Surface roughness and its application in the detection of offset plates

1 Introduction

Offset printing is a major form of lithography. Offset printing is based on the principle that water (foaming solution) in the blank area of ​​the printing plate and the ink in the graphic area are incompatible with each other, and can continuously achieve the balance of water and ink to achieve the transfer of graphic information. That is to say, the blank area of ​​the layout is hydrophilic and water-retaining through the rough pits of the sand, so that the blank area of ​​the layout is not contaminated by ink; the graphic area of ​​the layout is firmly attached to the physical force (van der Waals force) The photosensitive resin layer (dots) on the surface of the oxide film on the peak and valley of the plate base meshes to a certain amount of ink, thereby achieving the transfer of graphic information.

The rough surface of the printing plate is the pit layer, which is what we often say is the surface of the sand layer that has been oxidized.

The printing plate for offset printing is made of offset printing plate material. Offset printing plate materials including PS plates and CTP plates are made by coating a non-silver photosensitive thermal resin layer on an aluminum substrate that forms a dense sand layer after electrolytic oxidation on the surface.

For the support of offset printing plate, 0.30mm is 0.265mm ~ 0.300mm and 0.240mm ~ 0.250mm or 0.15mm is 0.140mm ~ 0.150mm thick (light printing) aluminum plate.

The grit layer on the surface of the aluminum base of the offset printing plate not only assumes the hydrophilic function during printing, but also plays the role of coating and curing the photosensitive resin layer. The quality of the sand layer is the basis of the overall quality of the PS and CTP versions.

The formation of the grit layer on the surface of the aluminum plate base of the offset printing plate is usually achieved by an electrolytic process. The surface will not form a more standardized and uniform micro-peak-valley structure like a machined part. It is generally a disordered and uneven three-dimensional spatial structure. However, its surface structure still has certain rules. When the concentration of the electrolyte is constant, the vehicle speed is uniform, and the current is stable, it should basically follow certain rules.

The surface sand layer on the aluminum substrate is actually composed of countless peaks and valleys. The height of the peaks may be uneven, and the depth of the valleys may be different. Therefore, the offset printing plate industry uses the concept of surface roughness adopted by the machining industry to describe the surface state of the aluminum plate base sand mesh layer of the offset printing plate material. The surface roughness measurement method is used to measure the roughness of the sand mesh and then evaluate the sand mesh . [next]

2 The meaning of surface roughness

Most parameters and concepts of surface roughness are based on the midline.

Any cross-sectional contour curve of the surface geometry of the aluminum plate is composed of three parts: roughness component, waviness component and shape error component.

Surface roughness refers to the surface micro-geometric features composed of less spacing (<1mm) and smaller peak and valley heights in the profile.

The surface waviness refers to the surface geometry feature composed of a large interval (1 ~ 10mm) and a significant periodic peak and valley height in the contour.

The shape error refers to the surface geometry feature with large wave distance (> 10mm) in the outline without obvious periodic change.

As the sand layer of the PS version and the CTP version, only the concept of surface roughness can be used to evaluate the problem. Of course, the evaluation of the surface waviness and shape error will be more comprehensive.

Each country has its own standard for surface roughness. The standards of different countries sometimes have different symbols for the same parameter. Some have the same symbols, but their meanings are different. The surface roughness testers used for the inspection of the mesh value of the offset printing plate industry in China include those made in Japan, those made in the United States, those made in Germany, and those made in China.

There are many parameters of surface roughness. China's current national standard GB3505-2000, which is equivalent to the international standard, stipulates that the centerline system is used to evaluate the surface roughness. Its parameters include 27 parameters such as the arithmetic average deviation Ra of the contour.

The parameters of surface roughness can be divided into:

â‘  Parameters related to the height characteristics of micro unevenness, including 13 parameters such as Ra, Rz, Ry and Rq;

â‘¡Parameters related to the characteristics of micro unevenness spacing, including 9 parameters such as Sm, S and D, Iq;

â‘¢Parameters related to the shape characteristics of micro unevenness: 5 parameters including Tp, Sk and Dq.

• Ra: average thickness, also known as the average value of the contour center line.

The Ra value represents the magnitude of the arithmetic mean deviation between the peak and the valley. The value is given by the measurement and calculation of the integration program of the precision roughness meter, so it has no intuitive measurement meaning.

• Rz: Ten-point height of microscopic unevenness, also known as average depth, or microscopic unevenness.

Rz refers to the sum of the average of the five largest contour peak heights within the sampling length L and the average of the five largest contour valley depths. See Figure 4.

Rz can also be expressed as the average value of the peak height Ypi of the highest 5 convex peaks and the valley depth Yvii of the deepest 5 concave valleys within the sampling length L = 1-5.

Rz only reflects the height of the peak of the micro-profile of the measured surface, and does not reflect the micro-geometric features.

