Page 242 - Basic Principles of Textile Coloration

MACHINES FOR DYEING FABRIC 231

being introduced. The other loose end of the tape is slowly withdrawn as the rope
feeds into the machine and is finally used to lift the end originally inserted back up
to the entrance port. The two ends are then sewn together to form a 300–400 m
continuous rope. After dyeing, the seam between the fabric ends is cut and the
tape reattached to the end that will pass through the jet. It is pulled into the
machine as the fabric is unloaded and remains there.

Because the interior of most jet machines is not easily accessible, they are
difficult to clean. In particular, polyester oligomer deposits may develop on the hot
surfaces and later dislodge. Cleaning with circulating NaOH and Na2S2O4
solution is common. The inside surfaces must be smooth to avoid snags.

Jet dyeing machines have liquor-to-goods ratios in the range from 5:1 to 15:1.
The lowest liquor ratios require using only partly filled machines and are useful for
dyeing cotton knits with reactive dyes because of the lower quantities of chemicals
consumed. Partly filled jet machines, however, tend to produce copious amounts of
foam from the turbulent passage of dye liquor and fabric through the venturi. Even
when low-foam chemicals are used in dyeing, an anti-foaming agent such as a
silicone or fatty alcohol will be required.

The venturi tube may be inside the main body of the machine, or in a separate
tube on its outside. The size of the actual orifice depends on the fabric weight and
width. For a given rope of fabric, too small a jet produces more creases because the
fabric rope is more compressed in the jet and may even jam. Too large a jet gives
inferior rope transport. In some machines, jets of varying diameter are
interchangeable, while others have adjustable jets. Accurate positioning of the jet
avoids giving a circular motion to the liquor that might cause twisting of the rope
and more creasing. Rectangular jets help to open up the fabric rope.

Jet machines can be quite hard on the fabric. Lifting the dripping rope from the
base of the machine to the jet, and the force of the jet, cause longitudinal tension
and creasing. Although the frequent fabric movement, and the ballooning of the
fabric after the jet, minimise lengthways creases, this type of machine is not
suitable for many delicate fabrics. Fabrics from spun yarns of staple fibres may tend
to become quite hairy in appearance because of abrasion.

Much gentler jet dyeing machines avoid some of these effects. Some limit the
fabric tension by providing mechanical assistance in moving the plaited fabric
through the machine and back to the jet. Fully flooded jet machines are useful for
more delicate fabrics. They will have a higher liquor ratio (15:1). In a fully flooded
machine, the fabric rope is always immersed in the dye liquor. Fabric from the