What is Hank Dyeing?
Hank dyeing is much less common than package dyeing but is still used for dyeing lofty yarns such as wool and high‐bulk acrylic yarns for knitting and carpet manufacture. The yarns are loosely arranged in hanks. Then they are hung over a bar and immersed in a dyebath. In this method, the color penetration is the best and the yarns retain a softer feel. The process of dyeing yarn in the form of hanks or skeins is carried out by both hand and machinery. It is also called skein dyeing. Hank dyeing is a simple but time-consuming process. Different types of hank dyeing machine are used for yarn hank dyeing.
First, the skein (skein is the length of a yarn wound in a loose coil) of yarn is looped over a hook and washed in water, opening the fibers to receive the dye. It is then dipped into the dye for up to 48 hours, washed and redipped. This procedure is repeated for several times. Once the desired color is achieved, the yarn is steamed to fix the dye to the fibers. Because it does not use as many chemicals as other forms of dyeing, hank dyeing is less damaging to the material. The final dye colors are also usually richer than those achieved by other dyeing methods. Hank dyeing is mainly done for carpet wool yarn dyeing. Besides, yarn for hand knitting and machine knitting is still predominantly dyed in hank form, although there are developments taking place which will allow these yarns to be package-dyed.
Why Hank Dyeing?
Although package dyeing route is much simple and easy to follow for dyeing of different types of yarn, but still the hank route is popular for certain qualities, there are two reasons for this,
- The quality and nature of the product does not allow to follow package route.
- The package route is non-practical and non economical.
Certain kinds of yarns which are so delicate that their physical properties such as strength appearance, texture and construction cannot be retained while following the package route in which yarn is processed under stretched conditions at elevated temperatures. The examples of such products are natural silk, viscose rayon, voluminous yarns, High bulk acrylic, shrink resist treated wool and its blends. Cashmere, its blends with wool and nylon, nylon and hand knitting yarns of different fancy constructions.
Economy of Process:
In case of mercerized cotton yarns, the yarn mercerizing is done in the hank form and material has to be neutralized before dyeing, therefore it is not economical to convert these hanks to packages for dyeing.
Process Sequence for Yarn Dyeing in Hank Form:
- Singeing (for mercerized cotton yarns)
- Mercerization (for mercerized cotton yarns)
- Hydro extraction or squeezing
- Hank to cone winding
Process Flow Chart of Yarn Hank Processing
First the yarns give as input. Then apply twisting and singeing reeling the yarn as hank mercerizing, bleaching, neutralization, hydro extraction, drying than hank yam to hank to cone.
Different Types of Hank Dyeing Machine:
Hank dyeing machines are relatively simple in their construction, though a number of different designs are available, including:
- Roller hank dyeing machine (Hussong type)
- Cabinet hank dyeing machine (Cabinet style)
- Spray hank dyeing machines (Hank spray type)
- Hank space dyeing machine (Space dyeing)
1. Roller type hank dyeing machine:
Most carpet yarn is dyed on single-stick Hussong machines (Figure 3). Heating is by open or closed-coil steam pipes, situated below a perforated false bottom. The dye liquor is circulated over a weir and through the yarn by means of a reversible impeller. In modern machines, the yarn is suspended from Vshaped sticks with perforations to prevent stick marking.
Improvements have been made to Hussong-type machines by the introduction of central impeller compartments to reduce the distance through which liquor has to flow. There is also the possibility of linking two machines to enable larger batches to be dyed, a development that is particularly relevant to the carpet industry.
Two-stick machines are a development from the one-stick machine. In onestick machines of the Hussong type the direction of flow is mainly up through the hanks, which causes the yarn to form a dense pack that impedes liquor flow. This causes unlevel dyeing, particularly if dyes of higher wet fastness (with inferior levelling characteristics) must be used. The use of a second stick at the bottom of the hanks prevents the mass being lifted by the flow and allows a greater rate of flow to be used without severe tangling. The distance between each pair of sticks must be adjusted according to the hank length, so that the yarn is not stretched tight during dyeing – this can cause severe stick marking at both the top and the bottom of the hanks. It is customary to leave about 4 cm free space between the bottom stick and the hank. This adjustment is especially critical when high-bulk yarns are being processed, since allowance must be made for the shrinkage which will occur in the hank as bulk is developed.
2. Cabinet type hank dyeing machine (Cabinet style):
The cabinet‐style hank dyeing machine is most suitable for dyeing delicate yarns such as wool, wool and nylon, wool and acrylic, shrink‐resistant wool, wool and silk, mercerised cotton, cashmere and linen. In these machines the liquor flow is controlled to be in the direction of the yarn, causing less chance of felting or disruption.
It is therefore a suitable method of dyeing fancy yarns such as chenille. The yarn is held both at the top and bottom on sticks to further reduce the movement of the yarn and maintain the feel of the yarn.
The advantages and disadvantages of cabinet type hank dyeing machines are as follows:
- Pressure dyeing cabinet for hanks
- Low liquor ratio steam and power consumption
- Low investment and running costs
- Multiple dyeing options
3. Spray type hank dyeing machine:
In spray dyeing machines the yarn is held on rotating arms. The rotating arms have the capability of spraying dye into the fiber both from the outside and the inside of the hank. Below the hanks is a collection tray incorporating a heating element to heat the liquor before recirculation to the spray arms. The spray pressure and flow rate can be varied to accommodate different yarn types to prevent any yarn distortion or damage. The machines are designed to be modular so that varying loads can be dyed. They can also be used to dye very delicate yarns.
Both cabinet machines and spray dyeing machines are designed with removable arms, so future batches can be prepared before the previous dyeing cycle is finished and loaded quickly to reduce the time the machine is not being used.
4. Hank space dyeing machine:
The space dyeing of hanks can be performed in one of two ways:
- Multiple strands of yarn sprayed with color at set intervals
- Multiple strands of yarn continuously printed using rollers as set intervals
After the color is applied, the dye is fixed in a steam oven and washed off before drying. This technique is used to create design effects in knits. Further processing can include doubling of the yarn. Space dyeing is also carried out when the fiber is in the form of a top or sliver and the effect of the different colors is then spun into the yarn. Similar techniques are also possible on packages of yarns. Other techniques to get the same result are available, such as the knit–de‐knit process.
Manual yarn dyeing is laborious and involves errors in dyeing which affects the major quality parameter of textile product. A simple machine is conceptualized for eliminating the problems associated with manual dyeing process considering the need of the beneficiary industry. Hank dyeing machine has revolutionary changed in manual yarn dyeing.
- Textile Dyeing By Dr. N. N. Mahapatra
- An Introduction to Textile Coloration: Principles and Practice By Roger H. Wardman
- Wool Dyeing Edited by D M Lewis
- The Coloration of Wool and other Keratin Fibres Edited by David M. Lewis and John A. Rippon
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.