Page 235 - Basic Principles of Textile Coloration

224 DYEING MACHINERY

friction and its own weight. Covering the winch with polypropylene or polyester
tape increases the friction.

Deep-draught winches have circular or only slightly elliptical winch reels and
the fabric falls into the back of the dyebath with little pleating. This type of winch
is preferred when fabric stretching may occur. Shallower machines have more
elliptical winches and plait the fabric into the back of the machine. The bottom of
the machine slopes from back to front so that the fabric accumulating behind the
winch easily moves forward. Tubular knitted fabrics, with a compact construction,
tend to form a balloon filled with air just below the jockey roll. This is desirable
since crease locations change with each cycle. An elevated driven roller at the
front of the machine is used to load and unload the goods.

A perforated baffle separates the liquor in the front of the machine from the
remainder. This part of the machine is called the salting box. Both dyes and
chemicals are added to it during dyeing and gradually mix into the remainder of
the solution. All additions must be made across the full length of the machine
with stirring. Heating is by steam injection from a perforated pipe running along
the length of the salting box. Careful design is necessary to ensure that steam
condenses before reaching the liquor surface. Direct steam injection causes some
bath dilution and can be quite noisy when steam bubbles collapse explosively on
rapid cooling, or when they strike the walls. The machine is preferably closed to
avoid steam and heat losses, particularly when dyeing at or near the boiling point.
The problem of non-uniform temperature in the bath, caused by the cooled fabric
falling into the back of the bath, can be minimised by use of a pump to circulate
the bath solution. The liquor ratio is typically about 20:1. During dyeing, the bulk
of the fabric is moving slowly in the dyebath. The rate of dyeing is partly
controlled by the rate of rope cycling and usually increases with increasing winch
speed. The winch is usually run quite fast but not at such a speed as to damage to
fabric, or promote its elongation or felting.

As for all forms of rope dyeing, the fabric must be fairly resistant to lengthways
creasing. Ideally, there should be constant lengthways folding and unfolding of the
rope as it cycles around the machine. Lubricants are often added to the dyebath to
promote this. A wide variety of materials that do not readily crease can be dyed
using the winch machine. The good mechanical action promotes crimp in the
yarns and improves the loop length of knitted materials. It gives thicker fabrics
with a fuller handle. Open-width threading of the fabric in a winch machine is
possible for heavy materials such as carpets and for fabrics that might be crushed
in rope form.