an improved continuous polymer filter of the screen belt type employs a valve arrangement to selectively apportion the extruder output flow through at least two filter paths . in the neutral valve position both filter paths receive polymer at equal flow rates creating a pressure across the screen belts that is too high to permit the screen belts to be advanced when a clogged belt section requires replacement with a clean section . to effect screen belt advancement , the valve is actuated to reduce the flow rate in the filter path requiring the replacement while increasing the flow rate to the same degree in the other filter path so that the net flow rate remains substantially constant . the screen belt receiving the reduced flow experiences a sufficiently reduced pressure drop thereacross to permit it to be advanced in a conventional manner , after which the valve is returned to its neutral state . the screen belts in both paths are advanced alternately while maintaining a constant net flow and assuring sufficient flow through the reduced flow path to avoid entry of air into the filter path . valve actuation and screen belt advancement may be effected automatically and in synchronization in response to a pre - set timer , to detected pressure conditions in the flowing polymer , or other parameters .