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 mesh, and the application conditions vary from country to country. However, it can be seen from the development trend of global labels in recent years that flexographic printing, narrow web printing, and digital printing have become new highlights in label printing in European and American countries, and they are also the development trend of label printing. In terms of pre-press processing, many customer-designed orders are mainly offset printing or gravure printing. If this kind of original is printed with flexographic printing, there will be many quality problems in the proofs, such as the color is not in place, the layers are not obvious, and hard edges appear. Wait. Therefore, to solve such problems, timely communication with customers is very necessary.
1) Today’s label printing is done manually, and many single-color ones are printed by machine (which can improve production efficiency. Although the overprinting accuracy of many newly developed label printing machines is not high, it improves printing efficiency. Efficiency and printing quality need to be improved. Label printing covers several main printing methods. In the plate making stage, different printing methods are selected. For different printing methods, the plate making process is also different. Take the plate making process as an example to give a brief introduction.
The flexographic plate making process is: original, film (negative), exposure, drying and post-processing.
1. . The original design suitable for flexographic printing should have the following characteristics: a large number of colors but less overprinting; the reproduction of particularly small details is not required; the screen line is not too high, but the color printing effect can be achieved; and packaging processing can be done online.
2. Film (negative film) meets the requirements of plate making, with clear graphics and accurate size specifications; use frosted film, and the density of the four corners of the film must be consistent; use a film with orthographic characters; measure with a transmission density meter, and the white bit density should be 0.06. below; the black level density is above 3.5.
3. Exposure includes back exposure and main exposure
① The support film of the photosensitive resin plate is laid upward and the protective film is laid downward. Receive exposure in the exposure drawer. The ultraviolet light passes through the support film to solidify the photosensitive adhesive layer. It can also control the washing depth and strengthen the bonding force between the support film and the photosensitive resin layer as needed. The thickness of the base is determined.
② Main exposure. The photosensitive resin plate support film is facing downwards and the protective film is placed flatly in the exposure drawer. , and then attach the film surface to the photosensitive resin plate. Apply the vacuum film flatly to the film (non-medicated film) to make the film and the photosensitive resin layer closely adhere to each other. Ultraviolet rays pass through the vacuum film and the transparent part of the film, causing the photosensitive part of the plate to polymerize and solidify. The length of the main exposure time is determined by the plate model and the intensity of the light source. If the exposure time is too short, the slope of the image and text will be too straight, and the lines will be curved. The small dots will be washed away. Otherwise, if the exposure time is too long, the printing will be blurred and the writing will be blurred. If there are large and small lines, thick and thin lines on the same printing plate, use black film to cover the small parts and expose them separately. It will not be lost due to washing to ensure the quality of the printing plate.
4. Wash and dissolve the unsensitized part, and retain the photopolymerized relief. The length of washing time depends on the thickness of the printing plate and the depth of the printing. If the plate washing time is too short, unsensitized resin will be left on the plate, which will affect the depth of the plate. If the plate washing time is too long, the plate will expand, causing the fine parts to deform or fall off.
5. Drying. Remove the plate washing solvent to restore the printing plate to its original size and thickness. The baking temperature is between 50-60°C. The baking time is determined by the thickness of the plate and the length of the plate washing time. Generally, it takes two hours for thick plates and one hour for thin plates. If the baking time is too long and the baking temperature is too high, the printing plate will become brittle and affect the printing life. If the baking temperature is too low, the drying time will be prolonged. If the baking time is too short, the plate will be damaged during printing.
6． Post-processing. That is, detackification and post-exposure. The photosensitive resin is completely hardened (polymerized) to reach the desired hardness index, and the viscosity of the printing plate is eliminated to facilitate ink transfer. The post-processing time is obtained by testing, and the purpose is to prevent cracking and sticking.
(2) With the development of economy, labels are widely used in commodities, which has led to the rapid development of label printing industry. However, people’s understanding of label printing inks is far less than their understanding of label materials, printing methods, etc. Label printing inks are like a mysterious field that urgently needs to be recognized and explored.
There are many types 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 uses letterpress printing, offset printing, flexographic printing and screen printing. From a technical point of view, these printing methods have their own advantages and disadvantages. Disadvantages: No printing method can replace another printing method.
Offset printing is a relatively common printing method. Domestic sheet-fed printing is basically dominated by offset printing. This printing method is fast, has high printing clarity, accurate overprinting, and full printing layers, but it is durable. It has poor performance and shallow ink layer, so it is not suitable for printing brightly colored label products.
Letter printing reproduces images with good clarity, higher printing speed, stronger color intensity and durability than offset printing, slightly worse than screen printing, but its layering performance and registration are better than those of screen printing. Printing and flexo printing, so most domestic label production uses letterpress printing.
The biggest advantage of screen printing is that the ink layer is thick and the covering power is good, but the speed is slow, the layering performance is poor, and the registration is low (this refers to the general domestic level, Wanzhen has developed it in recent years The registration accuracy of the screen printing machine has reached 10 lines, and the speed reaches 6,000 prints per hour), so the screen printing machine can be used to print labels with a weak sense of layering.
