Melt processing e.g. injection or blow molding of polymers may lead to molecular orientations and thus to non-uniform anisotropic thermal expansion when frozen in during solidification. In order to describe the anisotropic thermal expansion quantitatively an elementary volume consisting of a crystalline and an amorphous layer was used to calculate the ultimate coefficients of thermal expansion (CTE) with respect to the three space coordinates. This kind of modeling refers to the lamellar structure of semi-crystalline polymers. It allows for introducing the degree of crystallinity to account for processing effects and provides highly anisotropic CTEs which may be observed for highly oriented polymer fibers. Injection or blowmoulded polymer parts exhibit a much lower degree of preferential orientation. This means that a large number of lamellae exhibits a more or less pronounced preferential orientation. In order to describe their real thermal expansion behavior an orientation averaging with respect of the three ultimate CTEs has to be made. The temperature dependency of the CTE is introduced via the temperature dependent degree of crystallinity measured by DSC. Injection molded bars and blow molded model parts were investigated with respect to their anisotropic thermal expansion behavior to verify the model. First results are shown for different grades of polyethylene.