Injection apparatus for injecting a liquid formulation into a molten polymer at high pressure includes a reservoir containing liquid formulation at ambient temperature and pressure which is arrange toflood-feed pump body 26 of a first progressing cavity pump (pep) 4. The first pep 4 is driven by motor 6 and is arranged to accurately meter the liquid formulation into a second pep 8 which is downstream of the first pep and is arranged to increase the pressure of the liquid formulation by 200 bar or more. Downstream of pump 8 is a delivery valve 14 arranged to control passage of liquid formulation, via outlet 17, into pressurised molten polymer stream 75 which is present in an extruder 77.