Explained in Depth! Understand the Key Points of Label Flexographic Plate Control in One Article!

With the increasingly stringent national environmental protection policies, flexographic printing has become the fastest-growing printing method in the industry in recent years due to its unique green and environmentally friendly advantages. Coupled with the significant technological advancements in flexographic printing itself, more and more printing segments (such as labels, aseptic liquid packaging, paper cups, paper bags, cartons, breathable films, and flexible packaging) are adopting the flexographic process, which in turn has led to a booming market for flexographic printing plates. At the same time, the process and key control points of flexographic plate making have also received focused attention from printing companies, because without printing plates with stable quality, it is impossible to produce high-quality printed products. Currently, 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 thermosensitive plates, a branch of solid plates, as the entry point to provide a detailed introduction to the plate-making process and key control points of thermosensitive plates.PART.1 Plate-Making Process of Flexible PlatesFirst, let's understand the basic structure of flexible plates. Taking thermosensitive digital flexible plates as an example, as shown in Figure 1.

As can be seen from Figure 1, the structure of the flexible printing plate includes a protective film, a black film, a photosensitive resin layer, and a support film. The protective film is mainly used to prevent dust, damage, and oxidation that could reduce photosensitivity; the black film replaces the traditional composite barrier film on the surface of the plate and is directly engraved by an imaging machine; the photosensitive resin layer is composed of resin, monomer, photoinitiator, and other additives, and undergoes a polymerization reaction under the action of ultraviolet light; the support film is used to support the photosensitive resin layer and form a stable printing plate. Next, let's look at the plate-making process of the flexible printing plate. Taking dry thermal digital plates as an example, as shown in Figures 2 and 3.

According to the plate-making process shown in Figure 2, it is necessary to monitor every step of the plate-making process to ensure the quality of the plates and obtain stable printing plates. The specific plate-making requirements are as follows:(1) Before the raw plate material enters the plate-making stage, the model and batch number of the raw plates need to be recorded, and plates of the same model should be categorized as the same batch. Additionally, attention should be paid to the storage method of the plates, ensuring they are placed horizontally (vertical placement will produce streaks), keeping the environment dry, and avoiding direct sunlight.(2) The back exposure determines the thickness of the substrate and the height of the relief, so back exposure tests need to be conducted for each batch of plates. Exposure testing is done by adjusting the time according to the parameters provided by the plate supplier to select a back exposure time suitable for that particular batch and model of plate. During testing, special attention should be paid to the lamp energy, lamp preheating, and the temperature of the exposure table.(3) In the laser imaging stage, it is necessary to regularly calibrate the focus and engraving energy of the imaging equipment, which can be done according to the methods provided by the equipment supplier.(4) The main exposure, like back exposure, requires testing to obtain the main exposure time for that particular batch and model of plate. Similarly, attention should also be given to the lamp energy, lamp preheating, and the temperature of the exposure table.(5) During the plate washing stage, attention should be paid to whether the pressure on the front, back, left, and right of the plate is balanced. It is recommended to add 20mm solid strips on both sides in the washing direction when ganging plates to balance the pressure.(6) During the de-adhesion and post-exposure processing stages, it is crucial to control the timing and lamp energy.

It is worth mentioning that back exposure and main exposure testing can be fine-tuned based on different production batches during practical production. It is not recommended to strictly use raw material plates for testing because it wastes a lot of materials. Instead, testing can be combined with plates where high precision is not strictly required, such as solid tone plates or overvarnish plates. In short, during the plate-making process, it is essential to maintain the stability of the plate-making equipment, perform regular maintenance and monitoring, standardize operations, and reduce the impact of human factors.PART 2: Common Issues in Flexible Plate ProductionPlate-making is a crucial step in the entire printing process. The accuracy of plate-making directly affects the smooth operation of the printing process. In practical production, the plate-making process can help prevent some plate-related problems in advance, reduce downtime, and improve production efficiency. Below are some common plate-related issues and solutions encountered during production.(1) Mismatch between the printing plate and the plate cylinder, which may be caused by inconsistent thickness of the raw material plate or significant deviation in cutting dimensions.(2) If the exposure unit's lamp has aged and provides insufficient exposure energy, and the back exposure time is not suitable for the plate type, problems such as plate backing (printing the plate bottom), edge lifting or curling, backing detachment, relief height not meeting requirements (too thin or thick), horizontal stripes, gear marks, uneven printing with spots, or long-line wave patterns may occur. If the issue is with the main exposure, it may result in incomplete shadow lines (overexposure), bent or incomplete highlight lines (insufficient exposure), pinholes, color contamination in solid areas, edge-first printing (edges too hard), missing or distorted highlight dots.Once such issues occur, it is necessary to test the lamp energy or replace the lamp and then redo main and back exposure tests on the plates. As mentioned above, it is important to regularly monitor lamp energy to maintain exposure unit stability.(3) Problems such as missing or deformed highlight dots during printing may also be due to CDI laser issues, such as poor focusing or insufficient energy. In this case, a Focus Search is required to calibrate the laser focus, and an engraving energy calibration pattern should be used to calibrate the laser energy.(4) Improper de-adhesion treatment or post-exposure treatment can lead to poor ink transfer, dirty prints, rapid plate wear, loss of images, and plate stickiness during production.The above lists some plate-related problems that may occur during production, but this does not necessarily mean that these problems are always caused by the plate itself. Each situation should be analyzed specifically. Once a problem is detected, a process of elimination should be used to identify the root cause.PART 3: Flexible Plate InspectionAfter plate production is completed, the finished printing plate must be inspected before entering the printing stage. The plate maker needs to perform a visual inspection of the overall plate layout after production, checking registration information, document details, shadow and highlight lines and texts, isolated small dots, and long highlight lines. Additionally, random inspections of finished plates should be carried out, including checks of plate thickness, backing, relief height, and plate hardness. After the finished plates are ready, the plate maker should check 2% and 50% dot coverage, especially verifying visually that 2% dots are fully formed, to determine if the plate can be used on the press. Using a magnifying glass, other dot diameters can also be inspected for compliance, and control bars, like those shown in Figure 4, can be added to the plate.

It should be noted that when designing inspection and testing items, they must be set according to the company's actual situation, and practicality must be ensured.

This article takes the dry thermal plate-making process as an example to analyze the flexographic plate-making process, common issues, and key control points. For other materials, such as solvent-based or water-wash plates, the control methods are largely similar. It is hoped that the sharing in this article can be helpful to peer companies. It should be noted that different printing companies must develop control methods suitable for their own operations based on their specific plate-making processes, especially when designing inspection markings, as operational convenience must be considered. In addition, equipment must be regularly maintained, and records should be kept to facilitate tracking.