An apparatus and method for performing a print operation on a 3D printer (110) are described herein. A 3D printing apparatus comprising a 3D printer (110) is arranged to perform a print operation using build material from a build unit (120) unless a value that identifies a first build material stored in the build unit in a first register, which is set according to an initial build powder fill operation of the build unit (120) by a powder management station, is different from a value in a second register, which is set according to the initial build powder fill operation, or modified in response to, or following, a subsequent opening of the build unit (120).