For producing a bend or corner in a sealing strip 16, to match the shape of a bend or corner in its mounting flange, a portion of the sealing part 26 is removed between parts 26A and 26B, so as to produce a correspondingly shaped cut-out. The sealing strip is then placed within a mould cavity 54 which is curved to match the required configuration for the bend or corner and has an internal surface matching the external surface of the sealing part 26. A metal core 60, having a cross-sectional size matching that of the hollow interior of the sealing part 26, is placed in position within the cut-out by a swinging arm 58. Moulding then takes place whereafter arm 58 is swung outwards so as to withdraw core 60 (portion 26B has to be partially flattened to facilitate this). A small part D of the cut-out, which is blocked by, arm 58 during the moulding process, is left open. This is then closed off by a further injection moulding operation using a short core matching the cross-sectional shape of the core 60. This short core has a narrowed neck by means of which it can be pulled out of the sealing part 26B through a small hole in the sealing part which is thereafter closed by a grommet.