What is converting?
Tissue converting is the production process through which a 2-dimensional semi-finished product is transformed, generally into smaller reels (paper, fabric, nonwoven fabric, plastic film, aluminium, etc.), consumer goods or other semi-finished products, subsequently used for further industrial processing. The variety of processes is remarkable even within the paper sector alone. A few examples include:
- production of rolled products – tissue paper: household goods (toilet paper, paper towel) or non-domestic ones (e.g. tissue rolls for industrial dispensers or disposable sheets for medical stretchers);
- production of folded sheet products – tissue paper (napkins, handkerchiefs, paper towels);
- production of printing paper rolls or sheets – flat paper for the printing industry;
- production of plastic or metal-coated paper for the packaging industry.
Thanks to over 40 years of experience in the processing of carbon composites, Reglass designs and manufactures carbon fiber rollers specifically engineered for converting processes. The purpose of carbon cylinders is to increase the working speed of the machine; to reduce rotational inertia of the roller; to reduce stresses on mechanical parts, therefore increasing the endurance of components.
What follows is a brief illustration of the tissue paper converting processes.
The converting process: from the reel to the finished product
All kinds of finished products originate from specific processing lines, also known as converting plants, where paper undergoes several transformations. Here are the most important.
The unwinder is the area of the line where reels are unwound. Each reel is loaded on a reel-holder shaft, rotating at high speeds: carbon composite construction allow to increase working speed and/or avoid vibrations. The reel is tangentially driven by belts and paper may pass through several idle rollers before reaching the next processing stage.
In the tissue paper converting sector, the combination of high processing speeds, the low tensions that paper can sustain and the need for idle rollers having very low inertia and high smoothness makes carbon idle rollers the best choice for all high-performance machines manufacturers.
For some types of production lines, it may be necessary to include calendering of paper plies after the unwinders. Calendering consists of the passage of paper between two large steel cylinders with a high surface finish, at an accurately calibrated distance. This process compacts the plies and, when needed, it may grant paper a smoother and less creased finish.
While calendering cylinders are currently made of metal, return rollers and idle rollers can be made of carbon fiber composites. Thanks to carbon fiber composite rollers, a line can work at the highest possible speed with no vibrations, preserving the quality of the final product.
The purpose of the printing unit in the tissue converting lines is to imprint a design, even in several colours, to obtain a product that is aesthetically pleasant and recognizable. This is achieved with a flexographic method: through the anilox cylinder, ink is transferred from an ink chamber to the graphics embossed on a polymeric printing sleeve, which is secured onto a sleeve mandrel. The graphics then transfer the ink to the support, squeezed between the printing sleeve and the counter-pressure cylinder.
A carbon sleeve mandrel is the best choice for high-performance machines, also allowing sleeve change in an effective, fast and possibly automatable way. Its main features are:
- extreme rigidity, to obtain a uniform print on the whole board;
- lightness, to avoid vibration issues on lines that work at speeds above 500 m/min;
- precise and stable dimensions for easy sleeve change. This operation is carried out by pushing air through calibrated holes on the carbon shell, which makes it easy to remove the sleeve, creating a thin air cushion between it and the surface of the cylinder;
- safety for the operators who carry out the sleeve change operation.
Reglass has always produced sleeve mandrels using a proprietary technology that guarantees the stability of the junction between the metalheads and the carbon shell. Furthermore, for tissue paper spindles, Reglass relies on specific technology for air distribution, which allows the machinery to operate in complete safety, regardless of operating conditions, even if the cylinder should be damaged.
The combination of such features allows Reglass to offer CE certified sleeve mandrels with the best stiffness-to-weight ratio and of the highest quality, to support the most demanding manufacturers.
Embossing is an operation that makes it possible to permanently imprint on paper textures (micro-embossing) and/or designs (often specific to the brand) by pressing it between a special steel embossing cylinder and a counter-pressure roller. Each ply can be individually embossed and subsequently coupled with other plies. In the case of multi-ply tissue paper, this process is also useful to permanently fix the plies, ensuring better thickness and softness.
Engraved rollers are generally made of steel, which grants them an accurate and long-lasting engraving process. Even the counter-pressure cylinders, which are usually rubber-coated, are generally made of steel as the use of a different material usually isn’t required. The use of carbon, on the other hand, is optimal for the realization of guide rollers.
Perforation and winding
As for rolled products, the paper undergoes perforation, which allows the user to tear off the single sheets from the roll, followed by the winding phase. Paper is winded in logs (rolls having the same diameter as the finished product and width equal to that of the parent reel), with a central cardboard core. The log is subsequently cut into rolls of the final size, ready to be packaged and boxed.
Winding is one of the most critical phases in the production of roll products: the softness of the finished product and its appearance depends on the quality of this phase.
The process typically takes place with the use of two motorized driven rollers, in a fixed position, which tangentially drives the roll as it takes shape. A third roller, called the pressure roller, is used to gently squeeze the roll against the two drive rollers, preventing the log from slipping and guaranteeing a uniform winding. To achieve so, the press roller must be fixed on a movable arm that allows constant pressure to be applied on the log as it grows. Once it reaches the desired diameter, the log is ejected and the wrapping process begins all over again. The number of exchanges is in the order of tens per minute. The use of carbon pressure rollers, which may come with wear-resistant and high-grip coatings, is essential to reduce rotational and translational inertia, but also to ensure strength and stiffness.
For flat/folded products (napkins, wipes, handkerchiefs), there is then a cutting and folding unit, as well as the creation of the packages that will be sent to the packaging line. The machinery involved in these processes are seldom equipped with carbon rollers.
Packaging and palletization
Once the finished product is complete, it is sent to the packaging departments for the creation of the various kinds of packaging intended for wholesale.
Converting processes require advanced technologies to achieve high-quality finished products with efficient and cost-effective processes. The increasing demand for diversified and customized finished products (paper quality, product recognition, production cost management) and highly automatable processes, has favoured the use of carbon fiber cylinders, which guarantee high-quality standards, high machine speeds and simple handling. Reglass has developed specific technologies to build carbon fiber composite rollers destined for the converting process, paving the way for customizations that until now would have seemed unthinkable.