Understand the key points of label flexographic plate control in one article!

With the country's environmental protection policies becoming increasingly stringent, flexographic printing has become the fastest-growing printing method in recent years due to its unique green and environmentally friendly advantages. Coupled with significant advancements in flexographic technology, more and more niche printing sectors (such as labels, aseptic liquid packaging, paper cups, paper bags, cartons, breathable films, and flexible packaging) are using flexographic printing, which in turn has spurred the vigorous development of the flexographic printing plate market. At the same time, the plate-making process and key control points of flexible plates have also received focused attention from printing companies, because without printing plates of stable quality, it is impossible to produce printed materials of excellent quality. 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 focuses on thermosensitive plates, a branch of solid plates, and provides a detailed introduction to the plate-making process and key control points of thermosensitive plates.

PART 1: FLEXIBLE PLATE MAKING PROCESS

First, let's understand the basic structure of flexible plates. Taking a thermosensitive digital flexographic 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, black film, photosensitive resin layer, and support film. Among them, the protective film is mainly used to prevent dust, damage, and oxidation that can reduce photosensitivity; the black film replaces the traditional film laminated on the surface of the plate and is directly ablated for imaging by the engraving machine; the photosensitive resin layer is composed of resin, monomer, photoinitiator, and other additives, and undergoes a polymerization reaction under ultraviolet light; the support film is used to support the photosensitive resin layer and form a stable printing plate.

Next, let's take a look at the plate-making process of the flexible plate. Taking the dry thermal 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 every step in the plate-making process to ensure plate quality, thereby obtaining stable printing plates. The specific plate-making requirements are as follows:

(1) Before the raw plate materials enter the plate-making stage, the model and batch number of the raw materials need to be registered, and plates of the same model should be categorized according to the same batch. In addition, attention should be paid to the storage method of the plates to ensure they are laid flat (vertical placement will cause marks), kept in a dry environment, and protected from direct sunlight.

(2) Back exposure determines the thickness of the substrate and the height of the relief. Therefore, it is necessary to conduct back exposure tests for each batch of plates, adjusting exposure time according to the parameters provided by the plate supplier to select the appropriate back exposure time for that batch of that plate model. Special attention should be paid during testing to the lamp energy, lamp warm-up, and the temperature of the exposure table.

(3) In the laser imaging stage, it is necessary to periodically calibrate the focus and engraving energy of the imaging equipment, and calibration can be performed according to the methods provided by the equipment supplier.

(4) Main exposure, like back exposure, also requires testing to obtain the main exposure time for that batch of that plate model. Similarly, attention should be paid to lamp energy, lamp warm-up, and the exposure table temperature.

(5) During the plate washing stage, it is important to ensure that the pressure on the front, back, left, and right of the plate is balanced. It is recommended that when assembling the plates, add 20mm solid strips on both sides in the washing direction to balance the pressure.

(6) During the adhesive removal and post-exposure treatment stages, time control and lamp energy must be carefully monitored.

It is worth mentioning that the back exposure and main exposure test can be fine-tuned according to different batches in the actual production process, and it is not recommended to use raw materials for testing, because the waste is large, and it can be combined with some plates that are not very demanding when making printing plates, such as field plates, oil plates, etc. In short, in the process of plate making, we must maintain the stability of plate-making equipment, do a good job in daily maintenance and monitoring, form standardized operations, and reduce the impact of human factors.

PART.2 Frequently Asked Questions about 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 progress of the printing process. In the actual production process, the plate-making process can prevent plate-related problems in advance, reduce downtime, and improve production efficiency. The following are some common plate making-related plate problems and solutions in the production process.

(1) The printing plate does not match the mounting roller, the reason may be that the thickness of the raw material plate does not match or the cutting size deviates greatly.

(2) When the exposure energy is insufficient due to the aging of the lamp of the exposure machine, if the back exposure time setting is not suitable for this type of plate, the printing process may cause problems such as rubbing the bottom (underplate printing), the edge of the printing plate is raised or curled, the base base falls off, the relief height does not meet the requirements (too thin or too thick), horizontal stripes or gear printing, uneven printing with spots, long lines and wavy patterns, etc.; If it is the main exposure link, it may cause problems such as incomplete yin lines (overexposure), bent or incomplete positive lines (insufficient exposure time), pinholes, variegated or dirty spots printed on the field, printing plate edges first (too hard edges), high-gloss dots cannot be printed or dots are deformed.

Once the above problems occur, it is necessary to detect the energy of the lamp of the exposure machine or replace the lamp, and then re-test the main and back exposure of the plate. As mentioned above, it is necessary to constantly monitor the energy of the lamp to maintain the stable state of the exposure machine.

(3) During the printing process, the highlight dots cannot be printed or the dots are deformed, which may also be CDI laser problems, no focus or insufficient energy, at this time, you need to do Focus search for laser focal length calibration, engraving energy calibration pattern for laser energy calibration.

(4) When the de-sticking treatment and post-exposure treatment are improper, problems such as poor ink transfer, dirty print, rapid plate wear, loss of graphics and stickiness of printing plates will occur during the production process.

The above lists some problems that may be related to printing plates in the production process, but it does not mean that the above problems are necessarily problems with printing plates, and specific problems need to be analyzed. Once a problem is found, it should be analyzed using the elimination method until the root cause of the problem is found.

PART.3 Flexible plate inspection

After the plate is made, the finished printing plate needs to be inspected before entering the printing process. After the plate is made, the plate maker needs to inspect the overall layout of the plate, such as checking the layout information, document information, yin and yang lines and text, independent dots and long yang lines in the graphic part. In addition, it is necessary to conduct random inspection of the finished plate, and the test items mainly include the thickness of the printing plate, the base base, the height of the relief, the hardness of the printing plate, etc. After the finished plate is completed, the pattern maker should also inspect the 2% and 50% dots, especially visually check whether the 2% dots are complete, so as to judge whether the plate can be used on the machine, and at the same time combine with a magnifying glass to observe whether the diameter of other dots meets the requirements, and the control strip 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 ensure practicality.

This article takes the dry thermal plate-making process as an example to analyze the flexible printing plate-making process, common problems, and key control points. Other materials, such as solvent-based or water-washed plates, are largely similar in control methods. It is hoped that the sharing of this article can help peer companies. It should be noted that different printing companies must develop control methods suitable for their own company according to different plate-making methods, especially when designing inspection marks, it is necessary to consider operational convenience. In addition, equipment should be regularly maintained, records should be kept, which is beneficial for tracking.