Injector (14) for injecting a liquid formulation into a molten polymer includes outlet (21) at one end and, at its other end, is arranged to be connected to upstream conduit (25) via a coupling housing (26) so that liquid formulation can pass from conduit into the injector, and further includes an elongate conduit (27) in which an elongate pin is slideably arranged being capable of expelling all liquid formulation from conduit. To address the risk the outlet could become blocked in use, whilst avoiding the need to depressurize and/or stop the flow or polymer in extruder (19), the injection apparatus includes a spool (34) which is rotatably mounted within wall (35) of the extruder and is arranged to be rotated about an axis which extends substantially perpendicularly to the elongate extent of the extruder through which a polymer stream (18) flows. The spool may be moved between a first configuration and a second configuration wherein a flow path is interrupted, but wherein molten polymer continues to flow in said polymer flow conduit.