Polycarbonate microfoams produced by physical blowing agents usually have an unacceptable surface quality. The surface is rough and the visual difference in the surface quality is striking. However, the surface quality can be improved by the gas counterpressure technology. Polycarbonate has a high elongation at break but a low notched impact strength. Earlier, the microfoams showed higher notched impact strength, but a considerably reduced elongation at break. Foams produced by the gas counter-pressure technology have both these positive mechanical properties.

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.

In the review the principles of technology of microcellular polymers preparations were presented. The process consists of three steps: the bubbles nucleation, growth and stabilization. The examples physical foaming agents most often used such as carbon dioxide or nitrogen were given. Attention is paid to the fact that choice of foaming agent influences the structures of the foams obtained. An important group of chemical foaming agents, being organic or inorganic solid substances decomposing during the foaming process with carbon dioxide or nitrogen release, was also described. Three basic technologies used for microcellular materials preparation, i.e. MuCell (Fig. 1 and 2), Optifoam (Fig. 3) and ErgoCell (Fig. 4) were discussed in detail. The examples of applications of the materials prepared by the technologies mentioned above were given (Fig. 5-9).

The injection molding of microcellular polymers is expected to be increasable promise for engineering applications. The combined effect of precision mold opening and gas counterpressure process produced uniform microfoam structure with a maximal cell diameter less than 10 μm/1/. The nature of microcellular PP and PC/ABS was analyzed. It was observed that the mechanical properties, the morphology and the application of microcellular PP were influenced by the degree of crystallinity and the conditions of foaming process. The viscoelastic behavior of materials and their correlation between process, foam structure and properties of PP foam and PC/ABS foam were investigated by DMA.

Polycarbonate has the reputation of having a tough breaking behavior, but it is widely unknown that this applies only to special conditions. The impact strength of polycarbonate depends on the temperature, thickness (with a tough brittle transition as thickness increases), contribution of notch tip radius, impact speed, physical blowing agent, molecular weight of the polymer, and processing parameters. Research results indicate that microcellular foams produced by injection molding with physical blowing agent (MuCell TM Technology by Trexel) show a significantly higher notched impact strength than compact polycarbonate if the compact material is brittle under the same testing parameters. However, if the compact polycarbonate breaks toughly, the notched impact strength of the foamed material is always lower. Therefore, it is highly important to pay attention to the testing parameters and conditions when comparing the toughness of the foam with that of the compact material. The toughness of microcellular foams has similar properties like PC/ABS and PC/PP blend systems, which provides the possibility to combine the higher impact strength with the advantages of microcellular foaming such as weight reduction, lower shrinkage, shorter cycle times, lower clamp forces, and reduced melt viscosity. In order to use technologies and conditions, which are applied in the polymer industry as well, all materials were produced by an injection molding process. Special processing technologies such as gas counter pressure and precision mold opening were used in order to reach microcellular foam structures with cell diameters around 10 μm. These technologies yield exactly adjustable foam morphologies. Special morphologies are required to improve the notched impact strength of the foamed material. Two different equivalent models were extracted from the analyses, which indicate a significantly higher notched impact strength than the compact material under the same testing conditions. The knowledge of the ideal foam morphologies enables the industry to produce foamed materials with improved mechanical properties.