Tighter color standards help ensure each molded part looks as vibrant as the next.
Technical Specialists, Color Technology
PolyOne Plastic Compounds and Colors Group
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.)
SETTING COLOR STANDARDS THAT STICK
Many design projects in plastics are for components of larger assemblies. They can be relatively large collections of subassemblies such as automobile instrument panels or a matched set, such as an enclosure for one component of a computer system. Other assemblies might be as simple as half of an enclosure with a single control bezel for use on a handheld appliance.
These kinds of projects share a common color goal: Produce parts that match a given standard color. By far the most effective method for setting standards is to impose a top-down, design-driven approach and by defining colors using spectral curves.
The spectral curve describes a color outputted from a calibrated spectrophotometer in a controlled light environment. The approach permits closer color tolerancing and helps avoid metamerism, a phenomenon in which two objects appear to be the same color under one light source but markedly different under another.
The least-effective means of specifying a standard is to grab a part off the production line and attempt to match colors. There are simply too many variables. Differences in colorant batches, variations in metering or mixing, and molding conditions all can affect part color. Blending re-grind complicates color matching as well.
Defining a single, master standard provides everyone involved a measurable target. This is true even if it's only a working standard to be refined at a later date. Polyone's color matching capabilities, for example, uses the Speedecolor Management system to help designers see molded candidate color chips within 24 hr of request.
Portable, relatively low-cost spectrophotometers, in conjunction with custom software, lets designers describe the physical and mechanical attributes required of a part. Then, the spectrophotometer "reads" the color from a sample and defines its spectral curve. The software stores the application and spectral data and searches a database containing resin and colorant attributes and any possible color/resin interactions. The system next develops candidate color formulations. Lab batches are then mixed and color chips prepared and shipped for customer approval.
The software is said to automatically poll pigment, dye, and resin experts; double-checks how each colorant interacts with each resin; and factors in processing expertise gleaned from years of field experience. It does this within a few minutes of scanning the color sample. Alternative manual color-matching techniques generally take several days.
|Guidelines for expressing color standards|
Specify whether color input should be gathered at 2 or 10° aspecular (nondirect-reflecting) incident to sample surface. 10° is the most popular angle.
|Light source|| |
Specify illuminant. D65, a standard daylight source, or CWF, a standard cool-white fluorescent light source, are the most popular.
Capture and store the specular reflectance data.
|Color model |
Specify model used (CIE L*a*b* with CMC commercial tolerance the most popular)
|Process problems: What can the designer control|
EFFECT ON COLOR
|Color mixing|| |
Poor material feed can result in poor distribution and nonuniform color. Machine shortcomings such as too-short of a barrel often result in poor dispersion.
Work with processors to apply compatible colorant carrier systems and let down ratios. Work with tooling and process vendors to put the part on the right machine, with the right mixing equipment.
|Melt temperature|| |
(Too high) discoloration, burning (Too low) lack of mixing or dispersion of colorant
Work with color supplier to specify colors/colorants with thermal stability compatible with the melt temperatures of the base resin. When dispersion is the issue, work with color supplier to specify colorants in their most miscible forms.
|Injection fill rate during molding|| |
Slow fill better than fast Affected by shear at gates. Unwanted knit lines can be caused by poor placement of gates.
Work with tooling designers to create nozzles and gating that will create minimum knit lines or cause minimum color degradation due to shear heat.
|Screw speed (Injection molding)|| |
Slower speeds promote mixing
Little or no control
|Cracking, delamination, specks, plate-out (color migrating from part to mold surface, contaminating subsequent parts), contamination, and cloudiness.|| |
Little or no control
Some process problems can be minimized by careful resin selection, gating decisions, and other tooling considerations. Concurrent engineering helps minimize these, as does coordination with your color provider.
|Recommended analytical capabilities|
|Tensile and elongation testing|| |
|Melt flow|| |
|Izod impact|| |
|Differential scanning calorimeter (DSC)/ thermogravimetric analysis (TGA)|| |
Polymer identification/ Thermal stability
|Infrared spectrophotometry|| |
|Ultraviolet spectrophotometry|| |
Pigment/additive classification, identification
|Atomic absorption|| |
|High-performance liquid chromatography (HPLC)/ gas chromatography (GC)|| |
|GC mass spectrometer|| |
|Color suppliers should be able to provide all of the basic analytic capabilities required by today’s resins and colorants.|