Slider design is one of the vital tasks in achieving mold development automation. This paper proposes an approach to automating the determination of undercut regions and their releasing directions for complex parts in STL format. The end result of research will provide an impactful tool for manufacturing automation, and further enhance our advance mold design and manufacturing abilities. Specific research topics include: (1) automatic segregation of undercut regions and regular regions for each of the inner loops, (2) automatic selection of undercut regions for each of the inner loops, (3) automatic determination of releasing directions for each of the undercut regions, and (4) automatic grouping of undercut regions to produce sliders. Several real-life parts, such as cell phone component and car lamp component, are used as testing examples to implement the proposed approach and algorithms. While these example parts contain a large number of complicated free-form surfaces, the implementation results show that undercut regions and their releasing directions can be found within acceptable range of time.