Mechanical strength in polymer weld interfaces in semi-crystalline high density polyethylene (HDPE) and polypropylene (PP) is investigated. The welding method investigated is through transmission laser welding (TTLW). Utilizing the TTLW process with 0.4 wt % carbon black as absorber, a lap-joint is formed which is tested for mechanical properties using an Instron tensile testing machine. In contrast to earlier investigations, the tensile tests conclude that 89% of the strength of a HDPE/HDPE weld was developed in a PP/HDPE weld with HDPE as the absorbing part. This high weld strength is explained from: (1) a relatively low reptation time being in the millisecond range for both HDPE and PP; (2) a polymer mesh size (tube diameter (a)) being similar to the equilibrium interpenetration depth, determined from Helfand's theory and the interaction parameter (); (3) The selected HDPE and PP were both metallocene catalyzed and constituted a similar crystallization temperature which is required for high crystallinity in/near the interface. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013