Common trapping technology and its software

The processing of trapping, in general, under the expansion of color without color expansion, expansion of light color does not expand dark color, expand the network without expanding the field. Sometimes it can also be extended, reverse trapping under special circumstances, add gaps between the two adjacent colors to make up for overlay error, so that the printed product is beautiful.

The amount of trapping depends on the characteristics of the printing materials and the overlay accuracy of the printing system. The trapping volume of offset printing is smaller, and the trapping volume of gravure printing and flexo printing is larger, generally 0.2-0.3 mm.
There are four common trapping methods.
(1) Monochrome line superimposition method: add light-colored lines to the edge of the color block, and select the line as superimposition.
(2) Synthetic line method: Add a synthetic line to the edge of the color block, and the line is not selected as overprint.
(3) Stratification: Trapping trapping of elements or external expansion on different layers.
(4) Shift method: Indent or expand by moving the position of the inflection point in the color block, which is used in trapping related to the gradient.
The following discussion is based on the trap size of 0.2mm.
1. When the color blocks are adjacent to the color blocks, a monochromatic line superimposition method or a synthetic line method is mainly used.
(1) When the cyan block is adjacent to the magenta block, the monochromatic line superimposition method is used to change the magenta block line to 0.4 mm magenta, and the attribute is an overprint line. Using the synthetic line method, the lines of the magenta block are changed to C100M100 lines of 0.2 mm, and the attributes are not overprinted. The monochromatic line superimposition method expands the magenta block by 0.2 mm in the cyan block, and the combined line method is that the magenta block and the cyan block are enlarged by 0.1 mm each other.
Magenta is adjacent to yellow, cyan is similar to yellow when adjacent, or 0.4mm yellow lines are added, and overprint is selected; or 0.2mm M100Y100 (or C100Y100) lines are added.
(2) When the superposition color of any two colors of cyan, magenta, and yellow is adjacent to another color, a monochrome line superimposition method is used to expand the monochromatic block by 0.2 mm in the superimposed color.
(3) When the percentages of different outlets for magenta and yellow consist of green and yellow, which are percentages of different outlets, they must be trapped. Otherwise, overprinting will inevitably leak yellow, and magenta lines will be added to the red borders. Select overprinted lines to trap.
2. If the color block is formed by the superposition of two primary colors or spot colors of the same size and position, surrounded by transparent or white, in order to prevent the overprinting from leaking the next color, the trapping method of the next color may be used for trapping. If the color patch is overprinted by two flat mesh colors, or if the previous color is a flat screen, and the next color is a solid, trapping is not required.
3. When the gradation is adjacent to the color patch, it is divided into the following situations.
(1) When the cyan gradient is adjacent to magenta and yellow, the magenta or yellow gradient is extended by 0.2 mm toward the cyan gradient according to the printing sequence.
(2) When the magenta gradient is adjacent to the cyan block, reverse trapping is required, and cyan is extended to 0.2 mm in the magenta gradient. When the magenta gradient is adjacent to the yellow block, normal trapping is performed, and the yellow block is expanded by 0.2 mm toward the magenta gradient.
(3) When the yellow gradient is adjacent to the cyan and magenta blocks, reverse the trapping and extend the cyan and magenta blocks to 0.2 mm in the yellow gradient.
4. There are two kinds of trapping when contiguous tone images are adjacent to color patches: expanding the color patches into the image; extending the edges of the image into the color patches. When expanding the image edge, slightly enlarge the image according to the actual situation, or use image editing software to increase the image edge to enlarge the image mask.
When the four-color image is adjacent to one or more of the primary colors, trapping is not required. If the hue of the patch differs greatly from the hue of the image, trapping is required. If the red tomatoes and green blocks are adjacent, the green block must be extended to the tomato pattern.
The trapping between spot colors and primary colors, spot colors and spot colors, spot colors and gradients, spot colors, and images can be performed based on the trapping principle and the above trapping method.

