Page 227 - Basic Principles of Textile Coloration

216 DYEING MACHINERY

acidic, alkaline or salty solutions for dyeing. Some form of anti-corrosion treatment
may be required.

The dyeing machine must have some means of heating the dye solution.
Injection of live steam into the dye solution is the simplest method. Unfortunately,
this is does not afford the best temperature control and the excessive
condensation increases the liquor-to-goods ratio. For hot dyeing liquors, adequate
means for removal of escaping steam are important to prevent condensation
outside the machine. Heating with pressurised steam, circulated in a closed loop,
is the preferred method. If cold water can circulate through the heating loop, or
through an alternative set of tubes, controlled cooling of the dyebath is also
possible. Many dyeing processes involve controlled heating to temperatures close
to, or even over, the normal boiling point of water. The actual rate at which the
dye is transferred from the solution to the fibre, and its rate of diffusion into the
fibre, are frequently controlled either by the temperature of the dyebath, or
assisted by the addition of chemicals. The machine should therefore preferably
have some means of temperature control, and of adding the dye solution and any
solid or liquid dyeing assistants. Modern machines have automatic controls for
regulation of the dyeing temperature. These allow controlled heating at a given
rate and maintenance of a set maximum dyeing temperature. Older machines used
thermocouples and relays for this but today’s machines have programmable
microprocessors.

12.2 DYEING MACHINES FOR LOOSE FIBRE AND SLIVER

For the dyeing of loose staple fibres, level dyeing is less important than for piece
goods. Yarn forming processes usually mix the dyed fibres sufficiently well to give
satisfactory colour uniformity even when the initial colour of the fibres is not
completely level. The fastness to subsequent wet processing of the dyed fibres, as
well as to washing of the final product during the intended end-use, are more
likely to be significant.

Similar types of dyeing machine serve for dyeing raw stock and various types of
sliver. The most common types hold the goods in a conical pan with perforated
inward sloping sides, or in a perforated cage. The fibres are gradually packed into
the pan, then wetted and compressed by screwing down a solid plate laid on the
top. The top plate prevents the dye solution from flooding the top of the pack. A
flange on the bottom of the pan fits onto a central pipe delivering dye solution
from the pump to the inside of the fibre mass. The entire pan or cage sits in an