Positive Pattern PS Platemaking Process (3)

Second, the development

The PS plate is a plate coated with a lipophilic photosensitive layer on the surface of a hydrophilic aluminum substrate. Therefore, it is necessary to leave a photosensitive layer of the image portion after printing on the plate, and remove the photosensitive layer of the non-image portion to expose the hydrophilic plate. The process of removing the photosensitive layer of the non-image portion by the developer is called development.

1. The principle of development

The purpose of developing the positive PS plate is to remove the photo-degraded photosensitive layer on the plate surface. Commonly used 215, 214, and 124 photosensitive resins, see light decomposition reaction, generate tannic acid compounds dissolved in dilute alkaline solution. Therefore, using a dilute alkaline solution as a developer, the PS plate was subjected to a development process, leaving on the plate surface a lipophilic, slightly convex embossed image formed without a photosensitive layer.

The photosensitive layer of the non-image portion, which can be completely removed from the plate after development, first depends on the solubility of the photosensitive layer to dilute lye. If the photosensitive layer is fully exposed during the printing process, the photon-absorbing energy of the photosensitive material enables a certain amount of molecules involved in the photo-decomposition reaction, and better development effect can be obtained.

The development of the positive PS plate is essentially the dissolution process of the photoreceptor layer. The higher the temperature, the faster the development rate and the more the photosensitive layer is dissolved away. It is generally believed that the development speed increases by 1 time for every 12°C increase in temperature. When the temperature exceeds 30°C, the binding force between the photosensitive material (photosensitive resin such as 215, 214, and 124) and the film forming material (phenol resin, etc.) in the photosensitive layer is weakened, and the anti-alkalinity is reduced, and no phototext is found. Some of the sensitivity will be dissolved, so the temperature will increase further, the percentage of the area of ​​the layout will be reduced (Figure 6), and the reproducibility of the printing plate will be destroyed. In order to stabilize the development quality, the developer temperature should be controlled at 25 Nmousewheel="return bbimg(this)" alt="" src="" class="lazy" original="" onload="" border="undefined" />5°C

Nmousewheel="return bbimg(this)" alt="" src="" class="lazy" original="" onload="" border="undefined" />

Fig. 6 Development Conditions and Dot Reproducibility

If the development time is too long (developing speed is too slow), fine dots on the plate will be easily lost. If the development time is too short (developing speed is too fast), the photosensitive layer of the non-image portion cannot be completely dissolved, causing the layout to be dirty. In particular, the PS plate coated with a centrifugal coater has a large difference in thickness between the central portion and the peripheral portion of the photosensitive layer, and the quality problem caused by improper development time is more prominent. It is generally believed that the development time should be controlled from 30 seconds to 2 minutes. The better PS plate should have a development tolerance of 2 minutes. The composition and concentration of the developer should also match the above development time.

2. Developer

The developer consists of water, developer, inhibitor, wetting agent, and the like.

The developer is the main agent of the developing solution and is used to dissolve the light decomposing part of the photosensitive layer of the PS plate. The commonly used strong alkali developing agent is NaOH, KOH (potassium hydroxide), and the weak alkaline developing agent has Nmousewheel="return bbimg(this)" alt="" src="" class="lazy" original="" onload="" border="undefined" /> To prepare the developer, different alkaline developers are selected according to the method of aluminum plate surface treatment and the thickness of the oxide film. For aluminum base strong alkali resistance, such as the plate base after anodizing, high temperature sealing treatment, oxide film thickness of 5 g/m2, or plate base oxide film alkali resistance test, alkali resistance time of more than 50s For the PS version, NaOH or KOH is mostly used. For aluminum bases, the alkali resistance is weak. For example, the substrate is not anodized, only hydrophilic materials are coated on the surface of the aluminum plate, or the plate base oxide film is resistant Alkaline test, PS plate with less than 50s alkaline resistance time, more choice Nmousewheel="return bbimg(this)" alt="" src="" class="lazy" original="" onload="" border="undefined" /> Nmousewheel="return bbimg(this)" alt="" src="" class="lazy" original="" onload="" border="undefined" /> In addition to the developing effect, there is also the formation of a hydrophilic film layer. The role of buffer corrosion, not only makes the plate has a good hydrophilicity, but also the solubility of the graphic part is very small, is a stable performance, long service life of the developer.

