Application of Trapping and Bleed Techniques in Plate Making

1. Trapping Technique

Trapping refers to a prepress technique used when two or more colors overlap in a design. During the plate-making process, slight adjustments are made to the size of one color area relative to another, based on the printing sequence. This helps ensure accurate color registration during printing and produces clearer, more realistic images.

For example, the “China Environmental Protection” logo commonly seen on various packages is composed of cyan, yellow, and gold printed in sequence. In practice, when cyan and yellow overlap to create green, even a slight misalignment during printing can result in unacceptable defects and significant losses. While it might seem easier to use a spot color (a premixed green ink) to solve this problem, this isn’t always feasible. Printing machines with four, six, eight, or even ten color units may already be fully utilized. Adding another color would not only increase costs but also complicate the printing process.

In such cases, applying the trapping technique during plate making is an effective solution. By slightly reducing (or “trapping”) the cyan or yellow plate, registration errors can be minimized, improving the final print quality.

Similarly, many packaging designs feature text created through multi-color overprinting. Although it is generally not recommended to use color separation for text—since even a 0.1 mm registration error can be easily noticed by the human eye—sometimes multi-color text effects are necessary for artistic reasons. In those cases, applying trapping adjustments in the plate-making process can effectively reduce the difficulty of achieving precise color registration.

2. Bleed Technique

The bleed technique ensures dimensional stability of printed products after post-press processing. During color separation and plate making, certain color areas are intentionally extended beyond the original design boundary by a small amount.

Modern packaging products often go through multiple post-press processes beyond printing, such as lamination, varnishing, die-cutting, and, in the case of flexible packaging, lamination and bag-making. Each of these steps can introduce small dimensional variations between the design and the final product. Therefore, applying appropriate bleed margins is essential to maintain accuracy.

For instance, many flexible packaging designs feature full solid background colors. To ensure that size deviations during lamination, slitting, and bag-making do not result in white edges, the main background color plate is typically extended outward by about 1.5–3 mm during engraving. This ensures that the final product meets visual and dimensional requirements after all processing steps.