Rieter

Operating principle

Index

Several research teams, including some in the former Soviet Union, have investigated the possibilities of forming fiber strands with the aid of electrostatic fields. However, only the process proposed by the Battelle Institute has had a degree of success. The Electrospin Corporation (USA) demonstrated an experimental spinning machine based on this principle at the 1971 ITMA in Paris.

However, little has been heard about electrostatic spinning since then. In the process based on the Battelle principle (Fig. 2), a roving (2) taken from the roving frame is passed to a conventional double-apron drafting arrangement (3) and is subjected to a draft of up to 80-fold. The fibers exit freely from the front cylinder. They must then be collected to form a fiber strand and twisted to form a yarn. The first of these operations is performed by the electrostatic field, and twisting is carried out in a twist-imparting unit (6). Twisting presents no problems. The complexity of this method lies wholly in the electrostatic field generated between the front roller and the twist element (6) by earthing the front roller and applying a high voltage (about 30 000 - 35 000 V) to the twist element. This field has to accelerate the fibers and guide them toward yarn end (5) while maintaining the elongated configuration of the fibers. When the fibers enter this field, they take up charge and form dipoles, i.e. one end becomes positively charged and the other negatively charged. An open yarn end (5) projects from the twist element into the field. This yarn is negatively charged and is therefore always attracted to the front roller.
Due to the dipole pattern, there is thus a relatively high degree of fiber straightening between the front roller and the twist element. Fibers leaving the roller are accelerated and attracted to the yarn as a result of the charges carried by the two parts. They join continuously to the yarn. Since the yarn rotates, the fibers are bound in. A yarn is formed continuously and is withdrawn by withdrawal rollers (8), to be passed to a take-up device (9) for winding onto a crosswound package.
The problem associated with this process is the formation of a yarn in an electrostatic field, as follows:
(a) Charging of the fibers, and hence their behavior in the spinning zone, is dependent upon air humidity. Accordingly, for each fiber type, a specific and highly uniform environment must be created. The machine may need to be air-conditioned.
(b) The charge on each fiber, and hence its movement, is dependent upon its mass. Short fibers with low mass will therefore behave differently from long fibers.
(c) A limit must be placed upon the number of fibers in the electrostatic field, because otherwise they will cause mutual disturbance when charging and dipole formation takes place. Only fine yarns can therefore be produced.
(d) The same effect is observed with high throughput speeds; there is a corresponding limit on the production rate.

Due to these problems, electrostatic spinning has no chance of being used in spinning mills.

Fig. 2 – The electro-spinning principle