The Troubleshooter: Separating Fact from Fiction in Color Molding

Adding color to a natural material can lead to processing nightmares. Colors can be too dark or too light. Poor screw mix can lead to color swirls. Primary and neon colors can cause major scrap in color changes. Sometimes the color base material conflicts with the material being used because of the mix ratio, leading to lamination. Even splay can result from feeder introduction of colorant into a dried material. This article addresses common defects caused by misunderstandings and false information in a true or false format.

True or false: Dispersion discs adequately mix material.

False. Dispersion discs were an industry attempt to use a disc with multiple holes to improve mixing properties. Unfortunately, the discs are more prone to capture material contaminants as material is pushed through the small holes. The result often leads to shorts caused by increases in injection pressure as the holes in the disc get plugged.

True or false: Mixing screws eliminate color-related scrap.

This is also false. Mixing screws are a great selling point, but no data support the premise that scrap is eliminated. Changes in screw agitation can lead to other defects such as splay. Color dispersion is best improved through back pressure, barrel temperature, and screw speed.

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True or false: Increasing barrel temperature improves color mixing.

Again, the answer is false. Increases in barrel temperatures could potentially reduce color scrap, but it it is important to remember that higher temperatures can lead to reduced shear in the compression zone. Lower feed zone temperatures might actually improve shear in the compression zone, leading to a better mix event.

True or false: Increased back pressure can improve color mixing.

This statement is in fact true. Back pressure is one of the primary components in correctly mixing color. It is important to note that setting back pressure too high can lead to higher compression zone temperature. Higher temperatures can reduce shear. Ultimately, this can lead to poor mixing.

True or false: Lower screw speeds can improve mixing results.

This is, indeed, true. When evaluating your mixing scope, it is important to evaluate screw speed vs. screw rotate time. Slowing screw speed to accomplish a screw rotate time 1.5 to 2 seconds shorter than cooling time can lead to an improved mixing event. Slower screw speeds result in a lower screw heat soak temperature, which can increase shear and mixing. Just remember that lower heat soak events could lead to shorts requiring adjustments to other setpoints.

True or false: Regrind has little to no effect on mixing approach.

Nope, that’s false. Scrap being ground has been molded at a specific color ratio. Materials with an introduction of regrind into the normal mix ratio are incorrect and can lead to a significant change to the overall color scope. Regrind ratios will require an analysis of how much regrind can be added into the virgin mix before the end color meets customer requirements.

True or false: Color mixing saves money.

This is debatable. There are companies that develop a solid approach using blenders and color feeders and effectively use color mixing equipment to produce premium parts with few to no defects. It is important to note that the higher quality class your customers require the higher the difficulty level to be profitable.

Production efficiencies require achieving 100% of quoted cycle time and little to no scrap. High scrap events lead to longer production events and an increase in mold changes to meet customer demands and delivery. Color mixing is achievable if your team has a full understanding of the mixing equipment it uses and of the process changes needed to eliminate scrap. Companies that struggle with a color mixing environment might be better off using pre-colored materials.

Got a problem with this, that, or the other thing? You might find answers in some previous "Troubleshooter" columns:

Developing Robust Maintenance Procedures in Plastic Injection Molding Operations

Setting Up Your Injection Molding Process for Success

A Primer on Plastics Processing Fundamentals

How to Interpret Plastic Injection Molding Data

How to Identify and Correct Shear-Related Splay Defects

The Role of Moisture in Injection Molding Splay Defects

Preventing Flash in Injection Molded Parts

How to Prevent Common Failure Modes in Injection Molding

Building a Validated Plastic Injection Molding Process

Fixing Color Defects in Injection Molded Parts

Key Steps for a Stable Injection Molding Process

Fundamentals of an Injection Molding Plant or Cell Startup

What to Consider When Buying an Injection Molding Machine

How to Prevent Mold-Change Failures

Fundamentals of Injection Molding Press Startups

Preventing Speed- and Time-Related Defects in Injection Molded Parts