The product is continuously fed into the centre of the top screen, from where it spreads out evenly to the outside across the entire screening surface. The finer particles fall through the screen near the centre and the coarser ones successively towards the periphery. The material is moved in a spiral pattern with increasing acceleration of the particles the further they are from the centre, with the result that even those particles which are almost the same size as the screen mesh are screened. All the separated fractions are discharged through the lateral outlets. The screening process is repeated again and again on each deck.