Pinholes are also collectively referred to as screen surface micropores. It can produce print sticky, local ink layer deposition, thinning the drawbacks. Due to the uneven surface area of ​​the photosensitive adhesive on the surface of the screen, the level printing becomes more unstable, and the color difference becomes more apparent, which will also greatly reduce the service life of the screen.

Analysing the causes of pinholes and taking countermeasures in the entire process of screen printing will greatly benefit the quality of screen printing.

First, the reasons for pinholes

Pinholes appear in the screen after development, and are often caused by the following reasons:
(1) Incorrect screen degreasing treatment.
(2) Debris residues remain before rinsing and dry on the screen surface.
(3) Improper drying method after coating of the photosensitive paste (4) Improper drying method after coating of the photosensitive paste (5) Put the screen plate together with the developed screen plate or the degreasing screen plate after drying (6) Screen printing Exposure is not yet fully dry (7) Dust and dirt are adhered to the printing plate (8) Dust is adhering to the platen glass (9) The dust particles in the plate-making room air fall on the surface of the photoresist (10). Dust in the bucket (11) Unsuitable coating speed (12) Improper mixing of latex and diazo sensitizer (13) Immersion of sensitive adhesive for too long (14) Water is not clean enough

Second, pinhole solution

(1) Ensure that the correct degreasing treatment method is used. When the screen is degreasing, uniformly apply the degreasing agent to the surface of the screen and brush thoroughly with a brush. Start with the printing surface, and then brush the plate for 3–5 minutes. about.

(2) Chemicals such as degreasing agents may remain on the screen and cause damage to the screen. Manufacturers have detailed instructions for them. Before performing the next process, be sure to wash with pressure water to ensure that the screen is thoroughly cleaned. The standard of screen washing is that the screen surface can be evenly adhered to a layer of water suede, otherwise it must be re-degreased.

(3) Appropriate increase in the coating thickness, to the level of printing, the direct film or indirect film paste method should be adopted, and for the direct method, the thickness must reach 7-8μm, otherwise it is easy to pinhole.

(4) Drying after screen coating should be carried out in the correct way. The screen should be placed horizontally for drying. The drying temperature should be below 40 degrees Celsius. Otherwise, pinholes may appear on the upper part of the screen.

(5) Direct legal network edition. The post-coating drying of the film/emulsion system screen should be dried in a separate drying oven and cannot be combined with the screen after degreasing and the screen after washing. If put together, the evaporation of moisture on these screens may fall on the surface of the emulsion layer to form fisheyes and pinholes. If possible, two drying ovens should be prepared.

(6) Exposure cannot be performed until the photoreceptor has not completely dried. The screen is placed in a drying oven. The moisture on the direct screen plate is evaporated from both sides. The moisture on the film/emulsion screen can only evaporate from the scraping surface. The presence of water will reduce the photon activity and hinder the sensitivity of the photosensitizer. Sex. Sometimes it seems that both sides of the screen are dry, and in fact they are not dry inside and must be further dried. For film/emulsion screens, remove the film and dry it for about 2 minutes to ensure sufficient moisture evaporation.

(7) Stain on the printing film will have adverse consequences on exposure. All the fingerprints and dust traces on the printing film should be wiped clean with cloth and isopropyl alcohol, and electrostatic dust cloth should be used for dust treatment before use. When using screen printing software to paste the screen for exposure, be sure to The film layer of the printing plate is tightly compacted against the surface of the screen.

(8) Dust and dirt on the platen glass of the printer also affect the exposure. To wipe the surface with a suitable glass cleaner and a soft fleece, the glass transmittance must be replaced in time.

(9) Improper plate making environment can also cause defects in the quality of the coating process, and it is the main cause of pinholes. The establishment of equipment should be regularly cleaned; the floor should be painted with protective paint or paving bricks to prevent dust from escaping from the surface. From the beginning; regular cleaning work every day, with a damp cloth to wipe the ground or wipe the ground with an electrostatic cloth, conditional electrostatic precipitators in the plate room.

(10) Before the photosensitive adhesive is poured into the rubber bucket, wipe the rubber bucket with an electrostatic cloth, and use the gluing machine to apply glue. The glue roller must be cleaned.

(11) Pay attention to coating speed and coating pressure. The speed of moving the glue bucket on the screen surface should be controlled at about 4 seconds/3 inches. If the speed is too slow, it will cause uneven coating; if the coating pressure is too high, the central part of the screen during the coating process will When the edge of the glue bucket is separated from the contact, the coating thickness of the central portion is too thick, and the thickness of the glue layer on both sides of the glue bucket is too thin.

Since the length of the exposure time depends on the thickness of the photosensitive layer, the center portion often suffers from underexposure. Screens with loose mesh are not used in printing because this inevitably causes the glue bucket to be coated with a lower pressure and cannot control the thickness of the coating. Uneven thickness is a potential cause of pinholes.

(12) Many photosensitive adhesives need to be sensitized before use. Dissolve sensitization in distilled water first, mix well and then pour into emulsion and mix well. Sensitized sensitized adhesive needs to be placed for at least 1 hour, and sometimes it takes 12 hours to allow air bubbles to completely disappear. Then the coating operation is performed. Inhomogeneous mixing is easy to form uneven thickness during coating, and bubbles are the culprit of pinholes.

(13) Most sensitized adhesives can be placed for 4-8 weeks after being sensitized, and if the sensitized adhesive is stored in a refrigerator for a significantly longer period of time. The use of photoemulsions that are stored for a long time often causes problems.

(14) Fine particles of impurities in the water source will also form pinholes in the screen. To overcome this problem, a 5-micron screen and filter can be installed on the faucet.

Source: Henan Graphic Arts Longcheng Fly