This is a basic article written for inexperienced beginners, but the content contains an important revelation that in the current environment of the printing house, in order to obtain stable and high-quality prints, it is necessary to establish standards .
The most difficult is often to answer questions that seem simple. Some beginners often ask, why does the color change when printing? Why is it so difficult for printers to keep the colors of one job and another job exactly the same? The only answer is to adopt careful process control and automatic adjustment technology, so that the color change can be reduced to the minimum. This is a suitable answer when you have just started screen printing and know nothing about the process. But from the perspective of an experienced production and printing worker, they do not necessarily understand all the reasons for the color change.
If printing simple line work, this is a monochrome printing, this time you can ignore the four-color mesh tone, this answer makes sense within a certain range. Specifically, the main factors considered should be the following.
Ink preparation is not good
If it is assumed that the pigments in the ink are properly formulated, the usual reason for the color change is the added solvent. In a well-controlled workshop, the ink should be ready to be supplied to the printing machine at any time, that is to say, the printer should not go to the ink. In many companies, the ink is not adjusted and supplied to the printing machine, but is left to the printer to adjust, and they add and mix ink according to their own feelings. As a result, the pigment balance in the ink is broken. For water-based ordinary inks or UV inks, the water in the ink acts in the same way as the solvent in the solvent ink. Adding water will thin the dried ink film and affect the color of the ink, thereby reducing the color density . The reasons for such problems can be further traced. In the ink warehouse, the ink matching workers do not use the weighing device, but rely on their own judgment to add the appropriate amount of solvent, or the mixing is not suitable at the beginning, or the mixing amount of the ink changed during printing. Produce different colors. When this job is printed again later, this situation will become worse, unless there is enough ink ratio record, otherwise it is almost impossible to reproduce a color.
Inappropriate selection of wire mesh
If any screen printer thinks that the screen mesh is only a variable of the screen structure, which will only affect the deposition of printing ink, then he will be in trouble. The screen wire diameter and weaving method, 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 represents the amount of ink that passes through the screen mesh under certain printing conditions, is generally expressed in cm3 / m2. For example, a 150-mesh / cm screen with a screen diameter of 31 μm will pass 11 cm3 / m2 of ink. A mesh with a mesh diameter of 34 μm and 150 mesh will pass 6 cm3 of ink per square meter, which is equivalent to a wet ink layer of 11 and 6 μm thick. It can be seen from this that the simple expression of 150 mesh will make you get a significantly different thickness of the ink layer, and the result will cause a large difference in color.
With the improvement of wire mesh weaving technology, it is necessary to obtain a certain mesh of twill wire mesh instead of plain wire mesh. Although it is sometimes possible, it is very unlikely. Sometimes screen suppliers store some old twill screens. Generally speaking, the theoretical ink volume of these screens varies by 10%. If you use a twill weave screen to print fine-grained images, the phenomenon of thin lines breaking is more than using plain weave screens.
Screen tension
The lower tension of the screen will cause the screen to be detached from the printed surface slowly, which will affect the ink that stays on the screen and will have uneven color effects. In this way, it seems that the color has changed. To solve this problem, the screen distance must be increased, that is, the distance between the horizontally placed screen plate and the printing material. Increasing the screen distance means increasing the pressure of the squeegee. This will affect the amount of ink that passes through the screen and will cause further color changes.
Squeegee setting
We know more and more about how the squeegee works. Recent research shows that in the printing operation, the squeegee has some very interesting phenomena, which will be described in detail later. Based on the current technical situation, the softer the doctor blade used, the more ink will pass through the screen. The greater the pressure on the squeegee, the faster the squeegee blade wears during printing. This will change the contact point between the squeegee and the printed product, which will also change the amount of ink passing through the screen, and then Cause color changes. Changing the angle of scraping ink will also affect the amount of ink adhesion. If the doctor blade runs too fast, this will reduce the thickness of the attached ink layer.
Setting of the ink back knife
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 adhesion. The function of the ink return knife is to fill the screen mesh with a stable amount of ink. Adjusting the pressure, angle and sharpness of the ink return knife will cause the mesh to be filled with too much or too little ink. Excessive pressure of the ink return knife will force the ink to pass through the mesh, causing too much ink to adhere. If the pressure of the ink return knife is not enough, only part of the mesh will be filled with ink, resulting in insufficient ink adhesion. The running 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 a serious lack of ink, which is similar to the effect of changing the operating speed of the blade.
Machine settings
Serious process control is the most critical factor. Stable and consistent adjustment of the machine means stable and consistent color. If the machine regulation changes, then the color will be out of control. This problem usually occurs when the printing worker changes shifts, or later the printing worker changes the setting on the printing machine at will to adapt to his own habits, which will cause the color to change. The latest multi-color screen printing machine color is automatically controlled by a computer, eliminating this possibility. These stable and consistent adjustments should be made to the printing press, and these adjustments should be maintained throughout the printing operation.
The influence of printing materials
In the screen printing industry, an aspect that is often overlooked is the consistency of the printing materials to be printed. Paper, cardboard and plastic used in printing are generally produced in batches. A high-quality supplier can guarantee that the whole batch of materials it provides has a good surface smoothness, but things are often not the case. During the processing of these materials, any slight change in the process will change the color and Surface finish. Once this happens, the printed colors will look different, although nothing has changed in the actual printing process. When we want to print the same pattern on various materials from corrugated plastic board to fine art cardboard as a publicity advertisement, the printer will encounter these practical difficulties. Another problem that is often encountered is that our screen printing has to catch up with offset images. If we do not pay attention to process control, we will have no chance. Serious process control includes accurate color measurement, using a spectrophotometer to measure line colors, and a densitometer to measure the three primary colors, so that we can print stable and consistent images on various materials. If we still use traditional and intuitive methods, the color change may be so large that the process is completely out of control.
Observe the light source
Under different light sources, colors look different, and human eyes are very sensitive to these changes. This effect can be reduced by ensuring that the colors of the pigments used for ink matching throughout the printing operation are accurate and consistent. If you change suppliers, this may be a disaster. The measurement and perception of color is a very complex field. To achieve the best control, there must be a closed loop composed of the ink manufacturer, ink matching, proofing and accurate measurement during the printing process.
Effect of drying
Sometimes the color changes due to improper adjustment of the dryer. When printing paper or cardboard, if the drying temperature is adjusted too high, the general situation is white to yellow. The glass and ceramics industry suffers the most from color changes during drying or baking. The pigment used here must be completely changed from the printed color to the sintered color. The color of these sinters is not only affected by the baking temperature, but also by the air quality in the oxidation or baking area.
The above are just a few of the many reasons that cause color changes, and also one aspect that makes screen printing so interesting and challenging. All of this comes down to one point, which is good process control.
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