Milling drums for leather finishing: technology and innovation

The tanning of hides and skins is an articulated activity that involves numerous mechanical and chemical phases, starting with wet treatments and ending with dry treatments, passing from raw hides to wet blue or wet white, then to crust and finally to finished leather. Once dried, hides and skins are sent to finishing, one of the last of the leather making processes.

Leather finishing is essential to improve the performance, appearance and durability of the leather. This is where leather milling, a treatment that takes place in special drums for tanneries, is essential: over time, drum milling has evolved to become as important as it is today.

The application of new technologies and materials has allowed the creation of a new concept of leather drum milling, no longer carried out in old, reconverted dyeing drums, but in purpose-built machinery: the modern automated milling drums found in tanneries today.

As an integral part of the leather finishing process, let's find out how tannery drum milling is carried out today and what are the objectives in leather finishing.

Let's start with the basics: what is leather drum milling?

Leather milling is a mechanical processing of the leather carried out in drums, machines made up of large cylinders rotating along the axis, in which the hides are vigorously mixed and softened mechanically in an environment with controlled humidity and temperature.

This mechanical shaking and environmental conditioning leads to the separation of the leathers’ fibers and the distribution of the lubricating products introduced in the wet phases, thus softening the treated hides and skins.

It is possible to distinguish different types of leather milling:

• Dry milling: it takes place in milling drums where the hides are handled without humidity control. This is one of the earliest versions of leather milling, nowadays scarcely used, due to the unpredictability of the results.

• Drum milling: it takes place inside milling drums where air temperature and humidity are defined and controlled throughout the whole process. In concrete terms, this type of leather milling is the evolution of dry milling, allowing accurate control of the process and consequently of the results.

Leather drum milling, however, is not limited to the mechanical and conditioning phase: within milling drums it is possible to use recipes and formulations of chemical products to obtain specific technical or aesthetical effects in response to particular design and fashion requests.

We can also identify two other recent types of leather drum milling:

• Wet milling: here the machinery performs the process with wet hides, often ending with leather drying which gives the peculiar aesthetic characteristics of drum drying. Again, humidity and temperature controls are essential to control the results..

• Cold milling: in this case the treatment is carried out at lower temperatures than the normal temperature generated by the friction of the hides moving inside the drum. In this case it is even more important to control the temperature and humidity conditions inside the machine, as the hides are usually extremely sensitive to heat.

Goals and needs of leather milling in tanneries

Hides and skins drum milling is a very delicate process and its success depends on many factors. Over the years, many manufacturers of tannery machines have questioned the variables of leather milling and the overall leather finishing process.

Controlling the mechanical movement of the hides, temperature and humidity.

These variables have proven to be crucial in order to manage and repeat the leather milling process. As it is not possible to directly detect the humidity and temperature values on the hides, air temperature and humidity control becomes fundamental. The correct identification and selection of these parameters allows tanneries to control the environment in which leathers are treated, so that the hides can adapt to the new parameters of humidity and temperature. Hence, temperature and humidity are closely related variables and must be constantly monitored throughout the drum milling process.

Heterogeneity of leathers and larger production batches.

Tanneries often may find themselves working on batches made of different wet blue hides, or on already purchased goods in semi-finished crust. The leather finishing process becomes crucial to make hides and skins uniform, to reach the standards dictated by the production, and to guarantee an important added value given by the uniformity of the leather batch. Also, when processing large leather batches, it is sometimes necessary to divide them up among several machines: the need of tannery machines capable of perfectly replicating the leather finishing processes becomes crucial for the uniformity of the entire production batch.

Advanced technologies and emissions into the environment.

In the era of Industry 4.0, leather finishing cannot be left to chance: it is necessary to use tools and technologies capable of controlling performances and programming parameters in every detail. For as long as it concerns the environmental impact, dust from the milling process or released chemicals no longer represents a problem for air pollution. The quality of the emissions’ treatment from milling drums ensures sustainability.

Working shifts and conditions along with energy consumptions.

Large tanning groups have leather finishing plants all over the world, in very different climatic and geographical conditions: a real challenge for the industrialization of the leather making process. Machines must therefore be as untouched as possible by the influences of the external climatic environment, guaranteeing uniform treatment in all geographical areas. Also, tannery production is normally operating 24 hours a day, 7 days a week: the machinery must therefore be increasingly efficient, both in terms of leather production and consumption optimization.

Evolution of the tanning sector, from small to large productions.

Along with industrial leather productions there are many more requests for sampling and small scale experiments, with very small production batches to carry out trials and tests, extesive time is given to the research and development of the product. To offer the best solutions for leather milling, it is essential to adopt an approach to the leather making process that takes into account all the aspects involved: industrialization, standardization, technology, research and sustainability.

Duration, time and manpower of the milling drum process.

Being essentially a controlled atmospheric mechanical action, the quantity of hides being treated has a direct influence on the duration of the milling process. Leather milling departments have to cope with uncertain duration times that can last several hours. Human intervention is only required for loading and unloading: ensuring that staff are present at the correct time is crucial in order not to ruin the processed leathers with creases and wrinkles. Today, milling drums for tanneries are equipped with softwares able to solve these problems ensuring the maximum productivity.

Erretre: next-gen leather milling drums

Leathers and hides finishing with Erretre milling drums

Erretre was founded in 1974 to find innovative solutions for tanneries, in response to the leather sector which was then in the middle of growth and industrialisation. Erretre has always been dedicated to leather finishing, and the design of our leather milling drums comes from studies and collaborations with technicians, tanners, chemists and researchers: the result is a wide range of technologically advanced, industrial and laboratory leather milling drums, adaptable to the needs of tanneries and of the international market while respecting sustainability.

The strong points of Erretre milling drums for tanneries are:

• High efficiency and low consumption: the latest generation of Erretre milling drums is the Black Line series, the line of automated leather finishing drums that guarantees a greater efficiency and lower consumptions;

• Advanced technologies: as the international patent Chemical Milling ® for optimizing chemicals’ injection, or Erretre’s softwares L.C.I.S. and M.O.RE. that allow automatic management of tannery drums and finishing lines with simple and intuitive interfaces and memorization of the results;

• Processing of sensitive leathers: Cold Milling ® allows an easy milling treatment of the most delicate leathers, regardless of the working climatic conditions; it is compatible both with the Paca model, our drum with automated unloading where the doors and conveyor are automatic, and with the High model, our drum with manual frontal unloading;

• Efficient air dedusting: each Erretre milling drum is accompanied by a dedusting system for air purification, allowing the elimination of dust and volatile substances residual from leather treatment and the use of chemical additives;

• Customized accessories: in addition to an advanced temperature and humidity control system integrated in each machine, Erretre milling drums can be equipped with additional accessories to exploit their full potential and meet the different needs of tanneries;

• Laboratory machines: the same reproducibility of results guaranteed by Erretre drums is also offered by Ambra, the laboratory milling drum for tanneries with reduced dimensions, ideal for research and development departments and for smaller production batches.

Get in touch with Erretre's team