Research on Platemaking of Flexible Plate (I)

In recent years, flexographic printing has achieved great development in the global field with its unique advantages. In many application fields, flexographic printing has already challenged offset printing and gravure printing, not only in terms of efficiency but also in terms of quality. .

Today, in order to meet the growing demand for high-quality prints in the market, North America and European countries have adopted relatively thin plates. According to different terminal applications, the thinnest thickness can reach 0.76mm, and the thickest can reach 2.84mm. Industry insiders are expecting that thin-gauge process technology can bring greater development advantages to product quality and performance, including reducing raw material costs, shortening the plate making cycle, making it possible to use higher-hardness plates, with a shallow depth of relief, and improved image quality on the plates. Good ink transfer control, reduce plate wear, reduce press consumption, reduce distortion and so on.

At present, the thickness of 1.14mm plate in the folding cartons (such as cigarette boxes, Jiuhe, kits, health products boxes, cosmetics boxes, etc.) printing applications are more common. This is mainly due to the fact that the surface of the paperboard used for the folding cartons is relatively rough and the thickness is not uniform (errors up to 0.02 mm). If a thinner plate (such as 0.76mm) is used for printing, the printing process and print quality are difficult to control. In the study, we used 1.14mm thick typical plates: Asphahi APR series (made of liquid polybutadiene resin and other raw materials), BASF's Nylonflex series, Chase's Gold series, DuPont The company's Cyrel series and MacDermid's Flex-light series. For a particular plate, the process conditions for plate making are important in order to obtain high-quality, high-performance prints. Because in order to protect the interests of the company's property rights, few technical articles in this area are known to the world. This article will introduce in detail the plate-making research and related issues on various plates with a thickness of 1.14mm.

Experiment introduction

Experimental material

(1) Overcast image: The negative image used in the experiment was provided by Asahi (UK) Ltd. The image information on the overtone image contains three primary colors images in the field, lines and halftones. The halftone image information includes 1% to 90% of dot information of 100 lines/inch, 120 lines/inch, and 150 lines/inch. The width of the Yin and Yang lines on the Yin picture range from 0.05 to 0.50 mm. The size of the positive and negative characters are 6, 8, 10, and 12 points. There are 0.15, 0.25, 0.50, and 1.00mm sizes for the independent Yang and Yinshe points.

(2) Plate: The thickness of the plate used in the experiment was 1.14mm, in which AFP HD plate was supplied by Asahi (UK) Co., Ltd. The EPIC media is provided by the Huyton Truck Company, now the sales department of MacDermid UK.

2. Platemaking process

The plate-making equipment used in the experiment was the 920 Quick series produced by Asahi Chemical Co., Ltd. (Japan). The unit consists of a UV exposure unit (76" x 102", 193cm x 259cm), a rinse unit (90" x 120", 29cm x 305cm), a drying unit (76" x 102", 193cm x 259cm), and a rear exposure unit ( 76 inches x 102 inches, 193 cm x 259 cm) and UV light source plate refurbishment unit (76 inches x 102 inches, 193 cm x 259 cm).

The first step of the plate making process is to cut the plate according to the size of the negative image, and then back-expose it (also known as pre-exposure) in a semi-vacuum (360 mbar) state with a UV-a light source with a wavelength of 365 nm. The plate surface is accepted. The energy arrived is 254mW/cm2. The role of the back exposure is to determine the relief height on the printing plate, that is, the depth of corrosion, to enhance the printing plate base, ensure the relief depth, and increase the printing plate resistance to printing force. The back exposure time is determined based on the desired substrate thickness. The exposure time is too long, and the printing is easy to get dirty; the exposure time is too short, the substrate is too thin, and the ability to support small texts and the like is reduced.

The second step of the plate making process is to place the negative image on the plate and perform front exposure (also called main exposure) under full vacuum (700 mbar) to form the graphic printing portion on the plate and cure it. (Front exposure is the same as that used for back exposure). It is the key to ensure the quality of the plate, directly affecting the clarity of the plate image. In order to avoid halation of the photosensitive bottom layer of the negative image, the photosensitive emulsion layer should be brought into contact with the plate material during exposure. The length of the front exposure time is related to the type of the media, the strength of the light, the image thickness on the picture, and the complexity. Exposure time is too short, will make the version of the embossed slope is not enough, the lines are wavy bending, small points are washed away; exposure time is too long, it will lead to paste version, blurred writing. If there are large and small characters on the same printing plate, thick and thin lines, the black film can be used to cover the exposure separately. The fine parts will not be lost due to flushing, ensuring the plate quality.

The third step in the platemaking process is the plate wash. After the exposure, the latent image of the graphic remains on the plate, and the plate is still flat and cannot be used for printing. At this point, the unexposed portion is washed away with an organic solvent to harden the exposed portion of the resin, leaving the photopolymerized relief. Washing time depends on plate thickness and depth of relief decision, washing time is too short, the version will leave unsensitized resin and affect the depth of plate making, wash plate for a long time will make the plate expansion, resulting in deformation or deformation of the fine part . At the same time, the plate washing time is directly related to the solution temperature, concentration, drug concentration, brush pressure, liquid level, etc.

