Prepare for spinning
The top-making stage essentially prepares the wool for the spinning process, at which the wool is formed into a yarn, ensuring:
- the wool fibres are thoroughly blended to form a homogenous mixture
- that all fibres are parallel to each other
- any short fibres and residual contaminants have been removed; and
- the sliver (continuous stand of wool fibres) formed has a uniform weight per unit length (Grams/Metre) throughout its length.
The top-making process comprises three main steps, which includes Carding, Gilling and Combing.
Prior to carding, an accurate amount of synthetic processing lubricant and water is mixed together and applied to the now blended and conditioned scoured loose wool. The purpose of the lubricant is to reduce fibre to metal friction, and water reduces the formation of static. If either occurs during the carding process it is more than likely this will lead to excessive fibre breakages and reduce processing efficiency.
Typically the amount of residual grease remaining on scoured wool is about 0.3% to 0.5% by weight, which is actually a good thing, because wool grease in itself is a good processing lubricant. The amount of synthetic processing lubricant applied, taking into account the actual amount of wool grease present, is to increase the amount of Total Fatty Matter in percentage terms (%T.F.M) to the optimum for effective processing on the worsted system which is 0.8% +-0.2%.
Although during the scouring process one of the aims is to minimise the degree of fibre entanglement, a certain degree invariably occurs. The result is that the scoured loose wool tends to form in clumps or flocks of entangled fibre.
The purpose and objectives of carding is to:
- disentangle these clumps, by separating them into individual fibres
- align the fibres parallel to each other
- remove short fibres
- further blend the fibres by intermixing
- remove as much of any remaining contaminants, such as vegetable matter (seeds, grass, burrs etc.), as well as dirt and dust
- formation of a homogeneous web in terms of weight per unit area
- condense the web into a continuous length of fibre known as a card sliver
Although all occur, the success of carding is dependent upon achieving the above with minimal fibre breakage, minimal hook formations (fibres which take the form of a hook, rather than being straight), and minimal neps (very small clusters of short wool fibres).
The card, or carding machine, comprises a series of horizontally aligned rotating rollers or cylinders which are covered in pins. Some of the rollers or cylinders rotate in different directions - clockwise and counter-clockwise - whilst others in the same directions. Some rotate at the same speed (RPM), whilst others rotate at different speeds. Some are of the same size in terms of diameter, whilst others vary. Similarly, the pins are the same on some in terms of density, length and orientation, whilst on others they vary.
These different configurations enable differing types of mechanical actions to be exerted on the wool as it passes through the carding machine in order to achieve the objectives as described above. These differing mechanical actions, in order as the wool passes through the machine, include:
- a combing action as the wool enters the machine, to assist in parallelisation of the fibres, and removal of short fibres
- a carding action to disentangle the clumps of fibres, and thoroughly inter-mix the fibres to form a homogenous blend
- a stripping action, which works in conjunction with the carding action, assisting in further blending, parallelisation of the fibres and contaminant removal
- a doffing action to produce a homogenous web of fibres of uniform mass
- a condensing action which transforms the web into a continuous length of card sliver.
A series of gilling is carried out before (on card sliver) and after the combing stage (on combed sliver), to achieve the following objectives:
- Further blending of the wool fibres to produce a homogenous blend
- Further alignment of parallelisation of the fibres in the sliver
- Ensure that hooked-shaped fibres produced during the carding process are positioned in the best direction, such as a trailing position, in order to minimise fibre breakage during subsequent processing stages
- To even out or uniform the linear weight of the sliver (grams per metre) throughout.
The latter is achieved by passing the wool at least once through a special gilling machine which has an auto-levelling device, which continuously detects the linear weight of the slivers being fed into the machine, and speeds up or slows down the machine accordingly. For example, if the slivers entering the machine are on the heavy side then the machine will speed up and vice versa. The net result is that the sliver emerging from the machine has a uniform linear weight (grams per metre) along its entire length.
Gilling machines have two sets of rotating rollers - the feed or back roller, and the delivery or front roller. Positioned in-between these rollers is a bed of moving pinned combs.
The delivery rollers always are set to operate faster (RPM) than the feeding rollers, and the ratio in terms of their relative difference in speed is known as the draft. The draft can be adjusted to determine the linear weight (grams per metre) has it emerges from the gilling machine.
The feed rollers feed a number of slivers into the machine. Delivery rollers grab the slivers and draw them through the bed of moving combs, combining the fibre mass to result in a single sliver exiting the machine.
In the worsted system, wool is combed, and in the case of the production of fine wool yarns, is combed twice (known as re-combing) to ensure a better quality of yarn can be produced.
The objectives of combing are:
- To remove short fibres, known as noils, which if allowed to remain would result both in lower efficiency during the subsequent spinning operation, inferior appearance (less smooth fabrics) and wear performance (pilling)in the final articles of clothing
- Removal of neps (very small clusters of fibres), most of which are formed during the carding operation
- Removal of any traces of any remaining vegetable matter (such as burrs, seeds, grass).
Noils and Neps are invariably not wasted or thrown a way; they are sold for processing on the woollen system.
Card slivers which contain short fibres, neps and vegetable matter are fed into the comb, or combing machines, and emerges free from these in the form of a combed sliver.
The action of the combing machine is discontinuous and involves brushing a fringe of fibres through a series of fine combs.
A set of nipper rollers grip only the long combed fibres. The fringe of the long fibres is then laid on the previously extracted fringe and the new combed sliver is formed. The short fibres are discarded.