The basics of color: Part 2

In a cost cutting initiative, a midwest molder switched to a new supplier of virgin ABS resin for the second run of parts for an appliance OEM. Processing went fine, but when the parts were delivered, they were rejected by the customer's QC department for failure to meet its demanding color match standards. How could this have happened? The virgin resin that came from the second supplier was slightly more yellow than the ABS used in the first molding run, causing a subtle but costly shift in the color of the finished parts.

Base Resin Color
For many types of plastics, virgin resin produced by different suppliers can vary enough in yellowness index, tint, degree of translucency, compounded additives, and more, so that the same colorant applied to each will not produce the same results. Different colorant systems will be required to match the same critical color. Varying polymerization and finishing techniques are the primary culprits and can cause these differences, although the physical and rheological properties of two products may be virtually identical. Even the exact same grade of certain resins from a single supplier can vary enough from lot to lot to affect the color of molded parts. Over time, in fact, many stored resins may yellow due to oxidation. Molding the same color parts out of different resins presents even greater problems, trying to account for differences in base color, translucency, or gloss. However, it is possible to produce acceptable color matches if you keep in mind a few resin-specific tips.

All Resins are Not Created Equal
Virgin ABS, for example, can exhibit significant variations in yellowness as a result of polymerization technologies and additives. Other styrenics, such as high- and medium-impact polystyrene, may also vary but not to the same extent. SAN can cause unforeseen problems when regrind is used. Adding as little as 10 percent of unpigmented regrind to virgin resin will cause a decided shift to yellow. When molding white SAN parts, save the regrind for other components with darker or less sensitive colors, like browns or greys, where the shift will be less noticeable.

Polycarbonate will also exhibit a lack of uniformity from supplier to supplier in tint (blue to grey tones) that can affect color matches. Acrylics rarely display any differences between resin suppliers. Acetal is something to watch for lot-to-lot differences. Although the base colors of homopolymer and copolymer products are virtually the same clean, translucent white, even slight pigment impurities can result in unacceptable color.

Some types of thermoplastics, such as acrylics and transparent styrenics, permit the use of dyes that provide bright, clean colors. Polyolefins, on the other hand, require pigments because dyes tend to bleed out during processing and end-use.

Heat exposure can also affect the type of colorants that can be employed with certain resins. Polyethylene, for example, processes from 200 to 450F while polycarbonate processes between 500 and 600F. All color is affected by melt temperature and residence time. If you are running a part that processes at a high heat or that will run at a long cycle time, be sure to have your color supplier substitute a more heat-stable colorant to avoid burnout problems.

When coloring alloys, the same issues of base color mix ratios and additives apply. Critical color matches usually require the exact same grade of resin be used every time. This is especially true of PC/ABS products as well as nylon blends and modified PPO.

Fillers will affect color, too. A good example is polypropylene because of the talc and other mineral additives used in filled grades; these can vary from a light tan to a dark gray. Reinforced nylon will contain different combinations of glass reinforcement, minerals, and other additives that will vary from supplier to supplier and among grades from the same resin producer.

Making an Informed Decision
Maintaining the color consistency of molded parts requires an understanding of how to evaluate color from the initial design stage through commercial production. Many molders consider color to be the greatest variable in their work. Over this two-part series, we've given you a large amount of information. Here are some tips on using it to make decisions in your own operations.

Choose the right tolerance for the application. Color concentrates are generally developed for three levels of color match precision: commercial, close, and critical. Tighter tolerances usually mean higher costs, so most applications are specified in commercial tolerances. Close or critical tolerances are usually reserved for assemblies of parts molded of different thermoplastics where color differences can be easily perceived.

Many appearance parts, such as in cosmetic packaging, as well as more and more colors supplied to major toy manufacturers must meet tighter tolerances.

View color match samples under optimal conditions. Compare samples side by side against a neutral background (Munsell Gray N6 or N7 is recommended). Record observations under more than one type of light source to check for metamerism (the phenomenon whereby two identical colors appear different under different light sources) or at least under conditions closest to the intended product usage. Use glossy color standards for matches in glossy resins, matte ones for matte finishes. Don't evaluate samples under daylight because of variations in the quality of light due to weather or time of day. Never make color judgments under halogen, mercury, or sodium vapor light sources.

Let trained observers make final batch decisions. People who have been tested in the ability to discriminate colors using the Farnsworth-Munsell 100 Hue Color Test make the best judges of whether sample matches submitted by color concentrate suppliers meet requirements. While spectrophotometers are useful tools, they may register different readings for the standard and the resulting color match sample while an observer sees no variation. They also can have problems measuring fluorescent, metallic, and pearlescent colors as well as those of lightly tinted, transparent plastics. In addition, humans can better discern differences in surface texture as it relates to color.

Humans have their limitations as well. The eye is most sensitive to differences in light to medium colors and less sensitive to dark shades of blue, green, and brown. Also, if a qualified observer wears glasses, be sure there is absolutely no tint to the lenses. It is best to set tolerances for each color and application and stick with standard procedures.

Matching dissimilar plastics can be tough. Very close matches can be achieved between certain families of plastics, but other combinations can present differences in base color, morphology, degree of opacity, and inherent surface characteristics; all of which affect color matching. Also keep in mind the color opacity of sample matches in dissimilar materials may appear noticeably different at certain wall thicknesses, a crucial consideration for products with thin wall sections.

Spectrophotometers can be used for real-time QC. Spectrophotometers on the plant floor can be used to identify potential color consistency problems before they result in a large number of reject parts. To do this, standardized testing methods must be established. Sample parts must be measured at the same part area with the same measurement conditions each time.

Spectrophotometers enable you to create and maintain color standards and tolerances and to measure the product's color under a variety of illuminants. The color is shown both numerically and on a spectral graph.

Controlling color consistency in molded parts is increasingly important in terms of total quality control. Recognizing this is the first step in managing this critical but often misunderstood variable.