Page 247 - Basic Principles of Textile Coloration

236 DYEING MACHINERY

The pressure at the ends of the mandrel causes some bending. This gives higher
pressures towards the fabric selvages and a higher solution pick-up in the centre.
To counteract this, the surface camber of one or both bowls can be modified to
give even pick-up across the nip width under pressure. The rubber surface is
ground so that it tapers slightly towards each end. The camber introduced will
only give a uniform pressure across the width of the nip for one application
pressure. Alternatively, the axes of the two bowls can be skewed slightly so the
applied pressure is greater in the middle and less at the ends. The degree of skew
can be varied for different applied pressures.

The swimming roller developed by Küsters is a more sophisticated method of
compensation for mandrel deflection under pressure. This type of roller has a
stationary mandrel with an outer rubber shell that rotates on bearings. The space
between the mandrel and the rubber shell is divided into compartments in such a
way that the central region oriented towards the nip can be pressurised. In this
way, the surface pressure at the nip in the middle of the roll can be increased to
compensate for deflection of the mandrel. By controlling the pressure of oil
pumped in, and the load on the mandrel, the surface pressure at the nip is more
uniform across the width.

Some pads provide two dips into the dye solution. This may involve two sets of
pad rollers with two separate baths of dye solution, or two nips between a set of
three bowls with a single bath. When padding dry fabric using two baths, much
more solution is removed from the first bath during the first dip. The second bath
is fed from the supply reservoir and usually overflows into the first, which has a
level controller.

The wedge nip has the solution held in the nip between two horizontally
opposed bowls. Side plates at the ends just touch the rollers’ surface and hold the
solution above the nip. Using this type of pad, it is vital that the descending fabric
wets immediately on contact with the liquor as the immersion time is very short.
The wedge nip is useful for lightweight fabrics but tends to give excessive foam as
air is squeezed out of the fabric into the bath. It is most frequently encountered as
the second nip in a double pad system.

12.6.2 The Thermosol process

The Thermosol process is mainly used for the continuous dyeing of fabrics
containing polyester fibres. When polyester fabrics that have been padded with a
disperse dye suspension and dried, are heated to temperatures around 200–220 °C