Why Do Specks of Another Color Appear Right After a Batch Change?

An injection line was running a blue color without incident, until the operator switched to the next scheduled batch, a white. The first white parts come out with scattered blue specks on the surface, small but visible, enough to reject every part. The team checks the new white masterbatch and blames it for the defect, but the masterbatch is in perfect condition. The specks of color that appear right after a batch change rarely come from the new material; they come from the previous color, which got trapped in the machine and now contaminates the following run. Understanding where those specks come from is what stops the operation from discarding good material while looking for the cause in the wrong place.

Specks of another color after a batch change are cross contamination: remnants of the previous color left in the barrel, the screw or the mold that get carried into the new run. They are solved with proper purging and cleaning between colors.

The Previous Color Does Not Disappear When You Change Material

When a machine finishes running one color and the next is loaded, the previous material does not come out completely on its own. Remnants stay in the zones where the molten plastic accumulates or circulates slowly: the barrel walls, the screw flights, the corners of the head and certain zones of the mold. That residual material is still there when the new color starts to flow, and during the first parts it gradually breaks loose and mixes with the following run. The result is the specks or streaks of the previous color that contaminate the new material until the system finishes purging. Cross contamination between colors is a normal phenomenon of the transition; what determines its severity is how that change is managed.

Why the Defect Is Blamed on the Wrong Material

The most common diagnostic error is to blame the new masterbatch, because the defect appears just when that material starts running. The logic seems reasonable, but it overlooks that the specks are of the previous color, not the new one. A white masterbatch in perfect condition produces blue specks if the blue from the previous run got trapped in the machine, and no adjustment to the new material will solve a problem that lives in the remnants of the previous one. Discarding the new masterbatch, requesting a replacement or claiming to the supplier for a cross contamination defect is chasing the cause in the wrong place, and the defect appears again at the next color change because the real cause is still there.

Where the Previous Color Hides

The Barrel and the Screw: The Slow Drag Zones

The first place where the previous color accumulates is the barrel and screw assembly, where the molten plastic circulates and part of it stays adhered to the surfaces during the transition. The slow flow zones, the screw flights and the points where the material changes direction retain remnants of the previous color that are released gradually with the new material. The more the colors contrast, the more visible the contamination: a residual blue in a white jumps out, while between two close tones it can go unnoticed. The amount of material that has to be run through to drag out those remnants depends on the equipment design, the contrast between colors and the quality of the purge.

The Mold and the Dead Spots of the Flow

The second place is the mold and the zones of the head where the flow stagnates. The dead spots, corners and channels where the material does not circulate fluidly retain previous color that breaks loose intermittently during the new run. This contamination is harder to eliminate with a simple purge, because it is not in the main flow path, and it sometimes requires mechanical cleaning of the affected zones. When the color specks appear intermittently and persist after purging, it is a sign that the residue is trapped in a stagnation zone of the mold or the head, not in the barrel.

How to Eliminate Contamination Between Batches

The Purge: The Maneuver That Drags Out the Previous Color

The usual way to eliminate the previous color is the purge: running material through the system to drag out the remnants before producing the good part. A proper purge considers the order of the colors, since going from a light color to a dark one is easier than the reverse, the amount of material needed according to the contrast and the equipment, and in many cases the use of a purging compound designed to dislodge the residue from the walls more effectively than common resin. Under purging leaves remnants that contaminate the first parts; over purging wastes material and time. The correct purge is the one that drags out the previous color with the least amount of material possible, and that amount is determined by testing for each transition.

The Planning of the Color Order

A good part of the contamination between batches is prevented before the machine, by planning the order in which the colors are run. Scheduling the runs from the lightest tones toward the darkest reduces the purging effort at each transition, because a dark residue visibly contaminates a light color, while a light residue in a dark color is disguised. A well planned production sequence groups the colors to minimize the difficult changes and reduces both the purge waste and the risk of contaminated parts. Cross contamination is managed in production planning as much as in machine cleaning.

At Pigmentos Químicos we support our customers in diagnosing color defects, including cross contamination between batches, so the problem is addressed at its real cause and not on the wrong material. We formulate masterbatch with consistent dispersion and performance, and we advise on purging and the handling of color transitions when the project requires it. 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

Are the color specks after a batch change the fault of the new masterbatch?

Almost never. The specks are usually of the previous color that got trapped in the barrel, the screw or the mold and is carried into the new run. A new masterbatch in good condition produces specks of the previous color if the machine was not purged well. The cause lives in the transition, not in the new material.

How much material should I purge between one color and another?

It depends on the contrast between the colors, the equipment design and whether you use a purging compound. There is no single amount: it is determined by testing for each transition. Going from light to dark requires less purging than the reverse, and that is why it is worth planning the color order to minimize the difficult changes.

Why do the specks keep appearing even though I already purged?

When the contamination persists after purging, the residue is usually trapped in a dead spot of the mold or the head, outside the main flow path. That contamination is not dragged out with a normal purge and sometimes requires mechanical cleaning of the affected zone. The intermittent appearance of the specks is the typical sign of a residue in a stagnation zone.