22.214.171.124.4 Nip Design in Coating and Sizing
For coating, sizing or pigmenting in film transfer presses the primary purpose of the cover is both to provide a soft nip and to transfer the applied liquid film uniformly to the paper web. Abrasion resistance is important for keeping the nip geometry constant, as the film thickness of the applied coat/size is extremely thin and any nonconformity in the nip results in nonuniform film application. Good wettability and film splitting properties are also required.
In conventional puddle size presses the hard and elastic covers must mainly ensure a uniform nip, in which the size is pressed into the paper web. Both tem¬perature and chemical resistance of the covers are essential.
Coater backing rolls give elastic support and ensure transport of the paper web without slipping of the web, which is coated on the opposite side. Elasticity and optimized deformation behavior are the crucial parameters for roll cover design.