Offering a large variety of options, assembly injection molding represents an innovative and highly integrative technique of manufacturing mechatronic systems. This is due to the fact that it combines many technical functions such as casings and sealing of several elements made of different materials in one single manufacturing process. Particularly, the combination of various materials is a significant technological benefit and a big challenge at the same time. System tightness is a major target. This paper presents a model to improve the understanding of how leaking occurs and of the mechanisms underlying such as shrinkage and warpage behaviors related to material, process, and design. It shall also be the basis for investigations on the phenomena and development of strategies to improve the systems produced by assembly molding in terms of media tightness. Very important are the complex interaction and intereffects acting on thermal and structural shrinkages, the elements’ warpage entailed, and the adhesion between them.