Tissue

Converting in the tissue sector, optimising embossing

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The final quality of tissue paper is also determined by the embossing process. The evolution of a roll covering technology demonstrates the optimisation achieved, thanks to an innovative bonding system. But this is not the only innovation.

Staying on the market and remaining as competitive as possible is a complex challenge for industries, especially at times like the present when costs are rising dramatically. It is therefore essential to invest in the efficiency of these costs and to focus increasingly on product quality. Paper converting plays a major role in this.

SchäferRolls is a German company, founded in 1946 in Stuttgart and which has developed over time to the point where, in addition to its headquarters in Germany – now located in Renningen – it has set up two other production plants in Slovenia and the United States.

SchäferRolls produces polymer-based roller coverings specifically for various sectors, including the paper converting industry. For this market, it has developed some new technological solutions, presented at Miac 2021 by Federico Barsanti, senior engineer at SchäferRolls.

Evolution in embossing

Among these novelties, the most relevant is an innovative roll covering technology designed for the tissue embossing process, a fundamental phase of papermaking that can contribute significantly to the quality of the final product. It is called H.impress and is actually, explains Barsanti, an evolution of a previous SchaeferRolls embossing roll covering solution, the H55/H15.

The aim was to increase embossing performance under most of the often-challenging process conditions. In particular, the anchoring system and the functionality of the paper sheet layers have been improved. «The combination of these factors, » says the engineer, «makes it possible to reduce the temperature in the machine a lot».

The H55/H15 – from which the new technology is derived – is a solution that is already known and appreciated by the market. Consequently, the new design was created with the intention of maintaining the same mechanical characteristics as the previous model, such as resilience and elasticity, as well as a low level of «compression set»and high resistance to abrasion. «These characteristics allow the machine to operate reliably and to produce, in a completely safe manner, high quality tissue, not only in terms of softness but also in terms of volume. The areas of application are micro and macro embossing, suitable for all types of cover».

The most interesting feature of the new solution, however, is the aforementioned temperature reduction in heat generation. The combination of special materials specially developed by Schaefer Rolls ensures that the temperature is reduced to a level of around 10 to 15 degrees Celsius compared to the best performing materials in use today.

If one analyses and compares the data of the Tan Delta parameter of the two solutions, the factor that measures the dielectric dissipation of a material – i.e. how much energy is converted into heat and thus lost as electrical energy – one can understand the improvement that the engineers at SchaeferRolls were able to achieve with the new H.impress. «This is the first improvement that has been made by our company’s research and development engineers in Germany in the development of new functional layers. The results also depend on other parameters, » says Barsanti, «such as nip frequency, load, hardness, type of micro or macro embossing, as well as the characteristics of the material, such as the level of wear».

Another element that improves the functionality of H.impress is the improved physical and mechanical properties of the cover materials. These, in combination with an innovative new anchoring technology, will ensure that the roller will function for longer. «The new functional layers are a mix of synthetic materials as well as high-strength components, made with the same characteristics and properties as the H55/H15 model but with the ability to have a better machine performance and significantly reduce heat generation down to 15 °C, » it said.

The second major improvement to the new model is the bonding system, which ensures that the enclosure performs well under high mechanical stress, even with machines operating at full speed. «In terms of temperature, the new anchoring system is designed for safe operation up to 90 °C. This is an innovative multilayer technology that SchäferRolls has been applying for decades in the paper industry».

Between a first base layer – made of hard material and placed very close to the shroud module as an anchor layer – and the outermost functional layer, two intermediate layers were inserted to progressively switch from the shroud module to the machine functional module.

Results at the Lucca paper mill

H.impress H.impress was successfully applied in Italy at the Lucart mill in Diecimo, near Lucca.

«In October 2019, Lucart contacted our company’s experts to resolve some issues on the 216 line with micro-stapling and the 220 line with macro-stapling,» continues Barsanti. Difficulties on the first line, 216, were caused by positioning problems, machine downtime and operational failures, especially on the drive, while on line 220 the problems were related to lamination and finished product.

Analyses and thermal calculations carried out on the machines revealed the causes of their malfunction. «On both applications the temperature on the roof was very close to 70 °C and the heat distribution was not equally distributed between the driving and the functional part. We thought this was the perfect environment to test the performance of H.impress and so, in March 2020, the first cover was installed on line 216, on the micro-embossing operating station to be precise». According to the engineer, there was immediately positive feedback: «the temperature was reduced, and the heat distribution was quite uniform». This is a line that is always operational and on which the owner, after 9,000 hours of operation, has not experienced any problems, failures, or downtime.

«The mill subsequently installed the same technology on line 220, and the advantage in this case was much greater: it even dropped below 50 °C, with a difference in the distribution peak between before and after the intervention of around 5 degrees». Again, therefore, «the mill achieved a lower cover temperature and more uniform distribution». The paper mill technicians themselves emphasised the results and the ability of the machines to maintain performance even at maximum production speed.

From a cliché to nanotechnologies

H.impress is not the only innovation from SchäferRolls. Barsanti also presented two other innovations: Insitu, a new technological development designed for plate rollers, and Q.dura, a new type of cover for marry rollers.

«The concept behind Insitu, » continues the engineer, «is to make a cover by producing a base layer and a functional layer in a single step. The base layer is made of epoxy resin, in some cases with the addition of carbon fibres, and between the base layer and the functional polyurethane layer a special filling material is applied that is able, during washing, not to react with both materials. This allows a perfect anchorage between the two materials without any chemical or physical reaction». A solution that makes it possible to produce the covering much faster and also secure the anchorage between the functional base layer.

Q.dura, on the other hand, «is a polyurethane technology designed to minimise the intervals between machine cleaning and, at the same time, to optimise the useful life of the cover». On tissue converting lines, having a clean surface is very important, explains Barsanti. Dust, oil, fibres and glue are residues that come into contact with the rollers and must be eliminated if they are to affect their proper functioning. In addition, «on the cover surface, their presence could lead to an increase in temperature, and this would disturb the functional geometry of the machine with the deformation of the material, and generate other problems related to the phenomenon of thermal crowning (thermal expansion) of the roller».

Q.dura’s solution is based on a concept already applied in the paper industry and is based on the use of nanoscale technology. «We have mixed our polyurethane compound with nanotechnology. The modified material we have thus developed makes it possible to transfer important repellent properties to the cover, making it much more effective than rubber. The results are excellent: not only is cleaning very quick and easy, but it is also possible to eliminate all glue residues on the roller, which are in any case fewer in number than on a rubber surface». The solution therefore guarantees low thermal expansion of the cover, a dirt-repellent surface and optimum hardness for better lamination of the layers.

Results at the Kinda paper mill

The results of applying Q.dura to a tissue machine at Swedish Tissue AB‘s paper mill in Kinda, Sweden, proved to be in line with expectations. The machine produces 15-17 g/m2 tissue paper at speeds between 350 and 600 m/min.

«The Swedish Tissue mill has had major problems with deposits on its machine in the past: oil, grease, dust, dirt, » says Barsanti. «These residues were sticking to the paper fibres and being returned to the sheet, so that the mill was no longer able to produce defect-free tissue. It was forced to scrape the surface to remove the excess dirt, and the use of the scraper led to additional operational and machine problems». The solution was to install Q.dura with a hardness of 98 shore A (ShA). The repellent properties of the SchaeferRolls technology were proven and the mill solved its problems, while also improving the performance and service life of the cover. «Today, » concludes Barsanti, «the mill can operate without any oil residues and without a scraping system for the paper sheets, which are now clean and free of defects».