Forming tool for forming plastic preforms, comprising: an inner tool part with an outer face; an outer tool part with an inner face; wherein a circumferentially closed annular channel with a circular inlet opening and a non-circular outlet opening is formed between the outer face of the inner tool part and the inner face of the outer tool part, wherein the outlet opening has a largest outlet opening diameter (D9a) and a smallest outlet opening diameter (D9b); wherein the inner tool part has an axially adjustable mandrel element, with which the outlet opening can be varied over the entire circumference, wherein the mandrel element has at least one elongate mandrel outer face when viewed in cross-section; wherein a setting device (21, 21') with a setting element (22, 22') is provided at the outer tool part, which setting element extends along a partial section of the elongate mandrel outer face and laterally limits the outlet opening, wherein the setting element (22, 22') is adjustable transversely to the flow direction of the preform in order to change the thickness of the outlet opening in a partial region, characterised in that the smallest outlet opening diameter (D9b) is smaller than 0.8 times the inlet opening diameter, and the largest outlet opening diameter (D9a) is larger than 0.9 times the inlet opening diameter, wherein a first channel length (Ka) between the inlet opening and the outlet opening, viewed in a longitudinal section through the largest outlet opening diameter (D9a), and a second channel length (Kb) between the inlet opening and the outlet opening, viewed in a longitudinal section through the smallest outlet opening diameter (D9b), have a deviation of at most 5% with respect to a channel length averaged from the first and second channel lengths (Ka, Kb). an inner tool part with an outer face; an outer tool part with an inner face; wherein a circumferentially closed annular channel with a circular inlet opening and a non-circular outlet opening is formed between the outer face of the inner tool part and the inner face of the outer tool part, wherein the outlet opening has a largest outlet opening diameter (D9a) and a smallest outlet opening diameter (D9b); wherein the inner tool part has an axially adjustable mandrel element, with which the outlet opening can be varied over the entire circumference, wherein the mandrel element has at least one elongate mandrel outer face when viewed in cross-section; wherein a setting device (21, 21') with a setting element (22, 22') is provided at the outer tool part, which setting element extends along a partial section of the elongate mandrel outer face and laterally limits the outlet opening, wherein the setting element (22, 22') is adjustable transversely to the flow direction of the preform in order to change the thickness of the outlet opening in a partial region, characterised in that the smallest outlet opening diameter (D9b) is smaller than 0.8 times the inlet opening diameter, and the largest outlet opening diameter (D9a) is larger than 0.9 times the inlet opening diameter, wherein a first channel length (Ka) between the inlet opening and the outlet opening, viewed in a longitudinal section through the largest outlet opening diameter (D9a), and a second channel length (Kb) between the inlet opening and the outlet opening, viewed in a longitudinal section through the smallest outlet opening diameter (D9b), have a deviation of at most 5% with respect to a channel length averaged from the first and second channel lengths (Ka, Kb).