Raising or Napping Finishing | Working Process of Raising FinishingProcess

Raising or Napping

A finishing process that raises the surface fibers of a fabric by means of passage over rapidly revolving cylinders covered with metal points or teasel burrs. Outing, flannel, and wool broadcloth derive their downy appearance from this finishing process. Napping is also used for certain knit goods, blankets, and other fabrics with a raised surface.

The raising process is a very old technique known also to Romans (as pictured in some paintings found in Pompeii). This operation is particularly suitable for wool and cotton fabrics; it gives a fuzzy surface by abrading the cloth and pulling the fibre end to the surface. During those last years this process has also been applied on polyester/viscose blends and acrylic fabrics. 

By means of this 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 fibre 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. 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

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.

Raising (napping) machine

1: roller; 

2: rollers equipped with hooks;
3: fabric;

4: nib cleaning brushes;

5: fabric tension adjustment

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. 

Raising the face of the fabric

The pile extraction is easier when carried out on single fibres: it is therefore suitable to reduce the friction between the fibres by wetting the material or, in case of cellulose fibres, 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 fibres 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. 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. 

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 fibre 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 minimises the loss of fibres 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.