The interface instability of the co-extrusion flow of polyethylene and polypropylene is studied experimentally in a slit geometry. This is done by introducing disturbances of controlled wave length and amplitude on three-layer symmetric (A-B-A) polymer melts and performing a series of extrudate mechanical testing. In this study variations of the mechanical properties as well as wave interlocking have been related to the conformation of the interfacial waves (IW). By investigating the growing (IW) and tensile stress of extrudate samples a relationship between interfacial instability (II) and mechanical properties of polypropylene (PP) and high density polyethylene (HDPE) has been established. It has been shown that instabilities are associated with IW, and it turns out that IW amplitude is known as a mechanism for controlling the strength of three layer polymer products. It is shown that the mechanism of interfacial strength is related to interfacial instabilities and the interfacial wave interlocking.