Screen printing process technology and detailed operation (3)

Third, photosensitive printing plate printing method

Photosensitive platemaking has direct method, indirect film method and direct film method and other methods, direct legal plate is directly coated on the surface of the screen after the photosensitive adhesive exposure printing; indirect film law will be first printed in the photosensitive Photographic On the diaphragm, and then paste on the screen; Direct diaphragm method is to first paste the film on the surface of the screen, and then exposed to develop.

Direct-to-screen platemaking uses chemical methods instead of manual engraving. The direct legal plate can not be separated from the colloid with photosensitive effect, that is, photosensitive adhesive. Photosensitive adhesive is basically gelatin, polyvinyl alcohol, nylon and the like as a matrix. Most of the sensitizers use dichromate and also have diazonium salts.

1. Direct French printing plate recipe and production

1 Gelatin chrome salt photosensitive adhesive formula and production

formula

Special gelatin 180g

Potassium red 30g

Clear water 1000ml

Ammonium dichromate 10g

Citric acid 10g

Barium Sulfate 10g

Alcohol 25ml

Ammonia water 55ml

operating

The above medicines are heated to be dissolved and filtered, and the glue solution is applied to the screen in a dark room with an automatic glue applicator or a manual squeegee, and is left ready for use after being dried.

2 polyvinyl alcohol chrome salt photosensitive liquid formulation and production

formula

A liquid

Polyvinyl alcohol 40g

Sodium dodecylcumyl sulfonate 0.5g

Acetic acid 4ml

Water 250ml

Liquor

Ammonium dichromate 5g

Water 30ml

A small amount of red

Polyvinyl acetate (or dibutyl ester 4ml) 40ml

operating

Polyvinyl alcohol used 1788, the degree of polymerization degree of saponification is 88%.

The solution was prepared by mixing polyvinyl alcohol, sodium sulfonate, and water and melting in a water bath followed by filtration through a 100-mesh wire mesh. After the solution was dissolved, it was filtered with filter paper.

In the dark room, mix liquid A and B in two liquids. After fully stirring, apply the glue to the screen with an automatic coating machine or hand-squeegee, and wait until dry.

Note: glacial acetic acid can accelerate the dissolution of polyvinyl alcohol, the development is also convenient, can improve the sensitivity of the photosensitive adhesive, the red dye makes the colloid appear bright red after photosensitive curing, and the unexposed part is washed out by water to expose the screen for easy observation. Polyvinyl acetate makes the photosensitive adhesive firmly bonded to the screen and the resolution increases.

3 polyvinyl alcohol diazonium salt photosensitive liquid formula and production

formula

Polyvinyl alcohol (degree of polymerization 1400, degree of saponification 88%) 40g

Polyvinyl acetate (50% solids content) 160g

Water 240ml

Diazo resin 6g

A small amount of red

operating

At room temperature, the above components are stirred and dissolved in a dark room, and the glue is applied to the screen by an automatic coating machine or a hand-squeegee, and is left for use after drying.

2. Photosensitive adhesive coating

The coating of the photosensitive layer is to apply the prepared photosensitive liquid to the screen of the stretched frame. The coating methods include automatic coating machine coating and manual scrape coating. The medium and small screen printing manufacturers generally use the manual scraping glue to coat the photosensitive layer. The method of manually scraping the rubber is to scrape the glue twice. The first time the scraping starts from the scratched surface of the oblique screen, and the adhesive is scraped from the bottom upwards. The screen is reversed and then scraped again from the bottom to the back to make the entire panel glue. The thickness of the layer is uniform, and then put into a constant temperature oven, with the printing surface facing downward to make the printing film flat and dry for about 15 minutes. It is taken out from the oven, and then the second time the squeegee is scraped. The second squeegee scrapes twice on the scraped rubber surface. , And then scrape twice on the printing surface, wipe the excess glue on the plate edge, and then lay flat in the oven and dry for about 10 minutes.

After coating, the adhesive layer must be flat and uniform, and the thickness should be uniform. In particular, color screens must be prepared with 4 screens. The thickness of the 4 screens must be basically the same. As for the number of squeegees, depending on the requirements of the printing materials, the quality of the printing materials will be determined by the customer. The half-tone printing plate is generally dried on the printing surface after two times of squeegeeing, and then the squeegee and the printing surface are scraped twice. After drying can meet the requirements of printing.

