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Recently, R-PP (Rubber modified Polypropylene) has been often used in automotive exterior parts, but is difficult to use for longer parts because of the limit of it's dimensional stability. Studies of the thermal expansion mechanism of R-PP by the rule of mixtures and FEM indicated that optimization of the volume elasticity ratio of PP/rubber is a key point to design the low thermal expansion alloy. And the selection of rubber which is partially compatible with the PP is also important. The developed new alloys are used for automotive longer exterior parts such as the rocker panel and side moldings which have better dimensional stability than the usual R-PP and R-RIM(Reinforced Reaction Injection Molding).

Blending polypropylene (PP) with materials such as rubber or filler creates compound PP, which is widely used in auto parts because of its balanced physical properties, formability, and cost performance. Recently non-painting of parts, namely coloring the raw materials, is being investigated as a method of reducing the cost of exterior parts which have conventionally been painted to match body colors, which are mostly metallic now. However flow marks often form in long injection molded parts such as the side protection molding, causing remarkable decreases in exterior appearance quality. This research investigates the influence of metallic coloring of PP material (blending PP and metallic pigment) factors on flow marks, as well as countermeasures for those causes of flow marks. PP and metallic pigment characteristics were the factors investigated to evaluate flow mark level.