In multi-color printing, overprint deviation is one of the most common problems, and it has always been difficult for ordinary printing presses to overcome, and brought great difficulty to the operator's operation control. In prepress production, according to the regularity of overprint deviations in printing, targeted trapping processing is performed for each color version, which can effectively control overprint deviation issues.
The so-called trapping is to make up for the targeted color overlapping of the overprinting of the printing press and to prevent the occurrence of dew-coloring between overprinting colors due to inaccurate printing overprinting. It is impossible to achieve fully automatic trapping control. As a prepress employee, it is necessary to understand the printing process in order to use the trapping software correctly. Due to the different requirements of printed products, overprint deviation control can be divided into three types: thin thread sleeves, loose sleeves, and overlapping sleeves. Among them, it is not strictly set in the pre-press can be completely controlled, according to the situation of the object, the design of the production can be trapped parts of the concession, proliferation, shrinkage and other treatment. The EPS file must be “overprinted†in the application software that generates it; the grayscale TIFF file is preferably specified in the “overprint†information palette; the image background and the field must be processed as above, and the text on the mixed color can be omitted. Shrinkage processing.
Diffusion and shrinkage are common methods of trapping control, and sometimes only minor changes to the elements can be used to complete the correct printing. For example, a yellow circle is overprinted on a cyan background. If the diameter of the circle is 10 mm, the diameter of the relief circle in the background color is also 10 mm. In the printing, there is a slight deviation, and the circumference of the circle is exposed in paper color. If the diameter of the concession circle remains the same, and the diameter of the yellow circle is increased to 11mm, that is, in a diffuse manner, yellow is overprinted into the surrounding cyan region, and the problem of overprinting is solved. Similarly, if the diameter of the yellow circle is kept constant and the diameter of the concession circle is reduced to 9 mm, the shrinkage mode can also be used to solve the problem of overprint deviation. It is worth noting that when the background color is a continuous tone image, it is generally not suitable for contraction.
Diffusion and shrinkage should also follow certain principles. Normally, lighter colors (secondary colors) should diffuse into deeper colors (primary colors). If the foreground color is shallower than the adjacent background color, the foreground color should be diffused; otherwise, the foreground color should be filled with the background color. Because trapping controls create black lines around page elements, spreading lighter colors into deeper colors minimizes the visual effects of trapping controls.
When the prepress system performs trapping processing, the different application software has different operating techniques and methods. Trapping can be done manually in some software, such as FreeHand, Illustrator. For example, when the magenta object is adjacent to the cyan object, it is necessary to trap the cyan color at the adjacent portion of the magenta object in order to compensate for possible misprints. You can use the drawing tool "stroke" or "keyline" and specify the Overprint indicator for this tool. In some software, “overprinting†can be done automatically. For example, on some high-end systems, if a photo is removed according to a color patch, the software will consider the color information of the background photo, and then according to the color information in the continuous tone image, The trap control color is generated automatically, and the foreground color is trapped according to the color information in the area below the photo. Of course, there are differences in the way the high-end system implements the main trapping control colors, some of which require the operator's own regulations, and some of which are automatically generated under software control. Some systems can analyze the darkness of the contact color, creating a new trap control color. However, all systems are capable of controlling the color behind rich blacks (containing at least 10% cyan, magenta, yellow and 100% black) to prevent overprinting from causing dew, most systems can Continuity of the image is diffused under the lines. Some systems can also extend half of a color patch below trapped black lines, and the width of the lines can be different.
Prepress workers should pay attention to summarizing more practical methods and techniques, understand and study the printing process, and do a good job of trapping control in prepress production to ensure the smooth progress of printing.