IN LEATHER FINISHING, QUALITY IS DETERMINED BY SPRAYING AND DRYING: THE STRATEGIC ROLE OF THE NER TUNNEL

In leather finishing, the quality of the final result is determined by the balance between two closely related steps: the application of the finishing film and its correct drying.

The spraying phase defines the composition, uniformity and characteristics of the film applied to the leather surface. But for this finish to truly express all its properties, the next phase — drying — must take place under the right conditions.

In other words, good spraying needs equally accurate drying to be effective.

It is in this phase that one of the most delicate balances of the entire finishing process comes into play.

Drying the film without altering the leather

In the finishing tunnel, the goal is not simply to ‘dry’: the real task of the system is to remove water from the finishing film applied to the surface, while avoiding transferring too much heat energy to the leather underneath.

If this balance is not managed correctly, two undesirable effects can occur:

• overheating of the leather, resulting in stiffening of the fibres;

• shrinkage of the surface, leading to loss of area.

Drying therefore becomes a precision process, in which temperature, exposure time and ventilation must be carefully controlled.

Not just drying: the role of cross-linking

In many finishing processes, especially in more technical applications, the action of heat is not limited to evaporating water from the film.

Heat provides the energy necessary for cross-linking, the chemical process that connects the polymer chains and gives the film greater strength and stability.

For this reason, the drying tunnel must not only dry the finish, but also correctly activate the reactions that determine the final quality of the film.

Excess energy can damage the leather, while insufficient energy can leave the film incompletely cross-linked, resulting in the stickiness of it.

The limitations of traditional tunnels

In traditional drying tunnels, the process generally takes place inside a single chamber at a constant temperature.

Under these conditions, the leather enters the tunnel and, as it moves along the path, continues to accumulate heat. When the water in the film evaporates, the cooling effect of evaporation is lost and the temperature of the leather tends to rise rapidly.

This explains why many traditional systems use fans or coolers at the end of the tunnel.

The problem, however, is that the damage has already been done during the process..

The real constraint in tanneries: space and productivity

In theory, a simple solution would be to work with very long tunnels and lower temperatures, allowing more time for the finish to dry.

In reality, however, this approach is rarely feasible.

The space available in tanneries is often limited and the speed of the conveyor belt is closely linked to the productivity of the line. To compensate for shorter tunnels, the temperature is therefore tended to be increased to speed up the process.

And this is where the problem arises: higher temperatures to compensate for less space means increasing the risk of overheating the leather.

The NER concept: controlling the energy transferred

To address this delicate balance, Erretre has developed the NER drying tunnel, based on a modular architecture that allows for more precise control of the energy transferred during the process.

Unlike traditional tunnels, the system consists of physically separate modules, each of which can operate with independent parameters.

This allows the process to be progressively modulated along the tunnel, preventing the skin from accumulating heat in an uncontrolled manner.

The result is more accurate management of the balance between film drying and substrate protection.

Humidity control and fibre behaviour

Controlling the moisture content of the leather is another key factor in the quality of the process.

If the leather enters the subsequent stages — such as Milling — with an incorrect moisture content, the behaviour of the fibres can change significantly.

Leather that is too dry tends to cause the fibres to slide less between each other: electrostatic phenomena can occur and the mechanical action of drumming is absorbed unevenly, with some areas breaking down too much and others remaining hard.

Conversely, leather that is too moist can react in the opposite way, with fibres that are too mobile and keen to shrinkage producing therefore excessive pebbling.

For this reason, it is essential that the drying tunnel keeps the leather within an optimal humidity range, where the fibres remain plastic and workable.

An often underestimated but decisive phase

In leather finishing, attention is often focused on the quality of the spraying. However, as many tannery technicians observe, the real economic performance of the process often depends on the tunnel.

An experienced operator can greatly improve the results of a spraying booth by working on parameters and adjustments. The tunnel, on the other hand, works autonomously throughout the entire line.

If the drying process is not managed correctly, this is where the weight, quality and value of the material can be lost — often without being noticed immediately.

For this reason, the drying tunnel is not simply a technical step in the finishing line, but one of the most strategic points for ensuring stability and quality in the leather finishing process.