Reasons and Solutions for Expanding Screen Printing Dots (Part Two)

Printing pressure

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.

After determining the parallel relationship between the screen and the squeegee, the next issue to pay attention to is the ink-jet knife and stroke. The conventional ink-returning knife should have a reversed angle with the squeegee, which helps the ink scraping back to the place where the printing began. At present, there is a new type of ink-returning knife in the U.S. market, which utilizes the principle of hydraulic pressure and features ink from the wire. Instead of squeezing the ink out like a regular ink-jet knife, the ink is ejected from the mesh hole, and the hydraulic progress of the ink drop is also controlled. This new type of ink-returning knife works well and requires little pressure.

The focus of squeegee pressure is shear force instead of grinding ink. All the squeegee pressure must fall directly on the edge of the sharp squeegee. This is like if you put a hose on the garden watering hose. Let the pressure increase. It is not necessary to increase the squeegee pressure to the point of bending at all. Even if it is added to the bend, it is ineffective because it does not cut the ink.

The pressure needed to cut the ink is very small. Any pressure that exceeds this pressure value should cut the ink on the one hand and on the other hand it should strengthen the bond between the ink and the substrate. This further pulls the ink out of the mesh instead of pressing it out. The greater the pressure, the greater the possibility of ink diffusion. The sharper the squeegee, the higher the shear efficiency. General dot gains can often be resolved by sharpening the squeegee. Including the new squeegee also needs to grind. I think even a new squeegee is difficult to install to absolute uniform contact.

After other printing conditions are fixed, the ink parameters must be determined.

If no ink is selected, good printing results cannot be obtained. There are two points worth considering: The first point is the thixotropy of ink. The static viscosity of the ink should be significantly different from the viscosity of the ink at the time of printing. If the difference is large enough, the ink's adaptability and fluidity will be good.

The second point is the ink recovery rate. To obtain a good, clear halftone dot, the ink must also have a faster recovery rate. The so-called recovery rate refers to the time after the ink is printed on the surface of the substrate, the shear force is removed, and the viscosity of the ink returns to the static viscosity. If the rate is too low, the dots will spread slowly on the surface of the substrate. This is particularly the case when UV inks are used to print third and fourth colors. In order to avoid this phenomenon, the ink recovery rate should be faster. How to check the recovery rate? First stir the ink with a stainless steel blade, and then remove the mixing knife out of the ink tank, let the ink adhering to the blade flow naturally from the mixing knife, the ideal half-tone ink will stop flowing after 10 seconds, and at the same time Under the formation of 28.5 ~ 32mm ink line. If the flow length is shorter than this value, the ink is too thin. It is easy to cause dot expansion problems. If it is longer than 32mm, it means the ink is too sticky and it is not easy to print fine images with poor accuracy.

The next problem is the printing sequence during four-color printing. In the United States, the usual printing sequence is: magenta, yellow, cyan, black, or cyan, yellow, magenta, and black. There are many problems with these two printing sequences. Users in the United States prefer to use medium coverage first halftones and then print high coverage yellow. The third uses a medium coverage ink, and finally uses a low coverage of black. In this order, it often happens that you draw a map. For example, with UV inks, when the third color is printed, a moire phenomenon occurs. The moire is due to the fact that the dot cannot fully conform to the ink layer on the surface of the substrate. A portion of the ink at the dots is printed on the upper ink layer, and the remaining ink remains on the back of the screen, resulting in an increase in the amount of ink in some dots when the next layer is printed, resulting in a severe dot gain.

To minimize this kind of network expansion, I suggest using the order: black, cyan, magenta, and black. But make sure that you are using transparent yellow. Transparent yellow is more expensive, while translucent pigments are less expensive. If the latter is adopted, the color of the printed product will be yellowish and the effect will not be very good.

Finally, consider the substrate itself

Substrate surface tension and adsorption are two important factors. The surface tension determines the degree of wetting on the surface of the substrate. Be sure to choose a substrate that matches the ink rubber, which is especially important for plastic materials. Surface tension not only affects the ability of the ink to bond on the surface of the substrate, but also affects the flow of ink on the surface of the substrate. If you choose a plastic material, consult your ink supplier for suitable additives that improve ink flow and adhesion. In order for the ink to flow properly, the operator must weigh the effects of various factors on the print quality and may have to reduce some requirements in the thixotropic properties.

Adsorption of the substrate material may also have an adverse effect on halftone dot printing. In particular, materials such as textile surfaces and paper surfaces that have not been surface-coated are more troublesome. When printing textiles, there is a special white coating that can largely eliminate the problem of dot gain due to adsorptivity. This white coating is not reflective, but the tone is obvious, and the color separation of the fabric must be performed with a specialized color separation instrument. It is especially recommended to print a white coating on the fabric, which will reduce the dot gain by 50%.

Dot enlargement on paper that is not surface treated can be controlled by halftone linearization (changing the dot size on the film), or it can be controlled by a solid seal coating. This coating can be matte or white. Add a layer of white solid seal coating to help discern detailed differences in graphics and reduce the spread of displacement