Coated photoresist cannot simultaneously double-squeegee. The screen is a through-hole. During the squeegeeing process, the excess photosensitive adhesive will be squeezed behind. When the other side is scraped, the excess adhesive on the back side will be squeezed to the front. Some glue will be glued and will be squeezed. To the front, some glue will flow into the sizing device, and the film will not be thickened by the number of squeegees. The correct squeegee should be scraped on one side. After the glue seals dry the mesh, it is scraped on the second side so that the dry sensitized layer will swell with water-based glue during the second squeegee. Increase the thickness of the film.

3. Bake

When baking the plate, the screen of the good adhesive layer is flat and flat in the oven with constant temperature, and the screen cannot stand. Otherwise, the glue will flow downwards, and the adhesive will be thin and thick on the adhesive layer, which will affect the printing plate imaging when printing. the quality of. The oven temperature is generally controlled at 39-45°C, and the best is 39-40°C. The first squeegee was dried in an oven for 15 minutes and the second squeegee was dried in an oven for 10 minutes. Bake time is too short, the temperature is too low, the photosensitive layer is not completely dry, it will result in developing version or printing plate resistance to printing force is low; bake version is too long, the temperature is too high, the photosensitive layer will produce its own cross-linking, causing Development is difficult. Bake plate is the key to ensure the quality of the screen, we must strictly control the screen drying temperature and time.

It is worth noting that the constant temperature oven coated with sensitized adhesive should be used for special purposes. Bake the sensitized adhesive and bake the rinsing water. The vaporization of the washed version will affect the thorough drying of the sensitized adhesive layer, affecting the sensitivity of the sensitized adhesive.

4. exposure

Exposure, also known as printing, affixes the photographed positive film or the four-color common, magenta, cyan, black mesh mesh dot positives of the photographic film to the coated photosensitive emulsion screen. Placed in the printer, vacuum exposure exposure printing. When printing, negative film can not be reversed, otherwise the light through the film surface needs to pass through the film base and then exposed on the screen, easy to make the image level lost, the picture is made false, the color fades.

Multi-color screen printing before the plate to be regular positioning, respectively, the yellow, magenta, cyan, black four-color separation of the positive film placed in the same position on the four photosensitive plates, set a good rule line, and use transparent tape to be fixed.

The role of exposure is to make the non-graphic part of the photosensitive agent layer under the action of light cross-linking reaction, the loss of hydrophilic groups, forming a net-like structure and insoluble in water, leaving the web after development. The graphic part is not affected by light, and no reaction occurs. After developing and washing with water, the glue liquid takes off the wire mesh and forms a through hole. During printing, the printed material passes through the mesh hole and presents the desired graphic print on the substrate.

Screen printers use UV lamps as light sources, and self-made exposure boxes can also use fluorescent tubes as light sources. The exposure time of the heavy chrome salt or diazonium salt photosensitive adhesive under the ultraviolet light source is 3 to 4 minutes, and under the fluorescent light source, the exposure time is 7 to 8 minutes and the lamp distance is 50 to 60 cm. The exact exposure time is flexibly adjusted according to the light source color temperature, light source distance, film thickness, light-sensitive material characteristics, and development temperature.

5. development

After the photographic plate is exposed, the photographic negative film is peeled off, and the photographic printing plate is placed on the plate containing the developing solution face up and developed for 2-3 minutes. The development temperature is controlled at about 20°C. The water layer of the non-photosensitive part is inflated with water and pressurized with water, and the dissolved glue is washed away.

Mesh transfer printing plate development is best to use pots, should not use high-pressure spray gun water development, so that the screen made of graphic rich layers, dot lines smooth, fine points do not lose, large dots do not deform, the edge of writing clean. The development of high pressure water spray gun will make the local density of the screen thinner, the level will be damaged to varying degrees, the hairline will be rough, the dot will be broken, the fine point will be lost, the price will be lost in the level, and the color reproduction will be difficult.

6. Revised, closed net

Renovation is the repair work that is performed on the surface of the plate due to defects in the photographic film, printing plate dust, etc. due to defects such as thin film layers, blisters, and the like. The trachoma in the part of the halftone printing plate is generally not to be repaired, because the four-color overprinting is not obvious; if it is not repaired, overprinting will produce obvious ink spots.

Sealing net is to use sealer or photo-sensitive adhesive to seal the part of the board that should not be opened, to prevent the printed material from leaking and sticking to the printed product.