Optimization of injection moulding process of PP with metallic pigments

The coloring of plastics is essential in parts design and in the promotion of new products. As such, metallic pigments are used to color plastics aiming to replace the appearance of metals. However, the metallic look in plastics by the addition of metallic pigments prone the visual appearance of weld lines and flow lines. These defects are caused by the anisotropic character of flake like metallic pigments when there is the meeting of two flow fronts or there are melt flow disturbances caused when holes or castles are present in the part. There are some solutions to overcome this problem, but there are also numerous variables that constrain the elimination of these defects. Therefore, the coloration of plastics should be taken into account in part design. The number of gates, its position and type, abrupt changes in wall thickness and improper wall angles influence the melt flow, which in turn allows these defects. The effect of the amount, size and shape of the metallic particles in the appearance of weld lines and flow lines has been studied. Larger size of metallic particle and spherical/rounded particles attenuate these defects. However, the solution may consist in optimizing the injection process and in the appropriate selection of metallic particles or otherwise can even be necessary changing the mold design. This work aims at studying the influence of the physical characteristics of the metallic pigments (size and shape of the particles) on the aesthetic appearance of defects and on the optimization of the processing conditions to minimize the defects caused by the use of metallic particles. The part geometry used is a two gated box. Morphological analysis and optical characteristics of the part are analyzed in this work