A filtration apparatus for an extrusion or injection molding machine having an internal bore through which a polymer melt flows, said filtration apparatus including a slide plate and a sealing device for sealing said slide plate, said sealing device comprising: an annular seal having a sealing surface which abuts against said slide plate, said slide plate being translatable across the bore of the extrusion or injection molding machine, said annular seal having a thrust surface located at an opposite axial end of said seal from said sealing surface, said thrust surface being angled relative to said sealing surface; and comprising a first spring angled relative to and contacting said thrust surface of said annular seal for biasing said sealing surface of said annular seal against said slide plate, characterized by said first spring having first and second tangs (40A, 40B) at opposing ends, each of said first and second tangs (40A, 40B) having an outside surface and an underside surface, said first and second tangs (40A, 40B) are always positioned in overlapping contact relationship so that an outside surface of said first tang contacts said underside surface of said second tang to form a continuous barrier against passage of the polymer melt upon expansion and contraction of the first spring caused by a change in pressure of polymer melt in the bore, and wherein increasing pressure of polymer melt in the bore forces said first spring to expand due to radial forces exerted on the first spring which transfer against said thrust surface of said annular seal causing said sealing surface of said annular seal to be further biased against said slide plate to form a secure closure such that the polymer melt cannot contact the seal. an annular seal having a sealing surface which abuts against said slide plate, said slide plate being translatable across the bore of the extrusion or injection molding machine, said annular seal having a thrust surface located at an opposite axial end of said seal from said sealing surface, said thrust surface being angled relative to said sealing surface; and comprising a first spring angled relative to and contacting said thrust surface of said annular seal for biasing said sealing surface of said annular seal against said slide plate, characterized by said first spring having first and second tangs (40A, 40B) at opposing ends, each of said first and second tangs (40A, 40B) having an outside surface and an underside surface, said first and second tangs (40A, 40B) are always positioned in overlapping contact relationship so that an outside surface of said first tang contacts said underside surface of said second tang to form a continuous barrier against passage of the polymer melt upon expansion and contraction of the first spring caused by a change in pressure of polymer melt in the bore, and wherein increasing pressure of polymer melt in the bore forces said first spring to expand due to radial forces exerted on the first spring which transfer against said thrust surface of said annular seal causing said sealing surface of said annular seal to be further biased against said slide plate to form a secure closure such that the polymer melt cannot contact the seal.