Intensity of the pearlescent effect depends on how well the platelets align parallel to the surface of the plastic part. In injection molding, orientation can be disturbed by drag when platelets pass through gates, causing dark flow lines. Such flow lines are less noticeable than those that form with opaque, metallic pigments, however. Pigment "disorientation" also can occur at weld lines. Molders use a number of strategies to help prevent flow and weld lines:
Raise resin and mold temperatures as high as you can without harming part appearance. This lowers melt viscosity to allow better platelet orientation and reduces cooling, which causes weld lines. Molders often run molds 75-100° F above normal with pearlescents. They also use higher injection speed and pressure.
Use single-gate molds with wedge-shaped fan or tab gates. Or use a smaller gate that will generate more shear heating to keep the melt hot enough to minimize flow lines.
Place the gate in a thicker section of the part as far as possible from flow obstructions. In radially symmetric parts, such as caps, the gate is placed at the center of the part so that resin flows concentrically.
Minimize the length of sprue and runner to keep the melt hot.
Keep part thickness as uniform as possible.
Use overflow vents to remove flow lines.
Use concentrates rather than powdered pigment for easier handling and better dispersion.
Dies and mandrels must be clean and free of defects. Scratches, burrs, and burnt deposits can tip platelets and create surface marks .
Screen packs should be properly sized. As a rule of thumb, mesh sizes of 40 to 60 give good pigment dispersion. Backpressure can become excessive if screens are too small or too many are used, especially with large sized pearls.