Why does screen printing produce color shift? If we consider the mixture of several colors and consider only one of them, it may be easier to discuss the causes of color shift. This article shares several factors that affect the color deviation of screen printing. The contents are for your reference:
Screen printing refers to the use of silk screen as the base, and through the photosensitive plate-making method to make a screen printing plate with graphics. Screen printing consists of five major elements, screen printing plate, scraper, ink, printing station and substrate. The basic principle of screen printing is that the mesh part of the screen can pass through the ink, and the non-graphic part of the mesh cannot pass through the ink. During printing, pour ink into one end of the screen printing plate, apply a certain pressure to the ink portion on the screen printing plate with a scraper, and move at a constant speed toward the other end of the screen printing plate. The ink is moved by the scraper from the graphic during the movement Part of the mesh is squeezed onto the substrate.
Listed below are some of the most common factors that cause color shifts in screen printing: ink formulation, selection of meshes, stretched screen tension, pressure, drying, substrate characteristics, and viewing conditions.
Ink formulation assumes that the pigment used in the ink is a standard pigment, so the biggest cause of color shift is the addition of solvents such as inks to the ink. In a workshop with good color control equipment, the ink can be adjusted according to the control equipment. However, for most printing companies, it is impossible to have these facilities. They only use the experience of the workers to prepare the ink.
Generally, the ink is added to make the ink more suitable for printing. However, once the ink is added to the ink, the concentration of the pigment in the ink will change, which leads to changes in the color characteristics of the ink during printing. In addition, the excess solvent in the ink will form an ink film after drying, which will reduce the brightness of the color.
Before ink application, there is also the problem of ink dilution. For example, workers in the ink workshop make judgments based on their formulas when preparing or diluting inks, which leads to inevitable color cast. If they are prepared a few days ago After printing with good ink, the color shift caused by this situation is more obvious. Therefore, it is almost impossible to avoid color cast completely.
If you think that the screen mesh is the only factor affecting the ink transfer, you will encounter a lot of trouble. Mesh diameter and wrinkles also affect ink transfer. In general, the more ink attached to the screen ink hole, the more ink transferred to the substrate during the printing process.
In order to estimate in advance how much ink each mesh can transfer, many screen suppliers provide the theoretical ink volume (TIV) for each mesh transfer. TIV is a parameter indicating the amount of screen ink transfer, which refers to a certain How much ink will be transferred per mesh under specific printing conditions. Its unit is the volume of ink per unit area.
To ensure the consistent color tone in printing, it is not enough to keep the mesh number of the mesh unchanged, but also to ensure that the diameter of the mesh and its waviness remain constant. Changes in any of the screen parameters will result in changes in the thickness of the ink film during printing, resulting in color changes.
If the tension of the stretched net is too small, it will cause peeling of the film, so that too much ink remains in the mesh, which will make the printed matter sticky.
This problem can be solved by increasing the distance between the screen and the substrate, but increasing the distance between the screen and the substrate must increase the pressure, which will cause more ink to be transferred to the substrate In order to change the density of the color, the best way is to keep the tension of the stretched net uniform, so as to ensure the consistency of the color
Appropriate pressure settings are essential to maintain consistent colors, and it is critical to ensure uniform pressure levels during the printing process. Especially in high-volume repetitive printing operations.
When it comes to pressure, the first consideration is the hardness of the doctor blade. The squeegee has a small hardness, which is beneficial to the contact rate, but it is not conducive to excessive bending resistance, and the friction against the screen during printing is also large, which affects the printing accuracy. The second is the angle and speed of the doctor blade. The influence of the ink knife angle on the amount of ink transfer is more significant. The smaller the ink knife angle, the greater the amount of ink transfer. If the ink knife is too fast, it will cause insufficient ink filling and incomplete imprinting, which will affect the printing quality.
Once you have obtained the correct pressure settings for the print job and recorded them accurately, in the printing process, as long as you follow these settings correctly, you will get a consistent and satisfactory color print product.
Sometimes, the color looks the same just after printing, but after the finished product, it is found that the color has changed, which is often caused by the incorrect setting of the drying equipment. The most common reason is that the temperature of the dryer is set too high, which causes the color of the ink on the paper or cardboard to change.
One problem that screen printing masters often overlook is the surface characteristics of substrates. Paper, cardboard, plastic, etc. are all produced in batches, and high-quality substrates can ensure stable and consistent surface characteristics. But this is not the case. Minor changes in the surface characteristics of the substrate will cause color shifts in the printing. Even if the printing pressure is guaranteed to be uniform and even the processes are operated correctly, the inconsistency of the surface characteristics of the substrate will cause greater printing. Color cast.
When the same printing equipment is used to print the same product on different substrates, the influence of the surface characteristics of the substrate on the color is particularly obvious. Customers may ask for window advertisements to be printed on plastic, or they may be required to be printed on other cardboard. And customers may require the same color of the same work.
In such cases, the only way is to make accurate color measurements. Use a spectrophotometer or a spectral density meter to measure the color density. If there is a color shift, the density meter can clearly reflect it, and you can overcome this color shift by controlling other processes.
Human eyes are very sensitive to subtle changes in color, and colors can only be distinguished under light conditions. Because of this, it is necessary to ensure that colors are compared under the same light conditions. Otherwise, adjusting the amount of ink or pressure will cause more changes. Large color cast.
All in all, the key to maintaining consistent color lies in the stable control of each process to ensure the stable performance of the ink. The selection of mesh, the tension of the stretched net, the pressure, the surface characteristics of the substrate and the observation conditions all have a certain effect on the color deviation, but, Accurate setting records and stable control of each process are the key to ensure consistent color of screen printing.