Several factors affecting the process of screen making

In addition to the selection of plate-making materials, the key to improving the quality of screen printing products lies in the rationality of the screen printing plate production process. At present, screen printing plates are mostly hand-made, and there is an urgent need for the improvement of plate-making techniques. Here, some of the problems encountered in the manual platemaking process are discussed briefly.

We have used three kinds of plate-making materials: dichromate silk screen film (film paper), diazo film screen glass and alcohol-soluble nylon screen film (hereinafter abbreviated as F, D, N). In the production process, various kinds of materials should pay attention to the following issues:

First, the choice of screen mesh number and the impact of stretch net on the quality of the printing plate

l, the choice of screen mesh number

The number of screen meshes to be selected depends on the printed pattern and the requirements on the pattern. If the pattern is larger and the lines are thicker, and the requirements for the pattern are not as accurate, a mesh with a lower mesh number may be used. Conversely, if the pattern is small, the lines are narrow, and the pattern needs to be precise, a mesh with a higher mesh number should be used.

The number of screen meshes directly affects the printing quality. Because of the relatively low screen mesh, the printing ink permeability is good and the mesh is not easy to plug. However, the number of meshes is too low, and it is easy to infiltrate the printing material too much, resulting in unclear pattern edges and pattern distortion.

The high number of screen meshes can meet the requirements of clear edges and lines. However, the number of screen meshes cannot be higher than the number of printed materials. Otherwise, it will easily block the mesh. Therefore, a moderate number of meshes should be selected, generally between 120 mesh and 300 mesh. The dichotomy in the single-factor merit selection method can be used as a selection test.

2, stretch net

When the screen is stretched over the frame, the degree of tightness should be appropriate, and it should be noted that the tension at each point in the frame should be the same. If the screen is stretched too tight or too loose, or if the tension at each point is inconsistent, the following problems may occur:

The screen is stretched too tightly: to make the printing plate lose its elasticity, resulting in distortion of the pattern during printing;
The mesh is too loose or the tension is not the same:
1 fine lines cannot be developed;
2 The edges of the pattern appear zigzag, and the pattern distortion;
3 Leakage phenomenon during the printing process;
4 In terms of F plates, the adhesion between the film and the screen is poor, resulting in the phenomenon of film peeling during development;
For D and N versions, the layout is difficult to evenly flatten when squeegeeing. The glue cannot adhere tightly to the screen. D version of the film caused by the development of the phenomenon of smooth slipping off the N version of the layout is also very easy to produce trachoma.
5 In terms of F and D, the net is loose and it also affects the reuse of the screen.

Second, the temperature on the quality of the printing plate

The quality of the printing plate is related to the various temperatures during production. This temperature includes: 1, the operating room temperature; 2, the oven temperature; 3, the exposure chamber temperature; 4, the developer temperature.

l, operating room temperature

The operating room temperature should generally be maintained at about 25 °C. Winter and summer seasons should pay attention to the regulation and control of temperature.

For F version, the influence of room temperature is not obvious.
For D version, if the room temperature control is not good in hot weather, the following problems will occur:

1 Preparation of glue can easily produce thermal cross-linking, resulting in difficult or obvious development;
2 A large amount of air bubbles are generated in the glue solution, causing bubbles or blisters on the plate.
For the N version, when the room temperature is lower than 2O°C, the following problems will occur:
1 Difficult to develop or difficult to develop due to uneven thickness
Trachea appeared on the 2nd surface, which caused the phenomenon of slurry leakage in the printing process;
3 uneven layout, resulting in inconsistent print pattern color.

2, the oven temperature

According to the drying temperature set by the respective performance index of the plate-making materials, the temperature should be gradually tested from the lower limit of the specified range according to local conditions, and the optimum drying temperature should be selected. Normal drying N version without blowing; drying F or D version should be added to the blast, in order to speed up the speed of the layout of the water, as soon as possible to make the layout dry.
If the drying temperature is too high, the following problems may occur:

1Development difficulties, resulting in ignorance;
2 The screen loses its elasticity, resulting in distortion of the printed pattern.

If the drying temperature is too low, the drying time must be prolonged, otherwise the layout will not be fully dried. For F, D version, it will cause the phenomenon of film layer or glue layer falling off; for N version, the glue layer is also easy to thin when developing, thus shortening the service life of the printing plate.

3, the temperature inside the exposure box

This temperature is easily overlooked. The drying temperature is often better, and the temperature of the exposure box will eventually make the plate-making failure.

