It is the common aspiration of all the dyers to achieve an uniform dyeing. Unfortunately, no matter how well a given formula reacts in the laboratory dyeing equipment, But in the actual production environment, the scenario might alter. The formula may fail to click after all. Often the reason cited for such a failure is attributed to irregular dyebath flows. Therefore there is prime importance of maintaining uniformity in dye bath flow in a production equipment. If this is not followed in letter and spirit, there is every chance of an inconsistency in dyed material. It is imperative for any modern dye house to have sound dye bath controlling technology.
A dye bath control mechanism can be broadly divided into two steps.
Measurement- In this step, first a sample from a dye bath is taken. After that, the theoretical recipe is either fed into the computer, or is procured from the database. An optimal dilution required for the transmission analysis is worked out automatically. It is then carried out by the control mechanism. This is then followed by taking the measurement. This generally happens at a spectrum of 200-850 nm at 2 nm pitch. As soon as the measurement is over, the system automatically cleans itself. Finally the measurement results are checked for evaluation in a computer.
Evaluation and correction- In the second step after measurement, the control mechanism indicates whether the dye bath is rejected or accepted. In addition, the control mechanism also gives a detailed information regarding the mistakes. Total error, concentration error, shade error etc. are effectively pointed out. Tolerances may be adjusted based on the specific requirements. A multi component analysis is put to use for analyzing an error.
If the dye bath is rejected for any reason, there exists a possibility to put a proposal in place for rectification. This is calculated taking into consideration the volume of the dye bath.
There are various causes for a defective Dye Bath some of the important ones are given here under:
Delivery- It may happen, that there are fluctuations in the dyestuff lots as delivered by the supplier. Usually a tolerance of (+/-) 3-5% is quite acceptable.
Humidity/ Storage- This is one of the biggest and most basic problem. Depending on the condition inside the storehouse, there is likely increase or decrease in moisture content of the dyestuffs. It has been found that these errors are invariably the biggest sources for a defective dye bath. It has been seen that fluctuations can go up to 30% or more. The table below shows the effect of the humidity on dyestuff.
Preparation Errors- There can be preparation errors. This can happen with incorrect exchange of one dyestuff with the other. Weighing error (as in manual preparation) is also seen. Error of Dosing in automated color kitchens (like defective valves or agitators) is also seen. It has been seen that even for a system with full automation, chances of errors lie typically in the range of 2-3%.
Miscellaneous Errors- Other types of errors include contamination, dilution (preparation volume) or errors in the form of calculation.
It has been found in conventional dyeing that after dyeing, only the dye and a few of the speciality chemicals gets fully consumed during the operation. While most of the chemicals remaining in the dye bath are rejected. Increasingly due to tough environmental guidelines, the dye houses have been forced to study the feasibility of dye bath re-use. The dye bath reuse depends on a number of factors like dye, shade, colour, and if dyeing is carried out in a continuous or batch process. It has been found that in some cases, with a plan in place dye baths can be successfully re-used at least between 5 - 25 times.
Some of the processes where dye bath re-use has been very successfully implemented are shown in the following table.
Design of Dye Bath Reuse System
There are many alternative ways to design a system for Dye Bath reuse. Though limitations like the physical and production limitations of the dyeing process also affects the design for Dye bath re use. Two methods of Dye bath re use are discussed here.
Holding tank-pump system (dyebath transfer)- In this system, one pump and a tank are enough for two dye machines.
Additionally for three to four dye machines, there is a need for two storage tanks. As pumpings are generally accomplished within a time span of approximately five minutes, a single pump with properly operated valves is sufficient for dyebath transfer. In both the cases a proper production schedule is a must.
Material Transfer Methods- The Material transfer method for dye bath re use can take the form of two possibilities. First, where the the dyed material is moved to another dye machine for conventional rinsing. Second, where the the dyed material is moved to another location for nonconventional rinsing. If this method is implemented, provided it is physically and technically tenable to the production process. The material transfer method results in savings from the cost of design and installation of a pumping system.