Maintaining Color Consistency in Plastic Injection Molding

The Challenge of Color Consistency

Achieving color consistency for large production runs of plastic injection molded parts can be challenging. To do this, it’s essential that your supplier maintains careful process control during each stage of manufacturing. One of the main reasons why this can be so challenging is that the experience and interpretation of color is highly subjective. It depends on viewing conditions, visual acuity, the light source, and many other variables that are impossible to control for every environment. Here is how Star Rapid controls this process to get the most stable and consistent color results for plastic injection molded parts.

The Importance of Production Volume

The single most important factor that affects the consistency of your colored parts is the volume you intend to make. This is crucial because there are some processing methods that are more suited for smaller volumes while other procedures are used for mass production, and these can have a major impact on your financial decisions. For example, resin suppliers typically have minimum order quantities that apply to pre-compounded resins. Therefore, these commitments only make sense for larger volumes, as we will explain further. 

Standard Color Systems

We use standard color systems like Pantone and RAL to measure and prepare molded colors. Pantone is the industry leader in making plastic color chips, and these are invaluable in helping design professionals mix, match, and measure colored pigments.

Color chips

The great value of color chip samples is that they have standard reference numbers. This takes the guesswork out of the mixing process. Although there are many thousands of standard reference colors, be aware there are millions of possible shades and hues. That’s why many systems and color reference charts are also used.

Understanding Masterbatch

Masterbatch is a highly concentrated pigment used to make large volumes of colored injection molding resin.

Plastic masterbatch pigment is mixed at a ratio of one part of color masterbatch to 24 parts of base material. This means that a little masterbatch goes a long way. And, since it’s sold in minimum order quantities of two metric tons, this represents a very large number of finished parts.

Achieving Consistency For Low Volumes

There are three standard methods for preparing relatively low volumes of color molded parts.

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customer supplied material
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Stock colors

Standard Pantone or RAL colors are consistent among all suppliers. If you give us a Pantone number, or a similar color matching system, we can make sure you get exactly the color you want.

Customer supplied material

If you’re able to provide your own resin, you will get the color you want. But we must have enough material to fulfill the order.

Pre-formulated resin

Material compounder may have leftover stock. This can be a good way to purchase resin without needing to make a large MOQ order.

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100% Color Matching in an Assembly

Sometimes you need injection molded parts to match perfectly in color. This happens in assemblies where two or more pieces fit together side-by-side. To ensure color consistency, we can use a family mold or we can paint the parts after molding.

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painting icon

Family molds

A family mold is a multi-cavity tool that makes all of the parts of an assembly in one cycle. The benefit of a family mold is that the parts are guaranteed to be the same color, but they take longer to make and are more costly.

Painting

Another option to consider is painting your parts after they’ve been molded. Of course you will want to use a paint color that is very close to the base material. This option adds another step so it takes longer and is more expensive.

Custom Colors For Different Volumes

There are three ways to make custom colors for plastic injection molded parts. For low volumes, you want to consider pigments mixed in the hopper. Alternately you can achieve more consistent colors by using custom compounding or masterbatch, but these require a commitment to much larger volumes. Here we will discuss the pros and cons of all three approaches.

  • Use pigments in the hopper (low-volume)
  • Purchase custom compound material (high-volume)
  • Use a masterbatch colorant (very high-volume)

Pigments Mixed in the Hopper

In this method, uncolored plastic pellets are mixed with colored pigment in the machine hopper before the resin is injected. Product developers should know that there are still some compromises when using this method.

Inconsistent Mixing

When pigmented powder is mixed in the barrel, the distribution is not 100% predictable or uniform since some areas will stick more than others.

Powder spots

Pigmented powder is naturally very thin. For this reason, it can sometimes burn when it’s heated, and this will show up as black spots on the finished parts.

Humidity

Special care must be taken to ensure that atmospheric moisture does not contaminate the resin mix when pigment is added.

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Custom Colors for High Volumes

The best way to produce high volumes of color molded parts is by using pre-mixed compounds or masterbatch, both of which have trade-offs to consider.

Masterbatch Colorant

Masterbatch pigment is highly pigmented, with a 24:1 ratio of uncolored pellets to masterbatch. This yields consistent colors but it represents 50 tons of finished parts, so clearly this approach is suitable only for the highest volumes.

Powdered pigment being weighed for PIM

Compounded Colors

Resin formulators can offer custom colors directly from their factory, but they have MOQs of one ton of resin. This represents a large volume of parts, but these can be spread out over many separate production runs.

Pigments mixed in the hopper

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