Apparatus for sequentially introducing additives into a polymer granulate, comprising • a mixer, which comprises a housing and at least one mixer shaft, • at least one inlet opening for a polymer granulate, • at least one inlet opening for a first additive which is located in the transport direction downwards from the inlet opening for the polymer granulate, • at least one outlet opening for the polymer granulate mixed with the first additive, which is arranged in the transport direction downwards from the inlet opening for the polymer granulate and from the inlet opening for the first additive, so that a first mixing zone is formed, • at least one second inlet opening for a second additive which is located in the transport direction downwards from the inlet opening for the polymer granulate, the first inlet opening for the first additive and the outlet opening for the polymer granulate mixed with the first additive, • at least one second outlet opening for the polymer granulate mixed with the second additive, which is located in the transport direction downwards from the inlet opening for the polymer granulate, the first inlet opening for the first additive, the outlet opening for the polymer granulate mixed with the first additive and the second inlet opening for the second additive, so that a second mixing zone is formed, • Drive units for driving at least one mixer shaft, characterised in that • within the first mixing zone, on the surface of the mixer shaft, non-passable conveying mouldings are fixed in helical arrangement on the surface of the mixer shaft, said conveying mouldings effecting transport and mixing of the polymer granulates, and • between the outlet opening for the polymer granulate mixed with the first additive and the inlet opening for the second additive, between the mixing zones (12, 13) on the surface of the mixer shaft, there is arranged a section with a continuous conveying screw which is not permeable to solids flowing back from the subsequent mixing zone, and • in the subsequent mixing zone, non-continuous conveyor mouldings which effect transport and mixing of the polymer granulate are in turn fastened in helical arrangement to the surface of the mixer shaft. • a mixer, which comprises a housing and at least one mixer shaft, • at least one inlet opening for a polymer granulate, • at least one inlet opening for a first additive which is located in the transport direction downwards from the inlet opening for the polymer granulate, • at least one outlet opening for the polymer granulate mixed with the first additive, which is arranged in the transport direction downwards from the inlet opening for the polymer granulate and from the inlet opening for the first additive, so that a first mixing zone is formed, • at least one second inlet opening for a second additive which is located in the transport direction downwards from the inlet opening for the polymer granulate, the first inlet opening for the first additive and the outlet opening for the polymer granulate mixed with the first additive, • at least one second outlet opening for the polymer granulate mixed with the second additive, which is located in the transport direction downwards from the inlet opening for the polymer granulate, the first inlet opening for the first additive, the outlet opening for the polymer granulate mixed with the first additive and the second inlet opening for the second additive, so that a second mixing zone is formed, • Drive units for driving at least one mixer shaft, • within the first mixing zone, on the surface of the mixer shaft, non-passable conveying mouldings are fixed in helical arrangement on the surface of the mixer shaft, said conveying mouldings effecting transport and mixing of the polymer granulates, and • between the outlet opening for the polymer granulate mixed with the first additive and the inlet opening for the second additive, between the mixing zones (12, 13) on the surface of the mixer shaft, there is arranged a section with a continuous conveying screw which is not permeable to solids flowing back from the subsequent mixing zone, and • in the subsequent mixing zone, non-continuous conveyor mouldings which effect transport and mixing of the polymer granulate are in turn fastened in helical arrangement to the surface of the mixer shaft.