Method for sizing a multi-ply fiber web and a forming section for a multi-ply fiber web

A method for forming a multi-ply fiber paper or board web, comprising the steps of:

• forming a first fiber layer of the paper or board web on a first wire in a fiber web former so the first fiber layer has a first side in contact with the first wire, and a second side opposite the first side;
• forming a second fiber layer of the paper or board web on a second wire in the fiber web former so the second fiber layer has a first side in contact with the second wire, and a second side opposite the first side;
• applying a sizing agent, at least part of which forms a foam, by at least one curtain application device to at least one of the second side of the first fiber layer and the second side of the second fiber layer;
• forming a multi-ply fiber web in a twin-wire forming part of the former by bringing the second side of the first fiber layer on the first wire into engagement with the second side of the second fiber layer on the second wire so that the sizing agent is between the first fiber layer and the second fiber layer and the first wire and the second wire form the twin-wire forming part of the former;
• and
• conveying in the twin-wire forming part of the former the sizing agent from one of the first fiber layer of the paper or board web and the second fiber layer of the paper or board web over a boundary therebetween into the other of the first fiber layer of the paper or board web and the second fiber layer of the paper or board web so that the sizing agent enhances ply-bond strength.