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Curtain coating

 

New DynaLayer from Voith for better coating quality and greater cost efficiency

• More uniform application of several coating layers in one step
• Low mechanical stress on paper reduces web breaks
• Cost savings due to reduced coating color consumption

With its ease of handling and outstanding coating quality, the DF Coat has evolved into an efficient alternative to conventional coating units. Through several refinements, Voith is extending the range of uses for non-contacting coating application and increasing the economic efficiency of paper producers with the new DynaLayer.

During the ongoing development of the origin DF Coat, the focus was on improving the productivity of the coating unit and opening up new areas of application. Several new components make the system more compact and simplify rebuild measures. At the same time, the latest generation of the coater allows the production of even more uniform CD and MD profiles. This has a positive effect on the printability of the paper and produces a more homogeneous print image. In addition, the number of costly web breaks is reduced by the new generation of DynaLayer because of the non-contacting application of the coating.

The modular slide-die-concept of the DynaLayer allows the uniform application of two and more coating layers to the paper in one step. The individual nozzles can be opened for inspection, cleaning and profiling.

One example for the extended range of applications is the slide-die concept in the DynaLayer: When producing specialty papers, this new kind of nozzle technology allows the uniform application of two and more coating layers to the paper in one step. To this end, one nozzle initially directs each coating color upwards onto an inclined plane. All coating layers slide over a trailing edge without mixing and then form the optimally dosed curtain film that is applied to the paper web. All layers are applied in a single, time-saving process. Excess coating does not occur, so the color does not need to be recirculated. In addition, there is no need for a leveling element to achieve a uniformly high level of coating application. There is therefore less mechanical stress on the paper web, which significantly reduces the number of breaks.

The slide-die-concept of the DynaLayer secures that all coating layers slide over a trailing edge without mixing and then form the optimally dosed curtain film that is applied to the paper web.
 
As the suction is done extremely close to the paper web, the color coating deposits in the edge area that are typical of conventional systems are prevented. These are caused by the buildups of color coating resulting from the constriction of the color curtain at the edges.
Christoph Henninger, Product Manager Coater Stations at Voith

The modular slide-die concept of the DynaLayer makes it easier to access the individual nozzles for cleaning and inspection. Moreover, after the individual modules have been separated from one another, the user can easily adjust the slots to the ideal width using tension and pressure screws.

Thanks to its geometry, a newly designed trailing edge minimizes the film rebound, known as the “teapot” effect. This is noticeable in the edges of the paper web, which can be better controlled and are more stable thanks to the new technology.

Edge guides with thin film of water guarantee clean edges on the paper web and ensure that the coating layer is applied with a precisely defined coating width. Thanks to a new kind of feed and air removal system, the water is always free of bubbles, which is a basic prerequisite for the inboard mode of operation to function reliably. A vacuum suction removes the water from the edge deckle. “As the suction is done extremely close to the paper web, the color coating deposits in the edge area that are typical of conventional systems are prevented. These are caused by the buildups of color coating resulting from the constriction of the color curtain at the edges,” explains Christoph Henninger, Product Manager Coater Stations at Voith. Several manufacturers of specialty papers are already using the technology and are reporting much reduced color consumption or losses and the associated lower costs. Various board manufacturers have also achieved positive effects thanks to the water-based edge control that paper manufacturers can also retrofit as an option in the course of a rebuild.

The inboard mode guarantees that the color layers do not exceed the exactly defined width on the paper web. Since the extraction takes place extremely close to the paper web, the usual color deposits at conventional installations are prevented on following rolls.
 

The uniform color application by the new curtain coater is particularly useful for the production of white board with a multi-layer coating. To achieve a high surface opacity in these kinds of papers, most manufacturers nowadays use titanium dioxide. The pigment is expensive, so every reduction improves the economic efficiency of the system: For example, a paper manufacturer that has been using the DynaLayer for some time now, cut costs for titanium dioxide by about half for the production of board with a white protective layer, by completely dispensing the titanium dioxide in the bottom coating layer while ensuring the same quality of coverage. A similar effect was achieved by a manufacturer of specialty papers that is saving up to EUR 150,000 per year thanks to the uniform application of the coating of the expensive functional layers, because he was able to reduce the total color application as a result of better CD profiles.

With its new deaeration pump, Voith is offering several starting points for improving economic efficiency. The functional principle of the pump has already proven effective at the pilot plant of the Voith Coating Technology Center in Motomiya, Japan. In several test phases under realistic conditions, the deaeration pump was able to demonstrate the high quality of its air removal. The pump reliably prevents the formation of air bubbles in the color. This minimizes the number of surface defects in the coating layer and therefore reduces broke. “Our new deaeration pump has twice the air removal capacity of a conventional vacuum deaerator – but is much more compact in size,” emphasizes Christoph Henninger from Voith. Using the new pump therefore saves a lot of space on the paper machine and makes it easier to integrate its technology during rebuild measures.

The innovative DynaLayer can be operated without a hood. Purge air at the nozzle (1) removes moist air from the environment at the die lips and prevents condensation on the lips and water droplets on or in the curtain film. A thin air shield (2) protects the color curtain against disturbing external influences.

The further development of the slot-die nozzles of the DynaLayer also allows for a more compact design of the coating unit. Instead of the usual costly enclosure including an energy-intensive air conditioning function, a thin air shield protects the curtain against disruptive influences from outside. To be able to realize this air shield, Voith first had to resolve the problem of condensation on the nozzles. Because in conventional solutions, moisture deposits on the nozzle lips due to the temperature difference between the hot paper web and the cooler ambient air. The solution developed by Voith consists of two components: “Firstly, we improved the curtain nozzle for the top part. Secondly, uniform air flows avoid condensation at the nozzle,” says Henninger. Dispensing with an enclosure and air conditioning not only saves space and simplifies the process, but makes it easier for the operators to access the system for all work on the machine. The open design also provides a better view of the DynaLayer and a more flexible web run. The new curtain coater is therefore also ideally suited for rebuilds of existing systems.

Firstly, we improved the curtain nozzle for the top part. Secondly, uniform air flows avoid condensation at the nozzle.
Christoph Henninger, Product Manager Coater Stations at Voith

The effectiveness of the patented system has been proven by tests under a wide range of climatic conditions. A long-term test at a paper mill confirms this result: Since the installation of the new nozzle insulation more than nine months ago, there have been no deposits of condensation under any circumstances, which has led to more stable production processes and more uniform product quality.

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