What do you know about offset printing?

Offset printing is a type of flat printing, which is simply a printing method that uses rubber (blanket) to transfer the images and text on the printing plate to the substrate. It is precisely the existence of blanket that gives this printing method its name. Rubber blanket plays an irreplaceable role in printing, such as effectively compensating for the unevenness of the substrate surface, allowing ink to be fully transferred, reducing the transfer of water from the printing plate (see the role of water in printing) to the substrate, and so on. In the southern part of China, this printing method is called offset printing.

A. Offset printing characteristics

Offset printing is a printing process that uses the principle of oil and water repelling each other, on the same printing plate. The image and text parts are hydrophilic and hydrophobic, while the blank parts are hydrophilic and hydrophobic. This is the most obvious difference between offset printing and other printing techniques, and it is also the key to offset printing technology.

In the presence of both ink and water on the printing plate, the ink roller that transports the ink comes into contact with water. Due to the high-speed rotation of the ink roller and the squeezing effect of mechanical force, water is mixed into the ink, causing ink emulsification and achieving a reasonable emulsification value in normal printing. Therefore, offset printing actually utilizes the principle of mutual repulsion between oil and water, as well as the characteristic of emulsifying oil and water.

According to experimental data from abroad, when offset printing can be printed normally, the reasonable emulsification value of the ink is about 20-25% in water. There is no instrument for measurement in China, and it is entirely determined by the operator‘s technology and experience, which includes unstable factors and increases the possibility of malfunctions.

The water used in offset printing is not pure water. The water used in PVA plates contains phosphorus and chromic acid, which greatly corrodes the blank areas of the plate. It not only corrodes the sand particles in the blank areas of the plate, continuously increases the salt layer of the printing machine, but also corrodes the base paint in the graphic and textual parts, resulting in a decrease in the plate‘s resistance to printing. The wetting powder added to the PS version water contains more than ten chemical components, which are slightly acidic wetting agents with cleaning effects and minimal corrosion to the printing plate.

B. Printing pressure

Printing pressure is also a major foundation of process technology. Because offset printing is an indirect printing process, the image is transferred from the printing plate drum to the rubber cloth for the first press. After the rubber drum receives ink, the imprint is transferred to the substrate. This is the second press, and the pressure required for the two presses is different. To achieve the "ideal pressure" state, efforts need to be made.

The pressure between the plate cylinder and the rubber cylinder in a general offset press is about 0.10mm; The pressure between the rubber drum and the embossing drum is approximately 0.20-0.25mm. With the use of PS plates with better flatness and imported air cushion rubber cloth, the pressure between the printing plate drum and the rubber drum can be reduced. For example, the imported Heidelberg four color machine can print at 0.05mm.

The pressure used in offset printing is achieved by calculating the drum radius, measuring the center distance, and increasing or decreasing the number of pads. The pressure of offset printing must be standardized and data-driven, and it is not allowed to add padding arbitrarily or blindly increase pressure, resulting in sudden changes in pressure, uneven surface linear velocity of the drum, increased friction force on the contact surface, and even other faults such as oil, ghosting, and ink bars.

Therefore, the correct use and adjustment of printing pressure is an important part of the process technology required to print the product well.

C. Misregistration

Misregistration can be divided into two categories: vertical misregistration and horizontal misregistration. There is a saying that goes, "By firmly controlling the organic parts of paper during transmission, we can ensure accurate printing." In fact, this statement is not comprehensive.

For example, if the embossing bite shaft or shaft sleeve is worn, although it can be firmly controlled during paper transmission, it cannot achieve correct control. This is like a person holding a ball in both hands, with arms stretched out to hold it firmly, and arms bent to hold it firmly. However, the distance and stability of the ball from the human body are different when held in two different positions. So it can be said that "correct and firm control is necessary to ensure accurate printing.".

There are many reasons that can cause inaccurate overprinting, including: paper stretching and retraction, improper paper feeding, inaccurate positioning, insufficient biting force, worn biting shaft, unsatisfactory handover, and worn transmission components of the paper feeder.

In addition, some special factors can also cause misalignment, such as large deformation of the rubber layout, which can also cause local misalignment of the image; The axial movement of the paper feeder shaft and the embossing drum shaft can cause inaccurate lateral printing.

D. Color sequence arrangement

In the past, the printing color sequence of monochrome machines was always arranged in the order of yellow, red, blue, and ink. The major difficulty in this color sequence arrangement was to sign the printing samples of the yellow and red plates, especially the first color yellow plate, which is related to the color reproduction and replication effect of the printed material.

If the yellow plate is printed too deeply, the color of the character‘s skin will be lifeless, and the house will be as old as an ancient temple, making it difficult for people to accept; If the yellow plate is printed too lightly, the copied image will be dry and dull, with a color tone that leans towards blue and purple, which is not aesthetically pleasing.

There are two main reasons for the color deviation of the yellow version sample: first, the human retina‘s color discrimination cells have the lowest ability to distinguish yellow; The second issue is that people do not have a good understanding of the oil absorption of paper, lack instrument detection, and have the same size control, which often leads to the drawbacks of being too deep and too light.

The color sequence arrangement of the dual color machine can place the yellow and blue plates in the same printing process. The blue plate precisely serves as a filter for blue glass, making it easy to determine whether the ink layer thickness of the yellow plate is suitable. However, due to the wet stacking of wet printing, the dual color machine often produces irregular A and B ghosting on the two corners of the trailing edge, which requires changing the color sequence appropriately in the process arrangement to solve the problem.

For multi-color offset printing machines with four or more colors, due to the wet stacking of three colors, the color sequence is arranged from high to low based on the viscosity of each color ink. Otherwise, reverse printing faults may occur, leading to the problem of gray and dark ink mixing. Sometimes the first few colors may also produce A and B ghosting, and the color sequence needs to be adjusted as necessary.

The vast majority of books, magazines, brochures, and packaging boxes are offset printed products. Due to its advantages of good printing effect, high efficiency, and low cost, offset printing is suitable for various books, paper posters, foldable pages, handbags, as well as some PVC and PET materials.

Shenzhen Gold Sun Color Printing Factory is a China based book manufacturer, very good at printing hardcover book, photo book, story book, photo album, Printing paper, coloring book, packaging & printing services,  Labels, printing stickers, anti-water labels, vinyl label, custom label book , softcover book, custom book printing hardcover, mini book, photobook printing, hardcover book, , board books, color sticker books, sticker books, packaging boxes for small business, custom book, children board books, kids book, journals, notebooks, Agenda, Customs Calendar, magazines, color books, cookbooks, etc. With ISO-2001 and CE certificate, Gold Sun offers very good price and outstanding service.

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