In both woollen and worsted manufacturing there are three stages at which wool undergoes colouration:

• Fibre - loose wool or tops
• Yarn - as hank or packages
• Piece - in fabric or garment form

The requirements of the dyer are to produce goods that are of the correct colour with fastness properties appropriate to the end product and having uniformity of shade.

Fibre Dyeing

Raw wool is not dyed before scouring, because of the contaminants that it contains. Dyeing of loose wool is carried out in woollen processing but wool for worsted yarns is not dyed until after combing. The reasons for this are that coloured noils have less value than ecru noils and the combing of relatively small lots of dyed wool would not be cost effective. The dyes used for fibre dyeing must have good fastness to wet treatments, because the yarn into which the fibre is spun or the will normally undergo further processing. These processes almost invariably involving scouring (washing) to remove processing lubricants or other, more severe wet treatments, such as milling or potting.

Dyes having good wet fastness are generally less level dyeing i.e. more difficult to achieve uniformity of shade than those having lower wet fastness. However, levelness of shade is less critical in fibre dyeing than for yarn and piece, because any minor non-uniformity will be eliminated in carding or gilling, which thoroughly mix the dyed fibres. Shade matching is also easier to achieve in fibre dyeing, because each processing lot normally consists of several dye batches. If the initial dye batch is not quite the correct shade the dyer can adjust the recipes of subsequent batches so that, when blended together, the correct shade will be achieved.

The reasons for dyeing wool in fibre form are therefore:

• To produce large lots of yarn with good shade uniformity
• For mixture (mélange or heather) shades
• For high wet fastness properties
• For fibre blend products

A disadvantage of fibre dyeing is that wool may be damaged during the dyeing process, which reduces the efficiency of spinning in comparison with undyed wool. There is also a commitment to shade selection at an early stage in the manufacturing process. It should be noted that the time from fibre dyeing to the end product sale is a minimum of three months and probably much longer. Therefore there is more difficulty in predicting which shade will be fashionable, so far in advance.

Fibre dyeing is carried out in machines where the dye liquor is pumped through a compressed pack of loose fibre or tops and a typical machine is illustrated below.

Tops are dyed in “ball” form, where the sliver is wound like a bobbin, each one being of about 5kg. Alternatively, “bump” (coiled) tops may be dyed and this form is regarded as being rather easier to dye uniformly. Bump tops are produced by the top maker, without the need for rewinding into the ball form, and a typical weight for dyeing is 10kg. Top dyeing machines normally have, perforated spindles onto which the tops are placed and compressed. Each spindle will hold five or six tops and the number of spindles will vary, according to the machine capacity, up to about ten.

Yarn Dyeing

Wool yarns are dyed in the form of hanks (skeins) or packages. Examples of yarn packages and package dyeing machines are shown below.

For dyeing yarn packages are loaded onto perforated spindles in the dyeing machine, as shown. Uniformity of density of the packages is vital to achieve uniform flow of the dye liquor, which is pumped through the yarn. Packages are therefore either precision wound on rigid centres or random wound and then compressed as a column on the centre spindle. The latter form requires package centres that are either compressible or are biconical and can slide one inside the other.

For hank dyeing yarn must first be wound into the hank form. Special machines must are used for dyeing hanks, having “sticks” (metal rods) across the top of the machine on which the hanks are hung. “Sticks” are often also used at the bottom of the hank to keep it in an extended form, although some yarns, particularly for carpets, are dyed with only a top “stick”. Dye liquor is circulated through the mass of yarn to achieve uniformity of shade.

Dye selection for yarn dyeing is important if adequate shade levelness and fastness properties are to be achieved. Levelness of shade is much more critical than for fibre dyeing but there is more tolerance than in piece dyeing, particularly if the yarn is to be used in multicoloured, patterned products. However, yarn dyeing would seldom be used to produce, for example, plain coloured woven fabrics.

The advantages of dyeing in yarn form are that it allows efficient spinning of undyed fibre, shades can be selected nearer to the time of end product sale and smaller dyelots are feasible than for fibre dyeing.

Fabric Dyeing

Dyeing in fabric form minimises the time between shade selection and retailing for woven goods. However, it is limited to the production of plain shades.

Uniformity of shade is critical in piece dyeing and it is therefore essential to use dyes that are capable of producing level dyeings. These dyes generally do not have good fastness to washing or wet processes but this is not a serious problem, because most of the products that are manufactured from piece dyed material will require fastness to dry cleaning only.

Unlike yarn and fibre, fabric is dyed in machines in which the material is circulated through the liquor. The schematic diagram below illustrates the design of a modern fabric dyeing machine.

Traditional “winch” dyeing machines are being superseded by “overflow” and “soft flow” machines. In all these machines the fabric is fed over a winch reel and the two ends sewn together. The driven reel (and dye liquor in overflow and soft flow machines) circulates the fabric through the dye liquor. A time of 1.5-2 minutes for each revolution of the fabric rope is normally regarded as adequate to produce a level dyeing.

Garment Dyeing

Garment dyeing is a most critical operation but gives the manufacturer the opportunity to colour his product at the latest stage possible in the production process. For wool products this route is used only for knitwear (the dyeing of knitted panels, although not strictly garment dyeing, is also included here). The criticality of the process arises because much knitwear that is garment dyed is to be sold as machine washable or hand washable. This requires dyes of good wet fastness, which are generally not very level dyeing. However, because garment dyeing produces plain shades, it is vital that there is a very high degree of shade uniformity.

To resolve this dilemma specific dyeing processes have been developed that allow dyes of good wet fastness to be applied. The softer yarn twist of most knitwear is also more conducive to dye liquor penetration of the material than, for example, woven fabrics.

Wool garments are dyed in “paddle” machines or rotary drum machines. These have a gentle mechanical action to circulate the garments through the dye liquor and thereby avoid undue felting. The dyeing process is often combined with scouring, milling and shrink resist routines.