a sealing device for sealing a slide plate translatable across an extrusion bore in a polymer filtration apparatus includes an annular seal having a sealing surface which is biased against the slide plate by an annular spring . the seal has a thrust surface which is angled relative to the sealing surface and is located at an opposite axial end of the seal from the sealing surface . the annular spring exerts radial forces against the thrust surface of the seal to bias the seal in an axial direction against the slide plate . pressurized polymer in the extrusion bore further biases the seal against the slide plate .