NUMERICAL-SIMULATION OF BLADE COATING WITH SHORT DWELL AND ROLL APPLICATION COATERS

Paper and board are often coated at high speeds with a mineral-based a queous suspension in order to improve their printing properties. This suspension is usually called coating colour. The flow behaviour of the coating colour in the cavity of the short dwell coater (SDC) and in t he vicinity of the blade tip when paper is coated with a stiff blade h as been analysed using the finite element method. The models used to s imulate the flow incorporated free surfaces and shear-thinning colours . The Newtonian case was in some cases also included in the modelling. The viscosity level and the shear-thinning character of the coating c olour had a significant influence on the flow in the SDC cavity, altho ugh the overall behaviour was to a large extent governed by the speed of the coater. The pressure distribution along the paper surface in th e SDC cavity was also analysed. In agreement with earlier reported res ults, increasing the machine speed raised the pressure level in the co lour close to the blade tip. The rheological properties of the coating colour also affected the flow field and the pressure distribution in this region; e.g. it was found here that a colour with a high viscosit y level at low shear rates developed a high pressure level close to th e blade tip. The most interesting result revealed by the analysis was that changes in the configuration close to the blade tip (converging f low between the blade tip and the paper and compression of the substra te under the blade tip) and boundary conditions at the blade tip surfa ce had a very significant effect on the pressure distribution. The pre dictions of the numerical simulation were to some extent compared with experience from practical coating trials.