Screen printing color change
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The most difficult part is often to answer questions that seem simple. Some beginners often ask why the color changes when printing. Why is it so difficult for a printer to keep the color of one job exactly the same as another? The only answer is to use careful process control and automatic adjustment techniques to minimize color variations. This is a suitable answer when you are just beginning to work on screen printing and know nothing about the process. But from the point of view of an experienced production printer, they don't necessarily understand the color change.
If you print a simple line job, this is monochrome printing, you can not consider the four-color mesh tone at this time, this answer is reasonable within a certain range. Specifically, the main factors considered should be the following.
Ink is not well prepared
If, assuming that the pigments in the ink are properly formulated, then the usual cause of the color change is the added solvent. In a well-controlled workshop, the ink should be ready to be supplied to the press at any time, that is, the printer should not dispense ink. In many companies, the ink is not adjusted and supplied to the press, but left to the printer to adjust and add and mix the ink according to their own feelings. As a result, the pigment balance in the ink is broken. For water-based common inks or UV inks, the water in the ink acts in the same way as the solvent in the solvent ink. Adding water can thin the dried ink film and affect the color of the ink, thereby reducing the density of the color. . The reasons for such problems can be further traced. In the ink warehouse, the ink-filling worker does not use the weighing device, but only relies on his own judgment to add an appropriate amount of solvent, or the mixing is not suitable at the beginning, or the amount of ink changed during printing, so that the mixed ink will Produce different colors. This situation becomes worse when the job is printed again later, and unless there is enough ink ratio recording, it is almost impossible to reproduce a color.
Screen selection is not suitable
Any screen printer who finds that the mesh size is only a variable of the screen structure will only affect the deposition of the printing ink, then he will have trouble. The mesh diameter of the screen and the manner of weaving, that is, plain or twill, have a great influence on the thickness of the printed ink film. The screen supplier will provide detailed technical information on the screen, the most important theoretical ink volume, which indicates the amount of ink passing through the screen mesh under certain printing conditions, generally expressed in cm3/m2. For example, a 150 mesh/cm screen with a mesh diameter of 31 μm will be able to pass 11 cm3/m2 of ink. A wire mesh having a mesh diameter of 34 μm and 150 mesh passes through 6 cm 3 of ink per square meter, which corresponds to a wet ink layer of 11 and 6 μm thick. It can be seen that the simple representation of 150 mesh will give you a distinctly different thickness of the ink layer, which will result in a large difference in color.
With the improvement of the wire mesh weaving technology, it is possible to obtain a certain number of twill weaves instead of plain weaves, although this is sometimes possible, but the possibility is small. Sometimes the wire mesh supplier stores some old twill screens. In general, the theoretical ink volume of these screens varies by 10%. If you use a twill weave screen to print a fine-grained image, the fine lines break more than the plain weave.
Screen tension
The lower tension of the screen will cause the screen to detach from the printed surface slowly, which will affect the ink staying on the screen and cause uneven color, so that the color changes. To solve this problem, it is necessary to increase the mesh distance by increasing the distance between the horizontally placed screen plate and the printing material. Increasing the mesh distance means increasing the pressure of the squeegee, which will affect the amount of ink passing through the screen, which will cause further changes in color.
Squeegee setting
We are increasingly aware of how the squeegee works, and recent research has shown that there are some interesting phenomena in the squeegee in the printing operation, which are described in detail later. Based on the current state of the art, the softer the squeegee used, the more ink will pass through the screen. The greater the pressure acting on the squeegee, the faster the squeegee will be worn during printing, which will change the point of contact between the squeegee and the print, which will also change the amount of ink passing through the screen. Causes a color change. Changing the angle of the squeegee also affects the amount of ink attached. If the doctor blade is running too fast, this will reduce the thickness of the attached ink layer.
Ink knife setting
You will be surprised to find that many screen printers do not understand that the setting of the ink return knife will have a significant impact on the amount of ink attached. The function of the ink repellent is to fill the screen aperture with a stable amount of ink. Adjusting the pressure, angle, and sharpness of the knife back will cause the mesh to be too much or too little. Excessive pressure on the ink return knives forces the ink to pass through the mesh, causing excessive ink adhesion. The pressure of the ink returning knife is insufficient, which causes the mesh to be partially filled with ink, resulting in insufficient ink adhesion. The speed of the ink return knife is also very important. If it runs too slowly, the ink will overflow. If it runs too fast, it will cause serious ink shortage, which is similar to the effect of changing the speed of the blade.
Machine setting
Serious process control is the biggest key factor. The stable, consistent adjustment of the machine means that the color is stable and consistent. If the machine adjustments change, the color will lose control. This problem usually occurs when the printer is changing jobs, or later the printer is free to change the settings on the press to adapt to his own habits, which will cause color changes. The latest multi-color screen printing machine uses computer automatic control to eliminate this possibility. These stable, consistent settings are to be made to the press and these settings remain the same throughout the print job.
Influence of printing materials
In the screen printing industry, the often overlooked aspect is the consistency of the printed materials to be printed. Paper, paperboard and plastic used in printing are generally produced in batches. A high-quality supplier can guarantee that the entire batch of materials it provides has good surface smoothness, but things are not always the same. In the processing of these materials, any slight change in the process will change the color of the material and Surface finish. Once this happens, the printed color will look different, although the actual printing process has not changed anything. When we want to print the same pattern on a variety of materials, from corrugated plastic sheets to fine art cardboard, the printers will encounter these practical difficulties. Another problem that is often encountered is that our screen printing has to catch up with the offset image. If we don't pay attention to the process control, we have no chance. Careful process control includes accurate color measurement, the use of a spectrophotometer to determine the color of the line, and the use of a densitometer to measure the three primary colors, so that we can print a consistent color image on a variety of materials. If we also use traditional intuitive methods, the color change can be so large that the process is completely out of control.
Observation light source
Under different light sources, the colors look different and the human eye is very sensitive to these changes. This effect is reduced by ensuring that the colors of the pigments used in the printing process are accurate and consistent throughout the print job. If you change suppliers, this can be a disaster. Color measurement and sensation is a very complex area. To achieve the best control, there must be a closed loop consisting of accurate measurements by the ink manufacturer, the ink dispenser, the proofing and the printing process.
Drying effect
Sometimes the color changes due to improper adjustment of the dryer. When printing paper or paperboard, if the drying temperature is adjusted too high, it is generally white to yellow. The glass and ceramic industry is the most troubled by color changes during drying or baking. The pigment used here is completely changed from the printed color to the sintered color. These sintered colors are not only affected by the baking temperature, but also by the deterioration of the air quality in the oxidized or baked region.
These are just a few of the many reasons for the color change, and one aspect that makes screen printing so interesting and challenging. All of this comes down to one point, that is, good process control.