Inhibitors are used to suppress the speed of development. Because the aluminum oxide base film's oxide film has poor resistance to alkali, it is strongly recommended that it be formulated with developer. When used, the developing speed is very fast. In order to prevent the dissolution of the developer from the aluminum plate base, the developing speed must be reduced. The method is to develop Inhibitors are added to the solution. The commonly used inhibitors are KCl (potassium chloride), NaCl (sodium chloride), Nmousewheel="return bbimg(this)" alt="" src="" class="lazy" original="" onload="" border="undefined" /> (sodium phosphate), KCl and Nmousewheel="return bbimg(this)" alt="" src="" class="lazy" original="" onload="" border="undefined" /> In addition to suppressing the speed of development effect. Such as the use of formulated developer, due to Nmousewheel="return bbimg(this)" alt="" src="" class="lazy" original="" onload="" border="undefined" /> has its own inhibitory effect without the need for additional inhibitors.

The role of the wetting agent is to reduce the surface tension of the developer, increase the wettability of the developer, and evenly spread the developer onto the surface of the printing plate to facilitate uniform development. Common wetting agents include surfactants such as sodium dodecylbenzene phosphate and Tween. Sodium dodecylbenzene phosphate is an anionic surfactant and Tween is a trade name for sorbitan stearate polyoxyethylene ether, a nonionic surfactant. In addition to their wettability, these two surfactants also have emulsifying and dispersing properties, and they also help the printing plate in the development process.

The PS-versions produced by different manufacturers, the composition of the photosensitive layer, and the processing method of the plate base are all somewhat different, and their alkali resistance is also different. Therefore, the developer must be formulated with developers, inhibitors, wetting agents, and matching ratios based on the performance of the PS plate. Here are two commonly used developer solutions for reference during formulation.

Strong base dilute solution developer:

NaOH 1～4 g
Nmousewheel="return bbimg(this)" alt="" src="" class="lazy" original="" onload="" border="undefined" /> 25g
Sodium dodecylbenzenesulfonate 1～2g
Water 1000ml

Weak alkali dilute solution developer:

Nmousewheel="return bbimg(this)" alt="" src="" class="lazy" original="" onload="" border="undefined" /> 3~6g
Sodium dodecylbenzenesulfonate 1～2g
Water 1000ml.

The higher the content of the developer in the developer, the faster the development. If the development is excessive, the photosensitive layer without light will be dissolved, the percentage of the dots on the printing plate will be reduced, the grease of the plate and the resistance of the printing plate will be reduced. Even "off version" failure; the other hand, the lower the developer content, the slower the development speed, which not only to extend the development time, but also due to lack of development, resulting in an increase in the percentage of outlets, layout dirty. The content of the developer is usually determined according to the developer formulation provided by the manufacturer of the PS plate. However, since the development conditions have a direct influence on the development quality, the test development must be performed under a fixed development condition before the final development. Developer concentration. The development tolerance of domestically-made positive-working PS plates is generally 2 minutes. In order to accurately control the concentration of the developer, the PS plate with the drying plate can be developed for 30-60 seconds, and then used for 30-50 times. Observe the magnifier that if the hollow point of the printing degree step scale is 98 to 99%, the hollow point is not padded, and 1 to 2% of the small black spots are not lost, the developer concentration is appropriate.

Third, the development operation

Mechanical development is performed using an automatic developing machine. There are many types of automatic developing machines, but their structures are basically the same. They are composed of a liquid temperature control device, a speed adjustment device, and a circulation device for developing and other rinsing liquids. Various solutions to the layout of the role and circulation are carried out in their separate discrete slots, there can be no leakage between each other. FIG. 7 is a schematic view of a structure of an automatic developing machine that is currently widely used. Development and processing are performed by using a spray pipe to spray liquid onto a plate, followed by brushing.

The efficiency of the automatic developing machine is very high. A class (8 hours) can process more than 30 plates, and the development quality is also very stable. However, the developer will generally be aged during use. The aging phenomenon of the developer refers to a phenomenon in which the developing solution gradually fades in the process of continuously processing a large number of printing plates until it completely disappears. This aging phenomenon is the result of developing the developer for a long period of time. For the alkaline developer, it is also related to the occurrence of a neutralization reaction in the air absorbed by the NaOH in the developer. The ageing phenomenon of the developer can be observed on the gray scale stamped under the same exposure conditions when observing the blank progression (Fig. 8). Due to the effect of ageing of the developer, the number of plates that are qualified for development in production is limited, which requires a predetermined number of plates as a standard. The number of developed plates exceeds this standard, and the developer must be updated. The specific method for determining the standard printing plate number is as follows: The PS plate is cut into small pieces, and each block has a grey ladder ruler and a dot ladder ruler (or printing ruler ruler), and is printed under the same conditions to make a test block. (The quantity meets the test requirement, generally about 40-50 pieces) Stored in a place that is protected from light and not affected by temperature and humidity. Starting from the developer loaded with a new developer.