Page 232 - Basic Principles of Textile Coloration

MACHINES FOR DYEING YARN 221

Fully filled, closed machines allow bi-directional flow that assists level dyeing.
Even though the walls of a package may be of uniform thickness, the surface area
of the package increases with distance from its centre. With in-to-out flow, this
results in a decrease in the volume flow rate towards the outer edges of the
package. To obtain level dyeing of the yarn throughout the package, the initial dye
strike must be controlled. Less level dyeing dyes, of higher washing fastness, are
used than when dyeing hanks. Frequent flow reversal gives improved levelness
with such dyes of low migration ability. Package dyeing has many advantages over
hank dyeing. It allows higher loads, lower liquor ratios, more rapid dyeing and
easier unwinding of the dyed yarn.

Package preparation is a crucial step. Some of the factors influencing the
stability of a package and its permeability to dye solution are:
(1) the denier or tex of the yarns or filaments;
(2) the degree of twist of the yarn;
(3) the extent to which the yarn traverses the package (cross-winding) and its

tension;
(4) the degree of swelling or shrinkage that occurs in hot water;
(5) the actual shape of the package.

The yarn tension throughout the package must be even and winding must be as
uniform as possible. The packing density should give a moderate resistance to flow
but adequate and even penetration of the dye liquor throughout the package. The
initial rate of dye liquor flow through a viscose filament package may drop sharply
because of the fibre swelling that occurs in water. Non-textured nylon or polyester
filaments, that tend to shrink in hot water, may have to be initially relaxed and re-
wound before dyeing. Textured artificially made filaments give softer packages that
do not require pre-shrinking and for which the permeability is much less
dependent on yarn and filament characteristics.

All packages must have a stable construction and be as identical as possible.
Depending upon the type of yarn, the packages must withstand some swelling, or
alternatively some shrinkage. It is essential that the package is stable to the change
in pressure that occurs when the liquor flow reverses direction. If the yarns move
and become trapped, unwinding becomes difficult. The flow of liquor through the
package must not cause any distortion or blowing out of the yarns. A blow-out
results in an excessive local flow of dye solution and reduced flow to other
packages. Typical flow rates in a package dyeing machine are around
3 0 l k g–1 m i n–1, giving complete circulation of liquor every 30 s, or even less. A