Screen printing is the basis of screen printing. Screen printing is an important process of screen printing. There is no excellent screen printing. No matter how good the other conditions are, the strict control of various parameters is futile. Because the "mold" is not sophisticated and defective, the quality of the graphics on the printed matter cannot exceed the "mold". Therefore, in the process of screen printing, to ensure the quality of printed products, we must first have a good printing screen. There are many methods for making screens, but due to the differences in production methods, the impact on the print quality during the printing process is different.
Conventional plate making method
The commonly used plate making methods of screen printing screen can be divided into the following three categories:
â–² 1. Direct plate making
Direct plate-making is to directly coat (attach) the photosensitive glue or photosensitive film on the screen printing surface that has undergone pre-screen treatment. After drying, put it into the plate-making machine together with the screen frame to carry out the photosensitive imaging reaction. Version.
1) Photosensitive plate making
For the screen made by coating photoresist, the thickness of the photoresist layer on the web can be controlled by the operator, but in the process of making a plate, the flatness of the film is slightly worse due to the limitation of the operator's doctor coating method, that is, each screen The Rz value at the location varies. When the photosensitive adhesive layer is thin, the surface of the film is rough, and the Rz value is large. During printing, the ink may diffuse from the gap between the screen and the substrate (the low recess of the screen), resulting in uneven edges, lines and dots. The sharpness drops, etc., as shown in Figure 1.
Not only that, because the photosensitive adhesive layer on the web is not easy to control, especially in the case of a thin adhesive layer, when the screen is developed, under the impact of water pressure, pinholes, sand holes, etc. appear, which reduces the screen printing durability.
2) Photosensitive plate making
The photosensitive film is coated on the flat PET or PBT film base by the coating machine in advance
The flatness of the photosensitive adhesive layer, and controls the thickness and uniformity of the adhesive layer. Due to different printing requirements, different screen meshes are selected, and the specifications of the thickness of the photosensitive film are also different. High-mesh screen, high-precision printing, thin photosensitive film should be used, such as Ketutai ultra-thin Capillexcp and Kemeilai Cap15 #; low-mesh screen, used for rough graphic printing, should be thick Some photosensitive diaphragms. Compared with plate making with photosensitive film, plate making with photosensitive film has great advantages in controlling the Rz value of film and the uniformity of screen thickness. However, the screen made of photosensitive film is completely attached to the silk screen by the capillary action of the film adhesive layer on the water. The photosensitive adhesive can enter the mesh less, and the combination of the photosensitive film and the silk screen is low. Therefore, compared with the photoresist plate-making method, its durability is lower, as shown in Figure 2.
â–² 2. Indirect plate making
The biggest difference between the indirect method and the direct method is that the image must be printed on the photosensitive film first, and it needs to be developed with warm water, and then the developed film is immediately pasted on the printing surface of the screen, dried and exposed Remove the film base and print after sealing. The characteristic of this plate-making method is that the fineness of graphics and text is not affected by the color and weaving method of the screen, the precision of screen production is high, and the sharpness of the edges of the graphics and text is good. However, compared with other plate-making methods, the indirect method has the lowest screen resistance, because the photosensitive film is only bonded by the non-graphic parts in contact with the screen. Therefore, during the printing process, due to continuous scraping by the rubber scraping The part where the photosensitive adhesive and the screen are bonded will peel off a little bit, resulting in a reduction in the service life of the screen. It is only suitable for printing high-definition short version graphics.
The above commonly used screen production methods all have different degrees of shortcomings. For this reason, the United States and Europe have generally used mixed plate making methods to make screens, such as the common Comaray solvent-resistant blue CP2 photoresist plus Comaray blue photosensitive The hybrid plate-making method of the diaphragm (see Figure 4). Using this kind of special photosensitive adhesive to paste the photosensitive film to make a plate can avoid uneven coating thickness in the photosensitive plate coating method, the Rz value is too large due to uneven thickness and low flatness, and the screen is prone to pinholes, etc. Disadvantages; improve the combination strength of the photosensitive film and the screen, and solve the problem of low printing resistance of the screen; at the same time, it also makes up for the indirect film can not improve the printing quality, low printing durability and other shortcomings. [next]
â–² 1. Mixed plate-making process
Pre-screen treatment (roughening, stripping) → drying → transfer the photosensitive film on the printing surface → drying (<40 ℃, about 2 minutes) → scraping CP2 photoresist on the ink surface → dry thoroughly (35 ～ 40 ℃) → remove The base of the photosensitive film → back to baking (35 ~ 40 ℃, about 3 minutes) → exposure according to the time required by the test (printing) → flushing with pressurized water → drying → second exposure of the ink surface (2 ~ 3 minutes) ) → Revision, sealing → Printing.
