Page 245 - Basic Principles of Textile Coloration

234 DYEING MACHINERY

Nylon hose can be mounted on appropriately shaped boards that are placed in a
type of autoclave that allows dyeing, drying and heat setting to be carried out in
sequence. Garment dyeing machines are also available that allow pressure dyeing
of polyester articles.

12.6 CONTINUOUS DYEING EQUIPMENT

Some of the techniques and problems of continuous dyeing were outlined at the
end of Chapter 10. This section now considers the equipment routinely used in
continuous dyeing.

12.6.1 Padding mangles

During padding, the fabric passes into a solution of chemicals, under a submerged
roller and out of the bath. It is then squeezed to remove excess solution (Figure
10.7). The objective of this process is to mechanically impregnate the fabric with
the solution or dispersion of chemicals. This must be as uniform as possible both
along the entire fabric length and across its width. Pad impregnation is common
for the dyeing of fabric and for the application of finishing chemicals.

The uniformity of the distribution of any chemical in the padded fabric depends
on having a constant immersion time in the bath and between the bath and squeeze
rollers, an even pick-up of the solution, and uniform wetting of the fibres. The bath
conditions must also remain constant. These conditions are largely met by running
the fabric at constant speed through a bath maintained at constant level, and by
adjusting the pad roller pressure so that it gives uniform pick-up of solution. A key
requirement for successful pad dyeing is uniform preparation of the fabric.

The bath level must remain constant. A simple mass balance for the dye
solution shows that the rate of flow of solution (kg soln min–1) from a supply
reservoir into the pad bath must equal the rate at which solution leaves the bath
in the fabric. The latter is equal to the fabric pick-up (kg soln kg–1 fabric) times
the mass flow rate of fabric (kg fabric min–1). At a typical running speed of
15 kg fabric min–1 with 60% solution pick-up, 9.0 kg soln min–1 must flow into
the pad bath. Since the volume of solution in the bath is around 30–40 l, fresh
solution completely replaces it in about 3–4 min. The dye solution from the
supply reservoir feeds into a perforated pipe running along the bottom of the pad
bath at the back of the fabric, the holes pointing away from the fabric. A
controller maintains a fixed bath level or depth and activates the feed from the
dye solution supply reservoir.