Screen printing plate classification - Photographic plate (2)

c) Coating of the photosensitive liquid. There are many ways to apply the photosensitive solution to the web. The most common methods are the scraping method and the rotating method.

In spin coating, the plate is fixed on the applicator. The photosensitive liquid is poured into the center of the plate, and the fallen liquid is uniformly coated into the periphery by centrifugal force. Compared to coating with a blade, a film having a uniform thickness can be obtained. If the conditions such as viscosity, rotation speed, and mesh of the photoreceptor are the same, anyone can apply a fixed coating. However, it is only suitable for the coating of small plates. The area around the large plate cannot be coated (the range is 50 x 50 cm). See Figure 2-111.

Figure 2-111 Photosensitive coating

1- Photoresist 2-Motor

Scraping is a kind of coating method, and the front end of the scraper is brought into contact with the screen for coating. The scraper is a ship-type coating tool. One of the four sides functions as a doctor blade, and the inside of the bucket stores a photosensitive liquid. The shape of the scraper is various, but it should have two parts: the groove and the scraper.

Scrap coating includes hand coating and machine coating. As shown in Fig. 2-112, the scraper bucket with hand-applied method shows arcs of different radiuses on the side of the bucket, and the bucket edge is convex along the length to ensure that the thickness of the coating is uniform. Due to the strong acid reaction of diazo sensitized adhesive, the plastic hopper must be made of plastic or stainless steel. However, even if it is a stainless steel rubber bucket, the glue liquid should not be stored in the bucket for a long time, and the glue liquid will be destroyed due to oxidation.

The length of the bucket depends on the size of the coating surface. The stainless steel rubber bucket produced in China is composed of 6 different lengths, which can be used for different sizes of drawings. The relationship between the area of ​​the frame, the length of the rubber bucket and the area of ​​the squeegee is as follows:

Figure 2-112 Squeegee

Frame area = a × b (a is a short side, b is a long side);

Scratch area = (a + 4cm) × (b + 6cm);

Rubber bucket length=a+4cm.

In this way, both the glue and the work area are saved.

The edge of the edge of the scraper in contact with the wire mesh must ensure a high degree of flatness and no sign of bruising. If the flatness is low or there is a bump, the streak or thickness of the film layer will be uneven after the coating is applied, so that the burr and the thickness of the ink layer are uneven after the printing. The edge of the edge of the scraper should be smooth to prevent the consequences of scratching the screen during coating. Since the stretched mesh has a certain degree of elasticity, the thickness of the film tends to be uneven when the coating is applied, that is, the film thickness in the middle portion is thin and the film thickness is thin in the four sides. In order to avoid this type of problem, it is common to make the side of the contact screen slightly curved when making the scraper. This avoids uneven film thickness due to screen elasticity. Use rustless, corrosion-resistant, light-weight stainless steel as much as possible when making scrapers.

The sequence of the scraper coating is shown in Figure 2-113. In the figure: 1 Place the net frame of the stretch net at an inclination of 80-90°, and pour the photosensitive liquid with a capacity of 6-70% into the bucket. The front presses onto the web. 2 Tilt the tip of the placed bucket so that the liquid level contacts the screen. 3 Apply coating while keeping the tilt angle constant. If the coating speed is too fast at this time, air bubbles are likely to cause pinholes. 45 When applied to the side of the frame 1 to 2 cm, the tilt of the bucket is returned to near horizontal, and no liquid is applied until the coating is applied. After all of the sensitized adhesive is coated in this way, the frame is inverted upside down and re-applied once, and then dried. The first drying should be sufficient. If it is dried with hot air, the proper temperature should be mastered. If the temperature is too high, there is a possibility of generating hot ash. Attention must be paid to it. After drying, apply the same method again 2 or 3 times until gloss appears. The magnitude of the force of the scraper contact net varies depending on the coating speed. If the scraper reciprocation is counted as one stroke, the thickness of the photosensitive film applied once is 1.2 to 1.6 μm in the completely dried state. Therefore, a film thickness of 10 μm can be obtained after 7 to 8 strokes.

Figure 2-113 scraper coating

Usually, the screen surface to be coated is the surface that is in contact with the substrate. In order to increase the resistance to the printing force, the scraper can make one stroke or two strokes in the wind as one stroke.

This kind of coating hopper coating can be grasped with only a little practice. If the number of strokes is fixed, the film thickness can usually be obtained. However, when the film thickness requirement is quite strict, it must be measured with a film thickness meter.

The number of coats depends on the required stencil thickness. In order to make the coating surface smooth, a "wet-dry-dry" coating method is adopted, that is, coating and drying are alternately performed (FIG. 2-114). Each time it is called once again, the general film needs to be coated 2 to 3 times, 2 to 3 times each time. If the consistency of the glue, the rate of through-holes, and the operating parameters of the coating film change, the number of times of gluing should also change, and even “wet wet” can be completed all at once.

Figure 2-114 Coating sequence