Static and dynamic wire tension control techniques

The purpose of creating and maintaining a long-lasting high static tension is to overcome some of the changing factors in the dynamic process and the constant balance of ink film thickness. Static and dynamic tensions are closely related. With the increase of static tension, the dynamic tension is decreasing. . On the other hand, increasing the static tension can reduce the negative effects of changes in dynamic tension on the print surface. This is why people are particularly favored by high static tension. The printer discovered this through direct observation. They often call the static tension the ideal state in the screen, and it is also the core issue of the quality of the screen printing. In the center of the screen, the ink deposits are very clear, clean, accurate and flawless. When the image leaves the center of the screen, the precipitation of the ink becomes more and more difficult to control, especially for halftone images, and the dot enlargement phenomenon is the most serious in the region of the image near the edge of the frame.

The distance from the inner edge of the frame to the edge of the image is called the free mesh area of ​​the screen. The smaller the area without a mesh, the more difficult it is to control the image outside. Since the screen is fixed on the frame, there is no elasticity at the adhesion. The farther the squeegee is from the edge of the frame, the faster the tension changes.

Every change in the printing force in screen printing occurs, and the ink transfer speed also changes accordingly. The point at which the ink passes through the mouth of the mesh depends on the viscosity, viscosity, and rheology of the ink. If you want the ink to pass through the mesh, apply appropriate force to it to overcome the shear resistance of the ink. This force comes from the stored power and the pressure of the scraper when the screen is stretched. The tighter the wire mesh is, the more power it will store. Only when this power reaches and exceeds the ink's shear resistance, the ink will transfer. The balance between the two is called the yield point.

Another way of thinking about this problem is to think of the high mesh tension as a kind of resistance. When the squeegee puts pressure on it, the screen is stretched to create resistance. If the screen is taut, the squeegee will transfer the pressure to the mesh, increasing the power stored on the screen. When the resistance (stored power) reaches the yield point of the ink (yield value), the ink starts to transfer and achieves the desired printing effect.

In order to reduce the yield value, printers generally increase the clearance between the off-plate gap and the squeegee blade. This is the most common method for improving the print quality. In many cases, this approach works, because the level of detail and resolution of the image details are so low that people do not notice those additional pressures. Therefore, the printing companies believe that increasing the pressure of the squeegee and increasing the off-plan gap can improve the printing quality. However, being buried by these experiences is the exact value of pressure overcoming. The printer did not try to find this value by applying less pressure and off-gaps. Instead, it constantly increased the pressure on the squeegee and sometimes doubled before stopping. This greatly affects the quality of the production process and the product. It does not matter if the line is only produced.

Increase the screen tension. The company uses the scraper as a tensioning device to stretch the screen to the surface of the substrate. The greater the off-plate gap, the greater the force required. The printer may be able to get the results they want by this method, but this method is wrong.

The main problem is the uniformity of the pressure applied. The maximum inaccurate pressure for doing so is not uniform, and the smaller the static tension of the screen, the greater the off-plate spacing (grid distance) or squeegee pressure required to reach the ink yield point. More importantly, this will cause a greater change in the dynamic tension of the screen.

For example, if a screen is 30 x 40 inches in size, the static tension is 16 N/cm (static tension A). The length of the netless area between the edge of the frame and the edge of the image is 4 inches. The tension value is shown in Table 1. It can be found that the 32-inch image is too large for the 40-inch screen. If the off-gaps gap is 0.25 inches from the center of the screen, the dynamic tension on the print surface is 25 N/cm. Assuming a tension tolerance of ±2 N/cm, the screen's balance point is 4 inches. More importantly, as you move along the edge of the screen, you will see a noticeable change in tension. In this example, the tension of the edge reaches 60 N/cm. From the perspective of the screen, this value has exceeded the breaking strength of the screen.

In the second example, the same screen tension was 20 N/cm and the off-plate gap was reduced to 0.125 inches. The change in the tension in the image is still very obvious. At this time, the balance point is 16 inches wide, and the edge tension of the screen has been reduced to 45 N/cm, but since the surface tension of the screen has reached 20 N/cm, this design still cannot allow People accept.

In the last example, the same screen tension was 24 N/cm and the off-plate gap was 0.0625 in. The dynamic tension of the center is still 25 N/cm, but the change in the image has disappeared. The balance point at this time has also increased to 28 inches, and the amount of change in the image is only 2 N/cm.

There is another advantage to controlling dynamic tension by increasing the static tension in the screen: that is, he reduces the fatigue cycle, which is the total distance that the screen needs to move in contact with the substrate and then leave it. In the first example, the fatigue cycle is 0.5 inches, and the tension changes up to 44 N/cm in each cycle of the frame edge. In the third example, the fatigue cycle dropped to 0.125 in, and all tension cycles at the edge of the frame dropped to 6 N/cm. In this way, even if the screen lasts longer in production, it will be obvious.

Finally, when the initial tension is low and the clearance away from the plate is large, it will have a certain influence on the static mesh tension. Extreme fatigue cycles accelerate the relaxation of the frictional connection force at the mesh joints, ie, the frictional force generated by mesh weaving. When the mesh is tensioned, the friction at the joints increases with the interweaving of the threads, reducing the interweaving points at the joints and the slidability of the threads. This effect may reduce the static tension to only a few hundred sheets to 10-15 N/cm, which will further complicate issues such as registration, ink settling stability, and printing parameter settings.

With the increase of image resolution, especially when printing halftone images, the impact of dynamic tension changes on the integrity of the entire image is crucial. Halftone printing is based on the visual characteristics of the human eye. As a continuous tone image, this effect can be achieved by changing the dot size. If the dot size is not appropriate, the screen will be deformed according to that size to overcome the high off-grid gap, so the resulting printing effect is not ideal, these images may look too shallow, too dark or color distortion . The illusion arises from the halftone dot shape, ink layer thickness, and color change.

Uneven tension

A higher static tension can make the off-plan gap at the print surface smaller and reduce the amount of change. The stability of the dynamic tension on the print surface means faster printing, cleaner and cleaner products. Smaller squeegee pressures can also be used to reduce the squeegee friction, wear, and abrasion of mesh and stencils. In addition, the screen's resistance to printing will increase, and changes in images and color will also decrease. It is the people’s real purpose to protect the uniformity of dynamic tension as much as possible.

The stability of the tension is the key to high quality screen printing. To control the color, shape and printing plate, we must control all factors that influence the printing effect on the printing surface.


Source of information: CPNIC