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Scopes of Improvisation in Knit-Dyeing process of Cotton in Bangladesh to optimize the process-time

Abstract:

Knit dyeing is the shiniest star in the sky of textile dyeing in Bangladesh. Knit-Garments sectors hold more than 40% of the annual export items proportion, which shows how influencing the role of knit dyeing industries is in our economy. However, this vastly growing industry should be far-sighted and well prepared for all the upcoming challenges like instability in energy situation, environmental degradation and global economic crisis etc. The purpose of this investigation is to overview the whole conventional process of knit dyeing of cotton fabric and sort out the areas of concern which might be altered to establish new technologies also develop the process in order to compete with other counter parts. The main focus of this study is to optimize and reduce the processing time aiming RFT without any major change in the system.

Introduction

The dyeing of cotton knitted fabric is a very complicated as well as interesting process. This is the most important sector among the dyeing industries in Bangladesh. Through out the years the Knit-dyeing industries have created their own way of working involving technology & experience. So the process of dyeing is well established.

But in present adverse situation of the socio-economy and highly competitive business-world, things are threatening our existence in international market. Most importantly the lack of energy supply in our country, environmental degradation is getting worse day by day; moreover, the ever-degrading profit margin & extremely limited lead time for order are making it almost impossible for dyers to complete the task. So there has to be some sort of re-thinking & steps should be taken to cope with this adverse situation.

There are a lot of scopes where new innovation is required for example, in the dyeing method, machines which are currently used, the energy and utility used for dyeing etc.

One of the most important issues is the ‘process-time’; because the dying operation should be done in very precise time limit as the lead time of the orders are getting shorter and shorter. Also there should be enough time given for achieving accurate shade. This makes the whole process very difficult. Besides there is energy problems which some time gets so severe that the process can’t be carried out. Due to all these reasons we have to complete dyeing in minimum time with optimum quality. This is the point of discussion in this investigation.
The process time controlling is not an easy operation because shade matching is the most unpredictable task in dyeing. So the first objective is to match the shade in lab trial also implementation in bulk and also decide the specific RFT for the whole process. Then the actual controlling of the process sequence to achieve RFT comes into action.

Methodology:

The paper is based on case-study of number of selective factories of a selected zone of Bangladesh. It also includes some prospective ideas or suggestions based on probability-analysis of different factors. The whole work is a combination of theoretical as well as investigational work on currently faced problems by the industries.

Deciding RFT in Knit-Dying

The present situation of dying process in Knit sector is well established & successful. But there are new challenges arising everyday which are trying to unsettle this progress. The process can be improved more if some new technology is introduced under precise condition making sure that the quality of production is not degraded.
The knit dyeing process of cotton is almost entirely using Reactive Dyes & Exhaust methods, which are still undoubtedly successful. But there are some problems which were not previously taken into consideration due to flexible market situation, now the business is very complex and competitive. So now some of them are playing vitally in the ratio of profit with quality.

The first & the foremost target of any industrial production are to produce in the least time with maximum quality. At present days our knit dyeing industries are affecting with reduction of profit margin, economic crisis in countries, energy crisis etc. In such hostile condition the industries should concentrate very carefully on the duration of unit production, making the dyeing right in the minimum process steps. That’s why intense control of process should be done to get as much as close towards right-first-time dyeing.

Aiming RFT in Reactive dyeing in exhaust method [1]

Right first time dyeing in cotton knitted fabric is a very complex & hard to control. It varies from shade to shade, lot to lot, factory to factory. To minimize the process time a series of controlling of variables is required. It starts from choosing the right yarn lot to the finishing process of dyeing. The utility used in processing like – water, steam etc. plays a very inflective role in achieving RFT.

Variables of scopes of controlling RFT[1]

Assignable variables in dyeing[2]
Measurable factors that influence Dyeing Performance is controlled as below:

  1. Liquor Ratio
  2. Internal pH of Fabrics
  3. Dye-Fixation Temperature
  4. Specific gravity of Salt (Salt Concentration)
  5. Addition Profile (Dyestuff’s Addition behavior)
  6. Dye-fixation pH
  7. Dyeing Cycle Time

Random variables
Factors that require intervention of management to effect control of Impurities in:

  1. Cotton
  2. Water supply
  3. Chemicals e.g. QC of raw materials
  4. Controlled Coloration Concept

Checking List for exhaust dyeing process: [1]

  1. Water Hardness Test: With Hardness Test-strips.
  2. Bicarbonate in dye-bath: Tested with Methyl-Orange+HCI Test. Presence of NaHCO3 in dye-bath water works as buffer; So, increase NaOH (38°Bé) at 0.1 ml/l for each 0.1 g/l NaHCO3.
  3. Salt Hardness Test: With Hardness Test-strips & NaOH Test If total amount of hardness from water, salt & material is more than 10°dH, try to reduce.

For water: Increase Sequestrant
For salt: Change salt to good quality
For material: Demineralization can solve it.If not possible, try to dye with Only Soda ash method.

