Reasons and Resolutions of Screen Printing Network Expansion (3)

In order to control the dot enlargement, the ink volume must be controlled first. Minimizing the amount of undercutting can minimize the damaging effects of dot enlargement in halftone overlays. There is no doubt that the biggest cause of dot gain comes from the pressure imbalance that exists in the printing process itself. Part of the reason is that the system itself is inherently controllable to a large extent. In addition to non-impact printing processes (mostly inkjet), the transfer of ink from the stencil to the substrate requires a balanced transfer of various forces. For screen printing, all of the force's role is in the entire process of inking, full screen printing, applying pressure to make the ink pass through the screen, and finally attaching the ink to the substrate. In this process, the interaction between the squeegee and the wire mesh balances these forces. These forces determine when and how the ink flows. In order to minimize the proliferation of outlets, you must know how to balance and control these forces. Otherwise, the effect of printing will be unimaginable. The tension of the screen itself and the pressure applied to the screen are not uniform at different locations throughout the screen. The tension of the fixed frame can be controlled within the range of 1 Newton/cm2, but the tension is different everywhere on the surface of the substrate. The uneven tension on the surface of the substrate is directly related to the area of ​​the screen opening, the screen tension, the net spacing, and the rebound force of the net. Due to the uneven tension on the surface of the substrate, the requirements for the ink pressure at different positions are also different, and thus the flow rate of the ink is also different. As a result, the ink has been pressed through the screen to form a dot before being exposed to the substrate.

As long as we continue to use chemical fiber screens, the debate about screen tension will be endless. I personally have an important point about tension, which is different from 95% of the world's current counterparts. That is, it can have good printing effect under any screen tension, except that it is the productivity and the net pass rate. Quality is acceptable within a wide range. This article only deals with tensions greater than 20 newtons/cm2. There is one difference that needs to be considered. The tension of the screen is a static capability. Once we add the squeegee pressure and the net distance, the static tension becomes a dynamic tension. After setting various printing parameters, there is a particularly accurate dynamic tension value or a very different dynamic tension value. The most influence on the flow of ink and the amount of under ink is the difference between the static tension and the dynamic tension (Δt) and the non-uniformity of the dynamic tension. We want to keep the dynamic tension range within 2 newtons/cm2.

When the screen has a static tension of 22 N/cm2 and a screen pitch of 2 mm, the dynamic tension (Δt) is stable between 4 and 8 newtons (ie, the printing tension is 26-30 N/cm2). The pressure on the outer edge of the screen is relatively high. If the screen pitch is adjusted to 4.5 mm, the Δt value increases to 15 to 80 N/cm2. A simple pitch adjustment makes a great change in tension. This is one of the reasons why many printers are accustomed to increasing the squeegee pressure and increasing the net pitch in order to improve the printing quality. The higher the mesh distance, the greater the screen tension and the increased pressure of the squeegee, which also stretches the screen and increases the tension.

In order to understand the impact of various factors on the expansion of outlets, we have established a continuously changing tension distribution. For any ink, the ink's cut point is determined by the fluidity of the ink, the tension of the screen, and the pressure of the squeegee. At the cut point, the ink is squeezed out and dropped on the back of the screen. The ink that cannot be printed on the substrate after each printing builds up more and more, thus causing the dot to enlarge. The more ink, the more serious the dot expansion. At this time, the printer will find that the print quality of the first two or three times is OK, and the dot expansion problem will begin to appear in the fourth printing. By the sixth time, the screen printing needs to be erased.

The reading of the sample is read with half-tone lines between 65 and 100 lines. Most full-tone printed substrates have a surface tension of 28 to 35 N/cm2. The more viscous the ink, the wider the reproducible range and the greater the required tension. High-viscosity inks have less expansion than inks with lower viscosity. The higher the screen tension, the easier it is to control the dot gain. The larger Δt, the worse the tension uniformity at the squeegee blade tip, and the ink becomes thin, resulting in dot gain.

Another problem is the stability of the tension during printing. The higher the mesh distance, the greater the damage to the screen and the worse the tension stability. When the distance is 2.5mm, the fatigue cycle is 5mm; when the distance is 1mm, the fatigue cycle is 2mm. A fatigue cycle is a print cycle. This is like a paper clip bending forward and backward, after bending material deterioration, or even broken. Under the effect of pressure, the polyester filaments "cold flow," resulting in a drop in tension. The higher the pitch, the faster the tension drops.

Because of the presence of clips on the frame, the net distance is always a problem. The minimum mesh spacing is limited by the thickness of the frame clamp, and the thickness of the mesh clamp is rarely less than 3mm. It is difficult to control the expansion of the mesh point at this mesh pitch. There is a direct relationship between the percentage of network expansion and the network distance. Because of the increased network distance, the percentage of network expansion will also increase.

Another related to the netting distance is the peeling force of the screen, that is, the screen and the substrate are separated by the external force. As the squeegee moves toward the end of the screen, the screen springs back from the print screen and the substrate. The problem is that when printing to the end of the printing process, the net pitch increases, causing different tensions before and after the printing process. In order to minimize this difference in tension, we have adopted a method of pressurizing the end frame. When you increase the peeling force during screen printing, the screen is detached from the surface of the substrate, which helps to maintain the consistency of the screen and the consistency of the screen tension.

In addition, there is a relationship that is often overlooked. The parallel relationship between the screen and the squeegee must be kept in a good parallel to ensure the uniform consistency between the ink squeegee and the squeegee pressure. To check whether the parallel, the operator can adjust the ink return knife stroke, so that the ink return knife only scratches the screen, if the screen is parallel to the ink knife holder, the ink layer covering the screen must be uniform from front to back. Otherwise, it is easy to judge from the thickness of the ink layer whether the screen and the scraper are parallel. The result of inhomogeneous ink recoating is that the pressure around the part of the graphics is too large, the ink is squeezed out of the screen prematurely, and dot gaining occurs, which seriously affects the consistency of printing. If the screen is scrubbed after every six prints, it is easy to see that the ink has been prematurely squeezed out of the mesh.
(to be continued)