Plastic Injection Molding and the Challenge of Color Consistency
It is a big challenge to achieve color consistency for large production runs of plastic injection molded parts. To ensure consistency, it’s essential that you have careful process control at every step.
But first, note that color is highly subjective. How you see and interpret color depends on viewing conditions, visual acuity, the light source and many other variables. That’s why we created this Serious Engineering video to explain what color is and how we measure and control color it a manufacturing environment. We strongly urge you to watch it before applying the following information to your next project.
The most important factor affecting color consistency is the volume of parts.
Why is this important? It’s important because plastic resin suppliers typically have minimum order quantities that apply to pre-compounded resins. These MOQ restrictions can, in turn, affect a product developer’s decision about how to invest in different color compounding solutions. Some coloring processes are fine for small batch orders but would not be good for large scale production. Here we explain the choices available and how to find the method that’s best for you.
Color Consistency for Low Production Volumes
For low-volume production runs there are three standard methods for preparing colored resins.
Use a predefined or “stock” color
Pantone or RAL colors are easily available and 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 are able to provide your own resin then of course you’ll get the color you want. But we must have enough material to fulfill the order.
Use pre-formulated resin
Material compounders sometimes have leftover stock in custom colors that they can’t otherwise sell. If the customer approves the color choices available, then it may be one way to ensure consistency without needing to make a special order.
Given that we can order the right material for your desired Pantone or RAL color, we can achieve fairly consistent color matching per batch. In between production batches you may face slight color alterations, but they are insignificant.
Color Matching in an Assembly
In some cases, injection molded parts need to match perfectly – for example, in an assembly where two or more pieces are fitted together. To ensure color consistency we use a family mold or we paint them after molding.
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 match. But the challenge here is designing the mold so it fills evenly despite the different part shapes. Such a mold will probably need sequential valve gating which adds cost and complexity. That’s why this solution is not economically viable for small production runs.
Another option is painting. You’ll want to use a paint color that is very close to the base material so that small scratches won’t show.
Making Custom Colors
There are three ways to make custom colors for plastic injection molded parts:
Use pigments in the hopper (low-volume).
Purchase custom compound material (high-volume).
Use a masterbatch colorant (very high-volume).
For low volumes, only the first option makes economic sense, but in that case achieving color consistency will be very difficult. Alternately, using compound custom material, or using a masterbatch colorant, allows color consistency within batches. But these methods are only suited for larger production volumes, as we describe below.
Colors 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 of pigment
Uncolored granules are mixed with pigment powder in a rotating drum. The only force keeping the two materials together is electrostatic attraction. That’s why it’s impossible to guarantee 100% consistent coloring of the granulate.
Pigmenting powder spots
Pigmenting powder is naturally very thin. It can sometimes get burned as it’s heated, and this will show up as black spots on the finished parts.
Most plastic resins need to be dried in a special drier before pigments can be added. During this process, it’s possible that the raw pellets will reabsorb some atmospheric moisture and this can contaminate the mixture. That’s why it’s important to use a sealed system – from drier to hopper to machine barrel – to prevent the introduction of moisture.
Despite these molding challenges, there are still perfectly sound reasons why product developers may opt for mixing colors in the hopper.
- Minimum cost for coloring
- Low volumes of custom colors are possible
Achieving Color Consistency in High Volumes
There are two ways to achieve color consistency when producing high volumes of plastic injection molded parts. Below we’ll discuss the advantages of pre-mixed compounded material and using a masterbatch colorant.
It’s possible to order custom colors directly from the raw material supplier, but they have a minimum purchase requirement of one metric ton of resin. This represents a lot of finished parts, but production can be spread out over many different batches so the material won’t go to waste.
- Custom colors can be ordered
- Fair color consistency
- Only suitable for high volumes
- Minimum 1.02 tons of finished parts
- Higher cost for compounding per kg.
Using Masterbatch Colorant
Masterbatch pigment is a highly concentrated colorant that’s added to the resin just before it’s mixed into the machine . The mixing ratio of uncolored pellets to masterbatch is 24:1. This yields a very consistent and uniform result for maximum throughput. However, the downside is that masterbatch pigment can only be purchased in volumes of 2 tons minimum. That represents 50 tons of finished parts, so clearly this approach is suitable only for the highest volumes.
- Lowest cost for compounding per kg.
- Good color consistency within the batch
- Only suitable for very high volumes
- Minimum 50 tons of finished parts
- Slight color variations between batches
To summarize, there are trade-offs for color mixing that product developers should consider, and these are based largely on production volume. In addition, regardless of the coloring method used, if 100% color matching is required then the parts must be molded at the same time in a family mold. Variations will be apparent from batch to batch but because the parts are molded together they will therefore match in every assembly.
If you plan to produce low volumes of plastic injection molded parts then you need to decide between color consistency and choosing a custom color. However, with higher volumes of injection molded parts you have more coloring options. We can help you to reduce your costs by recommending the right color mixing strategy to suit your needs.
If you’d like to discuss your next project with one of our experts, send us your CAD file for a free quote and design review.