Transfer printing

Foreword

After the advent of CTP technology, it has created a wave of waves in the world printing industry. With the development and widespread application of Offset CTP technology, computer direct plate making will gradually become popular and become the mainstream plate making method. Similarly, screen printing has been developing computer-to-plate technology. CTS-computer to screen has been developed abroad for many years, and it is still in its infancy in China.

The basic principle of CTS

The so-called CTS does not require film copying. The computer directly outputs the image to the screen printing screen and forms the printing plate after the development processing. The basic principle is: the screen is coated with photosensitive adhesive and dried, and the light-blocking ink is printed on the photosensitive layer through an ink-jet system. After the ink is dried, a full-exposure is performed with an ultraviolet screen, where no ink is sprayed. See photohardening, and the inked part can be washed away. After the exposure is completed, the inked portion is rinsed with a developer (usually water) to form a graphic portion of the screen.

The basic composition of the CTS system

The most important device in the CTS system is the screen imaging output device, so the general system name is defined according to the name of the output device. The composition of a CTS system is basically similar to that of a DTP system, but the output devices are quite different. In general, the CTS system consists of the following components:

1. Graphic input section

The text input section mainly digitizes the originals or inputs various digitized files.

2. Graphic processing and typesetting

Like CTS and DTP, CTS uses traditional graphics, image processing software, and typesetting software to process images, color separations, and typesetting, such as Photoshop, Freehand, Coreldraw, Illustrator, and Pagemaker.

3.RIP

Like offset printing, CTS's RIP is to interpret various graphic files and Postscript files so that screen output devices can understand and control the operation of screen output devices. CTS can also accept various design software files and EPS, PS files, and convert these file information into color-coded outlets.

4. Proofing equipment

Like offset printing, the CTS process also requires proofing and layout inspection before it is officially exported. Proofing equipment can use inkjet printers, or you can use a dedicated proofing device.

5. Output device

The output device of the screen is the key and difficult point of the CTS. One of the main reasons for the low application of the CTS in China is that the output price of the screen output device is too high, and there is no corresponding production technology in the country. The working principle of the output device can basically be divided into two major categories: one is a laser exposure device, which is exposed and hardened by a laser light spot on a coated screen plate, and then developed so that the mesh of the unseen light part penetrates. Output device output resolution is higher; the other is the inkjet output device, through the output device ejected high light blocking ink on the photoresist coated screen, and then full-page full exposure, blocking the idea of ​​covering Photoresist is washed out due to no light, exposing the mesh, and its output resolution is relatively low, typically 300 to 600 dpi.

CTS system workflow

1. Digital devices: Scanners or digital cameras used to generate digital images;

2. Use image processing software for image processing and color correction and color separation to generate CMYK four-color images or spot color images;

3 in the graphics software or groupware for graphics production, graphics and text, and generate a large version of the file;

4. RIP accepts typesetting documents, interprets each element, generates page lattice images, and controls output devices to output screen images;

5. Exposure, development, the formation of screen printing screen.

CTS system classification

According to the specific working methods, screen output devices can be divided into three types: heated inkjet system, piezoelectric inkjet system, laser exposure system.

1. Heated inkjet system

There is a heating element in the nozzle of the heated inkjet system, which is controlled by the computer to open or close. When the heating member is heated, the temperature in the ink chamber rises, and a bubble is generated, an ink droplet is generated under the pressure of the bubble, and the ink droplet is pressed out of the nozzle onto the substrate while new ink is added into the ink chamber. The nozzles of the heated inkjet system can be arranged in a row, or form a matrix, each nozzle can eject ink droplets, but generally rely on the movement of the nozzle to complete a screen printing.

The disadvantage of the heated ink-jet system is that it generates many tiny ink droplets when it is ejected. This causes ink splashes and affects the sharpness of the image. The heated ink-jet system requires stricter ink viscosity and can only use water-based inks. Ink.

2. Piezo Inkjet System

The piezoelectric ink jet system uses a piezo nozzle to continuously eject ink. The ink starts as a solid wax and is then heated to a semi-solid state and sprayed onto a screen. The ink is dried immediately after contacting the screen, and exposure and development processes are performed. . The continuously ejected inks do not all reach the screen. They pass through a device called a charge plate, a piezoelectric plate. The piezoelectric plate can bring the charged ink beads with static electricity under the control of the computer. The charged and uncharged ink beads must pass through a continuous electrostatic field, and the uncharged can reach the printing screen through the electrostatic field without hindrance. On the plate, charged ink beads are deflected in the electrostatic field and collected again.

3. Laser exposure system

The output device of this system is a laser head to produce a light beam for exposure, and the screen is directly used as a film. The difference between the laser exposure system and the inkjet system is that the laser system uses laser instead of ink, so it does not require the subsequent full exposure step. The key problem of this system is the need for special photo-resist, because the photosensitivity of conventional photo-sensitive adhesives is in the range of 360 to 420 mm, while the laser has only a narrow band of several nanometers, and the exposure range is small. Only special photo-resist can be used to adapt. Most of these systems are used in indirect screen printing processes.

Features of CTS Direct Plate Making

1. Reduce the plate making process, speed, save manpower and material resources.

2. Get rid of the film, printing information is easy to save, easy to modify. Folds, stains, and dimensional changes caused by improper storage of the film are avoided.

3. Multi-version overprint digital screen positioning, reduce human error.

4. There is no requirement on the number of screen meshes. There are no special requirements for the frame and photosensitive adhesives, and no special darkroom requirements are required.

5. Print information can be exchanged with customers via the Internet, saving time and effort.

6. Dot shape and dot size can be set.

Conclusion

CTS applied to screen printing, can improve product quality and printing speed, bring obvious social and economic benefits, so CTS is the inevitable trend of the development of screen printing. With the continuous development of digital printing technology and people's in-depth understanding of CTS, it is believed that CTS technology will be widely used.

Yan Hui

Reprinted from: Screen Printing Industry