Slasher/sheet Dyeing;-

Slasher / Sheet Dyeing Process:

 What is Slasher Dyeing or Sheet Dyeing?

Slasher Dyeing is a special dyeing technique which is mainly used in the production of indigo dyed warp ends for denim fabric. Slasher Dyeing is also called sheet dyeing. It is a continuous process which combines dyeing and sizing in a single operation. Warp yarns are repeatedly passed in warp beam form through several baths of indigo dye before being sized and wound for weaving. The dyeing time is deliberately short to allow poor dye penetration and build-up. This results in interior washing and rubbing fastness of the dyed yarns, which accounts for the fashionable effect of denim after subsequent washes. The dyed yarns are then sized and wound onto a warp beam to be ready for use in the weaving process.

Figure 1: Slasher dyeing unit

In slasher dyeing, warp yarns are dyed as open-width form and dyeing, drying, sizing processes are performed in the same machine continuously. In continuous slasher dyeing and sizing machine, direct warping beams are used, instead of ball warping logs in case of Indigo rope dyeing system. The Slasher Dyeing machine is capable of handling Ne count form 9/s to 30/s (OE and Slub both). At the back end of the slasher dyeing range, the direct warping beams are creeled. The yarns sheet from each beam is pulled over and combined with the yarns from the other beams so that multiple sheets of yarns can be made.

In sheet dyeing range, the total no of required ends for a weavers beam are dyed, dried, sized and dried simultaneously. The back direct warping beam contains 380- 420 ends, similar to rope, but the ends are distributed evenly over the width of the flanges and the end lay parallel to each other.

This continuous slasher dyeing range eliminates a few intermediate processes of the rope dyeing, such as re-beaming, sizing. The yarn sheet from the back beam passes through wash boxes, where it is treated with caustic and subsequently washed with normal water. After squeezing the excess water; the yarn sheet passes through Dye baths and skied for oxidation as in the case of rope dyeing. This develops the indigo coating on the yarn. After dyeing, the dyed yarn is washed by passing through 3-4 wash boxes and finally squeezed before allowing it to pass through drying cylinders. The dyed yarn then enters into the sow box, where it is sized. Subsequently the yarn sheet is dried.

The yarn sheets then passes through a set of stainless steel split rods, which separate them into individual sheets, equivalent to the number of section beams in the creel. After passing through the split rods, the yarn sheets are collected into single sheet and passed through a expansion comb at the head stock, which separate individual yarns. The expansion comb can be adjusted to the desired loom beam width. Slasher dyeing range typically consists of 1-2 wetting vats, 4-8 dye baths and 3-4 rinsing troughs. The immersion and oxidation times lie between 10-20s or 45-60s.

Slasher Dyeing Range:

In this type of dyeing range, warp yarns are extended flat and separately and are arranged parallel to each other, with a millimetric gap between them in individual layers. The width of the range corresponds to the sum of all warp yarn. The number of yarns per beam ranges between 300 and 750, whereas the total number of threads that make up the warp yarn vary between 3,500 and 9,500, according to the thickness and diameter of the yarn, the type of warping range and the denim fabric design in the weaving process.

Process Sequence in indigo Slasher Dyeing:

The passage of flow of yarns in slasher dyeing is shown below-

Pretreatment

↓

Washing

↓

Dyeing

↓

Washing

↓

Drying

↓

Sizing

↓

Drying

Pre-treatment:

Pre-treatment process in sheet dyeing consists of treatment of the cotton yarn sheet with caustic and wetting agent. Pre-wetting is carried out in order to get proper dyeing of the sheet. Pre-wetting is carried out with a wetting agent, at room temperature. In some cases, if well penetration of the dye is required, the yarns are treated with strong caustic soda solution followed by hot wash and cold wash treatment prior to dyeing. The pH of the bath is 11.8-12.

Washing:

Cold washing is carried out at room temperature.

Dyeing (indigo blue dye):

The dyeing is carried out in 4 to 8 dye boxes. The dyeing is carried out with;

.Indigo powder

.Sodium hydrosulphite

.Caustic soda

Dispersing agent:

Typical dipping time of sheet in each tank is 15secs and oxidation time is about 90secs.

Washing:

The dyed yarn sheet is subsequently in order to remove the unfix dye from the yarn surface. However the number of washing tank may vary. Typical wash type is: cold wash at room temperature and hot wash at temperature of 70ºC.

Drying:

The dyed yarn sheet is dried by passing it through drying hot cylinders.

Accumulator:

The function of accumulator is to store the extra yarn sheet when the machine is stopped or at the time of size beam doffing, so the dyeing cannot be stop.

Sizing:

The yarns are sized in order to achieve the required strength.

Drying:

The dyed sheet is dried by passing through drying cylinders.

Slasher Dyeing Processing Parameters:

Items                    Unit.          Technological                                                   Specification

Dyeing

 temperature           °C              20-30

Dipping duration.    Sec.          15-20

Oxidizing duration  Min.        Each passage                                                    1- 1.5.   

Pick up.                    %.             20-25%

Padder pressure     Tonb.       5-10

Depth of shade        %.             2.5

Advantages of Slasher Dyeing:

Slasher dyeing ranges have a number of advantages. Slasher dyeing range produce sized beam directly which is ready to use in weaving. Sheet Dyeing method has the following advantages:

1)Slasher dyeing is more comfortable for producing lightweight denims.

2) These machines require less floor space,

3) Enable smaller production runs,

4) Have a quicker turn over time,

5) The technology is less capital intensive and the machinery cost is less;

6) The cost of production is less.

7)Other types of dyes can be use to dye the cotton in this range. Hence the slasher dyeing technique can produce a wide 8) variety of colors other than indigo blue.

8)Rope opening is avoided, as in the case of rope dyeing.

9) The immersion and oxidation times are much shorter than rope dyeing.

Disadvantages of Slasher Dyeing:

1) In Sheet dyeing, there is a problem of center to selvedge shade variation.

2);The hydrosulphite consumption is much higher owing to the greater surface.

3)Slasher dyeing is considered to be lower quality than rope dyeing – the dye does not penetrate well and the color tends to be uneven. However, recent mechanical improvements have helped make it a more viable option.

Advantages of Slasher Dueing over Rope Dyeing:

1)Slasher dyeing is normally used to assemble its denim fabrics, which has the following advantages over rope dyeing:

2) The innovation requires less capital venture.

3) The innovation is demonstrable.

4) The expense of generation is lower.

5) The process time is lower.

6) Sheet dyeing machines are easy to work with.

The main problem with sheet coloring is that there is an issue with choosing the selvedge shade variety.

Ref ;-https://textilelearner.net