Continuous or discontinuous processing, pre-treatment or post-treatment, a dyer is always in a dilemma while achieving scale or defining scope. Although everyone’s common goal is to achieve maximum utilization of equipment across different dyeing processes and different order volumes, the reality is hounded by traditional beliefs.
A dye house should not have both continuous and discontinuous machines
To most dyers and finishers, continuous and discontinuous processes are mutually exclusive concepts. In cotton finishing, dye houses that start off with exhaust dyeing (for batch/discontinuous dyeing) equipment usually remain with that concept, even if their daily output warrants a closer look at the economic benefits that a combination of the two concepts can offer.
Rather than being mutually exclusive alternatives, continuous and discontinuous wet processing equipments can work perfectly, as highly efficient and economically, compelling complementary solutions. One of the main reasons why such integral solutions have not been widely applied is that they have till date not been available from a single supplier and have thus, been seen as competitive rather than complementary process concepts. Only now the suppliers have created one-stop sales organizations (e.g. FONG’S) that offer both exhaust and continuous equipments.
When existing piece dyeing capacity is at its limits, always add more dyeing machines
Particularly in situations where existing piece dyeing capacity is at its limits, the usual reflex is to add more dyeing machines. However, there are alternatives, some with compelling arguments. The main consideration has to be whether more capacity can be added economically by separating pre-treatment from the core dyeing process.
By separating pre-treatment (e.g. scouring and bleaching) from the core dyeing process, the process time in dyeing machines is reduced by typically around 60 to 90 minutes (the time usually taken for bleaching). This means that in reactive cellulose dyeing, the exhaust process is shortened to 2 hours, allowing correspondingly more batches to be dyed per day. The bleaching is carried out on a continuous line instead.
Continuous processing lines may only make sense for extremely large daily outputs
When piece dye houses reach their maximum capacity, the usual opinion is to consider adding more dyeing vessels.
It would surprise most readers to know that a continuous bleaching range already makes a lot of sense with a daily dye house output of around 5 tonnes. That is, by any standards, not a large operation.
As we can see from the above figures (and quite general), it can also make a lot of sense to first look at which processes that are presently part of the exhaust dyeing process could be handled more economically on a continuous line. This holds particularly true for washing and bleaching. In addition, where vertical operations are concerned, it also makes sense to approach the issue of surface handle and appearance from the yarn construction and fibre blend issues, rather than devoting up to 30% of the dye house capacity attempting to increase lustre through enzyme treatment (and suffering substantial fabric weight loss in the process).
Exhaust dyeing is not the only concept for processing small and medium batches
Conversely, the commonly held opinion is that exhaust dyeing is the only concept for processing small and medium batches. As we can see from the simple point made above about a bleaching range, continuous processing is a term that does not only cover complete washing, bleaching and dyeing ranges, but also individual process steps. In this article, our focus is not on bed sheet plants, where thousands of metres of fabric are dyed in the same pastel shade for days on end. Far rather, our attention is on dyers and finishers that have smaller batch sizes in many shades but nevertheless use reasonably similar grey fabrics across their range. Can continuous equipment be of any use to them?
Let us look at a medium-size exhaust dye house with a daily (revenue) capacity of 10 tonnes of cotton knit fabric. To achieve this, an installed capacity of 2,500 kg is required (based on 4 batches a day, easily achievable with Airflow-type dyeing machines).
The typical process cycle on a 500 kg machine on medium shades include:
– Loading: 10 minutes
– Bleaching: 80 minutes (at 1100C)
– Dyeing: 120 minutes
– Washing off: 70 minutes
– Unloading: 10 minutes
This adds up to a total time of 290 minutes, assuming a load of 450 kg. Water consumption per kg of fabric is 31 litres. (Data based on THEN-AIRFLOW™ machine).
From the above, we see that almost 30% of the time spent for this all in-process is dedicated to bleaching. Conversely, if the bleaching processes were to be continuous, the daily dye house production would increase by 38%, i.e. from the current theoretical batch rate of 4.96 to 6.85. In tonnes per day, we would see an output of more than 15 tonnes – without the addition of a single dyeing machine.
In fact, adding another 5 tonnes of daily capacity would necessitate adding at least 2 dyeing vessels with 500 kg capacity and one with 250 kg (again based on THEN-AIRFLOW™ productivity).
Bio polishing can reduce dyeing output by 30% or more
Bio-polishing has become extremely popular over the last few years, possibly so without anyone in the value-addition chain working out the true cost of the process. Its main purpose is to achieve a smooth fabric surface even when lower quality yarn with a high content of short fibres has been used in knitting. Usually, this type of enzyme treatment is carried out as part of the piece dyeing process and slotted in-between bleaching and dyeing or between dyeing and washing off. (The advantage of bio-polishing between bleaching and dyeing is that the subsequent dyeing is carried out on the fabric post-weight loss).
Embedding the enzyme treatment in the overall dyeing process sounds convenient and cost-effective. However, the fact is that depending on the desired intensity of the bio-polishing this additional process step adds 60 to even 90 minutes to the batch time or, in other words, some 20-30% in terms of time.
Logically, and proving the point that nothing in life is free, poorer quality yarn needs more bio-polishing than better quality yarn, often offsetting the lower yarn price of the former. But, coming back to our example of a 10-tonne-per-day dye house, bio polishing costs us between 2 and 3 tonnes of daily output! Looking at the loss of dyeing capacity caused by bleaching and bio-polishing, we see that these two processes – when integrated into the exhaust dyeing process – reduce our dyeing output by 50%!
Considering only bleaching of knitted fabrics for the moment, one solution is to handle washing and bleaching on a continuous line, such as the SINTENSA and COMPLEXA ranges by GOLLER. These can be used for either RFD or for optical white. As opposed to even the most advanced piece dyeing machines using Airflow technology, this will result in a reduced specific water consumption of around 1 litre per kg of fabric on both RFD and on optical white.
Coming back to the issue of bio-polishing, the alternative is to singe and/or mercerize the cotton as part of the same continuous washing and bleaching process.For this an additional mercerizing module (e.g. CADENA) can be added to the washing/bleaching range mentioned above. For higher-quality fabrics, this process can be combined with a singeing unit.