Influence of the injection rate and mould block material of hybrid moulds on the morphological development of PP parts

For conventional injection moulding of semi-crystalline plastic parts it is common to obtain a skin/core type of structure through the wall thickness. The cause of this structure is the thermo-mechanical environment during the process. The size of the skin and also its degree of orientation strongly influences the mechanical properties of the plastics part. They can be controlled by many technological variables such as injection rate, mould temperature, melt temperature, wall thickness, mould material, etc. Hybrid moulds may use different materials in the moulding blocks which affects the skin thickness symmetry on both sides of the parts. This work aims at understanding the influence of different materials used for the moulding blocks of hybrid moulds on the structural properties of polypropylene parts. The material combination of steel/steel, steel/(epoxy+aluminium), steel/(epoxy+short steel fibres) for the moulding blocks is used to induce asymmetric structure development in the part. Simultaneously the influence of the injection rate is analysed as it is one of the more relevant variables in the skin formation. The morphology structure of the mouldings determined by the various conditions is characterized by polarised light optical microscopy. Moldflow simulations are used to predict the skin/core ratio in the mouldings.