Screen printing pressure uniformity control
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In screen printing, suitable and uniform printing pressure is an important guarantee for obtaining high quality screen printing. If the printing pressure is too small, the printing plate can not be printed without contacting the substrate, resulting in incomplete printing, uneven thickness of the ink, etc. When the printing pressure is too large, the deformation of the screen will be too large, and the printing pattern will be blurred. Printing problems such as screen and squeegee wear are severe. If the printing pressure is not uniform, more print defects will occur. Therefore, controlling the factors affecting the printing pressure and improving the printing pressure unevenness become the primary problem to be solved by the screen printing process. There are many factors affecting the uniformity of screen printing pressure. This paper analyzes the influence of screen resilience, mesh distance, frame deformation and screen printing machine on the uniformity of printing pressure, and explores the technology to improve the uniformity of printing pressure. Measures.
1. Influence of screen resilience on printing pressure uniformity
The printing pressure of screen printing is a result of the combined action of the squeegee pressure applied by the squeegee, the resilience generated by the deformation of the screen, and the adhesion of the ink. In screen printing, due to the existence of the mesh distance, the screen will produce elongation deformation under the action of the doctor blade pressure. The deformation of the screen is different depending on the position of the doctor blade. The resulting screen resilience is also different, under the same scraping pressure, the printing pressure will appear uneven. For a flat screen printing plate, the deformation of the wire mesh can be divided into deformation along the stroke direction of the doctor blade and deformation in the longitudinal direction of the blade.
As shown in Figure 1, the deformation of the squeegee is in the direction of motion, including the fixed distance of the mesh and the variable of the mesh distance.
Assuming that the mesh elasticity follows Hooke's law and the screen tension coefficient is, the formula of the screen tension in the stroke direction of the doctor blade can be expressed as:
Fixed distance:
Network distance variable:
The resilience of the screen is the component of the vertical and vertical tension of the screen on the left and right sides of the squeegee. From the above expression, when the squeegee moves on the screen, the deformation of the screen will follow the position of the squeegee. The difference in the resulting resilience of the screen is constantly changing. When the squeegee applies a constant squeegee pressure to the screen during the squeegee process, the screen printing pressure will follow the position of the squeegee. The change is different, which is the uneven printing pressure.
As shown in Figure 2, the deformation of the squeegee in the longitudinal direction, whether it is fixed mesh or variable mesh, the deformation of the wire mesh is the same, the formula of the wire mesh tension can be expressed as:
It can be seen from the above expression that when the squeegee is biased toward the side of the frame along the length direction, the amount of deformation on the left and right sides of the screen is different, and the generated return force is also different, resulting in uneven printing pressure along the length of the squeegee.
In summary, the uneven printing pressure along the stroke direction and length direction of the squeegee is different. If the compensation is performed for the two kinds of pressure unevenness, the squeegee can be placed on the printing plate along the length of the squeegee. The central position ensures that the distance between the sides of the squeegee and the sides of the screen is equal, so that the printing pressure along the length of the squeegee can be kept uniform; in the direction of the squeegee, the printing pressure and the position of the squeegee need to be calculated. Relationship, and then take corresponding measures to compensate.
2. Influence of mesh distance on printing pressure uniformity
The mesh distance is determined by the characteristics of the screen printing itself. Due to the difference in the mesh distance, the influence on the uniformity of the screen printing pressure is also different. According to the formula of the screen tension introduced in the previous section, the larger the mesh distance, the larger the range of the screen deformation amount, which will increase the range of the screen printing pressure along the stroke direction of the blade, thereby increasing Unevenness of printing pressure. However, if the mesh distance is too small, the separation between the screen and the substrate will be unsatisfactory, so that the screen cannot rebound immediately and affect the image clarity. Therefore, in order to improve the uniformity of the screen printing pressure, a smaller mesh distance should be selected as much as possible while ensuring that the screen can rebound immediately.
3. The effect of ink viscosity on printing pressure
When printing a batch of screen printing products, a part of the solvent in the ink will volatilize over time, making the print quality of the same batch inconsistent, affecting the uniformity of the printed product. In general, the ink required for the same batch of printed products needs to be matched at one time, and the viscosity of the ink should be adjusted in time during the printing process to ensure the uniformity of the printed product.
4. The influence of the frame on the uniformity of printing pressure
The screen is tight on the frame, and due to the tension of the screen and the asymmetric connection of the screen to the frame, the frame is deformed, resulting in uneven tension on the screen, which is detrimental to the uniformity of printing pressure. influences.
In the printing process, in order to minimize the deformation of the frame after the mesh is stretched, the frame can be pre-deformed. There are two methods for pretreatment: one is to make the frame into a convex shape according to the strength principle of the arch structure, as shown in Figure 3 (a), or to stretch the frame that has been made with special tools. In this shape; the second is to pre-deform at the same time as the stretcher, that is, the frame is pressed by the front end baffle of the puller, so that the frame is deformed as shown in Fig. 3(b), after the fixed net, the frame The magnitude and direction of the forces are basically the same. After the pre-deformed frame, after the printing plate is formed, the tension distribution on the printing plate is uniform, and the uniformity of the printing pressure can be improved in the printing process.
5. Influence of screen printing machine on printing pressure uniformity
The machine factors that affect the uniformity of screen printing pressure are various, mainly including the parallelism of the guide rail and the uniformity of the force applied by the doctor blade assembly.
5.1 The parallelism of the guide rails. In screen printers, the guide rails play a key role in printing pressure uniformity. Generally, the screen printing machine has two cylindrical or rectangular parallelepiped guide rails, and the squeegee assembly slides back and forth on the guide rail through the slider to realize the squeegee movement, and the parallelism of the guide rail and the printing platen is the uniformity of the application of the squeegee pressure. It is guaranteed that if the guide rail is tilted, the squeegee pressure applied by the squeegee assembly will be changed, resulting in uneven printing pressure. Increasing the parallelism of the guide rails allows the guide rails to be measured by the parallel gauge when the guide rails are installed, and is continuously tuned to minimize the influence of the parallelism of the guide rails on print uniformity.
5.2 Squeegee assembly force uniformity. The squeegee is the urging mechanism for the squeegee pressure, ensuring the uniformity of the squeegee force and the straightness of the squeegee surface during work are the key to ensuring uniform printing pressure. If the squeegee wears, it will affect the uniformity of the printing pressure, so it is necessary to check the squeegee surface wear frequently and scratch it in time. The uniformity of the squeegee pressure of the screen printing machine can be ensured by the structure of the squeegee assembly. At present, the screen printing machine often uses two cylinders of the same specification to control the squeegee, as shown in FIG. The two cylinders are supplied by the same gas source, so the forces acting on the two ends of the doctor blade are equal. In the whole printing process, the cylinder pressure is kept constant to ensure uniform and uniform printing pressure.
The problem of uniformity of screen printing pressure is a comprehensive problem. In addition to printed materials and printing machinery, there are also human factors and environmental factors, such as the technical level of operators, operating methods, and work attitudes. This requires continuously improving the technical level of the operators, accumulating operational experience, and reducing the influence of human factors on the uneven printing pressure. In addition, keeping the environment clean and stable, temperature, humidity, voltage and pressure are also the key to ensuring uniform printing pressure. In the actual printing process, it is necessary to control the main factors that adversely affect the printing pressure uniformity according to the printed products, so as to effectively improve the quality of the screen printing.