Flow Mark Reduction of Metallic Colored PP

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. As a result, flow marks are less likely to occur when PP has low melt viscosity and wide molecular weight distribution, and when the metallic pigment has small mean particle diameter and low concentration. Based on the results of this research, we were able to manufacture a non-painted metallic color side protection molding, thus lowering costs, improving environmental conservation (reducing volatile organic compounds (VOC) and recycling), and better protecting door surfaces from scratches.