Screen printing pressure uniformity control

In screen printing, proper and uniform printing pressure is an important guarantee for obtaining high quality screen printing. If the printing pressure is too low, the printing plate cannot touch the substrate and cannot be printed, resulting in incomplete printing and graphics, uneven ink thickness, etc. When the printing pressure is too high, the screen deformation will be too large and the printing image will be blurred. Screen and squeegee wear serious print failures. If the printing pressure is not uniform, more print failures will occur. Therefore, controlling the factors influencing the printing pressure and improving the printing pressure non-uniformity become the most important issues to be solved in the screen printing process. There are many factors that affect the uniformity of screen printing pressure. This article focuses on the impact of screen resilience, screen spacing, screen frame deformation, and screen printing machines on the uniformity of printing pressure, and explores techniques for improving the uniformity of printing pressure. Measures.

1. Effect of screen resilience on printing pressure uniformity

The screen printing printing pressure is the result of the combination of the squeegee pressure applied by the squeegee, the resilience generated by the screen deformation, and the ink tack. In the screen printing, due to the presence of the mesh distance, under the effect of the squeegee pressure, the wire mesh will have elongation and deformation. With the different positions of the squeegee, the deformation amount of the wire mesh is also different due to the deformation of the wire mesh. The resulting web resilience is also different. Under the same squeegee pressure, the printing pressure will appear uneven. For flat screen printing plates, the deformation of the screen can be divided into deformation in the direction of the stroke of the doctor blade and deformation in the direction of the length of the doctor blade.

Assuming that the screen elasticity obeys Hooke's law, and the screen tension coefficient is , the formula of the screen tension in the stroke direction of the squeegee can be expressed as a fixed pitch type and a variable pitch type.

The screen resilience is the component force of the screen tension in the vertical direction on the left and right sides of the squeegee. It can be known from the above expression that when the squeegee moves on the screen, the deformation of the screen will follow the squeegee position. Different and different, the resultant wire resilience also changes constantly. When the squeegee is applying a constant squeegee pressure to the screen during the squeegee process, the screen printing pressure will follow the position of the squeegee. Different changes, that is, the phenomenon of uneven printing pressure.

In the deformation of the squeegee in the length direction, whether it is a fixed net distance or a variable net distance, the deformation of the screen is the same. The formula of the screen tension can be expressed as:

It can be seen from the above expression that when the squeegee is biased toward the side of the screen frame along the length direction, the deformation amounts of the left and right sides of the screen are different, and the generated back force is also different, resulting in uneven printing pressure along the length of the squeegee. .

In summary, the phenomenon of uneven printing pressure along the stroke direction and the length direction of the scraper is different. If compensation work is performed for the unevenness of the two types of pressure, the scraper can be placed on the printing plate along the length of the scraper. In the central position, ensuring that the distance between the two sides of the scraper and the two sides of the screen is equal, the printing pressure along the longitudinal direction of the scraper can be kept uniform; and in the direction of the scraper movement, the printing pressure and the position of the doctor blade need to be calculated. Relationship, and then take appropriate measures to compensate.

2. The effect of network spacing on the uniformity of printing pressure

The net distance is determined by the characteristics of the screen printing itself. Because of the different net distances, the influence on the uniformity of the screen printing pressure is also different. From the formula of the screen tension introduced in the previous section, it can be seen that the larger the screen distance is, the larger the screen deformation amount is. This will increase the range of screen printing pressure along the stroke direction of the squeegee. Non-uniformity of printing pressure. However, the small net distance will cause the separation between the screen and the substrate to be unsatisfactory, which will prevent the screen from rebounding immediately and affect the image clarity. Therefore, in order to improve the uniformity of the screen printing pressure, a smaller distance should be used as far as possible to ensure that the screen can rebound immediately.

3. Effect of ink viscosity on printing pressure

When printing a batch of screen printing products, part of the solvents in the ink will volatilize with time, making the colors of the same batch of prints inconsistent and affecting the uniformity of the printed product. In general, the inks needed for the same batch of printed products need to be fully dispensed, and the viscosity of the ink should be adjusted in the printing process to ensure the uniformity of the printed products.

4. The influence of the frame on the uniformity of the printing pressure

The screen is tight on the frame. Due to the effect of the screen tension and the asymmetrical connection of the screen to the frame, the frame is deformed, resulting in uneven tension on the screen and adversely affecting the printing pressure uniformity. influences.

In the printing process, in order to minimize the deformation of the frame after stretching, the frame can be pre-distorted. There are two ways to pretreat: First, according to the strength principle of the arched structure, the net frame is made into a convex shape.

5. Effect of Screen Printing Machine on Printing Pressure Uniformity

The machine factors that affect the uniformity of the screen printing pressure are many, mainly the parallelism of the guide rail and the uniformity of the force applied by the squeegee blade assembly.

1 The parallelism of the guide rails. In screen printing machines, the guide rails play a key role in printing pressure uniformity. Generally, the screen printing machine has two cylindrical or cuboid type guide rails. The squeegee blade assembly slides back and forth on the guide rail to realize the squeegee movement. The parallelism between the guide rail and the printing platen is to apply the squeegee pressure uniformity. Ensure that if the guide rail tilts, it will cause the squeegee pressure applied by the squeegee assembly to change, causing uneven printing pressure. Improve the parallelism of the guide rails. When the guide rails are installed, the parallel guides are used to measure the guide rails, and the adjustments are continuously performed so that the parallelism of the guide rails minimizes the influence on print uniformity.

2 Squeegee assembly exerts evenness. The squeegee is a squeegee pressure squeezing mechanism. It ensures that the uniformity of the squeegee's force and the straightness of the squeegee's surface are the key to ensure the uniform printing pressure. If the squeegee blade is worn, it will affect the uniformity of the printing pressure, so it is necessary to constantly check the squeegee blade surface wear and timely wear and scrape. The uniformity of the squeegee pressure exerted by the screen printer can be ensured by the structure of the squeegee assembly. At present, screen printing machines often use two cylinders of the same size to control the squeegee.

The problem of the uniformity of screen printing pressure is a comprehensive problem. Apart from printed materials and printing machinery, there are also human factors and environmental factors, such as the operator’s skill level, operation method, and working attitude. This requires continuously improving the technical level of operators, accumulating operating experience, and reducing the influence of man-made factors on the uneven printing pressure. In addition, keeping the environment clean and stable, the stability of temperature, humidity, voltage, and air pressure is also the key to guaranteeing uniform printing pressure. In the actual printing process, it is necessary to control the main factors that adversely affect the uniformity of the printing pressure according to the difference in the printed products, so as to effectively improve the quality of the screen printing.