This operation characterizes the processing of longer and finer wool for the production of valuable yarns.
Combing produces wool slivers (tops) of 20-30 g/m, which are made up in bobbins or cans, which contents is successively pressed into bumps.
The aim of this operation is to eliminate the shorter fibres and at the same time to parallelize and to clean the fibre bundle.
The combing line envisages several passages.
Pre-combing
Pre-combing lines generally consist of three intersecting passages which enable to parallelize the fibres, so as to comb the fibres with minimum discarding of long fibres. When processing wool and long-stapled man-made fibres, pin drafters are used which have combs placed between the feed and delivery rollers to control the fibres during the drawing operation.
The combs are driven by chains or by rotating cams and the drawing frames can be fed by cans or by bobbins. The delivery units are automated both for can and bobbin delivery.
The drive systems use motors with frequency controlled speed (inverter), which permit to change the working speed according to the kind of processed fibres.
The drawing frame has at its delivery automatic devices for sliver threading, so as to facilitate sliver piecing in case of sliver breaking.
The geometry of the comb system is of primary importance, especially the distance between the points at which the comb quits the sliver and the nipping point of the delivery rollers, in particular when processing wool types containing high percentage of short fibres.
In the latest machines, the axis of the feed and delivery sides are driven by separate motors with electronic control of the speed and consequently of the change of draft ratios. These drawing frames can be equipped also with on-line control systems to ensure sliver regularity. The drawing frames with electrically driven axis enable to apply new self-adjustment systems, since the speed of the feed and delivery rollers are controlled and can modify the draft ratio depending on the electronic impulses coming from mechanical feelers which measure the mass of the input sliver.
The system includes also a memory which considers the time needed by the sliver coming from the feeler to reach the draft zone. These electronic systems have replaced complex mechanical systems.
Rectilinear combing machine The purpose of this machine is:
· To eliminate shorter fibres
· To obtain a sliver with a fibrous diagram showing fibres with a higher average length
· To clean the sliver
· To further parallelize the fibres.
This process is carried out with the single-headed, intermittent working. rectilinear combing machine (Fig. 10).
A fibre tuft is torn out of the feed sliver by a gripper system. The heads of the fibres are cleaned by a rotating comb and the fibre tuft is then gripped by a pair of detaching rollers. At this stage the rectilinear comb lowers itself and combs the back ends of the fibres. The tufts are then overlapped and taken up in form of sliver. A crimping device imparts consistency to the sliver, before its laying into a can. The working speed reaches 260 strokes/min, the feeding charge is up to 500 g/m and the delivered slivers weigh 23- 35 g/m.
The machine is provided with a suction system which removes impurities and collects the discarded short fibres, which are used within further processes.
Post-combing
The combing machine is followed by three passages of intersecting gill-box which, through the usual doubling and drawing actions, impart regularity to the sliver. A contribution in this direction is also given by the autolevelling system applied on one of the three intersecting passages. This operation is extremely important, as this is the last passage in which a change can be brought about to obtain a regular yarn.
In fact the following passages - the finisher drawing frame or the roving frame or the spinning machine - have only the function of thinning the count.
The pre-combing and post-combing stages can rely, in case of big production lots, on automated systems, although not neglecting the technological requirements which are essential for a good yarn quality:
· Doubling of the slivers
· Alternate combing of the top end and of the bottom end to eliminate fiber hooks.
Considerable are the economies in material handling (-30%) and in the occupied floor space (60%).
