There is provided a method of generating print data for use by an additive manufacturing system to generate a plurality of 3D objects within a build chamber having a build surface. First spatial data defining a first 3D object and second spatial data defining a second 3D object is received. First print data is generated to cause the additive manufacturing system to manufacture the first 3D object at least partly from a first build material. Intermediate print data is generated to cause the additive manufacturing system to manufacture a partition comprising a 3D object configured to fill the build chamber in a plane parallel to the build surface. Second print data is generated to cause the additive manufacturing system to manufacture the second 3D object at least partly from a second build material.