During injection molding process the macromolecules of the polymer material are thermal and mechanical forced. This force occurs in specific orientation of the molecules and structural behavior inside the injection molded parts. Especially at interfaces (weld lines) or in shear zones these orientations influence the optical and mechanical properties behavior of the material. This study deals with the process-structure-properties behavior of injection molded semi-crystalline thermoplastics. Focusing on the use of thermoplastics without further ingredients like nucleating agents. PEHD and PLA were compared which are distinguished in their macromolecular structure and therefore in the crystallization kinetic. The influence of the thermal and rheological process conditions and the thermal treatment of PLA after processing on the morphology and crystallinity degree were analyzed and the consequences on the mechanical behavior determined. For the determination of melt and crystallization behavior differential scanning calorimetry DSC was performed. The morphology was determined using polarized-light optical microscopy. The experimental results were compared with results from the simulation. From all specimens tensile test were performed to investigate the mechanical behavior. The presentation gives an overview about the thermal and rheological process influence on the structure development in the injection molded parts. It is shown how the structural behavior in injection molded parts is affected by the flow conditions during the injection and influenced by the thermal process conditions. Especially in case of shearing PEHD and PLA show similar structural behavior. An overview of the influence on the tensile strength is given.