Label printing

The wide application of labels and the continuous development of label varieties naturally promote the development of label printing technology. Label printing covers all printing methods such as flat, convex, concave and screen, and its application varies from country to country. However, from the development trend of global labels in recent years, it can be seen that flexographic printing, narrow rotary printing and digital printing have become the new highlights of label printing in Europe and America, and they are also the development trend of label printing. In prepress processing, many orders designed by customers are mainly offset printing or gravure printing. If this manuscript is printed by flexography, there will be many quality problems in the sample, such as the color is not in place, the level is not obvious, and hard edges appear. Therefore, it is very necessary to communicate with customers in time to solve such problems.

1) Now label printing is done by hand, and many monochrome labels are printed by machine, which can improve production efficiency. Although the overprinter accuracy of many newly developed label printing machines is not high, it improves the printing efficiency and the printing quality needs to be improved. Label printing covers several main printing methods. In the plate-making stage, different printing methods are selected according to different product properties, and the plate-making process is also different for different printing methods. This paper briefly introduces the flexographic plate-making process as an example.

The process flow of flexographic plate making is as follows: original film, exposure, washing, drying and post-treatment.

1. Original manuscript. The manuscript design suitable for flexible printing should have the following characteristics: more colors, but less overprinting; It is not required to reproduce particularly small details; The number of screen lines is not too high, but it can achieve color printing effect; You can do packaging processing online.

2. Film (negative film). Meet the needs of plate making, clear graphics and text, accurate size specifications; Use frosted film, and the density of the four corners of the film should be consistent; Use medicine membrane orthography; Measured by transmission densitometer, the white bit density is lower than 0.06; The bit density of black is above 3.5.

3. Exposure includes back exposure and main exposure.

① back exposure. The supporting film of the photosensitive resin plate is laid flat in the exposure drawer, with the supporting film facing upwards and the protective film facing downwards for exposure. Ultraviolet rays can cure the photosensitive adhesive layer through the support film, thus establishing a stable substrate, controlling the depth of plate washing and enhancing the bonding force between the support film and the photosensitive resin layer. Determine the back exposure time according to the required substrate thickness.

② Main exposure. Also known as front exposure, the supporting film of photosensitive resin plate is facing down and the protective film is facing up. Put it flat in the exposure drawer. Tear off the protective film once in a row, and then stick the film surface on the photosensitive resin plate. Cover the vacuum film flat mother 1 on the film (non-film vacuum, so that the film can be closely attached to the photosensitive resin layer. Ultraviolet rays penetrate the vacuum film and the light-transmitting part of the film, so that the photosensitive part of the plate is polymerized and cured. The main exposure time is determined by the plate type and light source intensity. If the exposure time is too short, the slope of the picture and text will be too straight, the lines will be bent, and the small words and dots will be washed away. On the contrary, if the exposure time is too long, the page will be applied and the handwriting will become blurred. If there are big, small, thick and thin lines on the same printing plate, you can use black film to expose them separately as appropriate, so that the small parts will not be lost due to washing, so as to ensure the quality of the printing plate.

Rinse. Clean and dissolve the unexposed parts, and keep the photopolymerization relief. The length of plate washing time depends on the thickness of the plate and the depth of the image. If the plate washing time is too short, unexposed resin will remain on the plate, which will affect the depth of plate making. If the washing time is too long, the plate will swell, resulting in the deformation or loss of fine parts.

5. dry. Remove the cleaning solvent to restore the original size and thickness of the printing plate. The baking temperature is between 50-60℃. Baking time depends on the thickness of the board and the length of washing time. Generally, two hours for thick plates and one hour for thin plates. Too long baking time and too high baking temperature will make the printing plate brittle and affect the printing life. Too low baking temperature will prolong the drying time, and too short baking time will lead to poor printing.

6. Post-processing. I.e. after debonding and exposure. The photosensitive resin is completely cured (polymerized) to reach the required hardness index, which eliminates the stickiness of the printing plate and is beneficial to ink transmission. The post-treatment time is obtained by experiments, and the purpose is not to crack or stick.

(2) With the development of economy, labels are widely used in commodities, which promotes the rapid development of label printing industry. However, people's understanding of label printing ink is far less than that of label materials and printing methods, and label printing ink is a mysterious field that we need to understand and explore.

There are many kinds of label printing inks, which can be divided into three categories according to the different solvents used-water-based label printing inks, solvent-based label printing inks and UV label printing inks.

(3) At present, domestic label printing mainly adopts relief printing, offset printing, flexographic printing and screen printing. From a technical point of view, these printing methods have their own advantages and disadvantages, and no printing method can replace another printing method.

Offset printing is a widely used printing method. Sheet-fed printing in China is basically offset printing. This printing method has the advantages of high speed, high printing clarity, accurate overprint and full printing level, but it is not suitable for printing bright label products because of its poor durability and shallow ink layer.

Relief printing has the advantages of good graphic clarity, fast printing speed, stronger color intensity and durability than offset printing, and slightly worse than screen printing, but its hierarchical performance and registration are better than screen printing and flexography, so relief printing is mostly used in label production in China.