The fourth step of the plate making process is drying the layout. The flushed version contains a lot of solvent and must be placed in a dryer to volatilize the solvent absorbed in the plate as quickly as possible to restore the plate thickness to a standard value. Bake plate temperature is generally between 50 ~ 60 °C. The baking time is determined by the thickness of the plate and the length of the plate-washing time, generally 2 hours thick and 1 hour thin. Bake plate temperature is too high, baking time shortened, so that the printing plate becomes brittle, deformation, printing will appear rotten version; baking temperature is too low, will extend the drying time, will make the resin aging hardening. After the plates are dried, there will still be a small amount of solvent remaining. If these solvents are not completely removed, the plate thickness uniformity will be affected. Therefore, the printing plate at this time is allowed to stand at room temperature for a certain period of time, during which time the residual solvent can be completely evaporated from the plate to ensure that the plate is cured well.

The fifth step of the plate making process is post-exposure. By irradiation with ultraviolet rays (wavelength 365 nm), the printing plate produces the greatest degree of photopolymerization. The energy received by the surface of the printing plate is 10.2mW/cm2, all the parts that have insufficient photosensitivity and incomplete curing are cured, so that the printing plate reaches the required hardness index, and the resistance of the printing plate to ink and solvent is increased, and printing is improved. Edition wear resistance.

The final step in the plate making process is to remove the tackiness. In order to eliminate the stickiness of the printing plate and increase the plate finish to facilitate ink transfer, the plate was exposed under a UV-c light source of 245 nm, and the plate received an energy of 2.9 mW/cm2.

3. Measurement methods and instruments used

(1) Mesh area measurement

Viviflex 330 is designed for dot area measurement of transparent flexographic and vulgar pictures. The operation steps are as follows: First select the correct version (if there is a covered printing plate and a non-covered printing plate, different imaging functions are required), then place the flexible printing plate on the transparent table to ensure that the part to be measured can be accurately lasered. Detected, preview the image, focus, capture the image and ensure that the dot can be clearly observed, and finally complete the analysis of the image. Under normal circumstances, the sample size required to be measured should be no less than 1.0cm x 1.0cm. Many useful information can be obtained in image analysis, such as dot area and screen specification parameters of printing plate (or vignette). In this stage of analysis, some outlets are automatically measured (which can also be obtained by manual measurement). The information retained by these records is statistically significant. It can also be used to detect defects in the shaded images or the printing plate outlets. As well as checking whether the dot size is uniform or not, whether there is any deviation in the detection of graphic and textual information from the Yin picture to the printing plate and the optimization of platemaking conditions.

(2) Image Contour Analysis

The instrument used was a Joel JSM-820 Scanning Electron Microscope (compared to an Oxford CXL type X-ray spectrometer). According to the selected detection position (such as the sample surface or sample edge intersection) and the product requirements, the excitation voltage value varies between 10 to 25 kV, and the magnification is 65 to 1000 times. A representative area of ​​each sample tested can be reflected by electron micrographs. After analyzing the developed photos, the following image outline information can be obtained: the structure of the surface of the printing plate, the information of the shaded lines, and the information of the shoulder points of the dots.

(3) Plate Relief and Plate Thickness Measurement

The instrument used in this test was a Kennedy tester with a resolution of up to 0.0001 inches (Kennedy Measurement Instruments, UK). Measurements are made on a horizontal plane.

Experimental results and analysis

1. Back exposure

In flexographic printing, the depth of embossing of the printing plate has a direct influence on the quality of the printing plate. Through the back exposure test, it is possible to obtain an important relationship between the selected exposure light source energy level and the emboss depth of the printing plate. In general, the length of the exposure time (in seconds) can be used to express the desired relief depth.

However, it is worth noting that when the exposure equipment is used for a period of time, the light intensity of the UV lamp will decrease, and the depth of relief represented by the aforementioned time value will be less accurate. If you still use the original exposure time you can not get the desired ideal embossing effect, then the back exposure time must be adjusted accordingly. The Asahi 920 Quick Platesetter uses advanced integrated exposure technology to represent various ideal embossed depths with different energy units, effectively controlling the quality of the back exposure.

We can select one type of plate from each company for back exposure experiments and analyze and evaluate the results.

A 2 cm wide strip was selected from the edges of the experimental plate and exposed on the front for 7 minutes. The rest of the plate (used in back exposure experiments) was partially covered with a cover strip. The purpose of doing this is to get a 2 cm wide hard ground on the plate sample. When we measure the relief depth of printing plates, the parameters of these solid areas can be used as a reference. The length of the back exposure time will affect the thickness of the plate base. As the back exposure time increases, the depth of the relief will become shallower. Relatively speaking, under the same exposure conditions, the depth of the relief of the Asahi HD plate is deeper than that of the Polyfibron EPIC plate, which indicates that these two types of plates have different photosensitivity coefficients for UV light sources. Repeat the experiment can draw the following conclusion: Under the same time and conditions, the back exposure of the two different types of photosensitive resin plates, the resulting relief depth is about 0.025mm.

2. Positive exposure

In this experimental study, a 1.14 mm thick relief plate with a depth of 0.52 mm was used as an Asahi AFP HD. The Yin pictures used in the experiment have also been carefully examined.