Converting of specialty papers comes with many challenges
Converting features needed to overcome challenges when processing thin, specialty papers. The converting of specialty papers, such as tea bags and technical filter paper, comes with many challenges. When converting these types of materials, the web widths of parent rolls constantly increase (i.e.: filter papers up to 2,500 mm and tea-bag paper up to 4,300 mm) while the thickness of the material steadily decreases. At the same time, the slitting width of the finished rolls gets narrower.
Furthermore, most customers want to produce the finished rolls in only one process step, rather than traditional, multiple steps. This article will cover web dimensions and advantages for production efficiency; the best Tension-Nip-Torque Recipe (TNT) and the best configuration to limit constraints and thus be able to deliver more productivity with the highest quality; the perfect combination of shear cut and perforation to produce the best slitting quality; and the advantages of high-quality components to achieve results quickly and easily.
The converting industry is a fast-growing business, and its products are becoming more and more part of our daily life along with new markets and new materials. Furthermore, to satisfy this quite demanding market, many applications in the converting industry require much more efficiency and a higher level of performance.
Generally speaking, the slitting technology for flexible materials is divided into three main categories: Paper-based, plastic films and aluminium foil. However, some markets present all of them, sometimes with special substrates for very specific applications, such as laminations, and coated and surface-treated materials.
These take advantage of the technical characteristics as well as benefit from each other for very balanced final materials. The specialty paper materials for filtering equipment represent one of the most significant among the categories mentioned above. Growth is strongly supported by an increase in health & safety awareness as well as strong demand for high-quality lifestyles, especially in developed countries. Filtering media include a wide range of products for food & beverage, automotive, industrial, heating & air-conditioning, civil, building applications, and many others.
Bigger, wider parent rolls of thinner materials
The slitting and rewinding process for filtering systems can be quite challenging. Many applications are so specific that an indepth understanding of the application, the materials’ physical properties and the performance of each component are required to understand and guarantee the most appropriate solution. One of the most challenging issues comes from today’s trend toward a continuous increase of the material dimensions (roll diameter, width and weight, both for the master roll as well as finished rolls), while the material thickness is steadily decreasing. A clear example is the market for tea-bag paper or coffee filters (see Figure 1), which demand equipment with widths up to 4,300 mm, along with a complete automated slitting and converting process that allows converters to drastically reduce downtime and consequently improve production efficiency. For the very sensitive materials, such as air-filtering and automotive-fluid filter media, some converters face working with increased web widths up to 2,500 mm and fully center-driven winders. In both cases, the maximum rewind diameter is often up to 1,500 mm.
Low density demands precise tension control
Not all filtering papers have the same requirements, but most have a common peculiarity: their low density, which is a direct consequence of maximizing the contact surface between the fluid being filtered and the quantity of filtered substance involved. The major challenge with this material is, due to its physical characteristics and in comparison with other sorts of paper, to manage the low tensile strength of the substrate and the strong sensitivity against higher tensions during converting. Slitting and rewinding low-density materials require an extremely precise and complex tension control for the whole machine and, in particular, for the rewinder and its parameters to avoid damage to the web or web breaks. This is especially true if the material is surface-treated or must be slit and perforated in one step. Moreover, for some filtering media, the low density, combined with its anisotropic web structure, makes web tension control a critical issue to preserve the dimensional precision of the finished rolls.
Control of contact force
Besides the individual control of the web tension, the contact force between nip roller and finished roll is an important value to achieve high-quality rolls. As the density of the filtering paper is quite low, accurate control of the contact force becomes essential, particularly with increasing diameter of the final wound rolls. Machines designed to convert sensitive materials, such as filter papers, are equipped with a closed-loop control for the nip force using a measuring roller as nip roller. In addition, challenges arise with increasing dimensions of the web width and the finished-roll weights. Winding narrow rolls (slitted from a 2,500-mm-wide web, on 3-in. cores) requires special equipment to minimize the deflection of the rewinding shaft. Following the goal to keep bending of the shaft within limits, a system to compensate the weight is installed in the machines. Thus, it can be managed, to balance the nip force, the bending of the shaft and the winding quality in such a way that numerous narrow rolls with large diameters, wound onto individual 3-in. cores, can still be separated easily after extraction from the shaft.
Once the best machine configuration for the application has been specified in terms of material and finished-roll dimensions, the limiting conditions to determine the so-called “TNT recipe” must be defined. TNT is the acronym for Tension-Nip-Torque. The right combination of these parameters is an important milestone to define the perfect configuration for a slitter/rewinder. These three fundamental physical components could be changed in a predefined range of values and are interconnected.
From the slitter manufacturer’s perspective, the right TNT recipe is a question that must be answered in two steps:
- It has to be defined which values are needed when working with specific materials, how they are contingent from each other, which kind of system is managing the process, and how the control of algorithms is taking into account all variables.
- It must be evaluated how the speeds and dimensions will be managed along with the acceleration, deceleration and the eventual reaction of the web during these operations. Finally, the slitter manufacturer’s engineers must guarantee the repetitiveness of the recipe. This means that they have to make sure that the machine is perfectly functional for the total range of materials, dimensions and speeds.
Specifying and working with slitter manufacturers
As each customer has his or her own material converted into certain dimensions, it is the machine manufacturer, making the choices mostly based on experience, who determines the best working parameters to maximize productivity while maintaining desired quality. Once a good working concept is obtained, an additional requirement should be examined: the individual system design. In most cases, standardized machines can’t meet the requirements mentioned above. Thus, a tailor-made solution for each customer is designed based on specific requirements. Through this customization, the machines may include unique features such as special hardware and software needed to produce specific finished products in the most efficient way or to better integrate slitting into the overall manufacturing process. Sometimes the machine customization changes so much the engineers must re-adapt the TNT specifications to guarantee perfect integration and correct functionality.
One customization example concerns an automotive development. The requirement was to separate the slitting unit into two independent units: one slitter with a shear-cut system and the other with a perforation unit with a score-type system. The target was to take advantage of two independent setup processes while still maintaining high precision and improving productivity; both operations would be done at once.
How do you define slitting “quality?”
And finally, “How does the converter define slitting quality?” The production quality and efficiency of the system are required to stay within a range of acceptance by the process requirements and to satisfy or even exceed customer expectations. Among all parameters, the precision in terms of roll dimension –such as tolerances in sizes and shape – is one important subject of the slitter that must be satisfied. Slitting tolerances can start as little as ±0.1 mm and will depend on the material properties, web dimensions and process quality.