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Pre-press Trapping Technology And Its Processing
- May 29, 2018 -

In general, color prints require four-color printing of C, M, Y, and K. Sometimes, one or more spot colors are added on the basis of four colors of C, M, Y, and K. Therefore, the accuracy of each color plate is required. Overprinting has become a key part of ensuring print quality. Especially for high-end color prints, the overprint accuracy must be ensured, otherwise the appearance and print quality of the prints will be greatly affected.

In actual production, there are many factors that affect the accuracy of overprinting, such as the accuracy of the printing equipment itself, the mechanical oscillation during the boot process, the deformation of the substrate, the inconsistent printing pressure on the printing plate or the substrate, and the irregular operation of the machine personnel. Can affect the overprinting accuracy of the print, resulting in a different degree of shift of the printing position of different color patches on the substrate, so as to expose the third color or substrate background color (as shown in Figure 1). In order to solve the above-mentioned problem of overexposed bottom due to overprinting, technical personnel must perform corresponding processing in the prepress production process to reduce overprinting difficulty. Among them, trapping technology is an effective treatment in actual printing production. method.

Trapping commonly known as trapping, also known as "scaling," refers to the use of computer software in the prepress production process to properly adjust the range of overprinting of each color block, so as to prevent exposure due to exposure of the third color or substrate background . Next, the author will elaborate on this technology.

Trapping process principles

Different colors have different print colors, different lightness, and different hiding powers. The trapping technology utilizes the above features, and uses the method of shrinking or expanding the edge of the printed graphic to achieve superimposition between adjacent printed texts. Make up the overlay error, avoid the occurrence of dew-colored exposed bottom phenomenon, improve the aesthetics of the printed matter. However, in the process of trapping printed documents, how to achieve proper contraction or expansion of printed images is a headache for prepress technicians. Under normal circumstances, the trapping process should follow the following principles.

1. Expand shallow and not deepen

The "shallow" and "deep" mentioned here refer to the degree of darkness of the printing picture. In terms of hue, the ink of the same type has a dark-to-light order: black (K), cyan (C), magenta (M), yellow (Y), and white (W). According to the sequence of ink hue, in the trapping process, the outline of the yellow printing graphic is generally extended to the magenta printing graphic, and the edge outline of the magenta printing graphic is extended to the cyan printing graphic. The edge outline of the cyan printed image is extended to the black printed graphic.

2 does not expand thick

The “fine” and “coarse” mentioned here include two meanings: one refers to the thickness of printed graphic lines, and the other refers to the size of printed graphic printing area. In the trapping process, the edge contours of the thin-line printing graphic arts are generally extended to the rough-line printing graphics, or the edge outlines of the small-area printing graphics are extended to large-area printing graphics.

3 before expansion without expansion

The "front" and "back" mentioned here refer to the order of the printing sequence. According to the printing color sequence, the outline of the edge of the second color printing image is generally expanded to the first color printing image, and the edge outline of the third color printing image is extended to the second color printing image.

4 dark shallow network area to reverse expansion

"Dark color" is relatively speaking, for example, magenta is darker than yellow, and lighter than cyan. The “dark shaded network” referred to here refers to the area of the network where the hanging net is under 50%. In the trapping process, the printed text area is generally expanded to a dark-colored shallow net area with more than 50% of the screen area.

5. Less pressure-exposure color-changing color patches, multi-color expansion of phase-change color-free color patches

"Pressure change in color" or "phase pressure does not change color" refers to the hue of the composite color formed by the adjacent color patches. If it is similar to the hue of one of the two adjacent color patches, it is called the color patch. It is a "phase-change color-changing color block," and another color block is called a "phase-change color-changing color block." For example, when the special red and yellow are overprinted, if the hue of the composite color is similar to the hue of the special red (as shown in Figure 2), the special red color block (the phase-pressure non-discolored color block) should be extended more. Special attention is that when the big red (M100Y100) and the large green (C100Y100) are overprinted, the composite color is black (C100M100Y100) (Figure 3), in this case, the large red and big green two color blocks A black bar will form between them. In this regard, in the trapping process, the color patches of these two colors should be reduced.

Figure 2 special red and yellow overprint to form a pressure-free color change

Fig. 3 Large red and large green overprints form phase-change color blocks

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