  1. Residual Peroxide: With Peroxide test-strips (if just bleached material)
  2. Alkali Concentration: Correct concentration to be used (if different, convert it)
  3. RecipeCalculation: Should be done for dyes & auxiliaries
    1. To measure weight, a balance is to be used.
    2. To measure volume, a cylinder or pipette is to be used.
  4. Material quality Checking:
    1. pH with pH test paper
    2. Hardness with Hardness test-strips
  5. Machine parameters:
    1. Dependability of thermometer
    2. Circulation pump’s pressure (in package dyeing of yarn)
    3. Winch speed (in rope dyeing for knit goods)
  6. Dyeing Liquor Ratio: the amount of water in Dyeing machine:
    1. Total amount of water
    2. Water needed for (dyestuffs + Alkali) Solution.
  7. Water contamination always changes & it adversely affects RFT performance. So here’s a standard water property for dye-house usage: [1]

pH                               : 6 – 8
Hardness                      : max. 5°dH
Suspended matter        : < 1 mg/l
Organic load                : < 20 mg/l
Residue on ignition      : < 50 mg/l
Iron                              : < 0.1 mg/l
Manganese                   : < 0.05 mg/l
Copper                         : < 0.01 mg/l
Nitrate                          : < 50 mg/l
Nitrite                          : < 5 mg/l
Free CO2 0 if possible (risk of corrosion)

Color control[2]

  • Selection of dyes
  • Behavior of dyes in combination
  • Accuracy of laboratory dyeing recipe
  • Accuracy of transfer to bulk-scale recipe
  • Batch to batch reproducibility in bulk-scale dyeing
  • Method of color assessment
  • Determination of metameric index

These Parameters Show the scopes of controlling the process of reactive dyeing which will eventually rise to RFT dyeing. The closer to RFT decides maximum productivity.

Reasons why Process time needs to be optimized in Bangladesh:

Current problems faced by the dyeing industries –

  • Short lead time
  • Huge competitive market
  • Most severe is the gas crisis, gas-pressure falls about less than 10 psi.

Possible Action for overcome the process time –

  • Changing the whole process techniques, building new method of work.
  • Slight change of process parameters for reducing the time.

Second one is discussed here which is more suitable at this stage for major number of industries in our country because most of the suffering industries in our country are still of mid-level standard.

A Statistical Time-Analysis of RFT in Different Knit-Dyeing Industries of Bangladesh

Here a detailed analysis of Time in Knit-dyeing process of three different well-established Knit-dyeing factories situated at Narayangonj, in Dhaka division. Due to the current hostile situation the mid-level industries are getting more affected, that’s why this region was chosen to analyze on there point of view.

For protecting confidentiality, the names of factories are not disclosed here. Here we will look at them as Factory A, B & C. A standard process of dyeing similar kind of fabric with similar type of shade for each Factory is studied and Statistics of time required for each steps collected for comparative study to find the reasons of fluctuating RFT if the dyeing is right without any addition or topping.

Standardization:

Water Quality Factory A – hardness > 5°dH         Line Pressure = 4 kg/cm2
Factory B – hardness > 5°dH         Line Pressure = 3 kg/cm2
Factory B – hardness > 5°dH         Line Pressure = 3 kg/cm2
Fabric Quality Single Jersey   gsm – 160-170 weight – 700-750 kg
Color Medium to dark color
M:L 1:7 for both Scouring & Dyeing
Dyeing Machine HTHP   Capacity – 800 kg   Nozzle Pressure – 4 to 6 psi
Used Chemical Almost similar quality in all three factories

 Comparison of RFT for scouring (with bio polishing) process in Three Industries

tth_2009_1stComparison of RFT for Dyeing process in Three Industries

tth_2009_1st_2

Analysis and Discussion

From above chart of RFT we can see the total time of thatparticular rightly dyed batch –

Factory ‘A’ – 260+325= 585 min i.e. 9 hrs& 45 min

Factory ‘B’ – 249+370= 619 min i.e. 10 hrs &19 min

Factory ‘C’ –252+334= 586 min i.e. 9 hrs &46 min

Basic difference of process parameters three factories

Operation-wise time-consumption during the whole process –
rakib_clip_image002Now we look into the individual difference & comparison of time required for specific process in the three industries –

  • Dyeing method:

Factory A, C uses ‘All in’ method where dyeing & fixation both happen in 60°c. On the other hand Factory B uses ‘Migration’ method, which takes9% more dyeing time than Factory A& C. rakib_clip_image002_0000

  • Dosing system:

In case of color & soda dosing, Factory A uses ‘Linear dosing’ all the way through according to cycle time. On the other hand, Factory B uses ‘Progressive dosing’; Factory C uses ‘fixed time dosing’.

Factory Total dosing type & time (min) Comparison in % Color & Soda dosing time (min.) Comparison in %

Remarks

A

Linear – 116

113%

66

120%

Overall time consuming, but precise dosing.

