The name “Loopdye” results from the method of skying or air passage for oxidizing the Indigo-dyed yarn and the method of passing through the Indigo dye. On the other 2 important Indigo machine types, the dyed yarn is passed through from 6-8 Indigo boxes on rope machines or 6-20 Indigo boxes on slasher (sheet) Indigo machines, multiple dye boxes being necessary for dark shades because only a small amount of Indigo can be applied in each immersion. After immersion in each Indigo dye box, the yarn is conducted through the air after each box, where the reduced Indigo (yellow-green) is oxidized or “fixed” by oxygen in the air returning to the original blue, then the yarn enters the next dye box, passes into the air and so forth until the required depth of shade is developed.
In the case of rope and sheet ranges, this oxidation takes place above each dye box. In the Loopdye process, there is only a single Indigo box through which the yarn passes 4-5 times. The white cotton is pulled into the front of the machine and passes first through the pre-treatment boxes, then moves through a reactor which can be used for steaming or additional reaction time for sulfur- bottoming or Mercerization, followed by washing. The wet yarn then enters the Indigo dye box. When the yarn exits the dye box, instead of moving forward, the yarn is carried to the rear of the machine, around the top and rear of the yarn creel from where it started, passes under the yarn creel where it is returned to the Indigo box for another dye passage. This continuous passage of yarn between the yarn creel and the dye box is in the form of a “loop” which is almost circular. After making multiple loops through the Indigo dye box the yarn is conducted through wash boxes and on to drying cylinders. The Loopdye machine is a simplified version of a “sheet” or “slasher” Indigo machine. After drying the Indigo-dyed yarn, the yarn passes directly to sizing where the yarn is prepared for weaving. Because the sizing part of the machine must stop in order to remove a completed weaving beam, in order to prevent the dyeing unit from stopping as well, there is a yarn accumulator between the drying cylinders at dyeing and the wet-size boxes. When the yarn stops moving on the sizing unit, a series of parallel cylinders begin to move apart allowing the yarn from the dye unit to continue through dyeing and allows the size machine approximately 2 minutes of time to install an empty weaving beam and re-start the sizing machine.
Loopdye Machines in the Denim Industry
In the early 1990’s, thee were approximately 30 Loopdye machines in use. Currently, the number is reported to be 60 or so. The biggest concentration of these machines is in Brazil. Vicunha employs 11 of these machines, Canatiba, Santana and Cedro have 2 units each, while Tavex, Tear, Textil Kafi, Santista have 1 each. There are 9 of these machines that have been equipped with nitrogen units which use nitrogen gas as protective blanket over the surface of the Indigo dye. The nitrogen gas prevents oxygen in the air from attacking sodium hydrosulfite resulting in more consistent dyeing and reducing consumption of hydrosulfite, lowering costs and pollution. There are other claimed advantages such as higher speeds and darker Indigo color.
Advantages and Disadvantages
- Productivity – When compared to a multi-box slasher machine, productivity is essentially equivalent since the yarn loading, start-up times and speeds are similar. Rope dyeing machines can produce up to 4 times as much dyed yarn.
- Capital Investment – The Loopdye machine has the lowest initial costs of continuous Indigo dyeing machinery, currently reported to be approximately 25% less than 8 dyebox slasher machine.
- Operating Costs – Maintenance and energy costs are reported to be approximately 20% lower with Loopdye when compared with slasher dyeing and even lower than with rope dyeing.
- Space requirements – The Loop machine with a single dye box requires less floor space than either sheet dyeing or rope dyeing. Rope machines also require higher ceilings because of the design of the airing arrangement.
- Indigo Dyeing Quality – The newer designs of Loopdye are reported to have little of the problems with Cross-Shade (side-to-side) shading than with slasher dyeing equipment. Indigo consistency from the start-to-finish of dyeing can be expected to be better with the inclusion of nitrogen units. Rope machines still have an overall advantage in terms of Indigo dyeing quality, but this may be overcome by employing improved chemical blending.
- Sulfur dyeing – The Loopdye machine can be equipped with a steamer for cold-pad sulfur bottoming which will provide greater consistency than a hot application in the 1st box. The Loop machine is not provided with enough boxes after Indigo dyeing for sulfur topping as the slasher dyeing is. With the newer methods for cold-sulfur dyeing, the Loop machine is ideal for sulfur colors since it the dye can be applied in only one box, which allows for faster color changes and less dye discarded after the dye lot is finished. Rope machines still have the greatest flexibility with regard to producing a full range of denim colors.
- Weaving Efficiency – The methods of dyeing, especially of sulfurs, has a direct effect on warp yarn breakage in weaving, which lowers operating efficiency as well as fabric and garment quality. Experience with the older design of Loopdye machines demonstrated higher levels of warp breaks in weaving than other Indigo machines. Rope dyeing results in the lowest-level of weaving stops, largely because yarn breaks in dyeing can be repaired at long-chain re-beaming.
- Versatility – In the higher denim fashion market, some companies like Vicunha have had success using a combination of Loopdye and slasher dyeing. Overall, the slasher dyeing with its greater number of application boxes offers more flexibility in product development, while rope dyeing provides the greatest flexibility for denim product development.
This is a guest post by Harry Mercer. Mr. Mercer has 30 years experience in the denim business including 3 prominent U.S. denim companies. He is an expert colorist for measurement and color matching as well as textile testing.