The moulded part has at least two different sections with different outer diameter and a void inside the moulded part. To make the void, a projectile is passed through the still molten injection mouldable material in a direction of flow. A pressurized fluid is exerted in the void in the direction of flow behind the projectile to move the projectile. To obtain constant wall thicknesses, the projectile has at least one elastic or plastic section, which is delimited by a space in which the pressurized fluid can enter. The volume of the space and the effective outer diameter of the projectile can be changed by the fluid pressure and is controlled in such a manner that the space and the effective outer diameter of the projectile is enlarged and/or reduced to make the moulded part with different diameters.