Coloring plastics at the press is one of the most functional, value-added features a molder can impart to a molded product. Self-coloring not only improves aesthetic properties, it can also improve UV stability and make processing easier. Also, adding color at the machine is almost always less expensive than purchasing precolored material from a supplier.

There's just one catch. Once the color system has been incorporated into the polymer matrix, it becomes an integral part of the material and may alter its engineering properties as well as its processability. As a result, it's important to be aware of some of the common problems involved with melt-coloring plastics and how to avoid them.

Generally, aesthetic flaws can be attributed to three different causes: equipment (machine, mold, ancillary equipment), molding (process), and design (part and tool)/formulation (base resin and colorant). In some instances, the flaw may be the result of problems in more than one category, but generally, there is always a predominant issue. Similarly, some problems may have more than one symptom, so several different "problems" may be showing up in parts that have the same root cause, which calls for the same corrective action.

When troubleshooting aesthetic problems in molded parts, it's important to begin with an open mind. Since successful molding is an interdependent combination of machinery, labor, material, and design, troubleshooting is often more complicated than people initially expect.

Identifying the Problem
The first step in determining if the flaw is caused by a molding- or colorant-related problem is to inspect parts visually to determine what the problem looks like. Some flaws are very distinctive and quickly lead you to their root cause. Others are less distinct and may be caused by a number of factors that fall into several categories. Consequently, some basic troubleshooting and root-cause/corrective-action work is required. The table with this article provides a list of common aesthetic flaws that could involve colorant and their most probable causes.

The next step is to find out when and where the flaws are being reported. Flaws that show up immediately upon molding require one approach to troubleshooting-for instance, processing and equipment issues are easier to check at this point. In this case, the troubleshooting process might start by asking the following questions:

However, field failures that occur after months or even years will suggest a different starting point-a focus that begins with either part design or formulation:

Regardless of the simplicity or complexity of a molding project, good documentation practices should be in effect and followed diligently by all team members. Processing parameters should be noted throughout. Not only will this practice help molders stabilize their process and create good parts faster, it will also provide a baseline for reference should the process drift and problems arise.

Common Sense- Often Uncommon
The best way to begin troubleshooting an aesthetic problem in which colorant and/or special-effects packages are involved is to rule out the colorant package as the source of the problem. This is done by running uncolored (natural) material through on the same process settings to see if the problem recurs. If it does reappear, then it's likely that the problem is not caused by the colorant package, although the base resin could still be part of the problem. If the problem disappears, then the colorant package is the proper place to begin your investigation. It's important to remember that aesthetic issues may exist that are difficult to see without color. In other words, a colored background will frequently magnify aesthetic problems.

While it is outside the scope of this article to cover the entire process, one thing that should be mentioned is that proper troubleshooting requires a sensible, methodical approach. Unless a Taguchi design of experiments study is being conducted, only one parameter at a time should be changed, and its effect should be carefully evaluated and documented. Then the parameter should be returned to its baseline setting before another parameter is changed.

While the following guidelines involve colorant-related problems, we will also touch briefly on processing, equipment, and design issues, indicating areas of concern that can affect colorant and/or aesthetics. If the troubleshooting process indicates that one of these categories is a likely cause, then there are a number of good resources, both human and written, that can and should be consulted for further help.

Troubleshooting Guidelines

As with any manufacturing process, molding a quality product consistently requires proper equipment, design, material, and processing procedures. The following list of considerations are provided as general guides when beginning to troubleshoot aesthetic problems.

1. First, check the following four equipment areas.

Screws/nonreturn valves. In most cases, a general-purpose screw will work well with colorants, depending on the letdown ratio, chromaticity, and base resin used. Although it's usually not necessary, a growing number of injection molders are using mixing screws to aid in color distribution. Processes with high throughput, short cycle times, and large shot sizes or those that use difficult-to-color resins will often benefit from this type of screw.

