A filtering arrangement of thermoplastics extruding apparatus has a rotatable cylindrical screen unit through which material flows radially inwards and is discharged axially and an outlet aperture for impurities which are washed from the screening unit by reverse flow, i.e. radially outward flow, of part of the filtered material. The outlet aperture for the impurities is variable in cross-section by moving an arcuate segment about the axis of the cylinder to control the extent of reverse flow washing.