Page 237 - Basic Principles of Textile Coloration

226 DYEING MACHINERY

stirring the bath ensures uniformity of its temperature. During dyeing, the
machine lid should be closed. This helps to maintain a high and uniform roll
temperature and reduces heat loss and steam consumption. Any condensation on
the lid must run into the dyebath without drops falling onto the fabric rolls. The
lid limits air access and avoids rapid oxidation of reduced leuco dyes. Dyeing under
pressure is also possible using jigs that roll into an end-opening autoclave.

The jig has a small bath containing the dye liquor (200 to 750 l) and gives a
very low liquor-to-goods ratio of around 1:1. It is therefore useful for those dyes
used on cellulosic fibres that normally give only low to medium exhaustion. Jig
dyeing is a form of pad–roll dyeing repeated several times. Only a few metres of
the total length of fabric are in the bath solution at any moment and dyeing occurs
almost entirely in the roll of fabric outside the bath. The rate of dyeing depends on
the amount of liquor retained by the fabric, the degree of exhaustion in the dwell
period, and the extent of liquor exchange during the next dip. The latter is usually
around 60–70%, but may be less for heavy fabrics.

The roll of fabric may vary in length from 1000 to 5000 m. The fabric must
have constant width and be wound up so the selvages align exactly one on top of
the other. The roll therefore has uniform, vertical sides, perpendicular to its axis.
Expander bars ensure that the fabric winds up free from creases. Each end of the
fabric has an attached end cloth. If several pieces of fabric are in the roll, the ends
are cut straight and sown end-to-end without overlap or spaces. A heavy seam
holds more dye solution and may transfer dye to adjacent layers of fabric that will
show more deeply dyed stripes.

The modern jig often has both the take-off and take-up rollers driven to
minimise lengthways tension. This is quite pronounced if only the take-up roller is
driven. Regulating the relative roller speeds, or braking the free take-off roller,
adjusts the tension. Lengthways tension elongates the fabric and reduces its width.
In subsequent processes, relaxation of the strain introduced may also result in
lengthways shrinkage. To promote uniform dyeing, and maintain a constant dip
time in the bath, the linear velocity of the fabric is maintained fairly constant by
gradually changing the angular speeds of the rollers.

There is little mechanical action in a jig machine and it is less suitable where
vigorous scouring is required before dyeing. The major problems are side-to-centre
colour variations, called listing, and lengthways colour variations, called ending.
Ending is more pronounced with slow dyeing or temperature-sensitive dyes. The
fabric at the end of the roll receives two closely spaced dips in the dyebath and
tends to be dyed paler with slowly exhausting dyes. The end of fabric close to the