A method of manufacturing a component of a roof window frame or sash, the method comprising the steps of: - providing a thermoplastic with a melting temperature in the range of 100 - 300°C; - providing a mould comprising a cavity, the cavity being defined by a plurality of boundary members, the volume of the cavity being adjustable; - heating the thermoplastic to a temperature at which it melts; - introducing the liquid thermoplastic into the mould cavity via an inlet; - increasing the volume of the mould cavity by moving at least one of the boundary members and providing relative motion between the inlet and the mould cavity whilst introducing further liquid thermoplastic such that the liquid thermoplastic conforms to the expanded volume of the cavity; - allowing the thermoplastic to harden so that it can maintain its shape without requiring support from the mould and releasing the hardened thermoplastic from mould; wherein the thermoplastic contains filler in an amount of 10 - 70 % by weight, and further wherein a mould insert is provided in the mould cavity, preferably before the liquid thermoplastic is introduced, the liquid thermoplastic being moulded around the mould insert, the mould insert being removed from the cavity after the thermoplastic surrounding the mould has hardened such that it can maintain its shape without requiring support from the mould insert, the removal of the insert forming a hollow region in the component of the roof window frame or sash. - providing a thermoplastic with a melting temperature in the range of 100 - 300°C; - providing a mould comprising a cavity, the cavity being defined by a plurality of boundary members, the volume of the cavity being adjustable; - heating the thermoplastic to a temperature at which it melts; - introducing the liquid thermoplastic into the mould cavity via an inlet; - increasing the volume of the mould cavity by moving at least one of the boundary members and providing relative motion between the inlet and the mould cavity whilst introducing further liquid thermoplastic such that the liquid thermoplastic conforms to the expanded volume of the cavity; - allowing the thermoplastic to harden so that it can maintain its shape without requiring support from the mould and releasing the hardened thermoplastic from mould; wherein the thermoplastic contains filler in an amount of 10 - 70 % by weight, and further wherein a mould insert is provided in the mould cavity, preferably before the liquid thermoplastic is introduced, the liquid thermoplastic being moulded around the mould insert, the mould insert being removed from the cavity after the thermoplastic surrounding the mould has hardened such that it can maintain its shape without requiring support from the mould insert, the removal of the insert forming a hollow region in the component of the roof window frame or sash. wherein the thermoplastic contains filler in an amount of 10 - 70 % by weight, and further wherein a mould insert is provided in the mould cavity, preferably before the liquid thermoplastic is introduced, the liquid thermoplastic being moulded around the mould insert, the mould insert being removed from the cavity after the thermoplastic surrounding the mould has hardened such that it can maintain its shape without requiring support from the mould insert, the removal of the insert forming a hollow region in the component of the roof window frame or sash.