In the whole textile finishing cycle, an important role is taken by all the preliminary treatments, both dry and wet, where all the natural impurities and the residuals of spinning and weaving processes are removed. Dry treatment is usually useful to remove the hairiness from the fabric to obtain a clean surface, which allows the structure of the fabric to be clearly seen.
Furthermore, making the fabric free of sizes allows to have the maximum possible penetration of dyeing and auxiliary chemical finishing products up to the fabric base. For these reasons, a preliminary treatment properly made gives a great influence on the quality of the final product.
Given that the edge cutting could not be enough or could be tricky especially working with very thin fabric, the hairiness is removed by burning with the “singeing process”. During this process, the fabric is in contact with incandescent metal items, exposed to a direct flame or forced to run close to highly heated ceramic items (direct singeing by irradiation). The system using direct flame with a singeing machine is still today the most popular one.
Usually, in the same line of the singeing process, there are also other dry auxiliary treatments such as:
On the other hand, an auxiliary wet treatment is the desizing process: in this treatment the fabric passes through a bath with two rows of rolls; the lower one is always under the bath level so the fabric is always in contact with the liquid.
The direct flame singeing system is the most used for his great efficiency in hairiness removing. This singeing process is usually made with a linear burner fed with methane, which should guarantee enough heating power with a uniform flame on all fabric width. The fabric should run close to flame tip (approx. 1,5÷4 mm) with a linear speed between 60 and 120 m/min.
In order to guarantee a certain flexibility of the process, in terms of different composition, weight and type of workable fabrics, it is possible to change the distance and the relative positions between fabric and flame.
In particular, the main working configurations are:
Here below there is a further classification under this working configuration:
– perpendicular singeing on cylinder: the flame acts against the fabric while it is contact with a cooled cylinder; the distinctive feature of this system is the high constraint on the fabric which suffers the highest flame pressure.
– perpendicular singeing on free fabric: the flame acts against the fabric while it is not in contact with a third elements. In this way, the fabric is not tied down so the pressure of the flame in this configuration is less than in the previous one.
During singeing process, a possible further action useful for having a wider range of workable products and for a better managing of the system is the regulation of flame intensity. Generally, in thermodynamics, the intensity is defined as the heat quantity that pass through a geometrical section in a specific time; so for changing the intensity it is necessary to act at least on one of these two considered elements (the heat quantity in a specific time or the geometrical section).
Usually it is easier to manage the quantity of the fuel that feed the burner by a mixing station. Of course there are some physical limitation in order to keep the flame stable and powered on (functioning burner field) but in general it is possible to change the heating flow in a range of tens kW per geometrical section units.
If we analyse the combustion side of the process, it is important to avoid an oxidising flame that may lead to undesired carbon and sooty residuals. In any case, it is necessary to remove these eventual residuals with proper washing cycles to be made after singeing process.
The singeing process typically is made on grey fabric just because all the combustion residuals should be removed by washing cycles.
Very often in a singeing machine you will find two burners that, also thanks to the different threading possibilities, allows to make the singeing process on both fabric faces or two times on the same side.
Of course, with the first layout it is possible to reduce the time needed for the singeing process while the second configuration gives a higher singeing power.