Uses ink temperature and mesh size to reduce adjustment presses

Improper ink viscosities and mesh options are more likely to cause printer problems than you might expect.

One thing that has plagued me has been that when I visited the screen printing shop, I noticed that the press operators always had to adjust the equipment. They could barely leave the press. Any kind of adjustment seemed like only The role of a few minutes. And most of the adjustments have little effect, because they do not understand how the ink is transferred to the substrate through the mesh.

The choice of UV ink or hot melt ink requires different mesh numbers. Textile screen printing companies have more choices than image printing companies. However, there are at least three different kinds of meshes for each type. Textile printing prefers to use more meshes. We must ask ourselves, "Does it really need so much?"

Let us think about what factors cause the ink to flow at different speeds. The most basic factor is heat. Whether it is UV ink or hot melt ink, the temperature has a direct effect on the viscosity of the ink, and the screen printing machine generates a huge amount of heat energy applied to the platen. The heat directly acts on the screen, gradually increasing the temperature of the ink. Since both inks use a hot-melt resin as a base material, the viscosity gradually changes with increasing temperature. The control of the heat in the printing system is not a major issue and the adjustment of the printing press must be taken as the main consideration. The main problem is to control the effect of the mesh on the flow of ink on the substrate and how we can use the size of the mesh to balance this flow.

Feeling heat

Let's define the heat in a very simple way. When we inspect the printing system, we need to consider the range of heat change in the printing operation. This is a problem that most press operators never thought of. The lower the ink temperature, the thicker the ink layer. As the temperature of the ink rises, the viscosity continues to drop. This requires constant adjustments to keep the machine consistent with the increase in ink film thickness and dot gain. There is an equilibrium point at which the viscosity of the ink begins to stabilize and there is no significant change with increasing heat. Only when the ink begins to cure does the viscosity increase significantly, both for cross-linked UV inks and gel-type hot melt inks. When the ink flow becomes stable, you do not need to make any adjustments to the press.

Most inks reach this stage when the temperature reaches approximately 90°F (32°C). This is only in a broad sense, but it also basically conforms to the law. Therefore, it can basically be concluded that printing can be completed when the ink temperature reaches at least 90°F. In order to achieve this condition, it is necessary to use UV ink or rapid curing unit, and use non-contact IR pyrometer measurement. These temperature heat detection guns are priced at no less than $100 and are an important control tool in any workshop.

Mesh effect

With the stability of the ink viscosity, we must begin to pay attention to the control of the ink delivery process by the number of meshes and mesh size. Most printer operators understand the technical diagrams through the mesh manufacturer's instruction manual. These complex and detailed charts tell us how to choose and use meshes. The three most important factors that should be considered are: the number of meshes, the diameter of the network lines, and the open area ratio. The number of meshes is the detail standard that we want to reproduce. This is only part of the selection criteria. The more lines per inch, the better it is not necessarily for you. It depends on whether you want to do half-tone printing or line printing. In general, the more lines per inch, the more obstacles the mesh opening encounters. Therefore, increasing the number of meshes does not play a big role in detail reproduction.

This is true for polyester yarns made of a polymer resin, and this line also gives us the ability to use a very fine thread to make a mesh with higher tension. Interactive development can balance and promote our strengths. In this way we can first select a line of a particular diameter that has the ability to affect each mesh count and has a higher tension. In the past 10 or 15 years, printers have improved the accuracy by changing the mesh tension, which has significantly increased the release and production speed of the ink, and reduced the pressure of the off-set gap and the squeegee. These are very reasonable and good practices, but we are still subject to some constraints, that is how to effectively deal with the high tension mesh. Roughening generally causes mesh errors and suppresses the achievement of the highest printing intensity.

The increase in diameter of the cable can reduce the open area per inch. For any kind of mesh number, the larger the diameter of the wire, the lower the proportion of the opening area. Larger diameters help us achieve and maintain higher dynamic tensions and static print tensions. Because the smaller the opening, the more we need the higher strength to allow the ink to transfer to the substrate against the attraction of the web. There is an inverse relationship between the opening area and the mesh tension: the larger the opening area, the less tension is required to complete the ink release. From this it can be understood that we use larger mesh openings for smooth ink flow. In addition, the less tension is better for a given tension wire, so a lower tension is also needed.

A mesh made of very fine wire is now available as a new low-coefficient, high-molecular-weight polyester. These materials allow us to achieve and support excellent tension standards. This is our latest opportunity.

A tension that all screens have

The day-to-day work of screen printing workers is accomplished using a number of different mesh counts. In general, the strength of each screen is different, depending on the diameter of the thread and the number of meshes. The simplest way to simplify the mesh tension is to calculate the relative support index (RSI) of the mesh. We can calculate the intersection of network lines by the number of meshes to define the RSI and divide them by 10,000 units. This data will tell us the density relationship of polyester per inch of mesh. The higher the value, the higher the intensity we can stretch and reach. Although this has some limitations, the most important issue related to it is how to make thinner lines undamaged.

When you test through a series of mesh numbers and wire diameters, it quickly shows that many mesh RSIs support tensions in the 30 N/cm range. This is even true for fine meshes. So we can think we can make each of these mesh values ​​reach the same value. This is a very ideal conclusion, because it can easily reduce the stretch of the mesh. For example: Set the same value for all mesh counts in production. This means that we can set standards for off-gaps and stripping because we will use the same static tension for all meshes. The same is true of dynamic sheets on the surface of prints.

The next step is to determine the proportion of open area through a mesh with RSI values ​​that support our selected tension criteria. Through the manufacturer, you will find a number of meshes with a stable static tension and an open area ratio varying only between 1-3%. This means that a group of mesh numbers will have the same opening area and printing tension. Therefore we can conclude that the ink will pass through these meshes with almost the same resistance. Because the diameter of the wire increases as the diameter of the wire increases and the number of meshes decreases, an increase in the impedance force is caused.

As the mesh strength and opening area gradually become equal, we need to significantly reduce the adjustments made to the press. When we select the number of meshes, we only need to adjust the pressure of the squeegee. Using this method, we can adjust the stencil thickness to repair the loss of detail due to line overlap effects or interference. For example, in the past, for white clothes, we might use a very rough mesh, such as a 160 thread/inch mesh made of a wire with a diameter of 54 microns. But now that we are producing the same product, we can use a 305-line/inch mesh made of a 30-micron diameter wire, and it has a higher sensitivity. This is a good way to achieve smooth, clear halftone printing, while also achieving good ink coverage and high detail reproduction and sharpness.

In either case, the static tension of the mesh is in the range of 30 N/cm, and the proportion of the opening area is between 38 and 40%. A 305-line/inch mesh made of a 40-micron diameter wire or a 305-thread/inch mesh that is a 34-micron diameter wire has an open area of ​​31-36%. High-density polyester can bring higher strength and wider openings. This means less printing pressure, clearer details and higher printing speed. Of all these advantages, the most important one is that we have a simplified printing system that does not require many adjustments to reach the printer's settings and mesh strength.

Ink temperature is the key

Remember that you must keep the temperature of the ink so that its viscosity does not decrease with increasing temperature. A higher viscosity means a greater flow resistance, so that we must adjust the pressure, off-plate clearance, speed, and stripping. Stabilizing the ink before printing can reduce the need for these adjustments. The ink can be made to pass through the mesh at the same speed as the mesh strength, the number of meshes, and the diameter of the thread are balanced. If you want to use shorter lead times for faster printing and longer production times per day, you should try these techniques and prevent printing confusion.

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