Page 239 - Basic Principles of Textile Coloration

228 DYEING MACHINERY

The winding of the fabric onto the beam is a critical step. The beam is usually
first wrapped with a few layers of cotton fabric. This provides a soft bed for the
material, allows dispersion of the solution, and filters out any particles. The fabric
is then wound onto the beam at constant speed with a uniform but not excessive
tension. It is essential to avoid creases and to have evenly aligned selvages without
any curling. A piece of polypropylene or calico is stitched to the end of the fabric,
rolled around a few times and re-stitched. This prevents the centre of the roll from
ballooning and keeps it clean. Tubular knit fabrics are usually slit along one crease
before beaming. Although successive lengths of fabric can be joined and dyed on
the beam, the number of seams should be kept to a minimum. As in jig dyeing,
heavy seams tend to retain more dye solution and can cause deeper dyeing on
adjacent layers in contact with the seam.

Beam barrels come in various diameters. They are smooth for about 20 cm at
each end and the rest of the cylinder is perforated to allow good liquor flow. The
perforations at the ends can be wrapped with flexible stainless steel sheets, and
therefore blocked, to accommodate narrower fabrics. The degree of overlap of the
fabric, beyond the end of the perforations at each end of the beam, must be
carefully controlled. If there is too much overlap, the selvages receive less dye
solution, but if the overlap is too small, much dye liquor escapes out through the
sides of the roll. Both give unlevel dyeing near the fabric edges, particularly on the
inner layers near the beam.

The fabric to be dyed must have good dimensional stability. If there is any
length extension during dyeing, solution may leak out of the roll sideways between
the layers of fabric giving unlevel edges. Shrinkage along the length of the fabric
during dyeing causes excessive pressure that can flatten the fabric. Width
shrinkage causes flow variations while width extension gives ballooning of the roll
and reduced flow. Prior to rolling up the beam, fabrics made from thermoplastic
synthetic fibres are therefore pre-set, using hot air or water, to relax tensions and
eliminate possible dimensional changes during dyeing.

The beam of fabric is mounted in a cradle that glides on rails into the autoclave
through an end door. When the door is closed, this seals the near end of the beam
and provides good contact of the far end as it engages the tapered outlet of the
liquor circulating pump. The machine is filled with water until water overflows
into the expansion tank. The circulating pump forces water from the beam interior
out through the layers of fabric. A wetting agent helps to eliminate air bubbles
within the fabric roll. De-aeration is essential to avoid paler dyed spots. The level
in the expansion tank rises as the temperature increases. Solutions of dyes and