Page 246 - Basic Principles of Textile Coloration

CONTINUOUS DYEING EQUIPMENT 235

The pad rollers are called bowls and may be horizontally or vertically opposed.
The region where they are in contact and exert pressure on the wet fabric is called
the nip. On leaving the bath, the fabric passes vertically into the nip of
horizontally opposed bowls, or over a small free-running smooth roller in front of
the nip of vertically opposed bowls (Figure 10.7). After the nip, the fabric should
leave both bowl surfaces at the same time to avoid back-to-face colour differences.
In fact, after leaving the nip, contact of the wet fabric with any roller or guide
should be minimal. The heavy padder must be on a firm floor with the fabric
accurately centred.

The bath volume should be as small as possible (30–40 l). This helps to
minimise waste residual solution after padding is over and promotes a rapid turn-
over of the bath solution. This ensures better constancy of the solution
composition in the bath and minimises initial colour tailing from the preferential
absorption of the dyes (Section 10.5.2). Ideally, padding should occur under
conditions for which dye substantivity is negligible. Padding at room temperature
is most common. For dyes with appreciable substantivity at room temperature,
padding is often done at higher temperatures where the dyes are less likely to
cause colour tailing. Various types of displacement devices are used to minimise
the volume of liquor in the pad. If the fabric is not as wide as the pad trough,
heavy, hollow metal blanks are added at the ends of the bath. A similar metal
displacer is often placed in between the descending and ascending fabric, above
the submerged guide roller. Such devices minimise the bath volume and ensure
rapid turn-over of the dye solution. Liquor circulation in wide troughs ensures
uniformity of the dye concentration across the width.

The two pad rollers are called bowls and their axes may be horizontally or
vertically opposed. They are typically about 2 m long and 35–40 cm in diameter.
The roller axes are usually exactly aligned. A pneumatic system applies a pressure
of up to about 50 kg cm–1 of bowl length at both ends of one of the mandrels, the
upper one when they are vertically opposed. This gives a contact width of about
1 cm where the rubber flattens. The higher the applied pressure, the greater the
degree of deformation of the rubber surface, the lower the solution pick-up, but
the greater the extent of wear over time. The rubber surface should be treated
with great care. Any surface damage holding dye solution may transmit repeating
colour spots onto the padded fabric. The bowls should be hosed down immediately
after use and the pressure released. The pressure should only be applied when the
rollers are rotating to avoid forming a permanent flattened zone that can cause an
irregularly coloured stripe on the fabric.