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The characteristics of CTP technology have certain requirements on the quality of CTP plates and CTP plates, such as apparent quality, physical properties and adaptability to electrolysis. The following is a detailed introduction to the aluminum plate base commonly used according to CTP plates. .

The overall requirements for printing on the plate are very high. Especially the current CTP plate is mostly used for high-grade color plate making. To ensure the quality of the plate, the apparent quality of the plate is the premise.

 

Requirements for apparent performance of the base

The basic requirements for the appearance of the aluminum plate base are clean, flat, no cracks, corrosion pits, spots, vent holes, scratches, creases, marks, peeling, pine branches, oil marks, etc.; Non-metallic indentation and sticking, horizontal skin, horizontal stripes and other defects; no slight chromatic aberration, bright strips, etc.; no bulge, ruffles and so on. In addition, CTP plate production has the following three specific requirements for plate flatness.

(1) The gap between the unrolling surface of the aluminum coil and the plane shall not be greater than 1 mm, and the number of waves per meter of unfolded surface shall not exceed 3;

(2) The side bends shall not exceed 1 mm per meter length;

(3) The aluminum coil should be tight, the end surface is neat, and no cracks or burrs are allowed.

CTP plates do not allow a little unevenness, because most CTP devices use scanning imaging. Unlike the PS plate, the vacuum is used to make the layout and film close. If the plate is flat, it will affect the laser imaging quality. . Of course, the flatness of the plate is related to the condition of the plate production equipment.

 

Requirements for the geometry of the aluminum plate base

Nowadays, the CTP platesetter market is mainly composed of thermal CTP and violet laser CTP. The thermal CTP is plated by an external drum type laser scanning method, and the plate material is fixed on the outside of the drum, and the plate is rotated at a high speed at the same time as the drum. The violet laser CTP plate making machine adopts the inner drum type laser scanning method to make the plate, and the plate material is placed on the inner side of the drum. When the plate making plate is stationary, the laser head is scanned at a high speed. The plate-making mechanism version has a high speed and adopts an automatic plate-up, which automatically performs positioning and punching, so the geometrical requirements on the plate base are very strict. The geometry of the substrate includes the thickness of the substrate, the width of the aluminum coil, the uniformity of the substrate, and the like.

1. Aluminum plate base thickness and tolerance

The thickness of the aluminum plate base for the CTP plate material is generally 0.280 mm, which is slightly thicker than the PS plate currently produced (the PS plate plate base thickness is 0.270 mm ± 0.005 mm). Some foreign companies use 0.275mm thick aluminum plate bases with thickness tolerances of ±0.005mm. The base of the aluminum plate is thicker, the plate stiffness will be better, and it is not easy to bend.

2. Aluminum coil width tolerance

China's plate production generally does not install online trimming device, so the width tolerance of aluminum coil is required to be ≤±0.5mm, preferably without tolerance.

There is also a clear requirement for the cutting quality of the aluminum coil, that is, there should be no burrs or ruffles on both sides of the aluminum coil after slitting.

Of course, the electrolytic edges on both sides should be cut before coating.

3. Vertical and horizontal uniformity of the base

The longitudinal and lateral thickness tolerances of the aluminum plate base shall be ≤±0.005mm. Now the plate making machine of each model sets the thickness of the plate to 0.280mm or 0.275mm; the thickness deviation will affect the plate making, and the parameters set by the CTP platesetter are generally not allowed to be adjusted.

 

Requirements for the performance of the base

The aluminum plate base for CTP plates must have good physical and mechanical properties.

CTP plates are often used in automatic plate-up, automatic positioning of the punching device, requiring the plate to have a certain degree of stiffness. If the base is too soft, the stiffness will be poor and it will be easy to bend and affect the upper version. The aluminum plate base is too hard, it is not easy to level, and it can also be adjusted on the machine. The aluminum plate used in the production of CTP plates in China is generally soft except for the quality of aluminum coils produced by a few manufacturers such as Southwest Aluminum. The aluminum coils produced by some Japanese companies are also weaker. It is currently recognized that the German Hydro Aluminicim company's status number is H19 and the grade is 1052 aluminum coil (hereinafter referred to as De Aluminum 1052), which is most suitable for the production of CTP plates. The mechanical properties of different grades of aluminum plate base are shown in Table 1.

Table 1 Comparison of mechanical properties of aluminum plates with different grades

Although the aluminum plate of Deco 1052 has high strength, the leveling property is very high, multi-roller leveling and simple leveling system can be adapted, and there are few problems such as bulge and ruffle, and the plate has excellent flatness.

 

Requirements for electrolytic properties of aluminum plate base

Because CTP plates are mostly used for high-end color printing, the CTP plate version is more compact than the PS plate. In particular, CTP platesetters use FM or FM/AM hybrid screening technology, and their fine mesh and network cable. The reduction is directly related to the surface roughness of the substrate, that is, the sand value of the plate base. The coarser the sand, the worse the reduction of the mesh and the wire.

The CTP plate material currently on the market has a sand value Ra of 0.45 to 0.60 μm and an ideal value of 0.45 to 0.55 μm, and the PS plate has a sand value Ra of 0.60 to 0.80 μm. In order to obtain a finer sand, the aluminum base must have good electrolysis properties. Most domestic aluminum plates are difficult to produce sand with a Ra value of 0.45 to 0.55 μm. Of course, the thickness of the sand should also be related to the electrolytic process conditions of the plate production.

A fine and uniform layer of sand is obtained by electrolysis, which is related to the apparent quality and surface roughness of the basic body of the plate. If the apparent quality of the base is too poor, it is impossible to eliminate the defects and defects of the layout without strengthening the electrolysis, resulting in rough sand. If the plate base Ra value is ≥0.30 μm, it is difficult to make a fine sand. The Ra values ​​of different plate bases are shown in Table 2.

Table 2 Ra value of different aluminum plate bases

 

To prepare a fine and uniform sand layer, the intrinsic composition of the aluminum plate is the key, which is related to the aluminum alloy composition of the aluminum plate, that is, the content and composition of the trace metal contained in the aluminum alloy. De Aluminum 1052 aluminum plate is because the content of special metal is more reasonable, so that its base has good physical and mechanical properties and can obviously improve its electrolysis suitability. It can be suitable for hydrochloric acid and nitric acid electrolysis; it can obviously improve the response sensitivity of electrolytic sand. A fine and uniform sand layer can be formed without reinforcement treatment.

In short, the overall quality of the aluminum coil for CTP plate production should be higher than that for the PS plate. At present, in order to ensure the quality of CTP plates, most offset printing plate manufacturers are using imported aluminum coils, of which German products are mostly.