Non Woven Fabrics

Nonwoven fabrics are made out of fibres, without any restriction, but not necessarily from fibres. These
can be very short fibres of a few millimetres length as in the wetlaid process; these can be “ordinary” fibres, as used in the traditional textile industry, or then very long filaments etc. Properties and characteristics of a nonwoven fabric de-pend for a large part from the type of fibre it is ultimately made of. These fibres can be natural or man-made, organic or inorganic; the characteristic of a fibre being that it is longer than its thickness, or diameter. Such fibres can also be pro-duced continuously in connection with the nonwoven process itself and then cut to length, or then extruded directly e.g. from polymer granules into a filament and then fibrous structure.are neither woven fabrics, nor such other textiles as knitted fabrics. Behind these statements lies a fundamental characteristic of nonwoven: contrary to woven or knitted fabrics, fibres that ultimately make up the nonwoven fabric need not to go through the
preparatory/transitory stage of yarn spinning in order to be transformed into a web of a certain pattern.

Non woven manufacturing processes

There are three main routes to web forming

• the drylaid system with carding or airlaying as a way to form the web;
• the wetlaid system;
• the polymer-based system, which includes spunlaying (spunbonding) or specialized technologies like meltblown, or flashspun fabrics etc.

Water jet weaving machines

These machines are produced only by few companies and are used for the manufacture of light and medium weight fabrics with standard characteristics and in water repellent fibre materials, primarily multi-filament synthetic yarns. Water jet machines are extensively used in East Asia, but have limited importance in other countries. They are characterized in particular by high insertion performance and low energy consumption.

Operation principle

Figure shows how the machine operates. The weft yarn, which is fed from cone 7, is drawn-off by a feeding and measuring device 2 and then passes through a tension regulator 3 and a weft clamp 4. When the insertion has to take place, the weft clamp loosens its hold and the thread inserted inside a nozzle 1 is struck by a jet of pressurized water and launched through the shed at high speed. After the insertion has taken place, while the weft is hold flat by the threads which are moved by the leno mechanisms 5, the thermal knives 14 enter into action on the launch side to cut the weft, and on the opposite side to trim the fabric. A yarn clamping device 13 holds the weft waste which is cut off by the right-handed thermal knife, while rotating gears arrange for its removal (centre selvedge). The water is conveyed by a pump 8, provided with a filter, the piston of which is controlled by a cam 10 producing the phases of water suction from the container 9 and of water supply to nozzle 1.

The sequence of the launch phases is the following: the pump 8 enters into action and the initial water jet serves only to straighten the residual small piece of weft, from nozzle 1 to thermal knife 14. This action, which has a duration time varying from 5 to 30 rotation degrees of the main shaft, depends on the yarn count and is named guide angle. The yarn flight forms a so-called flight angle, leaving clamp 4 open to permit to the pressurized water jet to insert the weft thread into the shed. The clamp opening time varies according to reed width and to loom running speed. On yarn exit from the shed, there is an electrical feeler or an infrared sensor which checks the presence of the weft end and makes the machine to stop in case of absence of the weft.A drying device removes the humidity absorbed by the fabric, sucking it through grooves produced in the front beam 6 of the machine. A maximum of two weft colours can be inserted (weft mixer).

The devices used for shed formation are either pedal eccentrics (cranks) or cams or dobbies; Jacquard machines are not used, because water jet machines are suitable for mass production of plain fabrics.