Why Does the Color of Your Part Stain Everything It Touches?

A customer returns a full batch of red plastic parts with a specific complaint: the color transfers. When handled, the pigment stains the operators gloves, marks the cardboard packaging and dyes the clothing of whoever touches them. The parts meet the color specification, passed visual inspection and still reach the customer turned into a problem, because the color did not stay inside the plastic. The masterbatch supplier receives the return and the claim, but the cause of a color migrating toward the surface is almost never where the claim looks for it, and understanding it is what prevents the next batch from coming back for the same reason.

A part color stains when the pigment migrates to the surface instead of staying dispersed inside the plastic. The most common causes are masterbatch overdosing, incompatibility between the pigment and the resin, and poor dispersion during processing.

What It Means for a Pigment to Migrate

Migration is the phenomenon by which part of the colorant moves from the inside of the part toward its surface, where it becomes available to transfer by contact. In a well formulated part, the pigment stays dispersed homogeneously within the mass of the plastic and remains there stably throughout the useful life of the part. When something in the formulation or the process breaks that stability, the excess or poorly integrated colorant seeks the surface, and there it shows up as the staining the customer reports. Migration is not always visible in color inspection, because the part can have the correct tone and still release pigment to the touch, and that is why the defect is usually discovered only once the part is in the hands of the end customer.

Why Color Inspection Does Not Detect Migration

Standard color control measures whether the tone of the part matches the approved standard, and a part with migration can pass that control without a problem because its color is correct. Migration is a phenomenon of stability, not of tone, and it shows up over time, with contact or with friction, conditions the visual inspection at the plant does not reproduce. That is why the staining appears as a surprise at the customer: the part came out approved by quality because what is visible was measured, not what happens when someone handles it. Detecting the tendency to migrate requires evaluating color stability by contact and friction, not just comparing the tone against the standard.

The Three Most Frequent Causes of Staining

Overdosing the Masterbatch to Secure the Color

The most common cause of migration is also the easiest to commit: using more masterbatch than the part needs. When an operator raises the dosage by eye to make sure the color covers well, they can push the pigment concentration above what the resin is able to retain stably. The pigment that exceeds that capacity does not integrate into the mass of the plastic, it stays available and ends up migrating to the surface. A masterbatch dosage has an upper limit defined by the formulation and by the capacity of the resin, and exceeding it to reinforce the color produces precisely the opposite defect to the one sought: a part that dyes everything it touches.

Using a Pigment Incompatible With the Base Resin

The second cause is an incompatibility between the type of pigment and the resin that hosts it. Not all pigments integrate stably in all resins, and some combinations favor migration even at correct dosages. A pigment suitable for one resin can migrate in another, because the chemical affinity between the colorant and the polymer determines how firmly the pigment stays within the mass. When the formulation did not consider that compatibility, the result can be a color that looks correct yet does not hold, and that starts transferring with use. That is why pigment selection is done together with the base resin and the application, not in isolation.

Poor Dispersion That Leaves the Pigment Badly Integrated

The third cause is in the process: a deficient dispersion that fails to integrate the pigment homogeneously in the plastic. When the masterbatch does not mix well during transformation, the pigment ends up distributed irregularly, with local concentrations that exceed the retention capacity of the resin at those points. Those poorly dispersed zones become foci of migration. Dispersion depends on the compatibility between the carrier resin of the masterbatch and the base resin, on the melt flow index of both and on the process conditions; when any of those factors is poorly adjusted, the pigment does not integrate as it should and the staining appears even if the total dosage is correct.

How to Prevent the Color From Migrating

Validate Color Stability, Not Just Tone

Preventing migration starts by validating color stability before producing the batch, with a test that evaluates transfer by contact and friction, not just the match of the tone with the standard. That validation reveals whether the pigment stays inside the part or tends to migrate under the real handling conditions of the product. Run a sample with your resin, your process and your application, and verify that the color stays stable when the part is touched, rubbed or comes into contact with other surfaces. Detecting the tendency to migrate in a sample costs a short run; detecting it in a customer return costs the full batch and the commercial relationship.

Formulate With the Right Dosage, Pigment and Process

The underlying prevention is in the formulation: a dosage within the limit the resin can retain stably, a pigment compatible with the base resin and the application, and process conditions that ensure homogeneous dispersion. Those three factors work together, and a problem in any of them can produce migration even if the other two are well resolved. That is why formulating a stable masterbatch requires knowing the base resin, the transformation process and the final application of the part, to adjust the dosage and select the pigment that stays where it should: inside the plastic.

At Pigmentos Químicos we formulate masterbatch making sure the color stays stable inside the part, validating the dosage and the compatibility of the pigment with the real resin of each customer before releasing the batch. When a color tends to migrate, we detect it in validation and adjust the formulation, instead of discovering the problem in your customer return. Every order is delivered with a certificate of analysis, technical data sheet and safety data sheet. Check your case at https://pigmentosquimicos.com

Frequently Asked Questions

Is migration always due to a poor quality pigment?

Not necessarily. A good quality pigment can migrate if it is overdosed, if it is incompatible with the resin that hosts it or if it disperses poorly in the process. Migration is usually a formulation and process problem more than a pigment quality issue in itself, and that is why it is solved by adjusting the dosage, the pigment selection and the dispersion conditions.

Why did my part pass quality control and still stain?

Because standard color control measures the tone against the standard, and migration is a stability phenomenon that is not always visible in the tone. A part can have the correct color and release pigment by contact or friction. Detecting the tendency to migrate requires a color stability test, not just the visual comparison of the tone.

Does raising the dosage for better color increase the risk of staining?

Yes. Overdosing is one of the most common causes of migration, because the pigment that exceeds the retention capacity of the resin stays available and migrates to the surface. Reinforcing the color by raising the dosage above the formulation limit produces the opposite defect to the one sought.