a method of producing an injection - molded plastics article having a hollow interior , such as a pipe section comprising the steps of forming a mold cavity having outer dye parts and an inner mandrill to define an annular mold cavity . then introducing plastics material into the mold cavity and delivering a pressurized fluid to the interface between the mandrill and the inner surface of the molded article through passages in the mandrill opening to the surface thereof , and withdrawing the mandrill while maintaining the outer dye parts in a closed condition . in addition , an apparatus for producing such hollow molded article , the hollow molded article itself , and a method of lining an aged , broken or damaged pre - existing pipeline are also described .