2012

Guidelines for Thermoplastic Color Control and Change Management

In order to make good color specifications, the OEM should gain an understanding of color technology, color measurement, and methods available to control color.

Color is an important factor in many aspects of medical devices, from design to how the device is used and by whom. In 2010, the FDA and regulatory bodies around the world increased their scrutiny of colors as additives in all materials and are paying special attention to the biologic testing performed on pigments used in plastic in an effort to reduce potential safety risks.

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Good color specifications create boundaries within a color space that indicate the exact color desired by OEMs for medical devices, but are not too tight to cause excessive rejections. Numeric color measurement plays an important role in color control for the plastics compounder and the molder. By establishing tolerances for acceptable color, OEMs can help processors, from raw material to finished parts, achieve consistent color quality across multiple lots and manufacturing sites around the world.

Color control tolerances are often driven by the application. For example, color used to identify different sizes and types of medical instruments usually does not have to be as stringent, since color is being used to differentiate one instrument from another. Devices incorporating various polymers of the same color can require tighter tolerances to maintain color harmony and devices incorporating brand colors are often driven by corporate guidelines using ink on paper.

An important first step in color control is a basic understanding of color technology, color measurement, and methods available to control color. Device color can be evaluated visually by trained colorists or instrumentally with color measuring devices and software. Visual evaluation can be limiting because it can be difficult to communicate or transfer color information and is usually not a good choice for consistent color control. A good practice is to measure color instrumentally and incorporate visual evaluation in the total color control process.

altThe two most common color-measuring instruments are the colorimeter and the spectrophotometer. Colorimeters are useful for simple color difference measurements when both the standard and the trial are made from the same substrate, the same colorants and using one illuminant. Spectrophotometers measure the reflected or transmitted light at specific wavelengths to provide color difference measurements as well as take into account gloss or texture.

Numerous measuring systems have been developed over the years to quantify color numerically using color-measuring instruments. These systems require the use of a defined color space that is usually three-dimensional and provides a means for color specification or identification and comparison. The most popular are based on the CIE L*a*b* color space. In the CIE L*a*b* color space, colors are identified by their L*, a*, and b* values:

L* defines the lightness/ darkness of a color
a* defines the redness/ greenness of a color
b* defines the yellowness/ blueness of a color

When a color is measured instrumentally using the CIE L*a*b* color space, the resulting numeric coordinates for L*, a* and b* can be used to identify a particular color and can also be used as a quality control tool. Numeric color values from current manufacturing lots of medical devices can be compared to stored standard values. Numerous color difference equations have been developed based on the CIE L*a*b* color space that actually improve the correlation between what is measured instrumentally and how it corresponds to what is actually perceived by the human eye. Visual evaluation should always be a part of a good color control program.