The dissymetry influence of structure and internal stresses on the deformation of polypropylene injected parts

The shear stresses and elongational stresses dictate on macromolecules a pr ivileged orientation from the melt flow in the mold. Most of flow stresses arise from the filling stage of the mold, but also from the packing and the post-filling stages. They have a low amplitude and they do not directly pa rticipate in the deformation on the plastic part, but they induce local ani sotropies of the material properties as Young's modulus or thermal expansio n that favour the load imbalance when shrinking. They superpose themselves upon thermal stresses which appear during the cooling of the successive pol ymer layers due to the fact of incomplete thermal retractions. The plastic part shows then a multilayer composite structure with a complex distribution of molecular orientations in each layer. The molecules are us ually unoriented in the core of part where the molecular relaxation is prep onderant due to a low rate of coiling, and in the flow direction in the she lls due to the shear stresses and fast cooling coupling. The injected plastic part is a 2 mm thick box in U shape. The material is a n isotactic homopolymer of polypropylene. The parts are fed by a film gate in the joint face of the mold, in order to generate a unidirectional melt f low. The molding conditions are based on the average parameters of injectio n; the influence of cooling condition on morphology and part deformation ar e especially studied when making local thermal dissymetries. A morphological gradient has been revealed by infrared dichroism method on microtomed slices cut through thickness. The molecular orientation measurem ents, represented by the second order moment of the orientation function of distribution, show the influency of flow length on molecular orientation i ntensities and the effect of cooling rates on the orientation profile. The thermal stress field, measured by thermal expansion method on microtome d slices, confirms the major effect of cooling rate on the stress repartiti on and the stress level in sample. We can notice internal thermal stresses between -8 MPa in the skin and +2 MPa in the core, with up to 3 MPa differe nce between each side of mold surfaces according to the cooling conditions of the part. A correlation between the shape of thermal stress field, molec ular orientation levels and the morphology of the polymer is allowed to est ablish from expansion measurements on microtome slices. In fact, a minimal thermal expansion was observed just under the skin at about 10 and 90 % of relative depth which is representative of a pronounced organization in the zone of great orientations where the positive thermal stress is maximum. A raised expansion appeared in the core where the polymer is isotropic and wh ere positive thermal stresses have a low range. From local measurements of orientation by infrared dichroism and from inter nal thermal stresses measurements by Dilatometrical Analysis of Image, corr elations between average orientation rates and local stiffnesses calculated from deflection tests on the one hand, and the linear thermal expansion co efficients calculated from results of dilatometrical tests between 30 and 8 0 degrees C on the other hand, have been established. The data integrated i n a deformation calculation of a part constituted as a multilayer composite material defined by its dilatometric profile versus thickness, have brough t to the fore the coupling of cooling stresses and those induced by structu ral gradient (orientations, crystallinity). Calculations have corroborated the deflection measurements made in temperature where the effects have been separated by a thermal treatment of the part at a temperature of 100 degre es C during 5 h in order to release all cooling stresses. The global deflec tion has therefore been expressed as the result of the superposition of the effects generated by thermal stresses whose dissymmetry induces a curvatur e, and the effects of a structural gradient itself dissymmetric, due to mol ecular orientations and crystallinity of polymer, equally responsible for a deformation.