Abstract: A method of manufacturing continuous ribbed pipes of thermoplastic resin material comprises the following successive steps. A length of pipe is injection moulded in a mould cavity between male and female moulds, the latter being shaped to provide integral ribbing on the outer surface of the pipe. After sufficient setting, the female mould is disengaged and the pipe length is ejected along the male mould whereupon the female mould is re-engaged with the one end of the formed pipe forming a closure for the mould cavity. Further material is again injected into the cavity which fuses with said one pipe end and which forms a further pipe length. These steps are repeated until the desired length of pipe is formed and plunger means are provided to effect the pipe ejection above and to close the other end of the mould cavity during injection.