Special decorative effects are hot trends in creating unique brand packaging. From consumer goods to automobiles, “pearlescent pigments” are widely used because they change appearance at different viewing angles, and they easily are incorporated into paints and inks.
Pearlescent pigments are based on mica flakes coated with titanium dioxide. As the coating thickness increases, the color varies from silvery white to yellow, red, blue and green. Different colors can be achieved by adding a second coating of iron oxide (gold and beige) or chrome oxide (green); and a range of metallic colors (bronze and copper) is achieved by replacing the titanium dioxide with iron oxide.
What makes these pigments special is not so much their color, but their pearl-like appearance, which can be “tuned” by adjusting the size of the flakes. Small flakes (about 5 microns) give rise to a satiny appearance with good opacity. Larger flakes (about 25 microns) give a lustrous effect with lower hiding power. Typically, you would blend different particle sizes to achieve a desired combination of luster and opacity.
Color stylists typically combine pearlescent pigments with transparent pigments and dyes to add luster to the base color. They not only make the finish more attractive, but they also protect it by reflecting harmful rays from the sun. Because the size and shape of these particles are so important to the resulting appearance, special care has to be taken when processing the pigments in plastics.
High shear forces can fracture the flakes and reduce the luster effect the surface of the pearlescent flakes reflect in light. In contrast, the edges scatter the light in all directions and are comparatively darker. This can lead to an undesired effect in injection molded parts.
When flow fronts come together, they may form a weld line. If pearlescent pigments are used, the flakes tend to flop over at the weld line, exposing their edges. The result is a dark line accentuating the weld.
Proper mold design places the gates in such a way that the weld lines are in less visible locations.