Factors to be considered in ink layer control in screen printing
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Before we start to discuss with you how to control the ink layer, we should first review some of the problems that we have seen in printing in the past: 1. The color of the printed color block is deep and shallow. 2. When the net is adjusted, the dot is enlarged (there is a moiré phenomenon). 3. The difference in color during batch printing. These problems will affect the production schedule and batch returns, which will bring inconvenience to the normalization of our work. So how to control and prevent these problems is worthy of discussion by our screen printing workers. The following are the variables we should understand in printing.
First, analyze the theoretical amount of ink:
There are two ways to analyze the amount of ink in the screen. One is to calculate the theoretical ink permeability of the screen, and the other is to actually print the amount of ink. Calculate the theoretical amount of ink that has the following formula:
Opening ratio: L
The thickness of the screen is: D
The theoretical ink penetration is: M = L × D ÷ 100%
The above formula is only a theoretical method for calculating the amount of ink in the screen. Once applied to practice printing, the data will change, but it is proportional. Let's look at the following factors:
1. The mesh number of the wire mesh (directly determined to the hole width and opening ratio). The higher the mesh size, the smaller the hole width, and the lower the opening ratio. For example, the 355 mesh screen has a hole width of 31 μm, an opening ratio of 26%, and a theoretical ink permeability of 14.3 cm 3 /m 2 . The 305 mesh screen has a hole width of 45 μm and an open cell ratio of 35%. The theoretical ink permeability is 16.3 cm 3 /m 2 .
2. The wire diameter of the wire mesh (directly affects the mesh thickness, opening ratio, mesh size). It refers to the diameter of the unwoven wire diameter, and the wire diameter after weaving becomes elliptical. Will affect the size of the mesh. The same mesh size of the mesh will affect the opening size and opening ratio.
For example, the 305 mesh wire diameter 31um opening ratio is 35%, the hole width is 49um, the mesh thickness is 49um, and the theoretical ink permeability is 17.2cm3/m2.
The 305 mesh wire diameter 34um opening ratio is 29.6%, the hole width is 45um, the mesh thickness is 55um, and the theoretical ink permeability is 16.3cm3/m2.
The 305 mesh wire diameter 40um opening ratio is 20.1%, the hole width is 37um, and the mesh thickness is 65um. The theoretical ink permeability is 13cm3/m2.
3. The thickness of the screen (directly affects the ink layer). We can use a thickness gauge to measure the thickness of the screen. Since the screens produced by various manufacturers have their own weaving process, the warp and weft threads are not bent to the same extent. The greater the degree of bending, the thinner the screen will become, and vice versa. During the stretching process, the wire diameter is stretched due to the influence of external force, which will eventually affect the thickness of the wire mesh.
4. The hole width of the screen (directly affects the amount of ink). We know that the wider the mesh, the larger the amount of ink. In the process of stretching the net, the higher the tension, the lower the mesh size, and the wider the mesh, the larger the amount of ink permeated.
5. The opening rate of the screen (directly affects the amount of ink). The open cell ratio refers to the white ratio of the area of the ink permeate per unit area of the screen to the total area. For example, for a screen having an opening ratio of 35%, the portion of the ink-permeable portion is 35%, and the portion that is not permeable to the screen is 65%. The higher the opening ratio, the larger the amount of ink discharged.
Calculation formula:
The hole width is: K
The number of meshes is: M
Inches are: Y
Opening ratio: L=K×M÷Y2×100%
6. The pressure plane of the wire mesh (directly affects the mesh thickness, hole width).
More than 355 mesh of imported wire mesh is used for flattening. The flattened screen will obviously become thinner and the mesh will become smaller. When the flattened wire mesh is stretched, let us look at the following example: 10%~15% ink can be saved when the pressing plane is stretched toward the scraping surface, and 15% of the ink can be saved by stretching the pressing surface toward the printing surface. ~25%.
