Possible Causes for Screen Unsatisfactory Printing

All screen printing plates will be damaged sooner or later. This is the characteristic of the network version. However, if the screen is always damaged in advance, it is likely that there is a problem with the screen: Perhaps the underexposure of the screen, may not be the performance of photosensitive adhesive and printing ink, such as the reason.

There are many factors affecting the durability of the screen. Grasping the control of these factors and some of the impact of printing parameters will greatly extend the life of the screen.

How to choose the type of screen

There are two main types of screens, namely screens made with photopolymer and screens made with capillary photo films. The photosensitizers used in all screens are the three types of diazo, diazo-photopolymer or single-component nonphotosensitizers. These screens are all sensitive to ultraviolet light before exposure and will polymerize and cure after irradiation with ultraviolet light.

Each type of screen has a different performance of photosensitive or capillary photosensitive film to meet the needs of different printing occasions, according to the type of printing needs to select the appropriate type of screen to ensure the normal life of the screen.

How to make a good screen

When choosing the type of screen, the first thing to consider is the screen. Regarding the screen, it is important to consider the cleaning of the screen in addition to the tension to be discussed later. Before the screen is coated with sensitized adhesive or affixed with a capillary film, ghosts and degreasing must be removed, and must then be thoroughly rinsed with water.
Some manufacturers use matte to roughen the printing surface of the screen to increase the bonding force between the screen and the photoresist. Moisturizers are sometimes used to increase the surface tension of the screen.

If photosensitive adhesives are used to make screens, the thickness of the coating is important. The thicker the photoresist, the longer the exposure time. Thicker screens are more likely to cause underexposure problems, and underexposure is an important cause of reduced screen life. Thick-film screens are used more often in the electronics industry and in the textile printing industry. In general, the coating thickness of the photosensitive adhesive does not exceed 20% of the thickness of the screen.

In addition, it should be noted that the photosensitive adhesive coating must be uniform, otherwise, a relatively thick portion of the photosensitive adhesive will cause underexposure, and the screen will not be durable. Of course, the above problem does not exist if a photosensitive photosensitive film plate is used.

Film

The blackness of dots or patterns on the film also affects the life of the screen. The blackness value (Dmax) of the film is generally required to be 4.0 or higher so that the UV light can be completely blocked. Blackness of negatives made with ink jet or laser printers is usually not sufficient.

Exposure time

Underexposure is the number one reason why screens are not durable. Underexposed screens are the most susceptible to solvent attack. Humidity in the working environment can also erode the screens, and the screens are not resistant to mechanical wear.

The factors affecting the exposure quality include the quality of the photosensitive film (capillary film), the screen performance, the humidity of the screen before exposure, the performance of the light source, and the distance between the light source and the screen, etc. It should be developed according to the actual situation. parameter.

Humidity in the air is also an important reason for the poor durability of screens. If the humidity of the screen is high before the exposure, the exposure of the screen can not be fully exposed no matter how long the exposure time is. The method of controlling the screen plate humidity is to maintain a constant humidity (45% to 55%) in the screen making environment and the drying temperature (35 to 40C) of the screen plate.

Many screen printers often do not adequately artificially expose screen resolutions. For example, when screens are made of white screens, the refraction of light will reduce the resolution. This method is used to compensate. Of course, using this method to make out the screen will certainly not wear the problem. Some people will use the method of re-exposure to make up for the above-mentioned shortcomings, that is, expose the exposed screen plate again or expose it to the sun for a while.

So how to ensure the correct exposure of the screen? The first thing to do is to ensure that the equipment performs well and that the luminous efficiency of the light source is checked regularly. Set the appropriate distance between the screen and the light source. If the distance between the screen and the light source is too short, it is impossible to expose at all. If the distance is too large, the exposure time is too long. The more appropriate distance is the length of the screen diagonal.

The method for determining the correct exposure time is to use an exposure scale, which can be purchased in the market. There are different transmittance percentage filters on the slide rule.

The Exposure Scale shows an exposure time that is 2 times the expected accurate exposure time. If a diazo-type photosensitive adhesive or capillary photosensitive film is used, the optimal exposure time should be the time corresponding to the pattern without color change; if a single-component photosensitive adhesive is used, the halftone pattern can be calculated from the ruler. Grayscale transitions and other character line copy performance determine the best exposure time.

Screen cleaning

The cost of the screen cleaning agent is not high, but do not use household detergents. This type of detergent may contain components that damage the screen. All kinds of photosensitive adhesive suppliers will provide corresponding screen degreasers (detergents), some are solvent-based, and some are biodegradable.

Other factors

The tension of the screen should be considered when making the plate. However, the effect of tension on the life of the screen will only be exhibited at the time of printing. If the tension of the screen is too low, the screen can easily be debonded in a continuous stretch. In addition, in order to enable the screen to quickly rebound, had to increase the distance between the network, and therefore require the pressure of the squeegee also increased accordingly, which is undoubtedly more likely to damage the screen, but also to deform the pattern.

Friction of the doctor blade and the printing substrate on the screen plate also affects the life of the screen plate. In order to reduce the wear of the blade, the two ends of the blade can be rounded and the stroke of the blade can be shortened. Some printing substrates, such as ceramic tiles, themselves have a lot of abrasiveness to the screens. Therefore, special plate-making materials must be used when making such screens.
In short, many parameters of the screen are determined during pattern design, such as the number of screen meshes, the type of photoresist, the type of printing ink, and the like. If these parameters are still untouched before exposure, the screens produced will definitely be a problem.

Author: Shanghai Yarn Screen Printing Equipment Co., Ltd.