Silk screen printing plate making process (3)

When developing, care should be taken with thinner lines. If a negative gray scale measuring the density of the standard density is measured with a densitometer, it is better to find the standard exposure time and the development time.

During development, when the specified development time is approached, the darkness of the image on the film becomes dark, and the blackness on both sides is almost the same. At this time, the film should be quickly pulled out of the developer and transferred into the stop-stop solution.

The stop solution is a 2 to 4% acetic acid diluent used to neutralize the alkalinity of the developer attached to the film. Stopped liquid plays a role of protecting the fixing liquid while stopping the development. If it is new liquid, it can only be immersed in several types. During use, the stoppage fluid continuously loses its acidity, so the new fluid should be replaced promptly.

The fixing solution is made of sodium sulfate as a main component, and then added with a protective agent, an antifouling agent, a hardening film, and a pH adjusting agent. It has the effect of dissolving light-sensitive silver salts. The film was removed from the stop solution and transferred to the fixing solution. The undeveloped residual silver halide was dissolved to make the film transparent. Do not remove it immediately after being transparent, but continue immersing it for the same amount of time. That is, the fixing time is twice as long as the start of fixing to the transparent time. If you use the new solution for 2 minutes, the ordinary solution for 10 minutes, and then you can extend some time, but it should not exceed one hour.

The fixed-finish film should be well washed in order to remove the remaining drug on the film, as shown in Figure 2-71. Rinse for about 15 minutes. If the film is washed too easily and unevenly, it is necessary to often dither the film to make it fully washed. At the same time, care should be taken not to soak in water overnight.

The washed film should be placed on a clean glass plate. After the water is removed with a squeegee, it is naturally suspended by a clip or placed in a drying oven and dried. The drying box is equipped with a heater, so that the temperature inside the box is heated to 40°C, and the film is hung in the drying box and dried.

The quality of the negative should be checked before drying. If the darkness is too dark or too dark, the fine lines can not be used, only re-shooting. If there is some ash, it can be removed with a mixture of red blood salt and sodium thiosulfate.

Films that are placed in a dry box and dried are sometimes spotted because they have not been dehydrated before being placed in the oven. They can be removed with a surfactant.

Development should pay attention to the following matters: You must use the developer specified by Reese film; follow the instructions for the preparation of the specified developer, accurately called the amount of the custom agent; attention to the development of liquid temperature does not change; according to the developer specified time development; developer placement It cannot be used when it is too long or discolored.

d. Production of positive films. The bottom plate of the screen printing plate is positive, different from the letterpress plate and lithographic plate. The negative film should be turned into a positive film. There are three methods: sealed printing method; camera re-irradiation method; enlarger magnification method.

The sealed printing method is a method of printing on a Reese film used for printing in a sealed printing machine. The principle of this type of plate-making machine is the same as that of the vacuum plate-making machine, and there is no big difference in the structure. However, the object of the printing is a film and a photographic paper, and the sensitivity is greatly different from the photographic solution coated on the plate. Therefore, the light source does not use a strong light source such as an arc lamp or a mercury lamp, and a light bulb of 300 W is used, and a voltmeter is installed, and it can be timed in seconds.

The film surface of the negative film and the Rees film of the printing plate are overlapped, vacuum-sealed with a vacuum pump, exposed and printed. The Reese film used in the printing and the film used in photography are made of hard-tone emulsion. There are two kinds of special photography and seal printing. Dedicated to sealed printing film, the photosensitive speed is very slow, can not be used for positive color film. The printing film can be developed and fixed as a normal film.

The precision of the sealed print is the same as that of the negative film of the same size. However, it is not economical to use two large films when printing large-size prints. If there is a transmission camera device, by multiplying the negative film, the positive film can be enlarged by two to three times, so that the negative film can be small even if it is small. The cameras produced in recent years are mostly equipped with document holders that can perform transmission photography and reflection photography, and the lighting can also be freely moved to the back.

However, the camera is limited by the magnification ratio, its magnification is only two to three times, and the replacement lens can be four to five times. The size of the positive film thus produced is limited. In order to enlarge the positive film, it must be enlarged with a magnifying machine.

Zoom cameras have vertical and horizontal types. Recently, there have been sold on the market dedicated to screen printing negative film and zoom cameras. The camera is installed in a dark room, the negative film is clamped on a glass plate, mounted on the original document holder of the camera, and the image is projected on the film with the illumination light on the back of the negative film for exposure. In this way, the negative film can be enlarged up to six times to make a 120×150 cm large positive film.

There is a method that does not need to shoot the original as a negative film, but can be directly made into a positive film. This method is a reversal developing method. However, the direction is opposite to that of the original document. Therefore, a thin photosensitive film should be used during the shooting and the photosensitive film should be back-illuminated. However, with this method, the tune is not very stable, and at the same time it is not convenient to insert text and enlargement, so it is not generally used.