The temperature in the exposure chamber depends on the type of light source. If it is a cold light source, it is generally not considered; if the source of heat is relatively intense, such as a high-pressure mercury lamp, special attention should be paid. Because the high-pressure mercury lamp cannot ignite instantaneously, several plates of continuous exposure methods are usually used. In this way, the temperature of the surface of the plate at the time of exposure can be as high as 85°C or higher. This temperature is extremely unfavorable for the three layouts of F, D, and N. It will cause similar problems such as development difficulties due to excessively high temperatures in the oven, and will seriously affect the quality of the printing plate. Therefore, exhaust equipment must be installed in the exposure box to reduce the plate surface temperature. In general, the surface temperature of the plate cannot exceed the drying temperature.

4, developer temperature

Developing: It is the last condition for the success or failure of the printing plate. When the process is inadvertently performed due to the previous process, the method of increasing the temperature of the developer is often adopted in order to accelerate the development speed. The temperature of the developer should be controlled with caution, and the drying temperature should generally be the upper limit.
If the developer temperature is too high, the following problems can easily occur:

For F and D plates, it is easy to make the plate swell, and a zigzag pattern appears around the pattern.
For the N version, it is easy to make the pattern edge frizzy, resulting in pattern distortion.

Third, the impact of time on the quality of the printing plate

The quality of the produced plate is also very much related to the time. The control of time is the same as the temperature, and the requirements are relatively strict, mainly including: 1, the drying time in the oven; 2, the exposure time; 3, the development time.

l, in the oven time

This time should first depend on the degree of drying of the layout, and then consider the drying of the specified range.
For D and N, since the squeegee is manually operated, the thickness of the squeegee is not completely controlled. In general, the glue layer is thicker and the drying time is also properly lengthened. Each time the glue is not too thick, it should be thin and uniform. Again, the squeegee should be done on the basis of the previous full drying, otherwise there will be inconsistent surface dryness.

For the F version, it should also pay attention to the amount of sensitizer. Otherwise the layout is difficult to dry. If the F and D editions are not fully dried, they will cause the film or adhesive layer to slip off during development. If the drying time is too long, it may easily cause difficulties in development or may not appear.

For the N version, since it does not have a drying limit, it depends only on its degree of drying. Therefore, under the normal drying temperature control, it can be stored in the oven for a long time to ensure that its layout is fully dried. If it is not fully dried, the layout is easily thinned and blisters appear.

2, exposure time

The process of exposure is a photochemical reaction process. Regardless of the type of media, the exposure time, despite having a specified value, must be tested several times. In the plate making process to be able to be aware of the entire process, estimate the problems that will occur after the development, so as to finally determine the appropriate exposure time.
To judge from the condition when developing: the exposure time is appropriate.

Fast development, clear patterns, crisp lines, proven exposure time is appropriate;
Difficult to develop or appear to show that the exposure time is too long;

The exposure time is not enough. For the F and D plates, the surface of the film or adhesive layer during the development is slack or easily peeled off. For the N version, the layout is blank and the non-patterned parts are glued.

3, development time

The most obvious indication of a successful printing plate is the relatively short development time. This means that the adhesive film has good adhesion and development. If the development time is long, it not only indicates that there are problems in the production process, but also affects the service life of the printing plate, and even results in failure of the production. Because the photochemical reaction is not sufficient during the exposure process, the developer dissolves the film of the non-light-transmitting portion and also affects the portion where the reaction is insufficient. Therefore, the long development time will affect the fastness of the printing plate. The development time is too long. For F and o editions, the following problems will occur:

1 pattern edge swelling, lines are not clear;
2 The surface of film or adhesive layer is smooth, and the adhesion to the screen is reduced;
3 film, adhesive layer easy to fall off.

The development time is too long. For the N version, the following problems will occur:

1 layout thinning, appear trachoma;
2 pattern edge hair, lines are not stiff.

The process of making a printing plate is a complex physical and chemical process that involves many factors. In addition to the above-mentioned factors, there are also exposure luminosity, humidity in the process of making plate, and so on. Especially in the plate making process, many processes are manual operations, and many factors are directly controlled by people. Therefore, many factors cannot be measured accurately in the manufacturing process. For example, the thickness of the squeegee is difficult to measure during operation. Therefore, how much time can be dried can not be specifically determined; for example, how thick the film after squeegee is, how long the exposure time should be, and it is not proportional. Incremental, but after repeated trials. However, due to the other preparation factors in the plate making process in the previous stage, the conclusions reached are only a reference. In short, to make the screen printing process to be as complete and reasonable as possible, it is necessary to continuously summarize, explore, and find out some rules through long-term practice.

Source: China-Hong Kong silk screen network