Second, the actual amount of ink printed:
1. The tension of the screen (affecting the size of the opening, mesh size, wire diameter, mesh thickness). During the process of stretching the net, as the tension increases, the technical parameters of the screen itself will change accordingly. First, it affects the mesh number of the mesh. The higher the tension, the larger the mesh count (until the mesh is plastically deformed). Next, it will affect the hole width of the screen, the mesh will become larger, the wire diameter will become thinner, and the mesh will become thinner. These factors will eventually lead to changes in the amount of ink.
2. The thickness of the film layer (determines the amount of ink). If we use a photosensitive adhesive to make a screen, we also need to consider the solid content of the photoresist itself. After the plate is pressed, the film will evaporate and the film will become thinner. Therefore, we can only use the thickness gauge to detect the overall thickness of the screen.
3. Ink type (indirectly affect the thickness of the ink layer). We know that after the solvent-based ink is printed, the solvent will volatilize and the ink layer will eventually become thinner. After the light-solid ink is printed, the resin is solidified instantaneously after being irradiated with ultraviolet rays, so the ink layer remains unchanged.
4. The viscosity of the ink (indirectly affect the thickness of the ink layer). The lower the viscosity of the ink, the thicker the ink layer will be in the printing process because the ink itself contains less solvent and, on the contrary, the thinner.
5. The hardness of the squeegee (indirectly affect the thickness of the ink layer). In the printing process, the higher the hardness of the squeegee, the less likely it is to deform, and the smaller the amount of ink, the larger the opposite.
6. The mouth of the scraper (directly affects the amount of ink). If the edge of the scraper is at a right angle, the amount of ink is small. If the ink is at an obtuse angle, the amount of ink is large.
7. The angle of the scraper. (Indirectly affects the thickness of the ink layer). At the time of printing, the smaller the angle between the blade and the screen, the larger the amount of ink, because the blade is in surface contact with the screen. The opposite is small.
8. The pressure of the scraper (directly affects the amount of ink). At the time of printing, the greater the pressure applied to the blade, the smaller the ink is, because the ink has been rushed before it is completely squeezed out of the mesh. The opposite is small.
9. The pressure of the ink return knife (direct ink volume). The greater the pressure applied to the squeegee, the greater the amount of ink that is due to the fact that a small amount of ink has been squeezed out of the mesh by the squeegee before it is smeared. The opposite is small.
10. The speed of squeegee (indirectly affects the thickness of the ink layer). The faster the printing speed, the smaller the ink is, because the ink has not been completely filled, and the ink has been squeezed out, causing the ink to be interrupted.
11. Printing materials. (Directly affects the thickness of the ink layer). The flatness of the surface of the substrate also affects the thickness of the ink layer, and the rough surface ink will penetrate (such as woven fabric, leather, wood). The opposite is big.
12. Printing environment (indirectly affecting the thickness of the ink layer). One problem that we have always overlooked is the change in temperature and humidity in the printing shop environment. If the temperature of the printing environment changes too much, it will affect the ink itself (such as ink viscosity, fluidity, etc.).
Third, how to calculate the thickness of the ink layer?
Through the above discussion, we know that if a certain link changes during the printing process, and eventually the amount of ink is inconsistent, how should we calculate the thickness of the ink layer? One method is to weigh the weight of the wet ink. First, try to control every link in the printing to remain unchanged. After printing, the weight of the substrate is printed, and then the original weight of the substrate is subtracted. The data obtained is wet ink. The weight, another method is the thickness of the ink-measuring layer. The thickness of the substrate after the over-inking is measured by a thickness gauge, and the original thickness of the substrate is subtracted, and the obtained data is the thickness of the ink layer.
Fourth, how to control the thickness of the ink layer?
How to control the thickness of the ink layer has become a problem faced by screen printing workers. First of all, we need to use the existing measuring equipment to ensure the accuracy and objectivity of the measured data; the factory can use automatic conditions. The coater completes the gluing process to ensure the thickness of the glue layer. The next step is to ensure that every step in the plate making and printing is kept as constant as possible. Each printing parameter should be recorded to provide the ideal data for finding the proper thickness of the ink layer.