B

Progressive – 102

100%

55

100%

Effective for good RFT and precise as well

C

Fixed time – 110

107%

70

128%

Time saving but not precise.

 

  • Temperature gradient:

rakib_clip_image002_0001Factory A uses gradual increment of temperature in 1.5 or 2°c/min. Factory B & C don’t usually uses gradient in temperature except in color steam factory B uses gradient. But Factory C never uses gradient. It saves time though it is not recommended for perfect dyeing.

Factory

Total time consumed in temperature rise (min)

Comparison & Remarks

A

56

133%

B

59

140%

C

42

100%

  • Rinse & Fill:

rakib_clip_image002_0002 Rinse & Filling consume a major amount of time in the whole process-time. It depends on the Line Water pressure of the factory. Here, line pressure of Factory A – 4 kg/cm2 on the other hand factory B & C have 3 kg/cm2 pressure which results great difference in the time consumption.

Factory & Water pressure

Rinse & Filling Time

Comparison in %

Portion(%) of total time

A

57 min

100%

8%

B

66 min

115%

12%

C

66 min

115%

13%

Possible Scopes of Improving RFT

  1. Effective Loading/ Unloading:
  • Around 35-40 min is lost during loading and unloading.
  • If the trolleys are rightly managed and a string is used to load the fabric, almost 40% time can be recovered, i.e. around 10-15 min. time can be saved.
  1. High Pressure improved Fill/Rinse system:
  • Around 11-12 times rinse & filling are done sequentially during whole process, which consume around 60-70 min. it depends on the line-water pressure.
  • If the water pressure can be maintained around 5 kg/cm2 by using the booster pump effectively, the rinse & fill time can be reduced to 40-50 min i.e. at least 20 min can be saved.
  1. Using Progressive Dosing:
  • Type of dosing affects the whole time of process as linear dosing is time consuming so progressive dosing is more effective.
  • In case of the auxiliaries injection can be easily implied.
  • This saves time around 20-30% i.e. 15-20 min.
  1. Using Multi-functional chemicals/ substitute:
  • Salt substitute is very effectively used in one of our studied factory. They uses substitute in almost 90% case, which reduces the cost per kg around 5-6 tk. & reduces time of process as number of auxiliaries is not required here also just 50% salt is required.
  • Using multi-functional agents also reduces the time consumption.
  • Around 15-20min can be saved by using MFC.
  1. Using Pre-heated water:
  • If the water in preparation tank is preheated to 50-60°c by using the used steam from heat-exchanger, it will result huge time savings.
  • Here a comparison table is shown –
Steps where temp. is needed

Time for rising temp if conventional normal water

Time for rising temp. if with pre-heated water 50-60°c

Scouring steam 95°c

10 min

5 min
(new water taken here)

Scouring hot 90°c

10 min

5 min

Enzyme 55°c

2 min

1 min

Enzyme steam 70°c

3 min
(without gradient)

2 min
(without gradient)

Color steam 60°c

5 min

1 min

Normal Hot wash 70°c

2 min
(without gradient)

2 min
(without gradient)

Chemical hot wash 90°c

 10 min
(without gradient)

5 min
(without gradient)

Fixing 60°c

5 min

1min

Total

47 min

23 min

Revised RFT if these factors would be used:

Total accumulated saved time is (10+20+15+15+20) = 80 min. So the Hole Dyeing process may be completed at 8 – 8.5Hrs

For light shades it would be 7.5 – 8 hrs

So it is quite clear that just precise controlling is enough to acquire a useful RFT. However dyeing is the most unpredictable operation. So there is never ever fully assured of the time limit.

CONCLUSSION & REMARKS

The improvisation is a very vast idea of working. It is a continuous innovation in the process. The discussion above is simply showing the scopes not the whole system involved. Optimizing time is never a fixed or standard type of work; it varies from shade to shade. But still it needs to be controlled very precisely for limiting the process time.

Acknowledgments:

  • AKM Mohsin Ahmed-GM, Knit Concern ltd.
  • M. Rezaul Karim Tutul – A.C.E.O Legend Dyechem
  • Prof. Dr. Zulhash Uddin-Head of Wet Process Department, CTET
  • Md. Monirul Islam – Associate Professor, Head of Textile Management Department, CTET
  • A S M Tareq Amin – Bangladesh Textile Today
  • Engr. Golam Rabbi Khan – Production Manager, Metro Knit ltd.
  • Engr. Shushanto – Senior P.O. Micro Fiber ltd.
  • Engr. Shihab – P.O. Tex Europe Bd. Ltd.

I cordially apologize if there is any misinformation regarding any factory data which is quite unintentional.

References

  1. Achieving RFT (Right First Time) in Exhaust dyeing – (compiled by) M. Rezaul Karim Tutul
  2. Evolution of right-first-time dyeing production (J.S.D.C.) – J Park and J Shore

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