As with screws, selection of non-return valves is often resin-specific. Ball check valves are more shear intensive and can aid in color distribution, although this method can prove problematic when processing parameters such as screw speed, backpressure, and barrel temperatures are pushed close to the base resin's limits, leading to both colorant and resin degradation.

Screw and barrel wear. A worn screw or barrel-regardless of the design-will inhibit mixing, causing a loss in plasticating quality and therefore color distribution.

Barrel capacity. As a general rule, using more than 60 percent of the barrel capacity with a GP screw is considered to be pushing the processing envelope for good color incorporation. Of course, this is cycle-time dependent. The barrel's L/D ratio is also important. For example, shorter barrels (18:1) are less conducive to a good mix, while longer barrels (24:1) provide a longer mix environment, but also longer residence time. It's important to keep an eye on material residence time, because excessive residence can degrade either the base resin or the colorant, or both, while insufficient time in the barrel may lead to distribution issues.

Colorant handling and dispensing. One of the keys to successful color processing is an accurate blending/distribution of colorant and base resin before both enter the molding machine. When blending equipment is used, production personnel must be aware of proper operation, calibration, and maintenance. If the equipment is not properly calibrated to the process, any money saved by coloring the raw materials will be lost in wasted product.

For solid (salt and pepper mix) colorants, there are many different options, such as drum tumblers or auger mixers. For blending concentrates and powders at the machine, either volumetric or gravimetric feeders are used. Volumetric feeders are both less expensive and potentially less accurate than gravimetric types because they feed colorant based on speed vs. time. Gravimetric feeders weigh and blend each component in the mix. For liquid colorant, special volumetric pumps are typically used to handle the color accurately and neatly. (See the sidebar on p. 87 for definitions of common colorant-system formats and their functions.)

2. Tooling design can also impact aesthetics.

Gates and runner systems. Proper gating and runner types should be considered at the design table. It's important to remember that additional heat generated by a hot manifold or pin gate during injection fill can affect the integrity of the color. If it's known ahead of time that such tooling is being used, a colorant supplier can formulate the color package differently.

Knitlines. Anticipating aesthetic issues during the early design stage is very important, especially if knitlines are a factor. When moldfilling analysis or experience indicates knitlines are likely, color formulation changes can be made with certain resin families that will improve the appearance of knitlines. Of course, this issue is more easily resolved during the early stages of part and tooling design.

Mold texture. Also an important consideration during the design stages is mold texture, since it can affect how the eye perceives color. The presence of gloss and/or texture in the tool can make what would otherwise be an exact color match appear very different. Gloss should be considered in relation to both the mold and resin being processed, since some materials naturally have higher gloss.

3. Take a good look at part design.

Color/special effects. It's important to understand that a joint blessing and challenge of molding color and/or special effects is that the color package may accentuate design or molding problems that are already present in a part, such as knitlines, slight sinks, or stress-whitening marks near the base of a snapfit. Taking color back out and running the part in natural resin may appear to make the problem disappear, but in reality the problem, if it is a molding or design issue, is probably still there, and just not as visible.

The problem will be accentuated in chromatic colors and high-gloss designs. So if it's something that can otherwise be lived with, slight design modifications may solve the problem. On the other hand, certain problems, like stress-whitening marks, can often be hidden by the correct use of color or possible reformulation.

When integrally colored, molded parts of dissimilar materials are mated with other molded or painted parts, they must often match in color, gloss, and texture. Here, it's important to work closely with the resin and color suppliers to ensure that the parts will match as closely as possible when they are new and that they will fade at the same rate throughout the lifetime of the assembly.

Bosses/ribs/knitlines/parting lines. Wherever there is a flow interruption in the part design-from bosses, ribs, or even converging flow fronts on multiple gated parts-there is a strong possibility of sink marks, knitlines, and compromised mechanical integrity because knitlines are always weaker than the surrounding resin. Since the orientation of the material in these areas shifts, the refractive index across this region of the part will also shift, posing aesthetic challenges for many special-effects packages. Therefore, these interruptions should be designed into nonvisible faces and noncritical areas of the part.