The print characteristics of flexographic printing are relatively average. This method is often used in foreign countries to print labels. However, this technology has high requirements on the processing of anilox rollers and the technical level of workers, although the machine is relatively simple. However, the process is quite complex, and not many labels are printed in this way in China.
If you want to print the most exquisite labels, you need to effectively combine various processes during the printing process and integrate the advantages of various printing methods to form a greater combination of advantages.
The purpose of combining multiple processes:
(1) Effective anti-counterfeiting effect: using multiple process effects for printing on the same label product, especially in some products that are easy to be counterfeited This printing method is used on the labels of cosmetics and food packaging, allowing consumers to see the authenticity of the product at a glance through the label, thereby protecting the interests of product manufacturers and consumers, and also winning an excellent reputation for the printing house.
(2) Improve the quality of printed matter: The combination of multiple 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, thereby satisfying people The growing sensory requirements also improve the printing quality of printing plants, thus bringing more vitality to them. To improve production efficiency and reduce costs, the combination of multiple processes also includes the combination of printing and post-printing processes. During product production, printing and post-printing are completed at one time, and the finished product is produced directly, which can greatly reduce costs.
Multi-process machines have the following two forms:
(1) One is a single machine combination: the most common one among trademark printing machines is the slant-back machine , printing, hot stamping, die-cutting, waste discharge and other functions can be completed on one machine, but because it is printed on a single machine, the printing size will be affected.
(2) The other is to combine the units into a production line: printing and post-press processing technology units such as letterpress printing, screen printing, flexo printing, hot stamping, and die-cutting are combined into a flow production line for printing production. Achieve the effect of combining multiple processes on the same product. This combined printing production method adapts to the needs of the development of the label market. The quality of printed matter has been greatly improved, and it has also brought huge profits to printing manufacturers. The development of unit-type production lines in China has become more and more popular. Be optimistic.
The combination of letterpress, flexo and screen printing in label printing
A practical example of label production is used to illustrate the combination of multiple processes in the production process. We want to print a label with color levels, blue solids, character avatars and corresponding text descriptions. From the above comparison, we know that printing patterns with richer color levels, letterpress printing is better than flexographic printing in label printing The layering is stronger. The best choice for printing actual graphics and text is screen printing. For character avatars, we can choose flexo printing. Therefore, on this label, we combined letterpress, flexo and screen printing to form a production line to print exquisite labels. In label printing, this combination of letterpress, flexo and screen printing is a typical example of a combination of multiple processes.
The process arrangement in the combination of multiple processes is also a science and requires a lot of practical experience. For example, screen printing is slow to dry and other issues, so comprehensive considerations must be made when combining.
(4) When people talk about laser tags, they must think of the word "need to be treated with care" because it is difficult to produce and transport. Of course it's a great product and there's a lot of demand for it. But if it can be replaced by other tags, then give up laser tag.
One problem is overheating. The fusing head on a laser printer reaches temperatures of 250-300 degrees Fahrenheit. To produce a perfect label, the base, adhesive and liner must all be assembled. Even if the ingredients are all right, if the label moves too slowly during printing, it can be damaged by heat. Another problem is that the label is dog-eared - which means the label is invalid. If the product is damaged, it must be repaired at the printing shop.
Paper suppliers and manufacturers must solve the above problems before labels are shipped to users.
Laser labels are generally processed on rotary flexographic printing equipment. According to Heinl, vice president of RLP Iowa. “Laser labels are difficult to produce on a press.”
Paper suppliers need to understand the moisture control requirements required for laser labels so they can resolve hemming and press jam issues. Whether curling occurs on 81/2 or 11 die cuts, manufacturers will have a hard time solving the problem.
Adhesives are another element. Heinl said, "If it is too soft, it will collect on the roller, which is expensive to repair. Unless you use the appropriate blade when trimming, dust collecting on the roller can cause the press to jam."
In addition, be careful when packaging finished products to avoid label folding and damage to printer information.
According to Illinois CD Company sales coordinator F, the company uses special anti-roll linings and sealable plastic bags to package laser tags to prevent moisture (moisture and heat will cause the product to curl). Since the adhesive takes one year to dry, the shelf life of the label is written on the packaging. F said that when users feed laser labels to the machine, they can put their hands into the feed tray to prevent curling, which can avoid malfunctions. 1. For cylindrical bottles, especially those with a diameter less than 30MM, materials must be selected carefully.
2. If the label size is too large or too small, attention should be paid to actual testing.
3. If the object to be attached has an irregular surface or even a spherical surface, there are specific considerations for the type of label material, 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, labeling tests can be carried out if necessary.
6. Even if the label is affixed at room temperature, pay attention to whether it has experienced high temperatures during export transportation and use.
7. A watery or oily environment will affect the characteristics of the adhesive. Pay attention to the environment and temperature of labeling.