Injection moulding system comprising an elongated manifold having a melt passage extending from a common inlet, branching to a number of outlets; each of which outlet is connected to a central bore (3) of a heated elongated nozzle (1) discharging into a gate (4) leading to a moulding cavity, which nozzle (1) is at least partly surrounded by a heating element (17); the heating element (17) is designed such that its diameter or cross-sectional shape is adjustable; said heating element (17) is encased by a mantle piece (21) of which the internal diameter or internal cross-sectional shape is adjustable too, and a device is provided to clamp fit the heating element (17) with respect to the nozzle (1) under the action of the mantle piece (21).