To obtain finer yarns, besides using longer fibres (30-38 mm) of smaller diameter, the sliver is submitted to a combing operation, which has a dual purpose:
· to eliminate the shorter fibres
· to further parallelise and clean the longer fibres.
Before combing, the sliver must be prepared with a passage of pre-combing drawing frame, followed by a lap winder, which has the task of assembling a certain number of slivers (24 or 36) in form of a roll (lap) suitable for feeding regularly the combing machine.
The lap winder is equipped with automatic systems for lap weighing and with differential pressure systems during lap formation, so as to reduce felting even at high production speeds.
The combing machine (Fig. 7) in which, through an intermittent movement, some fibre tufts are picked up, made to pass through two combs (a circular comb for combing the top ends of the tufts and a straight comb for the back ends), separates the short fibres according to the fibre diagram to be obtained. This operation causes also a reduction in the neps, the fibre entanglements which cause later on broken ends in the spinning machine and irregularities in the yarn.
The new models of combing machines are equipped with automatic lap feeding devices. Besides, systems have been developed for the automatic lap transport from the lap winder to the combing machines. After combing, the cycle continues with a drawing frame passage, the roving frame, the spinning frame, the winder and, if necessary, yarn doubling, twisting and finishing (see chapter 3.3).
