1. Characteristics of metal printing
1) Metal printed substrates are various metal sheets or metal packaging containers.
2) Coatings play a decisive role in the quality of metal prints. Coatings can increase the adhesion of metal substrates and inks and protect the ink film layer.
3) Use special metal printing inks. Because the surface of the metal printing substrate is smooth and does not have adsorption capacity, the metal printing ink is usually a thermosetting synthetic resin as the binder of the ink, and the heating is performed to oxidize the conjunctiva and volatilize to dry.
4) The printing operation of metal sheets or molded articles is also different from ordinary paper printing, and it has its own special printer structure and operating rules.
5) The printed metal plates or molded articles should be processed by different machines according to their uses. These products are required to have heat resistance, light resistance, abrasion resistance, and drug resistance.
2. Classification and Printing Methods of Metal Printing
Metal printing is generally classified according to the shape of the substrate and the type of substrate. The printing method also varies depending on the printed material. See Table 1.
3. Metal flexographic printing process
Flexo printing has become the fastest growing environmental printing method in the printing industry in the world. It is called green security printing, especially in the packaging field. Flexo printing has been widely used. At present, flexo printing is expanding in the field of metal printing. The development of metal flexo printing has played a pivotal role in the entire metal packaging and printing industry, and it has a profound impact on domestic product penetration and on the international market.
3.1 Metal printing materials pretreatment
Due to the non-absorption, high hardness, and thickness of the metal printing materials, and their uniqueness in surface reflection, material processing, and post-press processing, the metal flexographic printing process is quite different from conventional paper printing, including There are also differences in pretreatment processes such as undercoating, primering, and overcoating. The inner coating is to apply a layer of protective bright oil on the inner side of the metal printing material molding product to protect the metal printing material from the erosion of the contents, and also to protect the contents; the primer coating is the metal printing material and the ink. The layer of adhesive between the layers, its role is to make the ink or white paint can be firmly attached to the metal printing materials; coating is to print the surface, its role is to improve the adhesion of the metal surface and the ink layer.
The metal substrates coated with paint must be baked in a high-temperature oven before they enter the printing process. The baking temperature and baking time should be determined according to the specific conditions of each factory.
3.2 The speciality of pre-press design for metal flexo printing
The selection of the pattern, text, and color in the manuscript by the designer of the metal flexo package must first meet the production requirements for flexo plate making and printing; secondly, it should be suitable for the actual level of equipment, materials, and operations of the printing manufacturer; and also satisfy the user's product. The specific practical standards for packaging, in the prepress design for metal flexo printing, designers must pay attention to the following points:
1 Express the color number of creative ideas For the packaging design of metal flexo printing, on the one hand, it is necessary to take into account the number of printing machine color groups. On the other hand, from the process, it must be taken into consideration whether white ink is used or whether lacquer is used, so that it can finally be used. To express the color number of the idea. Because the flexographic printing is transmitted through the anilox roller, the number of the anilox roller determines the amount of ink transferred and the fineness of printing. Normal screen adjustments use high-line anilox rollers to make the dots clear; while field versions use low-number anilox rollers to achieve color saturation. In a color, there are both halftone images, and on the ground, and when they are visually prominent, this color is generally divided into a halftone edition and a field edition separately to facilitate the printing operation.
2 pay attention to the level of application: As the flexible version of the quality of the letter, the material is highly flexible and easily deformed, so the pressure after the increase in the network is more serious, and in the process of plate-making, 1% or 2% of the small outlets are easy to lose when washing , 3% of the outlets will expand to about 10% after printing, and it will be impossible to achieve a gradient of 100% to 0% or a very soft layer. Therefore, the areas of highlights and shades that flexographic products can express are relatively Smaller, better-printed products with high light can only reproduce 8% to 10% of outlets. When using gold or silver ink, if the pattern contains a halftone, it is better to leave more space in highlights and shadows. The best light spot is better than 20%, because such ink printing dots are easy to paste.
3 consider the printing overprint: metal flexo printing is the use of wet-press method to transfer the ink to the metal substrate, so only overprinting, can not be overprinted, so when designing colors as possible choose a single color. When the two parts of the text or pattern need to be overprinted, there should be a certain gap to prevent string ink, so the metal flexo printing must be trapped to solve this problem before printing.
The embossed pattern on the flexographic plate is formed by brushing off the unexposed portions with a solution of the printing solution and a brush, so the flexographic plate is required for the size of the smallest character and the finest line.
