A method and apparatus for the sequential injection moulding of plastic material into the mould cavity or by using a plurality of controlled open-close injectors, wherein the sequentially injected plastic material originates in mould flow fronts in different directions whose collision generates at least one weld line on the moulded article. The sequential injection is actuated with specific modes according to which the opening of the at least one of said injectors is anticipated or delayed with respect to other injectors and/or at least one of the injectors is closed partly and/or at least one of the injectors is gradually opened only partly. Such specific modes are managed to control and define the shape and/or the position of the weld line on the moulded article.