As legislation governing greenhouse gas emissions becomes more stringent around the world, the automotive industry is keen to identify new down-gauging and light-weighting opportunities. Plastics, and in particular polypropylene (PP), are key enablers for lightweight design in vehicles. Using low-density plastic materials not only results in lower component weight, but also offers additional benefits concerning design freedom, functional integration, and of course manufacturing costs. Consequently, more and more interior and exterior car components are being made of plastics. What is more, as new alternative car concepts are developed, there is increased need for novel, lightweight solutions, because weight reduction is directly proportional to range extension. Using more polymeric foams and plastic foam structures is the next logical step for the industry.
Plastic parts made in the foam injection moulding process offer additional weight savings potential of up to 20% compared to compact plastic parts. In the foam injection moulding process, blowing agents are dispersed and dissolved in the pressurised polymer melt. The two main types of foam injection moulding processes, differentiated according to type of blowing agent used, include: first, so-called physical foaming, where typically N2 or CO2 is incorporated in the melt in supercritical state; and second, the chemical foaming process, which uses citric acid and/or carbonate compounds added in solid state. The latter requires a thermally induced chemical reaction to release the actual blowing agent (in most cases, also CO2).
In both processes, gas bubbles are formed in the melt when the polymer blowing agent mixture is exposed to a pressure drop, e.g. during injection into the mould cavity. The resulting parts typically exhibit a solid skin layer, and an increasing number and size of foam cells towards the core of the part (the so-called integral foam).
In addition to the obvious benefit of weight reduction, foam injection moulding also permits the production of parts with high dimensional stability (reduced warpage); minimised sink marks; and the complex geometries not possible when using conventional injection moulding techniques. However, a major drawback of foam injection moulding is the often poor surface appearance of the parts. For example, the finished surface of the part may show characteristic streaks originating in the orientated and elongated foam cells in the skin layer.
Borealis has developed a PP compound that can be processed into foamed automotive parts that boast excellent surface appearance and mechanical properties. Foamed, glass reinforced PP components are around 15% lighter than non-foamed parts. Because here are no existing standardised methods to describe the properties of foamed parts, Borealis has focussed on the development of test standards, including a unique foaming mould. These tools are in use to systematically investigate the fundamentals of foaming, and tailor a material that meets stringent OEM requirements when foamed.