What is Raising or Napping | Working Process of Raising Finishing

Last Updated on 15/07/2023

What is Raising or Napping Finishing?
Raising or Napping is the finishing process that raises the fibers on a fabric to produce a mat of fiber ends, or nap. It may be used on knit or woven textiles made of staple fibers, such as wool and cotton, or with fibers cut to staple length and spun into yarns such as silk, rayon and polyester. Napped fabrics are usually made with loosely spun yarns in the filling direction (weft) so that the fibers can easily be pulled out to form the nap. Raising is also used for certain knit goods, blankets and other fabrics with a raised surface.

Raising can be done on one or both sides of the fabric. The technique can improve durability, hide defects or obscure the weave of the cloth. On the other hand, napped fabrics can also have increased pilling, abrade more easily even with care or flatten with wear. Common fabrics that are napped are wool and cotton flannel, flannel-back satin, polyester fleece, flannelette and outing flannel. Sueded fabrics are also napped through a process that includes an additional step to shear the nap close to the surface of the fabric to produce a smooth, soft finish.

Working Process of Raising Finishing:
In raising finishing process a hairy surface can be given to both face and back of the cloth providing several modifications of the fabric appearance, softer and fuller hand and bulk increase. This enhances the resistance of the textile material to atmospheric agents, by improving thermal insulation and warmth provided by the insulating air cells in the nap. The fuzzy surface is created by pulling the fiber end out of the yarns by means of metal needles provided with hooks shelled into the rollers that scrape the fabric surface. The ends of the needles protruding from the rollers are 45° hooks; their thickness and length can vary and they are fitted in a special rubber belt spiral-wound on the raising rollers (Figure 1). These rollers are generally alternated with a roller with hooks directed toward the fabric feed direction (pile roller), and a roller with the hooks fitted in the opposite direction (counterpile roller).

Raising rollers
Figure 1: Raising rollers

The machine also includes some rotating brushes, which suction-clean the nibs in pile and counterpile directions. Actually the trend goes towards a ratio of raising rollers/pile rollers equal or 1/3. The two series of rollers have independent motion and can rotate with different speed and direction thus carrying out different effects (Figure 2).

  1. Roller;
  2. Rollers equipped with hooks;
  3. Fabric;
  4. Nib cleaning brushes;
  5. Fabric tension adjustment
Raising machine
Figure 2: Raising machine

The action of these systems is almost powerful and the results depend upon the effects and the type of fabric desired . The raising effect can be obtained by adjusting the fabric tension (5) or by adjusting the speed and the roller rotation direction (2).

Once a certain limit has been exceeded, the excessive mechanical stress could damage the fabric: it is therefore better, when carrying out a powerful raising, to pass the wet fabric through the raising machine many times (dry when processing cotton fabrics) and treat the fabrics in advance with softening-lubricating agents.

The pile extraction is easier when carried out on single fibers: it is therefore suitable to reduce the friction between the fibers by wetting the material or, in case of cellulosic fibers, by previously steaming the fabric. For the same reasons, it is better to use slightly twisted yarns.

The same machine allows different options of independent motions:

  1. Fabric moving between entry and exit
  2. Motion of large drum
  3. Motion of raising rollers

The raising intensity can be adjusted by suitably combining the above mentioned independent motions, the tension of the textile material, the number of pilewise or  counterpile raising rollers and their relative speed. It is possible to obtain combed pile raising effect, “semi-felting” effect with fibers pulled out and re-entered in the fabric, and complete felting effect.

The raising machine is equipped with two overlapping drums each one featuring 24 rollers, which can process two faces or face and back of the same fabric (Figure 3). The drums assembled on a standard machine can rotate separately one from the other in the fabric feeding direction or in the opposite direction by carrying out a counter rotation. In this model all the functions are carefully monitored and controlled by a computer system; in particular all the commands are driven by alternating power motors controlled by “Sensorless” vector inverters.

Raising the face of the fabric
Figure 3: Raising the face of the fabric

The control electric system features:

  1. PLC programmable controller for machine and alarms automation;
  2. Touch screen to program and update all processing parameters;
  3. Operating conditions of each single raising process (up to one million recipes”) that can be stored to facilitate the batch reproduction.

Furthermore, a series of special pressure rollers can be assembled on the feeding cylinders to prevent the fabric from sliding, thus granting an extremely smooth raising. The raising process ability lies merely in raising the desired quantity of fiber ends without excessively reducing the fabric resistance. For this reason, the technique applying the alternated use of pile and counterpile rollers is the most widely used since it minimizes the loss of fibers from the fabric and the consequent resistance reduction.

Standard raising machines have been designed to work with fabrics powerfully tensioned essentially because they are not equipped with an efficient and reliable tension control.

This gives rise to the effects detailed below:

  1. The contact surface between the fabric and the raising cylinders is quite small;
  2. The hook nibs work only superficially on the fabric and the raising effect is quite reduced;
  3. The fabric width is drastically reduced.

The above mentioned inconveniences have now been eliminated thanks to the last generation of raising machines, which reduce the number of passages and carry out the raising process by gently tensioning the fabric.

Problems in Raising Finishing:
Following problems are occurred in raised knitted fabrics:

  1. Variations in temperature and humidity conditions can affect the raising effect. Cotton is usually processed in a warm, dry condition, although if it is badly creased it may be prewetted and re-dried.
  2. If the pile action is much greater than the counterpile, the cloth may tend to cling to the pile rollers. The cloth on the feed side then becomes very tight and on the backside very slack. This may result in creasing. The machine should be reset to a more balanced action.
  3. Bad setting of the cleaning brushes (‘fancies’) can do a lot of damage, but in general uneven raising should be corrected by re-grinding or replacing the wire.
  4. Changing the fabric width may produce lines due to wire damage at the selvedge of the previous cloth.
  5. Lateral striping can be caused by yarn variation, which may not show up before raising.
  6. Streaky or patchy raising may be due to traces of finishing agents.

You may also like:

  1. Textile Finishing: Beautification Process of Fabric
  2. Sueding Finishing Process: Mechanism, Application and Benefits
  3. Calendering Finishing Process in Textile Industry
  4. Advances in Textiles Finishing and Applications

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