At present, the diazo photosensitive emulsion (diazo sensitizer) used for screen printing is an amide, which is environmentally friendly and suitable for green printing and is therefore widely used. The advantages of diazo emulsions are that the coated stencils are stored for a longer period of time, which is environmentally friendly. One of the most important elements for producing a good quality direct emulsion template is that the photosensitive layer of the template must be uniform, the emulsion should completely cover the screen, and the adhesive layer of the stencil printing surface should be thicker. The stencil printing surface should be coated with 1 to 2 times, and then immediately apply 2 to 4 times on the squeegee side, wet to wet and then dry. After drying, apply another coating on the printed surface 1 or 2 times so that the photosensitive layer reaches the final desired thickness. Each sizing must be performed after the previous adhesive layer has dried.
The decisive factors for the number of stencil squeegees are the printing requirements and the fineness of the screen. In most cases, the main adhesive layer is wet-on-wet-coated, and after each layer is dried, the photosensitive adhesive layer is additionally coated to eliminate the unevenness caused by the screen structure.
The quality and fineness of the screen are another important factor in determining the number of times the main body of the stencil is coated. The screens with similar thicknesses generally have large differences in the percentage of open area. Larger pore sizes require more emulsion per mesh to pass through the mesh. In order to obtain the same layer thickness for the screen, different times of gluing are required. The screens with the same mesh and different quality have different thicknesses of the gluing layer because they have different apertures and different screen thicknesses. Therefore, the number of gluing is not the same. The sizing should be performed immediately after the stencil treatment of the screen to prevent the screen from getting contaminated with dust unless the degreased screen is stored in a dust-free environment.
The screen must be completely dry when gluing. Manual gluing usually uses a scraper bucket. The edge of the squeegee must be very sleek and tilt upwards over the entire length to ensure that the entire coating is even, even in the center of the screen. Since many diazo sensitizers and their sensitized emulsions have a strong acidic reaction, a non-oxidation type scraper bucket made of plastic should be used, and aluminum scraper buckets are easily damaged. The metal-plated rubber scraper will oxidize within a very short period of time, thereby destroying the composition of the photosensitive emulsion. At this time, fine foam and scum will appear in the emulsion. If the emulsion is stored in a stainless steel or aluminium scraper, similar phenomena occur after a few hours. Once the foam or scum appears, the emulsion can no longer be used. Pay special attention to the fact that the emulsion must not be left in the rubber squeegee for a long time. When it is placed, it must be covered with the emulsion so that it does not become dusty or dry.
For printing requirements, the following are the main points. 1 line. For high-resolution prints, use a mesh of 90 mesh/cm or finer. The thickness of the photosensitive layer on the screen should be 10 μm to 18 μm. In general, the thickness of the photosensitive layer is approximately 15% to 25% of the thickness of the screen. 2 halftones. Halftone printing requires a thinner ink layer, so that the photosensitive layer is also very thin, about 4 μm to 8 μm, and the thickness of the photosensitive layer is about 10% of the thickness of the screen. 3UV ink. Nowadays, UV inks are used for packaging and printing. When printing, care should be taken to minimize the amount of ink adhered. In general, the photosensitive layer of the stencil printing surface should not exceed 5 μm.