to produce a panel of variable thickness , a stratified assembly of a layer of fibers containing a non - polymerized binder and at least one cover sheet is placed between the mold and mold - backing of a molding press . the stratified assembly is caused to conform to the interior surface of the mold or mold - backing by forcing the cover sheet against the mold or mold - backing interior . hot air is blown into the fiber layer from the mold surface opposite the cover sheet , into at least the region that will correspond to the area of greater thickness . the direction of the heated air is reversed after a period of time , the blowing of the air being maintained and adjusted in order to secure polymerization of the binder . at the same time , the portions of the panel corresponding to areas of lesser thickness , generally the edges of an acoustical panel , are cured by contact heating means . to effect cross - over blowing , both the mold and mold - backing are provided with ports through which heated air may be blown or passed .