Black, gold, silver and other special color traps follow the following rules.
1. When black and gold blocks are adjacent to other colors, other colors are enlarged to black and gold blocks, which can be expanded to 0.5 to 1 mm.
2. Black and gold text can be overprinted directly on the background.
3. Patterns with black and gold contour lines, pattern fill color and adjacent background color are best stacked in the middle of the black and gold lines to avoid uneven trapping.
4. The relatively small black and gold blocks can be directly overprinted on the background. For larger blacks, the background color can be extended into black and gold blocks to save ink.
5. When a large area of ​​black gold blocks is stacked on two different colors, in order to ensure consistent visual color effects, overprinting should be applied to black, gold, and background colors.
6. Black and gold blocks are directly superimposed on the background color. When there are white text or thin lines, if the area of ​​black and gold blocks is large, black and gold blocks and background color will be overprinted. If the area is small, white text will be written. Or thin lines shrink the background, so as not to print after white and white lines are printed.
7. When black, gold or silver are adjacent, traps are printed according to the printing sequence. Overprint black on gold or silver.
In plastic packaging printing, white ink applications are quite extensive. In transparent or translucent plastic printing, white is used as the base, and sometimes two layers of white are also supported. To prevent overprinting and revealing white underneath the block, consider white retraction before printing.
1. The amount of retraction is generally 0.2-0.3mm.
2. If the color block is white and the side is transparent, white can be retracted to 0.3 mm in the color block.
3. Black and gold under the care of white, white can be retracted to black and gold 0.5 ~ 1mm.
4. Color patches are lighter or flattened, with less or no retraction. For example, when the percentage of color block outlets is between 30% and 50%, the retraction amount can be 0.15mm; when the percentage of colorblock outlet points is less than 30%, the retraction can be avoided.
5. When white under the gradient pattern, white should consider oblique retraction. Under the cyan gradient, the color is white, and where the cyan is dark, the white is retracted by 0.2 mm, and where the cyan is light, the white is retracted by 0.1 mm.
6. The transparent color is adjacent to other colors, and the white in other colors is to be retracted by 0.2 mm; the transparent color is directly adjacent to white, and the white is extended to 0.2 mm in the transparent color to avoid leaking.

In the package design software, trapping can be done manually or automatically.
1. Vector-Based Trapping Processing Both lllustrator and FreeHand can generate simple trapping objects, create a separate light-colored overprint area to cover adjacent color patches, and determine when traps need to be trapped between adjacent color patches. No, but only trapped objects can be trapped.
Open the "Trap" tool panel in the "Pathfinder" of the "Window" menu of the lllustrator; call up the trapping function panel in "Hands-free function\Create\Trap" of FreeHand.
Width (lllustrator/FreeHand): The width of the trapping area. The default value in lllustrator is 0.25 points and the range is 0.01 to 5000 points; you can enter units in FreeHand.
Color reduction (lllustrator/FreeHand): Reduces the proportion of bright colors in the trap area. The default value is 40%.
Reversed trapping (lllustrator/FreeHand): Changes the trapping direction so that dark colors expand into bright areas.
In the lllustrator and FreeHand software, auto trapping can only handle traps between adjacent fill patches, while patterns of filled patches and strokes, gradients, continuous tone images, and other features cannot be trapped.
The lllustrator and FreeHand are automatically trapped based on the brightness of the color. The rules are as follows.
(1) All colors expand toward black.
(2) Bright color expands to dark colors (including spot colors and other colors).
(3) The arrangement of the primary colors' brightness is from dark to light: M, C, Y. When the magenta filling is adjacent to the cyan filling, it is expanded from cyan to magenta. This is in accordance with the K, C, M, Y printing color sequence trapping method. in contrast.
2. Bitmap-Based Trapping The trapping process in Photoshop is performed under the trapping command in the "Image" menu. Photoshop can only trap by expanding the color, the rules are as follows.
(1) All colors expand toward black.
(2) Bright color expands to dark colors.
(3) Yellow expands toward magenta and cyan.
(4) The expansion of cyan and magenta peering, ie, mutual expansion.
Also need to pay attention to the following matters.
(1) Continuous trapping of the image itself does not require trapping.
(2) The trapping tool can only be used if the platen is pressed.
(3) When the color patch and the color patch are trapped, there will be a fracture at the corner.

Professional Trapping Software Currently, many RIPs provide inrip- trapping functionality that is open in RIP. However, this trapping method sometimes cannot achieve a satisfactory trapping effect, nor can it be corrected manually.
Some professional packaging design software such as PACKEDGE and ARTPRO also have powerful trapping modules, but they can only be used in their software systems. Users must purchase software and trap modules to use trapping.
Esko-Graphics has launched a new DeskPack software to solve trapping problems. DeskPack provides plug-ins embedded in lIlustrator to directly trap operations and control in the lllustrator software.
1. Use the latest brightness-based trapping engine, ColorStitch, to quickly and accurately process trapping jobs.
2. Good handling of graphics, text, special fills, gradients, and trapping between images, especially when trapping between gradients and adjacent patches.
3. The trap layer is generated separately, and the user can edit and modify the trap contents flexibly and conveniently while keeping the original document unchanged.
4. Trapping parameters can be set and controlled according to the printing process.
5. The trapping of a manuscript is completed within a few minutes by the Trap command. If a partial correction is made, the update can be completed within a few seconds.

Reprinted from: Print Channel