The basics of color: Part 1

By: 
October 10, 1998


In a two-part series, color experts from Teknor Color Co. and Minolta Corp. share some fundamental knowledge about color in molded parts that will help designers and molders avoid color problems. Part 2 will appear in the November issue of IMM.
Plastic colors can be accurately identified and compared using color instrumentation equipment such as spectrophotometers. To do so, these devices plot the exact location of colors in "color space," quantitative systems which define the universe of colors in their three elements: lightness, hue, and saturation. One of the most popular versions is the L*a*b* color space system (also referred to as CIELAB). As shown in three dimensions, the L* axis charts the progression of lightness from white to black. Together with the perpendicular chromaticity coordinates a* (red to green) and b* (yellow to blue), the exact location of every color in color space can be mapped with three numerical coordinates.

What do an eggplant, a piece of driftwood, a lapis lazuli stone half the size of a grain of rice, and a banana peel have in common? They are only some of the unusual and difficult to match items sent in by customers for developing color concentrates. Transforming these samples into well-defined color standards for molded parts is a critical element in ensuring color consistency throughout the product life cycle from initial design through production. Color uniformity indicates attention to detail and suggests product quality and consistency of performance.

Establishing Standards

To be truly effective, standards must be established in molded plastic chips, not paper swatches or other materials. Dyes or pigments used for color concentrates are usually very different from those employed in printed matter, textiles, paint, or other non-plastic substances. Plastic resins are processed at temperatures of 250 to 600F and require costly, heat-stable pigments. Compare this to the low temperature (and low-cost) dyes involved in printing.

Molded plastic parts are also three-dimensional, so base resin composition (transparent, translucent, or opaque), surface appearance (glossy or matte finish, texturing), and wall thickness all impact color perception. Color variations in virgin resin, from lot to lot or as a result of prolonged storage or exposure to light, are another factor.

The good news is plastic colors can be accurately identified and compared using color instrumentation equipment such as spectrophotometers. These devices plot the exact location of colors in "color space," a color system in which the universe of colors is defined in terms of three elements: lightness, hue, and saturation. However, while spectrophotometers can be valuable tools, there are other considerations in making final color decisions.

When a molded plastic chip (at least 2 by 4 inches) is set as a standard, it should be measured with a spectrophotometer, dated, marked as the master, placed in an opaque black envelope, and stored away from light sources. Another chip whose spectrophotometric reading is as close as possible to the master should be marked and identified as the "working standard." These instruments are so precise, it is unlikely identical readings will be obtained for two chips even of the same molding run. Each chip should be identified as to specific resin grade, lot number, and letdown ratio used. If a spectrophotometer is not available, standards should be defined as minimum lightness and darkness versus the master standard.

Thickness has a major impact on the tone of a color, as seen in this typical color chip with three steps of thickness.

The Heartbreak of Metamerism

A newly incorporated outdoor furniture manufacturer designed its first product with multiple plastic and painted metal parts all in the same custom color. Aiming at the high end of the market, the company carefully evaluated production parts, many of which were injection molded, under sunlight to assure close color matches of the assembled set. After the furniture was shipped to retail showrooms, however, the company immediately began to receive complaints that the colors of different parts of the sets didn't match.

The fledgling company was a victim of metamerism, the phenomenon in which two colors that look the same under one light source can appear quite different under another. The outdoor furniture matched perfectly outdoors in sunlight, but under fluorescent or incandescent lights, it didn't. Metamerism occurs when different colorant systems are used to match the same standard but have different spectral reflectance curves.

So why not just use the same colorant systems for all parts? As mentioned earlier, paints used on metal surfaces are not exposed to the high temperatures of processed plastic parts. Also, the opacity, translucence, or transparence of virgin resins adds a three-dimensional challenge to matching colors. Completely different colorants may be required to achieve the same color in dissimilar resins. Then there is that difficult surface finish effect. Another potential problem arises when standards submitted for critical color matching are the size of postage stamps.

As a result of these factors, many color matches are actually metameric under daylight, which contains wavelengths of UV light not found in incandescent or fluorescent light. In reality, perfect, non-metameric matches are sometimes impossible, but there are ways to avoid the problem and to develop good color standards.

Avoiding Problems

Here are some tips that will help you cut development time, avoid costly mistakes, and improve the color consistency of your molded parts.

Include color as a design element. While the type of resin, estimated melt temperature, additives, or planned surface appearance all impact the selection of an optimal colorant system for the component, there are special aesthetics that can be created by color as well.

Don't reinvent the wheel. In many cases, the color you plan to use has been developed for the resin specified and has been archived by a color concentrate supplier. As an example, Teknor Color has developed more than 75,000 colors since 1961. Tapping a standard color library such as this one could save time and money. So involve a color supplier early in the process. With full product information-specific resin grade, other materials to be matched, anticipated end-use environments, wall thicknesses, and so forth-a color concentrate producer can help identify metameric problems early in the development cycle.

Be wary of relying on paper swatches. Although color matches can be developed by concentrate suppliers from printed paper swatches, there is significant variation among these samples. There are literally thousands of printers capable of producing paper swatch booklets, all using different paper stocks and inks. Also, paper can yellow with age, so the paper sample your customer is using as a reference may be quite different from yours.

Set color standards and tolerances for all environments. Color matches, as in the outdoor furniture example, should have taken into account more than just outdoor and showroom light. How would colors match if a customer placed furniture on a screened-in porch? Standards should be specified for more than one or two lighting conditions to help control metamerism.

An inexpensive light box with Munsell Grey interior is desirable, but even this simple viewing sheet can be helpful.

Build or buy your own light box. A simple tabletop light booth with a neutral gray (Munsell Gray N6 or N7) interior and multiple light sources can be very useful for comparing parts to standards. Just be sure to compare at the same 45 deg viewing angle to minimize the effect of the surface finish.

Establish a system for your color use. Track color in your operation through some means of identifying all resins and concentrates used along with parts molded from those materials, being sure to date all entries into these archives. Another wise practice is to retain at least a 10 lb sample of each natural resin used and store it for future reference. This way color and consistency of resin lots can be quantified. It's also a good idea to annually review molded chips or plaques of every natural resin used.

Remember that resin is a critical element. For color-critical jobs, use only virgin resin. Regrind has additional heat histories that could affect the color of molded parts enough to warrant rejection by some customers. If you change resin suppliers, even if you switch to a different grade from the same producer, be sure to store accurate data on processing conditions, current color concentrate, and samples of good parts. Different producers may use different pigments or dyes to match the same color which can result in metamerism, and slight differences in base resin color from different producers can affect the color of parts even when the same concentrate is used.

Once you have established working standards, treat them like gold. They are the key to a total quality assurance program where everyone involved is working from the same page. The result will be higher production yields and early identification of out-of-spec parts.


Contact information

Teknor Color Co.

Pawtucket, RI

Jim Cafferty

Phone: (401) 725-8000

Fax: (401) 724-8520

e-mail: [email protected]
Website: www.teknorcolor.com

Minolta Corp.

Ramsey, NJ

Maria Repici

Phone: (888) 473-2656

Fax: (201) 825-4374

e-mail: [email protected]
Website: www.minolta.com

Comments (0)

Please log in or to post comments.
  • Oldest First
  • Newest First
Loading Comments...