Colouring in Plastics

The Pigments and dyes used in plastic are, chemically, similar to those used in other industries such as paint, paper inks etc. some are modified slightly for use in plastics but most retain exactly the same form. The differences lie in their application.

A major difference in plastics is heat stability. During processing an ink or paint is not subjected to any great temperature changes, where as a plastic closure moulded in a hot runner system can achieve temperatures over 300 degrees Celsius during processing. These conditions must be taken in to account when a colourist is using a pigment for a colour match. This can lead to difficulties when matching colours in plastics from printed inks. Ink formulations such as those provided to the printing industry may well be formulated using a combination that, whilst working well in inks, is not suitable for use in plastics.

Another difference is in saturation of pigments. Inks and paints can be used in high dosages allowing large amounts of colour to be applied to a surface finish. In plastic the colorant is carried inside the product limiting the amount of pigment that can be used. This can lead to difficulties in matching some printed colours.

When we visually assess a colour we look at it in terms of hue (colour shade) saturation ('strength' of colour) and brightness (light to dark). In plastics we can measure colour using a device called a Spectrophotometer.

This device can mathematically calculate the value of a given colour assigning it a numerical value. This can then be represented in 'colour space'. Colour space is a three dimensional representation of a colour often depicted in a sphere. It is essentially built up of a white to black axis ('L' value); green to red axis ('A' value) and a blue to yellow axis ('B' value). Using the mathematical values given to us from the spectrophotometer we can accurately depict where a colour lies within this space. This allows us to accurately measure the difference between any two colours as well as give a visual representation of their differences.