4. Take these processing tips to heart.

Screw speed. For color processing, a slower screw speed is usually better to avoid shearing and burning the colorant and to ensure proper color distribution, cycle time permitting.

Backpressure. Higher backpressure promotes more thorough mixing, especially with a GP screw, again, if cycle time permits. A maximum value of 300 psi (2 MPa) is generally recommended by machinery OEMs.

Resin temperature. Higher melt temperatures also promote better mixing, as long as the processor is careful not to begin melting too early in the feed section of the screw, although this too can be resin dependent.

5. Be aware of these colorant and formulation issues.

Resin and colorant compatibility. The melt index relationship between the base resin and color are very important and should be considered early in the design. Additionally, certain pigments react with particular resins to a greater or lesser degree.

For instance, some organic pigments tend to dissolve and disappear (or change color) in styrenic polymers. Similarly, many dyes tend to change color or disappear in nylon polymers, which are weak reducing agents that effectively destroy the chromophore component of the dye that provides color. Additionally, except in special cases, dyes are not recommended for use in olefins because of very limited solubility and a potential for color migration (crocking). Further, dyes can act as a plasticizer in some materials, reducing thermal properties or even reacting with the resin itself to produce various effects.

Phthalocyanine (phthalo) pigments can be problematic, since they can act as nucleators, causing often uncontrollable shrinkage and warpage problems in crystalline resins. Fluorescent dyes also can pose problems in certain environments. They lack thermal stability for use in many engineering resins, they chemically react with nylon, and they lack UV stability for use in most outdoor applications.

A final category of problems is often found with polymer blends and alloys. There can be phase boundaries between the various components of these materials that can cause light scattering and a resulting milkiness. Hence, obtaining chromatic colors can become a costly undertaking due to the large amount of colorant that needs to be introduced to counteract the milkiness-often at the expense of economic concerns and critical physical or mechanical properties.

Carrier specification. The specific carrier for the color and its flow properties vs. the base resin play an important role in color distribution, and must be considered during the initial design. The carrier should melt in the late-feed or early-transition sections of the screw to ensure proper distribution of the pigment or dye in the mix. If the carrier has either too high or too low a melting point, the colorant will not be properly distributed in the melt, and finally in the molded part.

Other additives. Special additives for the colorant package can help with some color-related problems. However, these ingredients are normally not incorporated until processing efforts have failed, primarily because they add cost that may make the end product less cost-effective.

Grade can make a difference. It's important to remember that not all resin families or even grades within the same family are interchangeable with a given color package, let alone with different pigment or dye packages in the same chemical family. Further, since most colorant suppliers custom formulate their packages to a specific resin grade by a particular manufacturer, if the resin supplier is switched after the color package has been formulated, it should be retested.

A good example of this is the fairly wide fluctuation that can be seen in the natural color of polypropylene, which can range from a yellow-white to a gray-white shade, depending on the quality and quantity of filler used.

An Ounce of Prevention
Like so many other problems in life, colorant issues are far easier to anticipate and prevent than to cure once they've occurred in molded parts. If color is important to the final performance of an application, then it should be designed in from the start via simultaneous engineering. To ensure the colorant package is properly formulated for the application, provide the color formulator with all available information on both the application and its material targets as soon as they are set.

If any of these specs change during the latter stages of the design process, inform both the resin and colorant suppliers. Changes may require a new formulation. Don't hesitate to call on the colorant supplier to analyze, retest, and reformulate a color formulation to make sure it will be suitable in the end-use environment.

It's also important to inform the colorant formulator of your equipment setup, in case you need to make changes in order to optimize the resin/colorant/throughput requirements of a particular application. Finally, watch your process parameters and once the process has stabilized, note any sudden changes that may signal the beginnings of a problem.

Special care is needed when self-coloring engineering resins because a significant portion of their perceived value-and therefore cost-lies in their superior physical, mechanical, and aesthetic properties, which can be sensitive to even small compositional changes. The more important color is in a given application, the sooner the colorant supplier should be involved in the design process.