With the increasingly strict national environmental policies, flexographic printing has become the fastest-growing printing method in the printing industry in recent years due to its unique advantages of green and environmental protection. In addition, with the significant progress of flexographic printing technology, more and more printing sub fields (such as labels, sterile liquid packaging, paper cups, paper bags, cardboard boxes, breathable films, soft packaging, etc.) are using flexographic printing technology, which has led to the vigorous development of the flexographic printing plate market. At the same time, the process and control points of flexible plate making have also received significant attention from printing enterprises, as it is impossible to print high-quality printed materials without stable quality printing plates. At present, the flexible plates used in the domestic printing market are mostly photosensitive resin plates, which can be divided into liquid plates and solid plates. In this article, the author takes the branch of thermosensitive plates in solid plates as the starting point, and provides a detailed introduction to the plate making process and control points of thermosensitive plates.

PART.1.Flexible Plate Production Process

Firstly, let's take a look at the basic structure of flexible plates. Taking thermosensitive digital flexible plate as an example, as shown in Figure 1.

From Figure 1, it can be seen that the flexible plate structure includes a protective film, a black film, a photosensitive resin layer, and a support film. Among them, protective film is mainly used for dust prevention, damage prevention, and oxidation resistance to reduce photosensitivity; The black film replaces the traditional film that is coated and obstructed on the surface of the plate, and is directly ablated and imaged by the engraving machine; The photosensitive resin layer is composed of resin, monomers, photoinitiators, and other additives, which undergo polymerization reaction under the action of ultraviolet radiation; The support film is used to support the photosensitive resin layer, forming a stable printing plate. Secondly, let's take a look at the plate making process of flexible plates. Taking dry thermosensitive digital plate as an example, as shown in Figures 2 and 3.

From the plate making process shown in Figure 2, it is necessary to monitor each step of the plate making process to ensure plate making quality and obtain stable printing plates. The specific plate making requirements are as follows: (1) Before the raw material plate enters the plate making process, it is necessary to register the model and batch number of the raw material plate, and classify the plates of the same model according to the same batch. In addition, it is necessary to pay attention to the storage method of the plate material, ensure that the plate material is placed horizontally (vertical placement can cause wrinkles), keep the environment dry, and avoid direct sunlight. (2) The back exposure determines the thickness of the substrate and the height of the relief, so it is necessary to conduct a back exposure test on each batch of plate material. The exposure test should be carried out according to the parameters provided by the plate supplier plus or minus the time, and the back exposure time suitable for a certain batch of plate material of that model should be selected. During testing, special attention should be paid to the energy of the lamp tube, the preheating of the lamp tube, and the temperature of the exposure table. (3) In the laser imaging process, it is necessary to regularly calibrate the focal length and engraving energy of the imaging equipment, which can be calibrated according to the methods provided by the equipment supplier. (4) Both the main exposure and back exposure require testing to obtain the main exposure time for a certain batch of this type of plate material. It is also necessary to pay attention to the lamp energy, lamp preheating, and the temperature of the exposure table. (5) During the plate washing process, attention should be paid to whether the pressure on the front, back, left, and right sides of the plate material is balanced. It is recommended to add 20mm solid strips on the left and right sides of the plate washing direction during plate assembly to balance the pressure. (6) Attention must be paid to time control and lamp energy during the debonding and post exposure processes.

It is worth mentioning that the back exposure and main exposure tests can be fine-tuned according to different batches in the actual production process. It is not recommended to specifically use raw materials for testing, as there is a lot of waste. When making printing plates, it can be combined with some less demanding plates, such as real plates and oil plates. In short, during the plate making process, it is necessary to maintain the stability of the plate making equipment, carry out daily maintenance and monitoring, form standardized operations, and reduce the impact of human factors.

PART.2

Common Problems in Flexible Plate Production

Plate making is an important part of the entire printing process, and the accuracy of the plate making process will directly affect the normal operation of the printing process. In the actual production process, the plate making process can prevent some printing plate related problems in advance, reduce downtime, and improve production efficiency. The following lists some common plate making related issues and solutions during the production process. (1) The mismatch between the printing plate and the plate mounting cylinder may be due to the thickness mismatch of the raw material plate or a significant deviation in the cutting size. (2) When the lamp tube of the exposure machine ages and causes insufficient exposure energy, if the back exposure time setting is not suitable for the type of plate material, the printing process may experience problems such as rubbing the bottom (plate printing), warping or curling of the plate edge, peeling of the bottom base, unsatisfactory relief height (too thin or too thick), horizontal stripes or gear printing, uneven printing with spots, and long line wavy patterns; If it is due to the main exposure process, it may cause incomplete negative lines (overexposure), bent or incomplete positive lines (insufficient exposure time), pinholes, miscellaneous colors, or dirty spots in the field printing, printing at the edges of the printing plate first (the edges are too hard), inability to print high gloss dots, or deformation of the mesh points. Once the above issues occur, it is necessary to test the energy of the light tube of the exposure machine or replace the light tube, and then re conduct the main and back exposure tests on the plate material. The above also mentioned the need to regularly monitor the energy of the lamp tube to maintain the stable state of the exposure machine. (3) During the printing process, if there are high light dots that cannot be printed or deformed, it may also be due to CDI laser issues such as lack of focus or insufficient energy. In this case, it is necessary to perform a Focus search for laser focal length calibration and carve energy calibration patterns for laser energy calibration. (4) When the debonding treatment and post exposure treatment are not appropriate, problems such as poor ink transfer, dirty prints, rapid plate wear, loss of graphics and text, and plate sticking can occur during the production process. The above list lists some issues that may be related to the printing plate during the production process, but it does not necessarily indicate that the occurrence of the above problems is a problem with the printing plate, and specific analysis is needed. Once a problem is discovered, it should be analyzed using exclusion methods until the root cause of the problem is found.

PART.3
Flexible Plate Inspection

After the production of the plate material is completed, the finished printing plate needs to be inspected before entering the printing process. After the production of the plate is completed, the plate maker needs to conduct a visual inspection of the overall layout of the plate, such as checking the positioning information of the image and text parts, document information, yin and yang lines and text, independent dots, and long yang lines. In addition, it is necessary to conduct spot checks on the finished plate material, which mainly include plate thickness, substrate and relief height, and plate hardness. After the completion of the finished plate material, the plate maker should also inspect 2% and 50% of the mesh points, especially visually to determine whether the 2% mesh points are completely formed, in order to determine whether the plate material can be used on the machine. At the same time, combined with a magnifying glass to observe whether the diameter of other mesh points meets the requirements, a control strip as shown in Figure 4 can be added to the plate material.

It should be noted that when designing inspection and testing projects, it is necessary to set them according to the actual situation of the company and ensure practicality. This article takes the dry thermosensitive plate making process as an example to analyze the flexographic plate making process, common problems, and control points. Other control methods such as solvents and water washed plates are similar and similar. It is hoped that the sharing of the article can help peer enterprises. It should be noted that different printing companies must develop control methods suitable for their own companies based on different plate making methods, especially when designing inspection marks, which must consider the convenience of operation. In addition, it is necessary to regularly maintain and maintain equipment, keep records, and facilitate tracking.