The biggest advantage of screen printing is that the ink layer is thick and the hiding power is good, but the speed is slow, the gradation is poor and the registration degree is low (this refers to the general level in China, and the registration accuracy of the screen printing machine developed in Wan Jian in recent years reaches 65,438+00 lines and the speed reaches 6,000 sheets per hour), so the screen printing machine can be used to print labels with weak gradation.

The printing characteristics of flexographic printing are moderate, and labels are printed in this way abroad. However, this technology requires the processing of anilox roller and the technical level of workers. Although the machine is simple, the process is quite complicated, and there are not many labels printed in this way in China.

In order to print the most exquisite labels, it is necessary to effectively combine various processes and integrate the advantages of various printing methods in the printing process to form a greater combination of advantages.

The purpose of multi-process combination:

(1) Effective anti-counterfeiting function: printing a variety of technical effects on the same label product, especially on the labels of some cosmetics and food packages that are easy to counterfeit, allows consumers to see the authenticity of the products at a glance through the labels, thus protecting the interests of product manufacturers and consumers and winning a good reputation for the printing house.

(2) Improve the quality of printed matter: The combination of various processes combines the advantages of various printing processes, so this combined printing method can ensure the printing effect of printed matter to the greatest extent, thus meeting people's growing sensory requirements and improving the printing quality of printing plants, thus bringing them more sources of life. Improve production efficiency and reduce costs. The combination of various processes also includes the combination of printing and post printing. Printing and post-printing in product production can be completed at one time, and the finished product can be directly produced, which can greatly reduce the cost.

There are two types of machines that combine multiple processes:

(1) First, single-machine combination: among trademark printing machines, we are more familiar with the oblique back machine, which can complete printing, bronzing, die cutting, waste removal and other functions on one machine, but because it is single-machine printing, the printing size will be affected.

(2) The other is to combine all the units into a production line: the printing and post-press processing units such as embossing, screen printing, flexography, bronzing and die cutting are combined into a production line for printing production, which realizes the effect of combining multiple processes on the same product. This combined printing production mode meets the needs of the label market development, the grade of printed matter has been greatly improved, and at the same time, it has brought rich profits to printing manufacturers. The development of unit production line in China is more and more optimistic.

Combination of relief printing, flexographic printing and screen printing in label printing

Combined with the example of label production, the multi-process combination in the production process is explained. We want to print a label, which contains color scale, blue area, portrait and corresponding text description. Through the above comparison, we know that printing patterns with rich color levels, in label printing, relief printing has a stronger sense of hierarchy than flexographic printing, and the best choice for on-site printing of pictures and texts is screen printing, and we can choose flexographic printing for portrait printing. Therefore, on this label, we combine letterpress printing, flexographic printing and screen printing to form a production line, thus printing exquisite labels. In label printing, the combination of relief printing, flexographic printing and screen printing is a typical example of multi-process combination.

The process arrangement in multi-process combination is also a science, which requires a lot of practical experience, such as slow drying of screen printing, so it should be considered comprehensively when combining.

(d) When people talk about laser tags, they will definitely think of the word "need to be treated with care" because it is difficult to produce and transport. Of course, it is a great product and there is a great demand for it in the market. But if you can replace it with other labels, you should give up the laser label.

One problem is overheating. The temperature of the fusion joint on the laser printer reaches 250-300 degrees Fahrenheit. In order to produce a perfect label, the substrate, adhesive and liner must be combined together. Even if all kinds of raw materials are prepared, if the label moves too slowly during printing, it will be damaged by heat. Another problem is the corner of the label-this means that the label is invalid. After the product is damaged, it should be repaired in the printing plant.

Before labels are delivered to users, paper suppliers and manufacturers must solve the above problems.

Laser labels are usually processed on rotary flexible printing equipment. Maine, vice president of RLP in Iowa, said. "Laser labels are difficult to produce on a printing press. “

Paper suppliers need to know the humidity control requirements of laser labels in order to solve the problems of curling and printer adhesion. Whether the curling occurs in 8 1/2 or 1 1 die cutting, it is difficult for manufacturers to solve this problem.

Adhesive is another element. Heiner said, "If it is too thin and soft, it will be on the accumulation drum, which is very expensive to maintain. Unless a suitable blade is used for trimming, dust will accumulate on the drum and the printing machine will get stuck. "

In addition, attention should also be paid to avoid folding labels and damaging printer information data when packaging finished products.

According to F, the sales coordinator of CD Company in Illinois, the company packs laser labels with special anti-roll lining and sealable plastic bags to prevent them from getting wet (moisture and heat can curl the products). Because the adhesive will not penetrate until one year later, the shelf life of the label is written on the package. F means that when users feed laser labels into the machine, they can put them into the paper tray by hand to prevent curling, which can avoid failures. 1. For cylindrical bottles, especially those with a diameter less than 30mm, the materials should be carefully selected.

2. If the label size is too large or too small, you should pay attention to the actual test.

3. If the surface to be pasted is irregular or even spherical, special consideration should be given to the label material type, thickness and adhesive.

4. Some rough surfaces (such as corrugated boxes) will affect labeling, and the varnish on the surface of corrugated boxes will also have an impact.

5. Automatic labeling machine labels, and labeling tests can be conducted if necessary.

6. Even if the label is labeled at room temperature, pay attention to whether it experiences high temperature during export transportation and use.

7. Too much water or oil will affect the characteristics of adhesive, and attention should be paid to the environment and temperature of labeling.