Physical and Chemical Foam Injection Moulding of Natural-Fibre-Reinforced Polypropylene—Assessment of Weight-Reduction Potential and Mechanical Properties

Abstract

Reducing weight not only consumes fewer resources for manufacturing but also requires less energy for transportation, thus preserving resources and reducing CO2 emissions. The latter part is of utmost importance in mobility applications. For example, in the automotive industry, the large-scale production of lightweight structural parts is becoming a main issue. An effective method to meet these requirements is foam injection moulding. In this study, physical (MuCell technology) and chemical foam injection moulding was used to produce plates made from wood-fibre- and cellulose-fibre-reinforced polypropylene, respectively. For both technologies, the used core-back method enabled precise mould opening during injection and thus allowed for variation in the plate thickness and density. The simpler short-shot technology, used only for the chemical foaming trials with differing shot volumes, provided plates with constant thicknesses. The foam structure and finally the mechanical properties of the plates depended on the filler type, the foaming method and the density. The latter was directly linked to either the plate thickness or the shot volume. Physical foaming appeared to be slightly more effective regarding the achievable density reduction (up to 37% reduction), but the physically foamed parts had worse mechanical properties at equal density than their chemically foamed counterparts. Besides the comparison of different foaming methods, this study provides the tensile, flexural and impact properties of natural-fibre-reinforced polypropylene composites over a wide density range, thus offering a good basis for evaluating weight-saving potential for various applications.