In the automatic packaging industries the needs for improving machine automation and productivity, with respect to functional, aesthetic and low noise requirements, as well as the adaptability to several box sizes, are more and more demanded. The mechanism revision in a packing machine requires a specific study of the above constraints and of the compatibility with all – often many – neighbouring components. When that study is performed with a CAD system, the availability of a skeleton structure, which captures all the main design constraints and allows to deliver, in a top-down fashion, the key characteristics of the design, is a very helpful tool. In the following, the engineering re-design of a carton-packing machine’s sub-system – which performs the carton flaps closure phase, after loading the product – will be analysed according to the top-down design approach. The key characteristics specifications have been firstly translated in plain 2-D functional schemes pointing out the kinematics of the proposed (semi-)automatic set-up. A parametric CAD system, with advanced tools for designing part and assembly, has been used for modelling and handling the whole assembly, and for evaluating the different proposed design solutions according to all configurations of the assembly based on several carton sizes. The final result has shown the validity of the proposed approach and it has pointed out how important the preliminary and plain functional analysis is before building the digital mock-up models.