• Ry: the maximum height of the outline

Within the sampling length L, the vertical distance between the top line of the profile peak and the bottom line of the profile valley. Or the distance from the highest peak to the bottom of the deepest valley within the sampling length.

In addition to the above parameters, there are Rp (maximum peak height of contour), Rm (maximum valley depth of contour), R3z (average height of third peak and valley), Rmax (maximum rough depth of contour), Rt (maximum depth of contour), Rpm (average peak height), Rq (root mean square deviation of contours, also known as average surface roughness) and other parameters. [next]

3 Surface roughness tester

The surface roughness of the plate base is a microstructure, which must be inspected by a special measuring instrument, that is, the commonly used surface roughness tester, also known as sand mesh tester.

There are many brands and models of the surface roughness tester for measuring the aluminum base of the offset printing plate. Harbin Measuring Tool & Cutting Tool Factory is the first one to produce surface roughness measuring instruments in China. There are various types of testers with complete functions, practical, microcomputer, portable, etc. Various offset printing plate production plants in China mostly use the SURTRON IC3 + type pocket surface roughness meter produced by the British Taylor company. The instrument is convenient to use, but it is not durable and easily damaged. The surface roughness measuring instrument used in China's offset printing plate industry is shown in Table 1.

The working principles of various surface roughness measuring instruments are basically the same, generally using air gap inductive sensors. During the measurement, the drive box drags the sensor to measure the movement in the direction perpendicular to the processing line of the workpiece (aluminum base). The diamond stylus moves up and down with the microscopic fluctuation of the measured surface, and converts this movement into a change in inductance, so that the oscillation circuit Amplitude modulation signal is generated. This signal is sampled and processed by the single-chip microcomputer, and then the measurement result is displayed or printed out.

The surface roughness measuring instruments used by various offset printing plate production plants in China mainly include the British SURTRON IC3 +, the Japanese E-30A and Harbin 2222, and the era TR-200. Although their working principles are basically the same, the actual The detected Ra values ​​vary greatly. We have tested the same aluminum plate base that formed the sand layer after electrolytic oxidation with the measuring instruments of the above brands.

Most of the surface roughness measuring instruments listed are simple and can only be used for general routine testing. As for the analysis and study of the impact of the grit state of the aluminum base of the offset printing plate material on the quality of the PS plate and CTP plate and the quality of the finished plate, a desktop surface roughness measuring instrument with complete functions and computer control must be selected.

In fact, the surface roughness tester has different models to choose from according to different requirements. For example, there are seven or eight models of surface roughness measuring instruments produced by Harbin Measuring Tool & Cutting Tool Factory. [next]

4 Application of the concept of surface roughness in the detection of offset plates

To evaluate the sand mesh status of the aluminum plate base, the preceding question should be based on the requirements of the offset printing plate material on the quality of the plate base mesh, to be precise, the printing requirements on the plate material. As a printer, I hope that the printing plate made of the plate material should have good dots, good hydrophilic and ink-friendly performance, and have the highest possible print resistance.

The surface roughness of the aluminum plate base for offset printing plates or CTP plates, that is, the state, depth, and spacing of the sand is determined according to the needs of printing and the quality of the aluminum body.

Printing requires good hydrophilicity on the surface of the substrate. The printing plate with ideal sand mesh state should contain sufficient fountain solution, not easy to get dirty, good dot reproducibility, and high printing resistance.

To ensure the normal transfer of lithographic printing ink, the water storage of the printing plate should be kept at about 1.25ml / m2. The water content of normal printing in China is generally 1.32ml / m2. To maintain this water storage capacity, the sand of the printing plate must have a certain depth, and a certain distance should be maintained between adjacent sands, and the distance should be kept at 3 μm (referring to PS version, CTP version may be shorter).

If the distance between two adjacent sand meshes is less than 3μm, the sand mesh of the printing plate is too fine and the water storage capacity is too low to meet the printing requirements. Of course, if the distance between two adjacent sand meshes is too large, the sand mesh of the printing plate is coarse and the water storage is too much. The moisture on the printing plate will be taken away by the rubber roller rotating at a high speed, but the water storage of the printing plate will be reduced. Causes the blank area to become dirty.

＋ Another important function of sand purpose is to support the particle base of ink. Ink is the material that forms the graphic part of the printed matter. Theoretical calculations show that each ink particle usually needs more than 5 sand meshes to support, in order to make the ink in a stable state without sliding.

The diameter of the offset printing ink particles, that is, the fineness of the ink is generally about 7.5 μm (now the ink particles are finer), and the area that can be covered is about 44.2 μm. According to the above assumption, the bottom surface of each grain of sand should be a circle with a diameter of 3 μm, then each grain of ink will be supported by 6 grains of sand, which is enough to keep the ink stable.

The size and state of the sand mesh are also related to the reproduction of the tone and color of the printed matter. The tone and color of the lithographic image is reflected by the size of the dot coverage area. The higher the dot number, the more dots per unit area, the richer the tone and color reproduction.

