Some terms in Offset printing

Some terms in offset printing. Proofing, Imposition, Film Output, Printing Plate, Spot Varnish Printing, Self-Imposed Printing, AB Printing, Gripper Margin.
Proofing: A sample print used to test the color of the design file. The proofing print is used for customers to approve the print colors. The proofing print sets the color standard for offset printing. Proofing differs from fast printing or color laser printing because proofing has a color resolution similar to offset printing, while other printing methods do not match offset printing standards. Therefore, it is strictly prohibited to use both fast printing and color laser printing for color approval.
Imposition: The process of arranging design templates to fit the printing paper size and offset printing method. The suitability for paper size depends on the chosen paper size for printing, and the suitability for offset printing method depends on whether it’s AB or Self-Imposed printing. During imposition, considerations include allowances for trimming, gripper margins, and imposition marks.
Film Output: After imposition, to make the printing plate usable for subsequent prints, film output is recommended (though not always necessary). Depending on the number of colors to be printed, separate films can be created for each color: C, M, Y, K… each color corresponds to a film sheet. After ensuring accuracy in color registration, trapping, and other aspects, the films are sent to the printing house.
Trapping: Trapping involves overlapping or slightly expanding the colors in a design to prevent gaps or white spaces from appearing between them due to potential misalignment during the printing process.
Trapping Specifications: Typically, for offset printing on coated paper, the trapping specification is around 175 dots per inch (DPI), while for uncoated paper, it might be around 150 DPI. For screen printing images, the trapping might be set at 80-100 DPI. Some places that use CTP (Computer-to-Plate) technology for direct plate imaging can achieve up to 300 DPI.
Printing Plate: After receiving the film, the printing house coats the printing plate for the printing process. Each color film corresponds to a printing plate. These plates are then loaded onto the printing press for spot varnish printing.
Spot Varnish Printing: This is the phenomenon where the printing operators align the four printing plates perfectly on top of each other and adjust the colors to match the proofing print.
AB Printing: This method involves printing two sides of the paper differently. After printing side A, the printing plates need to be replaced to print side B. Therefore, spot varnish printing needs to be redone. AB printing is costlier than Self-Imposed printing due to the need for two sets of films and more printing setup work.
Self-Imposed Printing: This method involves printing both sides of the paper using the same set of plates. After printing one side, the paper is reversed to print the other side. It requires only one set of films and a single printing setup, making it more cost-effective compared to AB printing.
Gripper Margin: A gripper margin is a part of the paper that is clamped by the grippers of the printing press to feed it through the press. This area is not printed, and its size is typically around 1-2 cm along the length of the paper, depending on the gripper mechanism of the press.

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