â–² 2. Process points
1) Pre-network processing
Any plate-making method must fully process the screen before the plate-making, otherwise it will affect the printing durability of the screen and the firmness of the fine graphics attached to the screen, and the pre-screen processing must be in place.
â‘ The surface of the new PET mesh with weak polarity must be roughened, which is conducive to improving the physical combination of the mesh and the photosensitive material;
â‘¡ In order to remove the influence of oil stain on the web on the adhesion of the photosensitive material, the roughened screen must also be degreased. While removing the oil stain on the web, the hydrophilicity of the screen is increased to make the water-soluble photosensitive adhesive (film) easy to handle Wetting and affinity of the silk screen to prevent the bubbles and pinholes that are easy to appear when applying (sticking) photosensitive glue (film), and avoid the loss of fine lines and dots due to poor adhesion when making fine screens. This process is very important when making fine lines and dot-printed screens.
2) Patch
The patch is divided into two steps. The first step is to transfer the patch from water to dry patch and wet patch. The effect is the same when using appropriate tools to stick the film, directly roll it up, or scrape it with Camry glue. The key is to stick it firmly, there should be no false stickers, and there should be no bubbles; while not destroying the original flatness of the membrane surface, some membrane glue should be properly infiltrated and adhered to the inner wall surface of the screen In order to ensure that the diaphragm and the wire mesh are firmly attached. In addition, when attaching the film, the time that the water used for attaching the photosensitive film stays on the plate adhesive layer should be corrected according to the change of the ambient temperature to control the swelling state of the photosensitive film adhesive layer, so as to achieve a better bonding effect.
The second step is to scrape CP2 photoresist on the ink surface to reinforce the attached photosensitive film, and scrape off the excess water on the ink surface in an appropriate time (the scraped water should be a little yellow and sticky) Light yellow photosensitive glue), and then dried at 35 ~ 40 ℃ temperature for about 2 minutes. In order to make the photosensitive film stick to the silk screen, it is necessary to apply CP2 photosensitive adhesive on the ink surface a second time (the method is the same as that of the direct plate making method, two times is enough), and finally at a temperature of 35 ～ 40 ℃ Dry thoroughly. If this process is not properly grasped, bubbles will be generated and the photosensitive film adhesive layer and the photosensitive adhesive will not be well combined. Note that if the temperature is high and the humidity is low, the photoresist can be diluted with purified water to reduce the viscosity of the photoresist a little, which is conducive to the affinity of the scraped adhesive layer and the photosensitive film, and improve the adhesion between them.
3) Drying of photosensitive adhesive layer
Whether the glue layer is dry is another key to controlling the quality of screen production. The glue layer is completely dry, which easily reaches the required exposure parameters, improves the printing durability of the screen, and is also conducive to the recovery of waste screens. It must be paid attention to. If the photoresist (film) layer is not dry or not completely dry, even if the exposure time is accurate, the wet photosensitive layer will be less sensitive to light due to its poor sensitivity to light, and the photosensitive adhesive layer will be easily washed out during development. Or printing is not durable. Therefore, attention must be paid to drying control.
3. CP2 + Capillary Photosensitive Film Plate-making Parameters The technical parameters involved in the CP2 + Cap series capillary photosensitive film plate-making method are shown in Table 2.
①Cap series photosensitive diaphragms are diazo-based photosensitive materials with thicknesses of 15μm, 18μm, 25μm, 30μm, 38μm and 50μm, which can be stored at room temperature for 18 months. Cap photosensitive film should be attached in the presence of water. The ink needs to be coated twice with photosensitive adhesive.
â‘¡CP2 photoresist is a two-component solvent-resistant photoresist, which has two colors of purple and blue. PL in the table is a single-component presensitized photoresist, which is characterized by excellent coating. Applying PL adhesive once is equivalent to coating 4 times the thickness of ordinary photoresist.
â‘¢It is recommended to test the exposure with Comerai or Kotute exposure test sheet according to your own conditions before exposure.
â‘£After the screen is completely dry, test the adhesion fastness of the photosensitive adhesive layer and the screen:
a. Use 3M600 # tape to stick the glue layer of the fine parts of the screen and text, it is better not to stick;
b. Scrape the photosensitive adhesive layer of the printing surface with nails, and observe the scratched parts of the ink, there should be no white scratch marks;
c. Use a magnifying glass to observe whether the screen hole on the ink surface is almost filled with photosensitive adhesive.
The screen made by CP2 + capillary photosensitive film (hybrid method) is suitable for UV ink, fabric / thermosetting plastic ink printing and water-based paste screen printing due to the smooth and uniform plate film and fine graphics. It is used in membrane switches and printed circuit boards. , Ceramic printing paper and fabric fine printing are widely used.
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