Apparatus is disclosed for producing thin polymeric film and includes a source (17) of a solution of polymer and solvent; a casting member (10) having an outer surface (12) for receiving a cast layer (18) of polymer and solvent, the layer having a transverse width; a casting die (16) for casting the layer of polymer and solvent onto the outer surface, the die including a transverse slot (20) with adjacent upstream and downstream lips (22, 24) between which the polymer and solvent flow during casting, the lips having lengths approximating the transverse width of the layer; and a device (11 ) for moving the outer surface past the casting die. Improved devices are provided for optically inspecting (26, 32) the lips to detect a presence of agglomerations of polymer on the lips; for directing a stream of solvent (28, 34) at the lip to wash away any detected agglomeration; for supporting (58, 116) the device for inspecting and the device for directing; and for traversing (44, 48, 52, 64; 90, 96, 106, 120) the device for supporting back and forth along the length of the lip to facilitate inspecting and washing of the lip.