A method of making mounting mats comprising the steps of: (i) supplying inorganic fibers through an inlet of a forming box having an open bottom positioned over a forming wire to form a mat of fibers on the forming wire, the forming box having rollers for breaking apart clumps of fibers and an endless belt screen; (ii) capturing clumps of fibers on the endless belt; (iii) conveying captured clumps of fibers on the endless belt so as to enable captured clumps to release from the belt and be broken apart by the rollers; (iv) transporting the mat of fibers out of the forming box by the forming wire; and (v) compressing and restraining the mat of fibers to thereby obtain a mounting mat having a desired thickness suitable for mounting a pollution control element in a pollution control device.