Correlation between injection moulding processing parameters and mechanical properties of microcellular polycarbonate

Since many decades, microcellular foamed materials have been produced basically to reduce the density of the materials in order to get lightweight parts. Meanwhile, it is well known that microcellular foaming by injection moulding offers many more advantages compared to compact injection moulding. Those are, e.g. lower shrinkage and warpage, shorter cycle times, lower clamp forces, reduced viscosity but improved properties of the foamed material in contrast to the compact material. These arguments are all known, but to improve the properties of the material, it is necessary to understand the interrelationship between the morphology and the mechanical properties. Furthermore, it is important to know how the processing parameters influence the morphology and the properties of the produced part. By understanding the relation between processing parameters and the consequential properties, it has become possible to create microcellular foamed parts with exactly defined properties. Through the variation of different processing parameters such as blowing agent concentration, injection velocity, mass temperature, mould temperature, weight reduction and different moulding processes like gas counter pressure injection moulded test, samples were produced to characterise the morphology and the mechanical properties. The experiments were performed with a polycarbonate type from Bayer MaterialScience. The cell size, thickness of the skin layer and distance between the cells were correlated to the processing parameters by means of nonlinear regression equations. Based on these equation, 3D graphs were created by variation of two parameters by fixing the remaining parameters to illustrate the relationships. Furthermore, the relation between the morphology and the mechanical properties was correlated, which makes it possible to produce parts through injection moulding with a well-defined Young's modulus or flexural strength.