Analysis of the Reasons for the Resistance to Force of the Positive Graph Type PS Plate

Fifth, the effect of coating photosensitive liquid.

Photosensitive liquids used in positive-type PS plates mainly include photosensitizers, film formers, solvents, and dyes, as well as some additives such as plasticizers and stabilizers. In this composition, sensitizers, film formers, and deep-sleep plasticizers are the main factors affecting the impact resistance.

1. Effect of photosensitizer

In the composition of the photosensitive liquid, the content of the photosensitive agent should be between 3 and 6%. If the content is less than 3%, the abrasion resistance and alkali resistance of the photosensitive layer will be greatly reduced, and when the content exceeds 6%, the photosensitive layer The alkali resistance of the layer is so strong that the sensitizer still remains in the sand spot after normal exposure and development, so that the blank portion has a strong sense of fat and causes dinging.

Secondly, the proportion of photosensitizer and film former should be appropriate, and the recommended ratio is: photosensitizer: film former = 1:16 to 1:2.0. In this ratio range, it is ensured that the formed photosensitive layer has sufficient abrasion resistance. , Alkali resistance and smooth development.

2. Effect of film formers

In the photosensitive liquid, the content of the film forming agent is about 6 to 8%. If the content is too high, the stability of the photosensitive layer will be affected; if the content is too low, the printing plate's resistance to printing and sensitivity will be reduced.

3. Effect of solvents

The amount of solvent directly affects the thickness of the photosensitive layer. If the photosensitive coating is too thin, the lipophilicity and printing resistance of the printing plate graphic part will be reduced. According to the regulations of the industry standard HG/T269495 issued by the Ministry of Chemical Industry, the photosensitive layer per unit area should be controlled at 1.60 to 2.50. %g/m2, in conjunction with this thickness, the amount of solvent should generally be controlled between 80 and 90%.

4. Plasticizer effect

The addition of a plasticizer helps reduce the brittleness of the photosensitive layer and improves the abrasion resistance of the photosensitive layer in printing.

In the above discussion, the factors affecting the printing force during the production process of the positive PS version were discussed. Through these production steps, positive type printing plates can be obtained (structure is shown in Figure 2).

The second part of the use of printing plate on the impact of the printing force

There are many factors that affect the printing plate resistance of the printing process. Here, some major factors are discussed in depth from two aspects.

First, the impact of printing

The printing process flow of positive PS version is:

Exposure→Development→Finishing→Roasting Processing→Maintenance

1. Impact of exposure

To understand the impact of exposure on the impact resistance, we must first understand the chemical reactions that occur during exposure and the changes in the photosensitive layer of the plate (Figure 3). The positive-tone PS photosensitive layer is an o-diazonaphthoquinone-based photosensitive material, which is insoluble in alkaline solution, but becomes an alkali-soluble, printed carboxylic acid compound after exposure.

The amount of exposure will directly affect the decomposition of o-dinitronaphthoquinone (lipophilic layer) and the formation of printed carboxylic acid compounds (hydrophilic substances), which also affects the lipophilicity and hydrophilicity of the printing plate.

If overexposed, it will lead to a reduction in the dot of the image part of the printing plate, and the oleophilicity of the graphic part will be weakened and the wear resistance will be reduced.

If the exposure is insufficient, part of the image lines will be thicker than the original, the dot area will be enlarged, and the hydrophilicity will be weakened to cause dingy.

There are two reasons for inaccurate exposure:

1 The density of the original graphic part is too low or too high.

The exposure time is inaccurate. The density of the original plate: the transmission density in the image area cannot be less than 2.8, and the transmission density in the blank area is less than 0.05.

To find a suitable exposure time, test each batch of PS plates with a Bruner test strip or a Fuji gray scale. Judging from the printing scale, the printed printing plate is controlled to be transparent to white in 3.5 to 4.5, and the printed printing plate is controlled to be translucent in 4.5 to 5.5 grade. If the actual printing operation, the distance between the light source and the printing plate changes, the exposure time should be adjusted according to the rule that the exposure time is proportional to the square of the distance from the light source to the layout.