Hard nip sizing with curtain application at MM Količevo BM3

In curtain sizing, the starch is applied in the form of a curtain to hard rolls and then pressed to the web. This method has many benefits compared to the conventional surface sizing methods. Curtain application is based on higher starch solids contents (20…30%) than film sizing (10…15%) or pond sizing (5…10%). This means that the curtain sizer applies less water with starch to the web.

Due to the high solids content of starch, the afterdryer section can be very compact after the curtain sizer, as less drying energy is needed after the sizer. A compact afterdryer section gives the possibility to increase the production of board making line remarkably, as it makes it possible to extend the predryer section without moving the coating section or reel location. In some cases, it is possible to install a curtain sizer to a board making line that has originally not had any surface sizer.

Also, the smoothness development of the sheet is enhanced with curtain sizer. This is related to the reduced wetting in surface sizing, which decreases the unwanted roughening effect of the base board. In curtain sizing, the optimized balance between the size coverage on the sheet and the size penetration into the sheet can be adjusted with the nip load. This is possible as the nip pressure can be adjusted with a deflection-compensated hard roll. Curtain sizer can also be used to apply pigments and several other chemical components to the web. For example, with pigment application it is possible to increase brightness and smoothness of the base board.

In October 2023 the first ever hard nip sizer with curtain application was started-up at MM Količevo BM 3 mill in Slovenia. The board machine BM 3 produces three different product classes, and it needed a sizer that would be flexible enough to fulfill the requirement of the wide product portfolio. The results from the first year of curtain sizer operation are presented as well as experiences from the start-up.