production cycle.
Wet Raising
Wet raising refers to the raising process performed when the fabric is in a moist state. Moist fibers have low rigidity and high elongation, making the raising action gentler than dry raising, causing less fiber damage, and facilitating the formation of a thick, dense layer of fluff. Due to the low wet strength of viscose fibers, polyester-viscose blended fabrics are not suitable for wet raising.
Water Raising
Water raising refers to the method of raising the fabric directly with water without dehydration after the fabric is immersed in water. This method is applicable to the raising of woolen fabrics and the raising of polyester sea-island fiber suede-like fabrics. For polyester sea-island fiber suede-like fabrics, water raising can maximize the absorption of fabric surface vibration caused by the raising machine during the process, thereby reducing fluff marks on the raised fabric surface and improving the qualified product rate.
4. Raising Process Analysis
(1) Fiber Specifications
Since different fibers vary greatly in physical and mechanical properties, the physical properties of fibers should be fully considered when selecting raising card clothing. Polyester fabrics have high strength and a smooth surface, so card clothing with good elasticity, thick needle shafts, and sharp needle tips is suitable for raising.
Generally speaking, the longer the fiber length and the higher the linear density, the more difficult it is to raise the fabric. Similarly, the higher the yarn twist, the more challenging the raising process becomes.
(2) Fabric Specifications
During raising, the raising machine mainly lifts weft yarns to form a layer of fluff on the fabric surface. Therefore, weft density has a significant impact on the raising effect.
Fabrics with low weft density are easy to raise, producing long and sparse fluff.
Fabrics with high weft density generate short and fine fluff.
For suede-like products, to achieve the effect of short, dense, and uniform fluff, the fabric weft density should be increased. The more weft floats there are, the more fibers will be hooked out by steel needles or burrs, making raising easier and the raised fabric richer in texture, but at the cost of greater fiber damage. The higher the fabric structure tightness, the more the weft floats protrude from the fabric surface, and the easier it is to raise the fabric.
(3) Moisture Regain
Polyester-cotton blended and polyester-viscose blended products contain cellulosic fibers. If the moisture content of cellulosic fibers is too high, the friction coefficient between fibers increases, which intensifies the raising effect. Therefore, polyester-cotton blended and polyester-viscose blended fabrics must be dried before raising.
Practice shows that the optimal moisture regain for polyester-cotton blended fabrics during raising is around 6.5%. Viscose fibers have low wet strength, so they should be kept at a lower moisture regain during raising. For polyester-wool or wool-polyester blended fabrics, a higher moisture regain (usually controlled at 60%–70%) results in better raising effect. Pure polyester fabrics have a compact structure, poor water absorption, and minimal fiber swelling when exposed to water, so moisture regain has little impact on their raising effect.
(4) Finishing Agents
Finishing agents also affect fabric raising. Agents that increase the dynamic friction coefficient between card clothing and fibers while reducing the static friction coefficient between fibers are beneficial to raising, such as cationic softeners. Conversely, agents with opposite properties are unfavorable to raising.
(5) Fabric Tension
Excessively low tension makes the fabric too loose, leading to fabric deformation and uneven raising, which deteriorates the fabric’s appearance.
Excessively high tension reduces the contact area between the fabric and the card clothing rollers. The card clothing rollers slide and rotate on the fabric surface instead of inserting into the fabric interior, resulting in poor raising effect.
Thin, low-density fabrics that are easy to raise should be given appropriately higher tension during raising, while fabrics that are relatively difficult to raise can be processed with appropriately lower tension.
(6) Raising Times and Fabric Speed
The stepwise raising method is usually adopted to reduce raising force and increase raising times. The fabric speed is generally controlled within the range of 5–20 m/min. Multiple raising machines can be connected in series to perform continuous multi-pass raising on a certain fabric. Usually, 2 to 3 raising machines are used in series; using too many machines in series will significantly reduce equipment utilization.
5. Comparison Between Raising and Sanding
Both raising and sanding are processes to produce a velvet surface on fabrics, but they have slight differences:
Raised fabrics are mainly processed with card clothing, and only the weft yarns are raised. The resulting fluff is sparse and coarse-long, with relatively little strength loss of the fabric. Raising generally requires multiple passes and has low velvet-forming efficiency.
Sanded fabrics are mainly processed with emery cloth, and both warp and weft yarns are sanded. The resulting fluff is dense and fine-short, but the fabric suffers greater strength loss. Sanding generally requires fewer passes than raising and has high velvet-forming efficiency.
In actual production, the fabric velvet surface processing method should be selected according to the desired velvet surface effect.
6. Shearing
After raising, the fabric surface is covered with fluff of varying lengths. To make the fluff and velvet surface of raised or napped fabrics neat, shearing processing is often required. Generally, woolen fabrics, velvets, artificial furs, carpets, and other products all need to undergo shearing. Sometimes, for non-velvet fabrics, shearing is also performed to make the weave texture clear and the fabric surface smooth.
Shearing is completed on a shearing machine, which is mainly composed of a helical cutter, a flat cutter, and a bracket. The relative positions of the shearing components can be adjusted according to shearing requirements.
Before shearing, the fabric first passes through a brushing roller to remove yarn knots and other impurities and lift the surface fibers. When the fabric passes through the tip of the bracket and bends sharply, the surface fibers stand upright, and the rotating helical cutter and fixed flat cutter cut the upright fluff. During shearing operation, the helical cutter reciprocates horizontally along the flat cutter through a gearbox with an eccentric wheel. This not only improves shearing efficiency but also minimizes shearing defects as much as possible. Shearing efficiency is proportional to the number of shearing times per unit length of the fabric.
There are two types of brackets on the shearing machine: solid brackets and hollow brackets.
Shearing with solid brackets produces neat fluff and has high efficiency, but it is prone to cutting small holes in the fabric when encountering yarn knots or other hard substances on the fabric surface.
Hollow brackets can avoid cutting small holes, but their shearing efficiency is relatively low.
According to the fabric type and the desired final velvet surface effect, the rotational speed of the helical cutter is generally controlled between 600–1200 r/min, and the fabric running speed is generally 8–20 m/min. During the shearing process, the operation status of each component of the shearing machine should be closely monitored to prevent or reduce the occurrence of the following defects:
Vertical stripes: The main cause is nicks on the flat cutter. Before shearing, the fabric is generally subjected to raising, sanding, or brushing, leaving a small amount of metal needles or other foreign objects on the fabric surface. These foreign objects can damage the flat cutter during shearing. Nicks on the flat cutter will result in vertical stripes on the sheared fabric surface. In addition, if foreign objects adhere to the bracket, causing local bulging of the grey fabric when passing through the bracket, vertical stripes are also likely to occur during shearing.
Horizontal stripes: When the rotational speed of the shearing helical cutter is too low or the fabric running speed is too fast, the fluff entering the shearing edge cannot be cut off in time, resulting in horizontal stripes on the sheared fabric.
Diagonal stripes: Irregular overlapping of the grey fabric when entering the shearing edge will cause diagonal stripes on the fabric surface.
Semicircular stripes: Severe wear of the helical cutter is prone to causing semicircular stripes on the sheared fabric.
Incomplete shearing: Insufficient pressure between the helical cutter and the flat cutter, or gaps caused by long-term wear, will prevent the fluff entering the shearing edge from being completely cut off, leading to incomplete shearing. In addition, if the shearing edge formed by the helical cutter and the flat cutter is not on the same vertical line as the bracket, incomplete shearing will also occur.