Page 229 - Basic Principles of Textile Coloration

218 DYEING MACHINERY

There are also machines for the continuous dyeing of loose fibre and sliver. For
loose fibre dyeing, suction onto a perforated drum or wire cage holds the fibres in
place during application of the dye solution and subsequent heating with a mixture
of steam and hot air to bring about dye fixation. This type of process requires large
runs of one colour for economic operation. Only dyes that absorb and fix rapidly
are suitable. In continuous sliver dyeing machines, the band of parallel fibres is
impregnated with dye solution and then passed through a heated tunnel. The
tunnel may be sectioned so that dyeing at high temperature occurs in the first
section, washing-off of unfixed and surface dyes takes place in the second, and the
fibres are dried in the third.

12.3 MACHINES FOR DYEING YARN

12.3.1 Hank dyeing

In a hank dyeing machine, the skeins of yarn usually hang from poles fitted into a
frame that can be lowered into the dyebath. Rods inserted at the bottom of the
frame keep the skeins fully extended. A spare frame allows preparation of one load
while another is in the dyeing machine. The frame has a perforated top and
bottom. The dye liquor gently circulates either down through the hanks, or in the
reverse direction if the machine is completely filled. At the front of the machine, a
compartment, separated from the hank frame, contains the impeller circulating
the dye liquor. The steam and cooling pipes are at the bottom of this
compartment. Dyes and chemicals are also added here. This type of dyeing
machine is frequently referred to as a Hussong machine.

For level dyeing, equal exposure of all the yarn in the skeins to the circulating
dye liquor requires even packing of the skeins. It is important that upward
circulation of the liquor in the machine lifts the yarns in contact with the upper
poles, otherwise these will dye paler because of poor contact with the liquor. The
liquor ratio is usually in the range from 24:1 to 30:1. Such high liquor ratios
require greater amounts of dyeing assistants and consume more steam than in
package dyeing machines described in the next section. The rates of heating and
liquor circulation are low.

Even though dyeing in a Hussong machine is less productive and less economic
than dyeing yarn wound into bobbins, it is still widely used for dyeing skeins of
wool and acrylic yarn. The Hussong machine preserves the yarn bulk and handle.
Acrylic yarns, in particular, may shrink in the hot dyebath, opening channels