In terms of the use of barcodes, since the flexographic plate is easily deformed during the printing process, the plate material is flatly attached to the circular cylinder and must be deformed. Therefore, for different printing machines and different bar code sizes, each bar code is before the plate making process. There must be a reduction in advance in order to be able to be tested after printing. At the same time, the direction of the bar code should be consistent with the printing direction.
4 Try to use vector software when designing: Most designers present their ideas using computers as tools. The design software has vector forms, such as Illustrator, Freehand, CorelDraw, etc., in the form of pixels, such as: Photoshop, Paint, etc. Photoshop is best used only to process images or make some special effects. Other lines, text, and color blocks are best made with vector software.
5 Particularities of the plate-making film: The traditional photosensitive plate plate-making uses frosted film, that is, the film surface of the film is frosted. The negative image output after separation requires that the density of the non-graphic portion be 3.5 or more and that of the transparent portion be 0.06 or less.
6 Flexible plate: The hand-engraved rubber plate, cast rubber plate, and photosensitive resin plate used in the traditional process each have their own suitable reproduction range. In today's digital process flow, the flexo platesetter CDI is the first laser output device for direct exposure of flexographic photopolymer plates. It spans the steps of stencil and film flushing, eliminating the need for film and ensuring consistent quality. Consistent, so the quality of digital flexo is more stable than the traditional process, and the minimum dot made into the cable version is 1%, which solves the problem of high-quality layered screen printing.
3.3 Metal Flexo Printing Preparation
1 The standard sample is the basis for printing. It should understand and master the color conditions of the standard sample, the main features of the image, and the main technical requirements.
2 consider printing color sequence.
3 Master and understand the performance characteristics of printed materials.
4 understand the printing ink, solvent characteristics and related information.
5 Understand layout, dimensions, positional relationship, registration requirements, printing pressure requirements, etc.
3.4 Metal Flexo Printing Jobs
When the pre-print preparation work is done and meets the requirements, it can be tested. The process is as follows:
1 Start the printing press to adjust the impression cylinder to the pressed position for the first time.
2 Observe the condition of the first proof-print proof check and the printing position on the product inspection table, and make supplementary adjustments to the printing press according to the problems that arise to achieve the correct registration.
3 start the ink pump to adjust the amount of ink sent to the ink roller to send ink.
4 Start the printer for a second trial printing. The printing speed is determined by a predetermined number. The speed of printing depends on past experience, printing materials, quality requirements of printed products, etc., and the use of prescribed official printing materials should be minimized.
5 Check the color difference and other related defects in the second sample and make corresponding adjustments. If the color density is not normal, adjust the ink viscosity or adjust the number of anilox roller lines.
6 When there is a color difference, the ink can be replaced or reconfigured as required.
7 Inspection When a qualified product is printed, a small amount of printing can be done before inspection.
After the trial printing and printing products meet the quality requirements, the formal printing is continued. In the printing, at the same time continue to check the situation of registration, color, ink, ink drying, etc. If there is a problem, it should be adjusted and corrected in time.
3.5 Post-printing of Metal Substrates
The printed metal substrate must be dried in an oven. According to the different ink anti-oxidation performance, choose different drying temperature and time. The principle is that the ink does not change color, the white paint does not turn yellow, and the printing surface is lightly rubbed with absorbent cotton with 50% alcohol, and the ink should not fall off. After the metal substrate is printed, it is generally subjected to two processes: coating and forming. The purpose of the brightening oil is to protect the ink film, increase the gloss of the printed material, make the product more beautiful, and can enhance the bending and the mechanism during the can manufacturing process. Impact capacity.
4. Inspection and quality control methods for metal flexographic products
4.1 Inspection Quality Requirements for Metal Flexographic Printed Products
1 The contents of conventional physical and chemical tests are: appearance inspection and physical and chemical indicators - appearance inspection is mainly based on visual observation, that is, the front and back of the product requires no obvious dirt, wrinkles, no major scratches on the main part of the printing surface, and minor parts allow Slightly scratched. Multi-color printing requires accurate overprinting, no leaks, no overlapping borders, solid and delicate details, neat surroundings, and no ghosting. The text lines are clear, complete, non-deformable, clear, complete, distinct, uniform ink, and accurate hue. Physical and chemical indicators include: impact strength, adhesion, gloss, whiteness, ink surface hardness, resistance to cooking and so on.
2 Special requirements inspection: Special requirements inspection refers to the product package