1 Reasonably choose the tension of the stretched net
The important parameters of screen printing are screen mesh number, screen diameter, screen thickness, opening size and ink consumption, screen diameter, mesh number and screen weaving method and the selected material directly affect the stretch net The size of the tension is tested according to these parameters when the net is stretched. When measuring tension, the measured tension point should be 10 cm away from the inner border of the frame, otherwise the measured tension is not accurate. Tension can be divided into warp and weft tensions. The tension in the warp direction when stretching the net is the tension in the winding direction of the entire wire mesh; the tension in the weft direction of the wire mesh is the tension in the width direction of the wire mesh. The amount of tension has a direct impact on the quality of the screen. The higher tension screen adopts a lower screen pitch, and a smaller pressure can be used during the printing process, the screen wear is reduced, and the service life is prolonged. At the same time, the smaller pressure is beneficial to avoid ink and deformation around the dots and improve the dots The sharpness of the edges guarantees the printing effect. When the screen pitch is increased by a factor of 1, it will increase the distortion of the printed image by a factor of three. When the screen tension is low, the ink drop is uneven and the dots expand and deform, which affects the hue. Uneven tension of the screen will cause uneven thickness of the ink layer, distortion of the pattern text, blurring of local graphics and text, and distortion or misalignment of the dots during dot printing, resulting in color shift. However, the maximum tensile force value that various specifications of wire mesh can bear is different. If it exceeds the set maximum tension value, the wire mesh will lose its elasticity, which is called plastic deformation or broken mesh, and should be paid attention to in the stretched mesh.
2 Correct use of the frame
The material, cross-sectional size, shape, flatness and strength of the screen frame directly affect the tension and stability of the screen. The screen frame must be deburred before and after it is used. The reusable screen frame should be cleaned with solvents such as acetone to remove residual ink, glue and screen fragments on the surface of the screen frame. Before stretching the net, pre-coat a layer of sticky mesh glue to the mesh surface of the frame. When sticking the mesh, apply another layer of glue to the wire mesh. Note that the glue must penetrate into the mesh hole on the side of the fitting surface of the wire mesh. The wire mesh is completely adhered to the frame. If necessary, when the glue is not completely dry, use a rubber scraper to press against the fitting surface to help the wire mesh to be firmly attached to the frame.
3 Stretching the net
(1) Standard stretch net method
First tear off the untidy end of the wire mesh, clip into the chuck in equal amount, and flatten the wire mesh, then clamp the other end of the warp wire mesh, and finally clamp the weft edge into the wire mesh clamp, then the wire mesh is not tension.
Pull the warp wire to 1/2 of the required tension value, and then stretch the weft direction to make the tension close to the required tension value. At the same time, adjust the weft and warp tensions to make them both reach the required tension value and rest for 10 minutes. Let it fully release the stress. After 10 minutes, adjust the tension in the warp and weft directions to reach the required tension, and rest for about 10 minutes. Repeat this operation twice, and then apply photosensitive adhesive to stick the screen on the frame to complete the entire stretch process. This method is a widely used standard stretching method.
(2) Stretching the net in 30 minutes
First make the tension in the warp and weft directions reach the required half, and then pull the net after stabilizing for about 1 ~ 2min, increase the tension to 80% ~ 90% of the required value, stop for 5min, and then pull the net to reach the required tension value, Stabilize the tension for about 10 to 15 minutes and then stretch again to the required tension value. After repeating again, stick the net and complete the stretch.
Pay attention to the following points in the process of stretching the net:
â— The maximum allowable gap between the chucks on the four sides of the stretched net table is within 3mm, so that all meshes are pulled and ensure that the meshes are uniformly stressed;
â— One-to-one correspondence between the side chucks of the stretched net table to ensure that the mesh hole is close to square when the net is stretched, and the tension is uniform everywhere;
â— The four corners have appropriate stretch margin according to the size of the screen frame, which can minimize the possibility of tearing the screen;
â— The screen printing plate can only be made after 24h, because the tension of the high-tension screen will drop by 10% to 20% after 24h, and the tension loss mostly occurs within the first 4h, and then gradually stabilizes;
◠Stretch net can be divided into straight stretch net and diagonal stretch net according to angle. In order to make the edges of the printed lines straight and smooth, without generating serrations, and at the same time avoid the moire (disturbance caused by the mesh and dots) during the plate-making process, when stretching the net, the corresponding edges of the screen's warp and weft lines and the frame should be At a certain angle, this method is called diagonal netting. There are many kinds of angles for the stretch net, the general angles are 15 °, 22.5 °, 45 °, 7.5 °, 30 °, etc. Among these kinds of stretch net angles, the edges of the lines obtained at 22.5 ° have better straightness and smoothness. The 90 ° stretch net is a straight stretch net, and the edge quality of the resulting line is the worst.
