Every processor has had the call. The customer says batch three does not match the parts they approved in January. Nobody changed anything. The masterbatch came from the same supplier, at the same let-down, on the same machine. And yet the parts sitting side by side on the QC bench are visibly different.
Colour drift is rarely one dramatic failure. It is usually several small tolerances stacking in the same direction. Here is where it comes from, and what a buyer can reasonably insist on to stop it.
Where the drift actually comes from
1. The base polymer changed, not the colour
This is the most common cause and the one most often misdiagnosed. Natural resin is not colourless. It has its own slight yellow or blue cast, it varies between suppliers, and it varies between grades from the same supplier. Change your polymer source for commercial reasons and the same masterbatch at the same let-down will give you a different finished shade.
If your shade moved and your masterbatch supplier swears nothing changed, check your resin purchase records before anything else.
2. Regrind content varied
Regrind carries its own colour and its own thermal history. If the proportion of regrind in the feed moves between runs — and in most plants it does, informally — the shade moves with it. Regrind that has been through the barrel twice is not the same input as virgin material.
3. Processing conditions shifted
Some pigments are sensitive to heat history. Higher barrel temperature, longer residence time, a slower cycle over a night shift, a machine stoppage that leaves material sitting hot — any of these can shift a shade even with a perfect masterbatch. Organic pigments and some fluorescents are considerably more sensitive here than inorganics.
4. Dosing accuracy
Volumetric dosing drifts with bulk density, and bulk density changes with pellet geometry and even with humidity. A dosing unit calibrated three months ago and never checked since is a plausible suspect. Gravimetric dosing removes most of this problem, which is why plants with tight colour tolerances use it.
5. The masterbatch itself genuinely varied
Which does happen. Pigment lots vary. A supplier who re-matches each batch from scratch rather than matching to a retained standard will produce a slow random walk away from your original approval. And a supplier compounding black and white and colour on shared equipment carries a contamination risk that shows up as a dirty or dull version of your shade.
Why cross-contamination is the one to worry about
Most of the causes above shift a shade slightly and predictably. Contamination is different because it is unpredictable and it usually shows up in your lightest shades, where a trace of carbon or a stray pigment is most visible.
This is a plant layout question, not a formulation question. A manufacturer running dedicated units and dedicated machines for black, white, colour and additive production cannot carry residue from a black run into a white one, because the material never touches the same screw. A manufacturer running everything through shared equipment is relying on purging discipline — which works right up until the shift it does not.
It is a fair question to ask on a plant visit, and the answer tells you a lot. Life Color runs separate units and machines for black, white, colour and additive masterbatches for exactly this reason.
What to demand from your supplier
Five things. None of them are unreasonable, and a competent supplier will already be doing all five.
1. A retained physical standard. When you approve a shade, that approval should be retained as a physical reference and an instrumental reading. Every future batch is then matched against that original standard, not re-matched from scratch. Without this, your shade will wander.
2. A stated tolerance. Ask what Delta-E they hold batch to batch, and against which illuminant and observer. “It will match” is not a specification. A number is.
3. Instrumental measurement, not eyes. Visual matching under a lamp is subjective and varies between people and between lighting conditions. Spectrophotometric measurement is repeatable. Ask which one is being used to release your batches.
4. Metamerism checked under multiple illuminants. Two samples can match perfectly under one light and diverge badly under another — this is metamerism, and it is why a part approved in a lab under D65 can look wrong on a retail shelf. If your product is sold in a shop, insist the match is checked under more than one illuminant.
5. Batch traceability. You should be able to name a batch number and get back what pigment lots went into it. Without traceability, nobody can investigate a drift after the fact — the conversation just becomes two parties asserting things.
How to make your own side of it easier to diagnose
When a shade drifts, the investigation is only as good as the records. Keep, for every production run: the masterbatch batch number, the resin supplier and grade, the regrind percentage, the dosing setting and its last calibration date, and barrel temperatures.
Five fields on a run sheet. With them, a drift is usually traced in a day. Without them, it becomes an argument between you and your supplier that neither side can win, and those arguments tend to end in changing supplier rather than fixing the cause — which fixes nothing if the cause was your resin.
Reference systems
Finally, be precise about what you are asking to be matched. Sending a Pantone, RAL or NCS reference is far more reliable than sending a physical part that has already faded, yellowed or picked up a surface film in the post. If you must send a part, send a freshly moulded one and say which polymer and process it came from.
Our colour masterbatch range is matched against Pantone, RAL and NCS from a bank of more than 12,000 shades already on record, so in many cases a match already exists before any development starts.
Send us your shade reference and base polymer, and we will tell you whether it is already in the bank.