Study on Exposure Test of Point-light Illuminating Flexographic Resin Plate

The flexographic printing of the "Ugly Duckling" was once regarded as a dazzling "white swan." According to a German magazine: In 1985, flexo printing accounted for only 15% of the global printing market, and by 1995 it had reached 27%. According to estimates, flexo printing will reach 38% by 2005. In stark contrast, both the plain and gravure markets have fallen significantly. Plain printing will be reduced from 40% in 2000 to 35%, and gravure will also be reduced from 18% in 2000 to 15%. This article mainly analyzes the effect of the light source equipped on the printing plate on the flexographic print. We will compare the point light source with the conventional traditional surface light source. The principle of point light exposure, point exposure, and surface light sources are easily explained. In the discussion, some basic physical concepts and theories will be cited.

Selection of printing plate light source

The stencil light source is one of the most important devices in plate making operations. It is the key to ensure that the photosensitive layer of the printing plate undergoes physical and photochemical changes in a short time. Choosing the right light source can reduce the exposure time, speed up plate making and improve the quality of the printing plate. The light source used for printing should have the following characteristics:

The wavelength emitted by the light source should be compatible with the absorption characteristics of the light-sensitive material and the color rendering properties.

The luminous efficiency of the light source is higher and the luminous intensity is greater. Because the other conditions remain the same, the higher the luminous efficiency of the light source, the greater the luminous intensity, the corresponding illumination is also large, so the photosensitive material to obtain the same exposure time is short.

UV content in the spectrum can cause damage to people's eyes and skin. Therefore, a UV-absorbing mask should be used for masking.

The light source will generate heat during use, and heat sources with small heat radiation should be used as much as possible.

The good workability and economy make sure the light source can be repaired and replaced, and the price is cheap.

The luminescence stability is high, ie, the luminescence intensity and the spectral distribution are relatively constant.

Light energy distribution uniformity is good.

It can ensure the reproducibility of the image after copying in the dried printing plate.

The role of point light

At present, the copying light sources mainly include surface light sources and high-pressure mercury lamps, fluorescent lamps, black light lamps, and iodine lamps of point light sources. The essence of point light source exposure is the use of direct light exposure lithographic printing plates to generate raised graphics, especially the formation of raised dots. Using only direct light to print flexographic plates can create the ideal vertical profile of embossed imprints, but it also has the obvious disadvantage that after the embossed imprint is formed, even a stronger light source or longer exposure time will not change the imprint. The geometric shape of the pattern means that a stable imprint structure cannot be formed (because the foundation at the bottom of the imprint is not very strong). For this reason, people often mix the use of direct light and scattered light (scattered light is usually inserted between the light source and the exposed layer of a mist of glass sheet) exposure printing plate. The plate is first exposed using direct light to form the upper side of the print. Then the scattered light is used to expose the printing plate to generate a wider imprinting substrate, so that the cross-linking curing effect occurs between the imprinting substrate and the imprint, and the firmness thereof is enhanced.

Light intensity and spectral distribution

The point light source has a distinct advantage over the light source constituted by a plurality of fluorescent lamps with respect to a general plate-setter, that is, it can precisely control the luminous flux. Since it is easy to control the luminous flux for a single beam of light, uniform control of the light sources constituted by a plurality of fluorescent tubes may be troublesome. It can also be achieved by attaching other equipment to the lamp, but the cost is relatively large.

The current problem of point light sources is that they are more expensive and practical in the field of flexo plate making. First of all, in the process of flexographic exposure must be able to print the highest depth? 3mm three-dimensional protruding prints; Second, to meet the needs of large format flexo drying. In the process of drying large-format printing plates, it should be ensured that the light intensity does not change with the increase of the printing plate width. This requires that the luminous efficiency of the light source increases with the increase of the printing plate web.

In addition, there is a difference in the distance between the center point of the printing plate and the edge point of the printing plate in the point source light printing apparatus. Therefore, the intensity of the light irradiated onto the printing plate also differs. According to actual measurements, the difference is about 25%. However, this difference can be partially corrected by an additional optical system such as a mirror.

It is also worth pointing out that the realization of high-speed and rapid exposure of large-format printing plates, people have tried to use two beams of light at the same time the exposure of the printing plate, want to use a solution to solve the problem. Unfortunately, such a solution cannot solve the problem perfectly. Dual light source in the exposure process of light diffraction phenomenon than the point light source is much larger. Due to the cross diffusion of light, the photosensitive edge under the non-transparent portion of the image is hardened, thereby reducing the sharpness of the printing plate, resulting in the narrowing and deformation of the dot, and even the loss of dots.

The photosensitive range of the printing plate

At present, the light we discuss is also limited to quantitative analysis. Now we start with quality, analyze the similarities and differences between point light sources and general surface light sources. Before analyzing the quality of a light source, it is necessary to first understand that the spectral distribution of the light source is extremely important. The specific flexographic plate is only sensitive to a small spectrum of light in the spectral range. It is this small wavelength that causes the chemical substance on the lithographic plate to decompose, thereby forming a convex graphic part and a hollow part of the depression. After many years of exploration and research, the spectral distribution of the surface light source and the photosensitive range of the printing plate have reached an ideal match. Displacement, widening, and narrowing of the wavelength range of the light source will change the sensitization performance of the printing plate.

In the process of flexo printing, a considerable amount of light is absorbed by the surface of the printing plate, resulting in incomplete decomposition of the photosensitive material in the deep layer of the printing plate, so that the printing plate material loses part of the resolving power, and even the image cannot be displayed well. Their corrosion resistance as well as wear resistance with respect to solvent materials becomes relatively weak. It is undesirable for light to be reflected or absorbed on the surface because these rays will adversely affect the depth of the resin.

Figure 1 shows the comparison of the emission wavelengths of point sources that are not suitable for flexographic exposure versus the emission wavelengths of typical fluorescent tube sources. It is not difficult to see that the point source emission spectrum is wider than the spectrum emitted by the fluorescent lamp. Since the photosensitive characteristics of a flexo plate mainly depend on the photosensitive material, different types of photosensitive materials are sensitive to wavelengths in different ranges. About most flexographic plates are sensitive to point light sources.

(to be continued)