A die for casting junctions of a coating of a pipeline is configured to be coupled to the pipeline to form an annular shaped closed compartment about a tubular joint portion comprising a tubular wall, which extends about a designated axis; a plurality of centering devices configured to align the axis of the tubular wall with a longitudinal axis of the pipeline; two annular walls arranged respectively at the opposite ends of the tubular wall; and two pluralities of vent openings arranged along the tubular wall respectively at each of the annular walls; and at least one feeding port configured to supply polymer material into the closed compartment.