A method of manufacturing a compressed rod of web material for the filter rod of an aerosol-generating article, comprising: supplying a feed of web material from a supply apparatus; drawing the web material through a filter rod manufacturing apparatus; introducing a curved ridge into the central portion of the width wise cross-section of the web material as it approaches contact with the interior of a folding funnel, by drawing the web material across a forming surface of a former that contacts a central portion of the web material, in which the forming surface deflects the central portion of the web material from a line extending between where the web material leaves the supply apparatus and the center of an outlet of the folding funnel; and folding the web material in the folding funnel to form a compressed rod of web material.