Corex’s (earlier Corenso) board mill in Pori, Finland is making investments and developing its production and equipment continuously. In 2019, one of the development projects was improving doctoring on the dryer section to meet the quality requirements of production. Special attention was paid to the cleaning of cylinder surfaces in the dryer section following the introduction of a new product group.
“For a few years now, we have been supplying our customers with board to be laminated. For an end product with a glossy surface, the quality of the base board needs to be flawless. For instance, dirty cylinders can cause depressions in the board,” says Minna Kurittu, Production Manager of the mill.
Sami Kari thinks that cooperation with Valmet has been very smooth. Pictured above with Antti Pitkänen from Valmet (photo above).
Based on a doctoring audit covering the whole board machine, the development of doctoring at the beginning of the dryer section was identified as the most important item. The problem was fines and impurities sticking to the surface of the dryer cylinders. Dirty cylinder surfaces weaken the drying effect, increase energy consumption and often cause the above-mentioned quality defects in the end product.
“We chose Valmet as our partner for our development project because we were convinced of the expertise of Valmet’s specialists in this area. The price was also competitive, as the proposed solution was eventually rather simple and did not require any large-scale rebuilds in the production line,” says Sami Kari, Project and Development Manager.
Development of the doctor concept was discussed in a kick-off meeting following the audit with both parties’ specialists. The shared view was that doctor beams that were in good condition would be used, bearing assemblies would be serviced and blade holders would be upgraded from gravity-loaded to hose-loaded Valmet Doctor Holder Compact holders. In addition, opening mechanisms would be removed from the doctors and their position would be locked with turnbuckles.
“Self-profiling holders improve the blade’s conforming on the roll surface and enable a precise and, if necessary, higher blade load than the old, gravity-loaded doctors. The doctors are operated from a separate pneumatic control unit. The decision was made to implement the modification on the three dirtiest cylinders at the beginning of the dryer section,” says Antti P. Pitkänen, Product Specialist, Valmet.
After start-up, cleaning the surfaces proved to be more difficult than anticipated. A breakthrough was reached on the third attempt by using thermally coated Valmet Doctor Blade Bronze nCT blades, which demonstrated their effectiveness right after start-up. After a couple of days in operation, the Bronze nCT blades made the surfaces shiny clean.
“The difference was remarkable! The Bronze nCT blades immediately started to carve the surfaces clean so that slivers were flying at the dryer group. After a couple of days, the change was hard to believe,” says Jonas Nordquist, Production Development Engineer.
The development project reached its goal: significant improvement was achieved in the cleaning of the cylinder surfaces. Blade replacement is also easier and safer now, and the Valmet Blade Trolley acquired during the project has improved the storage of blades.
“Our cooperation with Valmet has been very smooth. The product specialist actively searched for solutions during the project. We were confident that Valmet would have the necessary expertise and the decisions made would also meet the prerequisites of the equipment manufacturer and our production process. This allowed us to focus on other shutdown work ourselves. Encouraged by the project, we will next investigate how to improve doctoring on the calender,” Kari says.