This paper explores the application of Computer Aided Synthesis (CAS) to support the implementation of Set-Based Concurrent Engineering (SBCE) and Just In Time Decision Making (JIT-DM), which are considered as two of the cornerstones of the Lean Design method. Computer Aided Synthesis refers to a next generation computational tools that automates the process of generating candidate solutions to ill-defined design problems. The design of injection moulding is used to demonstrate the rationales of the approach. This is a well-known complex and time-consuming industrial design process that generally involves rework time as a consequence of the design iteration loops performed between mould design and part design. The paper presents a tool to automate the design of cooling systems and shows how it supports the application of SBCE and JIT-DM to enable an increase in design process efficiency and effectiveness. The paper finishes with a general discussion on the enabling role that CAS tools have on the implementation of lean design practices.