What you see is what you get

Susan Bates
Rick Johnston
Technical Specialists, Color Technology
PolyOne Plastic Compounds and Colors Group
Suwanee, Ga.

Edited by Jean M. Hoffman

One of the hottest topics in product design today is color. Thermoplastics make it possible to integrate color into new designs in imaginative ways. However, before a designer can realize a bold new esthetic vision, important design considerations must be addressed. For one, colors displayed on computer monitors may be perceived much differently than the actual parts. Part geometry also influences color as does the inter-action of base resins and colorant additives during the molding process. Moreover, colorants can change physical, chemical, and electrical properties of the plastic as well.

But, by far the most important consideration is establishing tight-tolerance color standards. This is particularly true for mated, assemblies made from multiple resins. Tighter color standards make subtle color shifts less likely.

New software lets designers select or create their own color palette. But these virtual color wheels viewed on the computer's monitor can lead to troublesome color matching problems down the road if their limitations are not understood.

CRTs create colors by selectively exciting red, green, and blue phosphorescent spots. Flat-panel displays accomplish the same task by illuminating red, green, or blue crystals. Both use luminance sources that result in direct transmission of light. This differs from most real-world colors in plastics in two key ways. First, most plastic parts depend on reflectance, not transmission, for color. And second, plastic parts are to some degree transparent or translucent.

Colors on screen look more vibrant and are literally suffused with an inner glow. Only backlit plastic parts will share the same luminescence. Screen colors are also a composite of colored dots, or pixels, that the human eye integrates. Because of this, colors on the screen will never be as pure as those of actual parts. Designers must realize that the screen representation, or a printout using three or four-color printing processes, is only a simulation of actual color.

Managing color on computer displays is a puzzle graphic artists have struggled with for years. Solving that puzzle generally involves scanning a calibrated standard target on a high-end scanner, followed by careful adjustment of the software, monitor, and printer to match the scanned input.

Plastic's transparent or translucent property may result in parts taking on color casts based on their geometry. Unfortunately, computer displays can't accurately simulate the phenomena. For example, a thin edge may look lighter in color than a thicker section. Esthetic features such as beltlines, sculpted corners, or transitions often take on different appearances depending how far below the surface the feature sits.

Designers often exploit resin transparency for special effects, such as frost, metal flake, opalescence, or edge-glow. Here, the depth of color or effect is literally that — depth of effect within the section of plastic that contains it. Interestingly, some of these effects simulate the luminescent glow of a CRT.

The play of light
In addition to color discrepancies caused by thick and thin sections, there are color differences created by the play of light on surfaces. This color dependence on part geometry has a couple of aspects. For one, panels positioned to reflect a light source strongly take on one color. Those at different angles assume another color, typically darker. Surface texture also affects color. Glossy surfaces generally give a brighter, more pure color. The rough surface of a matte finish diffuses light, dulling the color.

Differences in panel or feature-based reflections often helps the designer hide subtle shifts in color when an assembly is made from multiple resin systems. This is true even when the same pigment or dye colors each resin. One way to sidestep slight color mismatches is to design the assembly so that each resin follows a separate feature plane.

Contrasting colors as the assembly transitions from one resin to the next is another. This technique works well with today's soft sculptured look. Having no sharp transitions makes it harder to match colors because there is not a stark color shift when the light hits the different sections of the part at various angles.

Controlling material and process problems
Designers must also remember that resins interact differently with various colorants. For example, untinted, neat resins — particularly nylons, ABS, and filled resins — have base colors that interact with colorants to create colors that may be off spec. A common solution is to add enough colorant to hide the base resin. A better method, however, is to specify controlled-color resins. This lets the designer predict and, if necessary, avoid resin-based coloration problems.

In general, more consistent color matches will come with uniform chemistry and colorant formulations. This is especially true when components from different resins are clustered together. An automotive instrument panel is one example. One color formulator's standard phthalocyanine (phthalo) blue may be metameric (appear as different colors under various light sources) in relation to another's. Colorants made from identical raw materials or similar particle sizes that come from the same vendor, stand a better chance of matching across multiple resin carriers.

Specifying the right colorant and resin ratio (the letdown ratio), for products made from engineered materials is the final, and often critical, point to consider. Colorants can affect the engineering properties of resins. For example, colorants may change a viscosity which impacts the injection-molding process. Molecular-level interaction is also possible. Phthalo pigments, for example, act as nucleators in some resins which may cause the molded parts to shrink or warp differently than parts made from neat resins. Different compression or tensile strengths and electrical properties also may result.

Moreover, resins and colorants may interact in unpredictable ways. For example, some organic pigments dissolve — literally disappear — in styrenic polymers. Nylon polymers operate as weak reducing agents that can destroy the chromophore component of dyes. Likewise, olefins tend to have limited solubility and should be colored with pigments rather than dyes. Otherwise, color can migrate to adjoining materials in a process called crocking. Dyes also act as plasticizers in certain resins lowering their melting points.

Finally, variations or errors in processing, over which a designer may have little or no control can derail an otherwise successful project. Selecting a color and resin supplier that's able to address these pitfalls helps.

Relatively inexpensive and more capable spectrophotometers permit direct use not only on the design platform, but also in full production. Spectral curves, the fingerprints of color, let designers specify standards and give production personnel accurate and repeatable targets to match. The key is to understand and set the standard as early as possible in the design process. (For information on color basics including spectral curves see Coloring outside the lines, Sept. 7, 2000, MACHINE DESIGN.)