apparatus for filtering plasticized material between the inlet and the outlet of a housing has two parallel passages for reciprocable filter carriers each of which is movable between a first position and a second position . when in their first positions , the carriers permit plasticized material to flow from the inlet , through filters in their filter chambers and then into the outlet of the housing . if one of the carriers is moved to the second position while the other carrier remains in the first position , plasticized material which has been cleaned by passing through the filter of the other carrier is diverted in part into channels leading to the filter chamber of the one carrier , and such diverted material flows counter to the normal direction of flow of plasticized material through the chamber of the one carrier to entrain intercepted impurities and to expel the entrained impurities by way of a channel provided in the housing and being normally sealed from the chamber of the one carrier when the latter is maintained in the first position .