Right-First-Time - a necessity not a luxury


    Introduction



    The concept of right-first-time (RFT) processing in exhaust dyeing became established in the late 1970s and early 1980s. Originally it was termed ‘no addition’ dyeing if the dyeing was found to be on shade at the colour assessment stage, or ‘blind dyeing’ if colour assessment was carried out after the batch had been removed from the dyeing machine. Elimination of an inspection stage made a significant saving in itself. Both techniques were based on achieving a high percentage of bulk dyeings that did not need colour correction or reprocessing. The original benefits were seen as cost savings and increased productivity in the coloration process. However, the introduction of concepts such as just-in-time (JIT) production and quick response (QR) resulted in major financial savings by minimising stock holding by the retailer. RFT processing became an essential means of achieving these objectives. This has become even more necessary now that long supply chains have been established between the sources of raw materials and the retailer.



    Cost of non-conformance



    The batchwise dyer incurs severe financial penalties if RFT processing fails and a significant proportion of batches are wrong-first-time (WFT). A single colour correction adds between 24 and 36% to the cost of the initial dyed batch, depending on the dye/substrate system and the stage of manufacture at which dyeing is carried out. Stripping and re-dyeing can increase the initial dyeing cost by between 170 and 200%. These costs, however, are only some of the penalties incurred, since they can be equalled or exceeded by the loss in revenue and profit which occurs when machines which should be processing the next batches are occupied with corrective treatments, as shown in Table 1.



    Table 1 – Cost comparisons





































































    Process  CostProductivityProfit
    Blind dyeing  100      100  100
    Small addition  110        80    48
    Large addition  135        64   -45
    Strip and re-dye  206        48 -375


    The continuous dyer seldom has the opportunity to make corrections but it has been shown that when corrective treatment is possible, for example by tinting procedures, the penalties are of the same order as those shown in Table 1. The decrease in quality and the loss of customer goodwill are more difficult to quantify.



    Factors for RFT processing



    To achieve RFT processing, three objectives must be met:





    • a high level of within-laboratory reproducibility and accuracy


    • accurate transfer and scale-up from laboratory to bulk


    • repeatability between bulk batches dyed to the same colour.




    Various workers have emphasised the importance of undertaking an initial audit to identify those factors that must be brought under tighter control to meet the necessary limits of accuracy. The limits of accuracy can be defined as the practical process control limits that must be applied in the dyeing operation and in all the preceding manufacturing processes so that the sum of their effects on colour variation are equal to or less than the pass/fail colour tolerance limits. An adequate support laboratory is essential. The design, equipping, operation and procedures required include methods for achieving laboratory-to-bulk reproducibility in dyeing. Accurate laboratory dyeing is a highly cost-effective stage on the road to achieving RFT production since it has been shown that a bulk correction on a 300 kg batch costs as much as carrying out between three and twenty-five laboratory dyeings, or one sample dyeing of 3 to 10 kg, whereas a re-dyeing of such a batch costs the same as 25 to 85 laboratory dyeings, depending on the process and dyehouse infrastructure.



    Approximately twenty factors must be controlled in the dyeing process by the availability of standard operating procedures (SOP) or monitoring by the laboratory. These are in areas including dye selection, optimisation and standardisation; water quality; substrate dyeability, preparation and weighing; weighing and dispensing of dyes and chemicals; standardised dye application techniques and colour assessment.



    Controlled coloration



    The modern concept of controlled coloration depends on implementing all the aspects of the original RFT philosophy, discussed above. It is achieved through a combination of good management, dye and application technologies. Dye selection based on accurately standardised, robust, compatible, stable and consistent products is essential. Proper dye selection to ensure consistent laboratory-to-bulk reproducibility is very important. The dyer needs to follow systematic application methods allied to computer-aided optimised dye selection, according to substrate, equipment type and cost.



    The influence of dye standardisation on the likelihood of achieving RFT production has been demonstrated, based on the selection of reactive dyes for cellulosic substrates. Application methods for these dyes can be optimised to ensure RFT production using the Reactive Dye Compatibility Matrix (RCM), based on substantivity, migration and level dyeing factors. Improved dyeing machinery design, together with process control, is an additional factor in achieving RFT production. Presumably improved machinery design extends the range of dyes within an application classification that will give RFT results.



    On-line monitoring of key parameters has allowed new and highly productive dyeing and washing-off procedures to be developed. This led to the development of the concept of smart rinsing which results in significant savings in time and more effective use of water, improving the degree of control of the wet processing cycle without detracting from RFT performance or product quality. In the exhaust dyeing of reactive dyes, by using optimised preparation techniques, RFT dyeing based on controlled coloration methods and smart rinsing technology, it is possible to achieve a ‘load to unload’ production concept of five hours or less.



    Achieving RFT performance in continuous dyeing has received less attention. Selection of matched pairs of dyes for each fibre in the dyeing of polyester/cellulosic blends gave improved laboratory-to-bulk reproducibility with a reduction in dye inventory. A control strategy has been developed which is essentially a program to define the limits of accuracy required, to improve production efficiency in the continuous dyeing of polyester/cellulosic fabrics. It has been demonstrated that high levels of laboratory-to-bulk reproducibility are achieved with the Econtrol (Monforts) continuous dyeing process based on appropriate dye selection and laboratory equipment that accurately reproduces bulk conditions.



    Automation



    RFT processing is an essential pre-requisite to the introduction of automation and robotics. The major financial savings from the concept of RFT depend on firm discipline in a manually operated or semi-automatically controlled (SAC) dyehouse. This requires defining the limits of accuracy, standardising dye selection and application techniques, using computer colour matching (CCM) methods for recipe generation and colour matching. Total automation, however, yields major savings in direct labour costs.



    Conclusion



    The technology to ensure RFT production has been available for over three decades and has now been further developed into the philosophy of controlled coloration. Although rightly seen initially as a major method of saving costs and increasing productivity in the dyehouse, further major financial benefits accrue through the concepts of JIT processing and QR. RFT must therefore be the immediate goal for all dye application processes – this is no longer a luxury but a necessity!



    J. Park

    parkdy@globalnet.co.uk

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