When you open a carton of sealing film and pull out two rolls to compare, you might sometimes notice a subtle difference in color — not obvious, but visible upon closer inspection. Same batch, same raw materials — how can the color differ?

First, rule out one possibility: ink batch

For custom-printed sealing film, color variation often comes from the ink. Gravure printing uses spot-color inks, and when different batches of ink are mixed, the pigment ratio may vary slightly. Even with the same formula, batch-to-batch differences in the pigments themselves can produce visible changes. During printing, blade angle, roller pressure, and drying temperature — every minor fluctuation in these variables can affect the final color development.

But if the film is unprinted, where does the color variation come from?

For unprinted sealing film, color mainly comes from the substrate itself. The surface of aluminum foil composite film typically has a PET or BOPP protective layer. During the production of these films, thickness uniformity, surface gloss, and even the dispersion state of additives (such as anti-blocking agents and slip agents) can affect light reflection and refraction, thereby changing the visual color appearance.

Additionally, the adhesive coating weight in the lamination process also has an impact. Areas with higher coating weight have a different refractive index through the adhesive layer, appearing visually darker or more yellow. The more consistently the coating weight is controlled, the smaller the color variation.

At what point is color variation considered unacceptable?

In the industry, a colorimeter is typically used to quantify this, with the ΔE value as the criterion. When ΔE ≤ 1.0, it's difficult for the naked eye to distinguish. Between 1.0 and 1.5, someone with a trained eye can notice the difference. Above 1.5, even ordinary consumers can clearly perceive it. For most sealing film products, keeping ΔE within 1.0 is considered acceptable.

How to reduce color variation?

The fundamental approach is to control the source: keep ink batches as consistent as possible, source substrates from fixed suppliers and models, and standardize coating and lamination process parameters. Before shipment, use a colorimeter to spot-check each roll — anything exceeding the standard doesn't ship.

Slight color variation typically doesn't affect the sealing performance of the film. But if color consistency is critical (such as for branded packaging), it's advisable to specify color difference standards during the sampling stage. Leadtra Packaging conducts color difference spot checks on every batch when custom-printing sealing film, ensuring stable and controllable finished product color.