When you buy a textile product what you look for at first? What attracts you first is the colour of the item and then comes the other things - like the surface design, hand feel, comfort and its price. The eyes are our most trusted sense and hence we see the colour first from a distance. While textile was dyed extensively in the fabric form, some of the shortcomings of the process led to the technologists looking for alternatives. Thus the yarn dyeing techniques were introduced. In this post I will explain the HTHP yarn dyeing process.
HTHP yarn dyeing processes
Among the various yarn dyeing methods, the most common one is high temperature high pressure (HTHP) dyeing method. In this process, yarns are wound from paper cones to perforated plastic cones through which liquor can pass through. The process of winding from paper to perforated cones is an added pre-requisite.The perforated cones are mounted on stainless steel carriers that are built according to the capacity of the machines. The machine capacity varies from 1 kg to 1000 kg or even more, with denominations of 3 kg, 10 kg, 20 kg, 50 kg, 100 kg, 220 kg, 350 kg, 400 kg and 500 kg. The stainless steel carriers are also perforated throughout their bodies so as to facilitate the movement of liquor.
Also Read: Fabric Dyeing Process - an introduction for beginners
Yarn dyeing machines
The HTHP yarn dyeing machines typically has 3 tanks. The main tank is made to take in the carrier while the other two are subsidiary tanks. One of them is used to prepare the dye liquor that is dosed gradually as required. The other tank is used to add the remaining auxiliary chemical ingredients required for pre-dyeing and after-dyeing. |
| A typical carrier with dyed yarns |
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| HTHP Yarn dyeing machine |
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| Perforated stainless steel carrier |
The dyeing cycle
The dyeing cycle starts with gradual infusion of the dye liquor from the dye tank into the main tank which is half filled with water. After the dosing of the dye liquor that happens over 30 minutes, the rest of water is added into the main tank in order to fulfill the total volume.During the dyeing cycle, which is typically 60 minutes in case of reactive dyes, there is in-to-out and out-to-in movement of the dye liquor through the perforated yarn packages and the steel carrier. The out-to-in movement happens for 6 minutes followed by the in-to-out movement of 4 minutes, making the total time for one such complete cycle to be 10 minutes. The out-to-in time is higher than the in-to-out time because the dye liquor has to penetrate through a larger surface area in the former case, the radius of each yarn package being higher at the periphery.
Such a dyeing cycle requires a huge bit of energy and mechanical force that the main pump undergoes; hence its daily maintenance is mandatory.
Dyeing package density
The density of the yarn wound on to the perforated packages is extremely important. There is an optimum value that must be maintained both above and below which there can be uneven dyeing. Higher package densities can also enhance the chances of breakdown of the main pump due to excess of pressure during dye liquor movements. The yarn package density on the perforated package is typically kept between 0.30-0.35 kg per cubic metre.Evenness of shade
Like in case of fabric dyeing, the evenness of the shade is as important in yarn dyeing too. Two factors affect the shade evenness in case of yarn dyeing more than the others. One of them is the in-to-out and out-to-in movement and their ratio; a faulty ratio can result in deeper shades in the layers towards the centre of the yarn packages and lighter shades at the periphery layers. The other factor is dyeing package density, as described above.Weighing the colours
In most of the mills, colours for dyeing are weighed manually. The weighing must be accurate, since a slight variation can magnify itself at a larger scale while dyeing. The electronic weighing machine must therefore be calibrated frequently. Many mills have shifted to digitally controlled colour dispensers in order to achieve higher accuracy and lab-to-bulk reproducibility.Once I was dyeing a shade in pink with a bluish tone that required only 400 mg of blue dye to be added. Mistakenly, I weighed 400 gm of it and added to the dye liquor, only to find the machine operator running to me in about 5 minutes to say the shade has turned purple from the desired pink one. That was the first and last time I had been so careless while dyeing, but the incident serves a good example why we need to be so careful as well as accurate while weighing the dyes.
About the author
Arnab Sen is an Assistant Professor, Textile Design, National Institute of Fashion Technology (NIFT), Bhopal, India. He is a B.Sc.Tech. in Textile Technology from the University of Calcutta and an M.Tech. in Fibre Science and Technology from the Indian Institute of Technology Delhi. He has served the textiles industry of India as a teacher and a professional since 2001.