Selection of screen frame

Screen frame is one of the important materials for making screen printing plates. Whether the screen frame is suitable or not has a direct impact on the quality of plate making and printing quality. 
In order to ensure the requirements of plate making, printing quality and other aspects, the screen frame can be selected according to the following conditions. 
1. Strong resistance to tension. The material used as the screen frame should have sufficient strength to withstand the tension of the screen, because when the screen is stretched, the screen will exert a certain tensile force on the screen frame, which requires the screen frame to have tensile strength. If the strength is not enough, the frame will bend If you bend it, it will be deformed, and you will not be able to print good prints. 
Usually, the frame surface of the screen frame is required to be flat, the four corners are stable, and the frame arms (strips) are straight. However, due to the tension of the wire mesh and the asymmetric connection of the wire mesh to the frame, there are three kinds of moments (Fig. 2-31), namely the bending moment B, the angular moment C and the torque T . Among them, the bending moment makes the frame bar bend toward the center of the plate; the angular distance makes the opposite corners move closer, and the frame corners warp; the torque makes the frame bar rotate around the center of its fear surface, trying to break away from the wire mesh. These moments lead to the deformation of the screen frame, and the magnitude of the deformation depends on the quality of the material and the strength of the structure.

2. It should be prestressed. After the mesh is stretched, the bending deformation of the mesh frame will affect the tension stability of the mesh. In order to reduce this effect, the mesh frame can be prestressed. There are two processing methods: one is to make the screen frame into a convex shape as shown in the left of Figure 2-32 according to the strength principle of the arched structure, the deflection is about 4mm/m, and each inner angle is slightly larger than 90°; The made metal frame is stretched into this shape with special tools, and this pre-deformation treatment can resist the influence of wire mesh tension. Another method is to do the pre-treatment while doing the pneumatic pulling net, that is, the front end of the net pulling device is tightly pressed against the outside of the frame, and the multiple frames are bent by the top force (right in Figure 2-32). When the net is installed, although the force surface of the screen frame moves from the outside to the top, the force direction and size are basically the same, so no more bending will be added. Due to these advantages, the pneumatic stretcher has become the most popular stretcher at home and abroad.

3. Sturdy and durable. The screen frame is often in contact with water and solvent during use, and is affected by temperature changes. When affected by these external solids, the screen frame is required to be sturdy and durable, and no oblique phenomenon occurs, so as to ensure the repeated use of the screen frame to reduce waste and reduce costs. 
4. Easy to operate. Under the condition of ensuring the strength, the screen frame should be selected as light as possible, which is easy to operate and use. 
5. Good adhesion. The bonding surface between the screen frame and the screen must have a certain roughness to strengthen the adhesion between the screen and the screen frame. 
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6. The size is right. Screen frames of different specifications should be configured in production, and the appropriate screen frame should be determined according to the size of the printing size, which can reduce waste and facilitate operation. 
The inner size of the screen frame should be larger than the graphic part of the printing plate, so that it is not only easier to print, but also the size of the printed matter is accurate, and the accuracy of the amount of ink penetration will also be improved. For example, when the size of the printing plate pattern is a×b, and the inner size of the frame is 2a×2b, sufficient printing can be achieved. 
The selection of the screen frame size is mainly based on the printing area. At the same time, the factors to be considered are: the needs of the start and end parts of the squeegee and the ink return plate during printing; the need to store ink on the plate; to ensure that the screen tension of the film part of the printing plate is uniform. Required; the need for the screen printing plate to spring back during the squeegee printing stroke. 
Usually, the screen frame used in printing is mostly with a frame surface of 1m2, and the screen frame larger than 1m2 is called a large screen frame. The large screen frame is generally made of wood, but the tension it bears is often not ideal, and it is easy to absorb water and deform, so it has begun to be replaced by aluminum alloy. In order to solve the problem of insufficient strength of large screen frames made of profiles with smaller cross-sectional areas, reinforced aluminum alloy profiles have appeared on the market, and their cross-sectional shapes are shown in Figure 2-30. This profile is used to make large screen frames with the advantages of light weight, high strength and no deformation.