Residence Time

The amount of time a pigment system undergoes processing and fabrication can significantly affect appearance. Heat stability of colorants is not simply a function of temperature. Heat stability of colorants is typically a time-temperature relationship. Therefore, for a pigment pushed close to its heat limit, the time at that limit should be as short as possible. Pigments are chemical entities, carrying the potential to become involved in chemical reactions. If a reaction happens for any reason the pigments involved are considered to be failures for the application.

Polymers are also affected by the time-temperature relationship. Polymers face many of the same problems as pigments. Many polymer systems inherently will yellow and/or darken with extended exposure to temperature.

Shear

Shear is an important factor for pigments in plastics. Significant shear is required during processing to fully disperse pigments in a plastic. Shear, unfortunately, may also damage pigments or other particulate materials in the plastic such as fillers and other performance additives. Pigments, as particulate matter, may be fractured during the shearing process, but often, pigment color properties depend upon their specific particle size distribution. If that distribution is altered by particle fracture, the observed color can change.

Another possible dilemma for pigment particles undergoing shear is surface abrasion. Some pigments carry most of their color value on the surface of the particle. If the shearing process abrades the particle, a visible color change can result. It is imperative that the amount of shear in a conversion process be consistently controlled.

Nozzles and Screw Tips

Injection molding machine nozzles and tips present a potentially damaging shear and heat situation. Plastics pass through the nozzle/tip area at very high velocities and pressures during the injection cycle. These parameters can induce large shear forces that may damage the polymer matrix and other components. The high velocities also generate significant amounts of frictional heat. It's not unusual to have a material temperature rise at the nozzle in excess of 40° C (104° F). If this rise affects the heat stability of the pigments, polymer or any other component, failure of appearance or physical properties is possible.

Polymer Drying

Most polymers must be dried before molding or extrusion. Over-drying of a polymer for excessive times or at excessive temperatures can cause color shifts and/or a loss in physical properties.

Part Cooling in the Mold

The rate of cooling in an injection mold can affect color and appearance as well as physical properties. The cooling rate, driven by the mold temperature, will definitely affect the gloss of the molded part and possibly could affect physical properties and the clarity or transparency of some plastics.

Because many materials are thermochromic (changing color with temperature), color of molded parts can drift for a considerable amount of time after processing. Parts should be fully cooled and normalized to ambient temperature before assessing color. It is preferable to let parts cool naturally, rather than through compressed air or water quenching, to more closely simulate actual part conditions. Organic pigments are generally more susceptible to these phenomena than inorganic pigments.