4 Pre-net processing
Use special grinding paste and degreasing agent to roughen and degrease the screen, increase the porous and uneven surface and remove the oil stains on the surface, increase the bonding area of ​​the screen and the photosensitive material, and thus improve the attachment of the photosensitive material Focus on increasing the number of screen printing times and reducing pinholes on the photosensitive layer of the screen during the production process.
5 Apply photosensitive adhesive and paste photosensitive film
Photosensitive adhesive should have good thixotropy, proper viscosity, certain solid content and good bridging property. The photosensitive glue should be configured 12 hours before use, and must be fully stirred. It can be used after 12 hours of rest to prevent bubbles generated during the stirring process from affecting the quality of the glue. The gluing process of photosensitive glue is wet pressed on the printing surface and the ink scraping surface of the screen plate 1 + 2 times, and then the screen printing surface is laid flat and dried, the drying temperature is controlled at 35-40 ℃, and the degree of drying is 80 %. Wet and dry the screen printing surface and the ink surface by 1 + 1 times of scraping glue, and then lay the screen printing surface down to dry, the drying temperature is controlled at 35-40 ℃, and the degree of drying is 80%.
Finally, scrape the glue once on the printing surface of the screen, and then lay the screen printing side down to dry, the drying temperature is controlled at 35 ~ 40 ℃, and the drying is thorough.
When drying the screen, the coated screen and the pre-treated screen cannot be dried in the same oven. The water content of the pre-treated screen is very high. A large amount of water will evaporate during drying, so that the photoresist is mixed. The drying degree of the screen is reduced, and the drying time is extended. The screen printing plate should be thoroughly dried after applying the photoresist. If the photoresist layer contains a little water vapor, the water vapor will refract the light during exposure, thereby affecting the crosslinking degree of the photoresist; in addition, the photoresist is not completely dried. It will hinder cross-linking, resulting in pinholes, stripping, reduced adhesion between the adhesive layer and the screen, and reduced number of prints.
When coating photosensitive glue, pay attention to the coating speed first, because the coating speed is too fast, it is easy to produce pinholes; if it is too slow, the glue liquid will easily penetrate into the ink surface, and it is easy to cause uneven coating, so it should be at a moderate uniform speed Apply photosensitive glue.
When applying photoresist on a coarse mesh screen, pinholes and other phenomena often occur. To avoid these problems, the photoresist should be thicker. When printing for the first time, the printing surface should be scraped back and forth several times, so that The screen is fully moistened, and then the remaining glue on the ink surface is leveled. After drying, the coating is continuously scraped on the printing surface for 2 to 3 times, the speed is slightly faster. The capillary photosensitive film should have wider exposure latitude, higher resolution, good solvent resistance and strong adhesion to the screen, and the detailed technical process of screen-making of screen printing for stripped discs is good-good recycling and storage stability excellent.
Before attaching the capillary photosensitive film, the screen should be properly pre-treated (grinding, degreasing, etc.), the film is applied to the wet screen, and the excess water is removed with a scraper, and the net with the capillary photosensitive film is dried. Choose a temperature below 30 ° C during printing to prevent crosslinking during drying. After the screen is completely dried, remove the back protective film base.
6 Screen exposure
The dried photosensitive emulsion layer is exposed to UV light to harden the uncovered areas (polymerization) and is no longer water soluble, while the unexposed areas are still water soluble, and then rinsed with cold or lukewarm water Off.
Before exposure, determine the correct exposure time by using exposure time measurement film to do stepwise exposure. Insufficient exposure time, thinner photoresist layer, prone to pinholes, edges of graphics or wires are prone to appear jagged, screen printing times are reduced, screen reclaiming is difficult, shadows on screens that have been removed from the film are prone to shadows; exposure Excessively, the graphics will be distorted, the thin lines will be thinner, the low-key dots will expand, the high-gloss dots will shrink, and it will be difficult to show. The residual glue will appear in the leaked part, so that the ink will not pass through the mesh, and the membrane will become brittle and not durable.
7 Developer rinse
After the exposure, the screen should be washed and developed with water of appropriate pressure, especially the ink surface should be fully washed until the surface is not slippery. Use soft and clean paper, a clean cloth or a water absorber to remove excess water, put Dry in an oven at a temperature of 30 ° C to 40 ° C to prevent distortion of the screen when the temperature is too high.
8 Supplement
Use special sealing materials to repair non-inking areas or leaks, double exposure after drying to increase the strength of the screen, use silver dragon tape to reinforce the screen, and record the relevant data for making the screen .
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