The Troubleshooter: Injection Molding Defects Related to Color

Garrett MacKenzie | Apr 18, 2023

Among the most difficult processing challenges molders face on the production floor are color-related defects ranging from the appearance of color swirls to meeting customer requirements. These insights can help prevent color-related issues in injection molding.

The first consideration when mixing color concentrate with a base material is setting up the color feeder. The material manufacturer provides let-down ratios to be used when producing color blends. Be sure that the setup matches the manufacturer requirements and that the mixing equipment is clean and in good operational condition.

Poor mixing is often the primary cause of color swirls. Verify that adequate back pressure is being applied to produce the mixing and shear heat needed to achieve a good blend. Also, check that your mixing setup is correct and that the equipment is functioning properly. It's also important to note that a cracked check ring can sometimes cause swirl because of material shear as it passes through the crack.

Although an obvious solution for color that is too dark or too light simply might be adding more or less color to the blend, molecule alignment could also be the culprit. As chains align while the part packs, light can pass through loose alignment easier than if chains are packed tightly together. Temperature adjustments can be made using back pressure or barrel/mold temperatures. Less compactly packed chains let more light through, which makes the part color lighter. Chains that are compressed tighter let less light through, resulting in darker parts.

When molding using colorant blends, it is important to note that color should be viewed more as a shade than the color change itself. For instance, let's look at white as X1, light grey as X2, dark grey as X3, and black as X4. When climbing the scale — X1 to X2 or even X1 to X4 — changes in color are easier to accomplish. When moving down the scale from a darker to a lighter shade, the darker colors are stronger and make it more difficult to adjust the shade. X4 to X2 can be quite difficult, and X4 to X1 is even worse. Going down the scale will normally require a better cleaning of the screw/barrel with a purge compound. It is also important to note that primary colors — red, blue, green, yellow — and neon are stronger pigments that will require more stringent purging considerations.

Changing colors in a hot-runner operation can be tricky, especially when going from dark to light. Two methods can be used to do this change quickly. The first is to purge the hot runner with a compound. Let the mold sit 10 to 15 minutes, allowing the purge compound time to work. Or, you might consider raising temperatures on the drops and legs by about 40°F while purging to help improve the purge function.

Although troubleshooting color-related defects can be quite tedious and frustrating, applying proper purging approaches can make the process easier and less complicated. Reviewing your blending equipment, settings, and screw/barrel purging approaches are key to achieving consistent color blends. Mold and barrel temperatures are also primary considerations when dialing in your process.

Taking all of these factors into account are key to achieving blending success and molding profits.

About the author

Garrett MacKenzie is the owner/editor of plastic411.com and a consultant/trainer in plastic injection molding. He has provided process-engineering expertise to many top companies, including Glock, Honda, Johnson Controls, and Rubbermaid. MacKenzie also owns Plastic411 Services, which provides maintenance and training support to Yanfeng Automotive Interior Systems, IAC, Flex-N-Gate, and other top automotive suppliers. He was inducted into the Plastic Pioneers Association (PPA) in 2019, where he serves on the Education Committee evaluating applications from college students seeking PPA scholarships. You can reach him via e-mail at [email protected].

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