It has been found, in use of filtering devices of the type disclosed and claimed in my U.S. Pat. No. 3,471,017, that problems can arise in relation to the filtering of polymers having high frictional coefficients and, in particular, polymers which in addition exhibit negligible shrinkage or even positive expansion on solidification, e.g., polystyrene foam (produced for example by the pentane injection process). With such polymers the progressive solidification required to produce efficient sealing plugs in the inlet and outlet ports through which the filter band enters and exits from the filtering passageway would, in the conventional ports (especially the outlet port) as exemplified in my U.S. Pat. No. 3,471,017 cause a progressive setting up of stresses in the continuously forming sealing plugs which, being transmitted to the adjoining walls of the port, might result in the plug binding firmly in the port and, owing to the high frictional coefficient of the material, preventing forwarding movement of the sealing plugs and of the filter band.