Sand meshes with a bottom circle diameter of about 3 μm are ideal for containing fountain solution in the printing plate, ink particles supported by the sand mesh, and the dots of the printing plate supported by the sand mesh, ensuring the tone and color reproducibility of the image. So the ideal PS version of sand mesh, its surface roughness Ra value should be:

Ra = 0.375μm. Of course, this is the ideal state, in fact, the PS version of the sand mesh value is higher than this value.

In fact, the sand mesh of the PS version cannot be too fine, because the surface of the aluminum plate itself also has a certain roughness, generally 0.15 ~ 0.25μm, and the surface roughness of some aluminum plates even exceeds 0.35μm. In addition, there are always various defects on the surface of the aluminum plate, such as fine scratches and rough scratches can not be used, small impurity points, impurity lines, etc., these defects must be eliminated during electrolysis to cover up, so the PS version or CTP version The finest grit should also be above 0.40μm. [next]

Judging from the actual measurement of the surface roughness Ra value of the PS and CTP versions at home and abroad in recent years, it also shows that the surface roughness Ra value of the PS and CTP versions will not be less than 0.45μm. The range of the mesh value Ra of the offset printing plates of various brands is now available.

At present, the sand mesh values ​​of PS and CTP versions produced by domestic and foreign manufacturers are basically within the range of the above two tables.

The surface roughness Ra of the printing plate cannot be too low, although the theoretical requirement is that the lower the Ra value, the better; of course, the Ra value should not be too high, that is, the grit is too coarse, which obviously affects the quality of the dots and screen lines of the offset plate . It is better to keep the Ra value of the PS version of the aluminum base at 0.50 ~ 0.75μm, preferably not more than 0.80μm. In recent years, the CTP version has a Ra mesh value of 0.45 ~ 0.60μm.

The basic situation of the base sand is introduced above. The sand layer looks like a convex and concave surface composed of peaks and valleys. In fact, the knowledge of this three-dimensional structure is still great? The average height of the peak selection The average depth of the valley, the average spacing between peaks and peaks, the distance between peaks and valleys, and the state of peaks and valleys all have an impact on the performance and quality of the plate.

We have done three PS patterns, one with high Ra, one low, and one centered, and measured their Ry, Rz, Rpm, and Sm values ​​respectively.

The difference between the maximum height Ry of the contour and the ten-point height Rz of the micro unevenness is too large, indicating that there are too high peaks or too deep valleys in the sand mesh, which is detrimental to the quality of printing and the quality of printing.

The difference between Ry and Rz is small, which means that the sand is uniform, which is suitable for the requirements of printing and printing. Of course, the difference between Ry and Rz is too small, it is very likely to form a platform, which is not conducive to printing.

The Japanese E-30A roughness tester can measure the average peak height Rpm above the center line of the sand mesh. In theory, the Rpm value should be 1/2 of the Rz value, but the actual sand mesh value cannot be so ideal. Different, the internal composition of the aluminum base is different, the Rpm value may be greater than 1/2 Rz value, may be less than 1/2 Rz value.

Some roughness measuring instruments can measure Rp and Rm values, Rp + Rm = Ry. But here Rp, Rm and Ry are all maximum values, not average values.

If the Rpm value is close to and equal to 1/2 Rz value, it means that the peak height value above the center line and the valley depth value below the center line are basically equal, and the peak and valley are even up and down, so the sand should be said to be sensitive to the PS version Performance and water retention performance are more favorable.

If the Rpm value is less than 1/2 Rz value, that is, the top of the peak above the center line is relatively flat and the peak is thick, it is very likely that a platform will appear. This sand mesh has good wear resistance, but low resolution, poor dot reduction, and poor water retention.

If the Rpm value is greater than 1/2 Rz, the peak above the center line is steep. This kind of sand mesh should be said that the wear resistance is poor, such as not too high, the resolution, dot network cable quality should be good, if too high, it will not work.

In summary, the parameters related to the height characteristics of micro unevenness, such as Ra, Ry, Rz, etc., are obvious and crucial to the evaluation of the effect of sand mesh on the quality of PS plate.

It is also conceivable that the Sm value of the average pitch between the peaks and valleys of the sand mesh affects the quality of the PS version. The smaller the average distance between peaks and valleys, the more peaks and valleys supporting the dots and the greater the dot strength. Of course, the spacing should not be too small, too small will make the valleys of the sand mesh narrow, affect the water retention rate of the plate, and easily cause the printing plate to be dirty. If the average Sm value of the peak-to-valley interval of sand mesh is too large, it will obviously affect the quality of the dots and screen lines of the printed matter.

The influence of the surface roughness parameters on the quality of the PS or CTP version of the printing plate is very subtle, and this article is only a rough introduction and analysis.

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