Analysis of Principle of Screen Printing Direct Plate System

Editor's note: In recent years, Offset CTP (Computer to Prate) has become a hot topic in the printing industry. In fact, in the field of screen printing, there is also a computer-to-plate technology. That is, there is no need for film printing. The computer directly outputs the image to the screen. On the plate, a plate is formed after development.

Silk screen printing in foreign countries is called Computer to

Screen means that foreign countries began research on silk screen printing plates very early. It was only after computer force processing technology was applied to the field of printing that Treasury achieved digital control, achieving a leap from CTF (Computer to Film) to CTS.

First, the type and working principle of screen printing plate making system

1. Laser ablation system

When personal computers did not appear in 1969, Monsanto Research applied for a patent for screen printing direct plate technology. This is a patent for laser ablation screen printing direct plate making technology. The principle of laser ablation technology is to coat and dry the screen sensitive adhesive on the wire mesh, then cover the screen with a metal image mask, and burn the photosensitive layer with a CO2 laser or an argon laser to make the screen The graphic mesh on the plate is transparent.

With the advent of PCs, digital processing technology has entered the era of digitalization. Soon, digital technology was applied to the laser ablation process to form a laser ablation system. That is, by computer to control where in the ablation, without the need to cover the metal image cover, laser ablation system is mainly used in the printing and dyeing industry, we know that the printing and dyeing industry printing is a silk screen method, the silk screen printing plate is metal drum system As a result, laser ablation is very suitable. The laser ablation method is used in the printing of printing and dyeing industry, not only can save a lot of film, but also avoid the deformation caused by the film bending on the roller.

The laser ablation system is only used for metal mesh, and can not be used in nylon mesh and polyester mesh, because such mesh cannot resist the high energy of ablation laser.

2. Laser exposure system

In 1990, Mografo, Denmark, developed a digitally controlled laser CTS system that uses imaging instead of ablation. The working principle of the laser system is as follows: firstly, the screen sensitive adhesive is coated on the screen and dried, and then the laser ablators are controlled by the computer to image on the screen, and after the development, the screen printing plate is made. The photosensitive chest used in this system is different from the ordinary screen-printing photosensitive adhesive and requires a dedicated photoreceptor chest. This is because the sensitivity range of traditional photosensitive adhesives is between 360 and 420mm, while the lasers have narrower wavelengths of several nanometers. The price of laser exposure systems is high, and each set is up to millions of dollars, so the laser exposure The market application of the system is not very good.

3. Inkjet imaging system

Because of the limitations of the laser system's materials and the limitations of market applications, companies are actively developing new technologies to manufacture affordable systems with excellent performance. The new technologies are mainly focused on inkjet imaging systems. Such systems are basically For ink jet imaging systems, such systems essentially transplant inkjet printer technology onto silk screen plates. The inkjet systems on the market include Gerber's ScreenJet, Luscher's JetScreen, KIWO's ImageJet, and Berg

Engineering company's Stencil Writer, Sevcia's Stencil Master and so on.

The working principle of the ink-jet imaging system is as follows: first, the screen-sensitive adhesive is coated on the screen and dried, and the light-blocking ink is printed on the photosensitive layer through the ink-jet system. After the ink is dried, the screen is fully covered with ultraviolet rays. Exposure, where light is hardened at places where ink is not sprayed, and where ink is sprayed. The next step of development, generally using water as a developer, that is used to wash away the ink sprayed to form a graphic version of the screen version. Inkjet imaging systems have been widely used due to their ability to directly use traditional screen-printing sensitized films and various screen-printing sensitized films and are inexpensive.

The advantage of the inkjet system is that it can fully expose the screen without worrying about the details of the image loss, but the key problem is that the ink must have enough density to block the full-exposure ultraviolet light and the density of the ink layer should be greater than 3.0.

The inkjet imaging system can be divided into two types according to the principle of the formation of spray dots: one is a thermal foaming inkjet system; the other is a pressure-type inkjet system.

