151 1 INTRODUCTION In recent years emerged the interest in the industry to replace metal products by light weight and low cost products with the same appearance and perfor- mance. One solution is the addition of metallic pig- ments in polymeric matrices, which gives the metal- lic look to plastics, with no further post processing finishing of the part like paining or metallization. Pigments are available in the market with differ- ent supply forms, such as powder, pastes and pellets and also with different shapes like irregular shape (flakes), lenticular, glitter and spherical (Charvat 2003). They all provide a different metallic appear- ance to the parts arising from the ways they scatter the light. The type, shape, size and concentration of pigments (Edenbaum 1996) affects the metallic look of plastic parts. Pigments composed by particles in the form of flakes tend to reflect the entire incident light due to the high surface area (Harris 1999; Martins et al. 2012; Santos et al. 2013). However, at the edges the light is diffused, contributing to the re- duction of the metallic effect when the size of the flake pigments is reduced. Also spherical/rounded shaped pigments have low light reflection because only a very small area (single point) reflects light, decreasing the metallic effect (Charvat 2003). As a result, parts made of spherical aluminum pigments appear gray with a scattered sparkling effect, where- as with irregular-shaped pigment (flakes) display a very grayish silver effect (Charvat 2003; Martins et al. 2012; Santos et al. 2013). The pigment type and particle size defines the amount of pigment necessary to added to the poly- meric matrix. Typically, 0.01-1 wt% is the amount required for a pigment with a size of 5-30 μm, 0.5-2 wt% for a pigment with 30-60 μm and 2-5 wt% for a pigment with a size greater than 70 μm (Charvat 2003). Larger particles causes a greater amount of reflected light, thus a better metallic effect (Martins et al. 2012; Santos et al. 2013). In smaller particles there is a lower amount of reflected light, causing a decrease in brightness and in the metallic effect (Edenbaum 1996). Although the metallic pigments have numerous advantages in the production of parts with metallic effect, such as the elimination of post-processing techniques, the reduction of costs and production Optimization of injection molding process of polypropylene with aluminum metallic pigments I. Santos, A. J. Pontes, C. I. Martins Institute for Polymer and Composites/I3N, University of Minho, Guimarães, Portugal ABSTRACT: 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 con- strain 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 optimiz- ing the injection process and in the appropriate selection of metallic particles or otherwise can even be neces- sary 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 optimiza- tion of the processing conditions to minimize the defects caused by the use of metallic particles. The part ge- ometry used is a two gated box. Morphological analysis and optical characteristics of the part are analyzed in this work.