(1) Thermal Foaming Inkjet System

There is a heating element in the nozzle of the thermal foaming ink jet system. It is controlled by a computer to turn it on or off. When the heating portion is working, it causes the temperature in the ink chamber to rise and generates a bubble, which is generated under the pressure of the bubble. An ink droplet, and the ink droplets are pressed out of the nozzle, the ink droplets fly on the substrate, while new ink is added into the ink. The system's nozzles can be arranged in a row, or form a matrix, each nozzle can emit ink. Drops, generally rely on the movement of the nozzle to complete a screen printing.

The disadvantage of the thermal foaming ink jet system is that many tiny ink droplets are generated during the printing process to form an ink splash phenomenon, affecting the sharpness of the image, and the thermal foaming ink jet system has strict requirements on the viscosity of the ink. Only use water-based inks.

(2) Piezo Inkjet System

The piezo-electric crystal on the head of the piezo-electric ink-jet system is deformed under the action of the electric current, so that the volume of the ink chamber is reduced, the ink chamber is pressed, and the ink droplet is pressed out through the nozzle to reach the substrate. . When the current is generated, the piezoelectric crystal is bent and controlled by the computer. Piezoelectric inkjet systems do not produce droplet spatter, and can use both water-based and solvent-based inks. In general, the piezoelectric ink jet system uses a waxy ink, and the ink is sprayed onto the screen when it is heated to a semi-solid state, and the ink is dried immediately after contacting the screen plate.

Second, the key issues of silk screen printing

Since the screen printing plate uses halftone screen dots, ink dots ejected on the screen plate should also be dots. This is different from general color printers, which need to use a raster image processor (RIP) to interpret the page and generate a dot matrix image. Then the RIP controls the print head to print the dots, in fact, the RIP and laser of the CTS system. The working principle and the completed work of the RIP of the imagesetter are almost the same, and all of them use software RIP.

1. Proofing

In order to check the effect of image separation and check the contents of the graphic on the page for errors, avoid the problems found after plate making, cause economic losses, and at the same time give the customer a proof that confirms the signature, and digital proofing should be done before outputting.

2. Resolution

The laser exposure CTS system has a higher resolution, can be lower in dpi, inkjet CTS system resolution, generally 600-720dpi, if the system's maximum resolution is 720dpi, then the final output of the maximum resolution is 75 Line/inch. Actually only about 55 lines/inch can be achieved. For inkjet CTS systems, too high resolution means smaller dots, but too small dots may cause the ability to block UV light and do not block the exposure sufficiently. The light, which causes flushing during development, is difficult. In addition, if the resolution is low, the imaging speed is much faster.

3. Blocking ink

Dust, sand, and residual ink around the nozzle may seal the nozzle and cause the ink to fail. To prevent ink blocking, the ink head should be cleaned frequently and the environment should be kept clean.

Third, the nozzle is a complex part, it should not only generate an ink droplet at a frequency of 8-20KHZ, but also operate at a predetermined speed and direction on the screen at the same time. At this time, any influence of the outside world may result in missing printing. Or the idea is off the normal position. For example, due to the influence of dust, air current, and static electricity, it may cause the direction of the flight of the point to shift. The result is a bit of sub-overprinting and a bit of sub-printing. Another example is the effect of the electron impact on the adjacent mouth and the sound wave of the ink jet on the leading nozzle. Influencing can also cause some nozzles to spray no ink dots, and the heating elements of the nozzles will fail. Electron corrosion of the nozzles will also affect some nozzles that cannot spray ink dots. TS uses the following technology to solve the missing print

1. Microwav Technology

This method uses multiple nozzles to print multiple times to complete the continuous area of ​​the printing, each nozzle to complete a single print. If some of the previous nozzles fail to print, the subsequent printing will silence them. Even if there is no printing on some places, it is difficult to find among the dots that have already been printed. This method uses 1 pass, 2 passes, 3 passes, 4 passes, etc. However, the imaging speed is greatly slowed down.

2. Spray coverage technology

The idea generated by this method is so dense that the idea does not appear at all. This method is slow in imaging and consumes a large amount of ink.

3. Turn off some nozzle technology

Some CTS system drivers allow the operator to select the number of nozzles or nozzle arrays to be used for the job so that some inks are not "excited". The operator can use this command to remove nozzles that cannot be printed. In operation, the CTS system can use the remaining nozzles to re-image the area where the nozzle was turned off.